Micro-zooplankters in the plankton communities of the upper waters of the eastern tropical Pacific - PDF Free Download (2023)

Deep Sea Research, 1971, Vol. 18, pp. 861 to 883. Pergamon Press. Printed in Great Britain.

M i c r o - z o o p l a n k t e r s in the p l a n k t o n c o m m u n i t i e s o f the upper w a t e r s o f the eastern tropical P a c i f i c JOHN R. BEERS* and GENB L. STEWART* (Received 15 December 1970; in revisedform 11 March 1971; accepted 17 March 1971) AbstmetmThe numerical abundance, biomass, and taxonomic composition of the micro-zooplankton and chlorophyll, phaeophytin, and total seston levels were determined for six depth intervals within the upper 200 m at 12 locations from approximately 10°N to 12°S latitude on longitude 105°W. Data on the hydrography, phytoplankton inorganic nutrients, larger zooplankton standing stocks, and phytoplankton productivity at the various sites was available through the EASTROPAC program. Average micro-zooplankton volume over the euphotic zone showed a threefold range, from 15 mma/ms at the southerly extreme of our sampling to 47 mmS/ms near the Equator. The relative taxonomic composition of the micro-zooplanktonpopulations within the three size classes of material separated was similar to that described from earlier studies of California Current populations. Highly si~,nificantpositive correlations were found between micro-zooplankton abundance and chlorophyll levels in the various depth intervals when 'groups' of stations, delineated on the basis of similar hydrographic features, or all stations in the study were considered. Standing stock biomass of micro-zooplanktonaveraged 34 ~o(range, 23-66 ~o) of that of the phytoplankton, both estimated in terms of dry weight over the euphotic zone at the different sites. This was considerably greater than was earfier indicated for stations in the California Current. Calculations, based on assumptions regarding micro-zooplanktonfeeding rates, suggested that these small animal plankters might be consuming an average of 70Yo (range, 39-104~) of the daily phytoplankton organic carbon production at stations along the transect. The same calculation using data from weekly observations taken between April and September at a site five miles off the La Jolla coast in~eated an average of only 23 ~o (range, 7-52 ~ for this relationship. Micro-zooplankton biomass within the euphotic zone at the various locations relative to the larger zooplankton was suggested to be about an average 24 Yo, but this calculation was complicated by the different depths of sampling for the two populations and the methods of determining their volumes. INTRODUCTION

A SUOGESTIONthat micro-zooplanktont may be a more important component of food webs in oceanic areas than in nearshore regions was made in an earlier study (BEEPS and STEWART,1969a). This was based on a very limited number of observations of the relative standing stock abundance of several trophic levels in only one season and over a small geographical area. The present study has provided the opportunity to describe the composition of the micro-zooplankton populations and to relate their abundance to other living and non-living components of the environment at 12 locations over an extensive oceanic area transecting several water masses in the eastern tropical Pacific. The work was carried out as a special investigation in co-operation with the Eastern *University of California, Institute of Marine Resources, La Jolla, California 92037. tThe definition of 'micro-zooplankton' used here includes all animal plankters which by the methods employed passed 202/~ mesh filtering cloth and were preserved in recognizable form. This upper size limit is approximate to that proposed, for example, by DUSS~T (1965) and the 'working party' of the ICES, SCOR, and UNESCO group established in 1964 to evaluate zooplankton sampling (UNESCO, 1968).

861

862

JOHN R . BEERS a n d GENE L . STEWART . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tropical Pacific (EASTROPAC) program and complements their plankton observations. The 14-month series of EASTROPAC cruisest was designed to provide seasonal coverage on a wide range of hydrographic and biological variables in a broad area of the Pacific of major interest in recent decades due partly to a desire to extend commercially-productive fishing outward from the Central and South American coast. Findings for some of the biological variables considered have been presented by BLACKBURN, LAURS, OWEN and ZEITZSCHEL(1970) (chlorophyll a, zooplankton, and micronekton standing stocks) and OWEN and ZEITZSCHEL(1970) (phytoplankton and primary production). MATERIALS AND METHODS Samples of total seston material, including micro-zooplankters, phytoplankton, and detritus, were collected from five or six depth intervals over the upper 110-200 m at 12 approximately equidistantly-spaced locations during the EASTROPAC Cruise 76 of the R.V. David Start Jordan (Feb.-April, 1968). The stations, one per day, were occupied on consecutive dates from February 29 through March 1 l, 1968, and were between approximately 10°N and 12°S latitude on 105°W longitude. Pumping was from 1100 to 1630 hr each day. Station positions and the depth of their euphotic zone, estimated as three times the Secchi disc depth, are given in Table 1. Each location was adjacent to an EASTROPAC survey station site, being at the terminus of a 20 min, ½ m, 333 ~ mesh plankton net oblique tow from 200 m to the surface. The submersible pump-profiling hose system and deck-mounted plankton concentrator used here has been described by BEERS, STEWARTand STRICKLAND(1967). By a graded series of filtering cloths in the concentrating unit, the seston particles are separated into several size classes. A 363/z mesh cloth is used primarily to exclude the Table 1.

Positions and estimated euphotic zone depths o f station where micro-zooplankton collections were made.

Date 29 February 1968 1 March 1968 2 March 1968 3 March 1968 4 March 1968 5 March 1968 6 March 1968 7 March 1968 8 March 1968 9 March 1968 10 March 1968 11 March 1968

EASTROPAC Sta. number

Latitude

Longitude

Euphoric zone (3 × Secchi depth) (m)

76.021A 76.025A 76.029A 76-035A 76.044A 76-053A 76-062A 76.071A 76.079A 76.083A 76.087A 76.091A

10°38'N 8°25'N 6°12'N 4°22'N 2°30'N 0°39'N 1°13'S 3°22'S 5°30'S 8°09'S 10°16'S 12°21'S

105°03'W 105°13'W 104°52'W 104°56'W 105°08'W 105°06'W 105°09'W 105°00'W 104°58'W 105°00'W 105°06'W 105°01'W

102 87 99 84 75 69 60 72 66 78 75 78

tCopies of EASTROPAC Information Papers describing the expeditions, the nature of the observations and collections and station locations may be had from the EASTROPAC Coordinator, P.O. Box 271, La Jolla, California 92037.

Eastern tropical Pacificmicro-zooplankton

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larger organisms not quantitatively sampled by this method, while progressively smaller material is sorted out and concentrated on 202 /~ (+202 sample in text), 103 /z ( + 103), and 35 /~ (+35) filtering cloths. An unconcentrated sample of the particulate matter passing the 35 t~ mesh (--35 sample) is obtained by saving a small portion of the water passing this filter throughout the period of pumping over each depth interval. The sampling program was similar to that used previously (BEERSand STEWART, 1969a). The pump, delivering water on deck at approximately 140 litres per min was lowered through each interval in steps of one meter every 1.5 rain (depth intervals I-IV) or minute (V) or 2 m/l.5 min (VI). The volume of water sampled ranged from approximately 2 m 3 over depth interval I to 7.5 m 3 in the bottom interval (VI) or approximately 29 m 3 for the total column. Although various amounts of weight, from 100 to more than 900 lb, were used, hose angles developed during the sampling through the deeper strata. These were especially noticeable in the region of the equator where a particularly strong undercurrent exists. Depths of sampling were corrected, if necessary, using the pumped water temperature (adjusted for warming coming to the surface) related to data from a bathythermograph attached to the pumping unit and STD (Salinity, Temperature, Depth Measuring System, The Bissett-Berman Corp., San Diego, California)casts made immediately before and after the micro-zooplankton sampling. The uncorrected depth intervals were: I, surface9 m; II, 10--24 m; III, 25-49 m; IV, 50-74 m; V, 75-124 m; and VI, 125-200 m. At Sta. 76.062A a break in the electrical power leads to the pump restricted pumping to five depth intervals. Samples were fixed and preserved in an approximately 5 ~ formalin-sea water solution having the pH maintained at 8.0 - 8.2 with sodium borate. Enumeration of all recognizable micro-zooplankton, with the exception of possible particle-feeding flagellates, was based on the inverted microscopes method of Ua~RMOHL (1958). Taxonomic distinction, except for tintinnid ciliates to be reported on separately, is to major group only, e.g. Foraminifera, Radiolaria, Ciliata other than Tintinnida, etc. Sub-samples of the --35 material were examined at a magnification of 200 x until a minimum of 40 organisms were counted. Size measurements were made on all animals. Organisms in aliquots of the +35 fraction were counted at 200 x magnificationuntil a minimum of 100 each of Protozoa and Metazoa was reached. Size determinations of the first 20 organisms of each taxonomic group, except the Tintinnida where most individuals were measured, were made on all samples from approximately alternate days throughout the series. + 103 micro-zooplankton were counted until a minimum of 100 metazoans were seen. Measurements for subsequent volume estimates were done as with the + 35 size fraction. With a count of 100, assuming random distribution of organisms in the sub-sampling (generally found to be true of our procedures, but see BEERSand STEWART(1969a) for criticism), the accuracy in terms of repeat counts obtained at the 0.95 confidence level should be approximately -q- 2 0 ~ (LuND, Kn'LINC and LE CREN, 1958). At the level of 40 organisms counted in the --35 samples the limits of expectation would be about 4- 33 ~. Considering the non-random spatial distribution of these organisms in the field, their short generation times, etc., even though we pumped a relatively large amount of water per sample, these levels of counts probably yield as valid a picture of the momentary standing stock micro-zooplankton populations as is reasonably possible.

864

JorrN R. Bmm$and GENEL. STEWART

The volume of the body material of the various micro-zooplankton types was calculated using the size measurements made during the counting of the samples, usually estimating a third dimension (depth), and assigning a standard geometric shape (cone, cylinder, ellipsoid, etc.) or combination of shapes to the organism. Only complete animals were included except for the tintinnids and Radiolaria. Here, because of the ease with which the soft body material can be separated from the lorica or skeleton at several points during the sampling and fixing procedure, ' e m p t y ' organisms have been included. In both of these groups it has been estimated that the body of the animal normally occupies about one-half of the total volume of the ' structural clement.' This does vary, however, between species, e.g. smaller tintinnid forms generally appear to occupy a relatively greater proportion of the total lorica volume than larger species. Wide variations in the calculated volumes of organisms in any one sample resulted in detecting very few significant differences in the size of organisms in any given size fraction between the various depth intervals and/or locations of sampling, i.e. significant if the mean + 2 × the standard deviation of any given sample falls outside these limits for any other sample. Hence, in calculating the volumes an average value derived from all individuals measured has been used. The one exception to this was in the --35 samples where the estimated volumes of Foraminifera and copepod stages seen at the northern locations were significantly greater than the overall average. Thus, figures for' average' individuals of these groups in the --35 material were calculated separately for the first three dates of sampling and for the remaining nine days. Prior to adding fixative, aliquots were removed from each sample for the determination of chlorophyll a-phaeophytin by a fluorescence method based on YENTSCH and MENZEL(1963) and HOLM-HANSEN,LORENZEN,HOLMESand STRICKLANO(1965) and for total seston dry weight by a procedure modified from BANSE, FALLS and HOnSON (1963). Concentrated samples (Whatman GF/C filters) for chlorophyll, with added MgCOa, were deep-frozen until analyzed approximately two weeks later. Readings were made on a Turner Fluorometer (Serial B2017) and pigment concentrations were calculated using door factors determined by EASTROPAC for Cruise 76. A comparison of analyses by our procedure on fresh and frozen samples (6 replicates each) indicated an approximately 20 % lower value for chlorophyll on the frozen falters. Phaeophytin level was comparable on the two sets. Photosynthetic pigment levels were also determined at the 12 stations by EASTROPAC but at specific depths. Comparing results, both sets of data integrated and averaged over the euphoric zone (3 × Secehi disc depth), showed their levels to range from 73 % to 152 % of our determinations, the average over the 12 sites being 118 %. The phaeophytin concentrations were even somewhat more divergent. These differences could very possibly be due to the two methods of sampling, i.e. discrete depth (EASTROPAC) versus complete depth intervals, because of layering such as demonstrated by STRICKLAND (1968). Hence, no adjustments have been made to figures presented here. Pre-washed and pre-combusted Whatman GF/C filters were used for the dry weight determinations. Salts were removed from the filters and samples with brief rinses using ammonium formate isosmotic with sea water. Filters were dried to a constant weight at approximately 60°C. Weighings were done on a Cahn Electrobalance Model G. A data record which contains results on the numbers (including 'confidence limits' for each group based on the actual numbers of organisms counted in the

Eastern tropical Pacificmicro-zooplankton

865

sub-samples) and estimated volumes of all micro-zooplankton taxonomic groups examined, chlorophyll a and phaeophytin concentrations, and total seston dry weight for each of the size classes sampled in each of the depth intervals at the twelve stations has been prepared. Copies are available at the cost of reproduction from the Institute of Marine Resources, University of California at San Diego, La Jolla, California 92037. The data on the physical and chemical characteristics of the locations studied and their standing stocks of the larger zooplankton were supplied by EASTROPAC. In considering the micro-zooplankton populations in relation to other biological variables of their environment, as well as from station to station, it is valuable to have an 'ecologically-meaningful' basis for comparison. Previously we have used the euphoric zone for this purpose (e.g. BEERSand STEWART, 1969b). In the present study, this layer, calculated as being 3 × the Secchi disc depth, again appeared to be useful except perhaps at the more northern sites (Sta. 76-021A, 76.025A, and 76.029A) where relatively large amounts of small seston material in the sharp thermocline and nutricline below the Secchi depth suggests the compensation depth would be considerably higher in the water column than calculated. The average abundance of micro-zooplankton/m 3 was also examined from the surface to 100 m regardless of the distribution of any water column variables and over a euphoric zone calculated as 2.5 × the Secchi disc depth. Averaged over the top 100 m, the level of total micro-zooplankton varied from 72 % to 101% (average, 90 %) of that calculated on the basis of a 3 × Secchi depth euphoric zone. Values for a 2.5 × Secchi depth ranged from 98 ~o to 116 % (average, 107 %) of those for the 3 × Secchi depth. A 2-5 × Secchi depth was used by EASTROPAC as the basis for estimating euphoric zone depth. However, data on chlorophyll a and phytoplankton production at many of our stations around and to the south of the equator show significant rates of plant production below this level and suggests a better overall approximation is provided by a 3 × Seechi disc depth. RESULTS

Hydrography Various aspects of the gross hydrographic features of the eastern tropical Pacific have been described (e.g. WOOSTer and CROMWELL,1958; WYRT~, 1965). The present study of micro-zooplankton populations at sites between approximately 10°N and 12°S latitudes along 105°W longitude sampled in several different regions which can be delineated through their hydrographic characteristics. The three most northerly sites were in the westward flowing North Equatorial Current. Seven stations, from 76.053A to 76.091A, were across the South Equatorial Current. Stations at 4 ° 22'N (76.035A) and 2 ° 30'N (76.044A) showed some differences from the adjacent North and South Equatorial Current stations and were probably within the eastward flowing Equatorial Countercurrent system which at the time of year of this study is relatively weakly developed and at its southernmost point. The hydrographic and nutrient characteristics observed in the upper 200 m have served to separate the pumping sites into three groups of four stations each. Temperature-salinity diagrams and nitrate-N profiles (nitrate data from EASTROPAC) are shown in Fig. 1. Briefly, Group A sites (Stas. 76"021A, 76"025A, 76.029A, and 76.035A) had a mixed layer of at least 30-35 m with temperatures in excess of 26°C underlain by a generally sharp, strong thermocline in

866

Jomq R. BEERSand GL'NEL. Sa~WART SALINITY,%.

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76.071A

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150

200

Fig. 1. Tcmperatur~sallnity diagrams and nitrate-nitrogen profiles for Groups A, B, and C stations. which oxygen levels dropped below 1 ml/l. (except 76.035A) and nitrate increased from mixed layer concentrations of less than 1pg A N/1. (Between Stas. 76-025A and 76.029A there was a shoaling of the thermocline with mixed layer depths of only approximately 20 m (BLACKBURN, LArrY, OWEN and ZEITZSCHEL, 1970). Sites

Eastern tropical Pacific micro-zooplankton

867

included under Group B (Stas. 76.044A, 76.053A, 76.062A and 76.071A) showed mixed layers, if any, of less than 35 m and surface temperatures below 26°C. Their thermocline was not as sharp as seen under Group A; nitrate concentration in the surface water was greater than 1/~g A N/1. The Group C stations (76-079A, 76.083A, 76.087A and 76.091A) had mixed layers in excess of 35 m but with temperatures less than 26°C. Surface/mixed layer nutrients were relatively high (NOa-N, > 5 / ~ g A/1. The thermocline and nutricline was weak compared to the Group A sites. In relating this grouping of stations to the areas described by BLACKBURN(1966), Group A sites fall mainly into areas 4 and 7, Group B into area 9, and the Group C set into area 11. Micro-zooplankton

The abundance of the total micro-zooplankton is shown in Fig. 2 as biomass (volume) estimates for each depth interval at all stations. The levels attributable to each size class of sample are indicated. Amounts given are the averages for the various depth intervals sampled. The pattern of micro-zooplankton vertical distribution showed variations between stations which could be correlated in many instances with the hydrographic features of the different sites (see later). In general, the highest concentrations of microzooplankton material were found within the euphotie zone but not in the top depth interval sampled which included the upper 10 m of the water column. A markedly lower level of micro-zooplankton in all size classes was found below the compensation depth relative to the euphotic zone. The average standing stock of total micro-zooplankton (Protozoa and Metazoa) numbers and biomass over the euphotic zone at each site are presented in Table 2 and Fig. 3, respectively. In terms of biomass distribution an approximately threefold difference was found between the lowest (Sta. 76.091A, 15-0 mma/m 3) in waters at 12°S latitude and highest (Sta. 76.062A, 47.1 mm3/m 3) just south of the equator. Table 2.

The average numerical abundance o f protozoan and metazoan microzooplankters/m a within the euphotic zone. Protozoa

Metazoa

Sta.

--35 (No./m a)

+35 (No./m a)

+103 (No./m a)

76.021A 76"025A 76-029A 76"035A 76.044A 76"053A 76.062A 76.071A 76"079A 76.083A 76"087A 76"091A

350 ~00 400 000 190 000 220 000 380 000 520 000 890 000 530 000 490 000 470 000 240 000 370 000

13,000 11,000 8000 16,000 21,000 29,000 46,000 19,000 23,000 13,000 12,000 22,000

1700 2400 2200 2900 2700 3100 5500 2600 2300 3000 6000 3700

--35 (No./m 3) 37,000 20,000 9500 4100 7000 6400 9000 8300 8600 13,000 2800 1000

+35 (No./m a)

+103 (No./m a)

13,000 16,000 13,000 17,000 20,000 25,000 35,000 21,000 21,000 34,000 14,000 11,000

5000 5700 7300 5000 3900 6100 9800 4100 6100 5000 4900 2400

868

JOaN R. BEERS and GENE L. STEWART

Size classes

--35. As an average over the euphotic zone, numbers of animals passing the 35/~ mesh cloth ranged from 190,000/m8 (Sta. 76.029A) to 890,000/m 3 (Sta. 76.062A), averaging 430,000/m ~. Almost 98 % of these organisms were protozoans. The --35 fraction was approximately 90 % of the total micro-zooplankton numbers [i.e. (--35) + (+35) + (+103)] averaged from the 12 sites. In terms of its contribution to the total micro-zooplankton standing stock volume in the euphotic zone, the --35 fraction was small, averaging only 14% (range 6.3 %, Sta. 76.035A to 44 %, Sta. 76.021A; median, i1%) at the 12 locations. Approximately two-thirds of the --35 sample volume was accounted for by protozoans. This latter is strongly influenced by the relatively large amount of --35 metazoan material at the northern stations on the line. Post-naupliar copepods and heteropods (other Metazoa) were recovered in this size fraction only at Stas. 76.021A, 76.025A and 76.029A. In an earlier study (BEERSand STEWART,1967) across the California Current in December, 1965, significant numbers of these taxa were also found in this size sample, but were not seen in another study in a nearby part of the California Current during February, 1967 (BEERS and STEWART, 1969b). Omitting these three sites, the average of Stas. 76-035A through 76.091A showed metazoans to be less than 20% of the total --35 volume. +35. The relative abundance of protozoans and metazoans in this size class as average numbers over the depth of the euphotic zone was similar at many of the stations. While the number of Protozoa was generally an order of magnitude or more lower than in the --35 sample, that of the metazoans was usually several times greater. The volume of the individual protozoans was generally much less than that of metazoans in the same size class and, consequently, the biomass estimate for the former is relatively small. The protozoans accounted for an average 11% (range, 4-7 %, Sta. 76.083A to 21%, Sta. 76.091A) of the volume for this size class through the euphotic zone at all stations. + 35 micro-zooplankton as a fraction of the total microzooplankton population volume averaged approximately 31%. + 103. The animal plankton in this size class of sample was generally dominated, both in numbers and volume, by the metazoans. The exception to this was at the two most southerly stations where relatively large numbers of acantharians were found. The ratio of numbers of + 103 metazoans to protozoans through the euphotic zone averaged over all stations was 1.9:1 with the ratio of volumes being 27.5:1. Taxonomic groups

In general, the relative abundance of the various taxa in the different size classes of samples was similar to that previously reported for areas of the California Current (see BEERSand STEWART,1969b). Figure 4 illustrates the percentage composition over the euphotic zone at each site of the various taxonomic groups in the three size classes of micro-zooplankton. Protozoa Ciliata other than Tintinnida. These organisms were the most numerous of the

micro-zooplankters at all stations and in all depths sampled. They occurred almost exclusively in the --35 samples. Separation within the taxon was: (1) 'sheathed Oligotricha ' (e.g. Strombidium Clapar~de and Lachmann, 1859) of average volume

E a s t e r n t r o p i c a l Pacific m i c r o - z o o p l a n k t o n

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Fig. 4. The relative percentages of the"average micro-zooplankton numbers per cubic meter over the euphotic zone accounted for by the various taxonomic groups in the different size classes of samples.

872

JOHNR. BEEPSand GENEL. STEWART

5100 /z3 (723 organisms measured) equivalent to a sphere of 20-22 /~ diameter; (2) 'non-sheathed Oligotricha ' (e.g. Lohmanniella Leegaard, 1915) with an average volume of 2800 t~3 (760 organisms measured) equivalent to a 17-18 ~ diameter sphere; and (3) ' others' (e.g. Didinium sp., Mesodinium sp., etc.) also having an average volume of 2800 t~3 (1609 animals measured). This latter group included a relatively large number of poorly preserved specimens, some of which could have been 'damaged' oligotrichs but which, for the most part, appeared to have different ciliation from the Oligotricha. The desirability of examining eiliates in the living state is recognized but was not practical in the present quantitative survey-type study. Of all the 'others' counted almost 40% were in some respect questionable. In only approximately 10% of the cases were such poorly preserved forms seen among organisms in the first two categories. In terms of average numbers and their biomass equivalent through the euphotic zone the total of the ' Ciliata other than Tintinnida ' ranged from 119/1. (Sta. 76.035A) to 720/1. (Sta. 76.062A) averaging 288/1. or 0.4 mm3/m3 (Sta. 76.035A) to 2-5 mm3/m3 (Sta. 76.062A) with an overall average of 1.0 mm3/m3. The abundance o f ' sheathed oligotrichs ' as a percentage of the total averaged numbers over the euphoric zone ranged from 14% (Sta. 76.021A) to 43 % (Sta. 76.083A), averaging 26 %. Twenty-two to fifty-seven per cent (average, 37 ~o) of the total ' Ciliata other than Tintinnida ' volume was due to sheathed oligotrichs. The ' others' were an average 47 ~o (range, 35~o, Sta. 76.083A to 81Yo, Sta. 76-021A) of the total numbers or 26-73 ~o, average 40 ~o, of the total volume estimate. The ' non-sheathed oligotrichs ' were an average of 28 ~ and 23 % of the total numbers and volume, respectively. Tintinnida. Tintinnid eiliates, while being only 26 ~ (range, 10 ~A.%) of the average number of total ciliates (i.e. Tintinnida a n d " Ciliata other than Tintinnida ") [m s over the euphotic zone at all sites, accounted for more than half (average, 57 ~o; range, 28-72 %) of the total ciliate biomass. The most abundant rintinnids, e.g. species of Acanthostomella and Craterella, can pass 35 t~ mesh cloth. The range in numbers of tintinnids in this size class was from 37/1. (Sta. 76.029A) to 170/1. (Sta. 76.079A, 76"083A), averaging 9111. over the euphotic zone of all stations. Volume estimates for the --35 tintiunids were from 0.3 mm3/m3 to 1"5 mmS/ma. The --35 fraction accounted for approximately 9 0 ~ of the total tintinnid numbers in the micro-zooplankton and approximately 60 % of its volume when averaged over the euphoric zone at all sites. With the exception of sites just north of the equator, numbers of --35 tintinnids were generally higher in depth intervals relatively low in the euphoric zone compared to the top depth intervals. The maximum number within any depth interval sampled was more than 480/I. in depth interval IV (50-74 m) at Sta. 76.079A. Tintinnids in the +35 size fraction accounted for slightly less than 10% of the average total tintinnid number/m 3 over the euphotie zone but more than 38 Yo of the average total volume. The larger tintimaid species, i.e. those recovered in the -{-103 size class, were relatively scarce, contributing only approximately 1% of the total numbers and volume. The numbers of tintinnids not included in our micro-zooplankton samples is probably very small as most species reported from tropical waters have a dimension which would allow them to readily pass 202 t~ mesh cloth when oriented properly. Sarcodina. Sareodinians include the radiolarians, acantharians, and Foraminifera.

Eastern tropical Pacificmicro-zooplankton

873

The skeletons of some acantharians will go into solution readily in the fixative/ preservative used here ( B ~ m and Sa~WAaT, 1970b). Enumeration of these organisms was on a re-examination of the samples after it was determined that they could be recognized from the remains of the body. In our earlier studies these were not included in the counts. This omission is most serious in considering the + 103 Protozoa alone where the inclusion here of the acantharians added an average 159 ~o (median, 90 ~o; range, 28-368~o) to the average numbers and 19~o (range, 5-60~o) to the average volume over the euphoric zone at the 12 stations. Of the +35 Protozoa, inclusion of the acantharians added an average 8-7 ~ to the numbers/m 3 over the euphoric zone and 2"7 % to their biomass. The body volume of acantharians is small relative to other taxonomic groups in any size class since it is the extent of the spicular skeleton which dictates the fraction in which the organism is retained. The Acantharia added an average of only 3"2 ~o (range, 1.2-8.7 ~o) to the estimated total (i.e. (--35) + (+35) + ( + 103)) protozoan volume of the euphoric zone at the 12 sites and < 1 Yo (av., 0-4 ~o; range, 0.1-1.1 Yo)to the total micro-zooplankton (i.e. Protozoa + Metazoa) volumes. In some samples, particularly from the north, relatively large amounts of a simple spicular-type of material was found in the --35 samples from the upper depth intervals This may be remains of fragile collodarian or Polycyttaria radiolarians. No estimate of these was included in the count since most such forms, at least when mature, are of a size too large to be included as micro-zooplankton. Also, their role as particle-feeding phagotrophs, and that of the acantharians as well, can be questioned since they generally harbor abundant zooxanthellae and the nutritional relationships are not clear. In terms of their role in primary production, KHM~LEVA(1967) showed that in the surface waters of the Gulf of Aden, carbon production by zooxanthellae could be several times that of the free-living phytoplankton. Collozoum colonies, 5-7 mm in diameter, reached numbers of 16--20/I. in the Gulf of Aden waters. Radiolarians were generally most abundant in depth intervals around the base of the euphoric zone. Distinct from the ciliated Protozoa which were low in numbers in samples from below the compensation depth relative to the average over the euphoric zone, the radiolarians were generally still as abundant, or even more so, in these samples collected over mid-day. Total radiolarian numbers averaged over the euphoric zone ranged from 16,000/m 3 to 82,000/m a, averaging at the 12 sites 44,000]m 8 while their estimated volumes were 0.2-1 "3 mmS/m s (average, 0.7 mm3]ma). The division of numbers and volume into the three size classes was 82~o (--35), 1 6 ~ (+35), and 2 ~ (+103) by number and 18~ (--35), 4 3 ~ (+35), and 4 0 ~ (+103) by volume. Acantharia were most abundant in depth intervals within the euphoric zone relative to those below the photosynthesis compensation depth. The number of total acantharians was as high in depth interval I (0--10 m) as in any other depth interval sampled at approximately two thirds of the sites. In no ease was the maximum below depth interval III (25-50 m). Numbers of aeantharians ranged from 1100-5900]m 8 (av. 3100) over the euphoric zone at the 12 sites. Their average volume was 0"1 mm3]mS. Of the total Aeantharla over the euphoric zones of all sites, an average 49 ~ by number and 26 ~ by volume was in the +35 size fraction, the remainder being in + 103. Foraminifera, as most other protozoan groups, were.more abundant generally within the euphoric zone relative to sub-euphotic depths. While subject to variation, a tendency could be seen for the highest numbers and/or volume of these to be nearer the surface at the southerly stations than in the north. As with many of the other

874

JOHN R. BEERS and GENE L. STEWART

taxonomic groups, maximum abundance in the Group A stations was found just above the top of the thermocline. Numbers of Foraminifera averaged over the euphoric zone of the various stations ranged from 1300-16,000/m 3 (av. 8300/m3). --35 specimens accounted for an average 76~o of total numbers, ÷35 and +103 being 19~o and 5 ~ , respectively. Many of the --35 animals were juveniles of 3-5 chambers. In terms of biomass our estimates indicate approximately equal distribution among the three size classes. Metazoa Crustacea. In enumerating the copepods distinction has been made between naupliar stages and later stages, i.e. copepodites and adults = post-naupliar copepods. Size variation of the average individual nauplius in the three size classes was 87,000/~3 (--35, Sta. 76.044A - 76.091A); 330,000/~3 (+35); and 1,700,000 ~3 (+103). The average post-naupliar copepod was 810,000/~a (+35) and 3,300,000/~a ( + 103). The average numbers of naupliar copepods in the micro-zooplankton standing stock over the euphoric zone ranged from approximately 10,000/m a (Sta. 76.091A) to 44,000/m 3 (Sta. 76.062A, 76.083A), averaging 27,000/m a over the 12 sites. Postnaup[iar copepods ranged in number from 3400/m 3 (Sta. 76.091A) to 13,000/m a (Sta. 76.021A). Their average abundance over the study area was 6600/m a. Ratios of abundance of naupliar to post-naupliar copepods in the micro-zooplankton size range were from 2.4:1 to 7.3:1 (average, 4.3:1). Of the total naupliar numbers, about 95 ~owere small enough to pass 103/~mesh. Some nauplii of the relatively large copepod species may be retained on the 202/~ mesh ' exclusion' cloth. No estimate of these is available for the present study. Within the micro-zooplankton size classes, the percentage of the post-naupliar copepods through the euphotic zone that passed 103/~ mesh was considerably lower, averaging 43 Yo (range, 23-78 ~o) over the 12 sites. Although the numbers of post-naupliar copepods was lower than the nauplii, their volume was greater showing a ratio of 0.67 naupliar Copepoda: 1 post-naupliar Copepoda averaged over the entire study. Of the total naupliar volume, an average of 77 ~o (all stations, euphotic zone) was in the smaller size classes (--35 and +35) while only 17 ~ of post-naupliar copepod biomass was in this size range. Occasional ostracods were recovered in the + 103 samples but their numbers were never high and their contribution to standing stock volume was not significant. Other metazoans. Metazoans other than crustaceans which were recovered in the samples included heteropods, polychaetes, small species of chaetognaths, and larvaceans. Heteropods wero found in both the +35 and +103 cloth samples and occasional specimens (Group A stations only) were small enough to pass the 35~ mesh cloth. The additional' other Metazoa ' were almost exclusively of the + 103 sample, and with the soft-bodied forms at least, i.e. chaetognaths and Larvacea, theirinclusion here may be in error since generally the head and tail sections were separated and, if intact, may have been excluded from the sample due to size. As a component of the total micro-zooplankton populations the ' other Metazoa ' were relatively unimportant. Their numbers, averaged over the euphoric zone, ranged from 1-20% (average, 6"2~o) of the total metazoans and their biomass 3-12~o (average, 6"9 %) of the total metazoan volume. The dominant ' other Metazoa ', the heteropods, accounted for 52-99 % (av. 75 Yo) of the total numbers in this group over the 12 stations.

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876

Jom~ R. BEERSand GENEL. STEWART

Table 3. Chlorophyll a, phaeophytin a, and total particulate matter (seston) integrated and averaged over the estimated euphotic zone depth (3 × Secchi disc depth).

Sta. 76"021A 76"025A 76-029A 76"035A 76-044A 76"053A 76-062A 76"071A 76"079A 76"083A 76"087A 76"091A

Chlorophyll a (t~g/m a)

Phaeophytin a (p.g/m '~)

133 108 119 12l 113 152 223 110 105 84 45 55

140 217 116 103 141 124 183 119 95 68 41 49

Seston dry weight (mg/m 3) -414 214 246 241 429 314 341 225 223 210 228

Chlorophyll and Phaeophytin The concentrations of chlorophyll a and phaeophytin over the various depth intervals of the water column sampled at each station are presented in Fig. 5. Averaged over the depth of the euphotic zone chlorophyll standing crop ranged from a minimum of 44-8/zg/m 3 (Sta. 76.087A) to a height of 223.0/~g/m3just to the south of the equator at Sta. 76.062A (Table 3). Phaeophytin estimates through the euphotic zone ranged from 41.3 tzg/m3 (Sta. 76.087A) to 216.8/zg/m 3 at Sta. 76.025A where a sharp thermocline underlain by an oxygen-low layer was found within the euphotic zone (Table 3). With the single exception of depth intervals IV (50-74 m) and V (75-125 m) at Sta. 76.029A the chlorophyll-containing material was concentrated in the --35 samples, only a few percent being recovered in the +35 and larger material. Coscinodiscus centralis, a centric diatom of valve diameter of 145-260/z (CtJPP, 1943) was abundant in the -~-103 samples which retained 22 ~ (IV) and 8 °/ /o (V) of the total chlorophyll a at Sta. 76.029A. Phaeophytin in surface waters (i.e. depth interval I, surface-10 m) was generally low relative to chlorophyll (ratio: average, 1.8 chlor. : 1 phaeo; range, 1.1 : 1 to 2.7:1) but increased with depth such that at the base of the euphotic zone phaeophytin concentration generally exceeded that of chlorophyll. Depth intervals below the estimated compensation depth consistently showed high phaeophytin relative to chlorophyll. As with chlorophyll, by far the major fraction of phaeophytin was recovered in the size class of material passing 35/z mesh cloth. The ecological significance of measuring phaeophytin is very poorly understood since its further degradation rate, etc. is not known precisely. Phaeophytin has generally been considered to be a variable that can be used as an index of grazing pressure (see LORENZEN, 1967) as the ' chlorophyll pigment' of fecal material is often almost completely phaeophytin. The large fraction of the total phaeophytin in the --35 samples may be evidence that much of the phytoplankton is being grazed by the small animals. However, larger fecal pellets may be physically disrupted in the pumping system or may break up normally before the chlorophyll molecule degrades to phaeophorbide.

Eastern tropical Pacific micro-zooplankton

877

Seston The average abundance of total seston, i.e. phytoplankton, zooplankton, and detritus, as mg/m 3 of dry weight material over the euphotic depth at the various stations is shown in Table 3. Particulate material retained on the nets in the plankton concentrator, i.e. > 35/~ ,was a generally small percentage of the total, averaging only 8"4~o (range, 6.1-13.8 Yo) over all sites. Of the seston in the size classes which were examined for micro-zooplankton (i.e. --35, +35, and +103), approximately 90~o (average over all stations) was estimated to be detrital material. Phytoplankton was calculated to be between 4 and 14 ~o at the different sites while the animal plankters in this material were only 2-3 ~ at all sites (see BEERSand STEWART, 1969b) for methods of dry weight calculation). The levels of total particulate carbon were determined by EASTROPAC for selected depths at the various sites. Integrating and averaging their results over the euphoric zone and relating this to the average seston dry weight indicates the particulate carbon of the seston to be 10-28 Yo (average, 21 ~o). If organic material can be considered to be 40 ~o carbon, the total seston is estimated to be from 25-70 ~ organic material. The lowest value was found at Sta. 76-025A, a site where there was a strong thermocline and oxygen-deficient water within the estimated depth of the euphoric zone. GORDON (1970), examining several sites in the North Atlantic, showed the organic fraction of the total particulate matter to be 25-38 ~o. Calculated by the procedure used here, the figures for the North Atlantic sites would be 34-53 ~o. DISCUSSION

Micro-zooplankton, in general, probably derive a large part of their nutritional requirements directly from the phytoplankton. The relative abundance of the standing stocks of phytoplankton, as estimated from the chlorophyll a levels, and microzooplankton in the area of the eastern tropical Pacific under study indicates that the phytoplankton is supporting a generally large micro-zooplankton population when compared to the limited observations we have made in the waters of the California Current (BEERS and SXL~VARx, 1967, 1969b, and unpublished). This indicates that perhaps a relatively greater proportion of the primary production in this area passes through the micro-zooplankton component of the ' food web' in these oceanic areas than in the more coastal regions. A similar suggestion was made recently by PARSONS and LEBRASSEUR(1970). Calculations of the percentage abundance of micro-zooplankton material over the euphotic depth relative to that of phytoplankton, both in terms of calculated dry weight (see BEERS and SXEWAgT, 1969b, for method of dry weight estimation), ranged from 23~o (Sta. 76.035A) to 66Yo (Sta. 76.087A) averaging approximately 34 ~o over all sites. In general, the north and south ends of the transect showed higher relative abundance of micro-zooplankton compared to phytoplankton than near the equator (Group A, 33 ~ ; Group B, 25 ~o; Group C, 45 ~o). The range of micro-zooplankton standing stock abundance as a percentage of phytoplankton at five locations from ncarshore to oceanic in the California Current in February, 1967 (B~RS and STEWART, 1969b) was 14 ~o (station closest to the coast) to 34 ~ (oceanic site), averaging 23 %. When the protozoans of the micro-zooplankton populations are considered

878

JOHN R. BEERS and GENE L. STEWART

separately their calculated dry weight was 2-8 70 of that estimated for the phytoplankton. Stations along the northern end of the line and south to across the equator showed a range between 2-4 70 (Groups A and B, av. 3.4 700)while at the four sites in the south (Group C) values ranged from 5-4-7-7~, averaging 6"670. The four southerly stations were sites of relatively low standing crops of chlorophyll along the line we sampled. The data show the presence of more micro-zooplankton, both protozoan and metazoan, biomass per unit chlorophyll at the southern stations than in the north. On a numerical abundance basis the numbers of total micro-zooplankton ranged from less than 2000 (Sta. 76-039A) to more than 7000/t~g chlorophyll a (Sta. 76.091A). These numbers are comparable to or greater than the levels found at the shelf sites and at an oceanic station in the California Current and more than an order of magnitude greater than those at a neritic site (BEERSand STEWART, 1969a). Another comparison of ecological importance would be between the rate of phytoplankton production and the rate of utilization by the micro-zooplankters. An estimate of the possible grazing of the phytoplankton production (data from EASTROPAC) can be derived based on the assumption that the micro-zooplankton derive their nutrition from phytoplankton and furthermore that the protozoans, principally ciliates, ingest three times their own body carbon per day of phytoplankton carbon; that naupliar copepods consume their body weight in phytoplankton each day; and that for the post-naupliar copepods and' other Metazoa ' this relationship is 0-3. These latter figures were selected after examining data of MULLINand BROOKS(1967, 1970) on the amount of carbon ingested by various stages of Calanus helgolandicus and their weights. Other considerations entering into the assumptions have been discussed in BEERSand STEWART(1970a). The results of the computation indicate that the total micro-zooplankton through the euphotic zone could consume, on this basis, from 3970 (Sta. 76.062A) to 104~ (Sta. 76.035A), averaging 707o of the daily phytoplankton production at the stations in this study. Using the same assumptions made here on data for several sites just off the coast of La Jolla, California, the figure for micro-zooplankton consumption of phytoplankton productivity at a site about one mile offshore (Sta. 1) over 21 weekly intervals from April to September, 1967, ranged from 57o to approximately 125~, with an average of 2 2 ~ (median, 14700) (BEERSand STEWART, 1970a). A similar calculation for a site five miles off the coast of La Jolla (Sta. 3) showed a range of 7-52 70, averaging 23 ~o. Complicating factors to the above assumptions include, for example, the possible use by the micro-zooplankton of other sources than phytoplankton to meet nutritional requirements, and that the entire size spectrum of phytoplankton is probably not available to all of the various micro-zooplankton groups. Even though the absolute level may be in error due to our taking liberties with the assumptions, the relative difference should be valid and they support the premise of a more important role for the micro-zooplankton of the EASTROPAC oceanic areas than of the coastal regions examined previously. It is possible, however, that the populations are physiologically dissimilar enough between the oceanic and coastal regions considered here that different assumptions regarding feeding are warranted. The Protozoa consumption at a daily rate of three times their body carbon may be high, especially for the oceanic populations. Enough measurements of division rates are not available to provide a truly satisfactory figure. However, work (unpublished) in this laboratory has

Eastern tropical Pacificmicro-zooplankton

879

indicated the ability of tintinnids such as Favella cf. serrata and Stenosemella cf. nivalis to divide on the order of once a day or 1½ days. The non-loricate ciliates, i.e. ' Ciliata other than Tintinnida ', could be reasonably assumed to have an even shorter generation time. Even if the assumption regarding protozoan feeding were reduced to twice their body weight per day the calculations suggest the oceanic populations of micro-zooplankton were consuming a large fraction of the phytoplankton production; much greater than the comparable segment of the animal population in the inshore environment examined in 1967. Using 2 times for Protozoa the results for the total micro-zooplankton through the euphotic zone of the 12 stations ranged from a consumption of 33 9/oto 91%, averaging 59 %, of daily phytoplankton carbon production. Standing stock abundance of the larger zooplankton at the stations studied for micro-zooplankton was measured as part of the EASTROPAC program. Tows were made from approximately 200 m to the surface with a { m, 333 /~ mesh net. The average larger zooplankton volume/m ~, determined by the ' w e t ' displacement method (AI-ILSTRO~Iand TrmAILKILL,1963), at the various sites are given in Table 4. Micro-zooplankton standing stock volume/m3 over the euphoric zone was 12% (Sta. 76.087A) to 71% (Sta. 76.079A) of the larger plankton volume (0-200 m) at the various sites, the average being 24 %. A fraction of the larger zooplankton, i.e. that retained by 202/~ mesh but passing 333/~, is not accounted for in the above. If the weight of the +202 size class (i.e. +202-+363) particulate matter obtained by the pumping system over the euphotic zone is assumed to be all animal plankton and is added to the net zooplankton figures it would reduce the micro-zooplankton abundance relative to the larger forms by an average of less than 3 %. In addition, it is probable that a perhaps significant fraction of this +202 material is not animal plankton. There are other sources of probable error in this comparison which should be pointed out. First, it is probable that the average level of the larger plankton is greater within the euphotic zone than the average over the approximately 200 m depth used here. BLACKBURN(1966) reported data from Holmes (unpublished) on 140 m Table 4.

The volume of larger zooplankton and their standing stock relationship to the micro-zooplankton.

Sta.

76"021A 67"025A 76"029A 76"035A 76"044A 76"053A 76"062A 76"071A 76"079A 76"083A 76"087A 76"091A

' Wet' displacement volume, EASTROPAC 333/~ net zooplankton 0-200 m (mma/ma)

213 189 127 79 120 176 215 116 45 124 201 76

Percentage micro-zooplankton volume (mma/m3 euphotic zone) of net zooplankton

16% 17% 25% 29% 19% 17% 22% 20% 719/oo 21% 12% 2O%

880

JOHN R. BEERSand GENE L. STEWART

daytime tows in the eastern tropical Pacific which showed that approximately 60 of the plankton biomass was in the upper 70 m with 40 ~ between 70 and 140 m. For 0-300 m, BLACKBURN(1966) suggested an approximate vertical distribution of 0-70 m, 50~o; 70-140 m, 30Yo, and 140-300 m, 20~o. Secondly, the methods for determining plankton volume for the two collections are different. The ' wet' volume used with the larger zooplankton includes a relatively large percentage of interstitial water. The volume estimate for the micro-zooplankton derived from equating organisms to various geometric configurations, on the other hand, is probably ' conservative '. If the larger zooplankton volumes are reduced by one-third to account for the interstitial water content (see AHLSTROMand THRAILKILL, 1963) the micro-zooplankton volumes relative to the larger animals would be increased significantly. The food 'chain' concept while inviting because of the simple trophic relationships implied must be replaced by the food ' w e b ' when dealing realistically with most oceanic environments. Several interacting trophie levels are represented in most plankton samples collected with conventional towed nets. Of the microplankton which we studied, however, the phytoplankton can be defined on the basis of their chlorophyll content and the taxomic composition of the micro-zooplankton suggests a population highly oriented toward herbivore-type feeding. Even though our samples represent material from relatively large vertical distances it is of interest to examine the data for correlations which would have possible ecological meaning. Examining data averaged over the euphoric zone of the 12 sites, a highly significant positive correlation was found between chlorophyll a level and the total micro-zooplankton volume (r = 0.7587, n = 10). Chlorophyll and the metazoan component of the microzooplankton populations showed the same highly significant correlation whereas the relationship between chlorophyll and the total ciliates (Tintinnida + ' Ciliata other than Tintinnida ') was just significant (r = 0.5805, n = 10). Furthermore, total microzooplankton biomass over the euphoric zone had a positive significant correlation with phaeophytin concentration (r = 0.5961, n = 10) and a highly significant correlation with phytoplankton productivity (r = 0.9303, n = 10). Examining the abundance of chlorophyll a and micro-zooplankton, in terms of both numbers and volume, over the various depth intervals sampled, Table 5 shows that over the six depths intervals at any one station (5 intervals, Sta. 76.062A) significant or highly significant correlations were found at only slightly more than half of the 12 sites. When combined into the three groups of stations based on location and similarities of some of the important hydrographic characteristics, each group showed highly significant correlations between these variables as did the abundance of chlorophyll versus total micro-zooplankton numbers and volume in all depth intervals from all stations combined. Whether the micro-zooplankton are truly a more important component of oceanic food webs relative to nearshore areas cannot be decided on the basis of the present study since the hydrographic characteristics in the eastern tropical Pacific are dissimilar from the sub-tropical/temperate California Current waters previously examined and because the sampling here was restricted to oceanic waters. Tropical oceanic waters have generally been considered to be of relatively low (except in upwelling areas along the coast and the equator) and constant (over the year) biological production. It is probable that such is the situation in the area we studied, but that there is some seasonal variation can be seen, for example, from BLACKBURN(1966) and SOURNIA

Eastern tropical Pacific micro-zooplankton Table 5.

881

Correlation coefficients f o r standing stock abundance o f chlorophyll and micro-zooplankton (numbers and volume).

A. Depth intervals at each station; n = 4 (Sta. 76.062A, n = 3). Chl. a vs. Chl. a vs. Sta. micro-zooplankton numbers micro-zooplankton volume

76"021A +0.9094* 76-025A +0"9383t 76"029A +0"8588* 76"035A +0"6004 76-044A +0"8584* 76"053A +0"8863* 76"062A +0"8154 76"071A +0"8479" 76"079A +0"9173, 76"083A +0"8868* 76.087A +0"2639 76"091A +0-7146 B. Depth intervals within groups of stations; n = 22 (Group B, n = 21). Group A 76-021A+0.7471, 76.035A) Group B (76.044A)+0"8268t 76.071A) Group C (76.079A)+0-7745t 76.091A) C. All depth intervals at all stations; n = 69 +0"7455t

+0-8996* +0-9701* +0'7977 +0"9605t +0-8153 +0"9630t +0"6903 +0"9035* +0"9343t +0-7836 +0"1584 +0"7409 +0"8526t +0"8689t +0"7362t +0"8211t

All data log transformed: *positivecorrelation at 0.95 level; t positive correlation at 0"99 level. (1969). BLACKBURN, LAURS, OWEN and ZEITZSCrmL (1970) gave evidence for a significant seasonal variation in the 'day' zooplankton populations in the upper 200 m of an area of the tropical Pacific (105°-119°W longitude) but the magnitude was less than twofold. It is perhaps probable that any variations in micro-zooplankton populations due to seasonal effects would be of a similar magnitude if examined over the same time period and area. These considerations suggest a closer balance between trophic levels in tropical waters than elsewhere. Factors governing such a ' stable' equilibrium would undoubtedly be quite complex. Since we do not have data for any time sequence of a fine scale we must leave open such questions as why the standing stocks of phytoplankton and micro-zooplankton are relatively low at most o f the Group C stations where nutrient and hydrographic conditions would seem to be suitable to support higher levels while relatively greater populations occur in waters at some of the northern stations on the transect where the nutrients are very low and a strong thermocline exists. In terms of both numbers and volume, the average absolute abundance of microzooplankton was as great or greater over the sites sampled here as we have found in any location outside the neritic zone o f the California Current and, as discussed previously, relative to the standing crops of phytoplankton and plant production rates their abundance was high in these tropical waters. How typical these results are of other tropical oceanic regions is not known. Data on complete seasonal coverage of

882

JOHN R. BEERSand GENE L. STEWART

the micro-zooplankton are not available f r o m any oceanic region. In fact, the abundance of these small animal plankters in the open waters at higher latitudes is very little studied for any season. It is possible that in regions where there are pronounced cycles of phytoplankton abundance that some micro-zooplankters at least m a y have important roles in controlling the extent o f blooms. F o r example, the ciliates, both tintinnids and others, have been found to be relatively abundant in most plankton assemblages examined. L a b o r a t o r y observations on several tintinnid forms and an oligotrich suggest that division rates, while being quite variable, can under the proper conditions, be o f the order o f once a day. With the capacity of a rapid division rate and the assumption that there is a lag between their increase and that of their predators, their total food consumption could theoretically increase in an almost geometric manner. Several times during the coastal plankton survey off La Jolla (BEERS and STEWART, 1970a) there were indications t h a t the standing stock of ciliates m a y have been capable of consuming a greater a m o u n t o f organic carbon per day than was probably being produced by the size class of phytoplankters on which it was assumed they feed. Acknowledgements These studies on the micro-zooplankters and related components of Eastern tropical Pacific waters were supported mainly through National Science Foundation Grants GB-6357 and GB-12127. The able assistance of Mr. GARYP. OWEN with some of the laboratory plankton enumeration was appreciated. Miss BETSYFOCLISa~Rcontributed measurably to the production of the 'computerized' data records and the statistical analysis of the results. The work would not have been possible without the close cooperation of the EASTROPAC program which allocated considerable shiptime for this purpose on the R.V. David Starr Jordan, a vessel supported by the U.S. Department of the Interior. We wish to thank Dr. ALANR. LONOHURST, Coordinator of EASTROPAC; Dr. WILLIAMH. THOMAS,chief scientist of Cruise 76; and the many members of the scientific and ship's personnel who contributed to the success of the project. REFERENCES AHLSTROME. H. and J. R. THRAILKILL(1963) Plankton volume loss with time of preservation. Rept. CALCOFI, 9, 57-73. BANSE K., C. P. FALLS and L. A. HOBSON (1963) A gravimetric method for determining suspended matter in seawater using MiUipore® filters. Deep-Sea Res., 10, 639-642. BEERSJ. R. and G. L. STEWART(1967)Micro-zooplankton in the euphotic zone at five locations across the California Cm'rent. J. Fish. Res. Bd Can., 24, 2053-2068. BEERSJ. R. and G. L. STEWART(1969a) The vertical distribution of micro-zooplankton and some ecological observations. J. Cons. perm. int. Explor. Mer, 33, 30-44. BEERS J. R. and G. L. STEWART(1969b) Micro-zooplankton and its abundance relative to the larger zooplankton and other seston components. Mar. Biol., 4, 182-189. BEERS J. R. and G. L. STEWART(1970a) Numerical abundance and estimated biomass of micro-zooplankton. In: The ecology of the plankton off La Jolla, California, in the period April through September, 1967 (Part VI), J. D. H. STRICKLAND,editor, Bull. Scripps. Instn. Oceanogr., 17, 67-87. BEERS J. R. and G. L. STEWART(1970b) The preservation of acantharians in fixed plankton samples. LimnoL Oceanogr., 15, 825-827. BEERSJ. R., G. L. STEWARTand J. D. H. STR1CKLAND(1967) A pumping system for sampling small plankton. J. Fish. Res. Bd Can., 24, 1811-1818. BLACKBURNM. (1966) Biological oceanography of the eastern tropical Pacific: summary of existing information. Spec. Scient. Pep. U.S. Fish Wildl. Serv., - - Fish. 540, 18 pp. BLACKBtrRN M., R. M. LAtmS, R. W. OWEN and B. ZEITZSCnEL(1970) Seasonal and areal changes in standing stocks of phytoplankton, zooplankton, and micronekton in the eastern tropical Pacific. Mar. BioL, 7 (1), 14--31. CuPP E. E. (1943) Marine plankton diatoms of the west coast of North America. University of California Press, Berkeley. 221 pp. DUSSART B. H. (1965) I.es ditfdrentes catdgories de plancton. Hydrobiologia, 26, 72-74.

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