Production and respiration of overall plankton and ultraplankton communities at the entrance to the Gulf of Finland in the Baltic Sea

Primary productivity and respiration of the overall plankton community and of ultraplankton (organisms passing through a 3-m Nuclepore filter) were studied at the entrance to the Gulf of Finland during the growth season in 1982. Data of the respiration measurements from previous years are also presented. During the development of a diatom spring bloom, the algal component could be successfully separated from the bacterial component by size fractionation with a 3-m Nuclepore filter and thus the algal respiration could be approximated, being on the order of 10 to 20% of the gross production. After the phytoplankton spring maximum, bacteria played an important role in mediating the energy flow from phytoplankton exudates to higher trophic levels. Maximum values of 1 230 and 740 mg O₂ m^-2 d^-1 were recorded for overall and for ultraplankton respiration, respectively, during late July. High productivity values coupled with low phytoplankton biomass and low inorganic nutrient values were also recorded in late July, indicating effective nutrient regeneration and rapid turnover of the plankton community. During late summer, a considerable fraction (over 30%) of phytoplankton production was released as exudates, suggesting that much of the energy is channeled to higher trophic levels via bacterial pathways rather than by direct herbivorous grazing during this season. The summer development of phytoplankton community structure and functioning is strongly controlled by hydrographic conditions, i.e. by nutrient inputs via upwelling and by water temperature. A carbon budget for late summer indicated that bacteria may contribute only up to 50% of the overall respiration of the plankton community, which suggests that heterotrophs other than bacteria play an important role in nutrient regeneration. The present study stresses the importance of energy flow via the phytoplankton exudatebacteria-micrograzer pathway in relatively oligotrophic, brackish water ecosystems.     

The annual cycle of protozooplankton in the Kiel Bight

Protozooplankton (heterotrophic dinoflagellates and ciliates) composition and biomass was studied in a 20-m water column in the Kiel Bight on 44 occasions between January 1973 and April 1974. Both groups attained comparable biomass maxima during spring and autumn (0.3 to 0.7 g C m^-2 in the 20-m water column) and biomass levels were much lower in summer and lowest in winter. The spring protozooplankton maximum coincided with that of phytoplankton and during the rest of the year, protozooplankton stocks did not appear to be food limited as phytoplankton stocks were large throughout; many protozoans with ingested microplankton cells were observed, indicating that their potential food supply is not restricted to nanoplankton. Non-loricate organisms dominated biomass of the ciliates and tintinnids were of little importance. Tintinnids predominated in plankton samples concentrated by 20 m gauze indicating that most non-loricate ciliates, irrespective of size, were not retained. When phytoplankton sotcks were large (>3 g C m^-2) but those of metazooplankton small, as in spring and autumn, protozooplankton were the major herbivores with biomass levels comparable to those attained in summer by metazooplankton ( 0.5 g C m^-2). A highly significant negative correlation was found between protozooplankton and metazooplankton during the plankton growth season. Predation by the latter is thus an important factor regulating size of the protozooplankton population, although other factors also appear to be in operation. Loss rates of the pelagic system through sedimentation are highest in spring and autumn when protozooplankton dominate the grazing community and loss rates are much lower in summer when metazooplankton are the dominant herbivores. Apparently, the impact of protozooplankton grazing on the pelagic system is quite different to that of the metazooplankton.Publication No. 268 of the Joint Research Programme (SFB 95), Kiel University     

Relationship between phytoplankton cell size and the rate of orthophosphate uptake: in situ observations of an estuarine population

Orthophosphate uptake by a natural estuarine phytoplankton population was estimated using two methods: (1) ³²P uptake experiments in which filters of different pore sizes were used to separate plankton size-fractions; (2) ³³P autoradiography of phytoplankton cells. Results of the first method showed that plankton cells larger than 5 m were responsible for 2% of the total orthophosphate uptake rate. 98% of the total uptake rate occurred in plankton composed mostly of bacteria, which passed the 5 m screen and were retained by the 0.45 m pore-size filter. There was no orthophosphate absorption by particulates in a biologically inhibited control containing iodoacetic acid. Orthophosphate uptake rates of individual phytoplankton species were obtained using ³³P autoradiography. The sum of these individual rates was very close to the estimated rate of uptake by particulates larger than 5 m in the ³²P labelling experiment. Generally, smaller cells were found to have a faster uptake rate per m³ biomass than larger cells. Although the nannoplankton constituted only about 21% of the total algal biomass, the rate of phosphate uptake by the nannoplankton was 75% of the total phytoplankton uptake rate. Results of the plankton autoradiography showed that the phosphate uptake rate per unit biomass is a power function of the surface: volume ratio of a cell; the relationship is expressed by the equation Y=2x10^-11 X ^1.7, where Y is gP m^-3 h^-1 and X is the surface: volume ratio. These results lend support to the hypothesis that smaller cells have a competitive advantage by having faster nutrient uptake rates.     

Optical characterization of primary productivity across a coastal front

Methods for the remote estimation of phytoplankton biomass and production rates using multiplatform sampling strategies are essential for the better understanding of oceanic bio-geochemical cycles. Recent advances in remote sensing of ocean color have made synoptic estimation of phytoplankton biomass attainable. While considerable success has been achieved in the estimation of plant biomass, the synoptic estimation of phytoplankton rates of production has been inadequate. Rapid shipboard estimates of the vertical distribution of primary productivity, on mesoscale spatial scales and event-time scales, are needed to provide both surface validation and data for the development of bio-optical models linking production to the optical characteristics of the water column. This study details the primary productivity and optical properties of a frontal region in July 1985 along 35°50N in the Southern California Bight which is shown to be consistent with the concurrent high-performance liquid-chromatography pigment-analysis. We describe here a quasi-synoptic shipboard bio-optical sampling strategy across a frontal region as an example of time-corrected data for assessing phytoplankton production in highly variable ocean regions.     

Some experiments on phosphate assimilation by coastal marine plankton

A study of phosphate assimilation by coastal marine plankton revealed that both phytoplankton and microheterotrophs incorporated radioactive phosphorus (³³P). Size fractionation of the particulate matter (using 1 m pore diameter Nucleopore^® membrane filters), antibiotic treatment (using garamycin), and independent estimaties of photoautotrophic (¹⁴CO₂ uptake) and heterotrophic (³H-glucose uptake) activities were employed to separate phyto- and bacterioplankton phosphate uptake. Results indicated that phytoplankton ³³P-uptake was best estimated by the fraction of particulate matter retained on the 1 m membrane filters. Usually, less than 10% of the phytoplankton (based on chlorophyll a measurements) passed the 1 m pore-diameter filters, whereas about 90% of the heterotrophic activity passed. At least 50% of the ³³P-uptake was associated with the <1 m fraction. It may be possible to resolve the phytoplankton and bacterial contributions to ³³P-uptake by comparing the percent of total ³³P-uptake with the percent of total ³H-glucose uptake associated with the >1 m fraction.     

Effect of enclosure in large plastic bags on diurnal change in oxygen concentration in tropical ocean water

From November 1980 to February 1981 the concentration of oxygen dissolved in the surface mixed layer of the oligotrophic Caribbean Sea off Curaçao was quite constant (420.77±1.98 g at l^-1). However, immediately following enclosure in 4500-1 plastic bags reaching to a depth of 5 m the oxygen concentration began to decrease, down to values below saturation (405 g at l^-1) within 48 h. Autotrophic and heterotrophic nanoplankton cell numbers and algal pigments in bags remained constant or increased slightly during the first 24 h of enclosure. The rate of decrease in oxygen concentration in bags was significantly higher during daylight hours than in the night, which suggests that photo-oxidative processes were involved in the additional daytime loss of oxygen. The dramatic enclosure effect on the oxygen content of the water in the bags can be taken as evidence of the dependence of the oxygen concentration near the tropical ocean's surface on supply from below: in water freely circulating in the euphotic zone deviations from the mean oxygen concentration during a diurnal cycle were 0.47% at most, differential losses near the surface being counteracted through vertical exchange; while in water separated from the rest of the mixed layer in the plastic bags losses due to respiration of the enclosed plankton community plus an even greater loss, assigned to non-biological, photosensitized oxidation processes, were up to 10 g at O₂ l^-1 in 24 h. Although photo-oxidation is confined to the very surface the oxygen flux involved may be important enough to necessitate consideration of a photochemically induced loss factor in oxygen budget calculations, e.g. when primary production is to be estimated from diurnal oxygen curves.     

Spatial mesoscale patterns of West Pacific picophytoplankton as analyzed by flow cytometry: their contribution to subsurface chlorophyll maxima

Vertical distributions of picophytoplankton (ppp) (<2 m) were studied by ship-board flow cytometry during two cruises in Western Pacific waters to Palau and to Australia in 1990. Weak red-fluorescing small ppp, supposed to be free-living prochlorophytes (Chisholm et al. 1988), were abundant in the area surveyed. These ppp, designated the prochlorophytes, were abundant in the surface waters (>10⁴ cells ml^-1) at the northern region (27°03N; 7°11N) in November, whereas in December at the southern tropical stations (0°23.54S; 9°20.30S; 13°50.6S), they formed subsurface maximum layers (>10⁵ cells ml^-1) on a nitracline at a depth of 3.5 to 5.4% surface irradiation. Their fluorescence intensity increased with depth below 10% surface irradiation. The prochlorophytes at a depth of 1% surface irradiation had ten times higher fluorescence than those at the surface layer. The total fluorescence intensity of the prochlorophytes accounted for 32 to 63% of the sum of the total fluorescence intensity of all fluorescing phytoplankton detected at subsurface chlorophyll maxima in the tropical area. These results suggest that distribution of the prochlorophytes is greatly affected by nitracline and by light intensity and that their chlorophyll is a major contributor to the subsurface chlorophyll maximum in the pelagic West Pacific Ocean.     

The distribution of heavy metals in deposited urban dusts and sediments, Coventry, England

Samples of urban dusts and sediments have been collected and categorised into three compartment those which act mainly as sources, those undergoing transport and those which have been deposited. The sediments were fractionated into <63m to match similar sized lacustrine sediments and <2mm to represent natural and anthropogenic fluvially transported sediments. Preliminary results of total heavy metals analysis show that the <2mm fraction of the source group exhibits the greatest variabilty in metal concentrations. Overall, however, the <63m fraction dominates regardless of compartment. Some heavy metal concentrations decrease through the sourcetransportdeposit process, possibly due to selective onward transport of finer material or other anthropogenic processes unique to the urban environment (e.g. street cleansing). Sitespecificity is indicated in a comparison with other similar studies.     

The influence of water stability on the vertical structure of a phytoplankton community

The vertical distribution of the phytoplankton community in association with water column stability was examined for 1 year in an inshore area of the Southern Aegean Sea. An analysis of variance model (split-plot design) was applied to evaluate the variations in the vertical profile of diatoms, flagellates and coccolithophores. When either weak stratification or mixing conditions prevailed, diatoms in general were uniformly distributed throughout the water column while flagellates and coccolithophores appeared occasionally stratified. During the strong stratification period, all taxa demonstrated significant variations in abundance between depths in most cases. However, none of these taxa was confined to a single depth stratum during either the water mixing or the stratification period, but were all present at all depths during all seasons. The results demonstrate clearly that the parameter taxon is an important component in ecological observations on the vertical distribution of phytoplankton.     

Analyse des paramètres hydrobiologiques dans la remontée de Cabo Frio (Brésil)

South Atlantic Central Waters (SACW) upwell close to Cabo Frio (Brazil, Rio de Janeiro) shore. The resultant typical hydrobiological conditions were studied at an anchorage station over an annual cycle from February 1973–February 1974. Multivariate analyses of hydrological and planktonic data revealed the structure of the ecosystem and the factorial relations governing its dynamics. Alternation, superposition or mixing of the water masses of different origin (Brazil current, coastal, SACW) constitute the most important factors responsible for the great hydrobiological variability observed in the study area. Spasmodic changes in wind direction and force are superimposed on a seasonal trend of more frequent upwelling in summer than during the rest of the year. The deep water is characterized by temperatures of <18°C, nitrate contents of 10 g-at l^-1 and by organic matter mainly composed of detritic elements from the shelf. Temperature variations together with nutrient contents (NO₃ or PO₄) reflect variations in primary biomass at the surface but not at 50 m depth, where detrital matter precluded valid measurements. Water of the Brazilian Current (salinity 36.0) frequently mixes with deep water of the thermal front, or with coastal water (<35.0) which invades the area when south-west winds prevail. This lower-salinity water is rich in seston particles. During the study period, primary biomass was relatively low due to eutrophication. We observed less than 3 g l^-1 chlorophyll and 10⁶ phytoplankton cells per litre: the phytoplankton populations were highly diversified, indicating an advanced degree of complexity and evolution of this ecosystem. The observed formation of a thermocline constitutes an important inducing factor for an algal bloom. Simultaneous phyto- and zooplankton maxima would induce an increased grazing rate by herbivorous zooplankton which would also partly explain the relatively low level of primary biomass. Zooplankton is as abundant here as in other great upwelling regions: 100 organisms l^-1 and 200 mg organic matter m^-3.     

Zooplankton of tikehau atoll (Tuamotu archipelago) and its relationship to particulate matter

The standing stock and taxonomic composition of zooplankton (>200 m) were monitored in the lagoon of Tikehau atoll from April 1985 to April 1986. These data were supplemented by two 10 d studies on the variability, structure and functioning of the pelagic ecosystem. The biomass of animals >200 m comprised 50% of the total biomass of all organisms from 35 to >2000 m. The zooplankton populations were characterized by successive blooms of copepods, larvaceans, pteropods and salps, probably arising from the periodic input of detritus from the reef during windy periods. As a result, the ecosystem was not in a steady state and the data for the fluxes of organic matter are presented separately for April 1985 and April 1986. Using the C:N:P ratio method, net growth efficiencies, K ₂, were calculated for total mesozooplankton, mixed copepods, and two planktonic species, Undinula vulgaris and Thalia democratica. Combined with nitrogen and phosphorus excretion rates, these K ₂ values enabled the assessment of production rates. On a 24 h basis, P:B ratios (%) were close to 100 for the total zooplankton and 54, 34 and 800 for mixed copepods, U. vulgaris and T. democratica, respectively. These ratios are 5.7 times lower than that recorded for phytoplankton. High productivity may be ascribable to the high density of seston, the high temperature (29.5°C), and the kind of organisms present. Zooplankton production equalled 38 and 30% of ¹⁴C uptake during April 1985 and April 1986, respectively. Ingestion of animals >35m was calculated by means of assimilation efficiencies and amounted to 17 and 7% of particulate organic carbon, 100 and 38% of living carbon, and 64 and 140% of primary production during the two periods, respectively. Finally, inorganic exduring was 32 and 18% of phytoplankton nitrogen and phosphorus requirements. A model based on the dimensional structure of the pelagic food-web, has been drawn to illustrate the biomass and carbon, nitrogen and phosphorus fluxes in the study area. The lagoon appears to export part of its planktonic biomass, which is 4.2 times lower one sea mile outside the main pass connecting the lagoon to the open ocean.     

The effect of male calling on female locomotor activity of Teleogryllus commodus

Summary The calling song of the -cricket, Teleogryllus commodus, attracts for mating. Both calling and -locomotor activity are under circadian control and the behaviors occur approximately at the same time. Daily playback of calling songs for 3–12 h in constant light failed to entrain -running. Instead, calling acts as an external stimulus which can release -locomotion at any time of the 24-h period, while the clock-induced activity continues to run freely. The stimulatory effect of the calling song causes flexible extension of -locomotor activity, but a subsequent restorative period is necessary. The ecological consequences of acoustical stimulation are discussed.     

Effects of nitrogen and phosphorus enrichement on in vivo symbiotic zooxanthellae of Pocillopora damicornis

The reef coral Pocillopora damicornis (Linnaeus) was grown for 8 wk in four nutrient treatments: control, consisting of ambient, unfiltered Kaneohe Bay seawater [dissolved inorganic nitrogen (DIN, 1.0 M) and dissolved inorganic phosphate (DIP, 0.3 M)]; nitrogen enrichment (15 M DIN as ammonium); phosphorus enrichment (1.2 M DIP as inorganic phosphate); and 15 M DIN+1.2 M DIP. Analyses of zooxanthellae for C, N, P and chlorophyll a after the 8 wk experiment indicated that DIN enrichment increased the cellular chlorophyll a and excess nitrogen fraction of the algae, but did not affect C cell^-1. DIP enrichment decreased both C and P cell^-1, but the decrease was proportionally less for C cell^-1. the response of cellular P to both DIN and DIP enrichment appeared to be in the same direction and could not be explained as a primary effect of external nutrient enrichment. The observed response of cellular P might be a consequence of in situ CO₂ limitation. DIN enrichment could increase the CO₂ (aq) demand by increasing the net production per unit area. DIP enrichment could slow down calcification, thus decreasing the availability of CO₂ (aq) in the coral tissue.Hawaii Institute of Marine Biology Contribution No. 920     

Fractionated phytoplankton primary production,exudate release and bacterial production in a Baltic eutrophication gradient

The distribution of phytoplankton primary production into four size fractions (>10 m, 10-3 m, 3-0.2 m and <0.2 m), the utilization of algal exudates by bacteria and the bacterial production were studied in a eutrophication gradient in the northern Baltic proper. The polluted area exhibits substantially increased nutrient, especially nitrogen, levels while only minor differences occur in salinity and temperature regimes. Total primary production was 160 g C · m^-2 · yr^-1 at the control station and about 275 g C · m^-2 · yr^-1 at the eutrophicated stations. The estimated total exudate release was 16% of the totally fixed ¹⁴CO₂ in the control area and 12% in the eutrophicated area (including the estimated bacterial uptake of exudates). The difference in¹⁴CO₂ uptake rates between incubation of previously filtered water (<3, <2, <1 m) and unfiltered water was used to estimate bacterial uptake of phytoplankton exudates which were found to contribute about half of the estimated bacterial carbon requirement in both areas. Bacterial production was estimated by the frequency of dividing cells (FDC) method as being 38 g C · m^-2 · yr^-1 at the control station and 50 g C · m^-2 · yr^-1 at the eutrophicated stations. To estimate the mean in situ bacterial cell volume a correlation between FDC and cell volume was used. The increased annual primary production in the eutrophicated area was due mainly to higher production during spring and autumn, largely by phytoplankton cells (mainly diatoms) retained by a 10 m filter. Primary production duringsummer was similarin the two areas, as was the distribution on different size fractions. This could possibly explain the similar bacterial production in the trophic layers at all stations since the bulk of bacterial production occurs during summer. It was demonstrated that selective filtration does not quantitatively separate photoautotrophs and bacteria. A substantial fraction of the primary production occurs in the size fraction <3 m. The primary production encountered in the 3-0.2 m fraction was due to abundant picoplankton (0.5 to 8 · 10⁷ ind · l^-1), easily passing a 3 m filter. The picoplankton was estimated to constitute up to 25% of the total phytoplankton biomass in the control area and up to 10% in the eutrophicated area.     

The structure of plankton communities of the Indian Ocean

Species diversity and trophic structure of plankton communities have been considered as most important biological features. The Fisher () and Simpson () indices and the information index H=- pi log₂ pi (Margalef) were used for evaluation of species diversity. The indices have been calculated within the group of filter feeders by numbers ( and H) and biomass ( and H) of the species; the best results were obtained in the latter case. The index H=- pi log₂ pi is proposed for the quantitative evaluation of trophic structure, where a portion of an individual trophic group is taken as pi. The plankton material was collected by the R.V. Vityaz in the northern Indian Ocean in January through February, 1960. Analysis of characteristics of zooplankton distribution, species and trophic diversity in connection with characteristics of the water regime resulted in the definition of 3 types of the plankton communities: (a) those in intensive divergence zones; (b) those in poor divergence zones; (c) those in regions with stable water stratification or with feebly marked convergence, distiguished by different values of biomass, species diversity, and trophic group ratios.     

Salinity tolerance of benthic estuarine diatoms as tested with a rapid polarographic measurement of photosynthesis

Measurements of net photosynthesis of benthic estuarine diatoms were made by polarographic registration of oxygen saturation. A measuring cell was constructed in which media with salinities of 2.0 to 100.7 were pumped over the algae between measurements. Diatoms from unialgal cultures and mixed populations from intertidal flats appeared to be highly tolerant of extreme salinities. During short-term exposures (20 min) the net photosynthesis of the algae did not drop below 70% of the initial values, within the salinity range 4.0 to 60.0. Prolonged exposure (up to 6 h) gave essentially the same results. Populations of benthic diatoms, sampled from field stations with mean salinities of about 30, 12, and below 5, showed only gradual differences in their tolerance of salinities between 2.0 and 33.7. Two species, Navicula arenaria and Nitzschia sigma, were cultured in media ranging in salinity from 8.0 to 45.0 and from 2.0 to 45, respectively. The tolerance to changing salinity was only slightly affected by the salinity of the medium in the preculture. The role of salinity in the production and distribution of intertidal diatoms is discussed.     

Swimming pattern as an indicator of the roles of copepod sensory systems in the recognition of food

The roles of copepod sensory systems in the recognition of food were investigated using the Bugwatcher, a video-computer system designed to track and describe quantitatively the swimming patterns of aquatic organisms. The swimming behavior of the copepodPseudocalanus minutus in the presence of phytoplankton is characterized by a decrease in average swimming speed and an increase in pause behaviors compared to its swimming behavior in filtered seawater. Copepods exposed to chemosensory stimulation alone (filtered phytoplankton exudate) exhibited an increase in average swimming speed and an increase in the number of burst swimming behaviors. When exposed to a novel, non-food chemosensory stimulus (morpholine), no change in swimming behavior was observed unless the copepods had been conditioned to this odor in the presence of phytoplankton. Copepods exposed to mechanosensory stimulation alone (plastic spheres) exhibited a decrease in swimming speed and an increase in pause behaviors. When exposed to both forms of stimulation simultaneously (phytoplankton exudate and plastic spheres), a further decrease in swimming speed and increase in pause behaviors occurs, yielding a swimming pattern similar to that found in the presence of phytoplankton. This analysis of swimming pattern indicates that both chemoreception and mechanoreception contribute to the recognition of food inP. minutus.Contribution No. 406 of the US EPA Environmental Research Laboratory; Narragansett, Rhode Island 02882, USA     

Interactions between nutritional status and long-term responses to ultraviolet-B radiation stress in a marine diatom

Influences of nutritional status on the photoinhibitory effects of ultraviolet-B radiation (UVBR: 290 to 320 nm) on the specific growth rates (_obs) and biomass of Phaeodactylum tricornutum were determined using nutrient-replete batch cultures and nutrient-limited continuous cultures. P. tricornutum cultures were exposed to UVBR doses representative of current mid-latitude and ozone-depletion intensities. Specific growth rates and biomass were inhibited from 2 to 16% by UVBR during nutrient-replete growth. However, no effect of UVBR on _obs or biomass was detectable when nutrient limitation exceeded the potential for limitation by UVBR. Thus, a competitive interaction appears to occur between macronutrient stress and UVBR stress, such that _obs and biomass will be determined by the most limiting factor. Our results suggest that measurable decreases in phytoplankton _obs and biomass from UVBR are most likely in nutrient-rich areas of the ocean, while these parameters may not be appropriate for measuring UVBR stress in regions of nutrient limitation.     

Size composition of particulate organic matter in the lagoon of Tikehau atoll (Tuamotu archipelago)

Suspended particulate matter was comprehensively investigated from 6 to 17 April 1986 in the lagoon of Tikehau atoll (15°00S; 148°10W). Dry weight (DW), particulate organic carbon (POC), adenosine triphosphate (ATP), and chlorophyll a were measured for five size-classes (0.2 to 0.8 m, 0.8 to 3 m, 3 to 35 m, 35 to 200 m, and 200 to 2000 m). Taxa were identified and counted for the whole plankton (both autotrophic and heterotrophic). Particles <3 m accounted for 81% of the total POC (192 mg m^-3), and detritus comprised 82% of the total POM. Phytoplankton (cyanobacteria plus algae) accounted for 35% of the living carbon, 75% of which consisted of heterotrophic bacteria and cyanobacteria. The zooplankton biomass was composed of 31% nano-, 26% micro-, and 43% mesoplankton.     

Dependence of dissolved carbohydrate concentrations upon small scale nanoplankton and bacterioplankton distributions in the western Sargasso Sea

Significant correlations between trophic components of the <20 m microbial plankton and the concentrations of total dissolved carbohydrate (TCHO) and polysaccharide (PCHO) have been found at two drogued buoy stations which were sampled at 3 or 4-h intervals over diel cycles. An attempt was made to sample on two pre-selected isotherms (approximating isopycnals) at each station. Significant inverse correlations of TCHO and of PCHO with numbers of phototrophic nanoplankton (PNAN) were found along both isotherms at a station off the Carolina coast. Off central Florida, TCHO and/or PCHO were significant multiple linear functions of PNAN, heterotrophic bacterioplankton (BAC) and sometimes heterotrophic nanoplankton (HNAN) counts. Partial regression coefficients for PNAN were always negative and those for HNAN were positive. Inverse trends dominated the relations of BAC with PCHO and TCHO. The combined data from the two stations (300 nautical miles apart) produced very similar multiple linear relations. This suggests that the observed relationships are real, resulting from general physiological processes and interactions of the microbial plankton groups rather than from chance occurrences at a particular station, and that the combined activities of the <20 m plankton actively regulate dissolved carbohydrate concentrations in the Sargasso Sea.