Non-loricate Ciliates Research Articles

Diversity and distribution of heterotrophic dinoflagellates from the coastal waters of Port Blair, South Andaman.

The interaction between the environment and heterotrophic dinoflagellates inhabiting coastal waters of South Andaman was studied based on year round collections made during September 2012-August 2013 in the bay, eastern, and western region of South Andaman. The distribution pattern of microzooplankton in South Andaman showed high abundance in eutrophic waters (bay region) and gradually decreased towards the off shore region. Microzooplankton community comprised of six different taxa, viz. tintinnids, heterotrophic dinoflagellates, non-loricate ciliates, Foraminifera, Rotifera, and Copepoda (nauplii). Tintinnids were the major component of the microzooplankton (43.8±7%) followed by heterotrophic dinoflagellates (34±12%) and copepod nauplii (18.8±4.0%). This study focused on heterotrophic dinoflagellates which ranked next to tintinnids in overall abundance and contributed 38-42% in the bay, 22-37% in the eastern, and 15-29% in the western region to the microzooplankton community. Dinoflagellates showed a positive correlation with salinity and a negative correlation with dissolved oxygen and chlorophyll a (r=-0.3). Abundance of heterotrophic dinoflagellates in this area may be due to their diverse and advantageous mode of nutrition. A total of 35 species belonging to 8 genera of heterotrophic dinoflagellates were recorded during the study period. Heterotrophic dinoflagellates showed a great potential to thrive in low oxygenated and low productive area (p<0.001, Mann-Whitney test). Relatively higher diversity (H') in the dinoflagellates population was found in the bay region (avg. H'=3.46).

Abundance and biomass of planktonic ciliates in the sea area around Zhangzi Island, Northern Yellow Sea

We investigated the abundance and biomass of planktonic ciliates in the sea area around Zhangzi Island, Northern Yellow Sea, from July 2009 to June 2010. Ciliates were sampled monthly from surface to bottom with a 10m depth interval at 13 sample stations along three transects. A 1L sample of water from each depth was collected with a 2.5L Niskin water sampler and fixed in 1% acid Lugol’s iodine solution. Water samples were pre-concentrated using the Utermöhl method and observed using an Olympus IX51 inverted microscope at 100× or 200x. The dimensions of the ciliates were measured and the cell volume of each species was estimated using appropriate geometric shapes. The carbon:volume ratio used to calculate biomass was 0.19pgC/μm3. Abundance and biomass of the ciliate in water column were calculated as the integral of the abundance and biomass from bottom to surface, respectively. The classification of tintinnids was based on taxonomic literature. The average abundance of non-loricate ciliates was 3066±2805ind/L, ranging from 165ind/L (50m depth of St. B6 in July) to 26,595ind/L (surface of St. C1 in September). The average biomass of non-loricate ciliates was 2.88±2.68μgC/L, ranging from 0.05μgC/L (10m depth of St. A6 in July) to 20.51μgC/L (surface of St. A5 in August). The average tintinnid abundance was 142±273ind/L, ranging from 0ind/L (monthly) to 2756ind/L (surface of St. A1 in July). The average tintinnid biomass was 0.84±2.19μgC/L, ranging from 0.00μgC/L (every month) to 37.64μgC/L (20m depth of St. C5 in July). The results showed that the average abundance of total ciliates was 3208±2828ind/L, ranging from 166ind/L (10m depth of St. A6 in July) to 26,625ind/L (surface of St. C1 in September); the average biomass of total ciliates was 3.73±3.55μgC/L, ranging from 0.05μgC/L (10m depth of St. A6 in July) to 38.29μgC/L (20m depth of St. C5 in July). Abundance and biomass were vertically homogeneous in February, November and December, but decreased dramatically from the surface down to the bottom in other months. 23 tintinnid species were identified, 12 of which were in genus Tintinnopsis. Tintinnid species were more abundant in February, July and August. Tintinnids occupied 6.6±10.2% and 19.7±23.3% of the total ciliate abundance and biomass, respectively, which increased during the warm season and at coastal stations, and decreased during the cold season and at offshore stations. Large non-loricate ciliate species were prevalent in spring, while smaller species dominated in summer and autumn. The average abundance of total ciliates in water column was 132±72×106ind/m2, with increases during spring and autumn. The average biomass of total ciliates in water column was 152.57±93.10mgC/m2, with increases during spring and summer. The average abundance and biomass of total ciliates in water column were greater at offshore stations than at coastal stations during spring and autumn, and were lower during summer and winter. Non-loricate ciliates, tintinnids and total ciliates showed significant positive correlation with temperature and significant negative correlation with salinity (p<0.01). Non-loricate ciliates and total ciliates showed significant positive correlation with Chl a concentration (p<0.01); however, relationship between Chl a concentration and tintinnids was not significant.

Can Nonloricate Ciliate Assemblages be a Surrogate to Analyze Taxonomic Relatedness Pattern of Ciliated Protozoan Communities for Marine Bioassessment? A Case Study in Jiaozhou Bay, Northern China

Congruency using nonloricate ciliates as a surrogate to analyze taxonomic distinctness pattern of ciliated protozoan communities for assessing water quality was studied. The results showed that (1) with significant consistency with the total ciliate communities, the spatial pattern of nonloricate ciliates was significantly correlated with the changes of environmental status; (2) four taxonomic relatedness measures were significantly correlated with those of the total ciliate communities; (3) spatial variations in four taxonomic diversity indices of nonloricate ciliates were significantly related with the changes of environmental variables, especially nutrients; and (4) the paired taxonomic biodiversity indices of nonloricate ciliates showed a clear decreasing trend of departure from the expected taxonomic breadth in response to water quality. These results suggest that nonloricate ciliate assemblages can be used as a potential surrogate of ciliate communities to assess marine water quality using taxonomic distinctness measures, especially the paired indices based on presence or absence data.

Are non-loricate ciliates a primary contributor to ecological pattern of planktonic ciliate communities? A case study in Jiaozhou Bay, northern China

The contribution of non-loricate ciliate assemblage to the ecological pattern of a ciliated protozoan community was studied based on a 1-year (June 2007–May 2008) dataset collected from Jiaozhou Bay, northern China. Samples were collected biweekly from five sampling sites. Results showed that: (1) the non-loricate ciliate assemblages were the primary components and significantly correlated with the total ciliate communities in terms of species number, abundance and biomass; (2) the ecological pattern of non-loricate ciliate assemblages was significantly related to that of both total ciliate communities and variations in environmental variables; and (3) spatio-temporal variations in biodiversity (richness, diversity and evenness of species) indices of non-loricate ciliate assemblages were significantly correlated with those of total ciliate communities and the environmental conditions, especially nutrients nitrate nitrogen, nitrite nitrogen and soluble reactive phosphorous. These results suggest that the non-loricate ciliates are a primary contributor to the ecological pattern of total ciliate communities and might be used as a potential bioindicator for bioassessment in marine ecosystems.

An approach to determining potential surrogates for analyzing ecological patterns of planktonic ciliate communities in marine ecosystems

In order to identify a potential surrogate of planktonic ciliate communities for marine bioassessments and evaluating biological conservations, the different taxonomic/numerical resolutions and taxa as surrogates were studied in Jiaozhou Bay, northern China during a 1-year cycle (June 2007-May 2008). Samples were collected biweekly from a depth of 1 m at each of five sites. A range of physicochemical parameters were also measured in order to determine water quality. The genus- and family-level resolutions maintained sufficient information to evaluate the ecological patterns of the ciliate communities in response to environmental status. The non-loricate oligotrichous ciliate assemblages in both abundance and occurrence may be used as a surrogate of planktonic ciliate communities. Heavy data transformations were an optimal strategy for the species level of planktonic ciliates, while mild data transformations were for the higher. The ordination patterns based on species biomass, occurrence, and biomass/abundance ratio matrices were significantly consistent with that of species abundance data. The results suggest that the use of simplifications at both taxonomic and numerical resolutions are time-efficient and would allow improving sampling strategies of large spatial/temporal scale monitoring programs and biological conservation researches in the marine ecosystem with a relative paucity of scientists.

Quantitative variability of the copepod assemblages in the northern Adriatic Sea from 1993 to 1997

Quantitative variability of the copepod assemblages in the northern Adriatic Sea was investigated at two stations, during 43 cruises, from January 1993 to October 1997. Samples were taken at 0.5, 10, and 20 m, as well as near the bottom, using 5-l Niskin bottles. For inter-annual variation in the density of copepod assemblages data were presented as total number of nauplii and copepodites with adult copepods of the following groups: Calanoida, Cyclopoida-oithonids, Cyclopoida-oncaeids and Harpacticoida. Moreover, hydrographic conditions, both fractions of phytoplankton, non-loricate ciliates and tintinnids were taken into consideration. Nauplii are the most numerous fraction at both stations with an average over 74% in the total number of all copepod groups. Their numbers were significantly higher at the western eutrophic station, while at the eastern oligotrophic station, an absolute maximum of 693 ind. l −1 was noted. The maximum values of calanoids and oithonids occur generally during summer and these copepods are always more numerous at the western station: 33–50% and 50–63%, respectively. The most abundant taxa identified were the calanoid Paracalanus parvus and the cyclopoid Oithona nana. Oncaeid species Oncaea waldemari and Monothula subtilis dominated during late autumn and winter. An atypical increase in the abundance of oncaeids during the summer of 1997 could be related to an invasion and mass occurrence of the calycophoran siphonophore Muggiaea atlantica. It can be concluded that these dominant copepods are responsible for the stabilization of very complex processes. Atypical appearances of major copepod groups and disturbances in the copepod population structure itself can significantly influence changes in the ecosystem of this very sensitive region.

Zooplankton community and hydrographical properties of the Neretva Channel (eastern Adriatic Sea)

Temporal and spatial variability of micro and mesozooplankton was studied in 1998 and 1999 at four stations in the Neretva Channel area influenced by the Neretva river and the open waters of the south Adriatic Sea. The area is orthophosphate limited, but an excessive accumulation of land derived nitrogen is prevented by phytoplankton uptake and the general circulation pattern. Microzooplankton was dominated by ciliates, with average abundances comparable to other Adriatic channel areas (122–543 ind. l−1). Non-loricate ciliates (NLC) generally peaked in the warmer periods, but a winter increase was evident towards the inner part of the channel. Tintinnid abundances generally increased in autumn. A significant relationship with temperature was not recorded for either protozoan group. An inverse relationship between NLC and salinity might be indirectly caused by their preference for the food abundant surface layer. Mesozooplankton was dominated by copepods, with distinct summer maxima throughout the area and pronounced winter maxima of >10,000 ind. m−3 at the inner stations. The community was predominantly neritic but the open sea waters were important in structuring the mesozooplankton assemblage at all stations during the autumn–winter period. Although temperature regulated the seasonal dynamics of most metazoans and the species succession in the copepod community, small omnivorous copepods (Oncaea media complex, Oithona nana and Euterpina acutifrons) dominated regardless of the season. A trophic link between copepods and ciliates was evident in winter during low phytoplankton biomass.

The role of ciliates within the microbial food web in the eutrophicated part of Kaštela Bay (Middle Adriatic Sea)

Interactions among phytoplankton, bacterioplankton, heterotrophic nanoflagellates (HNF), ciliated protozoa and copepod nauplii were studied in the eutrophicated part of Kas?tela Bay from May 1998 to November 1999. Special emphasis was placed on relationships between size categories of nonloricate ciliates (NLC) and other microbial food web components. Biomasses of phytoplankton and bacteria were primarily influenced by abiotic parameters. Temperature indirectly controlled variation in HNF biomass through the changes in biomass of bacteria and the smaller phytoplankton fraction. Besides HNF, bacterial biomass was affected by the NLC 3 (Cell Length 3 , CL 104 µm 3 (CL >40 µm) had a strong mutual correlation and they seemed to be controlling the microphytoplankton fraction. During the colder part of the year, HNF abundance was regulated by ciliate grazing. The high impact of ciliates in summer 1998 could have been influenced by the taxonomic composition of the phytoplankton community changing, as well as the higher eutrophication level in the study area. Predation by copepod nauplii on ciliates and carbon transfer to higher trophic levels appear to be of relative importance only in the period when they are most abundant. This paper outlines the dominant relationships within the microbial food web and suggests that a significant amount of bacterial production, phytoplankton and HNF biomass could be transferred to higher trophic levels through the microbial food web.

Feeding by ciliates on two harmful algal bloom species, Prymnesium parvum and Prorocentrum minimum

We have focused on ciliates as potential grazers on toxic phytoplankton because they are major herbivores in aquatic food webs. Ciliates may exert top down control on toxic phytoplankton blooms, potentially suppressing or shortening the duration of harmful algal blooms (HABs). We measured the growth rates of several ciliate species on uni-algal and mixed diets of both HAB and non-HAB algae. The tintinnids Favella ehrenbergii, Eutintinnus pectinis and Metacylis angulata and the non-loricate ciliates Strombidinopsis sp. and Strombidium conicum were isolated from Long Island Sound (LIS), and fed HAB species including the prymnesiophyte Prymnesium parvum (strain 97-20-01) and the dinoflagellate Prorocentrum minimum (strains Exuv and JA 98-01). Ciliates were fed algal prey from cultures at various growth phases and at varying concentrations. We observed no harmful effects of P. minimum (Exuv) on any of the ciliates. However in a comparison of strains, P. minimum (Exuv) supported high growth rates, whereas P. minimum (JA 98-01) supported only nominal growth. P. parvum was acutely toxic to ciliates at high concentrations (2×10 4–3×10 4 cells ml −1). At low concentrations (5×10 3–1×10 4 cells ml −1), or in culture filtrate, ciliates survived for at least several hours. In mixed diet experiments, as long as a non-toxic alga was available, ciliates survived and at times grew well at concentrations of P. parvum (5×10 2–3×10 4 cells ml −1) that would otherwise have killed them. The present study suggests that prior to the onset of toxicity and bloom formation ciliates may exert grazing pressure on these HAB species, potentially contributing to the suppression or decline of P. minimum and P. parvum blooms.

Seasonal and vertical distribution of the ciliated protozoa and micrometazoa in Kaštela Bay (central Adriatic)

Seasonal and vertical distribution of tintinnids, non-loricate ciliates and micrometazoa were studied in Kastela Bay (central Adriatic Sea) throughout 1995. The species composition of tintinnids and copepods were studied as well. This is the first estimation of non-loricate ciliate biomass in the coastal area of the central Adriatic. Non-loricate ciliates were quantitatively the best represented ciliated protozoa, whereas nauplii were the most numerous micrometazoan organisms. Temperature affected the distribution of most micrometazoan components of the zooplankton and that of non-loricate ciliates. The temperature-dependent presence of individual size categories of non-loricate ciliates was also established. Apart from the interaction between microzooplankton groups, the influence of biotic factors, such as phytoplankton, bacteria, non-pigmented nanoflagellates (NNF) and mesozooplankton, was also discussed. The abundance of ciliates was controlled by both food supply (phytoplankton and NNF) and micrometazoan grazing. The results point to very complex trophic relationships within the planktonic community, suggesting that microzooplankton could be an important link between the microbial food web and higher trophic levels.

Seasonal distribution of the ciliated protozoa in Kaštela Bay

Seasonal distribution of the ciliated protozoa was studied in the west part of Kaštela Bay during 1995. These are the first data of the annual distribution of four size categories of non-loricate ciliates and tintinnines in the coastal area of the central Adriatic Sea. The maximum densities of ciliated protozoa were recorded during the spring and autumn, when they could act as important factors of ‘top-down’ control. Contributions of non-loricate ciliates and tintinnines to the total of ciliated protozoa were 72 and 28%, respectively. The highest density of non-loricate ciliates was registered at the surface in September (1400 ind l−1). Annual variability of the non-loricates density was mostly influenced by changes in numbers of the second (103–104 μm3) and third (104–105 μm3) size categories and they constituted 86% of the total non-loricate counts. Non-loricates with biovolume &lt;103 μm3 were the most abundant during the colder part of the year while those with biovolume &gt;105 μm3 dominated during the summer. The highest density of tintinnines (718 ind l−1) was recorded at 5-m depth in May. Quantitatively, the most important tintinnin species was Helicostomella subulata. The results suggest a complex influence of biotic and abiotic factors on the annual distribution of ciliated protozoa.

Grazing by the Antarctic sea-ice ciliate Pseudocohnilembus

Potential uptake and clearance rates of fluorescent microspheres (FM) from 0.25 to 4.05 μm diameter were determined for the non-loricate ciliate Pseudocohnilembus sp. from Antarctic sea ice. The percentage of ciliate cells that ingested FM after 20 min incubation decreased with increasing particle diameter. Pseudocohnilembus sp. ingested FM between 0.25 and 4.05 μm in diameter. We offered FM at concentrations less than natural concentrations for plankton plus detrital material and obtained clearance rates less than those previously reported for bactivorous ciliates. Clearance rates were 3.6–5.4 nl cell−1 h−1 for FM 0.5 and 1 μm diameter, respectively, but decreased to 1.1 nl cell−1 h−1 for 1.97 μm diameter and 1.4 nl cell−1 h−1 for 4.05-μm-diameter FM. Clearance and uptake rates of FM 0.5 and 1 μm diameter indicate that Pseudocohnilembus sp. principally grazes on bacteria-sized particles. However, it can also ingest organisms as large as nanoplankton and may graze particles as small as femtoplankton and colloids. This suggests a feeding strategy that may suit the temporal and spatial changes in food availability in the sea-ice habitat.

Predation by Oithona spp. on protozooplankton in the Ross Sea, Antarctica

We investigated predation rates of small copepods, primarily species of Oithona, on microprotozooplankton and net growth rates of these prey at several locations in the Ross Sea, Antarctica, during an austral summer (January 1997; U.S. JGOFS Process Study II). Ciliates, particularly non-loricate ciliates, contributed substantially to the carbon ration of Oithona spp., averaging 90% body C d −1, while dinoflagellates were much less important (1% body C d −1) despite the latter's higher abundances. We found no significant difference in net growth rates among non-loricate ciliates, tintinnid ciliates and dinoflagellates when zooplankton predators >64 μm were removed. The overall average growth rate for each protozoan taxon across the main transect line (76°30′S) was 0.1 d −1 (rates ranged from −0.5 to 1.0 d −1). Our findings also suggest that copepod predation has a minimal impact on the regulation of protozoan abundances. We estimated that predation by Oithona spp. could account for the removal of only 0.3–4.8% d −1 of ciliate standing stocks, and even less (<0.05–0.2% d −1) of the dinoflagellates. Low mortality from predation may help explain the relatively abundant populations of microprotozooplankton in the Ross Sea despite their low average net growth rates.

Immunochemical Detection of Predation on Ciliate Protists by Larvae of the Northern Anchovy (Engraulis mordax).

New approaches are needed for investigating planktonic prey-predator interactions in situ, free from enclosures and long term incubations. Lengthy incubations can lead to several artifacts, including selective mortality of planktonic organisms (1) lysis of some taxa upon fixation (2), alteration of prey-predator encounter rates through perturbations of the turbulent flow field (3) and modification of natural search and avoidance behaviors of predators and prey (4). Microscopic analysis of the gut contents of predators is a feasible alternative only if prey leave digestion-resistant hard parts. Here we describe a different approach involving immunochemical methods. We report the development and first application of an immunoassay that permits detection of predation on soft-bodied, nonloricate ciliates (Strombidium sp.). Polyclonal antibodies raised against Strombidium sp. recognize both intact ciliates and partially assimilated ciliate antigens occurring in a predator’s gut. We demonstrate, by both immunochemical and conventional methods, unequivocal predation by first-feeding larvae of the northern anchovy (Engraulis mordax) on nonloricate ciliates. The intensity of the immunochemical reaction, quantified by enzymelinked dot blots and reflection densitometry, is proportional to prey density and to the predator’s ingestion rate. Polyclonal antisera were produced in New Zealand white rabbits (5) against Strombidium sp. cultured on a diet of the bacterium Vibrio natriegens (6). Immunoglobulin G (anti-Strombidium IgG) was isolated from the antiserum by precipitation in 45% (w/v) ammonium sulfate, desalted by gel filtration, then fractionated by diethylaminoethyl ion exchange chromatography. Detailed

Grazing by the jellyfish, Aurelia aurita, on microzooplankton

The medusa of the scyphozoan, Aurelia aurita, is a seasonally important predator of copepodj and fish larvae in coastal waters but little is known about its grazing on mkrozooplanltton. Medusae preferen- tially remove large (£50 /un) oligotrichs and copepod nauplii from natural microzooplankton assemblages. Clearance rates for phytoplankton, tintinnids, rotifers and polychaete larvae are usually low. In feeding ex- periments in which mixtures of cultured prey were used, medusae selected small non-loricate ciliates over similar-sized algae, non-loricate ciliates over tintinnids, and large non-loricate ciliates over smaller ones. Observations of feeding medusae confirmed that post-capture selection of particles is important during microphagous feeding.

Tintinnids and other microzooplankton — seasonal distributions and relationships to resources and hydrography in a Maine estuary

Species composition and abundance of tintinnids and other microzooplankton were studied in the Damariscotta River estuary, Maine, USA during the period March 1981 to May 1982. Peak tintinnid abun- dances occurred during spring and summer and exceeded 7 x 101 I1. Spearman rank correlation coeffi- cients indicated that temperature, nanoplankton chlorophyll and phaeopigments passing a 20 jun filter were important factors correlating with total tintinnid density. Species composition of tintinnids changed seasonally and was similar in the spring of both years. Non-loricate ciliates were also most abundant in the spring, reaching peak densities of 4.5 x 10 4 l~l in April 1982. Rotifers were most common upestuary in the spring.

Particle production by planktonic ciliates1

Favella sp. (a tintinnid) and Balanion sp. (a nonloricate ciliate) produce fecal aggregates which overlap in size with their dinoflagellate food. These aggregates have a higher C:N ratio than the dinoflagellates. As the aggregates disintegrate, smaller particles are produced. For Favella grazing on the thecate dinoflagellate, Heterocapsa triquetra, the volume of the fecal material produced is 22–23% of the volume of algae consumed.The similarity in size of microplanktonic algae and ciliate fecal aggregates makes the Coulter Counter inappropriate for use in determining clearance or ingestion rates in ciliate feeding experiments unless egestion rates can be estimated. Visual counts were compared with Coulter Counter data for an experiment in which Favella and Balanion grazed Heterocapsa. The Coulter Counter underestimated ingestion rates by 30% for Favella and by 18% for Balanion and confounded the results in respect to particle size selectivity. However, because electronic particle counting measures the net change in particle concentration (ingestion — egestion) it can be used to measure assimilation rates. The assimilation efficiencies for Favella and Balanion eating Heterocapsa were thus estimated to be 70 and 82%.

Growth of Favella sp. (Ciliata: Tintinnina) and other microzooplankters in cages incubated in situ and comparison to growth in vitro

Microplankton cages with porous polycarbonate membrane sides were used to investigate the population growth of Favella sp., a large tintinnid which preys on dinoflagellates, Balanion sp., a non-loricate ciliate which also preys on dinoflagellates, and two other tintinnids, Eutintinnus pectinis and Tintinnopsis kofoidi, at close to in-situ conditions in a small estuary during a spring dinoflagellate bloom. The effects of temperature and food concentration on the growth of Favella sp. and Balanion sp. were also investigated in culture. Growth rates in the field were variable from day to day. The highest net growth constant (base e) observed for Favella sp. in the cages was 0.032 (generation time 21.7 h). This was lower than growth constants which can be achieved in culture. Food availability, parasitism by the dinoflagellate Duboscquella sp., and perhaps life cycle events all contributed to the lower net growth rate of Favella sp. in the field. The highest net growth constant observed in the cages for Balanion sp. was 0.068 (generation time=10.7 h), which is also lower than growth constants achieved in culture. The growth of Balanion sp. populations in the cages was limited by the availability of small-sized dinoflagellates and by predation. The highest net growth constants observed for E. pectinis and T. kofoidi were 0.030 and 0.068, respectively; we know little about the factors controlling the growth of these tintinnids.

Estimating the grazing impact of marine micro-zooplankton

This paper describes a dilution technique for estimating the micro-zooplankton grazing impact on natural communities of marine phytoplankton. Experiments performed in coastal waters off Washington, USA (October, 1980), yield estimates of micro-zooplankton impact equivalent to 6 to 24% of phytoplankton standing biomass and 17 to 52% of production per day. Indirect evidence suggests that most of this impact is due to the feeding of copepod nauplii and tintinnids; in contrast, non-loricate ciliates, comprising 80 to 90% of numerical abundance, appeared to contribute little to phytoplankton mortality.

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 μm 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.