Impact Of Zooplankton Grazing Research Articles

Hydrography of the Southern Ocean fronts – The zooplankton link: A detailed study in the Indian Ocean sector

Despite its importance, zooplankton diversity, biomass and abundance are still poorly known in the remote Indian sector of the Southern Ocean (SO). Therefore, systematic observations of zooplankton and hydrography have been made in the Indian sector of the SO, with a focus on Copepoda. We collected the data along 57° E transect during austral summer and examined various oceanographic locations in the central Indian Ocean sector of the SO lying in the east of the Kerguelen Plateau. The contribution of copepod abundance was substantial (85 %) to the total zooplankton biomass across the entire study region. We observed surface stratification in the Subtropical Front (STF) and Subantarctic Front (SAF); on the other hand, subsurface stratification was encountered in the Polar Front (PF) and South of PF (S-PF). As a result, nutrients were lower in warmer waters than in colder waters. On the other hand, zooplankton abundance was higher in colder waters than in warmer waters. The highest zooplankton grazing impact was recorded in the PF zone, where zooplankton removed ∼ 81 % of the daily phytoplankton production, while north of the PF zooplankton grazing was generally low. Four copepod species were shown to be highly connected to oceanic fronts. The vertical distribution of zooplankton abundance and species composition was chiefly driven by physical and biological conditions in the Indian sector of the SO. As a result, our findings indicate that physical processes play an important role in organizing plankton diversity, which ultimately determines the fate of the foodweb structure and the biological carbon pump in the studied area.

Impact of zooplankton grazing on phytoplankton in north temperate coastal lakes: changes along gradients in salinity and nutrients

Impact of zooplankton grazing on phytoplankton in north temperate coastal lakes: changes along gradients in salinity and nutrients

Structural-functional characteristics оf zooplankton in the ob estuary and adjacent Kara sea shelf in summer

The study was carried out in the Ob estuary and the adjacent shelf of the Kara Sea in July 2016. For the first time data on the species composition, abundance and distribution as well as on feeding of zooplankton were obtained during the period of intensive river runoff. The biomass of zooplankton in terms of wet weight (without jelly animals and chaetognaths) varied over a wide range from 40 to 1880 mg/m3. The biomass was dominared by the Limnocalanus macrurus population, represented by all age stages, and Pseudocalanus spp. (an average of 60% and 17% of biomass, respectively. Maxima of zooplankton biomass were associated with the boundary of the distribution of river waters and with the southern periphery of the estuary frontal zone (EFZ). The short-term temporal variation of the abundance of zooplankton was observed when the transect was repeated after 2 days: the biomass at the southern periphery of the EFZ decreased from 1900 to 250 mg/m3). The observed changes is supposed to be caused by the shift of the southern boundary of the EFZ under the change in the wind regime. At high phytoplankton biomass in the estuary (>7 μg Chl-a/l), the daily ingestion of autotrophic phytoplankton of different copepodite stages of Limnocalanus macrurus was 5.77, 4.46, 2.59 and 1.4 μg C/ind/day in CVI, CV, CIV and CIII, respectively. Energy intake by younger copepodite stages not only covered the metabolic requirements, but also enabled the copepods to growth. At a relatively low Chl-a concentration (<2 μg/l) on the shelf, the mean values for older stages were lower (1.56 μg C/ind/day and 1.17 μg C/ind/day in CVI and CV, respectively) and hardly met the metabolic demands. The zooplankton grazing impact estimated with the gut fluorescence method varied from 0.2 to 7.3% of phytoplankton standing stock and from 2.2 to 30% of primary production. The results are discussed in relation to the specific hydrophysical conditions in the summer period and in comparison with the data obtained earlier for the autumn season.

Structural and Functional Characteristics of Zooplankton in the Ob Estuary and Adjacent Shelf Areas of the Kara Sea in Summer

The study was carried out in the Ob estuary and the adjacent shelf of the Kara Sea in July 2016. For the first time data on the species composition, abundance and distribution as well as on feeding of zooplankton were obtained during the period of intensive river runoff. The biomass of zooplankton in terms of wet weight (without jelly animals and chaetognaths) varied over a wide range from 40 to 1880 mg/m3. The biomass was dominaned by the Limnocalanus macrurus population, represented by all age stages, and Pseudocalanus spp. (an average 60% and 17% of biomass, respectively). Maxima of zooplankton biomass were associated with the boundary of the distribution of freshwater and with the southern periphery of the estuary frontal zone (EFZ). The short-term temporal variation of the abundance of zooplankton was observed when the transect was repeated after 2 days: the biomass at the southern periphery of the EFZ decreased from 1900 to 250 mg/m3. The observed changes is supposed to be caused by the shift of the southern boundary of the EFZ under changing in the wind regime. At high phytoplankton biomass in the estuary (>7 μg Chl-a/L), the daily ingestion of Limnocalanus macrurus was 5.77, 4.46, 2.59 and 1.4 μg C/day in CVI, CV, CIV and CIII, respectively. Energy intake by younger copepodite stages not only covered the metabolic requirements, but also enabled the copepods to growth. At lower Chl-a concentration on the shelf (<2 μg/L), the mean values were lower (1.56 μg C/day and 1.17 μg C/day in CVI and CV, respectively) and hardly met the metabolic demands. The zooplankton grazing impact estimated with the gut fluorescence method varied from 0.2 to 7.3% of phytoplankton standing stock and from 2.2 to 30% of primary production. The results are discussed in relation to the specific hydrophysical conditions in the summer period and in comparison with the data obtained for the autumn season.

Does Zooplankton Control Phytoplankton Development in White Sea Coastal Waters in Spring?

Grazing impact of dominant zooplankton species on phytoplankton was estimated in the coastal areas of the White Sea in spring. The material was collected on expeditions of the R/V Ekolog in June 2012 and 2015 in Kandalaksha Gulf and Onega Bay of the White Sea. The data on the composition and biomass of zooplankton showed that depending on the intensity of the warming of the water column at the beginning of productive season, the zooplankton community was at the early-spring or late-spring stage of seasonal development. Zooplankton grazing impact varied from 1 to 90% of phytoplankton biomass and from 7 to 470% of primary production. The results are discussed in relation to the hydrothermal regime and the phase of coastal plankton community succession in different years.

Distribution and Feeding of Herbivorous Zooplankton in the Laptev Sea

Sampling was conducted along the quasi meridional transect at 130° E from the Lena River estuary to northern deep-sea regions of the Laptev Sea in September 2015. The latitudinal zonality and the impact of river runoff are manifested in the temperature and salinity distribution, concentration of particulate organic matter, and the structure of plankton communities. The differences in the chl a concentration and primary production along the transect are insignificant. The feeding rate of mesozooplankton herbivores was assessed by a fluorescence technique. The total consumption of phytoplankton biomass and primary production are estimated based on the feeding rate, abundance of zooplankton species, and their diel migrations. The daily grazing impact of zooplankton on phytoplankton biomass increases from 2% on the inner shelf to 3% on the mid-shelf, 5% on the outer shelf, and 10% in the deep-sea part of the basin. The consumption of primary production also increases: 1, 4.5, 5.7, and 13.9%, respectively. In the fall, the consumption of phytoplankton does not compensate the energy demands for respiration. The latitudinal zonality of the Laptev Sea appears not only in the hydrophysical water parameters and the structure of plankton communities, but also in their functional characteristics.

Feeding of dominant zooplankton species and their grazing impact on autotrophic phytoplankton in the Yenisei Estuary in autumn

Feeding of dominant mesozooplankton species was investigated in freshwater zone, estuarine frontal zone of Yenisei Estuary and adjacent inner shelf area during autumn season. Ingestion rate was estimated based on gut fluorescence measurements. It was shown that in spite on the end of productive season daily ingestion of phytoplankton for the most of the investigated freshwater, brackish and marine zooplankton species was higher than their metabolic requirements. Total phytoplankton consumption by zooplankton differed in different zones. In freshwater zone under high level of autotrophic phytoplankton biomass and primary production zooplankton grazing impact was low: 1.5% of standing stock and 20% of primary production. In the estuarine frontal zone 3.2–14.3% of standing stock and 150–290% of primary production was grazed by zooplankton per day, in adjacent inner shelf: 1.4–7.0% and 130%, accordingly. Based on comparatively analysis of obtained data and results of investigation of zooplankton feeding in Ob Estuary during the same season some general patterns of the zooplankton role in organic matter biotransformation in the large arctic rivers estuarine areas were revealed.

Photosynthetic pigments as indicators of phytoplankton development during spring and summer in Adventfjorden (Spitsbergen)

The study was carried out from April 30 until July 13 of 1997 in Adventfjorden (Spitsbergen). Formation of less saline and warmer surface water (∼1 m thick) caused by melting of the fast ice was observed in the fjord during the first days of May. In summer a less saline surface layer was about 3 m thick. Euphotic depth measured under ice sheet reached 12 m, whereas load of mineral matter brought with riverine discharge in summer (the content of total particulate matter in the fjord reached 1.66 kg m−2) dramatically reduced euphotic zone depth to 0.35 m. By pigment measurement three phases of phytoplankton development in Adventfjorden were distinguished: (1) spring bloom that has started under fast ice and reached maximum in the mid of May, (2) stagnation period in June, (3) increase of pigment concentration in July, what could indicate a start of the next algae bloom. Analyses of chlorophylls and carotenoids revealed that diatoms (chl c, fucoxanthin), and green algae (chl b, lutein) dominated phytoplankton community in the fjord. Moreover, the presence of peridinin indicates the presence of Dinophyta and alloxanthin—the occurence of Cryptophyta. In May and June 1997 phytoplankton appeared mainly in the surface of water, while in July, as a result of inflow of turbulent riverine waters into Adventfjorden, algae cells were pushed down and the highest numbers were observed at the depth ∼20 m. Great phaeopigments to chl a ratio (= 0.54) found in the fjord seston in June and July probably shows strong impact of zooplankton grazing on phytoplankton development. High contribution of chlorophyllide a in porphyrin a poll in samples collected under fast ice (chlorophyllide a/chl a ratio = 0.18) reflects the final stage of algal communitie succession in ice, just before spring ice melt and release of biota to oceanic water. Chloropyllide a content during summer was minor or not detectable, demonstrating that diatom cells were in good physiological condition. High chl a allomer/chl a ratio (average = 0.11 for the period investigated) confirms high oxygen concentration in environment of Adventfjorden.

Estimates of bacterial and phytoplankton mortality caused by viral lysis and microzooplankton grazing in a shallow eutrophic lake

Summary1. Since viral lysis and zooplankton grazing differ in their impact on the aquatic food web, it is important to assess the relative importance of both mortality factors. In this study, an adapted version of the dilution technique was applied to simultaneously estimate the impact of both viral lysis and zooplankton grazing on the mortality of heterotrophic bacteria, eukaryotic algae, unicellular cyanobacteria, prochlorophytes and especially filamentous cyanobacteria in a shallow eutrophic lake.2. Four dilution experiments were performed in December 2004, January 2005, and March and April 2006. Viral and heterotrophic bacterial abundances were obtained by epifluorescence microscopy and abundances of different phytoplankton groups by flow cytometry and light microscopy.3. Viral lysis was identified as the main mortality cause during the December 2004 and January 2005 experiments, apparently removing between 84% and 97% of the potential filamentous cyanobacterial production and up to 101% of the potential heterotrophic bacterial production. Microzooplankton grazing was estimated to remove between 90% and 99% of the potential unicellular cyanobacterial production and up to 46% of the potential heterotrophic bacterial production during the spring 2006 experiments.4. In some cases, no significant impact of viral lysis or zooplankton grazing was detected. Contrary to expectations, the apparent growth rate of filamentous cyanobacteria was even sometimes observed to decrease significantly upon dilution of microzooplankton.5. The dilution technique can give valuable insight into the impact of zooplankton grazing and viral lysis on the mortality of different plankton groups but require some caveats require special care to be taken when comparing and interpreting results.

Impact of zooplankton grazing on Alexandrium blooms in the offshore Gulf of Maine

Abstract Zooplankton grazing was investigated by shipboard experiments during natural blooms of Alexandrium spp. in the offshore Gulf of Maine in spring and/or summer of 1998, 2000, and 2001. Grazing studies were done in conjunction with studies of accumulation of Alexandrium toxins in the zooplankton, as part of the ECOHAB-Gulf of Maine regional program. Several species of copepods, marine cladocerans, and appendicularians were allowed to graze upon natural phytoplankton assemblages, at ambient temperatures (14–17 °C). Grazing was measured by quantitative microscopic analyses of disappearance of phytoplankton cells in initial, control, and experimental food suspensions. Thus, we were able to examine grazing upon Alexandrium in comparison to grazing on other co-occurring phytoplankton taxa. Even during Alexandrium “blooms,” this dinoflagellate was a minor component of the overall phytoplankton assemblage. It was present at stations where grazing experiments were conducted at levels of 0.12–7.57×103 cells l−1, or 0.03–3.93% of total phytoplankton cells. Maximum ingestion of Alexandrium accounted for only up to 3.2% of total cells ingested. Phytoplankton assemblages were dominated by athecate microflagellates, and to a lesser extent by diatoms and non-toxic dinoflagellates. Microflagellates were present at abundances of 159.62–793.93 cells ml−1, or 60.6–95.56% of total cells. Grazing on microflagellates accounted for 35.59–98.21% of total grazing. Grazing on Alexandrium spp. and microflagellates was generally non-selective, with these taxa being ingested in similar proportions to their availability in food assemblages. Grazing on diatoms was selective, with diatoms being disproportionately ingested, compared to their proportions in food assemblages. There were no apparent adverse effects of Alexandrium on grazers during incubations of 18–24 h, and grazer survival was 100%. Estimated daily zooplankton grazing impact on Alexandrium spp. field populations by field populations of experimental grazers averaged 5.79% (range=0–117%). Extrapolating experimentally determined grazing rates to total zooplankton assemblages increased potential grazing impact to 0–667.77% (mean=114.7%). However, these potential impacts are likely overestimations, because toxin accumulation data indicated that many of the most-abundant zooplankters (Oithona similis copepodites and copepod nauplii) likely graze only minimally upon Alexandrium spp. Thus, antipredation effects of high concentrations of Alexandrium on some grazers reported from some laboratory studies may only occur rarely in nature, because of low individual zooplankter grazing rates on Alexandrium, and dilution of grazing upon it by grazing on other food sources such as abundant microflagellates and diatoms.

Zooplankton dynamics in the eastern Atlantic sector of the Southern Ocean during the austral summer 1997/1998—Part 2: Grazing impact

Zooplankton grazing impact using the gut fluorescence technique was investigated in the top 200 m water layer along the 6°E meridian between 49°50′ and 60°25′S at six Biostations during a Scandinavian/South African Joint Global Ocean Flux Study (December 1997–January 1998) on board the SA Agulhas. Copepods were found to be the most conspicuous grazers along the entire transect, generally accounting for ≈40% of total zooplankton grazing. Pelagic pteropods, Limacina helicina and Clio sulcata, were the second most important grazers within the Seasonal Ice Edge (SIE) region, while the tunicate Salpa thompsoni and euphausiids, mainly Euphausia superba and E. frigida, were the second and third most important consumers of the phytoplankton production within the Winter Ice Edge (WIE) and Antarctic Polar Front (APF) regions. The overall ingestion rates of the zooplankton community ranged from 24 to 277 mg C m−2 day−1. The lowest and highest ingestion rates were recorded within the SIE and APF regions. The zooplankton grazing impact was low within the SIE region accounting for <10% of daily primary production. The highest phytoplankton consumption rate (56% of daily primary production) was observed in the WIE region, while the APF region was characterized by modest (23–33%) grazing impact of zooplankton.

Trophic coupling across the St. Lawrence River estuarine transition zone

The objective of this study was to analyze the coupling between trophic levels of the frontal area of the St. Lawrence estuary transition zone, which is the site of an estuarine turbidity maximum (ETM) and is an important nursery area for the juveniles of Atlantic tomcod Microgadus tomcod and rainbow smelt Osmerus mordax. A detailed series of measurements and sampling were conducted over 6 tidal cycles within the frontal zone. An inverse relationship between the abundance of the 63 µm-net plankton and that of autotrophs indicated the impact of zooplankton grazing on autotrophic biomass, which was largely composed of diatoms. Within the 63 µm-net plankton, nau- plii, copepodites and the adults of Eurytemora affinis appeared to be the most important grazers of autotrophs. First-order calculations illustrated that the primary production observed in the ETM is capable of supporting the biomass of this copepod and that its grazing pressure is capable of reduc- ing autotrophic biomass in the brackish waters of the transition zone. Heterotrophic species were a small component (<20%) of total microplankton in the freshwater samples, but dominated the total community biomass at higher salinities. This shift towards heterotrophic dominance implies a spatial coupling between upstream autotrophic production and downstream consumer processes. Analysis of stomach contents showed that the calanoid copepod E. affinis was the primary food source for larval fishes and mysids, although the combined ingestion rates of the 2 fish species are unlikely to have any impact on copepod standing stocks. By far the most important predators of the zooplankton are Neomysis americana and Mysis stenolepis. Estuarine circulation and associated entrapment processes ultimately control the trophic relationships and gradients in community structure within the ETM.

Zooplankton biomass and indices of feeding and metabolism in relation to an upwelling filament off northwest Africa

Zooplankton biomass and indices of grazing (gut e uorescence), respiration (electron transfer system activity, ETS) and growth (aspartate transcarbamylase, ATC) were studied in relation to an upwelling e lament off northwest Africa during August 1993. The e lament extended 150 km offshore into the oligotrophic waters. It was generated by a trapped, quasi-permanent cyclonic eddy located between the Canary Islands and the African shelf. High biomass, specie c gut e uorescence and electron transfer system activity in zooplankton were observed along the e lament structure. In contrast, low values of biomass, gut e uorescence, ETS and ATC specie c activities were found in the center of the trapped cyclonic eddy. Assuming a 50% of pigment destruction, the calculated grazing impact of zooplankton on primary production varied between 16 and 97%, a high range compared to other oceanic systems. Ingestion, estimated from indices of metabolism and growth, accounted for 47- 296% of the primary production (assuming an herbivorous feeding). Mesozooplankton trans- ported offshore into the oligotrophic area fule lled their metabolic demands with nonpigmented food as observed from the increase of omnivory from the coastal waters to the open ocean. The progressive decay of grazing and metabolic indices along the e lament suggests that advection, rather than local enrichment processes, is mostly responsible for the high biomass values in this physical structure.

The occurrence of faecal material in relation to different pelagic systems in the Southern Ocean and its importance for vertical flux

Abstract:During the SO-JGOFS-Polarstern-cruise in Oct/Nov 1992, faecal pellet abundance and distribution were determined in order to assess the impact of zooplankton grazing and defecation within the following three typical Antarctic plankton regimes in the Atlantic sector: the Marginal Ice Zone (MIZ), the southern Antarctic Circumpolar Current (ACC) and the Polar Frontal region (PFr). In contrast to the more southern regions, the PFr was characterised by the occurrence of relatively dense phyto-plankton blooms and high copepod concentrations. Faecal pellets were relatively abundant in the MIZ reaching up to > 105 µg faecal pellet carbon (FPC) m-3, whereas the values in the more northern regions were about one to two orders of magnitude lower: about 6 µg FPC m-3 in the southern ACC and less than 1 µm FPC m-3 in the PFr. Thus, the region with the highest phyto- and zooplankton concentrations showed by far the lowest faecal pellet standing stock concentrations. These results were compared to other regions in the Southern Ocean and to other regions in the world oceans and possible reasons for this situation and the potential ecological impact are discussed. Our investigations show, that not only the biomass of phytoplankton and zooplankton, but also mainly the structures of the plankton communities are decisive for sedimentation potentials of carbon and silica via faecal pellets in the different regions of the ocean.

Zooplankton grazing dynamics: top‐down control of phytoplankton and its relationship to an estuarine habitat

AbstractTop‐down control of algal blooms was investigated in the Swan–Canning Estuary, to determine whether the resident zooplankton community was able to substantially reduce phytoplankton biomass through grazing, or whether high phytoplankton biomass effectively inhibited grazing. In situ grazing by zooplankton at two depths in the water column was measured over a diel period on six occasions and examined in relation to the biotic and abiotic habitat conditions.Measured weight‐specific ingestion rates varied with phytoplankton and zooplankton species composition, but were significantly greater at the surface than at 5 m, close to the bottom (ranges of 1·59 to 252·41 and 0·51 to 28·99 µg phytoplankton C µg zooplankton C−1 day−1, respectively). There was no evidence of significant diel changes in ingestion rates of zooplankton. Phytoplankton biomass removed by in situ community grazing ranged between 0·9 and 45·0% of standing stock per hour. The maximum loss occurred when phytoplankton biomass ranged from 249·0 to 320·8 µg phytoplankton C l−1. Zooplankton grazing impact was negatively associated with high phytoplankton and low dissolved oxygen in bottom waters. The implications of, and mechanisms associated with, top‐down control of algal biomass in this estuary are discussed in relation to plankton succession and the physical environment. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Modelling the impact of zooplankton grazing on phytoplankton biomass during a dinoflagellate bloom in the Swan River Estuary, Western Australia

Ingestion rates of zooplankton were measured in the Swan River Estuary and in the laboratory. These data were used together with data from the literature on phytoplankton and zooplankton physiological parameters, to provide input data to a model of phytoplankton and zooplankton dynamics in the Swan River Estuary. The model also used measured environmental data (nutrient concentrations, light, water temperature and salinity) to simulate phytoplankton biomass (differentiated as four groups) and zooplankton biomass (differentiated as three size classes) at a site in the Swan River Estuary. Zooplankton grazing was shown to be highly important in attenuating a dinoflagellate bloom that occurred over a 3-week model simulation period.

Multisensor sampling of pelagic ecosystem variables in a coastal environment to estimate zooplankton grazing impact

Sampling was conducted on the west Florida continental shelf ecosystem modeling site to estimate zooplankton grazing impact on primary production. Samples were collected with the high-resolution sampler, a towed array bearing electronic and optical sensors operating in tandem with a paired net/bottle verification system. A close biological–physical coupling was observed, with three main plankton communities: 1. a high-density inshore community dominated by larvaceans coincident with a salinity gradient; 2. a low-density offshore community dominated by small calanoid copepods coincident with the warm mixed layer; and 3. a high-density offshore community dominated by small poecilostomatoid and cyclopoid copepods and ostracods coincident with cooler, sub-pycnocline oceanic water. Both high-density communities were associated with relatively turbid water. Applying available grazing rates from the literature to our abundance data, grazing pressure mirrored the above bio-physical pattern, with the offshore sub-pycnocline community contributing ∼65% of grazing pressure despite representing only 19% of the total volume of the transect. This suggests that grazing pressure is highly localized, emphasizing the importance of high-resolution sampling to better understand plankton dynamics. A comparison of our grazing rate estimates with primary production estimates suggests that mesozooplankton do not control the fate of phytoplankton over much of the area studied (<5% grazing of daily primary production), but “hot spots” (∼25–50% grazing) do occur which may have an effect on floral composition.

Temporal and vertical dynamics of phytoplankton net growth in Castle Lake, California

The impact of nutrient additions, zooplankton grazing and light intensity on phytoplankton net growth with depth and season was studied with five microcosm experiments in meso-oligotrophic, subalpine Castle Lake, California, during the period of summer stratification in June-September 1994. The incubations (4 day) were performed at 5 m intervals from the surface to the bottom using natural phytoplankton and zooplankton assemblages, with enrichments of phosphorus and nitrogen. The phytoplankton community was only limited by nutrients in the upper 5 m (epilimnion), as indicated by change in chlorophyll concentration. Nutrient enrichments had the greatest effect on the phytoplankton net growth in June and July. High light inhibited the phytoplankton net growth at the surface. Low light intensities limited phytoplankton at 20 m and below, and at the end of the growing season already around 10-15 m. A deep chlorophyll maximum in the hypolimnion in June-August was not limited by either light or nutrients. The results showed variation in grazers' impact on phytoplankton. These results suggest the importance of nutrient limitation only in the epilimnion with light inhibition at the surface, light limitation in the hypolimnion, and varying impact of zooplankton grazing in influencing the development of the phytoplankton in Castle Lake.

Seasonal dynamics of free and attached heterotrophic nanoflagellates in an oligomesotrophic lake

1. The seasonal development of heterotrophic nanoflagellates (HNF), bacteria, rotiferans and crustacean zooplankton was studied in the epilimnion of Lake Pavin, an oligomesotrophic lake in the Massif Central of France.2. HNF abundance varied from 0.1 to 2.5 × 103 mL–1. Free‐living HNF reached their highest density in spring when the copepod Acanthodiaptomus denticornis dominated the metazooplankton. They were present in low numbers when rotifers and cladocerans were numerous.3. Attached HNF, consisting of bicoecids and choanoflagellates, were fixed to large diatoms and to the colonial cyanobacterium Anabaena flos‐aquae. The abundance of attached HNF was significantly correlated to bacterial abundance, which fluctuated between 1.1 and 2.7 × 106 mL–1. Highest abundance of these epiphytic protists was recorded when free‐living heterotrophic nanoflagellates declined.4. The comparison of the dynamics of heterotrophic nanoflagellates, bacteria, and the impact of zooplankton grazing suggested that prey abundance, the presence of suitable attachment sites and limited competition from the free‐living forms were the main factors controlling the development of the epiphytic flagellate protists. In contrast, the low abundance of free‐living forms during the period of rotiferan and cladoceran development suggests the prevalence of a top‐down control by predation of the metazoopankton.

Toxic marine phytoplankton, zooplankton grazers, and pelagic food webs

Interactions between toxic phytoplankton and their zooplankton grazers are complex. Some zooplankters ingest some toxic phytoplankters with no apparent harm, whereas others are deleteriously affected. Phycotoxins vary in their modes of action, levels of toxicity and solubility, and affect grazers in different ways. Beyond effects on direct grazers, toxins may accumulate in and be transferred through marine food webs, affecting consumers at higher trophic levels, including fish, seabirds, and marine mammals. Grazers of toxic phytoplankton include protists as well as metazoans, and the impact of zooplankton grazing on development or termination of toxic blooms is poorly understood. In most interactions of toxic phytoplankters with grazers and other marine food‐web components, outcomes are situation‐specific, and extrapolation of results from one set of circumstances to another may be inappropriate.