Phytoplankton Consumption Research Articles

Biochemical prey recognition by planktonic protozoa

Planktonic flagellates and ciliates are the major consumers of phytoplankton and bacterioplankton in aquatic environments, playing a pivotal role in carbon cycling and nutrient regeneration. Despite certain unicellular predators using chemosensory responses to locate and select their prey, the biochemical mechanisms behind prey reception and selection have not been elucidated. Here we identify a Ca(2+)-dependent, mannose-binding lectin on the marine dinoflagellate Oxyrrhis marina, which is used as a feeding receptor for recognizing prey. Blocking the receptor using 20 microM mannose-BSA inhibited ingestion of phytoplankton prey, Isochrysis galbana, by 60%. In prey selection studies, O. marina ingested twice as many 6 mum diameter beads coated with mannose-BSA as those coated with galNac-BSA. When pre-incubated with mannose-BSA, O. marina was no longer able to discriminate between different sugar-coated beads. Thus, these findings reveal molecular mechanisms of protozoan prey recognition. Our results also indicate the functional similarity between cellular recognition used by planktonic protozoa to discriminate between different prey items, and those used by metazoan phagocytic blood cells to recognize invading microorganisms.

Biogeochemical fluxes through mesozooplankton

Mesozooplankton are significant consumers of phytoplankton, and have a significant impact on the oceanic biogeochemical cycles of carbon and other elements. Their contribution to vertical particle flux is much larger than that of microzooplankton, yet most global biogeochemical models have lumped these two plankton functional types together. In this paper we bring together several newly available data syntheses on observed mesozooplankton concentration and the biogeochemical fluxes they mediate, and perform data synthesis on flux rates for which no synthesis was available. We update the equations of a global biogeochemical model with an explicit representation of mesozooplankton (PISCES). We use the rate measurements to constrain the parameters of mesozooplankton, and evaluate the model results with our independent synthesis of mesozooplankton concentration measurements. We also perform a sensitivity study to analyze the impact of uncertainty in the flux rates. The standard model run was parameterized on the basis of the data synthesis of flux rates. The results of mesozooplankton concentration in the standard run are slightly lower than the independent databases of observed mesozooplankton concentrations, but not significantly. This shows that structuring and parameterizing biogeochemical models on the basis of observations without tuning is a strategy that works. The sensitivity study showed that by using a maximum grazing rate of mesozooplankton that is only 30% higher than the poorly constrained fit to the observations, the model mesozooplankton concentration gets closer to the observations, but mesozooplankton grazing becomes higher than what is currently accounted for. This is an indication that food selection by mesozooplankton is not sufficiently quantified at present. Despite the amount of effort that is represented by the data syntheses of all relevant processes, the good results that were obtained for mesozooplankton indicate that this effort needs to be applied to all components of marine biogeochemistry. The development of ecosystem models that better represent key plankton groups and that are more closely based on observations should lead to better understanding and quantification of the feedbacks between marine ecosystems and climate.

Trophic ecology of Calanoides acutus in Gerlache Strait and Bellingshausen Sea waters (Antarctica, December 2002)

We measured ingestion rates of Calanoides acutus on different microbial components of the Gerlache Strait (GE) and Bellingshausen Sea (BE) waters during December 2002. At the time of the study the abundance of both zooplankton (42–133 ind m−3) and phytoplankton (0.76–1.5 µg chlorophyll a l−1) were low, indicating that the spring phytoplankton bloom was still not fully developed. C. acutus showed high clearance rates along the study (up to 432 ml ind−1 day−1), selecting for large motile organisms such as ciliates and the dinoflagellate Gyrodinium spp., although their feeding impact was always <0.1% of the standing stock of any of their prey. The total daily rations were low (∼2% body carbon per day), mostly the result of phytoplankton consumption (except for station GE3 in which heterotrophic flagellates contributed to 73% of the diet), and barely enough to cover metabolic demands. Based on the relationship between oxygen (carbon) consumption and ammonia excretion (considered as indicative of the metabolic substrate) it seems that standard metabolic demands were supplied, apart from the diet, by the use of their own non-structural proteins, whereas the remaining reserve-lipids were used to produce eggs.

Cladoceran and rotifer grazing on bacteria and phytoplankton in two shallow eutrophic lakes: in situ measurement with fluorescent microspheres

Metazooplankton grazing on bacteria and on the phytoplankton of various sizes was estimated in shallow eutrophic lakes Kaiavere and Vortsjarv (Estonia) by in situ feeding experiments with fluorescent microspheres (diameters 0.5 μm for bacteria and 3, 6 and 24 μm for phytoplankton). Zooplankton community composition, abundance and food density were important factors determining grazing rates in these lakes. Cladocerans and rotifers filtering rates (FR) and ingestion rates (IR) on bacteria and phytoplankton were several times higher in Lake Kaiavere where bacterivorous rotifers and Daphnia contributed more to zooplankton assemblage. While cladocerans were generally the main phytoplankton consumers, both lakes differed with respect to the groups of bacterivores. Based on consumption of fluorescent microspheres, the metazooplankton grazing rates were relatively low and had low impact on production and standing stock of bacteria and ingestible phytoplankton (<30 μm). On average, 0.5 and 0.1% of standing stock of bacteria and 2.6 and 1.0% of standing stock of ingestible phytoplankton was grazed daily by metazooplankton in lakes Kaiavere and Vortsjarv, respectively. That corresponded to daily grazing of 4.1% of the bacterial production and 0.43% of the total primary production (PP) by metazooplankton in Lake Kaiavere compared with 4.3 and 0.06% in Lake Vortsjarv, respectively. The results suggest that the majority of consumption of the bacterial and phytoplankton PP is most likely channelled through the microbial loop.

Interannual variability in the distribution of the phytoplankton standing stock across the seasonal sea-ice zone west of the Antarctic Peninsula

The spatial distribution of phytoplankton cell abundance, carbon (C) biomass and chlorophyll a (Chl a) concentration was analysed during three summers (1996, 1997 and 1999) in a seasonal sea-ice area, west of the Antarctic Peninsula. The objective of the study was to assess interannual variability in phytoplankton spatial distribution and the mechanisms that regulate phytoplankton accumulation in the water column. Phytoplankton C biomass and Chl a distributions were consistent from year to year, exhibiting a negative on/offshore gradient. The variations in C concentration had a close and non-linear relationship with the upper mixed layer depth, suggesting that the vertical mixing of the water column is the main factor regulating phytoplankton stock. The magnitude of C gradients was 5-fold higher during 1996 than during 1997 and 1999. This was ascribed to interannual variations in the concentration of diatom blooms in the region influenced by sea-ice melting. Vertical distribution of the phytoplankton, as estimated from Chl a profiles, also varied along an on/offshore gradient: Chl a was distributed homogeneously in the upper mixed layer in coastal and mid-shelf stations and concentrated in the deep layer (40–100 m) occupied by the winter waters (WW, remnants of the Antarctic surface waters during summer) in more offshore stations. The region with a deep Chl a maximum layer (DCM layer) was dominated by a phytoplankton assemblage characterized by a relatively high concentration of diatoms. The extent of this region varied from year to year: it was restricted to pelagic waters during 1996, extended to the shelf slope during 1997 and occupied a major portion of the area during 1999. It is hypothesized that iron depletion in near surface waters due to phytoplankton consumption, and a higher concentration in WW, regulated this vertical phytoplankton distribution pattern. Furthermore, we postulate that year-to-year variations in the spatial distribution of the DCM layer were related to interannual variations in the timing of the sea-ice retreat. The similarity between our results and those reported in literature for other areas of the Southern Ocean allows us to suggest that the mechanisms proposed here as regulating phytoplankton stock in our area may be applicable elsewhere.

Influence of ciliated protozoa and heterotrophic nanoflagellates on the fate of primary production in the northeast Atlantic Ocean

Heterotrophic nanoflagellates and ciliates and their herbivorous activity were studied within the framework of the Programme Océan Multidisciplinaire Méso Echelle (POMME) in the northeastern Atlantic between 16°–22°W and 38°–45°N during winter, spring, and late summer/autumn 2001. Ciliate ingestion rates of Synechococcus and eukaryotic algae were measured using fluorescently labeled prey. Heterotrophic nanoflagellate ingestion rates of Synechococcus and Prochlorococcus were also estimated. Heterotrophic nanoflagellate and ciliate standing stock within the surface layer (0–100 m) showed seasonal variation, with maximal values in spring (866 mg C m−2 and 637 mg C m−2, respectively). Oligotrichs dominated the ciliate assemblages, except at one site visited during spring, where a tintinnid bloom was observed. Ingestion of photosynthetic cells less than 10 μm in size was positively correlated (r = 0.7, p &lt; 0.05, n = 12) with primary production and accounted for 2–94% of this. Phytoplankton consumption reflected differences in the evolution of the phytoplankton bloom and in the structure of the microbial food web, both associated with the strong mesoscale hydrodynamic variability of the study area. In that context it is worthy to note that when tintinnids reached high abundances locally (1260 cells L−1), their impact as phytoplankton grazers was important and reached 69% of primary production. Generally, heterotrophic nanoflagellates and ciliates were relatively more important in determining the fate of phytogenic carbon during spring. Another interesting feature of primary production consumption was that during the autumn, when Prochlorococcus dominated the phytoplankton community, the protozoan grazing activity was ineffective in regulating the fate of primary producers.

Do Dreissenid Mussels Affect Lake Erie Ecosystem Stability Processes?

Ecosystem stability processes such as constancy, resilience and persistence are important, but often neglected, topics of invasive species research. Here we consider how invasive dreissenid mussels affect ecosystem stability processes in Lake Erie through both consumptive and excretory processes using the stability landscape heuristic (Gunderson, 2000). Consumption of phytoplankton by dreissenid mussels adds complexity to the system and potentially slows energy transfer from lower to higher trophic levels decreasing system constancy and lowering system resiliency. Excreting soluble waste products at low nitrogen to phosphorus ratios exacerbates these impacts on stability processes because low nutrient ratios favor growth of cyanobacterial blooms, less preferred food of zooplankton, further decreasing the transfer of energy from lower to higher trophic levels. We also provide evidence for recent changes in Lake Erie's stability landscape including a return towards eutrophy.

The impact of a benthic filter feeder: limitations imposed by physical transport of algae to the benthos

We used an acoustic Doppler profiler to investigate the hydrodynamics of a nearshore site in western Lake Erie, and we incorporated the measured parameters in numerical simulations of phytoplankton consumption by benthic zebra mussels (Dreissena polymorpha) to examine the link between pelagic production and benthic filter feeders. Daily-averaged eddy diffusivities varied from 10–5 to 10–4 m2·s–1 at our site. Our simulations demonstrate that diffusivities of this order decrease near-bed algal biomass, while algal biomass in the pelagic remains relatively unaffected. Between 8% and 67% of the algal biomass in the water column could be consumed daily, depending on the shape and magnitude of the diffusivity profile. Correspondingly, in situ vertical biomass profiles showed a near-bed zone of algal depletion, but no impact was observed near the surface. The impact of the zebra mussel in nearshore regions is expected to be stronger than in deeper open water. The flow of algal biomass into the benthos was tightly coupled with turbulent mixing, suggesting that open water algal consumption by zebra mussels is small compared with previously published estimates that ignored vertical turbulent mixing processes.

북극해 하계 중앙 바렌츠해에서 종속영양 원생동물의 군집구조와 공간적 분포

2003년 8월동안 중앙 바렌츠해의 23개 정점에서 표층생태계를 대상으로 하여 원생동물 플랑크톤의 공간적 분포와 군집구조에 대하여 조사하였다. 조사수역은 물리-화학, 엽록소-a 농도의 분포특성에 의해 고온 고염의 대서양 수괴의 영향을 받는 수역(수역 I), 저온 저염의 북극수괴의 영향을 받는 수역(수역 III), 두 수괴가 혼합되어 분포하는 수역(수역 II)으로 구분하였다. 조사수역의 엽록소-a 농도는 수역 I에서 비교적 높게 나타났으며, 수역 III에서 낮은 분포를 보였다. 조사기간 동안 원생동물 플랑크톤은 <TEX>$10{\mu}m$</TEX> 이하의 종속영양 미소 편모류, 무각 섬모충류와 유종 섬모충류를 포함하는 섬모충류, 무각 와편모류와 유각와편모류로 구성되어 있는 종속영양 와편모류로 구분하였다. 원생동물 생물량은 <TEX>$11.3{\sim}38.7{\mu}gC\;l^{-1}$</TEX>로 평균 <TEX>$21.0{\mu}gC\;l^{-1}$</TEX>로 나타났으며, 원생동물의 군집은 수역에 따라 다른 특성을 보였다. 고온 고염의 수역 I에서는 섬모충류에 의해 50% 이상의 높은 우점율을 보였으며, 저온 저염의 수역 III에서는 종속영양 와편모류에 의해 평균 50%의 우점율을 보였다. 종속영양 미소 편모류는 원생동물 군집에서 적은 그룹으로 나타났으나, 수역 III에서는 10% 이상의 기여율을 보여 다른 수역에 비해 비교적 높은 기여율을 보였다. 섬모충류의 생물량중 무각 섬모충류는 유종 섬모충류에 비해 3배 이상 높은 생물량을 보였으며, 주로 strombidium spp.와 Strobilidium spp.에 의해 높은 생물량이 유지되었다. 종속영양 와편모류중 무각 와편모류는 유각 와편모류에 비해 10배 정도의 높은 생물량이 나타났다. 따라서 조사기간 동안 섬모충류와 종속영양 와편모류는 원생동물의 중요한 부분을 차지하는 것으로 나타났다. 또한 종속영양 원생동물의 생물량과 엽록소-a 농도 사이에 상관관계 분석 결과 이 두 그룹의 생물량 사이에는 높은 상관관계를 보였다(R=0.82, p<0.0005). 이것은 종속영양 원생동물과 식물플랑크톤 사이에 잠재적 피식-포식자의 관계가 있음을 암시하며, 특히 종속영양 와편모류와 소형식물플랑크톤 사이의 밀접한 관계는 북극해 해양 생태계의 미세생물 먹이망에서 종속영양 원생동물이 일차생산의 중요한 조절 요인이 될 수 있음을 시사하였다. To investigate the spatial distribution and community structure of heterotrophic protists, we collected water samples at 23 stations of central Barents Sea in August, 2003. This study area was divided into three area with physico-chemical and chi-a distribution characteristics: Area I of warm Atlantic water mass, Area III of cold Arctic water mass and Area II of mixed water mass. Chl-a concentration ranged from 0.18 to <TEX>$1.04{\mu}g\;l^{-1}$</TEX> and was highest in Area I. The nano-sized chi-a accounted fur more than 80% of the total chi-a biomass in this study area. The contribution of nano-sized chi-a to total chi-a was higher in Area I than in Area II. Communities of heterotrophic protists were classified into three groups such as heterotrophic nanoflagellates (HNF), ciliates and heterotrophic dinoflagellates (HDF). During the study periods, carbon biomass of heterotrophic protists range from 11.3 to <TEX>$38.7{\mu}gC\;l^{-1}$</TEX> (average <TEX>$21.0{\mu}gC\;l^{-1}$</TEX>), and were highest in Area I and were lowest in Area III. The biomass of ciliates ranged from 4.2 to <TEX>$19.3{\mu}gC\;l^{-1}$</TEX> and contributed 31.5-66.9% (average 48.1%) to the biomass of heterotrophic protists. Ciliates to heterotrophic protists biomass accounted fur more than 50% in Area I. Heterotrophic dinoflagellates biomass ranged from 5.7 to <TEX>$18.4{\mu}gC\;l^{-1}$</TEX> and contributed 27.1 to 56.3% (average 42.8%) of heterotrophic protists. Heterotrophic dinoflakellates to heterotrophic protists biomass accounted fur about 50% in Area III. Heterotrophic nanoflageltate biomass ranged from 0.5 to <TEX>$3.4{\mu}gC\;l^{-1}$</TEX> and contributed 3.2 to 19.6% (average 9.2%) of heterotrophic protists. Heterotrophic nanoflagellates to heterotrophic protists biomass accounted fur more than 10% in Area III. These results indicate that the relative importance and structure of heterotrophic protists may vary according to water mass. Heterotrophic protists and phytoplankton biomass showed strong positive correlation in the study area The results suggest that heterotrophic protists are important consumers of phytoplankton, and protists might play a pivotal role in organic carbon cycling In the pelagic ecosystem of this study area during the study period.

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.

Interactions between planktonic microalgae and protozoan grazers.

For an algal bloom to develop, the growth rate of the bloom-forming species must exceed the sum of all loss processes. Among these loss processes, grazing is generally believed to be one of the more important factors. Based on numerous field studies, it is now recognized that microzooplankton are dominant consumers of phytoplankton in both open ocean and coastal waters. Heterotrophic protists, a major component of microzooplankton communities, constitute a vast complex of diverse feeding strategies and behavior which allow them access to even the larger phytoplankton species. A number of laboratory studies have shown the capability of different protistan species to feed and grow on bloom-forming algal species. Because of short generation times, their ability for fast reaction to short-term variation in food conditions enables phagotrophic protists to fulfill the function of a heterotrophic buffer, which might balance the flow of matter in case of phytoplankton blooms. The importance of grazing as a control of microalgae becomes most apparent by its failure; if community grazing controls initial stages of bloom development, there simply is no bloom. However, if a certain algal species is difficult to graze, e.g. due to specific defense mechanisms, reduced grazing pressure will certainly favor bloom development. The present contribution will provide a general overview on the interactions between planktonic microalgae and protozoan grazers with special emphasis on species-specific interactions and algal defense strategies against protozoan grazers.

Ecological Modeling for Estimation of Environmental Characteristics in Masan Bay

The ecosystem model was applied to estimate the regional distribution of the net production(or consumption) of phytoplankton and the net uptake(or regeneration) rate of nutrients in Masan Bay for scenario analysis to find a proper management plan. At the surface level, net production of phytoplankton is 200 mgC/㎡/day at the entrance of the bay, and 400∼1000 mgC/㎡/day at the center of the bay. The inner area of the bay showed more than 2000 mgC/㎡/day. All areas of the bottom level have a net consumption, with the center of the bottom level showing more than 600 mgC/㎡/day. For dissolved inorganic nitrogen, the results showed a net uptake rate of 100∼900 mg/㎡/day at the surface level. It showed that the net regeneration is above 50 mg/㎡/day at the bottom level. For dissolved inorganic phosphorus, the net uptake rate showed 10.0∼80.0 mg/㎡/day at the surface level, and the regeneration rate showed 0∼20.5 mg/㎡/day at the bottom level. Therefore, in order to control the water quality in Masan Bay, it is important to consider the re-supplement of nutrients regenerated in the water column.

162 Interactions Between Planktonic Microalgae and Protozoan Grazers

For an algal bloom to develop, the growth rate of the bloom‐forming species must exceed the sum of all loss processes. Among these loss processes, grazing is generally believed to be one of the more important factors. Based on numerous field studies it is now recognised that microzooplankton are dominant consumers of phytoplankton in both open ocean and coastal waters. Heterotrophic protists, a major component of microzooplankton communities, constitute a vast complex of diverse feeding strategies and behaviour which allow them access to even the larger phytoplankton species. A number of laboratory studies have shown the capability of different protistan species to feed and grow on bloom forming algal species. Because of short generation times, their ability for fast reaction to short‐term variation in food conditions enables phagotrophic protists to fulfil the function of a heterotrophic buffer, which might balances the flow of matter in case of phytoplankton blooms. The importance of grazing as control of microalgae becomes most apparent by its failure; if community grazing controls initial stages of bloom development, there simply is no bloom. However, if a certain algal species is difficult to graze, e.g. due to specific defence mechanisms, a reduced grazing pressure will certainly favour bloom development. The present contribution will provide a general overview on the interactions between planktonic microalgae and protozoan grazers with special emphasis on species‐specific interactions and algal defence strategies against protozoan grazers.

Diel horizontal migration of zooplankton: costs and benefits of inhabiting the littoral

1. In some shallow lakes,Daphniaand other important pelagic consumers of phytoplankton undergo diel horizontal migration (DHM) into macrophytes or other structures in the littoral zone. Some authors have suggested that DHM reduces predation by fishes onDaphniaand other cladocerans, resulting in a lower phytoplankton biomass in shallow lakes than would occur without DHM. The costs and benefits of DHM, and its potential implications in biomanipulation, are relatively unknown, however.2. In this review, we compare studies on diel vertical migration (DVM) to assess factors potentially influencing DHM (e.g. predators, food, light, temperature, dissolved oxygen, pH). We first provide examples of DHM and examine avoidance byDaphniaof both planktivorous (PL) fishes and predacious invertebrates.3. We argue that DHM should be favoured when the abundance of macrophytes is high (which reduces planktivory) and the abundance of piscivores in the littoral is sufficient to reduce planktivores. Food in the littoral zone may favour DHM by daphnids, but the quality of these resources relative to pelagic phytoplankton is largely unknown.4. We suggest that abiotic conditions, such as light, temperature, dissolved oxygen and pH, are less likely to influence DHM than DVM because weaker gradients of these conditions occur horizontally in shallow lakes relative to vertical gradients in deep lakes.5. Because our understanding of DHM is rudimentary, we highlight potentially important research areas: studying a variety of systems, comparing temporal and spatial scales of DHM in relation to DVM, quantifying positive and negative influences of macrophytes, focusing on the role of invertebrate predation, testing the performance of cladocerans on littoral versus pelagic foods (quantity and quality), investigating the potential influence of temperature, and constructing comprehensive models that can predict the likelihood of DHM. Our ability to biomanipulate shallow lakes to create or maintain the desired clear water state will increase as we learn more about the factors initiating and influencing DHM.

Horizontal transport and seasonal distribution of nutrients, dissolved oxygen and chlorophyll -a in the Gulf of Nicoya, Costa Rica: a tropical estuary

The distributions of salinity, temperature, nutrients, dissolved oxygen and chlorophyll -a (chl -a) concentrations in the Gulf of Nicoya, Costa Rica, during the rainy and dry seasons are presented. In the rainy season, the entire Gulf is strongly stratified due to high riverine discharge; surface temperature decreases and salinity increases towards the sea and most of the Gulf is undersaturated with dissolved oxygen. In the dry season, the Gulf is still stratified although without the strong fresh water signal identified in the rainy season. The lowest surface temperatures appear in the middle of the Gulf whilst the salinity generally decreases towards the upper Gulf. Only the deep waters (below 30 m depth) are undersaturated with dissolved oxygen. In the lower Gulf, oversaturation reaches up to 134% at the surface. The concentration of Si(OH) 4 in the Gulf is much higher during the rainy season than in the dry season, whilst PO 4 is not seasonally dependent. Surficial concentrations of NO 3+NO 2 in the upper Gulf are higher in the dry season than in the rainy season; whilst in most of the lower Gulf, the concentrations are lower in the dry season. Surficial chl -a concentrations in the Gulf are higher in the rainy season, in particular, close to the Tarcoles outflow. A three-component mixing diagram describes the spatial distribution of the nutrients, during both seasons. Riverine waters from the Tempisque (high nutrients and low salinity) are mixed with surface waters from the lower Gulf (higher salinity and lower nutrients). The resulting water then mixes with oceanic water. Salinity in relation to PO 4 is seasonally dependent in the upper Gulf; the riverine end member during the dry season is higher, by a factor of 4, than during the rainy season. There is a significant correlation between NO 3+NO 2 and salinity only during the dry season in the upper Gulf; this is probably a result of phytoplankton consumption of N, in the rainy season. The calculated NO 3+NO 2 riverine end member in the rainy season was 3.4 times lower than in the dry season, which is similar to that found for PO 4, indicating anthropogenic sources of N and P. The Si(OH) 4 riverine end member is higher in the rainy season; this is a result of natural rock weathering and increased transport, due to higher riverine discharge. The productivity in the Gulf of Nicoya is potentially limited by the availability of nitrogen, probably as the direct consequence of the introduction of riverine waters with high phosphorus concentration (low N/P ratios). Si(OH) 4 is always in great stoichiometric excess over N and P.

Summer feeding activities of zooplankton in Prydz Bay, Antarctica

Grazing of dominant zooplankton copepods (Calanoides acutus. and Metridia gerlachei), salps (Salpa thompsoni) and microzooplankton was determined during the austral summer of 1998/1999 at the seasonal ice zone of the Prydz Bay region. The objective was to measure the ingestion rates of zooplankton at the seasonal ice zone, so as to evaluate the importance of different groups of zooplankton in their grazing impact on phytoplankton standing stock and primary production. Grazing by copepods was low, and accounted for less than or equal to 1% of phytoplankton standing stocks and 3.8-12.5% of primary production for both species during this study, even the ingestion rates of individuals were at a high level compared with previous reports. S. thompsoni exhibited a relatively high grazing impact on primary production (72%) in the north of our investigation area. The highest grazing impact on phytoplankton was exerted by microzooplankton during this investigation, and accounted for 10-65% of the standing stock of phytoplankton and 34-100% of potential daily primary production. We concluded that microzooplankton was the dominant phytoplankton consumer in this study area. Salps also played an important role in control of phytoplankton where swarming occurred. The grazing of copepods had a relatively small effect on phytoplankton biomass development.

Feeding ecology of the mysid, Mesopodopsis wooldridgei, in a temperate estuary along the eastern seaboard of South Africa

Feeding ecology of the mysid, Mesopodopsis wooldridgei, was investigated at six stations in the Kariega estuary during summer (November) 1999 using in vitro incubations and the gut fluorescent technique. Three functional feeding groups were examined, adults (>15 mm), immatures (6-8 mm) and juveniles (<4 mm). Individual levels of gut pigment concentration for adults and immatures ranged from 0.8 to 1.4 ng pigment per individual (ind. -1 ) and between 0.3 and 1.2 ng pigment ind. -1 , respectively. Among the juveniles, gut pigment concentrations ranged from 0.1 to 0.3 ng pigment ind. -1 . Gut evacuation rates for adults and immatures were 0.90 and 0.98 h -1 respectively. Juvenile gut evacuation rates were equivalent to 1.38 h - 1 . Losses of pigment during digestion were 53, 67 and 78% for adults, immatures and juveniles. Carbon derived from the consumption of phytoplankton for adults, immatures and juveniles rangedfrom 1.06 to 1.77, from 1.40 to 3.39 and from 0.19 to 0.56 μg C ind. -1 day -1 . Clearance rates of the three size classes feeding on microzooplankton were on average 29, 12 and 6 ml ind. 1 h -1 for adults, immatures and juveniles. These rates correspond to an ingestion rate of between 0.7 and 1.66 μg C ind. -1 day -1 for adults and between 0.21 and 0.63 μg C ind. -1 day 1 for immatures. Total carbon ingested by juveniles feeding on microzooplankton corresponded to between 0.12 and 0.31 μg C ind. 1 day 1 . Carbon derived from the consumption of copepod nauplii and copepodids were equivalent to between 0.8 and 3.5 μg C ind. -1 day -1 for adults and between 0.4 and 1.05 μg C ind. 1 day 1 for immatures. Juveniles did not feed on copepod nauplii. Results of the investigation suggest that the diet of M. wooldridgei in the Kariega estuary changes with development. This is probably due to the inability of larger individuals to feed on the small phytoplankton cells (<10 μm) which dominate total chlorophyll a in the estuary.

Seasonal Changes in Zooplankton Biomass and Grazing in a Temperate Estuary, South Africa

Seasonal changes in size-fractionated chlorophylla (chla) and primary production, zooplankton biomass and zooplankton community grazing were investigated at six stations in the temperate Kariega Estuary, South Africa. Variations in chla, primary production and zooplankton biomass during the study demonstrated a seasonal pattern with minimum values recorded in winter and maximum values in summer. Total chla and primary production during the study ranged from 0·11 to 2·01μg l−1and between 13·9 and 37·7mg Cm−3d−1, respectively. Throughout the study, picophytoplankton (<2μm) dominated total chla and primary production. Zooplankton biomass during the study ranged from 7·98 to 59·31mg Dwt m−3. Pearson correlation analysis indicated that total zooplankton biomass was significantly correlated to total chla concentration (P<0·05). Community grazing rates of the zooplankton also demonstrated a seasonal pattern and ranged from 0·91 to 6·93ml l−1h−1. The elevated grazing rates recorded during summer could be related to higher seawater temperatures and zooplankton biomass. Grazing impact of zooplankton community corresponded to a loss of between 1·4 and 57·8% (mean=9·8%) of the total daily phytoplankton production. Specific ingestion rates of the two dominant copepod species, Acartia longipatella and Pseudodiaptomus hessei, indicated that carbon derived from the consumption of phytoplankton alone was sufficient to meet the basic metabolic requirements of the two copepods during summer and during spring for A. longipatella. The low contribution of phytoplankton to the total daily ration of the two copepods appears to be related to the size structure of the local phytoplankton which is too small to be grazed efficiently by the two copepod species.

Identification of model structure for aquatic ecosystems using regionalized sensitivity analysis

The Regionalized Sensitivity Analysis (RSA) was developed in 1978, for identifying critical unknown processes in poorly defined systems, thus directing the focus of further scientific investigations. Here, we demonstrate its application to model structure identification, by ranking the constituent hypotheses and identifying the critical elements for progressive revision of the model. Our case study is Lake Oglethorpe--a small monomictic impoundment in South-eastern Georgia, USA. Recent studies indicate that the warm temperate regional climate affords an extended growing season--typically from March to October--which promotes bacterial productivity in the lake. The result is a summer food web dominated by microbial processes, in contrast to the conventional phytoplankton-dominated food chains typically observed in the cold temperate lakes of Europe and North America. Starting with a simple phytoplankton-based food web model and a qualitative definition of system behaviour, we use the RSA procedure to establish the critical role of bacteria-mediated decomposition in Lake Oglethorpe, thus justifying the inclusion of microbial processes. Further analysis reveals the importance of size-dependent selective consumption of phytoplankton and bacteria. Finally, we discuss important practical implications of this novel application of the RSA regarding sampling efficiency and statistical robustness.

Feeding Studies on Selected Zooplankton in a Temperate Estuary, South Africa

Total chlorophyll- a (chla), primary production and grazing impact of selected zooplankton were estimated at six stations in the Kariega Estuary in summer 1999. Total surface chla and production ranged from 1·13 to 2·12mg chla m−3and from 18·1 to 37·7mg C m−3d−1, respectively. At all stations both chla and production were dominated by small picophytoplankton (<2μm). Total zooplankton abundance and biomass ranged from 126 to 16468ind m−3and from 4·7 to 58·2mg m−3, respectively. Throughout the study zooplankton (>200μm) were dominated by the copepods Acartia longipatella and Pseudodiaptomus hessei which comprised >75% of total biomass. Also well represented in the zooplankton assemblages were representatives of the genus Halicyclops and the mysid, Mesopodopsis wooldridgei which generally comprised <5% of total abundance. At all stations, the highest biomass and abundance values were recorded at night which could be related to the distinct vertical migrations of the zooplankton. Individual gut pigment concentrations forP. hessei ranged from 0·2 to 1·3ng (pigm) ind−1and between 0·1 and 0·5ng (pigm) ind−1for A. longipatella. For M. wooldridgei, gut pigment concentrations ranged from 0·8 to 1·3 ng (pigm) ind−1. Gut pigment destruction rates for A. longipatella, P. hessei and M. wooldridgei were estimated at 92 (±5), 79 (±7) and 53 (±9)%, respectively. Ingestion rates of the two copepods ranged between 17·6 and 31·1ng (pigm) ind−1d−1for P. hessei and between 19·7 and 38·4ng (pigm) ind−1d−1for A. longipatella. These rates correspond to a carbon ingestion rate of between 0·9 and 1·6μg C ind−1d−1for P. hessei and between 1·0 and 1·9μg C ind−1d−1for A. longipatella. Total daily ingestion rate of M. wooldridgei ranged between 21·1 and 35·2ng (pigm) ind−1. This corresponds to a carbon ingestion rate of between 1·2 and 1·8μg C ind−1d−1. Carbon derived from the consumption of phytoplankton was sufficient to meet the basic metabolic requirements of the two copepod species. The combined grazing impact of the zooplankton corresponded to between 4·3 and 20·8% of the total phytoplankton production.