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Data from 7 mesocosms with a gradient in daily nutrient loading rate (LN) and inverse modelling were used to estimate all C, N and P flows in an idealised food web. The gradient in LN created autotrophic production (AP) of 7 to 314 μg C l−1 d−1 (mean values over 18 d). The production of dissolved organic C (DOC) decreased from 240 to 40% of AP, and was dominated by heterotrophs (80 to 58%), with heterotrophic bacteria (BAC) producing 48 ± 8% (means ± SD) by excretion or lysis. The consumption of DOC by BAC was 133 to 86% of AP, and DOC accumulated when experimental LN ≥ natural LN. C of BAC origin constituted 5.5 to 0.2% of the C consumed by copepods. The production of dissolved N and P was mainly by heterotrophs (92 ± 2% of DN, 92 ± 4% DP), but autotrophs produced 26 ± 7% of dissolved organic N (DON) and 21 ± 8% of dissolved organic P (DOP). For the production of inorganic N and P (DIN and DIP, respectively), BAC predators produced >50% of DIN and >70% of DIP at low or moderate LN (averages 37 ± 16% of DIN and 66 ± 14% of DIP). The contribution by BAC was low for DIN (12 ± 6%) and zero for DIP. For the re-mineralisation of N and P, the significance of BAC was as N and P rich food particles. DOP accumulated more than DOC, despite the high demand for P. Turnover time of dissolved organic matter (DOM) decreased from around 80 to 12 d with increasing LN. These data provide new insights into the role of various functional groups of organisms for the cycling of DOM, and to the differences in C, N and P cycling.

The objective of the present study was to examine how N and P availability interact with C metabolism in marine heterotrophic plankton and whether or not heterotrophic groups are likely to be subjected to essential nutrient limitations in natural coastal waters. The nutrients stud- ied were mineral P and N; the latter is a proxy for essential amino acids. We present a general the- oretical framework and criteria for evaluating essential nutrient limitations in heterotrophs. Data were derived from a comprehensive mesocosm experiment, where food-web flows were esti- mated using inverse modelling. Bacteria of the mesocosm communities were severely P-deficient throughout. The heterotrophic nanoplankton was most likely limited by another essential nutrient or by food C availability. Ciliates were most likely P-limited, whereas copepods might experience P limitation, but were more likely limited by another essential nutrient or food C in the present experiments. The N contents of the food were close to the theoretical requirements for copepods and ciliates. All planktonic groups released dissolved inorganic nitrogen (DIN), and all except bacteria released dissolved inorganic phosphorus (DIP). Our results suggested that P limitation might be readily experienced by bacteria, ciliates and perhaps also by copepods, but not by het- erotrophic nanoplankton in natural North East Atlantic coastal waters. A predator must balance its energy metabolism and growth under the variable availabilities of essential nutrients, and we pro- pose that C growth efficiency is a dynamic variable mainly dependent on the availability of the most limiting nutrient. We support the view that C availability alone cannot be used as a proxy for the food limitation of bacterial and zooplankton growth. Specific essential nutrients should be regarded as potential limiting factors, as for phytoplankton. The variable nutritional requirements of heterotrophic predators will then represent a major driver of heterotrophic species diversity, allowing a broad diversity of heterotrophic species in plankton communities.

The objective of the present study was to study the dynamics of protozoan and metazoan zooplankton food webs in coastal NE Atlantic waters exposed to variable nutrient input. Data were derived from a mesocosm experiment (7 units, 40 m 3 , 12 m deep) receiving variable nutrient input. The food web included 3 autotrophic groups based on size, and 4 functional heterotrophic groups mainly based on trophic position. Inverse modelling was used to construct networks of carbon flows for the planktonic food web. Heterotrophic nanoplankton, microplankton and mesoplankton (HNP, CIL and COP, respectively) were found to be equally important contributors to grazing and carbon release during undisturbed summer situations. The release of dissolved organic carbon by zooplank- ton was comparable to that of phytoplankton. Autotrophic food was generally more important for zoo- plankton than heterotrophic (mean 75%). Assimilation and growth efficiencies (AE and GE, respec- tively) of zooplankton groups in undisturbed situations were in the range of 33 to 69% and 10 to 41%, respectively. Values were inversely related to gross primary production (GPP). Sedimentation rates of carbon were low. High nutrient input rates increased food availability and most CIL and COP carbon flows. HNP did not respond, and neither did its food, that is, bacteria and picoautotrophs. The response in biomass was generally lower than that for the flows. Values of AE and GE of the zoo- plankton during high nutrient input and food availability varied between 11 and 29% and 5.7 and 19%, respectively, and throughout were lower than at low nutrient input. The sedimentation rate of particulate carbon increased strongly, resulting in an enhanced organic input rate in deep water.

Appendicularians, an important group of marine gelatinous zooplankton, are highly efficient filter feeders of small phyto- and bacterioplankton. It is not known which factors regulate their abundance and biomass in the field. In a mesocosm experiment, we manipulated nutrient load- ing and initial densities of calanoid copepods (almost exclusively the genera Temora sp., Centro- pages and Pseudocalanus) and followed plankton dynamics over 2 wk. Peak appendicularian densi- ties were inversely related to initial copepod densities. We observed more than 100-fold variation in Oikopleura dioica density among treatments, even though our experimental variables were kept within ecologically relevant magnitudes. The differences between copepod-reduced and copepod- enhanced treatments were much larger at high and moderate nutrient loadings than in treatments without nutrient enrichment. Thus, appendicularians are under strong pressure from calanoid cope- pods. Calanoid copepods appear to structure the plankton community via direct negative effects on large algae, ciliates and appendicularians and indirect positive effects on small algae resulting from the release of these from predation by ciliates and appendicularians.

Copepods and ciliates are 2 functionally different types of herbivores that dominate marine ecosystems. Ciliates have a higher reproductive capacity than copepods, but copepods can graze on ciliates. Hence we have an omnivorous system in which the inferior grazer can graze on its competitor. We conducted a 3 2 factorial mesocosm experiment with natural plankton communities to explore how the copepod -ciliate interaction affects algal development on the timescale relevant for algal blooms. We used nutrient load and copepod biomass as the experimental variables. Ciliate bio- mass was positively correlated to nutrient addition and negatively correlated to copepod biomass, with copepod biomass as the predominant factor at the highest densities tested. Both increasing copepod biomass and nutrient addition resulted in increased algal biomass, with the strongest effect on nanoalgae. The mechanism involved was carnivory by copepods and a resulting trophic cascade through ciliates to algae. We conclude that even a moderately high biomass of copepods may pro- mote algal blooms when the initial size distribution of the algal community is appropriate (i.e. domi- nance of algae eaten primarily by ciliates).