ALBERT

Description: Zooplanktonic organisms are often vulnerable to fluctuations in food supply. Their population dynamics is directly influenced by changes in phytoplankton availability and nutritional quality, which in turn is affected by changes in parameters such as nutrient loading. The aim of this study was to investigate how nitrogen (N) limitation in prey (i.e., food quality) affects the performance of zooplankton. Females of the copepod Temora longicornis and larvae of the polychaete Lanice conchilega were sampled in May and June 2016 off the German island of Helgoland, in the southern North Sea, for five-day laboratory feeding experiments. They were fed with diets of different quality - diatoms and dinoflagellates cultured in nutrient-replete (Diat and Dino , respectively) and in N-depleted (Diata and Dinoa , respectively) conditions. Sodium bicarbonate enriched with the 13C isotope (NaH13CO3) was added to prey cultures in order to label dietary fatty acids (FA) and to follow carbon (C) transfer into copepods and polychaetes. Zooplankton performance was assessed by analysis of the elemental and biochemical compositions and of the assimilation and turnover of C in copepods and polychaetes, and by measuring copepod physiological rates. Copepods feeding on Dino had the highest investment in somatic and reproductive growth. Copepods feeding on Diata had the highest N excretion rates. Egestion was a major pathway for eliminating excess C, and low food quality affected respiration rates and the intensity and speed with which dissolved organic carbon leaked from faecal pellets. Copepod physiological rates indicated that dinoflagellates are a food source of superior or similar quality to diatoms under nutrient-replete or N-depleted conditions, respectively. In addition, copepods feeding on Diata showed the highest lipid C assimilation and turnover rates. These results suggest a shift in copepod resource allocation (reproductive output or lipid accumulation) depending on food quality. Experiments with the polychaete revealed that larvae are able to regulate their lipid C content (homeostasis) regardless of the availability of dietary FA via selective accumulation and biosynthesis of FA. Lipid C assimilation results from both species, together with literature data, were used to formulate a hypothesis on different patterns of lipid homeostasis in zooplankton. These results present a robust contribution towards a better understanding of how zooplanktonic organisms might be affected by changes in the quality of their prey in the near future.