Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

Abstract

SummaryGeneralist fish species commonly act as important links between littoral and pelagic habitats and food‐web compartments in lakes. However, diet and habitat links may depend significantly on seasonal availability of, and qualitative differences between, littoral and pelagic prey and on fish size. Despite increasing interest in food‐web dynamics, little is known about the seasonal changes in, or qualitative differences between, littoral and pelagic trophic pathways supporting generalist fish species in high‐latitude lakes.We used stomach contents together with analyses of stable carbon and nitrogen isotopes and fatty acids to study the winter and summer diet of generalistArctic charr and determine the qualitative differences between littoral and pelagic prey items. We were particularly interested to determine whetherArctic charr are able to utilise abundant and lipid‐rich winter zooplankton resources in subarcticLakeSaanajärvi, northernFinland.Arctic charr fed actively on cladoceran zooplankton in both seasons, despite the higher abundance and higher lipid content of calanoid copepods. Although the stomach contents consisted mainly of zooplankton in summer, the isotopic compositions of muscle and liver suggestArctic charr relied more on littoral carbon sources throughout the year. Fatty acid analysis indicated thatArctic charr had lower amounts of body fat and total and essential fatty acids in winter compared with summer.Observed seasonal feeding activity and dietary shifts were partly related toArctic charr size. Small (<200 mm)Arctic charr had more empty stomachs in winter, but higher amounts of zooplankton in stomachs and of essential fatty acids in muscle tissue in summer compared with larger (>200 mm) conspecifics that had more seasonally stable feeding activity and diet.Fatty acid analysis indicated that both littoral and pelagic food sources provided similar fatty acids toArctic charr, but in general, zooplankton had higher percentages of essential fatty acids compared with zoobenthos. PelagicEudiaptomus graciloidescalanoids and littoralGammarus lacustrisamphipods had the highest concentrations of total and essential fatty acids, but only the latter prey item was found in Arctic charr stomach contents.Our study demonstrates that lipid‐rich zooplankton can subsidise the predominantly benthivorous diet of top consumers (hereArctic charr) in subarctic lakes. The results also demonstrate that littoral and pelagic trophic pathways can be highly integrated in high‐latitude lakes, as a result of the flexible foraging behaviour of top consumers such as Arctic charr.