Abstract
Climate change is proceeding rapidly at high northern latitudes and may have a variety of direct and indirect effects on aquatic food webs. One predicted effect is the potential shift in phytoplankton community structure towards increased cyanobacterial abundance. Given that cyanobacteria are known to be a nutritionally poor food source, we hypothesized that such a shift would reduce the efficiency of feeding and growth of northern zooplankton. To test this hypothesis, we first isolated a clone of Daphnia pulex from a permafrost thaw pond in subarctic Québec, and confirmed that it was triploid but otherwise genetically similar to a diploid, reference clone of the same species isolated from a freshwater pond in southern Québec. We used a controlled flow-through system to investigate the direct effect of temperature and indirect effect of subarctic picocyanobacteria (Synechococcus) on threshold food concentrations and growth rate of the high latitude clone. We also compared the direct effect of temperature on both Daphnia clones feeding on eukaryotic picoplankton (Nannochloropsis). The high latitude clone had a significantly lower food threshold for growth than the temperate clone at both 18 and 26°C, implying adaptation to lower food availability even under warmer conditions. Polyunsaturated fatty acids were present in the picoeukaryote but not the cyanobacterium, confirming the large difference in food quality. The food threshold for growth of the high latitude Daphnia was 3.7 (18°C) to 4.2 (26°C) times higher when fed Synechococcus versus Nannochloropsis, and there was also a significant negative effect of increased temperature and cyanobacterial food on zooplankton fatty acid content and composition. The combined effect of temperature and food quality on the performance of the high latitude Daphnia was greater than their effects added separately, further indicating the potentially strong indirect effects of climate warming on aquatic food web processes.
Highlights
The Arctic is currently warming at much faster rates than the global average and many physical effects including reduced seasonal ice cover over lakes and seas, a deepening of the permafrost active layer, and changes in snowfall and hydrology, have become apparent in northern environments [1]
The strain of Nannochloropsis used in this study contained almost twice the quantity of fatty acids (FA) per unit biomass as the subarctic Synechococcus (43.7 and 24.0 μg FA mg-1 dry mass-1, respectively), including polyunsaturated fatty acids (PUFA) that were completely absent from the picocyanobacteria (Fig 2)
This is consistent with previous observations that a large number of subarctic clones of this species are triploid while temperate clones, including the Daphnia-T used in our experiments, are diploid [26]
Summary
The Arctic is currently warming at much faster rates than the global average and many physical effects including reduced seasonal ice cover over lakes and seas, a deepening of the permafrost active layer, and changes in snowfall and hydrology, have become apparent in northern environments [1]. High latitude lakes have been identified as systems that are vulnerable to warming because of the wide-ranging influence of low temperatures and persistent ice cover on their ecosystem structure and function [2]. Temperature effects have been examined by observation, experimental manipulations and modeling [6,7,8]. These studies imply that climate change has the potential to directly affect aquatic communities and processes through changes in light and temperature conditions, but may exert indirect effects via changes in species composition and trophic relationships. Despite increasing interest in climate impacts on high latitude ecosystems, the combined influences of such direct and indirect effects on trophic processes, and phytoplankton– zooplankton interactions, remain poorly understood
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