Abstract
During past decades, many lakes underwent drastic human‐caused changes in trophic state with strong implications for population dynamics and food web processes. We investigated the influence of trophic state on nutrient allocation into Daphnia resting eggs. The production of resting eggs is an important survival strategy, allowing Daphnia to cope with unfavorable environmental conditions. Allocation of essential nutrients into resting eggs may crucially influence embryonic development and offspring survival and thus is of great ecological and evolutionary interest. The capacity of Daphnia to adjust the allocation of nutrients into resting eggs may depend on the dietary nutrient supply, which may vary with trophic state‐related changes in the phytoplankton community composition. Resting eggs were isolated from sediment cores taken from Lake Constance, a large prealpine lake with a distinct eutrophication and reoligotrophication history, and analyzed for elemental (carbon, nitrogen, and phosphorus) and biochemical (sterols and fatty acids) nutrients. Carbon allocation into Daphnia resting eggs continuously decreased over time, irrespective of changes in trophic state. The allocation of nitrogen into Daphnia resting eggs followed the changes in trophic state, that is, nitrogen concentrations in resting eggs increased with eutrophication and decreased again with reoligotrophication. The allocation of phosphorus, sterols and long‐chain polyunsaturated fatty acids, such as eicosapentaenoic acid, into Daphnia resting eggs did not change significantly over time. Changes in trophic state strikingly influenced all trophic levels in Lake Constance. However, nutrient allocation into Daphnia resting eggs was mostly resilient to changes in lake trophic state.
Highlights
Anthropogenic activities, such as industrialization, urbanization, and intensive agriculture, have resulted in increased nutrient loads and intense eutrophication of many lake ecosystem across the globe (Jeppesen et al, 2010; Smith, Tilman, & Nekola, 1999; Vitousek, Mooney, Lubchenco, & Melillo, 1997)
The generalized additive model (GAM) model revealed that the allocation of sterols into Daphnia resting eggs was unrelated to the temporal changes in phosphorus concentrations (GAM, p > .05; Figure 5), to summer phytoplankton biovolume, and the relative abundance of taxonomic phytoplankton groups
Correlations of allocation of total fatty acids into Daphnia resting eggs was unrelated to the temporal changes in phosphorus concentrations, to summer phytoplankton, and the relative abundance of taxonomic phytoplankton groups
Summary
Anthropogenic activities, such as industrialization, urbanization, and intensive agriculture, have resulted in increased nutrient loads and intense eutrophication of many lake ecosystem across the globe (Jeppesen et al, 2010; Smith, Tilman, & Nekola, 1999; Vitousek, Mooney, Lubchenco, & Melillo, 1997). Nutrient allocation into sexually produced resting eggs has been studied (Abrusán, Fink, & Lampert, 2007; Putman, Martin‐Creuzburg, Panis, & Meester, 2015) but potential effects on offspring performance have not been demonstrated yet. Changes in lake trophic state can result in fundamental shifts in the phytoplankton community composition (Jochimsen et al, 2013), which may affect the performance of zooplankton through changes in the availability of essential biochemicals, such as long‐chain polyunsaturated fatty acids (PUFA) and sterols (Hartwich et al, 2012; Martin‐Creuzburg & Merkel, 2016; Müller‐Navarra et al, 2004). To assess the impact of trophic state on nutrient allocation, resting eggs of Daphnia were isolated from the sediment egg bank of Lake Constance and analyzed for elemental (carbon, nitrogen, and phosphorus) and biochemical (sterols and fatty acids) nutrient concentrations. We hypothesized that nutrient allocation into Daphnia resting eggs is affected by temporal changes in trophic state and the associated changes in food quantity (i.e., phytoplankton biovolume) and quality (i.e., relative proportions of phytoplankton taxonomic groups)
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