Something’s fishy about those nutrients: The effects of stickleback morphology on nutrient recycling

Abstract

In aquatic ecosystems, fish can increase the levels of nitrogen (N) and phosphorus (P) directly through the excretion of dissolved inorganic nutrients, which can have consequent effects on ecological processes. This is known as nutrient recycling. Ecological stoichiometry provides a mass balance framework that predicts the effect of traits on nutrient acquisition, assimilation, and recycling. A hypothesis based on ecological stoichiometry states that a fish that assimilates more of its diet in constructing P-rich bone should excrete less P than a fish that invests more of its diet in P-poor tissues, such as muscle. We tested this hypothesis in the threespine stickleback, Gasterosteus aculeatus, a species known for its immense diversity in bony morphology. To determine the relative effects of different variables on the elemental composition of the threespine stickleback and its excretion, we sampled two populations in Victoria, BC, between April and August 2015: one marine and heavily armoured population in the Gorge Waterway and the other freshwater and poorly armoured in Swan Lake. Initial comparisons of the populations followed the predictions of organismal stoichiometry however, when we used a general linear model to further examine these trends, we found that armour had a relatively small effect on excretion. We concluded that while ecological stoichiometry can potentially allow for simplified predictions about roles in ecosystem function, it needs to be supplemented with additional contextual factors, such as environment, consumption rate and life stage in order to make accurate predictions about factors that control nutrient recycling in aquatic ecosystems.