Animal-Mediated Nutrient Cycling in Aquatic Ecosystems of the Cuatro Ciénegas Basin

Abstract

Consumers can alter nutrient cycling and create biogeochemical hotspots by excreting nutrients such as nitrogen and phosphorus at varying rates and stoichiometric ratios. Variation in these rates and ratios is in turn driven by traits such as body size, diet, and nutrient assimilation efficiency. These traits often vary considerably among and within species. As aquatic ecosystems in the Cuatro Ciénegas Basin are nutrient-poor, animals may play a large role in recycling key nutrients such as nitrogen (N) and phosphorus (P). We investigated this question by studying nutrient excretion rates of snails (Mexithauma quadripaludium and Nymphophilus minckleyi), a poeciliid fish (Gambusia marshi), and two cyprinodontid fishes (Cyprinodon atrorus and C. bifasciatus) over a variety of environments. Snails were collected from oncolites in the Río Mesquites, C. atrorus were measured in Laguna Intermedia, C. bifasciatus were measured in Poza La Becerra, and G. marshi were measured at nine sites around the basin as well as in laboratory experiments with fish from Poza La Becerra and Los Hundidos. We found substantial variation in nutrient excretion rates both among and within species and suggest that differences in diet and feeding rate could explain much of this variation. We also calculated the potential contribution of recycled P to supporting oncolite production and found that snails are likely not a major source of P to primary production in oncolites. Finally, we discuss how changes in these environments may lead to shifts in consumer-mediated nutrient recycling.