The influence of environmental parameters on spatial variation in zoobenthic density and stable isotopes (δ¹³C, δ¹⁵N, and δ³⁴S) within a large lake

Abstract

The use of baselines in stable isotope studies to interpret food web structure is essential, but baseline isotope values are often assumed to be spatially homogeneous, even in large aquatic ecosystems. To test this assumption in large lakes, we quantified spatial variation in δ13C, δ15N (deposit‐feeding Oligochaeta and filter‐feeding Dreissena spp.), and δ34S (Dreissena spp. only) and density in Lake Erie between 2014 and 2016. Lake Erie's three distinct basins differ in size, bathymetry, and nutrient loading, making it an excellent system for exploring spatial variation in stable isotopes of baseline organisms. Dreissena spp. densities were highest in the western and lowest in the seasonally hypoxic central basin, while Oligochaeta densities were relatively consistent throughout Lake Erie. Values of δ13C, δ15N, and δ34S exhibited distinct spatial trends that were not related to population densities but followed the west to east direction of water flow within the lake. For both taxa, δ13C was lower in the deeper, oligotrophic east basin than the shallow, mesotrophic west basin, and δ15N and δ34S increased from west to east. Spatial patterns of low δ34S in Dreissena spp. in the western and central basins were likely related to hypoxia, whereas patterns of δ15N in both taxa were probably related to the greater influence of agricultural land uses in the western basin. Spatial trends of stable isotopes in large lake zoobenthos are driven by complex interactions of environmental gradients, which could introduce bias in evaluations of trophic structures within aquatic ecosystems that use stable isotopes.