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

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 delta C-13, delta N-15 (deposit-feeding Oligochaeta and filter-feeding Dreissena spp.), and delta S-34 (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 delta C-13, delta N-15, and delta S-34 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, delta C-13 was lower in the deeper, oligotrophic east basin than the shallow, mesotrophic west basin, and delta N-15 and delta S-34 increased from west to east. Spatial patterns of low delta S-34 in Dreissena spp. in the western and central basins were likely related to hypoxia, whereas patterns of delta N-15 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.

Automated summary

The use of baselines in stable isotope studies is essential for interpreting food web structure. This study tested this assumption in Lake Erie by quantifying spatial variation in stable isotopes and density of benthic organisms. The results showed distinct spatial trends in stable isotopes, which were not related to population densities.