Spatial variation in the littoral vertebrate community of a reservoir relative to physical and biological gradients.

Nathan Ruhl,Jessica J Soski,Willem M Roosenburg

doi:10.7717/peerj.693

Abstract

Reservoirs possess gradients in conditions and resources along their long (deep-shallow) axis, but the response of littoral vertebrates (fish and turtles) to these gradients is poorly understood. We have quantified the littoral vertebrate communities throughout a small reservoir in Southeastern Ohio during July and August using traps, and related community composition to environmental variables using NMDS ordination. Ordination revealed that fish and turtles were broadly separated in ordination space, and three distinctly different environmental gradients were significantly associated with the underlying observed species abundances. Observed turtle abundance was explained by measurements of bathymetry, turbidity, and benthic resources, but none of these environmental variables were a reliable predictor of observed fish abundance. Temperature was a poor predictor of observed abundance for both fish and turtles independently, but when fish and turtles were considered together, it became apparent that there were cold areas of the reservoir where observed fish and turtle abundances were different than in other areas of the reservoir. These results suggest that the predictor (environmental) variables we used were appropriate for investigating turtle ecology in reservoirs, but that observed fish abundance is mediated by factors that were not modeled. The efficacy of using traps, the ecological implications of considering fish and turtles together as sympatric and potentially competing species, and directions for future study are discussed.

Highlights

Many studies have investigated the effect of environmental variation on differences between fish populations (e.g., Carey & Mather, 2009) and communities (e.g., Tonn & Magnuson, 1982; Mehner et al, 2005) among lakes
Because the stress of an Non-metric multidimensional scaling (NMDS) ordination will vary depending on the starting point, the analysis was run a number of times in order to ensure that the metaMDS function was performing well with the final stress falling between 18 and 20 in all cases
Characterization of the littoral vertebrate community using NMDS ordination revealed that fish and turtles were broadly separated in ordination space and three distinctly different environmental gradients were significantly associated with the underlying observed abundances (Figs. 4 and 5)

Summary

Introduction

Many studies have investigated the effect of environmental variation on differences between fish populations (e.g., Carey & Mather, 2009) and communities (e.g., Tonn & Magnuson, 1982; Mehner et al, 2005) among lakes. These studies typically treat lakes as homogenous with respect to environmental conditions and attempt to relate population or community dynamics to representative environmental variables This same approach has been used to distinguish fish assemblages of reservoirs as a function of the environmental conditions in the lacustrine zone (e.g., Godinho, Ferreira & Castro, 1998; Irz et al, 2002; Hoxmeier, Aday & Wahl, 2009). The presence of environmental and resource gradients in reservoirs has led to a simple classification scheme of functionally different areas within a reservoir (Kimmel & Groeger, 1984): the riverine zone is the shallow, well-mixed portion, the lacustrine zone is the stably stratified lake-like area, and the transitional zone is the area of mixed dynamics that separates the other two zones (Fig. 1) This zonation scheme has not been widely adopted by researchers studying vertebrates (fishes and turtles), but is often used in the context of understanding reservoir plankton dynamics (Bernot et al, 2004; Wang et al, 2011)

Methods

Results

Conclusion