Thresholds in macroinvertebrate biodiversity across water quality gradients in central plains lotic ecosystems
Michelle A. Evans-White1, Walter Dodds2, Donald Huggins3, and Debra S. Baker3. (1) Biological Sciences, University of Arkansas, 601 SCEN, 1 University of Arkansas, Fayetteville, AR 72701, (2) Divison of Biology, Kansas State University, 104 Ackert, Manhattan, KS 66506, (3) University of Kansas, Central Plains Center for Bioassessment, Kansas Biological Survey, Lawrence, KS 66047
We hypothesize that resource quality (i.e., carbon (C):nitrogen (N): phosphorus(P)) or quantity could drive threshold patterns by altering competitive interactions among consumers with differing resource demands. We examined total N (TN), total P (TP), and turbidity thresholds for total macroinvertebrate richness, functional feeding group richness, and mean taxa body carbon C:P in Central Plains streams to examine our resource hypothesis. We found a significant TN-richness (0.943 mg TN/L), TP-richness (0.070 mg TP/L), and turbidity-richness (9.9 NTU) thresholds. Richness decreased up to the threshold and remained low above it. Primary consumers were more sensitive to TN and TP than secondary consumers and threshold patterns were more evident in spring and autumn than summer. Turbidity reduced richness regardless of feeding mode and was less seasonally dependent. Mean body C:P of shredders and collector-gatherers were negatively correlated in spring and autumn samples (threshold = 0.056 and 0.259 mg TP/L, respectively) indicating that the thresholds may partially be driven by changes in food quality. Bioassessment in summer in temperate streams may miss negative effects of eutrophication on stream communities. If nutrients exceed historic ranges, the conditions that most organisms evolved under may be exceeded lowering diversity, and pushing the stream into a new state.