Effects of water column processes on the use of sediment traps to measure zooplankton non-predatory mortality: A mathematical and empirical assessment

Abstract

Zooplankton populations can at times suffer mass mortality due to non-predatory mortality (NPM) factors, and the resulting carcasses can be captured by sediment traps to estimate NPM rate. This approach assumes sinking to be the primary process in removing carcasses, but in reality, carcasses can also be removed by ingestion, turbulent mixing and microbial degradation in the water column. We present mathematical formulations to calculate NPM from sediment trap data by accounting for carcass removal by processes in addition to sinking, and demonstrate their application in a study in Lake Shira, Russia. Carcass abundance of the major calanoid copepod Arctodiaptomus salinus decreased with depth, indicating the effect of carcass removal from the water column. The estimated NPM values (0.0003-0.103 d-1) were comparable with previously reported physiological death rates. We further used independent data to partition carcass removal due to detritivory, turbulent mixing and microbial degradation. Estimated ingestion by the amphipod Gammarus lacustris could account for the disappearance of copepod carcasses above the traps. Wind-driven turbulence could also extend the carcass exposure time to microbial degradation. Collectively, these water column processes would facilitate the remineralization of carcasses in the water column, and diminish the carcass carbon flux to the benthos.