Mesocosm evidence for sandprawn-mediated shifts in pelagic resource ratios and phytoplankton traits

Abstract

Bentho-pelagic coupling is a key determinant of multifunctionality in marine ecosystems, influencing critical processes such as production and trophic dynamics. However, knowledge gaps regarding biotically mediated bentho-pelagic coupling limit our understanding of ecosystem responses to global change, and the functional significance of biota nested therein. This is especially relevant for endobenthic ecosystem engineers, which rank amongst the most dynamic of sediment and porewater manipulators in marine systems. Based on this rationale, we tested the consequences of ecosystem engineering by South African endobenthic deposit-feeding crustaceans (sandprawns) for pelagic microbial assemblages, to make inferences about bentho-pelagic coupling. Findings from a factorial 18-day mesocosm experiment indicated a density-dependent, sandprawn-induced decline in phytoplankton relative abundance, but negligible effects on bacterial groups. Picophytoplankton appeared more susceptible than nanophytoplankton to sandprawn density, since relative abundance of the former group declined more in the presence of sandprawns than the latter. By the end of the experiment, there was evidence of a trait shift in phytoplankton assemblages in response to sandprawn presence, with nanophytoplankton being more dominant in the presence of sandprawns. Our findings suggest that pelagic microbes may have differential susceptibilities to endobenthic deposit-feeders and provide useful information on pathways by which these engineers influence bentho-pelagic coupling. These pathways need further evaluation to better understand secondary food-web ramifications and ecosystem responses to global change.