Deposition of terrigenous sediment on subtidal marine macirobenthos: response of two contrasting community types

Changes in patterns of land-use associated with human population growth throughout the world have altered the regime of terrigenous material export from catchments to rivers and, subsequently, to estuarine and marine communities. Although terrigenous sediment is now widely recognised as a disturbance agent in estuarine and marine communities, experimental studies on its impacts in subtidal habitats are rare. Terrigenous deposits in the intertidal zone de-water at low tide and are subjected to erosion by waves during emersion, so experimental results in spatially widespread subtidal habitats where these processes are muted may differ substantially. We deposited terrigenous sediment into replicate experimental plots at 6 m depth at 2 subtidal sites, creating 3 treatment levels (magnitudes of terrigenous material addition) inside and outside a small harbour in northern New Zealand. We tracked the persistence of the terrigenous deposits (3 and 7 mm thickness) over time and sampled macrobenthic communities at both sites on Days 7, 14 and 30, to compare and contrast their responses relative to controls. The diverse coarse sand community outside the harbour (Site MI) was more sensitive to terrigenous materials than that which lived in muddier sediments inside the harbour (Site TK), as indicated by multivariate and univariate analyses: both the 3 and 7 mm treatments caused significant change at Site MI, whereas only the more severe 7 mm treatment caused significant change at Site TK. The terrigenous sediments we added matched the grain size of Site TK sediments better, and macrobenthic animals living in turbid tidal estuaries are probably better conditioned to cope with high suspended sediment concentrations and sediment deposition rates. However, beyond a critical threshold, terrigenous sediment had a negative influence on communities at both sites, and they had not recovered by the time the experiment was terminated 30 d later