Terrestrially derived sediment: response of marine macrobenthic communities to thin terrigenous deposits

Abstract

Coastal marine habitats adjacent to catchments with encroaching human development are likely to experience increased sediment loadings in ensuing decades. Thus, sedimentary distur- bance regimes in which coastal marine benthic communities have evolved may be shifting as depo- sitional events exceeding critical thresholds become more frequent. To understand the threat posed by terrigenous sedimentation in an embayment with increasing urban development, we determined the thickness and frequency at which terrigenous sediment deposits begin to affect the benthos. We performed manipulative experiments involving layers of terrigenous sediment <1 cm thick in a vari- ety of intertidal habitats in the Whitford embayment, North Island, New Zealand. Results of 3 sepa- rate experiments performed at 5 sites were largely consistent. While experimental plots were never completely defaunated, as little as 3 mm of the terrigenous material was sufficient to significantly alter macrobenthic community structure (measured after 10 d, relative to 0 mm controls). The direc- tion of change was predominantly negative; the number of individuals and taxa declined as a result of sediment application, as did the densities of nearly every common species. Large bivalves were less affected than smaller ones, and deeper-dwelling species were less affected than ones at the sed- iment surface. With repeated applications of thin terrigenous layers (3 mm thickness, monthly over a 6 mo period), the sandflat sediments gradually became finer (clay volume % increased), and macro- faunal community composition progressively diverged from controls. To summarise, macrofauna were negatively affected by extremely small amounts of terrigenous sediment, and repeated deposi- tional events did more damage than single ones. With increasing defoliation and excavation of catch- ment hillsides, the frequency of depositional events of a given intensity is likely to quicken, indicat- ing an enhanced likelihood of macrofaunal disturbance and degradation in estuarine tidal flats. Management decisions that protect coastal catchments may partially ameliorate the threat to the benthos in coastal receiving waters.