Assessing far-field effects of terrigenous sediment loading in the coastal marine environment

Abstract

Major rain events erode coastal catchments, and muddy plumes of terrigenous sediment can extend far offshore. Surface waters gradually clear as terrigenous sediments sink, although near- bed turbidity may remain high due to resuspension by waves and tides. This may adversely affect large suspension-feeding benthic epifauna, structurally and functionally important organisms in coastal marine systems, by clogging their filtration structures and decreasing their feeding efficiency. While terrigenous sediment concentrations likely decrease with distance from the coast, sensitivities of suspension feeders to this stressor may increase. We tested this hypothesis using controlled addi- tions of terrigenous sediment at estuarine and coastal sites in northern New Zealand. None of the large, solitary suspension feeders (pinnid bivalves Atrina zelandica, sponges Aaptos spp., and ascid- ians Styela plicata) were completely buried or killed by experimental deposition of terrigenous sediment. However, after living in the deposits for 3 wk, the condition of all 3 taxa declined relative to controls, and clearance rates of A. zelandica and Aaptos spp. were reduced (averaging about 40% less). A. zelandica outside the estuary (Site MI, coarse ambient sediment) were more sensitive to terrigenous material than A. zelandica inside the harbour (Site TK, fine ambient sediment), which was probably related to the greater background suspended sediment concentrations at TK to which the A. zelandica were accustomed. Impacts to large, structure-forming species such as A. zelandica, Aaptos spp., and S. plicata may eventually affect ecosystem structure and function, particularly if the frequency or magnitude of terrigenous sediment loading and resuspension increases.