Effects of terrigenous sedimentation on mangrove physiology and associated macrobenthic communities

摘要

In New Zealand, one species of mangrove, Avicennia marina var. australasica, forms a characteristic and often an extensive feature of the tidal mudflats of harbours and estuaries. Within their natural range, mangroves have generally increased in abundance in New Zealand over the last 100 yr in response to increased sedimentation associated with concurrent changes in catchment land use. However, little information is available about the ecological consequences of changes in the extent of mangrove habitats under varying sedimentation regimes. We therefore conducted a study to determine the effects of high sedimentation rates on mangrove plant communities and associated ben-thic community composition. We selected an estuary that is experiencing high rates of sedimentation (as high as 23 mm yr~(-1)) and associated increases in mangrove area. We recorded clear differences in both plant and benthic communities along a gradient of decreasing sedimentation. Mangrove architecture (such as height and density of plants) and health (as measured by chlorophyll a fluorescence) were linked to high mud content of the sediment and elevated sedimentation patterns. Mangrove plants at the upper landward sites, characterised by a high percentage of mud and high total nutrients (total phosphorus [TP] and total nitrogen [TN]) and organic content, were taller than those at the seaward sites and had a larger number of pneumatophores as well as the greatest number of new seedlings. However, benthic macrofaunal diversity and abundance within the mangrove habitats were lower than expected, and clear functional differences were found between habitats with differing sedimentation patterns. Sites with high sedimentation rates had lower numbers of suspension feeders, low macrobenthic diversity, and were dominated by deposit-feeding polychaetes and oligochaetes. The diversity and density of benthic macrofaunal communities was, however, lower than that of sandflat communities for both mangrove habitats and adjacent intertidal mudflats in these sheltered areas, suggesting a response to the increased silt/clay from sedimentation rather than to the mangroves themselves. Our study demonstrates the potential for functional and structural effects on benthic communities on a larger spatial scale in estuarine areas experiencing high rates of sedimentation.