Disturbance of marine soft sediment environments by terrestrial sediments : recolonisation by post settlement macrofauna

Abstract

This thesis investigated the reasons for the slow macrofaunal recolonisation of estuarine sandflats following disturbance by deposition of terrestrial sediment. It focussed on post settlement life stages, with particular emphasis on the response of common New Zealand bivalves that are known to undergo frequent, behaviourally initiated dispersal. Field and laboratory experiments were conducted to determine why post settlement bivalves that are frequently dispersing over sandflats do not become established in areas impacted by terrestrial sediments, and the extent to which this pattern is influenced by behaviour. Investigating their response to this disturbance also required information on: distances and rates of dispersal, how dispersal mode changes with bivalve size and ambient flow conditions, and potential cues used to determine the (un)suitability of a substrate for colonisation. To identify potential settlement cues, changes in biogeochemical characteristics of terrestrial sediment deposits were measured in a 4.5 mo field experiment, and measurements of chemical properties in the sediment-water interface were made in the laboratory. Significant relationships between sediment food (chlorophyll a) content and macrofaunal community composition were demonstrated in the field, and reductions in O2 and porewater pH were correlated with reduced bivalve burial in the laboratory. The potential for a sub-lethal effect of chemical species (reduced end-products of anaerobic metabolism) on the burrowing and dispersal behaviour of these small bivalves was also indicated. This work illustrated the transitory nature of individual post settlement bivalves at a given point in time and space on a sandflat, and the size dependent nature of their dispersal behaviour. Despite their ability to respond quickly to disturbance, they do not colonise terrestrial sediments, and the survival potential, burial and dispersal abilities of individuals that are directly exposed to the deposits are negatively affected. Given the detrimental influence of terrestrial sediment deposits on post settlement bivalves, it would be advantageous for an individual to avoid settling in the deposits in the first place. Details of how these bivalves detect settlement cues are needed to extend these findings and enable more accurate predictions of the population-scale consequences of terrestrial sediment deposition to be made.