MEPS 314:35-47 (2006)  -  doi:10.3354/meps314035

ABSTRACT: Infaunal community structure is sensitive to both the range of salinity values and timing of exposure wrought by the time course and magnitude of freshwater input to estuaries. We sampled macro-infauna (>1 mm) from soft sediment habitat at 8 shallow sites (2 m below MHW; mean high water) within 4 regions in the Doubtful–Thompson Sound complex, a fjord in southwestern New Zealand. This fjord receives a large influx of freshwater from natural and anthropogenic sources that form a distinct low salinity surface layer and drive mean estuarine circulation. Shallow water soft sediment habitat in the fjord is highly fragmented and subject to strong environmental gradients shaped by freshwater input patterns. Infaunal community structure in the inner region of Doubtful Sound was consistently different from other regions of the fjord. This was evidenced by a low abundance of bivalves, decapods, and polychaetes in the family Orbiniidae, and elevated abundance of amphipods and polychaetes in the family Neredidae. An analysis of taxonomic richness among regions revealed that the inner regions of Doubtful Sound are also the least rich in the complex. Both taxonomic richness and patterns in abundance of the major taxa covary with physical information on the return times for low salinity events, thickness, and patterns in mixing in the freshwater layer. These results are consistent with a large impact of the persistent discharge of freshwater from the Manapouri power station on shallow water infaunal communities in the fjord. The loss of bivalves in particular has large implications for patterns in food-web structure and function in this system.

KEY WORDS: Fjord · Infauna · Diversity · Salinity