The benthic invertebrate fauna of the Peel-Harvey Estuary of southwestern Australia after completion of the Dawesville Channel

Abstract

-During the 1970's the Peel-Harvey Estuary experienced severe symptoms of nutrient enrichment. There were large inputs of nutrients to the system, and only limited oceanic flushing occurred, via the long, narrow Mandurah Entrance Channel. In 1994 the Dawesville Channel was constructed to increase oceanic flushing within the Peel-Harvey Estuary; reduce the occurrence of toxic phytoplankton blooms (eg. blue-green Nodularia spumzgena) and decrease macroalgal growth. The present paper compares the benthic invertebrate fauna in the Peel-Harvey Estuary before and after the Dawesville Channel was constructed. One hundred sediment cores were collected during early autumn and late winter 2000, yielding a total of 17,443 benthic invertebrates, comprising 52 species. Polychaetes, molluscs and crustaceans dominated species richness and density in both seasons. There was a clear difference in benthic invertebrat~ species richness and density between seasons. During early autumn, benthic invertebrate communities in the Peel-Harvey Estuary had a low species richness (27 species), but this increased substantially (46 species) in late winter. Total mean density guadrupled from 6397m· 2 in early autumn to 26,180m· 2 in late winter. The increased species richness was due to increases in small crustaceans with a short lifespan and, to a lesser extent, chironomid insect larvae. With the exception of one site, the increased density was general throughout both Peel Inlet and the Harvey Estuary. There was no apparent correlation between increased densities and proximity to the entrance channels. Pre-Dawesville Channel data on benthic invertebrates are few. Molluscs provided the best comparisons between pre and post Dawesville Channel benthic invertebrates. In the late 1970's mollusc diversity was low and was dominated by two small estuarine species: the bivalve Arthritica semen and the gastropod Hydrococells brazieri. A two-year study at one site in Peel Inlet recorded a maximum density of 45,491 m 2 for A. semen and 19,959m 2 for H. brazieri. By the 1980's the de~sity of both species had declined; H. brazieri had nearly disappeared from the Peel-Harvey Estuary. In 2000, the mean densities of A. senzen and H. brazieri remained low. Several marine species that were recorded in the post Dawesville Channel samples were not recorded in the 1970's. Data suggests that the Peel-Harvey Estuary has become more marine, and that increased recruitment of planktonic larvae through the short, narrow Dawesville Channel and improved water guality has probably enabled these species to establish themselves. All of these species were recorded in very low densities, and they have not established the dense populations previously attained by H. brazierz and A. semen.