Report: TR2006/44
Author: Nathan Singleton, Vernon Pickett

Abstract

In April 2001 Environment Waikato initiated the Regional Estuary Monitoring Programme at five permanent monitoring sites in both the southern Firth of Thames and Whaingaroa (Raglan) Harbour. It is a long-term programme with the objective of monitoring the temporal changes in intertidal sediment characteristics and benthic macrofauna communities which may occur as a direct or indirect consequence of catchment activity and/or estuary development. This report presents the results of monitoring the sediments and a suite of 26 'indicator' taxa characteristic of the intertidal benthic communities.

Five permanent sites in the southern Firth of Thames and Whaingaroa Harbour were sampled in October 2004 and April 2005. Two sites from each harbour were additionally sampled in July 2004 and January 2005. Sampling the benthic macrofauna communities involved collecting 12 randomly located core samples from a permanent monitoring plot at each site. On each sampling occasion, replicate bulked sediment samples were collected for grain-size analysis, total organic carbon and total nitrogen content, with surface scrapes collected and analysed for chlorophyll-a and phaeophytin content.

Results indicate that there are distinct differences in the benthic macrofauna communities between sites in the Firth of Thames, but less so in Whaingaroa Harbour. In both the Firth of Thames and Whaingaroa Harbour there were slight changes in assemblage composition over time. The most consistently common taxa found at sites in the southern Firth of Thames included the polychaetes Aonides oxycephala, capitellids and Magelona dakini; and the bivalves Austrovenus stutchburyi, Nucula hartvigiana, and Paphies australis. The exotic ‘Asian date mussel’, Musculista senhousia, was also common at most sites (except Kuranui Bay), predominantly in April 2005. For Whaingaroa Harbour, consistently common taxa included the polychaetes Aquilaspio aucklandica, Cossura sp. and capitellids; and the bivalves Austrovenus stutchburyi, Nucula hartvigiana and Arthritica bifurca.

The median grain size was quite consistent at all sites in both the Firth of Thames and Whaingaroa Harbour. In the Firth of Thames similar high levels of mud were found at Kaiaua and Miranda, and during previous sampling periods (from April 2001 to October 2002) Kaiaua was generally the muddiest of the monitoring sites. Sites in Whaingaroa Harbour were generally muddier than those in the Firth of Thames. In Whaingaroa, the highest amount of mud occurred at Haroto Bay, the mud content at this site has been steadily increasing since October 2002.

The sediment variable or combination of variables that best explained the assemblage composition for sites in the southern Firth of Thames was the dry weight of shell-hash and the grain size fraction of 500 to 1000 µm. For Whaingaroa Harbour, the best combination of variables was the grain size fraction of 500 to 1000 µm, the proportion of mud, chlorophyll-a content, dry weight of shell-hash and phaeophytin content.

Southern Firth of Thames and Whaingaroa (Raglan) Harbour [PDF, 1.1 MB]