Regional Estuary Monitoring Programme (REMP) Data Report: Benthic Macrofauna Communities and Sediments – July 2006 to April 2007 - Southern Firth of Thames and Whaingaroa (Raglan) Harbour

Report: TR 2008/51

Author: Nathan Singleton

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 for the period July 2006 to April 2007.  It is envisaged that the Regional Estuary Monitoring Programme will provide relevant information useful in setting policy and assisting with the sustainable management of estuaries in the Waikato region.

Five permanent sites in the southern Firth of Thames and Whaingaroa Harbour were sampled in October 2006 and April 2007. Two sites from each harbour were additionally sampled in July 2006 and January 2007. 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. For each of the permanent monitoring sites, changes in the assemblages of monitored benthic macrofauna taxa over time were shown.

Results from the July 2006 to April 2007 monitoring period 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 Habour there were only slight changes in assemblage composition over time. Overall bivalves were found to be more abundant in Whaingaroa Harbour, whereas polychaetes were more abundant in the Firth of Thames. Gastropods were also relatively more abundant in Whaingaroa Harbour. 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, occurred at most sites in Firth of Thames (except Te Puru), being most common at the Gun Club. 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 limpet, Notoacmea sp. was also common in Whaingaroa Harbour.

This report presents selected sediment results from July 2006 to April 2007. The median grain size was quite consistent at all sites in both the Firth of Thames and Whaingaroa Harbour. In the Firth of Thames peaks in mud content were found at Kuranui Bay and Miranda in April 2007. Sites in Whaingaroa Harbour were generally 3-4 times muddier than those in the Firth of Thames. In Whaingaroa, the highest amount of mud occurred at Haroto Bay, which remained consistent over the one year of sampling, after steadily increasing since October 2002. The shell-hash content was also consistent over the sampling period in both estuaries.

This report documents the data from one year of the monitoring programme. Detailed discussion and analysis of trends or patterns of change over time in the benthic macrofaunal communities and sediment characteristics are reported on every five years in a separate trend report series for the Regional Estuary Monitoring Programme.

A review of the monitoring program will be undertaken in 2008/2009 to assess all aspects of the sampling protocol. It is strongly advised that the formal quality control assessment protocols for the sorting, identification and enumeration of benthic core samples continue to be rigorously implemented.

Regional Estuary Monitoring Programme (REMP) Data Report: Benthic Macrofauna Communities and Sediments – July 2006 to April 2007 - Southern Firth of Thames and Whaingaroa (Raglan) Harbour [PDF, 579 KB]