Report: TR 2010/37
Author: Nathan Singleton

Abstract

In April 2001 Environment Waikato initiated the Regional Estuary Monitoring Programme (REMP) 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. 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. This report presents the monitoring results of sediment characteristics, and a suite of 26 'indicator' taxa and abundant non-indicator taxa of the intertidal benthic communities for the period July 2008 to April 2009. 

This report documents the data from the eighth year of the monitoring programme. In addition to annual reports, detailed discussion and analysis of trends or patterns of change over time in the benthic macrofaunal communities and sediment characteristics are reported every five years in a separate trend report series for the Regional Estuary Monitoring Programme.

In the southern Firth of Thames five permanent sites were sampled in October 2008 and April 2009. In Whaingaroa Harbour five sites were sampled in October 2008 and, due to sampling at Te Puna Point being discontinued, only four in April 2009. Two sites from each harbour were additionally sampled in July 2008. 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. Surface scrapes were collected and analysed for chlorophyll-a and phaeophytin content.

Results from the July 2008 to April 2009 monitoring period indicate that changes in assemblage composition in both the Firth of Thames and Whaingaroa Harbour were mainly small. Overall bivalves were found to be more abundant in Whaingaroa Harbour than in the Firth of Thames, whereas polychaetes were more abundant in the Firth of Thames. Gastropods were more abundant in Whaingaroa Harbour and crustaceans in the Firth of Thames. The most consistently common taxa found at sites in the Firth of Thames included the polychaetes Aonides oxycephala, capitellids, polydorids and Scoloplos cylindrifer; and the bivalves Nucula hartvigiana, Austrovenus stutchburyi and Paphies australis. The exotic ‘Asian date mussel’, Musculista senhousia, occurred at the sites Kaiaua and the Gun Club. In Whaingaroa Harbour, consistently common taxa included the polychaetes Aquilaspio aucklandica, Cossura sp. and capitellids; and the bivalves Austrovenus stutchburyi, Nucula hartvigiana, Macomona liliana and Arthritica bifurca.

Between July 2008 and April 2009 the median grain size remained consistent at all but one site in the Firth of Thames and all sites in Whaingaroa Harbour. Sites in Whaingaroa Harbour were generally 2-3 times muddier than those in the Firth of Thames. In Whaingaroa Harbour the highest amount of mud occurred at Haroto Bay, which decreased slightly over the one year of sampling. The mud content was relatively consistent over the study period in Whaingaroa Harbour and more variable in the Firth of Thames. The shell-hash content was also consistent at most sites over the sampling period in both estuaries.

Continued monitoring will identify any patterns of temporal change in sediment characteristics and the associated benthic communities. From these time-series data we will be able to distinguish changes which may occur as a direct or indirect consequence of catchment activity and/or estuary development from natural variability.

Regional Estuary Monitoring Programme (REMP) Data Report: Benthic Macrofauna Communities and Sediments – July 2008 to April 2009 Southern Firth of Thames and Whaingaroa (Raglan) Harbour [PDF, 545 KB]