Wharekawa Estuary sediment sources Technical Report
- Author
- Gibbs, M., Bremner, D.
- Year
- 2007
- Journal / Source
- Environment Waikato
- Keywords
- Sediments, Benthic Communities (including shellfish)
- Summary
- Sedimentation rates in the Wharekawa Estuary are higher now than prior to human land use changes in the catchment. The major land uses in the catchment are exotic pine forestry (managed by Rayonier), Department of Conservation (DOC) indigenous forest and agriculture. As part of the consent conditions, Rayonier (and, previously, Carter Holt Harvey) is required to carry out sediment and biological monitoring of the Wharekawa Estuary. This monitoring has been ongoing for a number of years and is showing a decline in invertebrate species sensitive to sedimentation at the monitored sites within the estuary.The source of sediments causing these ecological effects was unknown. Initially the data was modelled to evaluate the sources of soil contribution to the estuarine sediments by land use. The results indicate that terrigenous soil contributions were present at all estuarine sites from pine (1-23%), pasture (<1-10%), native forest (<1-3%) and slip (<1-13%) land use sources. When the data were modelled to evaluate the soil contributions by sub-catchment, the results showed that the major sediment contributions in the mid-to-upper estuary came from the Wharekawa River sub-catchment (20-60%) with the Kapakapa sub-catchment producing 7-50% and the Tawatawa and Wahitapu sub-catchments each producing about 1-10%. While sedimentation rates in the Wharekawa Estuary may be higher now than prior to human land use changes in the catchment, the results of this study highlight the effects of changing weather patterns and the impacts of extreme weather conditions, the extremity and frequency of which have been increasing in recent years. Land use practices which remove the protective cover of plants on steep land will exacerbate the production of sediment during extreme events. Furthermore, flood material deposited in the river and stream channels during extreme events may continue to be discharged into the estuary over extended periods as chronic loads long after the extreme event has passed. This chronic sediment load may adversely affect some invertebrate species such as the cockle, Austrovenus stutchburyi, which are sensitive to sedimentation or enhanced suspended solids. The accumulation of water sorted sands near the inflows may also adversely affect species such as the mud snail, Amphibola crenata, that favour muddy habitats. Purpose: Environment Waikato contracted NIWA to determine the sources of sediments deposited in the Wharekawa estuary, using a forensic stable isotope technique. This report presents the results from a study of the Wharekawa Estuary using this technique to identify and apportion the sources of soil, by land use and by sub-catchment, contributing to the sediments at three locations along each of seven monitoring transect lines across the estuary and the ocean beach. The samples were collected in April 2007, and the results are indicative of the sediment source contributions at that time