IC41_NZ

Worldwide estuaries are experiencing recognized increase in suspended sediment concentration, caused by a combination of natural events and human-induced activities, resulting in decrease of water quality. In this context, our project focuses on understanding how background turbidity and turbid dredging plumes change the availability and properties of light in estuarine ecosystems and how they affect seagrass photosynthesis and the feeding behaviour of key species. Seagrass form highly productive ecosystems and provide a habitat for a range of other organisms, and suspension feeders play an import role in the benthic-pelagic coupling.

Through the application of the acoustic method, we tracked the plumes created during maintenance dredging in Tauranga Harbour, with the objectives of describing their dynamics and development with time and distance from the dredging area, and compare suspended sediment concentrations from water samples with background levels. Low frequency (2-month interval) light measurements, using a PAR sensor have been carried out in Tauranga Harbour from 2014 to 2016 in addition to high frequency (1-minute interval) turbidity data being provided by the Port of Tauranga. The PAR measurements were used to compute the light attenuation coefficient, Kd, according to the Beer-Lambert equation. To determine the light reduction that would occur for a particular turbidity limit, a joint probability approach was used, which describes the probabilistic distribution of variables subject to of the constraints imposed by other related variables. To understand how sediment colour change light quality, we tested a range of sediments samples in laboratory. Physiological laboratory experiments were also carried out to test the effect of elevated turbidity on the feeding behaviour of a suspension-feeding bivalve.

Our research outcomes are potentially of interest to the port industry and resource managers given the implications for key species and water quality parameters that are influenced by port activities. Improving our knowledge of how human activities influence water quality is also beneficial to local residents, fishermen and recreational users of harbours.