New Zealand Journal of Marine and Freshwater Research abstracts
Sedimentation, suspension, and resuspension in Tasman Bay and Beatrix Bay,
New Zealand, two contrasting coastal environments which thermally stratify in
summer
MAX M. GIBBS
National Institute of Water & Atmospheric
Research Ltd
P. O. Box 11 115
Hamilton, New Zealand
email: m.gibbs@niwa.cri.nz
Abstract The influence of density stratification on
sedimentation, suspension, and resuspension in Tasman Bay and Beatrix Bay, New
Zealand, two contrasting coastal environments, was studied with specific
reference to the implications for modelling aquaculture sustainability. Tasman
Bay, an enhanced scallop (
Pecten novaezelandiae Reeve) fishery, is a
very large coastal indentation gently shelving to c. 20 m deep c.
10 km from shore and open to the Tasman Sea, whereas Beatrix Bay, an area
of intense Greenshell
(TM) mussel (
Perna canaliculus Gmelin)
aquaculture, is a small (22 km
2 x 30-40 m deep) enclosed
embayment off the side of the nearby Pelorus Sound. Sediment trap arrays were
used to determine the vertical fluxes of suspended solids and the associated
chlorophyll component. Benthic chambers were used to investigate sediment
nutrient regeneration. In summer, salinity gradients in both bays are minimal
or non-existent because of low inputs of fresh water and density stratification
is mainly controlled by water temperature. The data from mid summer exhibited
different spatial distribution patterns for detritus and phytoplankton biomass
(as indicated by chlorophyll) in these two very different bays, although they
had similar turbulent environments. The density discontinuity at the
thermocline had a strong influence on settling of phytoplankton. There was
evidence of upwards entrainment of suspended particulate matter into the upper
water column from the thermocline in Beatrix Bay. Benthic resuspension was
estimated to contribute up to 90% of the suspended solids caught in sediment
traps near the sea floor in both bays. The trapping rate of phytoplankton was
thought to be dependent on species dominance. Possible mechanisms of
resuspension included turbulence in the benthic boundary layer, and high
velocities below the thermocline associated with internal seiches. The presence
of a mid water column chlorophyll maximum in Beatrix Bay is discussed in terms
of nutrient and light regimes at the thermocline, and species composition. In
Tasman Bay, the chlorophyll maximum was thought to be caused by resuspension of
benthic microphytes and their subsequent confinement in a thin layer (2-4 m
thick) of high turbulence between the thermocline and the seabed.
Keywords stratification; turbulence; sedimentation;
resuspension; Pelorus Sound; Tasman Bay
New Zealand Journal of Marine and Freshwater Research, 2001, Vol. 35
M00048
Received 10 July 2000; accepted 22 May 2001
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