Morphometrically induced estuarine phytoplankton patchiness in Pelorus Sound, New Zealand

New Zealand Journal of Marine and Freshwater Research

2001

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™ mussel (Perna canaliculus Gmelin) aquaculture, is a small (22 km 2 × 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 paniculate 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.

Section: Background and Review Of Previous Workmentioning

confidence: 99%

“…1981;Cloernetal. 1985;Gibbs 1993) or, in nutrient rich environments, the salinity stratification induced by the freshwater inflow (Cloern 1991 a;Ross et al unpubl. data).…”

Section: Background and Review Of Previous Workmentioning

confidence: 99%

Sedimentation, suspension, and resuspension in Tasman Bay and Beatrix Bay, New Zealand, two contrasting coastal environments which thermally stratify in summer

New Zealand Journal of Marine and Freshwater Research

2001

“…The nutrients nitrogen (N), phosphorus (P), and silicon were generally present at non-limiting concentrations (Gibbs et al 1992, unpubl. data), although reduced vertical mixing may regulate N and P availabilityand thus phytoplankton growth-at certain times of the year (Gibbs et al 1991(Gibbs et al , 1992Gibbs 1993). There is also some evidence that zooplankton grazing helps control phytoplankton in the Sounds, with the grazing pressure reducing markedly in the presence of the jelly fish ^wre/w, which prey on the zooplankton (Bradford et al 1987).…”

Section: Introductionmentioning

confidence: 99%

Seasonal changes in factors controlling phytoplankton growth in Beatrix Bay, New Zealand

New Zealand Journal of Marine and Freshwater Research

Vant

1997

Self Cite

Phytoplankton biomass and growth rates were measured in Beatrix Bay, Marlborough Sounds, during 1994-95. In spring and summer, nitrate and chlorophyll a concentrations in the mixed layer were low (typically < 1 mg N m 3 and < 1 to 3 mg Chi. a m -3 , respectively), and phytoplankton growth rates were moderate (average 0.3 d -1 ). Growth rates increased several-fold in response to the experimental addition of inorganic N (but not to inorganic phosphorus). Higher nitrate concentrations were found below the pycnocline. During autumn and winter, nitrate concentrations in the mixed layer were high (up to 60 mg m 3 ), but phytoplankton growth rates were lower than in summer (average 0.2 d -1 ), and did not respond to added nutrients. Chlorophyll a concentrations, however, were highest during the winter (typically 3-6 mg m -3 ). Even so, low light levels meant integral photosynthesis per unit chlorophyll a was low in winter. Phytoplankton growth appears to be controlled by the availability of nitrogen in summer and by light in winter.

“…In Pelorus Sound, major sources of N have been attributed to riverine inflows, sediment nutrient release, and oceanic exchange via estuarine circulation (Bradford et al 1986(Bradford et al ,1987MacKenzie et al 1986;Gibbs et al 1991Gibbs et al , 1992Gibbs 1993;Proctor & Hadfield 1996;Ross et al 1998). The recent construction of a mussel carrying capacity model (James & Ross 1996) requires a more detailed parameterisation of the fluxes and processes associated with the N sources and sinks for Beatrix Bay.…”

Section: Introductionmentioning

confidence: 99%

Nutrient cycling and fluxes in Beatrix Bay, Pelorus Sound, New Zealand

New Zealand Journal of Marine and Freshwater Research

Ross

Downes³

2002

Nitrogen (N) and light availability limit phytoplankton growth and hence regulate food supply for mussel farming in Pelorus Sound, New Zealand. In Beatrix Bay, a large 35-m deep embayment off Pelorus Sound, the supply of N is heavily influenced by physical factors. Bottom intrusions of N-rich or depleted oceanic water occur in the main channel of Pelorus Sound and Ndepleted, phytoplankton-enriched Pelorus River water characterises the near-surface low salinity field. Inside Beatrix Bay, nutrient cycling via phytoplankton uptake, sedimentation, and subsequent regeneration from the sediments by mineralisation, also has an important influence on the availability of N in the water column. However, almost continuous salinity or thermally induced density stratification, or both, within Beatrix Bay meant that the upper and lower water columns were often decoupled so that N regenerated from the sediments was not readily available in the upper water column, and phytoplankton settling from the upper water column may not reach the sediments directly below because of large horizontal transport. The N supply in the upper water column of Beatrix Bay, where the mussels are farmed, is most likely regulated by advection in and out of the bay from external sources. Conversely, the N concentrations below the pycnocline could be almost entirely supplied via mineralisation from the sediments. The rates of sediment N regeneration were seasonally dependent. During winter, when sedimentation exceeds mineralisation, N accumulates in the sediments at a rate of up to 10 mg m -2 day -1 , whereas in summer, when mineralisation exceeds sedimenation, N is lost from the sediments at rates of up to 9 mg m -2 day -1 .