Water column stratification and the spatial and temporal distribution of phytoplankton biomass in Tasman Bay, New Zealand: Implications for aquaculture

Abstract

Temporal changes in the abundance and horizontal and vertical distribution of phytoplankton biomass (chlorophyll a) in Tasman Bay, New Zealand were associated with changes in water column stratification, because of seasonal climatic effects and variations in the magnitude of freshwater inflows. The water column throughout the bay was usually strongly stratified. Vertical gradients in salinity, the result of outwelling river plumes overlying deeper more saline waters, usually played the major role in stabilising the water column. However, during summer low river‐flow periods, temperature stratification was also important. A persistent east to west gradient in Simpson's potential energy anomaly (an index of the strength of stratification) existed across the bay at most times, because of the dominant influence of the Motueka River plume. From late spring to late summer most of the nutrient depleted euphotic zone contained low chlorophyll a (Chl. a) concentrations while below the base of the pycnocline, turbid water containing re‐suspended sediments and relatively high Chl. a levels persisted. It is debatable, given the susceptibility of scallops to high levels of suspended silt, whether this condition is beneficial to these commercially important bivalves. Diatom blooms in autumn and spring, representing the annual productivity maxima, occurred when the water column beyond the extent of the Motueka River plume became well mixed. At this time the conditions for shellfish nutrition are probably at their best. At most other times flagellate‐dominated phytoplankton communities within subsurface concentrated Chl. a layers were associated with a mid water column (10–15 m), bay‐wide, pycnocline. This is a common feature of the structure of the water column of Tasman Bay that coincides with the depth range within which scallop growth and survival is highest and mussel nutrition and spat catching is probably optimal. Aquaculture management options and strategies for fisheries enhancement may be developed using these data such as the timing and depth of installation of bivalve spat catching gear to optimise shellfish nutrition and successful spat falls. The future lies in the installation of real‐time monitoring equipment within the bay and development of adaptive phytoplankton productivity models based on hydrodynamic models of water column circulation and vertical structure.

Keywords:

• Tasman Bay
• phytoplankton biomass
• seasonality, Simpson's potential energy anomaly
• chlorophyll a
• stratification
• nepheloid layer
• aquaculture