Standing stocks and productivity of phytoplankton off Westland, New Zealand, June 1979

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Abstract
Winter phytoplankton concentrations inshore off Westland were greater than those recorded off Kaikoura in the same season. Levels of chlorophyll a (B) and primary production (P) were comparable to those recorded off the Washington and Oregon coasts. Off Westland, the stratified inshore region had extinction coefficients (k) > 0.1 m‐1, a photic zone < 35 m, and a mixed layer depth < 40 m. Concentrations of B were greatest compared with the other 2 regions and were vertically well stratified, diatoms were in their largest proportions, and the phytoplankton carbon (PPC): B ratio decreased down the water column in a way that was consistent with low light adaptation at the bottom of the photic zone and high light adaptation at the surface. The maximum production per unit chlorophyll (PB) was > 1.5 g C g Chi‐1 h‐1 and maximum carbon specific growth rates (n) were usually > 0.2 doublings day‐1. The upwelling inshore region off Wanganui Bluff differs from the stratified inshore area in that the PPC : B ratios were uniformly low indicating a low light‐saturated photosynthetic capacity, which is supported by a low maximum PB(0.96 g C g Chi‐1 h‐1), although the maximum μ was > 0.3 doublings d‐1. Deeply mixed oceanic waters had k 40 m, and a mixed layer depth > 100 m. The concentrations of B were lowest with very little vertical stratification, and dinoflagellates were present in largest proportions. The maximum PBwas < 1.00 g C g Chi‐1 h‐1and maximum μ was < 0.17 doublings d‐1. Off the Westland coast the depth of mixing appears to be related to the broad regional pattern of distribution of chlorophyll. Evidence is presented which indicates the relationship probably reflects the effect mixing has on population composition rather than dispersion of chlorophyll down the water column. Although the effects of temperature and nutrients on P cannot be completely discounted, light (I) was the only parameter obviously related to PB. The size of PB is governed not only by I but also by the extent to which phytoplankton populations are concentrated near the surface and the light history of the cells. River water contributes to P by stabilising surface waters. The extent of vertical mixing, including upwelling, also governs light adaptation which is often evident in PPC : B ratios and the extent of surface inhibition of P.