MacKenzie et al.--Nutrients, microplankton off NW SI, NZ

New Zealand Journal of Marine and Freshwater Research abstracts

Nutrients, and microplankton biomass off the New Zealand South Island north-west coast, January 1982

A. LINCOLN MacKENZIE HENRY F. KASPAR PAUL A. GILLESPIE

Cawthron Institute Private Bag, Nelson New Zealand

Abstract A survey during the summer of 1982 revealed riverine outwelling and the upwelling of waters from aphotic depths at different locations on the Westland shelf. This led to nutrient enrichment of the euphotic zone and resulted in elevated phy to-plankton and bacterioplankton biomass estimates. At most stations subsurface chlorophyll a maxima existed near the base of anitrogen- depleted euphotic zone and near the boundary of the nitracline. Total N and total P concentrations in the euphotic zone (2.4-10.6 and 0.3-1.0 mmol m~³, respectively) never exceeded the concentrations within the aphotic zone (9.0-16.1 and 0.7-1.3 mmol nr³, respectively) at any station. Molar ratios revealed the larger proportions of the total nitrogen and phosphorus pools that existed in an organic form in the euphotic than in the aphotic zones. The relative depletion of dissolved inorganic nitrogen (DIN) compared to dissolved reactive phosphorus (DRP) as shown by DIN/DRP ratios in the euphotic zone (1.5-12.5) versus the aphoticzone(10.0-33.0) was apparentatall stations. Large pools of combined N and P existed in the top 4 cm of the sediments relative to the quantities within the water column, though organic N relative to organic P was depleted. A discussion of the relative magnitude of the distribution of sediment nitrogen flux and river-borne nitrogen loading to the nitrogen pools in the coastal water column is presented.

Keywords phytoplankton; bacterioplankton; chlorophyll a; heterotrophic activity; upwelling; outwelling; nutrient concentrations; nutrient ratios; nutrienl pools; sediments; South Island; New Zealand Received 30 September 1987; accepted 10 February 1988

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