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

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 a nitrogen‐ 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 m^‐3, 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 aphotic zone (10.0–33.0) was apparent at all 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.

Key words:

• phytoplankton
• bacterioplankton
• chlorophyll a
• heterotrophic activity
• upwelling
• outwelling
• nutrient concentrations
• nutrient ratios
• nutrientpools
• sediments
• South Island
• New Zealand