Role of zooplankton in the nitrogen cycle off the west coast of the South Island, New Zealand, winter 1987

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Abstract
Zooplankton abundance, feeding, and ammonia regeneration rates were measured along three transects off the west coast of the South Island, New Zealand, in winter (July) 1987. Crustacean nauplii and copepodites dominated the 55–200 μm fraction of the microzooplankton. Tintinnid population dynamics were related to the degree of freshwater influence. Tintinnid species with silica‐encrusted bowls were common close inshore and species with no encrustations were common offshore. The most abundant macrozooplankton were the small copepods Acartia ensifera, Paracalanus indicus, Oithona similis, Centropages aucklandicus, and Clausocalanus jobei. A. ensifera dominated at the inshore stations, whereas the outer stations had a mixture of neritic/outer shelf and oceanic species. Zooplankton biomass was highest at the outer station off Hokitika. Offshore displacement of coastal water and alongshore wind‐driven flow were important determinants of zooplankton dynamics. Ingestion rates of particulate nitrogen by zooplankton > 55 μm were equivalent to 0.3–20% of the daily uptake of nitrogen by phytoplankton and were highest close inshore. Regeneration of ammonia‐nitrogen by zooplankton > 55 μm averaged 12% of ammonia remineralisation. Small oligotrichs and heterotrophic nanoflagellates are suggested as the major agents for remineralisation; nanoflagellates may be an important additional link in the food chain between picoplankton and zooplankton > 55 μm.