The effect of coastal processes on phytoplankton biomass and primary production within the near-shore Subtropical Frontal Zone

This study evaluated drivers of phytoplankton net primary production (NPP) rates and chlorophyll-a (chl-a) concentrations within the coastally oriented Subtropical Frontal Zone (STFZ) off the South Island of New Zealand. Time series measurements of hydrographic parameters, macronutrients, size fractionated NPP and chl-a were conducted on a bi-monthly basis from July 2009 to November 2010. This study found that nutrient limitation in these waters is controlled by the dual influx of silicate inputs from riverine sources in coastal neritic water (NW) and oceanic inputs of nitrate from the high nutrient, low chlorophyll (HNLC) region of the offshore Sub-Antarctic Surface Waters (SASW). Total chl-a concentrations and primary production rates were perennially higher in near-shore NW and modified Subtropical waters (STW) than in the SASW, with highest indicators of biological production observed in the Austral spring and summer seasons (October to March). These periods of peak production and biomass were dominated in both parameters by microphytoplankton (>20 μm) size fractions. The coupled dominance by these large phytoplankton and the near depletion of silicate in all characterised waters within the frontal system indicate the importance of silicic diatoms as drivers of bloom production. The influence of coastal waters on the STFZ system is most pronounced with the intrusion of neritic water beyond the shelf boundary during periods of surface water thermal stratification and riverine dilution through flooding events. These two events were notably observed during the Spring 2009 sampling cruise in December 2009 and in the flood event in May 2010.