ENSO and riverine control of nutrient loading, phytoplankton biomass and mussel aquaculture yield in Pelorus Sound, New Zealand

Multi-year time-series were used to describe oceanic and riverine nutrient supply and primary biomass in Pelorus Sound, a 50 km long estuary supporting most of New Zealand's $200 mil- lion per annum mussel Perna canaliculus aquaculture industry. In the summer half-year (October to March), when the Southern Oscillation Index (SOI) was negative (El Nino), NNW along-shelf wind stress strengthened and sea surface temperature (SST) at the Sound entrance cooled, indicating upwelling. This triggered increases in phytoplankton biomass, particulate nitrogen (PN) and per capita yield of farmed mussels in the Sound. In the winter half-year (April to September), wind stress was unrelated to SOI, but during NNW winds Pelorus River flows increased, along with NO3 - , phyto- plankton biomass, PN and mussel yield. During an extended period of positive SOI (La Nina), SSE winds and drought during 1999 to 2002, seston (PN) abundance and its food quality decreased, con- comitant with a mussel yield decrease of ~25% throughout Pelorus Sound. Seston and mussel yield had recovered by 2003 without reductions in farming intensity, so over-grazing by mussels did not cause the yield minimum. Instead, climatic forcing of oceanic and riverine N supply and seston bio- mass underlay the inter-annual variation in mussel yield. As tracers of the relationship of nutrient loading and production, PN and mussel yield appeared more reliable than NO3 - or chl a concentra- tions. In Pelorus Sound, oceanic and riverine N supplies are seasonally complementary and sustain year-round mussel yield, although their inter-annual variability, linked to wider climate forcing, can drive considerable fluctuation in yield over the decadal scale.