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

Abstract

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 million per annum mussel Perna canaliculus aquaculture industry. In the summer half-year (October to March), when the Southern Oscillation Index (SOI) was negative (El Niño), 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 NO 3 -, phytoplankton biomass, PN and mussel yield. During an extended period of positive SOI (La Niña), SSE winds and drought during 1999 to 2002, seston (PN) abundance and its food quality decreased, concomitant 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 biomass 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 NO 3 -or chl a concentrations. 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. KEY WORDS: Climate · Estuaries · Mussel aquaculture · ENSO · Nutrients · Primary biomass Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 371: [131][132][133][134][135][136][137][138][139][140][141][142] 2008 Figueiras et al. 2002, Huang et al. 2008. These potentially confounding effects mean that it is important to have a good understanding of factors affecting estuarine productivity to enable the aquaculture industry and resource managers to separate natural climatic variability from impacts due to the farming activity itself.As a case in point, here we consider Pelorus Sound, New Zealand (see Fig. 1). This estuary supports ca. 3 ⁄ 4 of the $ 200 million per annum, 75 000 t green weight, national production of New Zealand greenshell mussels Perna canaliculus, which are grown on hundreds of individual farms throughout its main channel, sidearms and embayments (New Zealand Marine Farming Association 2007). The mussels are grown using suspended rope culture (Zeldis et al. 2005) and feed on phytoplankton and other suspended particles drifting through the farms. Starting in early 1999, farm productivity in the Sound declined by ca. 25% (measured in terms of per capita meat yield) followed by recovery during 2002, with substantial economic impacts and distortions within the in...

Influence of climate on Pelorus Sound mussel aquaculture yields: predictive models and underlying mechanisms

“…Instead, bottom-up effects of oceanic and river nitrogen (N) supply were proposed as drivers of the variation in mussel yield, consistent with other studies demonstrating relationships of marine secondary production with N supply (reviewed by Nixon & Buckley 2002). Zeldis et al (2008) correlated the yields with physical environmental variables (e.g. ENSO, sea surface temperature [SST], along-shelf wind stress and river flow) and chemical and biological variables (e.g.…”

“…1). Riverine input, dominated by the Pelorus River, and estuarine exchanges with Cook Strait both affect the dynamics of the sound (Bradford et al 1987, Gibbs et al 1992, Dupra 2000 with seasonally varying influence (Zeldis et al 2008). Pelorus River flow rates average 18 and 22 m 3 s −1 and NO 3 − concentrations average 140 and 170 mg N m −3 in summer and winter, respectively (Shearer 1989), such that winter river NO 3 − loads are twice those of summer.…”

“…Major and minor wind axes were calculated by the methods of Preisendorfer & Mobley (1988) to describe the mean wind variability at the 2 locations. Monthly mean SST was ob tained using AVHRR satellite radiometer data (NIWA SST Archive, NSA: Uddstrom & Oien 1999) Zeldis et al (2008). Total microphytoplankton carbon biomass was calculated as the sum of these taxa and ac counted for an average of 71% of total phytoplankton carbon (Safi & Gibbs 2003).…”

“…In disentangling the drivers of mussel yield in this ecosystem, Zeldis et al (2008) showed that overgrazing by mussels (i.e. top-down effects: Prins et al 1998, Ogilvie et al 2000, Møhlenberg et al 2007, Huang et al 2008 could not explain the 1999−2002 yield minimum.…”

“…1), Zeldis et al (2008) examined correlations among physical, chemical and biological variables and variation in mussel aquaculture yield. This large estuary supports ca.…”