Wind as a driver of zooplankton abundances and community structure in Storm Bay, Tasmania

Abstract

Zooplankton are highly sensitive to environmental change, therefore shifts in species abundances and community composition can often be linked to anomalies in one or more environmental variables. A study in Storm Bay (south-east Tasmania, Australia) during the 1980s, led to the suggestion that, in years of strong westerlies, the productivity and zooplankton biomass in the bay increased by a factor of ten. Moreover, the increased influence of the East Australian Current has recently been shown to negatively impact total zooplankton abundances within the same region. The aim of this study was to build further on these studies by identifying the effects of wind stress and temperature, along other environmental variables on zooplankton abundances in Storm Bay over the period between 2009-2015. Generalized Additive Models (GAMs) indicated wind stress (p=0.007), chlorophyll-a (p=0.02), and the Southern Oscillation Index (SOI) (p=0.002) to be significantly correlated with total zooplankton abundance. The significance of wind stress was mainly attributed to its effect on primary production, through the supply of nutrients, induced by advection and mixing. Wind stress was also considered to be of direct impact through wind-induced currents, caused by surface stress. When modelled separately (using GAMs and Canonical Analysis of Principal coordinates), species that correlated significantly with windstress included Cladocera, a range of gelatinous species, some copepods and larvae. The positive response of Cladocera and Gelatinous (i.e. Tunicates) was attributed to their ability to show explosive growth during periods of increased food availability (i.e. chlorophyll-a), and their susceptibility to wind-induced currents. The variability in wind stress correlation (negative or positive) across the taxon of Copepoda, is explained by the species-specific response to wind-induced turbulence. The observations on zooplankton abundances in Storm Bay in relation to wind stress, chlorophyll-a and SOI were consistent with those observed in other regions of the world. The predicted increase of Cladocera and gelatinous species with increasing windstress may have cascading effects on higher trophic levels in Storm Bay.