Variation in environmental conditions in a subtidal prey refuge: effects of salinity stress, food availability and predation on mussels in a fjord system

Abstract

Prey refuges are fundamental structural features in communities. We investigated variability in environmental conditions within a subtidal prey refuge for the blue mussel Mytilus edulis galloprovincialis formed by the persistent low-salinity layer (LSL) in Doubtful Sound, New Zealand. Multi-year observations and fine-scale oceanographic surveys along the axis of Doubtful Sound show strong spatial gradients in salinity, temperature, chlorophyll a (chl a) and nitrate concentrations. Mean surface salinity ranged from ~5 in the inner fjord zone to 15 in the mid-fjord, and 25 to 30 in the entrance zone. A marked subsurface maximum in chl a was observed below the LSL at 3 to 7 m depth. Adult blue mussels were confined to the LSL with a sharp decline in abundance from the entrance to the inner regions of the fjord. In contrast, mussel recruitment was observed both within and below the LSL to 10 m depth, with highest recruitment in the mid-fjord zone at 6 m depth. To test whether patterns in growth and survival in the absence of predation were coincident with food supply and salinity stress, we transplanted mussels in predator exclusion cages at depths of 2, 4, 6, and 8 m within inner, mid-, and entrance fjord zones and measured growth over 213 d. Variation in salinity and temperature, rather than food availability, had the largest impact on mussel growth rates and explained 89% of the variability in mussel growth. Salinity alone explained 87% of the variability in mussel growth. Adult survival in the absence of predation did not significantly differ with depth or fjord zone. Environmental stress, primarily in the form of stress associated with low salinity, exerted the greatest influence on growth of mussels within this critical subtidal prey refuge.