Links between Rain, Salinity, and Predation in a Rocky Subtidal Community

Abstract

Physical control of predation appeared likely in subtidal habitats of fjords in southwestern New Zealand because they intersect a shallow low-salinity layer (LSL) where marine predators might be excluded by low-salinity stress. We investigated links between predator distributions, predation intensity, rainfall, the depth of the LSL, and sessile invertebrate community structure at two sites in Doubtful Sound, New Zealand. Sessile invertebrate communities living on the rock walls of the fjord showed striking patterns of vertical zonation corresponding to the depth of the LSL. Barnacles and mussels, Mytilus edulis galloprovincialis, were most abundant at the 1–3 m depth range commonly bathed by the LSL, while sponges, bryozoans, and ascidians dominated space below it (6–18 m depth). Mussel abundance declined sharply with depth at both sites, from maxima of 52–80% cover in the LSL (3 m) to <2% cover just below the LSL (6 m). To test the hypothesis that the LSL influences the lower limit of the mussel zone by excluding predators and reducing predation from mobile invertebrates living below it, we measured predator densities and tracked predation on transplanted mussels, M. e. galloprovincialis, in the LSL (2.5 m depth) and at 3–4 depths below. Predators were most abundant just below the lower boundary of the LSL during February and April 1993. No transplanted mussels were consumed in the LSL at 2.5 m, but 20–80% of them were consumed in experiments conducted below it, indicating that the LSL represents a spatial refuge from predation. The shallowing of the LSL to 1.5 m depth during low rainfall in November 1993 provided an additional opportunity to evaluate the influence of the LSL on predation. Repeated predator surveys showed that sea stars and urchins moved up with the ascending LSL to create a pattern of high predator abundance at 2.5 m depth in November, suggesting that the shallow zone previously buffered from predation by the LSL was not a predation refuge in November. Rainfall and salinity at 2.5 m depth were inversely related. A significant regression of rainfall one day prior to salinity profiling and the depth of the LSL boundary (30 mg/g salinity) explained 61.5% of the variation in the depth of the boundary, which represented the maximum depth of the predation refuge. The regression enabled us to predict the depth of the predation refuge over a 1.5-yr period beginning in October 1992. The average depth of the refuge was 2.35 m, ranging from 1.5 to 5.6 m as a function of rainfall. Daily tidal variation increased the vertical excursion of the refuge by 0.9–1.7 m. Distinguishing between a partial predation refuge where the LSL occurred at a given depth for a part of a day, and a complete refuge where a depth occurred in the refuge for the entire day, we estimated that the 2.5-m habitat was a complete refuge during 10% of the 18-mo period and a partial predation refuge 90% of the time. The close correspondence between the average depth of the predation refuge (2.35 m) and the depth of maximum mussel cover (3.0 m) suggests that the mussel zone is restricted to the LSL by high predation at the lower boundary of the layer. The alternative hypothesis, that the lower limit of the mussel zone (6 m depth) was determined by a failure of mussel recruitment to this depth, was tested by a 1-yr recruitment experiment. Mussels recruited to the lower edge of the mussel zone at 6 m, although overall levels of mussel recruitment during May 1993–April 1994 were too low to maintain the observed zonation pattern. By increasing the spatial extent of the low-salinity layer, we speculate that changes in climatic conditions leading to increased freshwater runoff may alter salinity gradients, bathymetric distributions of predators, and the structure of shallow subtidal communities, especially in fjords.