Spatial synchrony of amphipods in giant kelp forests

Abstract

Amphipods are abundant in marine ecosystems worldwide and are important as prey and as consumers of macrophytes and detritus in food webs. Due to the spatially complex and dynamic nature of giant kelp (Macrocystis pyrifera) forests, assessment of the abundances of giant kelp and amphipods through time and space should provide insight into their potential interactions within the system. In an extensive field study within the surface canopy of giant kelp, the abundance of amphipods was quantified on artificial substrates at an array of 18 sites within kelp forests along Point Loma, California, USA, from July to October 2009 and 2010. Biomass of giant kelp canopy was estimated using remotely sensed imagery, and the spatial synchrony (autocorrelation through time) of kelp canopy was compared with synchrony of caprellid and non-caprellid amphipods. Caprellids exhibited high spatial synchrony that did not decrease with distance, while non-caprellids were synchronous on local scales, indicating high spatial heterogeneity in abundance through time. Gammarids showed a rapid exponential decrease in synchrony within the first 550 m that was consistent with synchrony of giant kelp. This suggests a local-scale biotic link between non-caprellids and giant kelp canopy, whereas caprellid synchrony is more likely to be influenced by regional-scale environmental variables. Caprellids and other amphipods are important prey resources for common kelp forest fishes, so these differences may in turn affect the spatial distributions of these predators. Moreover, excretion by amphipods may be an important source of nitrogen to giant kelp during periods of nitrogen limitation.