Spatial heterogeneity and synchrony of amphipods in giant kelp forests
The giant kelp (Macrocystis pyrifera) is a foundation species on temperate rocky reefs, that is both dynamic and spatially heterogeneous. The surface canopy of giant kelp has been shown to be spatially autocorrelated on local scales through time, a phenomenon known as synchrony. Within kelp forests, amphipods comprise a dominant component of the invertebrate assemblage associated with giant kelp fronds (both numerically and in terms of biomass) and are central in kelp forest food webs. Consequently, knowledge of amphipod synchrony within kelp forests will address a large gap in our understanding of ecological processes within kelp forests. Our objective was to assess the spatial and temporal heterogeneity of abundant amphipod groups and giant kelp. In an extensive 2 yr field study, we assessed the spatial synchrony of amphipods and giant kelp canopy biomass off Point Loma, California, USA. We estimated kelp canopy biomass using 30 m resolution multispectral imagery from Landsat 5 TM and Landsat 7 ETM+ sensors, and quantified abundance of gammarid and caprellid amphipods using artificial substrates at an array of 18 sites off Point Loma.
Caprellid amphipods were most abundant and exhibited high synchrony throughout our sampling array (autocorrelation among sites was high through time), indicating low spatial heterogeneity within our study area. Gammarids were synchronous across smaller scales, indicating high spatial heterogeneity in abundance through time. Gammarids exhibited a rapid exponential decrease in synchrony within the first 500 m that was consistent with synchrony of giant kelp canopy. This suggests that the processes structuring gammarid abundance and giant kelp canopy biomass may operate over similar spatial scales, while caprellids may be influenced by forces operating over scales at least as large as our sampling array. The differences in synchrony between caprellids and gammarids may be linked to differences in feeding strategies, and may translate to variation in community level processes within kelp forests, particularly those involving microcarnivorous fishes and grazing of kelps by gammarids. This study demonstrates the utility of sampling across multiple spatial scales and the application of spatial statistics to improve our understanding of the kelp forest community at the landscape scale.