MEPS 515:123-132 (2014)  -  DOI: https://doi.org/10.3354/meps11025

ABSTRACT: Benthic primary producers such as seaweeds and seagrasses are often inhabited by high densities of small arthropods, but little is known about the impact of these animals on their hosts, which could be positive if they keep them free of fouling epiphytes. We assessed the impact of small arthropods on algal epiphytes growing on coralline algal turf on a shallow, wave-exposed, rocky reef in warm temperate northeastern New Zealand. Plaster blocks impregnated with the insecticide carbaryl were used to reduce arthropod densities (by 96% relative to unmanipulated controls in the case of amphipods). By the end of the ~3.5 mo experiment total epiphyte cover was 88% when arthropods were excluded compared to 38% in unmanipulated controls, a 2.3-fold increase. Brown (Colpomenia spp.), green (Ulva spp.) and ‘filamentous and microscopic’ algal epiphyte taxa were responsible for the increase, while the cover of red foliose algal epiphytes decreased slightly. Hyalid amphipods (Protohyale spp.) were likely responsible for suppressing the epiphytes, as they were the most abundant arthropod mesograzer taxon and ate both Colpomenia spp. and Ulva spp. in a laboratory no-choice feeding assay. During the field experiment 2 large storms removed most of the epiphytes that had grown in the mesograzer exclusion treatment. Overall, our results indicate that the amphipods prevented overgrowth of the turf by epiphytes during calm periods. These results provide the first in situ cageless example of arthropod mesograzer-exerted control on the abundance and composition of primary producers on a subtidal rocky reef.

KEY WORDS: Amphipod · Epiphyte · Herbivory · Hyalidae · Mesograzer · Storm · Temperate reef · Turf