The role of small mobile epifauna in subtidal rocky reef ecosystems

Abstract

This thesis documents abundance patterns of mobile seaweed-dwelling macro invertebrates (epifauna) in space and time, and examines the contribution of epifauna to trophic fluxes on subtidal temperate rocky reefs. Research was carried out at sites near the University of Auckland's Leigh Marine Laboratory in northeastern New Zealand. Subtidal brown seaweeds were found to be inhabited by a diverse range of mobile epifauna (> 1 mm sieve size), with the most abundant groups being peracarid crustaceans such as gammarid amphipods and isopods. Amphipod densities were highest on finely-structured plants. Total isopod densities were not correlated with plant structure, but there was a trend for isopods with tubular body shapes to live on algal species with narrow fronds, and for dorso-ventrally flattened isopods to live on algae with wide fronds. Epifaunal taxa were generally not specific to one seaweed species, although the assemblages on most of the ten seaweed species surveyed were distinct from one another in multivariate space. In a dense seaweed bed, epifaunal abundances and community composition varied little on diel and lunar time scales, although results of recolonization experiments indicated turnover of up to 70 % of individual amphipods per plant each night. Movement was mainly by crawling from plant to plant, rather than swimming in the water column. Over 2-3 years animals inhabiting two fucalean seaweed species underwent seasonal fluctuations in abundance, with densities per unit seaweed mass tracking incident sunlight. Epifauna on the laminarian Ecklonia radiata showed no clear seasonal abundance patterns. A survey of all animals> 0.5 mm sieve size in four rocky reef habitats revealed that epifauna (< 10 mm in length) were extremely abundant in finely-structured vegetated habitats, where they contributed up to 99 % of total secondary productivity. The epifaunal contribution was lower in less structurally complex habitats such as the urchin barrens, but still amounted to ~80 % of total secondary productivity on the scale of the entire reef. In each habitat the most productive fish guild was that which preyed upon epifauna, but these fish only consumed ~20 % of epifaunal production. The fate of the remaining 80 % is unknown, and merits further research, as does the source(s) of primary production which fuels the epifaunal productivity. This study clearly identified epifauna as the major secondary producers on the reef, demonstrating the need to include them in trophic models of subtidal rocky reefs, from which they have to date been excluded. Ammonium excreted by mobile epifauna (> 0.355 mm sieve size) was shown to be a potentially important source of nitrogen for the host plant, conservatively estimated to provide up to ~80% of the plant's requirements under conditions oflow water turnover.