Soft-sediment benthic community structure in a coral reef lagoon - the prominence of spatial heterogeneity and 'spot endemism'

An encompassing view of coral reef ecosystems needs to extend beyond the prominent and attractive hard substrata to include soft-sediment habitats associated with the reefs. Focusing on the soft-sediment assemblages within the lagoon of the Great Astrolabe Reef (Fiji), we quantified pat terns, dines and variability of community structure across space and evaluated models for marine biodiversity conservation based on the spatial distribution and rarity of benthic species. Water depth sampled ranged from 17 to 42 m over a spatial sampling scale of 18 x 11 km with 25 localities on average 2.4 km apart. Both plant diversity and biomass were poor predictors of zoobenthic diversity. In contrast to the commonly held view that sediment characteristics are the prime factors in structuring benthic assemblages, spatial variability of the benthos was overriding the generally weak relationships between sedimentary features and the biota. High spatial heterogeneity in community structure is a key feature of the benthic biota in the lagoon. Part of this pronounced spatial heterogeneity stems from the marked patchiness in individual species distributions, here operationally coined 'spot endemism', Out of a total of 211 recorded taxa, 42% were rare, being restricted to a single site. No species' range spanned the entire lagoon; in fact, the maximum species range was 16 out of 25 sites sampled. Furthermore, the number of taxa common to any 2 localities was not strongly linked to spatial distance, with adjoining sites having no more taxa in common than more distant localities. The commonness of rarity, prevalence of highly compressed species' range sizes, and patchiness in benthic diversity all combine to have profound implications for conservation strategies of these marine benthic habitats.