Seagrass habitat disturbance: how loss and fragmentation of eelgrass Zostera marina influences epifaunal abundance and diversity

Abstract

Seagrass habitats commonly display evidence of anthropogenic disturbances such as propeller scars, mooring and anchor damage, trampling, and plant harvesting. These physical distur- bances may lead to habitat loss and fragmentation, yet effects of this habitat removal on seagrass epi- faunal communities are not well understood. We investigated how the degree of eelgrass Zostera marina L. loss influenced the abundance, diversity, and community composition of epifauna within experimental seagrass plots in San Diego Bay, California, USA. We established replicate small (4 m 2 ) and large (16 m 2 ) plots within existing eelgrass habitat, and removed all aboveground and below- ground plant material within randomly chosen cells to establish 4 to 6 levels of eelgrass clearing. We then sampled for epifauna up to 8 wk after eelgrass harvesting. We found no correlations between seagrass loss and epifaunal species richness, total epifaunal density or epifaunal diversity in small plots. In large plots, however, plots with 90% habitat removal had significantly lower epifaunal spe- cies richness and total epifaunal density than plots with 0, 10 or 50% habitat removal, suggesting that beyond a threshold level of eelgrass disturbance, species richness and abundance rapidly decline. Multivariate ordinations revealed that the 90% removal plots also had significantly different species composition than plots with less habitat loss. Our results support previous theoretical models predict- ing threshold levels of habitat loss for faunal communities and broaden our understanding of the response of marine epifauna to seagrass habitat degradation.