Invasive species and low dissolved oxygen (DO) threaten the biodiversity and ecosystem health of estuaries worldwide. To test the hypothesis that exposure to low DO reduces resistance of epifaunal community to invasion in the Chesapeake Bay, we conducted experiments using standardized settling panels, including 1) controlled experiments exposing epifaunal communities to low DO; 2) measurement of the short term response of motile and sessile epifauna to low DO; 3) survey of multiple sites in which community structure was correlated with low DO and other environmental variables; and 4) evaluation of the biological and structural effects of an invasive hydroid and a cryptogenic tunicate, both with high tolerance for low DO, on recruitment and development of epifauna.

Periodic hypoxia was correlated with an increased cover of the native serpulid polychaete, <i>Hydroides dianthus </i>. Cover of invasive and cryptogenic species increased with exposure to moderate low DO. Cover and incidence of bryozoans, sabellid polychaetes, and cnidarians differed among DO treatments. Nematodes, caprellids, and harpacticoid copepods vacated epifaunal communities in response to low DO.

In the multi-site survey, > 50% cover of invasive and cryptogenic species was associated with exposure to chronic low DO. Six of eight sites in the survey experienced periodic low DO (< 4 mg/l), but only one experienced chronic low DO ( > 40% of days below 4 mg/l DO). Shifting cover of <i>Hydroides dianthus </i>, barnacles, and invasive species was correlated (> 50%) with percent of days experiencing low DO. Epifaunal heterogeneity reflected environmental differences among sites.

Species richness and diversity at local sites declined with increasing abundance of certain taxa in higher salinity, higher diversity areas. Heightened cover of <i>Molgula manhattensis</i>, <i>Hydroides dianthus</i> or barnacles led to reduced local diversity but regional species diversity was maintained through environmental heterogeneity across sites. Conversely, in lower salinity, lower diversity zone, <i>Cordylophora caspia</i>, an invasive hydroid, had a positive effect on some species. Temporal and spatial shifts in cover of dominant species and in species diversity in response to low DO disturbance and other environmental factors may facilitate persistence of less competitive native or invasive species.