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Hypoxia is increasing in marine and estuarine systems worldwide, primarily due to anthropogenic causes. Periodic hypoxia represents a pulse disturbance, with the potential to restruc- ture estuarine biotic communities. We chose the shallow, epifaunal community in the lower Chesa- peake Bay, Virginia, USA, to test the hypothesis that low dissolved oxygen (DO) (<4 mg l -1 ) affects community dynamics by reducing the cover of spatial dominants, creating space both for less domi- nant native species and for invasive species. Settling panels were deployed at shallow depths in spring 2000 and 2001 at Gloucester Point, Virginia, and were manipulated every 2 wk from late June to mid-August. Manipulation involved exposing epifaunal communities to varying levels of DO for up to 24 h followed by redeployment in the York River. Exposure to low DO affected both species com- position (presence or absence) and the abundance of the organisms present. Community dominance shifted away from barnacles as level of hypoxia increased. Barnacles were important spatial domi- nants which reduced species diversity when locally abundant. The cover of Hydroides dianthus, a native serpulid polychaete, doubled when exposed to periodic hypoxia. Increased H. dianthus cover may indicate whether a local region has experienced periodic, local DO depletion and thus provide an indicator of poor water-quality conditions. In 2001, the combined cover of the invasive and crypto- genic species in this community, Botryllus schlosseri (tunicate), Molgula manhattensis (tunicate), Ficopomatus enigmaticus (polychaete) and Diadumene lineata (anemone), was highest on the plates exposed to moderately low DO (2 mg l -1 < DO < 4 mg l -1 ). All 4 of these species are now found world- wide and exhibit life histories well adapted for establishment in foreign habitats. Low DO events may enhance success of invasive species, which further stress marine and estuarine ecosystems.

Epifaunal disturbance by periodic low levels of dissolved oxygen: native vs. invasive species response