Distribution and potential larval connectivity of the non-native Watersipora (Bryozoa) among harbors, offshore oil platforms, and natural reefs
Non-native marine species thrive on artificial habitat. Expanding coastal infrastructure has led to concerns that increasing artificial habitat will facilitate the spread of nonnative species overall and to natural ecosystems. In the Santa Barbara Channel (SBC), California, USA, the non-native bryozoan Watersipora subatra (Ortmann, 1890) has previously been reported only in harbors and on offshore oil platforms. To assess the distribution and potential for spread of W. subatra between coastal infrastructure and natural habitat in the SBC, we surveyed 61 open coastal sites, including natural and artificial habitat, and evaluated the potential dispersal connectivity of W. subatra larvae from harbors and oil platforms to natural reefs using survey results and three-dimensional biophysical modeling. We discovered that this bryozoan has invaded natural reefs in the region. W. subatra was present on approximately 50% of the oil platforms and mainland reef sites, but only 17% of offshore island reef sites. Modeling indicated high potential connectivity from one harbor to the closest reef with W. subatra, 4–5 km distant from the harbor mouth, but dispersal to the more distant sites would likely require intermediate stepping stone sites or anthropogenic transport of spawning adults. Populations on offshore platforms, in contrast, were virtually unconnected to reef sites through ocean circulation. The survey data indicated potential opportunities to manage current and future invasions through the control of larval sources. In particular, manual removal of small isolated populations on reefs at the northern Channel Islands would reduce this source of larvae to uninvaded areas. Consideration of potential connectivity in the placement of mariculture infrastructure along the coast with respect to harbors and other sources of non-native propagules could also help to reduce the potential for stepwise spread of non-natives to natural reef habitat.