Drifting bryozoans increase nekton diversity in the north- central Gulf of Mexico unvegetated muddy bottom seascape

Abstract

Bryozoans are sessile suspension-feeders; however, some species become dislodged and represent drifting, ephemeral habitat. Beyond those on drift algae, there are relatively few studies describing the fauna associated with ephemeral habitats and the functions these habitats maintain in estuarine systems. Given the highly structured morphology of bryozoans and the muddy-bottom seascape of the north-central Gulf of Mexico, we hypothesized that the presence of bryozoans in nearshore waters provides additional temporally and spatially ephemeral habitat for invertebrates, small mobile fishes, or early life-stages of estuarine nekton. We examined seine and trawl samples from a long-term fisheries monitoring program operating within Mississippi Sound, Mississippi to test our hypothesis. Overall, we collected 71 and 85 invertebrate taxa in seine (2012–2017) and trawl (2013–2017) samples, respectively, with 40 taxa in common. We also collected 86 and 74 fish taxa overall in all seine (2012–2017) and trawl (2013– 2017) samples, respectively, with 53 taxa in common. We documented highest species richness and species diversity indices when bryozoans were present and in large volumes; however, most of the samples with bryozoans present were collected between September and November. Our results suggest bryozoan mats likely serve as a dispersal mechanism and refugia for estuarine nekton within a soft-sediment seascape as mats pulse shoreward with winds, tides, and currents.