Mobile corridors in marine landscapes: enhancement of faunal exchange at seagrass/sand ecotones

Abstract

Field experiments utilizing peak nighttime activity levels of amphipods were performed to determine if the spatial location of drift macroalgae in seagrass landscapes affected amphipod abundance on drift macroalgae. The study was conducted in Tampa Bay, FL over two nights in August 1995, within a Halodule wrightii Ascherson Phillips seagrass bed. Amphipod utilization of drift algae was assessed by defaunating the drift alga, Hypnea cervicornis J. Agardh, and recording net faunal movement onto algal clumps which were (1) stationary in bare sediment for 120 min; (2) stationary in H. wrightii for 120 min; (3) moved through H. wrightii for 90 min; and (4) moved through H. wrightii for 90 min and then across the seagrass sand ecotone and into bare sediment for 30 min. Amphipods, almost exclusively epiphytic taxa, were recovered from all experimental algal clumps placed both in seagrass and sand areas. Fourteen amphipod species were recovered with Ampithoe longimana and Cymadusa compta making up 77% of the total abundance. Greater abundances (but not diversity) of epifaunal amphipods were found on algae when they were located within a seagrass bed compared to bare sediment. Drift algal clumps that passed through the seagrass/sand ecotone had significantly higher amphipod abundance than (1) clumps that did not move through seagrass and were stationary in bare sediment and (2) clumps which moved through, but not out of, a seagrass bed. Demographic and ontogenetic differences were also detected among drift algal treatments. Gravid females were less abundant on algae that did not remain within or was not inside of the seagrass bed and proportionately more males and juveniles were found on algal clumps anchored away from seagrass vegetation. These findings demonstrate that drift algae acting as a mobile corridor provide epiphytal amphipods with a mechanism for dispersal within a seagrass landscape. Moreover, the drift algae that serve as a mobile corridor also act as a mechanism whereby the exchange of fauna is enhanced as drift algae move across the seagrass/sand ecotone. Thus, factors controlling drift algae movement, such as regional meteorological conditions, should have implications for interpatch dynamics of amphipods within a seagrass landscape.