Characterization of soundscapes in shallow water habitats of the Florida Keys (USA) and their influence on the settlement of larval fish and invertebrates

Abstract

In recent decades, changes in climate and water quality in Florida Bay and the Florida Keys (FL, USA) caused expansive cyanobacteria blooms that in turn precipitated massive sponge die-offs that drastically altered sponge-dominated hard-bottom communities in south-central Florida Bay. This area served as a model system to explore the effect of ecosystem change and habitat restoration on underwater soundscapes and larval recruitment. I had four main objectives: (1) characterize the underwater soundscapes of three near-shore, benthic habitats: mangrove islands, seagrass meadows, and hard-bottom (Chapter 2); (2) quantify larval settlement within healthy, degraded, and restored hard-bottom areas to test whether habitat degradation altered larval settlement (Chapter 3); (3) empirically test the role of sound in promoting larval recruitment to hard-bottom habitat (Chapter 3); and (4) employ the passive sonar equation and distance sampling techniques to evaluate how the loss of large sponges affected the densities and abundances of snapping shrimp (Chapter 4). I found that near-shore habitats exhibit distinct soundscapes, that habitat degradation alters those soundscapes, and that habitat restoration can reestablish natural soundscapes. Habitat type and time of day significantly affected soundscapes, whereas lunar phase did not. Healthy hard-bottom and mangrove habitats exhibited louder spectra and more snapping shrimp snaps than did degraded hard-bottom or seagrass beds. However, four years after restoration, the acoustic spectra and numbers of snapping shrimp snaps on restored hard-bottom were similar to those of healthy hard-bottom. Habitat quality and moon phase both significantly affected larval recruitment. Overall, healthy hard-bottom habitat attracted significantly more larvae than either degraded or restored hard-bottom, particularly during full moon. Playback of healthy hard-bottom soundscapes within degraded hard-bottom areas prompted higher larval settlement, particularly during the full moon. Estimates of snapping shrimp populations within degraded areas were significantly lower than estimates within healthy areas. Shrimp abundance estimates on healthy hard-bottom sites were one to two orders of magnitude greater than those on degraded sites. These studies demonstrate that tropical coastal habitats differ in soundscape characteristics, that habitat degradation affects soundscapes and the ecological process of larval settlement and recruitment, and that restoration of hard-bottom habitat can aid in returning these functions.