Loss of foundation species from nearshore hardbottom communities in the Florida Keys

Abstract

Nearshore hardbottom (NSHB) habitats occur in shallow coastal waters throughout the Florida Keys and support sessile benthic invertebrate communities dominated by sponges and octocorals. In turn, these sessile benthic invertebrates provide unique structural habitats, supporting motile invertebrate and fish communities. We examined changes to NSHB faunal communities in the Florida Keys under two different hydrological regimes (bayside and oceanside) in 2006 and 2023. Stony coral and fish richness, density, and community composition did not change during the study period in either regime, with weak evidence for declining octocoral density bayside (p = 0.067) and declining richness oceanside (p = 0.065), possibly due to increasing thermal stress. However, sponge community composition showed moderate evidence of change during the study period in both regimes (bayside p = 0.024, oceanside p = 0.040), indicating the loss of large structure-forming species, such as Spheciospongia vesparium (loggerhead sponge), likely driven by repeated cyanobacteria blooms. Bayside motile invertebrate mean density increased threefold (p = 0.022) over the study period, driven by increases in Panulirus argus (Caribbean spiny lobster) and Menippe mercenaria (Florida stone crab). Together, these findings provide evidence of a decoupling between habitat structure and higher trophic levels and suggest a lagged ecological response to the loss of foundation species. Documenting the current state and shifts in NSHB diversity and community composition is critically important to understand how nearshore systems are changing in the Anthropocene and can assist in setting restoration priorities and performance targets.