Implications of hardbottom sponge community decline for ecosystem function in the Florida Keys, Florida (USA)

Human damage to the marine ecosystem of the Florida Keys may have begun in pre-Columbian times but expanded with European colonization about 400 years ago, particularly through overfishing of large species such as seals, turtles, and groupers. Around the turn of the 20 th century, draining of the Everglades and construction of the overseas railway altered forever the flow of fresh and saltwater in the ecosystem. But by and large, the resilience of the ecosystem remained intact until the late 1970’s-early 1980’s when it began to collapse, exemplified by phytoplankton blooms, seagrass and sponge die-offs, mass urchin mortalities, and coral disease. Today, seagrass meadows are relatively healthy but coral reefs harbor few corals and inshore hardbottom habitats have been stripped of sponges and octocorals among other taxa. Changes to hardbottom sponge communities between 2002 and 2018 throughout the Florida Keys are documented here, as are the ramifications of sponge loss on local benthic scouring, underwater soundscapes, and the effect on waterborne microbial communities and water chemistry by sponge filtration. From 2002-2018, sponge abundance and diversity declined by 66% and 34% (respectively), even in regions that did not experience sponge-killing cyanobacteria blooms. Without the presence of large sponges, benthic scouring is reduced, which promotes sedimentation and may hasten the transition of hardbottom habitat to seagrass. Loud, acoustically complex hardbottom soundscapes become acoustic dead-zones without soniferous species associated with large sponges, dampening the sonic signatures that larvae and other animal life stages use to navigate to hardbottom. The decline in sponges and the effects of their lost filtration is estimated to have reduced ecosystem-level removal of DOC by 30 – 56% and NH 4 by 40% along with an approximate 30% reduction in the release by sponges into the water of PO 4 and NO 2 -NO 3. Like that on the region’s coral reefs, the ecological phase-shift in shallow, hardbottom communities of the Florida Keys has been catastrophic and the dearth of sponges has had effects that cascade through the ecosystem.