Abstract

In the early 1990s, widespread sponge mortality events occurred in the Florida Keys, USA. These mortality events were coincidental with successive blooms of the picoplanktonic cyanobacterium Synechococcus sp. Although the specific cause of sponge death remains unknown, we conclude that bloom conditions caused the death of these sponges. We documented the effects of the mortality events on sponge community biomass and followed the sponge population response of 23 species for up to 15 years (1991 - 2006). In doing so, we provide an unprecedented, long-term, and detailed view of sponge population dynamics in the Florida Keys, following a set of environmental conditions that caused widespread mortality. Abundance of many sponge species following the mortality events was dynamic and contrasts with work done on deep-water sponge communities that have shown such communities to be stable over long periods of time. Recovery of the sponge community biomass in the Florida Keys was slow, taking 10 - 15 years. Some species showed consistent long-term trends but many others displayed constantly changing abundance in time and space. We conclude that sponge deaths resulting from relatively infrequent bloom conditions can chronically reduce sponge community biomass and have potentially long-term and widespread ecological consequences. Hurricane Wilma directly hit the study area in 2005, allowing us to also document the effects of a hurricane on sponge populations. Although this hurricane event significantly impacted sponge abundance, in general, massive, long-lived sponges that dominate sponge community biomass in the area tended to be more resistant. Hence, we conclude that sponge community biomass will recover more quickly than following bloom-related mortality events.

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