Abstract
Coral reefs across the world are under threat from a range of stressors, and while there has been considerable focus on the impacts of these stressors on corals, far less is known about their effect on other reef organisms. The 1997–8 El-Niño Southern Oscillation (ENSO) had notable and severe impacts on coral reefs worldwide, but not all reef organisms were negatively impacted by this large-scale event. Here we describe how the sponge fauna at Bahia, Brazil was influenced by the 1997–8 ENSO event. Sponge assemblages from three contrasting reef habitats (reef tops, walls and shallow banks) at four sites were assessed annually from 1995 to 2011. The within-habitat sponge diversity did not vary significantly across the study period; however, there was a significant increase in density in all habitats. Multivariate analyses revealed no significant difference in sponge assemblage composition (ANOSIM) between pre- and post-ENSO years for any of the habitats, suggesting that neither the 1997–8 nor any subsequent smaller ENSO events have had any measurable impact on the reef sponge assemblage. Importantly, this is in marked contrast to the results previously reported for a suite of other taxa (including corals, echinoderms, bryozoans, and ascidians), which all suffered mass mortalities as a result of the ENSO event. Our results suggest that of all reef taxa, sponges have the potential to be resilient to large-scale thermal stress events and we hypothesize that sponges might be less affected by projected increases in sea surface temperature compared to other major groups of reef organisms.
Highlights
Coral reefs around the world are under threat from a range of local- and global-scale threats [1,2,3]
We found that there were higher seawater temperatures, lower sky cover and lower turbidity during the 1997–8 El-Nino Southern Oscillation (ENSO) period compared to non-ENSO years (Fig. 2), likely resulting in higher levels of UV radiation reaching the reef associated invertebrates in 1998 than in non-ENSO years
Rainfall was significantly lower during ENSO conditions and this resulted in reduced freshwater and sediment outflow from the local rivers and, significantly clearer water. 1998 was characterized by warmer air and sea temperatures, reduced cloud cover and rainfall, higher incoming solar radiation, and reduced turbidity
Summary
Coral reefs around the world are under threat from a range of local- and global-scale threats [1,2,3]. At local scales, these include habitat destruction, overfishing, pollution, sedimentation and invasive species. These include habitat destruction, overfishing, pollution, sedimentation and invasive species While these threats can have devastating impacts on reefs, in most cases management intervention and mitigation is possible at the scale of the impact. While the effects of large-scale El-Nino Southern Oscillation (hereafter ENSO) events on coral communities have been well described, as a result of the 1997–8 event that had devastating impacts of many coral reefs ([5,6] many others), the influences of ENSO events on other dominant reef organisms are much less well known
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