Struan R. Smith

Recent Region-wide Declines in Caribbean Reef Fish Abundance

Profound ecological changes are occurring on coral reefs throughout the tropics, with marked coral cover losses and concomitant algal increases, particularly in the Caribbean region. Historical declines in the abundance of large Caribbean reef fishes likely reflect centuries of overexploitation. However, effects of drastic recent degradation of reef habitats on reef fish assemblages have yet to be established. By using meta-analysis, we analyzed time series of reef fish density obtained from 48 studies that include 318 reefs across the Caribbean and span the time period 1955-2007. Our analyses show that overall reef fish density has been declining significantly for more than a decade, at rates that are consistent across all subregions of the Caribbean basin (2.7% to 6.0% loss per year) and in three of six trophic groups. Changes in fish density over the past half-century are modest relative to concurrent changes in benthic cover on Caribbean reefs. However, the recent significant decline in overall fish abundance and its consistency across several trophic groups and among both fished and nonfished species indicate that Caribbean fishes have begun to respond negatively to habitat degradation.

Caribbean-wide, long-term study of seagrass beds reveals local variations, shifts in community structure and occasional collapse.

The CARICOMP monitoring network gathered standardized data from 52 seagrass sampling stations at 22 sites (mostly Thalassia testudinum-dominated beds in reef systems) across the Wider Caribbean twice a year over the period 1993 to 2007 (and in some cases up to 2012). Wide variations in community total biomass (285 to >2000 g dry m−2) and annual foliar productivity of the dominant seagrass T. testudinum (<200 and >2000 g dry m−2) were found among sites. Solar-cycle related intra-annual variations in T. testudinum leaf productivity were detected at latitudes > 16°N. Hurricanes had little to no long-term effects on these well-developed seagrass communities, except for 1 station, where the vegetation was lost by burial below ∼1 m sand. At two sites (5 stations), the seagrass beds collapsed due to excessive grazing by turtles or sea-urchins (the latter in combination with human impact and storms). The low-cost methods of this regional-scale monitoring program were sufficient to detect long-term shifts in the communities, and fifteen (43%) out of 35 long-term monitoring stations (at 17 sites) showed trends in seagrass communities consistent with expected changes under environmental deterioration.

Introduction to Bermuda: Geology, Oceanography and Climate

The Bermuda islands and shallow platform lie on the top of an extinct Meso-Cenozoic volcano. The islands and the Bermuda Platform were created by reef building corals, vermetid snails and calcareous algae that colonized the eroding seamount. Coral reefs even played a significant role in the settlement of Bermuda and now human impacts of the well-populated islands are important to the ongoing health of the reefs and coral communities. The economic underpinnings of Bermuda – financial services and tourism, along with limited agriculture and small fisheries have created a society that has a moderate impact on the reefs and has a significant and direct economic interest in maintaining a healthy marine environment. Throughout the history of Bermuda, the distribution of coral reefs, and coral communities on the Platform have been determined by the interactions of several environmental factors. Pleistocene formations underlie many modern reefs and there is evidence of submerged reef tracts at depths below 60 m, all around the Bermuda Pedestal. The positions of these formations correspond to stable sea level positions attained during glacial and interglacial periods. Today, there is great variability in the marine environment on the Platform, with the most extreme conditions occurring in inshore waters. The most extensive reefs are developed around the shallow rim of the Platform and, seaward from this, down a gradually descending slope to about 50–60 m depth. Algal-vermetid cup reefs are particularly abundant in Bermuda. On the Platform, seagrass and calcareous green algae beds are closely associated spatially and ecologically with coral reefs, forming unique communities; these are widespread across the lagoon and larger inshore water bodies.

Threats to coral reefs of Bermuda

Bermuda’s reefs have endured the impact of 400 years of human settlement and resource extraction. Although the reef system has benefited from pro-active regulation and control of fishing and pollution since the twentieth century, the nearshore environment and lagoon reefs are threatened by ongoing and planned activities. Coastal development, including cruise ship ports, marinas and shipping channel expansion are significant potential threats through reef removal and sedimentation. The dense human population on Bermuda has produced chronic chemical and nutrient pollution in nearshore bays and harbours. Sewage has reduced water quality in some enclosed bays but is generally not a major threat. Coral bleaching has occurred repeatedly since the 1980s, in response to elevated seawater temperatures, but these events have not resulted in significant mortality. Corals diseases are prevalent at low levels of infection in a large number of species but do not appear to have caused significant mortality. The invasive lionfish (Pterios volitans) is present and the population is growing but culling and harvesting efforts are conducted. There is great concern for the potential impacts of climate-related changes, in particular ocean acidification. Bermuda’s corals grow at reduced rates compared with Caribbean conspecifics and there is evidence that some corals are already growing slower, under the current condition of declining aragonite saturation state in reef waters. The potential for reduced coral and reef growth, in combination with rising sea level, may compromise the effectiveness of the reef as a natural barrier to storm waves, resulting in greater coastal erosion.

Biogeography, Biodiversity and Connectivity of Bermuda’s Coral Reefs

As the most northern ecoregion within the Tropical Northwestern Atlantic biogeographic province, Bermuda’s reef biodiversity is a reduced complement of that found within the other ecoregions of the TNA. A characteristic of the Bermuda marine fauna is the absence of species otherwise ubiquitous in the TNA province (i.e., Acropora spp.). Notable differences in Bermuda’s species diversity is attributed to both geographic and physical forcing agents that include isolation, temperature, currents, bathymetric or coastal complexity, and environmental seasonality. Pleistocene sea level changes also may have been important to the development of Bermuda’s current diversity. Shallow-water scleractinian and octocorallian species diversity is currently considered well documented, however information is still lacking on the depth limits of many species, including in and extending beyond the mesophotic zone. The shallow-water azooxanthellate coral, Rhizopsammia bermudensis, is the only endemic scleractinian. Bermuda’s Symbiodinium diversity is comparable to the Caribbean in that clades A, B and C predominate in anthozoan hosts, but there is a notable absence of Clade D which has been recorded from several Caribbean conspecifics. In Bermuda, octocorals harbour only clade B. Most fishes in Bermuda have a western Atlantic distribution, but amphi-Atlantic and more widely distributed species are also common. High levels of genetic variation and unique Bermudian haplotypes have been determined for several species, spanning several higher taxa – not just cnidarians and fishes. Studies indicate that Bermuda’s marine populations are panmictic and self-seeding. Population connections with upstream reef systems have been inferred genetically for some but not all of the few species investigated. For Bermuda, population connectivity characteristics are so diverse, even among species with apparently similar reproductive and dispersal patterns, that best practices for management and conservation should be developed on a species by species basis.

Biology and Ecology of Corals and Fishes on the Bermuda Platform

Bermuda’s reefs support populations of corals and fishes, derived from the Caribbean fauna, which show distinctive characteristics in regards to reproduction and growth. Bermuda’s corals and fishes have an attenuated summer and early fall reproductive season, that appears to be controlled by cool water temperatures in the winter and spring months. Reef fishes show a clear shift in reproductive output to the summer months compared to the winter spawning of many Caribbean conspecifics. Coral recruitment is dominated by brooding species (e.g. Porites astreoides) across all reef zones although framework species (Diploria spp; Montastraea spp) are common. Settlement and recruitment rates are comparable to Caribbean reefs. The recruitment of reef fishes has been studied intensively and both near-shore and lagoonal reefs appear to be nursery habitats for many reef fish families, perhaps substituting for the paucity of coastal mangroves in Bermuda. The strong seasonality of water temperature appears to reduce growth rates in both corals and reef fishes but may facilitate longevity. Many reef fishes attain greater sizes than conspecifics in the Caribbean. The patterns of distribution of corals, fishes and other reef taxa have been quantitatively assessed over the complex reef lagoon, rim reef and fore reef terrace and data incorporated into GIS databases.

Widespread local chronic stressors in Caribbean coastal habitats

Coastal ecosystems and the livelihoods they support are threatened by stressors acting at global and local scales. Here we used the data produced by the Caribbean Coastal Marine Productivity program (CARICOMP), the longest, largest monitoring program in the wider Caribbean, to evidence local-scale (decreases in water quality) and global-scale (increases in temperature) stressors across the basin. Trend analyses showed that visibility decreased at 42% of the stations, indicating that local-scale chronic stressors are widespread. On the other hand, only 18% of the stations showed increases in water temperature that would be expected from global warming, partially reflecting the limits in detecting trends due to inherent natural variability of temperature data. Decreases in visibility were associated with increased human density. However, this link can be decoupled by environmental factors, with conditions that increase the flush of water, dampening the effects of human influence. Besides documenting environmental stressors throughout the basin, our results can be used to inform future monitoring programs, if the desire is to identify stations that provide early warning signals of anthropogenic impacts. All CARICOMP environmental data are now available, providing an invaluable baseline that can be used to strengthen research, conservation, and management of coastal ecosystems in the Caribbean basin.

Correction: Widespread local chronic stressors in Caribbean coastal habitats

[This corrects the article DOI: 10.1371/journal.pone.0188564.].