Samantha J. de Putron

Comparing Chemistry and Census-Based Estimates of Net Ecosystem Calcification on a Rim Reef in Bermuda

Coral reef net ecosystem calcification (NEC) has decreased for many Caribbean reefs over recent decades primarily due to a combination of declining coral cover and changing benthic community composition. Chemistry-based approaches to calculate NEC utilize the drawdown of seawater total alkalinity (TA) combined with residence time to calculate an instantaneous measurement of NEC. Census-based approaches combine annual growth rates with benthic cover and reef structural complexity to estimate NEC occurring over annual timescales. Here, NEC was calculated for Hog Reef in Bermuda using both chemistry and census-based NEC techniques to compare the mass-balance generated by the two methods and identify the dominant biocalcifiers at Hog Reef. Our findings indicate close agreement between the annual 2011 census-based NEC 2.35±1.01 kg CaCO3•m-2•y-1 and the chemistry-based NEC 2.23±1.02 kg CaCO3•m-2•y-1 at Hog Reef. An additional record of Hog Reef TA data calculated from an autonomous CO2 mooring measuring pCO2 and modeled pHtotal every 3-hours highlights the dynamic temporal variability in coral reef NEC. This ability for chemistry-based NEC techniques to capture higher frequency variability in coral reef NEC allows the mechanisms driving NEC variability to be explored and tested. Just four coral species, Diploria labyrinthiformis, Pseudodiploria strigosa, Millepora alcicornis, and Orbicella franksi, were identified by the census-based NEC as contributing to 94±19% of the total calcium carbonate production at Hog Reef suggesting these species should be highlighted for conservation to preserve current calcium carbonate production rates at Hog Reef. As coral cover continues to decline globally, the agreement between these NEC estimates suggest that either method, but ideally both methods, may serve as a useful tool for coral reef managers and conservation scientists to monitor the maintenance of coral reef structure and ecosystem services.

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.

Environmental controls on modern scleractinian coral and reef-scale calcification

In situ coral calcification was primarily controlled by temperature and relatively insensitive to seawater CO2 chemistry. Modern reef-building corals sustain a wide range of ecosystem services because of their ability to build calcium carbonate reef systems. The influence of environmental variables on coral calcification rates has been extensively studied, but our understanding of their relative importance is limited by the absence of in situ observations and the ability to decouple the interactions between different properties. We show that temperature is the primary driver of coral colony (Porites astreoides and Diploria labyrinthiformis) and reef-scale calcification rates over a 2-year monitoring period from the Bermuda coral reef. On the basis of multimodel climate simulations (Coupled Model Intercomparison Project Phase 5) and assuming sufficient coral nutrition, our results suggest that P. astreoides and D. labyrinthiformis coral calcification rates in Bermuda could increase throughout the 21st century as a result of gradual warming predicted under a minimum CO2 emissions pathway [representative concentration pathway (RCP) 2.6] with positive 21st-century calcification rates potentially maintained under a reduced CO2 emissions pathway (RCP 4.5). These results highlight the potential benefits of rapid reductions in global anthropogenic CO2 emissions for 21st-century Bermuda coral reefs and the ecosystem services they provide.

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.

Brooding Corals: Planulation Patterns, Larval Behavior, and Recruitment Dynamics in the Face of Environmental Change

The brooding reproductive mode in scleractinian corals is often associated with high recruitment success facilitating replenishment of populations following disturbance events. Thus, if conditions continue to deteriorate on coral reefs from anthropogenic impacts and global climate change, a clear understanding of patterns of planulation, larval behavior and recruitment by brooding species is needed to accurately predict future population dynamics and the overall resilience of coral reefs. Here, we review the current knowledge of these topics with specific emphasis on the effects of environmental factors and discuss implications for reproductive success and population stability. Brooding corals typically release mature larvae during planulation events that vary in synchrony on a seasonal, monthly and daily basis linked to various environmental conditions, such as seasonal sea surface temperature, the lunar and diel cycles. Release time dictates the environmental conditions that larvae will experience, such as light availability and wave action, and thus affects dispersal potential and recruitment success. Differences in larval size exist among species, as well as within broods released during the planulation events of a single species; a possible strategy to maximize the chance of fitness in unpredictable habitats. Upon release, brooded larvae are typically competent to settle within hours to days and respond to a variety of environmental cues, such as type of benthic cover and irradiance, to facilitate settlement choice. Species-specific larval photosensitivity aids in depth and substrate selection promoting survival and can ultimately influence adult distribution patterns. Studies indicate that shifts in benthic cover from environmental changes, such as algal abundance and sedimentation, may inhibit or change patterns of larval settlement, which will affect species composition and reef resilience. Research on the effects of ocean acidification and rising sea surface temperatures on the physiology, settlement and early calcification of larvae of brooding corals show mixed results, but patterns suggest that if global climate change continues at or beyond projected scenarios over the next century, recruitment may be compromised. Thus, an increased understanding of planulation patterns, larval behavior, and recruitment dynamics of brooding species will be essential for conservation and management in the face of environmental change.