Bakker, D. de

Coral reefs are declining worldwide. The abundance of corals has decreased alongside the rise of filter feeders, turf and algae in response to intensifying human pressures. This shift in prevalence of functional groups alters the biogeochemical processes in tropical water ecosystems, thereby influencing reef biological functions. An urgent challenge is to understand the 15 functional consequences of these shifts in order to develop suitable management strategies that aim at preserving the biological functions of reefs. Here, we quantify biogeochemical processes supporting key reef functions (i.e. net community calcification (NCC) and production (NCP), and nutrient recycling) in situ for five different benthic assemblages currently dominating shallow degraded Caribbean reef habitats. To this end, a custom made tent was placed over communities dominated by either one of five 20 functional groups: coral, turf and macroalgae, bioeroding sponges, cyanobacterial mats or sand, to determine chemical fluxes between these communities and the overlying water, during both day and night. Measured fluxes were then translated into responsible biogeochemical processes by solving a system of differential equations describing the contribution of each process to the measured chemical fluxes. Estimated processes are low compared to those known for reef flats worldwide. No real gain in primary habitat is recorded, with negative or very modest net community calcification rates by all communities. Similarly, 25 net production of biomass through photosynthesis is relatively low during the day and remineralisation of organic matter at night is relatively high in comparison, resulting in net heterotrophy over the survey period by most communities. Estimated recycling through nitrification and denitrification are high but denitrification does not fully counterbalance nutrient release from aerobic mineralisation, rendering all substrates sources of nitrogen. A multivariate pairwise analysis revealed that there is no significant difference between processes occurring on any of the assemblages, suggesting functional homogenisation 30 between distinct substrate types. We infer that the amount and type of organic matter released by abundant algal turfs and cyanobacterial mats on this reef, likely enhances heterotroph activity, and stimulates the proliferation of less diverse copiotrophic microbial populations, rendering the studied reef net heterotrophic and the overall biogeochemical ‘behaviour’ similar regardless of substrate type.

Anthropogenic pressures threaten the health of coral reefs globally. Some of these pressures directly affect coral functioning, while others are indirect, for example by promoting the capacity of bioeroders to dissolve coral aragonite. To assess the coral reef status, it is necessary to validate community-scale measurements of metabolic and geochemical processes in the field, by determining fluxes from enclosed coral reef patches. Here, we investigate diurnal trends of carbonate chemistry, dissolved organic carbon, oxygen, and nutrients on a 20 m deep coral reef patch offshore from the island of Saba, Dutch Caribbean by means of tent incubations. The obtained trends are related to benthic carbon fluxes by quantifying net community calcification (NCC) and net community production (NCP). The relatively strong currents and swell-induced near-bottom surge at this location caused minor seawater exchange between the incubated reef and ambient water. Employing a compensating interpretive model, the exchange is used to our advantage as it maintains reasonably ventilated conditions, which conceivably prevents metabolic arrest during incubation periods of multiple hours. No diurnal trends in carbonate chemistry were detected and all net diurnal rates of production were strongly skewed towards respiration suggesting net heterotrophy in all incubations. The NCC inferred from our incubations ranges from −0.2 to 1.4 mmol CaCO3 m−2 h−1 (−0.2 to 1.2 kg CaCO3 m−2 year−1) and NCP varies from −9 to −21.7 mmol m−2 h−1 (net respiration). When comparing to the consensus-based ReefBudget approach, the estimated NCC rate for the incubated full planar area (0.36 kg CaCO3 m−2 year−1) was lower, but still within range of the different NCC inferred from our incubations. Field trials indicate that the tent-based incubation as presented here, coupled with an appropriate interpretive model, is an effective tool to investigate, in situ, the state of coral reef patches even when located in a relatively hydrodynamic environment.

The capacity of coral reefs to maintain their structurally complex frameworks and to retain the potential for vertical accretion is vitally important to the persistence of their ecological functioning and the ecosystem services they sustain. However, datasets to support detailed along-coast assessments of framework production rates and accretion potential do not presently exist. Here we estimate, based on gross bioaccretion and bioerosion measures, the carbonate budgets and resultant maximum accretion potential (RAPmax) of the shallow reef zone of leeward Bonaire – between 5 to 12 m depth – at unique fine spatial resolution along this coast (115 sites). Whilst the fringing reef of Bonaire is often reported to be in a better ecological condition than most sites throughout the wider Caribbean region, our data show that the carbonate budgets of the reefs and derived RAPmax rates varied3 considerably across this ~58 km long fringing reef complex. Some areas, in particular the marine reserves, were indeed still dominated by structurally complex coral communities with high net carbonate production (> 10 kg CaCO3 m-2 year-1 35 ), high live coral cover and complex structural topography. The majority of the studied sites, however, were defined by relatively low budget states (< 2 kg CaCO3 m-2 year-1 36 ) or were in a state of net erosion. These data highlight the marked spatial heterogeneity that can occur in budgets states, and thus in reef accretion potential, even between quite closely spaced areas of individual reef complexes. This heterogeneity is linked strongly to the degree of localized land-based impacts along the coast, and resultant differences in the abundance of reef framework building coral species. The major impact of this variability is that those sections of reef defined by low-accretion potential will have limited capacity to maintain their structural integrity and to keep pace with current projections of climate change induced sea-level rise (SLR), thus posing a threat to reef functioning, biodiversity and trophic cascades. Since many Caribbean reefs are more severely degraded than those found around Bonaire, it is to be expected that the findings presented here are rather the rule than the exception, but the study also highlights the need for similar high spatial resolution (along-coast) assessments of budget states and accretion potential to meaningfully explore increasing coastal risk at the country level. The findings also more generally underline the significance of reducing local anthropogenic disturbance and restoring framework-building coral assemblages. Appropriately focussed local preservation efforts may aid in averting future large-scale submergence of Caribbean coral reefs and will constrain the social and economic implications associated with the loss of reef goods and services.
Read more at https://ore.exeter.ac.uk/repository/handle/10871/38706#2F5gySpesqKcswbw.99

Excavating sponges are among the most important macro-eroders of carbonate substrates in marine systems. Their capacity to remove substantial amounts of limestone makes these animals significant players that can unbalance the reef carbonate budget of tropical coral reefs. Nevertheless, excavating sponges are currently rarely incorporated in standardized surveys and experimental work is often restricted to a few species. Here were provide chemical and mechanical bioerosion rates for the six excavating sponge species most commonly found on the shallow reef of CuracËao (southern Caribbean): Cliona caribbaea, C. aprica, C. delitrix, C. amplicavata, Siphonodictyon brevitubulatum and Suberea flavolivescens. Chemical, mechanical and total bioerosion rates were estimated based on various experimental approaches applied to sponge infested limestone cores. Conventional standing incubation techniques were shown to strongly influence the chemical dissolution signal. Final rates, based on the change in alkalinity of the incubation water, declined significantly as a function of incubation time. This effect was mitigated by the use of a flow-through incubation system. Additionally, we found that mechanically removed carbonate fragments collected in the flow-through chamber (1 h) as well as a long-term collection method (1 wk) generally yielded comparable estimates for the capacity of these sponges to mechanically remove substratum. Observed interspecific variation could evidently be linked to the adopted boring strategy (i.e. gallery-forming, cavity-forming or network-working) and presence or absence of symbiotic zooxanthellae. Notably, a clear diurnal pattern was found only in species that harbour a dense photosymbiotic community. In these species chemical erosion was substantially higher during the day. Overall, the sum of individually acquired chemical and mechanical erosion using flow-through incubations was comparable to rates obtained gravimetrically. Such consistency is a first in this field of research. These findings support the much needed confirmation that, depending on the scientific demand, the different approaches presented here can be implemented concurrently as standardized methods.

From 19-27 October 2013, IMARES (Wageningen UR) organized a research expedition to the Saba Bank, to investigate the ecological functioning of the Bank. The expedition is a follow up of a survey of the bank in 2011 and is part of the “The Saba Bank Research Program 2011-2016” initiated by the Dutch Ministry of Economic Affairs (EZ). The bank is the largest submarine atoll in the Caribbean Sea, spanning an area of 2200km2. It is a Marine Protected Area and is acknowledged by the Convention of Biological Diversity as an Ecologically and Biologically Significant Area. The project is part of the implementation of the Exclusive Economic Zone management plan for the Dutch Caribbean.

If the Saba Bank is to serve as a source of healthy larvae for the neighboring reefs, a key question is how populations of reef organisms on the bank are connected with populations in the region and in the Wider Caribbean. The aim of the current report is to investigate the health status and the population genetic structure of two common native benthic species, Xestospongia muta (giant barrel sponge) and Montastrea cavernosa (great star coral), and an invasive species, Pterois volitans (lionfish). With the aidof molecular techniques and species assessments, we aim to assess:

• the level of genetic diversity within the populations of two common benthic species (X. muta and M. cavernosa) on the Saba Bank;
• the degree of genetic connectivity between populations on Saba Bank and surrounding reefs, based on newly obtain genetic sequences and sequences obtained from GenBank from populations across the Wider Caribbean;
• the current density and health status of the populations of X. muta and M. cavernosa on Saba Bank.
• the genetic connectivity, population size and the dispersal direction of the invasion of the lionfish on the Saba Bank, in relation to the Eastern Caribbean populations. 

To follow the health of the coral reefs of Bonaire and Curacao a number of locations on each island are photographed each year since 1973. At each location a 3 m square at up to 4 different depths is recorded and analysed.

Please contact Wageningen Marine Research for more information.

Tropical coral reefs are among the most biologically diverse and economically important ecosystems on earth. Nevertheless, we found dramatic changes in coral communities on the reef slopes of Curaçao and Bonaire since 1973. Cover and abundance declined for virtually all coral species. The data show a shift from communities dominated by framework building species (e.g., Orbicella spp.) to communities consisting of small opportunistic, phenotypically plastic, species, including few remaining structural colonies. Madracis mirabilis, Porites astreoides, Diploria strigosa, and Agaricia lamarcki are at present modest winners in the coral assemblage, although overall cover declined also for these species. Increased frequency and intensity of events inducing coral mortality and ongoing reduction in suitable hard substratum, provided by the remnants of large colony building species, could reduce the chance of these species to remain winners in the longer run. The observed loss in coral cover and the shift from larger structural to smaller opportunistic species reduced reef carbonate production by 67% and therewith, in combination with a trend toward smaller coral colonies, reef complexity. Alarmingly, reefs at upper-mesophotic depths (30–40 m) did not escape the general degradation of the coral community. The negative effects are larger around densely populated areas where local stressors are adding to degradation caused, for instance, by region wide mass bleaching. Without proper conservation and management this already dramatic degradation will continue and turn more and more coral species into losers.

Saba Bank, a submerged atoll in the Caribbean Sea with an area of 2,200 km2, has attained international conservation status due to the rich diversity of species that reside on the bank. In order to assess the role of Saba Bank as a potential reservoir of diversity for the surrounding reefs, we examined the population genetic structure, abundance and health status of two prominent benthic species, the coral Montastraea cavernosa and the sponge Xestospongia muta. Sequence data were collected from 34 colonies of M. cavernosa (nDNA ITS1-5.8S-ITS2; 892 bp) and 68 X. muta sponges (mtDNA I3-M11 partition of COI; 544 bp) on Saba Bank and around Saba Island, and compared with published data across the wider Caribbean. Our data indicate that there is genetic connectivity between populations on Saba Bank and the nearby Saba Island as well as multiple locations in the wider Caribbean, ranging in distance from 100s–1000s km. The genetic diversity of Saba Bank populations of M. cavernosa (π = 0.055) and X. muta (π = 0.0010) was comparable to those in other regions in the western Atlantic. Densities and health status were determined along 11 transects of 50 m2 along the south-eastern rim of Saba Bank. The densities of M. cavernosa (0.27 ind. m-2, 95% CI: 0.12–0.52) were average, while the densities of X. muta (0.09 ind. m-2, 95% CI: 0.02–0.32) were generally higher with respect to other Caribbean locations. No disease or bleaching was present in any of the specimens of the coral M. cavernosa, however, we did observe partial tissue loss (77.9% of samples) as well as overgrowth (48.1%), predominantly by cyanobacteria. In contrast, the majority of observed X. muta (83.5%) showed signs of presumed bleaching. The combined results of apparent gene flow among populations on Saba Bank and surrounding reefs, the high abundance and unique genetic diversity, indicate that Saba Bank could function as an important buffer for the region. Either as a natural source of larvae to replenish genetic diversity or as a storehouse of diversity that can be utilized if needed for restoration practices.

In 2014 a part of the Curacao reef was captured by underwater photography. From this photo collection a 3D model and an ortho-photo of that part of the reef was constructed. The ortho-photo shows a unique view on the reef that cannot be captured in a single underwater photograph.

Please contact the DCBD administratorfor more information.

To follow the health of the coral reefs of Bonaire and Curacao a number of locations on each island are photographed each year since 1973. At each location a 3 m square at up to 4 different depths is recorded.

Please contact the DCBD administratorfor access to the raw digital photographs.