Corals

Over the past decades, numerous studies have reported declines in stony corals and, in many cases, phase shifts to fleshy macroalgae. However, long-term studies documenting changes in other benthic reef organisms are scarce. Here, we studied changes in cover of corals, algal turfs, benthic cyanobacterial mats, macroalgae, sponges and crustose coralline algae at four reef sites of the Caribbean islands of Curaçao and Bonaire over a time span of 40 years. Permanent 9 m2 quadrats at 10, 20, 30 and 40 m depth were photographed at 3 to 6-year intervals from 1973 to 2013. The temporal and spatial dynamics in the six dominant benthic groups were assessed based on image point-analysis. Our results show consistent patterns of benthic community change with a decrease in the cover of calcifying organisms across all sites and depths from 32.6% (1973) to 9.2% (2013) for corals and from 6.4% to 1% for crustose coralline algae. Initially, coral cover was replaced by algal turfs increasing from 24.5% (1973) to 38% around the early 1990s. Fleshy macroalgae, still absent in 1973, also proliferated covering 12% of the substratum approximately 20 years later. However, these new dominants largely declined in abundance from 2002 to 2013 (11% and 2%, respectively), marking the rise of benthic cyanobacterial mats. Cyanobacterial mats became the most dominant benthic component increasing from a mere 7.1% (2002) to 22.2% (2013). The observed increase was paralleled by a small but significant increase in sponge cover (0.5% to 2.3%). Strikingly, this pattern of degradation and phase change occurred over the reef slope down to upper-mesophotic depths of 40 m. These findings suggest that reefs dominated by algae may be less stable than previously thought and that the next phase may be the dominance of slimy cyanobacterial mats with some sponges.

Want to know more about this project? Check out the whole thesis.

MSc report

Land based nutrient input from anthropogenic sources is a worldwide issue and a threat to coral reef health. Corals found in high nutrient environments are less resistant to global stressors such as rising temperature and extreme weather events. The absence of sewage treatment on St. Eustatius and its erosion prone coastline suggests poor water quality. Chlorophyll-a concentrations were monitored at 11 locations around St. Eustatius twice a month from May to August 2022 (n=7). Preliminary results on spatial and temporal variation of chlorophyll-a show an overall good water quality. Average concentrations did not exceed the environmental threshold of 0.3 μg/L at any of the sampled locations. Concentrations varied between locations and over time. Highest average chla values were found close to STUCO's brine outlet, the oil terminal and Golden Rock Dive & Nature Resort. Towards the end of the sampling period, chlorophyll-a concentrations were more variable between locations, which coincided with the onset of more frequent rain showers. No heavy rainfall occurred during the sampling period, so it was not possible to study the effects of higher precipitation on chlorophyll-a concentrations. Current results do not show reason for concern. However they should be re-evaluated when data on stable isotopes and nutrient concentrations are available. Long-term monitoring that includes the rainy season will show if the water quality around St. Eustatius is suitable for coral reefs or if action is needed to improve it.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Abstract: A biodiversity survey on three corallivorous snails (Mollusca: Gastropoda) was performed at 28 sites around the island of Bonaire to assess their distribution patterns and associated host corals. The snails and their hosts were identified and counted in three depth zones: 5–10, 10–20, and 20–30 m. The snails were Coralliophila galea and C. salebrosa (Muricidae: Coralliophilinae), and Cyphoma gibbosum (Ovulidae: Simniinae). All three species were widespread around the island without apparent interspecific geographical variation. Coralliophila galea was found exclusively on scleractinian corals, Coralliophila salebrosa almost exclusively on octocorals, and Cyphoma gibbosum only on octocorals. Coralliophila salebrosa showed more dietary overlap with Cyphoma gibbosum than with Coralliophila galea. Coralliophila galea was the most commonly encountered species with the largest number of host species. Owing to its hosts distribution, this species also showed a greater maximum depth and a wider bathymetrical range than the other two snails. The other two snails were shallower and their depth ranges did not differ significantly. Host-coral size did not seem to have influence on the number of snails per host. Coral damage caused by the snails was visible but appeared to be low, causing no mortality in Bonaire, which suggests that the relation with their hosts is more parasitic than predatory. Because these three corallivores have occasionally been reported to occur as outbreaks in other Caribbean localities and may act as vectors in the dispersal of coral diseases, it is recommended that future studies should focus on their population dynamics.

Abstract

Reef-building corals are ecosystem engineers that compete with other benthic organisms for space and resources. Corals harvest energy through their surface by photosynthesis and heterotrophic feeding, and they divert part of this energy to defend their outer colony perimeter against competitors. Here, we hypothesized that corals with a larger space-filling surface and smaller perimeters increase energy gain while reducing the exposure to competitors. This predicted an association between these two geometric properties of corals and the competitive outcome against other benthic organisms. To test the prediction, fifty coral colonies from the Caribbean island of Curaçao were rendered using digital 3D and 2D reconstructions. The surface areas, perimeters, box-counting dimensions (as a proxy of surface and perimeter space-filling), and other geometric properties were extracted and analyzed with respect to the percentage of the perimeter losing or winning against competitors based on the coral tissue apparent growth or damage. The increase in surface space-filling dimension was the only significant single indicator of coral winning outcomes, but the combination of surface space-filling dimension with perimeter length increased the statistical prediction of coral competition outcomes. Corals with larger surface space-filling dimensions (Ds > 2) and smaller perimeters displayed more winning outcomes, confirming the initial hypothesis. We propose that the space-filling property of coral surfaces complemented with other proxies of coral competitiveness, such as life history traits, will provide a more accurate quantitative characterization of coral competition outcomes on coral reefs. This framework also applies to other organisms or ecological systems that rely on complex surfaces to obtain energy for competition.

Spirobranchus giganteus is a tube-dwelling polychaete more commonly known as the Christmas tree worm. S. giganteus larvae are planktonic, which is followed by benthic settlement and development into a sessile adult. It has been shown that chemical and physical cues produced by live coral and adult S. giganteus attract larvae to settlement sites. In fact, it has been postulated that there may be a mutualistic relationship between S. giganteus and live coral. However, settlement of S. giganteus on coral rubble has been noted in the field. The aim of this study is to investigate whether there is a relationship between size of S. giganteus and substratum settlement type on the coral reefs of Bonaire. The hypothesis under examination in this study states that S. giganteus settled on live coral will be larger than S. giganteus settled on rubble. The measurement chosen for this study was the diameter of the orifice of the calcareous tube, since it has been shown that this is a good estimate for overall size of the polychaete. The diameter of the orifice of S. giganteus was significantly greater for individuals living on rubble (0.510 cm) than individuals living on live coral (0.457 cm). The species of coral that S. giganteus settled on also appeared to affect the orifice diameter. Individuals on Agaricia agaricites were significantly smaller than individuals settled on both Montastrea annularis and Siderastrea siderea. These results may indicate that the interactions between S. giganteus and live coral, such as extracoelenteric digestion and sweeper tentacles, may be disruptive enough to cause a lower size distribution of polychaetes on that substrate when compared to rubble.

This student research was retrieved from Physis: Journal of Marine Science III (Spring 2008)19: 25-30 from CIEE Bonaire.

Tubastrea coccinea is an invasive coral species found on the reefs of Bonaire. These corals are typically seen at various densities (up to 80% m-2) on hard, vertical substrata suggesting that biotic resistance could be one possible biological factor preventing settlement of T. coccinea elsewhere (e.g.,,, horizontal substrata). The impact potential competitors have on the successful invasion, recruitment and growth of T. coccinea was experimentally assessed by establishing replicated 15 x 15 cm plots of substrata already inhabited by single species or combinations of native species (0-3 and 3 seeded with adult T. coccinea) at the Harbor Village jetty, Kralendijk, Bonaire, which had the necessary vertical substrata. Monitoring occurred over a period of three weeks to assess percent cover change of the studied organisms. Additionally 15 vertical, 5 m transects were run to evaluate mean percent cover of all sessile species that inhabited the surveyed locations for a general representation of species diversity at the jetty. T. coccinea was not observed to settle in any of the experimental plots nor did the seeded adult conspecifics show any evidence of growth or recruitment. Observational data indicated that an algal turf had the highest mean percent cover, but in areas around T. coccinea, algal turf percent cover decreased by almost 20%, suggesting competition between the two organisms. No firm conclusions could be drawn about T. coccinea recruitment or growth, but results suggested that the presence of an invasive species may negatively affect the growth of native species when they are found in close proximity to it.

This student research was retrieved from Physis: Journal of Marine Science IV (Fall 2008)19: 2-6 from CIEE Bonaire.

The orange cup coral Tubastraea coccinea has expanded its range from the Indo-Pacific into the western Atlantic region. It grows on a wide variety of natural and man-made substrates, including rock ledges, docks, and shipwrecks. Its early reproductive age, fast growth rate, and ability to thrive where other species cannot could potentially make T. coccinea a valid threat to native species. The goal of this study is to provide baseline data on the size, depth, and range of T. coccinea on the island of Bonaire in the Dutch Caribbean. Substrate and light intensity preferences were also investigated by estimating percent cover on a variety of substrates and light conditions. A clear preference was observed for concrete substrates and low light conditions. Interactions between T. coccinea and a variety of other coral and sponge species were also investigated for potential harmful effects, but despite documentation of harmful coral-coral interactions in Brazil, no evidence of T. coccinea exhibiting harmful effects on native coral species was found in the study area of Bonaire. While T. coccinea does not currently appear to exhibit negative effects on native species in Bonaire, it may simply be in the early stages of expansion and this expansion needs to be monitored for future development of harmful effects on native species.

This student research was retrieved from Physis: Journal of Marine Science IX (Spring 2011)19: 70-78 from CIEE Bonaire.

This study investigates the relationship between yellow band disease (YBD) infected Montastrea annularis coral colonies and its potential to spread by contact with transect tapes, as commonly used in research. M. annularis plays an important role in maintaining reef complexity and diversity in Bonaire as it is a structural reef-building coral, yet the recent spread of YBD has created an degrading the reefs (Bruckner 2006). The study has two primary goals, to investigate whether the use of transect lines had the potential to transfer YBD from one coral to another, and whether a simple cleaning protocol can reduce this transfer. Transects placed on the YBD infected colonies of M. annularis had the most percent bacterial growth, though not statistically significant. It also showed that even though specific species of bacteria were unable to be identified, the transect lines are indeed capable of carrying bacteria. Although the difference was not significant in this study, cleansing treatments may have an effect on lessening the growth of bacteria.

This student research was retrieved from Physis: Journal of Marine Science X (Fall 2011)19: 11-15 from CIEE Bonaire.

Yellow band disease (YBD) is a bacterial infection affecting corals of the Montastrea annularis species complex. Recent mortality rates of M. annularis spp. on Bonaire have risen due to YBD and other biotic and abiotic factors. The loss of staghorn coral, Acropora cervicornis, the preferred habitat of the threespot damselfish, Stegastes planifrons, has caused the damselfish to inhabit M. annularis spp. Unfortunately, M. annularis spp. are slower growing corals that take longer to reach reproductive maturity and are thus less able to withstand S. planifrons biting their tissues and creating algal gardens on exposed skeleton. This weakens the coral and makes it more susceptible to diseases like YBD. The objective of this study was to examine the relationship between S. planifrons gardening and YBD. Sample sites of healthy and diseased colonies of M. annularis spp. were established across depths at Yellow Sub dive site in Kralendijk, Bonaire. Sites were monitored for damselfish inhabitants and signs of coral biting. Pictures were taken of each site to chart the progress of the disease over the course of the study, and ImageJ was used to determine percent cover of healthy versus unhealthy coral. No significant relationship was found between S. planifrons activity and YBD, although S. planifrons seemed to select healthy colonies. The increase of damselfish populations and their detrimental effects on Bonaire’s reef calls attention to the need for fishing regulations of predatory species and a heavier focus on the conservation of A. cervicornis thickets.

This student research was retrieved from Physis: Journal of Marine Science XI (Fall 2012)19: 83-87 from CIEE Bonaire.

A coral is made up of key associations between endosymbiotic zooxanthellae, protists, bacteria, archaea, viruses, and fungi. These microbe-coral interactions can be very beneficial, some associations providing key functions in reproduction, nutrition, and antimicrobial protection. However, as a coral becomes thermally stressed, the ability to regulate microbe growth in its surface mucus layer becomes diminished and opportunistic pathogens are able to colonize. Corals may be able to adapt for the changing reef ecosystem by selecting for more beneficial associations: one of the facets of the coral probiotic hypothesis. The invasive azooxanthellate coral Tubastraea coccinea is able to colonize very shallow, hot and turbid areas that are not favorable for settlement by other species. However, not much is known about T. coccinea other than its invasive nature in the Caribbean. The purpose of this investigation was to determine if the surface microbial communities of T. coccinea are one of the factors aiding its survival. Culturing on Thiosulfate Citrate Bile Salt plates was used to visualize and compare the overall culturable Vibrio spp. communities present in T. coccinea and other widespread shallow corals. At each site, the numbers of Vibrio spp. were not significantly different between the three species, but numbers of a gram-positive bacteria, Enterococcus spp., were found to be significantly higher in T. coccinea.

This student research was retrieved from Physis: Journal of Marine Science XII (Fall 2012)19: 73-79 from CIEE Bonaire.