The global trend of unprecedented losses in coral reefs is particularly striking in the Caribbean, where dense algal assemblages are commonly replacing corals. So far, hardly anything is known about the ecology of the dominant algal groups. The present study compiled records of Chlorophytes from nine studies in the shallow reefs of Curaçao in the years preceding the onset of coral reef decline (1908–1978) and compared them with records from three recent (2007–2009) expeditions conducted at the same and nearby study locations along the south-west coast of the island. A total of 107 species were encountered, including seven new records for Curaçao (Anadyomene saldanhae, Bryopsis hypnoides, Chaetomorpha minima, Derbesia fastigiata, Ulva flexuosa subsp. paradoxa, Ulvella scutata and Ulvella lens). Sampled material revealed a higher species number during the dry seasons than during the wet seasons, indicating a seasonal variation in algal growth. Most species grew on hard substratum or were epibiotic, and 13 species were found growing on more than one substratum. Comparisons with earlier studies suggest an extension in depth range for nine species. The present work provides a comprehensive overview of the distribution of Chlorophytes of the island and can serve as an important baseline for further research on coral reef ecosystem changes.
With increasing stressors to coral reefs, defining tools that evaluate their dynamics and resilience is important to interpret system trajectories and direct conservation efforts. In this context, surveys must go beyond conven- tional monitoring approaches that focus on abundance and biomass of key groups and quantify metrics that better assess ecological processes and ecosystem trajectories. By measuring a variety of conventional (e.g. proportional cover of broad benthic groups, biomass of herbivorous fish) and complementary resilience-based metrics (e.g. algal turf height, coral recruitment rates, juvenile coral densities, herbivorous fish grazing rates), this study evaluated the ecosystem responses to community-based management in Fiji. The study was conducted across three paired tabu areas (periodically closed to fishing) and adjacent fished sites. Conventional metrics reflected no management effect on benthic or herbivorous fish assemblages. In contrast, the complementary metrics generally indicated positive effects of management, particularly within the benthos. Significant differ- ences were observed for turf height (33% lower), coral recruitment rate (159% higher) and juvenile coral density (42% higher) within areas closed to fishing compared to adjacent open reefs. In addition, turf height was in- versely related to coral recruitment and juvenile coral density, and longer turfs (≥5 mm) were more competitive in interaction with corals. These results emphasise that conventional metrics may overlook benefits of local management to inshore reefs, and that incorporating complementary resilience-based metrics such as turf height into reef survey protocols will strengthen their capacity to predict the plausible future condition of reefs and their responses to disturbances.
Disease outbreaks have been involved in the deterioration of coral reefs worldwide and have been par- ticularly striking among crustose coralline algae (CCA). Although CCA represent important cues for coral settle- ment, the impact of CCA diseases on the survival and settlement of coral planulae is unknown. Exposing coral larvae to healthy, diseased, and recently dead crusts from three important CCA species, we show a negative effect of disease in the inductive CCA species Hydrolithon boergesenii on larval survivorship of Orbicella faveolata and settlement of O. faveolata and Diploria labyrinthi- formis on the CCA surface. No effect was found with the less inductive CCA species Neogoniolithon mamillare and Paragoniolithon accretum. Additionally, a majority of planulae that settled on top of diseased H. boergesenii crusts were on healthy rather than diseased/dying tissue. Our experiments suggest that CCA diseases have the po- tential to reduce the survivorship and settlement of coral planulae on coral reefs.
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 yr. Permanent 9 m2quadrats at 10, 20, 30 and 40 m depth were photographed at 3- to 6-yr 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 yr 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 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.
Despite the increasing dominance of turf algae in coral reefs, few studies have investigated their physiological and ecological responses to changes in abiotic factors. We tested the effects of depth and ultraviolet radiation on turf algae at different levels of successional stages using two experiments. Depth-related differences were found for all turf algal communities, characterized by a higher amount of the cyanobacteria taxonDichothrix and the red filamentous genera Poly-/Herposiphonia in the shallow and the appearance of oscillating cyanobacteria in deeper waters. In the first experiment, cross-depth transplantation of 153 days old communities influenced percentage cover, biomass and taxa composition. Downward transplantation lowered overall biomass and abundance of the foraminifera Sorites, whereas the crustose green alga Pringsheimiellaand filamentous cyanobacteria colonized the communities. A nearly reverse pattern was observed in upward transplanted communities. Overall we distinguished between sensitive taxa, like Oscillatoria, and taxa able to acclimate to alterations in their environment, like Pringsheimiella, Poly/Herposiphonia and Dichothrix. In the second experiment, algae grown for 285 days at 5 m were exposed together with a set of sterile settlement tiles to three UVR regimes at 2 m for 22 days. UVR had no effect on turf algal communities regardless of successional stage. This study highlights the presence of high light and UV tolerant species. The high UV tolerance of turf communities may confer a competitive advantage over other more sensitive coral reef biota, such as corals. This study demonstrates that turf algae are dynamic communities exhibiting species-specific resistance to environmental changes.
Benthic cyanobacterial mats (BCMs) are increasing in abundance on coral reefs worldwide. However, their impacts on biogeochemical cycling in the surrounding water and sediment are virtually unknown. By measuring chemical fluxes in benthic chambers placed over sediment covered by BCMs and sediment with BCMs removed on coral reefs in Curaçao, Southern Caribbean, we found that sediment covered by BCMs released 1.4 and 3.5 mmol C m(-2) h(-1) of dissolved organic carbon (DOC) during day and night, respectively. Conversely, sediment with BCMs removed took up DOC, with day and night uptake rates of 0.9 and 0.6 mmol C m(-2) h(-1). DOC release by BCMs was higher than reported rates for benthic algae (turf and macroalgae) and was estimated to represent 79% of the total DOC released over a 24 h diel cycle at our study site. The high nocturnal release of DOC by BCMs is most likely the result of anaerobic metabolism and degradation processes, as shown by high respiration rates at the mat surface during nighttime. We conclude that BCMs are significant sources of DOC. Their increased abundance on coral reefs will lead to increased DOC release into the water column, which is likely to have negative implications for reef health.
Abstract Distribution and abundance of coral diseases have been well documented, but only a few studies con- sidered diseases affecting crustose coralline algae (CCA), particularly at the species level. We investigated the spa- tiotemporal dynamics of diseases affecting CCA along the south coast of Curac ̧ao, southern Caribbean. Two syn- dromes were detected: the Coralline White Band Syndrome (CWBS) previously described and the Coralline White Patch Disease (CWPD) reported here for the first time. Diseases were present at all six study sites, and our results did not reveal a relationship between disease occurrence and human influence. Both diseases were more prevalent on the shallower reef flat than on the deeper reef slope, and during the warm/rainy season than during the cold/dry season. The patterns observed were consistent with a positive link between temperature and disease occurrence. Reef flat communities were dominated by Neogoniolithon mamillare and Paragoniolithon solubile, whereas deeper habitats were dominated by Hydrolithon boergesenii. Dis- eases affected all the species encountered, and no prefer- able host was detected. There was a significant relationship between both disease occurrences and CCA cover. Moni- toring of affected patches revealed that 90 % of lesions in CWBS increased in size, whereas 88 % of CWPD lesions regenerated over time. CWBS linear progression rate did not vary between seasons or species and ranged from 0.15 to 0.36 cm month-1, which is in the same order of mag- nitude as rates previously documented. We conclude that diseases have the potential to cause major loss in CCA cover, particularly in shallow waters. As CCA play a key role in reef ecosystems, our study suggests that the emer- gence of diseases affecting these algae may pose a real threat to coral reef ecosystems. The levels of disease reported here will provide a much-needed local baseline allowing future comparisons.
Benthic cyanobacterial mats (BCMs) are impacting coral reefs worldwide. However, the factors and mechanisms driving their proliferation are unclear. We conducted a multi-year survey around the Caribbean island of Curaçao, which revealed highest BCM abundance on sheltered reefs close to urbanised areas. Reefs with high BCM abundance were also characterised by high benthic cover of macroalgae and low cover of corals. Nutrient con- centrations in the water-column were consistently low, but markedly increased just above substrata (both sandy and hard) covered with BCMs. This was true for sites with both high and low BCM coverage, suggesting that BCM growth is stimulated by a localised, sub- strate-linked release of nutrients from the microbial degradation of organic matter. This hy- pothesis was supported by a higher organic content in sediments on reefs with high BCM coverage, and by an in situ experiment which showed that BCMs grew within days on sedi- ments enriched with organic matter (Spirulina). We propose that nutrient runoff from urban- ised areas stimulates phototrophic blooms and enhances organic matter concentrations on the reef. This organic matter is transported by currents and settles on the seabed at sites with low hydrodynamics. Subsequently, nutrients released from the organic matter degra- dation fuel the growth of BCMs. Improved management of nutrients generated on land should lower organic loading of sediments and other benthos (e.g. turf and macroalgae) to reduce BCM proliferation on coral reefs.
Lobophora variegata occurs in the eulittoral zone and in deep water on coral reefs in Curaçao. An analysis of the long-term (1979–2006) changes in the vertical distribution of the macroalga in permanent quadrats indicated a significant increase in cover of the deepwater community. In 1998, Lobophora covered 1 and 5% of the quadrats at 20 and 30 m, respectively. By 2006, these values had risen to 25 and 18%, precipitating a shift in abundance of corals and macroalgae at both depths. This increase coincided with losses in coral cover, possibly linked to bleaching, disease and storm-related mortality in deep water plating Agaricia corals. In contrast, macroalgae remained relatively rare (<6% cover) on shallower (10 m) and deeper (40 m) reefs despite declines in coral cover also occurring at these depths, illustrating the depth-dependent dynamics of coral reefs. Several hypotheses are suggested to explain these changes.