van Duyl, F.C.

Student Report 

The abundance of CyanoBacterial Mats (CBMs) at a depth of 50-90m was mapped along the west coast of Bonaire. Simultaneously local conditions (i.e. nutrient concentrations, temperature, salinity, pH, and light intensity) were measured and compared with locations where CBMs were absent. Most CBMs were found near Kralendijk. Differences in temperature and salinity levels between sites with and without CBMs were found, but nutrient concentrations were similar. The species composition in samples of the deep water CBMs were compared with samples of CBMs at 15m depth. All CBMs had a rich species consortia of bacteria, averagely consisting out of 7821 different species (OTUs). The composition between shallow and deep CBMs was found to be significantly different. Moreover, a gradient was seen in the abundances of cyanobacterial genera between the different locations of deep CBMs, which suggests that all deep CBMs have a comparable species composition with different abundances adapted to the local conditions. Overall, the presence of the deep CBMs seems to be related to a combination of multiple parameters: sufficient nutrients which are likely caused by eutrophication events, a relatively flat sandy bottom and low wave energy. 

keywords CyanoBacterial Mats, CBM, deep, genera, Illumina sequencing, species composition

A Commentary on
Managing Recovery Resilience in Coral Reefs Against Climate-Induced Bleaching and Hurricanes: A 15 Year Case Study From Bonaire, Dutch Caribbean

by Steneck, R. S., Arnold, S. N., Boenish, R., de León, R., Mumby, P. J., Rasher, D. B., et al. (2019) Front. Mar. Sci. 6:265. doi: 10.3389/fmars.2019.00265

A closer look at the methods of Steneck et al. (2019) reveals two issues of concern in making comparisons between the authors' estimate of coral cover and other published estimates.

• First, their method of estimating coral cover differs from conventional methods.
• Second, contrary to the sites sampled by Steneck and co-workers many of our sites have a coral cover of less than 10 percent and none are above 60%. The sites sampled by Steneck and colleagues appear to be a non-random selection of sites and as such should not have been presented as an average for Bonaire

Health of tropical coral reefs depends largely on the balance between constructive (calcification and cementation) and destructive forces (mechanical-chemical degradation). Gradual increase in dissolved CO2 and the resulting decrease in carbonate ion concentration (“ocean acidification”) in ocean surface water may tip the balance toward net mass loss for many reefs. Enhanced nutrients and organic loading in surface waters (“eutrophication”), may increase the susceptibility of coral reef and near shore environments to ocean acidification. The impacts of these processes on coral calcification have been repeatedly reported, however the synergetic effects on bioerosion rates by sponges are poorly studied. Erosion by excavating sponges is achieved by a combination of chemical dissolution and mechanical chip removal. In this study, Cliona caribbaea, a photosymbiont-bearing excavating sponge widely distributed in Caribbean reef habitats, was exposed to a range of CO2 concentrations, as well as different eutrophication levels. Total bioerosion rates, estimated from changes in buoyant weights over 1 week, increased significantly with pCO2 but not with eutrophication. Observed chemical bioerosion rates were positively affected by both pCO2 and eutrophication but no interaction was revealed. Net photosynthetic activity was enhanced with rising pCO2 but not with increasing eutrophication levels. These results indicate that an increase in organic matter and nutrient renders sponge bioerosion less dependent on autotrophic products. At low and ambient pCO2, day-time chemical rates were ~50% higher than those observed at night-time. A switch was observed in bioerosion under higher pCO2 levels, with night-time chemical bioerosion rates becoming comparable or even higher than day-time rates. We suggest that the difference in rates between day and night at low and ambient pCO2 indicates that the benefit of acquired energy from photosynthetic activity surpasses the positive effect of increased pCO2 levels at night due to holobiont respiration. This implies that excavation must cost cellular energy, by processes, such as ATP usage for active Ca2+ and/or active proton pumping. Additionally, competition for dissolved inorganic carbon species may occur between bioerosion and photosynthetic activity by the symbionts. Either way, the observed changing role of symbionts in bioerosion can be attributed to enhanced photosynthetic activity at high pCO2 levels.

During a recent reef coral survey at the submarine Saba Bank (Eastern Caribbean), an uncommon and diverse assemblage of unattached scleractinian corals (coralliths) was encountered, which has not been reported from the Atlantic before. Four different types of these free-living (unattached) corals were distinguished. They were observed on a relatively flat seafloor (15–20 m deep) with poor coral cover and full exposure to oceanic swell. Much of the substratum was not consolidated and consisted mainly of sand and fragments of branching coralline algae. One of the four types is the (1) anthocyathus stage in the life history of the free-living species Manicina areolata and Meandrina danae. The other three are coralliths formed as ecophenotypic varieties: (2) spheroidal–amoeboidal (= globular and (sub)massive) in Porites astreoides, Siderastrea radians, S. siderea, and Stephanocoenia intersepta; (3) tumbleweed-like (= globular and ramose) in Porites divaricata and P. furcata; and (4) discoidal (flat and circular with short branches) in Madracis decactis and possibly in M. cf. auretenra. This assemblage of free-living corals is likely related to a combination of abiotic factors consisting of wave exposure (swell), depths that waves can reach, a horizontal sea floor with little relief, an unconsolidated substratum, and low coral cover.