Duyl, F. van

The Saba Bank is a large (ca 2400 km2 ) submerged carbonate platform of 15-40m depth rising up from 800-1000m depth and fringed with coral reefs. It extends into a carbonate peninsula of ca 80m deep (Luymes Bank) which is pockmarked by sinkholes. More than twenty drowned sinkholes were distinguished in this peninsula based on available bathymetric data. Diameters of sinkholes vary from 70 to 1100 m and depths ranges between 10-300m. The area of the Luymes Bank with sinkholes is ca 66 km2 . During the NICO cruise in 2018 two sinkholes were visited in the Luymes Bank. In one of the two shallow sinkholes, which were only briefly explored with camera’s in 2018, we found peculiar pillar-like, probably calcium carbonate accretions with diameters of 40-60cm and protruding up to 1m from the sandy bottom. Pillars were found to stand neatly ordered on the bottom at a depth of ca 110 m. Based on the pink color on top, pillars look like features formed by crustose coralline algae of unusual size and density, almost in a stromatolitic fashion. In the second sinkhole such pillar-like structures were not found. Very little is known about these structures, their distribution and the conditions under which they are formed. Moreover, no information is available of the benthic communities and environmental conditions in the very deep sinkholes of more than 150m m depth. Therefore, the sinkhole expedition was completely dedicated to the sinkholes and the platform in which they occur (Luymes Bank).

The aims of the expedition were:

1. To study the distribution and environmental conditions (e.g. nutrients O2, particulate organic matter, water movement) of benthic communities on the platform between sinkholes and in the sinkholes with emphasis on areas with regularly distributed pillar-like structures in sinkholes.
2. To take high resolution pictures of the benthic communities with high-resolution camera system and NIOZ video frame in order to describe the benthic communities.
3. To collect bottom samples in order to determine the species diversity of these communities.
4. To collect pillars and assess the species consortia producing the pillars, their life history strategies, accretion rates and stratigraphic history.
5. To survey and investigate the carbonate chemistry of sinkholes of different size and depth and detect the effects of possible stratification in sinkholes.
6. To determine metagenomics and metabolomics in water samples from sinkholes of different size and depths.
7. To investigate Light-Dark shifts in metagenomics and metabolomics in near bottom water samples in relation to nutrients, O2, carbonate chemistry and POM in shallow sinkholes (20- 40m deep) with and without pillar-like structure and the platform community at approx. 80m depth.
8. To collect plankton samples for closer studies of plankton communities over the Luymes Bank

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

Sponges are ubiquitous on coral reefs, mostly long lived and therefore adaptive to changing environmental conditions. They feed on organic matter withdrawn from the passing water and they may harbor microorganisms (endosymbionts), which contribute to their nutrition. Their diets and stable isotope (SI) fractionation determine the SI signature of the sponge holobiont. Little is known of spatio–temporal variations in SI signatures of d13C and d15N in tropical sponges and whether they reflect variations in the environment. We investigated the SI signatures of seven common sponge species with different functional traits and their potential food sources between 15 and 32 m depth along the S-SE and E-NE side of the Saba Bank, Eastern Caribbean, in October 2011 and October 2013. SI signatures differed significantly between most sponge species, both in mean values and in variation, indicating different food preferences and/or fractionation, inferring sponge species-specific isotopic niche spaces. In 2011, all sponge species at the S-SE side were enriched in d13C compared to the E-NE side. In 2013, SI signatures of sponges did not differ between the two sides and were overall lighter in d13C andd15N than in 2011. Observed spatio–temporal changes in SI in sponges could not be attributed to changes in the SI signatures of their potential food sources, which remained stable with different SI signatures of pelagic (particulate organic matter (POM): d13C -24.9‰, d15N +4.3‰) and benthic-derived food (macroalgae: d13C-15.4‰, d15N +0.8‰). Enriched d13C signatures in sponges at the S-SE side in 2011 are proposed to be attributed to predominantly feeding on benthic-derived C.

This interpretation was supported by significant differences in water mass constituents between sides in October 2011. Elevated NO3 and dissolved organic matter concentrations point toward a stronger reef signal in reef overlying water at the S-SE than N-NE side of the Bank in 2011. The depletions of d13C and d15N in sponges in October 2013 compared to October 2011 concurred with significantly elevated POM concentrations. The contemporaneous decrease in d15N suggests that sponges obtain their N mostly from benthic-derived food with a lower d15N than pelagic food. Average proportional feeding on available sources varied between sponge species and ranged from 20% to 50% for benthic and 50% to 80% for pelagic-derived food, assuming trophic enrichment factors of 0.5‰ ± sd 0.5 for d13C and 3‰ ± sd 0.5 for d15N for sponges. We suggest that observed variation of SI in sponges between sides and years were the result of shifts in the proportion of ingested benthic- and pelagic-derived organic matter driven by environmental changes. We show that sponge SI signatures reflect environmental variability in space and time on the Saba Bank and that SI of sponges irrespective of their species-specific traits move in a similar direction in response to these environmental changes.