Erik H. Meesters

Summary

Habitat mapping is crucial for understanding habitat connectivity and for spatial planning, environmental management, conservation, and targeted research, including long-term change monitoring. However, such information has been lacking for many Dutch Caribbean islands, especially regarding marine habitats. This study used 2144 georeferenced images from different surveys to develop habitat models predicting the distribution of habitat types within the Saba Bank National Park. The habitat models link environmental factors to species or habitat occurrence, enabling predictions in unsurveyed areas with known covariates. Machine learning techniques (Random Forests, Gradient Boosting, and weighted K Nearest Neighbor) were applied to interpret and predict ten habitat types over the Bank. Three models were created for each technique: 1) utilizing only geographic coordinates; 2) incorporating covariables such as depth, distance to the edge of the Bank, Topographic Position Index (TPI), and Terrain Ruggedness index (TRI); 3) a combination of the previous two models. All models performed well, accurately predicting habitat types between 67 and 74% of the georeferenced images. However, the most natural representation occurred with models combining geographic and covariate variables. Predicted habitats include coral reef, patch reef, gorgonian reef, sargassum fields, cyanobacteria-dominated fields, Lobophora fields, Neogoniolithon- Lyngbya habitat, other macroalgae fields, sand with a mix of species, and bare sand. Habitat distribution appears to be related to the main currents in the area and depth, with coral reefs occurring mainly along the southern and eastern edge of the Bank, with gorgonians and other soft corals dominating there the shallow areas. Macroalgae, including fields of Sargassum, dominate the back-reef area. Extensive sand plains dominate the center of the Bank, and along the north-western and northern edge of the Bank, between 40 and 60m depth Lobophora fields can occur. In the south-eastern back reef area a number of mounds built up by the coralline alga Neogoniolithon occur. The Luymes Bank, the northeastern part of the Saba Bank, was the only area that was not correctly predicted, indicating that additional field-based observations are needed to refine results in this area.
 

This study aimed to identify ocean-and land-based sources of nutrients to the coral reef communities surrounding the Southern Caribbean islands Aruba, Bonaire, and Curaçao. The composition of water masses around these islands were assessed to depths up to 300 meters and three distinct overlying water masses were identified, separated by mixing zones. A fluctuating pycnocline separating surface from deeper (> ~50 m) water indicated the presence of internal waves. Nutrient profiles were typical of tropical waters with oligotrophic waters occurring above the pycnocline and a deep chlorophyll maximum (DCM) just below it (~65 m). Concentrations of dissolved nutrients differed among islands. Inorganic nitrogen (DIN) and phosphate concentrations were respectively lowest around Bonaire and Curaçao. The spatial distribution of chlorophyll-a (indicative of phytoplankton abundance), rather than nutrient concentrations, suggested the presence of higher-than-average nutrient concentrations in islands with higher population densities, near urbanized/ industrial areas, and near upwelling areas.

Abstract.

Submarine sinkholes are found on carbonate platforms around the world. They are thought to form and grow when 15 groundwater interactions generate conditions corrosive to carbonate minerals. Because their morphology can restrict mixing and water exchange, the effects of biogeochemical processes can accumulate such that the sinkhole water properties considerably diverge from the surrounding ocean. Studies of sinkhole waters can therefore reveal new insights into marine biogeochemical cycles, thus sinkholes can be considered as ‘natural laboratories’ where the response of marine ecosystems to environmental variations can be investigated. We conducted the first measurements in recently discovered sinkholes on 20 Luymes Bank, part of Saba Bank in the Caribbean Netherlands. Our measurements revealed a plume of gas bubbles rising from the seafloor in one of the sinkholes, which contained a constrained body of dense, low-oxygen ([O2] = 60.2 ± 2.6 μmol·kg−1), acidic (pHT = 6.24 ± 0.01) seawater that we term the ‘acid lake’. Here, we investigate the physical and biogeochemical processes that gave rise to and sustain the acid lake, the chemistry of which is dominated by the bubble plume. We determine the provenance and fate of the acid lake’s waters, which we deduce must be continuously flowing 25 through. We show that the acid lake is actively dissolving the carbonate platform, so the bubble plume may provide a novel mechanism for submarine sinkhole formation and growth. It is likely that the bubble plume is ephemeral and that other currently non-acidic sinkholes on Luymes Bank have previously experienced ‘acid lake’ phases. Conditions within the acid lake are too extreme to represent coming environmental change on human timescales but in some respects reflect the bulk ocean billions of years ago. Other Luymes Bank sinkholes host conditions analogous to projections for the end of the 21st 30 century and could provide a venue for studies on the impacts of anthropogenic CO2 uptake by the ocean.

Poster

Significance and Relevance

Multiple stressors (e.g., pollution, eutrophication, sedimentation, coastal development, overfishing, coral disease, ocean warming, and ocean acidification) are threatening the health and survival of coral reef ecosystems globally • Healthy coral reefs are more resistant to adverse effects of multiple-stressors • Guidance is needed to apply reef resilience to support coral reefs and the benefits reefs provide (ecosystem services).* • A resilience-based approach monitors the stress tolerance of coral reef ecosystems, promotes recovery and facilitate adaptation by integrating all aspects of the coupled social-ecological system*

Background

The Saba Bank, west of the Caribbean island of Saba, is a large (2400 km2) submerged carbonate platform of 15-40m depth rising from 800-1000m depth and fringed with coral reefs along the eastern and southern sides. Saba Bank is the largest protected area of the Kingdom of the Netherlands and a hotspot of biodiversity. In 2018 during the NICO expedition we discovered that part of the Saba Bank, called the Luymes Bank, contains a number of large and deep sinkholes. In 2019 NIOZ and WMR returned to the bank to study these sinkholes and made some extraordinary discoveries.

Objectives

• To study the distribution and environmental conditions (e.g. nutrients, O2, particulate organic matter, water movement, CO2 chemistry) of benthic communities on the platform between sinkholes and in the sinkholes with emphasis on areas with regularly distributed pillar-like structures in sinkholes.

• 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.

• To collect bottom samples in order to determine the species diversity of these communities.

• To collect pillars and assess the species consortia producing the pillars, their life history strategies, accretion rates and stratigraphic history.

• To survey and investigate the carbonate chemistry of sinkholes of different size and depth and detect the effects of possible stratification in sinkholes.

• To determine metagenomics and metabolomics in water samples from sinkholes of different size and depths.

• 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.

• To collect plankton samples for closer studies of plankton communities over the Luymes Bank.

Abstract
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 estimated
accretion rates (EAR) of the shallow reef zone of leeward Bonaire – between 5 and
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 EAR varied 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), 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) or were in a state of net erosion. These
data highlight the marked spatial heterogeneity that can occur in budget 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 landbased
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 rates 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 and biodiversity,
potentially leading to 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 rates 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 above reef
water depth increases on Caribbean coral reefs and will limit the social and economic
implications associated with the loss of reef goods and services.

K E Y W O R D S
Acropora cervicornis, bioerosion, Bonaire, calcification, carbonate budget, Caribbean, climate
change, sea‐level rise