Fleur C. van Duyl

Dissolution of a submarine carbonate platform by a submerged lake of acidic seawater

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.

Date
2021
Data type
Scientific article
Theme
Research and monitoring
Document
Journal
Geographic location
Saba
Saba bank

Restoration of Resilience of Nature and Society in the Caribbean Netherlands

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*

 

Date
2021
Data type
Media
Theme
Education and outreach
Research and monitoring
Geographic location
Aruba
Bonaire
Curacao
Saba
Saba bank
St. Eustatius
St. Maarten

Saba Bank: a scientific surprise

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.

Date
2021
Data type
Media
Theme
Research and monitoring
Geographic location
Saba bank

Extreme spatial heterogeneity in carbonate accretion potential on a Caribbean fringing reef linked to local human disturbance gradients

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

 

Date
2018
Data type
Scientific article
Theme
Education and outreach
Research and monitoring
Document
Geographic location
Bonaire