Fleur C. van Duyl

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.

Excavating sponges are among the most important macro-eroders of carbonate substrates in marine systems. Their capacity to remove substantial amounts of limestone makes these animals significant players that can unbalance the reef carbonate budget of tropical coral reefs. Nevertheless, excavating sponges are currently rarely incorporated in standardized surveys and experimental work is often restricted to a few species. Here were provide chemical and mechanical bioerosion rates for the six excavating sponge species most commonly found on the shallow reef of Curaçao (southern Caribbean): Cliona caribbaea, C. aprica, C. delitrix, C. amplicavata, Siphonodictyon brevitubulatum and Suberea flavolivescens. Chemical, mechanical and total bioerosion rates were estimated based on various experimental approaches applied to sponge infested limestone cores. Conventional standing incubation techniques were shown to strongly influence the chemical dissolution signal. Final rates, based on the change in alkalinity of the incubation water, declined significantly as a function of incubation time. This effect was mitigated by the use of a flow-through incubation system. Additionally, we found that mechanically removed carbonate fragments collected in the flow-through chamber (1 h) as well as a long-term collection method (1 wk) generally yielded comparable estimates for the capacity of these sponges to mechanically remove substratum. Observed interspecific variation could evidently be linked to the adopted boring strategy (i.e. gallery-forming, cavity-forming or network-working) and presence or absence of symbiotic zooxanthellae. Notably, a clear diurnal pattern was found only in species that harbour a dense photosymbiotic community. In these species chemical erosion was substantially higher during the day. Overall, the sum of individually acquired chemical and mechanical erosion using flow-through incubations was comparable to rates obtained gravimetrically. Such consistency is a first in this field of research. These findings support the much needed confirmation that, depending on the scientific demand, the different approaches presented here can be implemented concurrently as standardized methods.

One mechanism giving fleshy algae a competitive advantage over corals during reef degradation is algal-induced and microbially-mediated hypoxia (typically less than 69.5 µmol oxygen L⁻¹). During hypoxic conditions oxygen availability becomes insufficient to sustain aerobic respiration in most metazoans. Algae are more tolerant of low oxygen conditions and may outcompete corals weakened by hypoxia. A key question on the ecological importance of this mechanism remains unanswered: How extensive are local hypoxic zones in highly turbulent aquatic environments, continuously flushed by currents and wave surge? To better understand the concert of biological, chemical, and physical factors that determine the abundance and distribution of oxygen in this environment, we combined 3D imagery, flow measurements, macro- and micro-organismal abundance estimates, and experimentally determined biogenic oxygen and carbon fluxes as input values for a 3D bio-physical model. The model was first developed and verified for controlled flume experiments containing coral and algal colonies in direct interaction. We then developed a three-dimensional numerical model of an existing coral reef plot off the coast of Curaçao where oxygen concentrations for comparison were collected in a small-scale grid using fiberoptic oxygen optodes. Oxygen distribution patterns given by the model were a good predictor for in situ concentrations and indicate widespread localized differences exceeding 50 µmol L^-1 over distances less than a decimeter. This suggests that small-scale hypoxic zones can persist for an extended period of time in the turbulent environment of a wave- and surge- exposed coral reef. This work highlights how the combination of three-dimensional imagery, biogenic fluxes, and fluid dynamic modeling can provide a powerful tool to illustrate and predict the distribution of analytes (e.g., oxygen or other bioactive substances) in a highly complex system.

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