Biogeosciences

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.

Coral reefs are declining worldwide. The abundance of corals has decreased alongside the rise of filter feeders, turf and algae in response to intensifying human pressures. This shift in prevalence of functional groups alters the biogeochemical processes in tropical water ecosystems, thereby influencing reef biological functions. An urgent challenge is to understand the 15 functional consequences of these shifts in order to develop suitable management strategies that aim at preserving the biological functions of reefs. Here, we quantify biogeochemical processes supporting key reef functions (i.e. net community calcification (NCC) and production (NCP), and nutrient recycling) in situ for five different benthic assemblages currently dominating shallow degraded Caribbean reef habitats. To this end, a custom made tent was placed over communities dominated by either one of five 20 functional groups: coral, turf and macroalgae, bioeroding sponges, cyanobacterial mats or sand, to determine chemical fluxes between these communities and the overlying water, during both day and night. Measured fluxes were then translated into responsible biogeochemical processes by solving a system of differential equations describing the contribution of each process to the measured chemical fluxes. Estimated processes are low compared to those known for reef flats worldwide. No real gain in primary habitat is recorded, with negative or very modest net community calcification rates by all communities. Similarly, 25 net production of biomass through photosynthesis is relatively low during the day and remineralisation of organic matter at night is relatively high in comparison, resulting in net heterotrophy over the survey period by most communities. Estimated recycling through nitrification and denitrification are high but denitrification does not fully counterbalance nutrient release from aerobic mineralisation, rendering all substrates sources of nitrogen. A multivariate pairwise analysis revealed that there is no significant difference between processes occurring on any of the assemblages, suggesting functional homogenisation 30 between distinct substrate types. We infer that the amount and type of organic matter released by abundant algal turfs and cyanobacterial mats on this reef, likely enhances heterotroph activity, and stimulates the proliferation of less diverse copiotrophic microbial populations, rendering the studied reef net heterotrophic and the overall biogeochemical ‘behaviour’ similar regardless of substrate type.

Abstract

In this study we analyzed the impact of seawater carbonate chemistry on the incorporation of elements in both hyaline and porcelaneous larger benthic foraminifera. We observed a higher incorporation of Zn and Ba when pCO2 increases from 350 to 1200 ppm. Modeling the activity of free ions as a function of pCO2 shows that speciation of some elements (like Zn and Ba) is mainly influenced by the formation of carbonate complexes in seawater. Hence, differences in foraminiferal uptake of these might be related primarily by the speciation of these elements in seawater. We investigated differences in trends in element incorporation between hyaline (perforate) and porcelaneous (imperforate) foraminifera in order to unravel processes involved in element uptake and subsequent foraminiferal calcification. In hyaline foraminifera we observed a correlation of element incorporation of different elements between species, reflected by a general higher incorporation of elements in species with higher Mg content. Between porcelaneous species, inter-element differences are much smaller. Besides these contrasting trends in element incorporation, however, similar trends are observed in element incorporation as a function of seawater carbonate chemistry in both hyaline and porcelaneous species. This suggests similar mechanisms responsible for the transportation of ions to the site of calcification for these groups of foraminifera, although the contribution of these processes might differ across species.