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

. 2019 Dec;25(12):4092-4104.

doi: 10.1111/gcb.14800. Epub 2019 Sep 30.

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

Didier M de Bakker et al. Glob Chang Biol. 2019 Dec.

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 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 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.

Keywords: Acropora cervicornis; Bonaire; Caribbean; bioerosion; calcification; carbonate budget; climate change; sea-level rise.

© 2019 The Authors. Global Change Biology published by John Wiley & Sons Ltd.

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Figures

Figure 1
Figure 1

Reef characteristics by subregion. Main descriptive reef characteristics related to reef accretion for each subregion (1:12, see Figure 2 for the location of each region). Points represent the mean of all transects within each cluster, segments define the 95% confidence limits. Mean coral cover (a, e) shows the percentage of the total three‐dimensional transect length covered by live coral (including Millepora spp.). Net carbonate (b, f) production (gross production minus gross erosion), given in G (kg CaCO3 m−2 year−1). Measures for rugosity (c, g) are based on the chain and link technique (Risk, 1972). Mean reef height (d, h) from the bottom (in cm) was collected every 2 m along each transect line. Overall means for each zone are given as ‘Mean’ within each panel

Figure 2
Figure 2

Reef accretion rates of the shallow leeward fringing reef of Bonaire. Top panels (a,b) show the location and the mean reef accretion (EAR in mm/year) of the surveyed sites (n = 115) at each individual site and within each reef zone (lower terrace; drop‐off). EAR is displayed by means of a colour‐coded scheme. IPCC projections for sea‐level rise (RCP 2.6, RCP 4.5, and RCP 8.5) are included on the colour scale as well (in mm/year). Symbol size reflects mean live coral cover (LCC), shape indicates whether a community is dominated (>50% cover) by framework builders or opportunistic coral species. Bottom panels (d,e) show EAR by subregion (1–12), number of sites per subregion is given in brackets. Subregions are defined in the top panels and separated by dotted line insets. Blue points indicate mean EAR, error bars define the 95% confidence interval. Open circles represent EAR on a transect level. The dotted horizontal line indicates the point where EAR equals zero. Altitude maps are adapted from Mücher et al. (2017) [ASTER Global Digital Elevation Model Version 2]. The location of the town of Kralendijk and Klein Bonaire is indicated on the map. Inset in the top left panel (c) specifies the position of Bonaire in the southern Caribbean Sea

Figure 3
Figure 3

Relation between the degree of anthropogenic disturbance and estimated reef accretion. The degree of anthropogenic impact is classified from minimal to extreme (colours and symbols). Left and middle panels (a, b) display mean EAR (mm/year) per impact level (large coloured symbols) for both reef zones, error bars indicate the 95% confidence intervals. Grey open symbols indicate EAR for the individual transects. Right panel (c) shows the map of Bonaire with the degree of disturbance for each site given in colours and symbols corresponding to those of the other two panels. Dotted lines and numbers (1–12) define the coastal subregions

Figure 4
Figure 4

Relation between estimates accretion rate (EAR) and the percentage of live coral cover on the shallow reef of Bonaire. The black curve represents the fitted response (EAR) versus live coral cover for coral assemblages dominated by framework builders (dark grey) or dominated by opportunistic coral species (light grey). Points represent the measured means for each site (lower terrace sites as solid circles, drop‐off sites as open circles). Grey bands represent the predicted 95% confidence interval. The horizontal black dashed line represents an EAR of zero. The point where this line intercepts the fitted curve can tentatively be regarded as the approximate minimum required coral cover percentage to maintain the current depth elevation of the reef framework. Coloured dashed lines indicate the estimated accretion rates necessary to match IPCC‐projected sea‐level rise scenarios for Bonaire: RCP 2.6 (green, 4.9 mm SLR per year), RCP 4.5 (yellow, 6.9 mm SLR per year) and RCP 8.5 (red, 9.4 mm SLR per year; Slangen et al., 2014). Blue points indicate sites containing Acropora cervicornis stands (all lower terrace)

Figure 5
Figure 5

Decline in Acropora cervicornis since the 1980s. Estimated percentage cover of A. cervicornis along the leeward coast of Bonaire and Klein Bonaire based on data collected between 1981 and 1983 (a) (Van Duyl, 1985) and in 2017 (b) (this study). Dots show the 115 sites as surveyed in 2017 (this study), colours represent ranges in percentage cover of the total benthic community. Source of map: Google Earth v7.3

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