Didier M. de Bakker

Over the past decades, numerous studies have reported declines in stony corals and, in many cases, phase shifts to fleshy macroalgae. However, long-term studies documenting changes in other benthic reef organisms are scarce. Here, we studied changes in cover of corals, algal turfs, benthic cyanobacterial mats, macroalgae, sponges and crustose coralline algae at four reef sites of the Caribbean islands of Curaçao and Bonaire over a time span of 40 years. Permanent 9 m2 quadrats at 10, 20, 30 and 40 m depth were photographed at 3 to 6-year intervals from 1973 to 2013. The temporal and spatial dynamics in the six dominant benthic groups were assessed based on image point-analysis. Our results show consistent patterns of benthic community change with a decrease in the cover of calcifying organisms across all sites and depths from 32.6% (1973) to 9.2% (2013) for corals and from 6.4% to 1% for crustose coralline algae. Initially, coral cover was replaced by algal turfs increasing from 24.5% (1973) to 38% around the early 1990s. Fleshy macroalgae, still absent in 1973, also proliferated covering 12% of the substratum approximately 20 years later. However, these new dominants largely declined in abundance from 2002 to 2013 (11% and 2%, respectively), marking the rise of benthic cyanobacterial mats. Cyanobacterial mats became the most dominant benthic component increasing from a mere 7.1% (2002) to 22.2% (2013). The observed increase was paralleled by a small but significant increase in sponge cover (0.5% to 2.3%). Strikingly, this pattern of degradation and phase change occurred over the reef slope down to upper-mesophotic depths of 40 m. These findings suggest that reefs dominated by algae may be less stable than previously thought and that the next phase may be the dominance of slimy cyanobacterial mats with some sponges.

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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