Recent dynamics on turbid-water corals reefs following the 2010 mass bleaching event in Tobago
Introduction
Hard coral communities underpin many ecological functions (e.g., carbon cycling, energy fluxes, productivity and nutrient cycling) and ecosystem services (e.g., fisheries, tourism and coastal protection) that are crucial to the wellbeing of millions of people worldwide (Moberg and Folke, 1999; Woodhead et al., 2019). However, the confluence of global ocean warming and local-scale disturbances are driving profound changes on coral reefs worldwide, which is being experienced as shifts in hard coral dominance (Hughes et al., 2018) and/or losses in hard coral cover (e.g., Bruno and Selig, 2007; De'ath et al., 2012; Jackson et al., 2014). Additionally, where coral loss occurs the subsequent dominance of macroalgae or other sessile invertebrates reduces the structural complexity of coral reef habitats (Alvarez-Filip et al., 2013), with feedbacks that jeopardize biodiversity (Darling et al., 2017; Graham and Nash, 2013; Newman et al., 2015), reef growth (Green et al., 2008; Perry et al., 2013), fisheries production (Rogers et al., 2014), wave energy abatement (Harris et al., 2018; Sheppard et al., 2005) and tourism revenues (Spalding et al., 2017). Therefore, given current trends and future predictions of continued lossed in coral populations due to more severe disturbance events (Hoegh-Guldberg et al., 2007; Hughes et al., 2010; Nyström et al., 2000), predicting how coral populations might change, by evaluating past and present responses to disturbance events, is critical to developing our understanding of coral reef stability and strategies that facilitate resilience, while also preventing further loss.
Studying marginal reefs is particularly important to advancing our understanding of how hard coral communities might respond to possible future environmental change (Camp et al., 2018). Unlike typical coral reefs which thrive in clear and oligotrophic water, hard coral communities on marginal reefs thrive under conditions of high turbidity, high nutrients and/or high temperature variability (Kleypas et al., 1999; Perry and Larcombe, 2003). As a result, marginal reefs act as natural laboratories, allowing for the examination of coral reef communities, species interactions and ecological functioning under conditions that cannot be easily simulated (Burt et al., 2020).
In this study, we examined the spatial and temporal variability of benthic composition on the turbid-water (marginal) reefs of Tobago (Fig. 1). The coral communities that surround Tobago are distinguished from other Caribbean coral reefs by thriving in waters characterized by high turbidity (Muller-Karger et al., 1988), as well as elevated nutrients and reduced salinity (Hu et al., 2004). Owing to large differences in the hydro-geomorphology between the east and west sides of the island, due to water movement (Chollett et al., 2012), siltation (Buglass et al., 2016; Mallela et al., 2010), elevation and geology (Snoke et al., 2001), one can also presume that reef habitats may also differ between hydro-geomorphic settings. Elucidating this difference may to help focus important conservation considerations and strategies at the scales which may differ between hydro-geomorphic settings. However, much of our understanding of these reefs stem from small-scale studies that address short-term local issues, which can result in misleading interpretations of broad scale processes that affect reef stability and recovery (Hughes and Connell, 1999). In the last decades, successive mass bleaching events have placed Caribbean coral reefs on dynamic trajectories of degradation. We use the long-term coral reef assessment data record from 1985 to 2016, to provide insights on the stability (or resilience) of coral reefs following mass coral bleaching. Specifically, the purpose of this study is (i) to understand the influence of hydro-geomorphic setting on the spatial patterns of coral reef communities, and (ii) to evaluate the local dynamics coral reefs in the years following the 2010 mass bleaching event against a 2007 baseline.
Section snippets
Study area
The current assessment was conducted at 26 coral reef locations around Tobago (the smaller of the twin island republic of Trinidad and Tobago) (11.23°N, 60.70 °W; Fig. 1), across two hydro-geomorphic settings, separating study sites into east and west. The eastern sites (hereafter called wave-exposed) are characterised by strong, year-round trade-winds which expose coral communities to powerful ocean swell. Most coral growth on these reefs occurs along the upper slope of the narrow, rocky and
General condition and spatial patterns
Overall, CTB was the dominant benthic component with an average of 57.62% ± 1.09 (all sites combined) (Fig. 2a). Mean hard coral cover was 14.83% ± 0.85, which ranged between 2 and 37% (ESM Table 2). Generally, the maxima of coral cover was associated with the remote and wave-exposed locations (e.g. Sanger Rock [37%] and St. Giles [30%]) and the minima were associated with easily accessible, sheltered reefs (e.g. Belmont Bay [2%]) or areas that historically have supported low amounts of hard
Discussion
The current assessment highlights the heterogeneity in benthic compositon across the Tobago coral reef landscape, with five spatial trends emerging. Firstly, CTB (crustose coralline algae, algal turf and bare substrate) is a major contributor to benthic cover. This generalized condition is consistent with the historical record for Tobago from the 1980s to present (Alemu I and Clement, 2014, Buglass et al., 2016, Laydoo, 1985a, Laydoo, 1985b, Mallela et al., 2010), which may be the result of
Conclusion
It is clear that in an era of climate change that reef management should prioritize key coral reef sites for targeted and well-resourced management. It is impossible to make management recommendations without considering the critical role that hydro-geomorphic and biophysical settings play in understanding and managing for resilience. Coral reefs are also an integral part of global and local economies for tourism, recreation and food (e.g., Beck et al., 2018; Burke et al., 2008; Spalding et
CRediT authorship contribution statement
Jahson Berhane Alemu I: Conceptualisation, Investigation, Data curating, Formal Analysis, Visualisation, Writing – original draft, Writing – reviewing and editing. Jennie Mallela: Conceptualisation, Investigation, Writing – original draft, Writing – reviewing and editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the Institute of Marine Affairs, Environmental Policy and Planning Division and the University of the West Indies, and funding under the Global Environmental Facility technical contracts (PCA/2010/DEPI/004, GCP/TRI/003/GFF and GFL/6030-05–01). We thank the many people that contribubted to data collection and analysis over the years: R. Laydoo, R. Parkinson, R. Hubbard, G. Marley, M. Rampaul, K. Sampson, A. Titus and J. Gomez. Lastly, we thank the Department of
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