Mass mortality events in atoll lagoons: environmental control and increased future vulnerability

Abstract

Coral reefs and lagoons worldwide are vulnerable environments. However, specific geomorphological reef types (fringing, barrier, atoll, bank for the main ones) can be vulnerable to specific disturbances that will not affect most other reefs. This has implications for local management and science priorities. Several geomorphologically closed atolls of the Pacific Ocean have experienced in recent decades mass benthic and pelagic lagoonal life mortalities, likely triggered by unusually calm weather conditions lasting for several weeks. These events, although poorly known, reported, and characterized, pose a major threat for resource sustainability. Based on a sample of eleven events on eight atolls from the central South Pacific occurring between 1993 and 2012, the conservative environmental thresholds required to trigger such events are identified using sea surface temperature, significant wave height and wind stress satellite data. Using these thresholds, spatial maps of potential risk are produced for the central South Pacific region, with the highest risk zone lying north of Tuamotu Archipelago. A regional climate model, which risk map compares well with observations over the recent period (r=0.97), is then used to downscale the projected future climate. This allows us to estimate the potential change in risk by the end of the 21st century and highlights a relative risk increase of up to 60% for the eastern Tuamotu atolls. However, the small sample size used to train the analysis led to the identification of conservative thresholds that overestimated the observed risk. The results of this study suggest that long-term monitoring of the biophysical conditions of the lagoons at risk would enable more precise identification of the physical thresholds and better understanding of the biological processes involved in these rare, but consequential, mass mortality events.

Atolls of the World: Revisiting the Original Checklist

“…Nonetheless, most retain a distinctly marine fauna in the lagoon, although some are dominated by just a few species of bivalve mollusks and holothurians (Adjeroud et al, 2000;Salvat, 2009). Because of dependence on wind and wave-driven mixing, such closed atolls are more susceptible to mass mortalities than open systems, likely triggered by several weeks of unusually calm weather conditions (Andréfouët et al, 2015). This may contribute to the depauperate condition of their lagoons.…”

The perfect calm: Reoccurring mass die-offs on a remote coral atoll

“…If these die-off events were indeed caused by hypoxia, then to our knowledge this is the first report of such events on a submerged coral atoll without land separating lagoon and offshore waters [12] [13]. Dongsha Atoll is a marine protected area far from river discharges or human pollution, the common drivers of marine hypoxia [2].…”

“…Rather, these die-offs occurred under the strong 2014-15 El Niño, which brought anomalously high temperatures and calm weather to the South China Sea. Hypoxia events on Dongsha Atoll, and potentially other reefs, could become more common as the oceans warm and weather patterns change [13].…”

Hypoxia shapes coral reefs

“…The reason the Maze did not progress to medium and large-scale morphology, according to the conceptual model, is that, compared to most other cellular reefs, the Maze is a relatively open and well-flushed system. In more enclosed lagoons, where water residence times are significantly higher, stratification and hypoxia are likely to be even more prevalent and influential (Adjeroud, Andréfouët, & Payri 2001;Andréfouët et al 2015). In cellular lagoons, the most restricted of all, this could result in stratification emerging as the dominant geomorphological process, with occasional severe stratification events capturing multiple adjacent small-scale cells within large pools of hypoxic bottom water (Fig 10, section E).…”

“…Detailed bathymetric maps of Atafu and other large-scale cellular lagoons, such as Fakaofo (9.38°S, 171.22°W), and Manihiki (10.42°S, 161°W), would constitute a good test of the model, particularly if combined with depth-integrated water quality logging and benthic habitat mapping (cf. Andréfouët et al 2015).…”