Predicting seagrass recovery times and their implications following an extreme climate event

Abstract

Extreme temperature events are predicted to become more frequent and intense as climate change continues, with important implications for ecosystems. Accordingly, there has been growing interest in what drives resilience to climatic disturbances. When a disturbance overwhelms the resistance of an ecosystem, it becomes vulnerable during recovery, with implications for ecosystem function and persistence. Understanding what influences ecosystem recovery is particularly important in seagrass ecosystems because of their functional roles, vulnerability, and divergent recovery strategies. Seagrass cover was monitored for 3 yr following a large, heatwaveassociated mortality event in Shark Bay, Australia. Although the ecosystem's historically dominant foundational seagrass, Amphibolis antarctica, is capable of rapid disturbance recovery, this did not occur, likely because of the failure of mechanisms which have driven rapid recovery in other systems (persistence of rhizome beds, sexual reproduction among neighboring beds). Instead, a tropical early successional seagrass, Halodule uninervis, became more common, increasing diversity. These changes in the structure of the Shark Bay seagrass ecosystem, and reduction of biomass and structural complexity, will have important implications for ecosystem services and community dynamics and indicates that this ecosystem is highly vulnerable to future disturbances. More generally, our work suggests that seagrass ecosystems typified by a mix of early and late successional species may be particularly likely to exhibit a mismatch between recovery of cover per se and recovery of function following disturbance. As such, extreme climatic events have the potential to abruptly alter seagrass community dynamics and ecosystem services.

A marine heatwave drives massive losses from the world’s largest seagrass carbon stocks

“…22 and macroalgae: − 18.1 ± 1.8‰, ref. 21 ) indicated that seagrasses were the main sources of sediment C as allochthonous matter (that is, terrestrial inputs, seston or macroalgae) could not account for the 13 C-enriched C pools stored in seagrass sediments (Supplementary Table 2). Using a three-source mixing model and literature values for putative sources, the average contribution of seagrass to the entire depth of the sediment C stocks was estimated to be ~65% ( Supplementary Fig.…”

“…After the event, water clarity decreased progressively and significantly due to the loss of sediment stabilization. In addition, widespread phytoplankton and bacterial blooms were observed in both gulfs of Shark Bay as a result of increased nutrient inputs to the water column from degraded seagrass biomass and sediment erosion 13 , providing favourable conditions for CO 2 emissions 36 .…”

“…This can be compared to the 14.4 Tg CO 2 estimated to be released annually from land-use change in Australia 39 , which did not account for emissions associated with seagrass losses, and hence would have increased the national landuse change estimate by 4% to 21% per annum. Cumulative emissions due to seagrass die-off could range between 4 and 21 Tg CO 2 after 40 years, assuming no seagrass recovery during this period, a reasonable assumption given that the recovery of A. antarctica and P. australis has been shown to take decades (> 20 yr) 40,41 or not occur over contemporary timescales 13 . If damaged seagrass meadows recover, the estimates of CO 2 emissions after 40 years might be lower than reported here.…”

“…Climate change impacts, such as ocean warming and extreme events (for example, El Niño/Southern Oscillation), are exacerbating this trend. Marine heatwaves have led to losses of foundation seagrass species that form organic-rich sediment deposits beneath their canopies (for example, Posidonia oceanica in the Mediterranean Sea 10 and Amphibolis antarctica in Western Australia [11][12][13] ). Seagrass losses and the subsequent erosion and remineralization of their sediment C stocks are likely to continue or intensify under climate change 9 , especially in regions where seagrasses live close to their thermal tolerance limits 14 .…”

“…The two most notable seagrass banks, the Wooramel Bank and the Faure Sill, are the result of ~8,000 yr of continuous seagrass growth 16 . Despite seagrasses having thrived over millennia in Shark Bay, unprecedented widespread losses occurred in the austral summer of 2010/2011 in both the above-and below-ground biomass of the dominant seagrass A. antarctica and to a minor extent Posidonia australis 12,13 , the two species forming large continuous beds. For more than 2 months, a marine heatwave elevated water temperatures 2-4 °C above long-term averages 17 .…”