Drought, Mutualism Breakdown, and Landscape-Scale Degradation of Seagrass Beds

Abstract

In many marine ecosystems, biodiversity critically depends on foundation species such as corals and seagrasses that engage in mutualistic interactions [1-3]. Concerns grow that environmental disruption of marine mutualisms exacerbates ecosystem degradation, with breakdown of the obligate coral mutualism ("coral bleaching") being an iconic example [2, 4, 5]. However, as these mutualisms are mostly facultative rather than obligate, it remains unclear whether mutualism breakdown is a common risk in marine ecosystems, and thus a potential accelerator of ecosystem degradation. Here, we provide evidence that drought triggered landscape-scale seagrass degradation and show the consequent failure of a facultative mutualistic feedback between seagrass and sulfide-consuming lucinid bivalves that in turn appeared to exacerbate the observed collapse. Local climate and remote sensing analyses revealed seagrass collapse after a summer with intense low-tide drought stress. Potential analysis-a novel approach to detect feedback-mediated state shifts-revealed two attractors (healthy and degraded states) during the collapse, suggesting that the drought disrupted internal feedbacks to cause abrupt, patch-wise degradation. Field measurements comparing degraded patches that were healthy before the collapse with patches that remained healthy demonstrated that bivalves declined dramatically in degrading patches with associated high sediment sulfide concentrations, confirming the breakdown of the mutualistic seagrass-lucinid feedback. Our findings indicate that drought triggered mutualism breakdown, resulting in toxic sulfide concentrations that aggravated seagrass degradation. We conclude that external disturbances can cause sudden breakdown of facultative marine mutualistic feedbacks. As this may amplify ecosystem degradation, we suggest including mutualisms in marine conservation and restoration approaches.

Identifying knowledge gaps in seagrass research and management: An Australian perspective

“…What are the threats and implications from changes in the ecosystem for seagrass habitat? Threats to seagrasses may arise due to changes in other components of the ecosystem; For example, range shifts (Mazaris et al, 2013), species introductions (Williams, 2007), or species loss (de Fouw et al, 2016;Jackson et al, 2001) and changes in the behaviour of resident species can threaten seagrass habitat through changes in interactions such as through increased grazing rates (Verges et al, 2014). We need to improve our understanding of interactions in seagrass ecosystems.…”

Mutualistic interactions amplify saltmarsh restoration success

“…Such interactions can be intraspecific—for example, seagrass and marsh vegetation increasingly attenuate currents and trap sediment and detritus with higher shoot density (Maxwell et al., ; van de Koppel, van der Wal, Bakker, & Herman, )—or interspecific. Examples of the latter are the facilitation of cockles by intertidal mussel beds in the Wadden Sea (Donadi et al., ) and bidirectional facilitation in coral‐zooxanthellae and seagrass‐lucinid clam mutualisms (de Fouw et al., ; Hay et al., ; van der Heide et al., ). Despite their ecological importance, the consideration of positive interactions for coastal restoration has thus far remained limited.…”

“…Examples of the latter are the facilitation of cockles by intertidal mussel beds in the Wadden Sea (Donadi et al, 2013) and bidirectional facilitation in coral-zooxanthellae and seagrass-lucinid clam mutualisms (de Fouw et al, 2016;Hay et al, 2004;van der Heide et al, 2012). Despite their ecological importance, the consideration of positive interactions for coastal restoration has thus far remained limited.…”

Chemosymbiotic bivalves contribute to the nitrogen budget of seagrass ecosystems

“…Stable isotope probing with 13 C-NaHCO 3 and 15 N-N 2 was used to quantify C and N 2 fixation by the chemosynthetic symbionts. An isotope pool dilution experiment with 15 N-NH 4 Cl, was conducted in October to quantify gross and net NH 4 + fluxes by the bivalve symbiosis. Finally, elemental and natural stable isotope analyses were conducted to study the stoichiometric and isotopic niche of host and symbiont under the two contrasting seasons.…”

“…However, their relevance for ecosystem functioning has received limited attention due to the assumption that chemosynthesis plays a minor role in shallow-water ecosystems. Recent studies are challenging this assumption [2][3][4]. In seagrass sediments, chemosymbiotic bivalves of the family Lucinidae consume sulfide, allowing more plant growth while relying on the seagrass to stimulate sulfide production by free-living sulfate-reducing microorganisms [3].…”