Abstract
Abstract Salt marshes suffered large‐scale degradation in recent decades. Extreme events such as hot and dry spells contributed significantly to this, and are predicted to increase not only in intensity, but also in frequency under future climate scenarios. Such repetitive extreme events may generate cumulative effects on ecosystem resilience. It is therefore important to elucidate how marsh vegetation responds to repetitive stress, and whether changes in key species interactions can modulate vegetation resilience. In this study, we investigated how moderate but repetitive desiccation events, caused by the combined effects of drought and high temperatures, affect cordgrass (Spartina alterniflora), the dominant habitat‐forming grass in southeastern US salt marshes. In a 4‐month field experiment, we simulated four consecutive desiccation events by periodically excluding tidal flooding and rainfall, while raising temperature. We crossed this desiccation treatment with the presence/absence of ribbed mussels (Geukensia demissa) – a mutualist of cordgrass known to enhance its desiccation resilience – and with grazing pressure by the marsh periwinkle (Littoraria irrorata) that is known to suppress cordgrass’ desiccation resilience. We found that each subsequent desiccation event deteriorated sediment porewater conditions, resulting in high salinity (53 ppt), low pH‐levels (3.7) and increased porewater Al and Fe concentrations (≈800 μmol/L and ≈1,500 μmol/L) upon rewetting. No effects on porewater chemistry were found as a result of snail grazing, while ribbed mussels strongly mitigated desiccation effects almost to control levels and increased cordgrass biomass by approximately 128%. Importantly, although cordgrass generally appeared healthy above‐ground at the end of the experiment, we found clear negative responses of the repetitive desiccation treatment on cordgrass below‐ground biomass, on proline (osmolyte) levels in shoots and on the number of tillers (−40%), regardless of mussel and/or snail presence. Synthesis. Even though the mutualism with mussels strongly mitigated chemical effects in the sediment porewater throughout the experiment, mussels could not buffer the adverse ecophysiological effects observed in cordgrass tissue. Our results therefore suggest that although mussels may alleviate desiccation stress, the predicted increased frequency and intensity of hot dry spells may eventually affect saltmarsh resilience by stressing the mutualism beyond its buffering capacity.
Highlights
Anthropogenic forcing is inducing both gradual and acute changes in climate patterns world‐wide (IPCC et al, 2007, Myhre et al, 2013)
We investigated how moderate but repetitive desiccation events, caused by the combined effects of drought and high temperatures, affect cordgrass (Spartina alterniflora), the dominant habitat‐forming grass in southeastern US salt marshes
In addition to the foreseen continued warming, predictions are that many ecosystems will continue to be increasingly exposed to extremes, such as heavy rainfall, heat spells or droughts that will likely increase in both severity and frequency (Hansen, Sato, & Ruedy, 2012, IPCC et al, 2007)
Summary
Anthropogenic forcing is inducing both gradual and acute changes in climate patterns world‐wide (IPCC et al, 2007, Myhre et al, 2013). In addition to the foreseen continued warming, predictions are that many ecosystems will continue to be increasingly exposed to extremes, such as heavy rainfall, heat spells or droughts that will likely increase in both severity and frequency (Hansen, Sato, & Ruedy, 2012, IPCC et al, 2007) Such climatic changes affect individual species and ecosystem structure and can result in species shifts, extinctions and ecosystem degradation (Grimm et al, 2013; Urban, 2015). On the other hand, the mutualism between seagrass, bivalves and their endosymbiotic bacteria, prevents accumulation of the phytotoxin sulphide, but, at the same time, may make the system more vulnerable to drought‐mediated desiccation and rapid ecosystem collapse (de Fouw et al, 2016; van der Heide et al, 2012) Understanding how such dominant interspecific interactions influence ecosystem sensitivity to climatic changes is key to predicting temporal and spatial variability in ecosystem resilience. We crossed the presence of mussels and snails – species known to interact with cordgrass during desiccation events – with the desiccation treatment to test the hypotheses that: (a) multiple chemical stressors accumulate after each successive desiccation event, (b) mussels facilitate cordgrass by mitigating chemical stressors and (c) snails reduce cordgrass’ resilience to withstand these stressors over time
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