Repetitive desiccation events weaken a salt marsh mutualism. (22nd April 2019)

• Record Type: Journal Article
• Authors: Derksen‐Hooijberg, Marlous; Angelini, Christine; Hoogveld, Jasper R. H.; Lamers, Leon P. M.; Borst, Annieke; Smolders, Alfons; Harpenslager, Sarah Fay; Govers, Laura L.; van der Heide, Tjisse
• Editors: Randall Hughes, A.
• Is Part Of: Journal of ecology. Volume 107:Number 5(2019:Sep.)
• Page Start: 2415
• Page End: 2426
• Publication Date: 2019-04-22
• Subjects: drought -- Geukensia demissa -- Littoraria irrorata -- mussel -- mutualism -- salt stress -- Spartina alterniflora -- water stress; Plant ecology -- Periodicals; 577.05
• DOI: 10.1111/1365-2745.13178
• Languages: English
• ISSNs: 0022-0477

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. Abstract : In an 4‐month field experiment, we investigate how repetitive desiccation events affect the interaction between cordgrass and ribbed mussels, two dominant, habitat‐forming species, in southeastern US salt marshes. Mussels increased cordgrass biomass and buffered the gradual deterioration of sediment porewater conditions that arose over repeated desiccation events. However, mussels could not fully negate the negative effects of desiccation on cordgrass' tillers production, below‐ground biomass and proline levels in shoots. These experimental results highlight that although mussel mutualists may alleviate desiccation stress, an increased frequency and intensity of hot dry spells may eventually erode saltmarsh resilience by stressing the mutualism beyond its buffering capacity.