DOI: 10.5061/dryad.6h0k74n
title: Data from: Repetitive desiccation events weaken a salt marsh mutualism

description: 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.
description: Dataset-Repetitive
 desiccation events weaken a salt marsh mutualismThis dataset contains
 the data from a field experiment that was conducted from April 2014 till
 August 2014. We crossed the presence of mussels and snails with a
 desiccation treatment to test the hypotheses that: 1) multiple chemical
 stressors accumulate after each successive desiccation event, 2) mussels
 facilitate cordgrass by mitigating chemical stressors, and 3) snails
 reduce cordgrass’ resilience to withstand these stressors over time. The
 first data sheet shows data on cordgrass growth, mussels, chemical
 porewater and plant characteristics averaged per experimental plot. The
 second data sheet displays chemical porewater concentrations in four
 consecutive months. The third data sheet shows data about temperature en
 relative humidity.