What do seagrass experience? Snapshot versus time-integrated measures of sediment-sulfide exposure

Seagrasses are routinely exposed to sulfides associated with anoxic sediment, whether in pristine or highly eutrophic environments. Sulfide is a phytotoxin, but seagrasses have the capacity to reoxidize the sulfide and... [ view full abstract ]

Seagrasses are routinely exposed to sulfides associated with anoxic sediment, whether in pristine or highly eutrophic environments. Sulfide is a phytotoxin, but seagrasses have the capacity to reoxidize the sulfide and ameliorate the negative effect. The health of seagrass has been monitored in the Swan-Canning estuary since 2011/12, using a suite of indicators one of which informs on sediment sulfide-stress. Sub-lethal reductions in seagrass growth were associated with increased uptake of sediment-sulfide in a study conducted in 2011/12 (published in Ecological Indicators 2014) and the current study (2014) aims to further understand the temporal variability in sulfide exposure and plant responses. At six sites, we report on seagrass growth, and snapshot and integrated measures of seagrass sulfide exposure (diffusive thin gradient (DGT) and d34S in seagrass tissues respectively). Snapshot porewater sulfide concentrations were greater in seagrass-vegetated sediment (max 19-121 uM) than in unvegetated sediment (max 4-23 uM). Maximum porewater sulfide was positively correlated with seagrass growth at the time of sampling (R>0.8, p<0.001). Consistent with the relationships reported in 2011/12, integrated metrics of sediment-stress were negatively correlated with metrics of seagrass growth (R= -0.59 to -0.67, p<0.01). It is intriguing that the snapshot measures of porewater sulfide show a positive relationship with seagrass production, while the integrated measures of sulfide exposure (e.g. Fsulfide) show a negative relationship with growth. These seemingly contradictory results suggest that high seagrass production favours sulfate-reducing bacteria (possibly due to plant exudation of DOC during photosynthesis), which then increases porewater sulfide concentrations. However, while DGT provides a snapshot of potential sulfide exposure at a given time, they do not account for the potential for seagrass to simultaneously counter exposure by oxidising the sulfide either within the rhizosphere or within the root tissues, as both root DOC and oxygen release are coupled to photosynthesis. In contrast, Fsulfide provides a time-integrated measure of when this oxidizing potential is overcome by the supply of sulfide and growth becomes compromised. This disconnect between exposure and effect on plant health highlights the challenges with measuring porewater sulfide pools in either bare or vegetated sediment to inform on how sediment influences seagrass.