Shallow ponds are biogeochemically distinct habitats in salt marsh ecosystems

Abstract

Runaway expansion of shallow ponds can catalyze the conversion of vegetated marshes into open water environments. Predicting how this transition affects ecosystem functioning is difficult because little is known about pond biogeochemistry. We characterized sediment organic matter sources and transformations in three ponds with different plant communities, over alternating periods of tidal isolation and flushing, during summer and fall, using a combination of stable isotopes, lipid biomarkers, and benthic fluxes. Sediment respiration rates (1.66 ± 0.09 mmol C m−2 d−1 to 28.53 ± 7.76 mmol C m−2 d−1) were comparable to shallow estuaries and driven by sulfate reduction. Rates varied across ponds, reflecting differences in summertime Ruppia maritima and macroalgae abundances, but were similar between seasons. Interactions between aboveground plant and sediment bacterial communities translated into distinct biogeochemical processes across the three ponds. Tidal isolation and summer weather intensified plant and bacterial community effects on pond carbon dynamics, resulting in algal biomass and lipid δ13C values that were 3–12‰ enriched, compared to nearby habitats. Surface sediment organic matter mainly derived from pond microalgae and was compositionally distinct from tidal creeks and marshes. Surprisingly, sediment bacteria were not tightly coupled to benthic microalgae but decomposed multiple carbon sources in surface sediments and became increasingly reliant on buried peat at deeper horizons. Pond development over time could largely be explained by sediment respiration and the simultaneous accretion of the surrounding marsh platform. The role of decomposition in pond expansion is consistent with previous assessments based on whole‐pond metabolism rates. Consequently, future pond expansion could alter ecosystem biogeochemistry and reduce carbon storage.

Salt Marsh Pond Biogeochemistry Changes Hourly‐to‐Yearly but Does Not Scale With Dimensions or Geospatial Position

“…Sulfide concentrations were low, even at their peak, despite abundant sulfate availability and consecutive hours of low DO. This hints that sulfur is rapidly recycled, until the ponds become anoxic, which is consistent with sediment flux measurements (Spivak et al, 2018), pore water dynamics (Rigaud et al, 2018), and the presence of active sulfate reducing and sulfur oxidizing bacterial communities (Kearns et al, 2017;Rao et al, 2016;Salman et al, 2015;Wilbanks et al, 2014). For instance, the activity of bacteria in the orders Desulfobacterales and Chromatiales, which include members of the pink berry consortia, changed over diel cycles in a nearby PIE pond (Kearns et al, 2017).…”

“…Diel changes in surface water chemistry were measured over two summer days (25 and 26 July 2016) in Ponds 1 and 4. We focused on these ponds because we had previously characterized primary producer communities, metabolism rates, and organic matter dynamics over tidal stages and seasons (Spivak et al, 2017(Spivak et al, , 2018.…”

Shallow Salt Marsh Tidal Ponds–An Environment With Extreme Oxygen Dynamics

“…The sediment therefore fulfills an important function in the tidal ponds as a CO 2 reservoir for the benthic primary production as the water column was almost devoid of CO 2 in the afternoon (Figure 4). This supports previous observations from deeper tidal ponds in the same marshes, where Spivak et al (2018) found a significant DIC efflux from the sediment to the water.…”

Using Noble Gases to Compare Parameterizations of Air‐Water Gas Exchange and to Constrain Oxygen Losses by Ebullition in a Shallow Aquatic Environment

“…We infer that ebullition is driven by photosynthesis in this setting (see section 3.4), rather than methanogenesis. Methanogenesis should be strongly limited by high rates of sulfate reduction that occur in this pond (Spivak et al, ). Therefore E was evaluated over a large range of possible bubble sizes and initial enrichment of O 2 ; these parameters control the initial concentration and evolution of Ne in an O 2 ‐enriched bubble (Ne b ).…”

“…The study location was a salt marsh pond in the Plum Island Ecosystems Long Term Ecological Research site in Massachusetts, USA (42.7411°N, 70.8309°W), and has been previously described in Spivak et al (, ). The pond was circular, covered 7,000 m 2 , and had 25 cm mean depth during the study period.…”