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Shallow ponds are expanding in many salt marshes with potential impacts on ecosystem functioning. Determining how pond characteristics change over time and scale with physical dimensions and other spatial predictors could facilitate incorporation of ponds into projections of ecosystem change. We evaluated scaling relationships across six differently sized ponds in three regions of the high marshes within the Plum Island Ecosystems‐Long Term Ecological Research site (MA, USA). We further characterized diel fluctuations in surface water chemistry in two ponds to understand short‐term processes that affect emergent properties (e.g., habitat suitability). Primary producers drove oxygen levels to supersaturation during the day, while nighttime respiration resulted in hypoxic to anoxic conditions. Diel swings in oxygen were mirrored by pH and resulted in successive shifts in redox‐sensitive metabolisms, as indicated by nitrate consumption at dusk followed by peaks in ammonium and then sulfide overnight. Abundances of macroalgae and Ruppia maritima correlated with whole‐pond oxygen metabolism rates, but not with surface area (SA), volume (V), or SA:V. Moreover, there were no clear patterns in primary producer abundances, surface water chemistry, or pond metabolism rates across marsh regions supplied by different tidal creeks or that differed in distance to upland borders or creekbanks. Comparisons with data from 2 years prior demonstrate that plant communities and biogeochemical processes are not in steady state. Factors contributing to variability between ponds and years are unclear but likely include infrequent tidal exchange. Temporal and spatial variability and the absence of scaling relationships complicate the integration of high marsh ponds into ecosystem biogeochemical models. Plain Language Summary: The spatial extent of shallow ponds is expanding in many salt marshes, due to hydrologic management practices and sea‐level rise, among other factors. Accounting for ponds in ecosystem biogeochemical models is important for predicting how marshes may change in the future. It is impractical to characterize every marsh pond because they can account for a large fraction of the landscape. Developing predictive relationships between pond properties and easily measured attributes, such as dimensions or distance from marsh landscape features, could facilitate integration of ponds into ecosystem models. We found that pond biogeochemistry changes dramatically day to night, reflecting a combination of primary production and heterotrophic (i.e., microbial) respiration. However, abundances of primary producers, and their effects on whole‐pond oxygen metabolism, did not change predictably with pond surface area or volume. Pond properties also did not vary according to location within the marsh. Instead, each pond was unique. The processes affecting pond development are therefore highly localized and might reflect long periods of tidal isolation in the high marsh. Key Points: Spatial and temporal variability in the biogeochemistry of ponds makes it difficult to estimate their contributions to salt marsh functioningPond water oxygen levels swing from supersaturation to anoxia over daily cycles, causing successive shifts in redox‐sensitive metabolismsBiogeochemical processes reflect localized conditions and do not scale with dimensions or vary predictably with marsh spatial properties [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To test how benthic biogeochemistry responds to changing nutrient availability and estuary trophic state, I conducted mesocosm and field experiments. In the mesocosm experiment, sediment cores from eutrophic and oligotrophic estuaries were incubated under high or low nutrient levels. Net production rates were greater in eutrophic vs. oligotrophic cores while respiration rates were greater in high nutrient treatments, indicating that sediment metabolism was affected by trophic history and nutrient availability. Sediment organic matter (SOM) composition was insensitive to the nutrient treatments but varied with trophic history as carbon : nitrogen ratios were lower, δ15N was higher, and algal and bacterial fatty acids (FAs) were more abundant in eutrophic sediments. In the field experiment, intact cores were reciprocally transplanted between eutrophic and oligotrophic estuaries and changes in SOM composition were followed over four months. SOM responses were more dramatic in cores transplanted to the oligotrophic vs. eutrophic estuary. In sediments moved from the eutrophic to oligotrophic estuary, carbon and nitrogen content as well as abundances of algal and bacterial FAs fell within one month. Conversely, sediment δ15N increased sharply while algal and bacterial FAs rose slightly in cores transplanted from the oligotrophic to eutrophic estuary. Different SOM responses between the experiments were likely due to direct vs. indirect effects of nutrient enrichment. Nutrient levels were manipulated in the mesocosm experiment while the estuaries differed in nutrients, macroalgal biomass, benthic animal communities, and oxygen levels. Consequently, sediment biogeochemical responses to watershed nutrient mitigation will likely follow broader changes in estuary habitat quality. [ABSTRACT FROM AUTHOR]