Shallow ponds are heterogeneous habitats within a temperate salt marsh ecosystem

Abstract

Integrating spatial heterogeneity into assessments of salt marsh biogeochemistry is becoming increasingly important because disturbances that reduce plant productivity and soil drainage may contribute to an expansion of shallow ponds. These permanently inundated and sometimes prominent landscape features can exist for decades, yet little is known about pond biogeochemistry or their role in marsh ecosystem functioning. We characterized three ponds in a temperate salt marsh (MA, USA) over alternating periods of tidal isolation and flushing, during summer and fall, by evaluating the composition of plant communities and organic matter pools and measuring surface water oxygen, temperature, and conductivity. The ponds were located in the high marsh and had similar depths, temperatures, and salinities. Despite this, they had different levels of suspended particulate, dissolved, and sediment organic matter and abundances of phytoplankton, macroalgae, and Ruppia maritima. Differences in plant communities were reflected in pond metabolism rates, which ranged from autotrophic to heterotrophic. Integrating ponds into landcover‐based estimates of marsh metabolism resulted in slower rates of net production (−8.1 ± 0.3 to −15.7 ± 0.9%) and respiration (−2.9 ± 0.5 to −10.0 ± 0.4%), compared to rates based on emergent grasses alone. Seasonality had a greater effect on pond water chemistry, organic matter pools, and algal abundances than tidal connectivity. Alternating stretches of tidal isolation and flushing did not affect pond salinities or algal communities, suggesting that exchange between ponds and nearby creeks was limited. Overall, we found that ponds are heterogeneous habitats and future expansion could reduce landscape connectivity and the ability of marshes to capture and store carbon.

Shallow ponds are biogeochemically distinct habitats in salt marsh ecosystems

“…Long stretches of hydrologic isolation facilitate the accumulation of metabolites and depletion of critical resources (e.g., inorganic nutrients), thereby altering pond biogeochemistry. Tidal flushing may only partially alleviate these effects because exchange is often incomplete (Spivak et al ). Ponds also differ from other coastal habitats because they are embedded in the marsh platform and surrounded by organic‐rich peat.…”

“…Respiration measurements suggest that decomposition of marsh peat is an important process contributing to progressive pond development (Johnston et al ; Spivak et al ). However, respiration data are scarce and reflect the combined metabolism of the animal, plant, and bacterial communities that inhabit ponds (Johnston et al ; Moseman‐Valtierra et al ; Spivak et al ). Moreover, bacteria can assimilate OM from multiple sources and decomposition rates and pathways are affected by local plant communities.…”

“…We focused on three high marsh ponds, where we had previously characterized surface water chemistry, plant communities, and oxygen‐based metabolism rates (Fig. ; Spivak et al ). We predicted that carbon dynamics would vary across ponds due to interactions between aboveground primary producers and sediment bacteria, which would be intensified by tidal isolation and summer weather.…”

Pond Excavation Reduces Coastal Wetland Carbon Dioxide Assimilation

“…Because gas exchange from wetland ponds has rarely been measured (Moseman‐Valtierra et al, ), and no relationships were found with environmental variables measured, it is difficult to attribute GHG exchange in ponds to specific factors measured in this study. Shallow ponds in salt marsh landscapes have been described as very heterogeneous in terms of their gas exchange by Spivak et al (). Our data agree with the notion that these semi‐isolated ponds either import or export organic material (Spivak et al, ).…”

“…Shallow ponds in salt marsh landscapes have been described as very heterogeneous in terms of their gas exchange by Spivak et al (). Our data agree with the notion that these semi‐isolated ponds either import or export organic material (Spivak et al, ). Therefore, ponds in wetland landscapes—either natural or anthropogenic—should not be included in C inventory calculations until properly researched, as these ponded features are still understudied in terms of C assimilation (Spivak et al, ; Berkowitz et al, ).…”