Award: OCE-1233678

Coastal ecosystems sit at the interface of terrestrial and marine habitats and are extraordinarily valuable to human populations. Yet, these fragile ecosystems are rapidly deteriorating as the result of human activities and this holds important consequences for the economic stability of coastal communities as well as the availability of usable freshwater and nutritious food resources. Understanding how coastal ecosystems function and respond to human activities is the first step in predicting future resource availability and is key for benchmarking ecosystem recovery from disturbances. In this research, we focused on how the widespread disturbance of coastal nutrient enrichment (i.e., eutrophication) impacts the functioning of the tidal creeks that connect salt marsh ecosystems to the coastal ocean. We found that nutrients delivered to tidal creeks have little net effect on the benthic microalgae that colonize sediments at the bottom of the creeks. Instead, microalgae are tightly coupled to sediment bacteria. A significant fraction of carbon fixed by microalgae is leaked out of the cell and rapidly assimilated by nearby sediment bacteria. In turn, these bacteria break down algal exudates and respire the inorganic carbon back into the environment where it is taken up by benthic algae. Adding inorganic nitrogen to the water overlying benthic microalgae did not enhance rates of algal production or bacterial decomposition. This suggests that metabolism by benthic microalgae and sediment bacteria in the tidal creeks are largely insensitive to nutrient levels in tidal creek waters. In contrast, producers (including macroalgae and tall-form Spartina alterniflora) that line the tops of the creek banks and the edge of the marsh platform, may be sensitive to nutrient enrichment; oxygen data revealed enhanced autotrophy during the day and enhanced heterotrophy at night, accompanied by a change in the active microbial population. Combined, these data indicate that it is unlikely that nutrients that escape uptake by marsh grasses are intercepted by benthic microalgae in tidal creeks before they are exported to the coastal ocean. On the other hand, it is likely that inorganic nitrogen released via sediment decomposition will be captured by benthic microalgae and retained in the system and will not contribute to further eutrophication of coastal waters. Human activities have also impacted the density and spatial extent of shallow, saltwater ponds that naturally form in the platform of salt marshes. Expansion of these habitats at the expense of salt marsh grasses has important implications for the functioning of marsh ecosystems, including their ability to act as long term carbon and nitrogen sinks. We found that the ponds are heterogeneous habitats and their metabolic rates reflect complex interactions between plant community composition, dissolved and particulate organic carbon pools, and environmental conditions (i.e., temperature and light). Importantly high rates of respiration in pond sediments can account for pond formation and expansion, which has implications for long term carbon storage in marshes. However, inorganic nitrogen released via sediment decomposition is nearly matched by benthic microalgal demand. Consequently future expansion of pond habitats will likely reduce the ability of marshes to sequester carbon but tight recycling between benthic microalgae and sediment bacteria will likely prevent the export of inorganic nitrogen that was previously bound in buried organic matter. Overall, our research reveals that benthic microalgae in tidal creeks may not be an important sink of inorganic nitrogen washed from the watershed but tight coupling between benthic microalgae and bacteria reduces export of inorganic nitrogen liberated by the decomposition of organic matter buried in sediments. Additionally, the work suggests that ponds may have an out-sized effect on the overall salt marsh carbon cycle and are important to study further. Last Modified: 11/30/2016 Submitted by: Amanda Spivak