Sediment subsidies to coastal marshes, whether delivered during storm and flood events or via restoration activities, are critical for marsh elevation maintenance and persistence in the face of sea-level rise. These sedimentation events also introduce nutrient subsidies, the concentrations of which are increasing as a result of anthropogenic activity. Together, these subsidies may alter biological processes that regulate plant health and productivity, thereby representing an important feedback to surface elevation maintenance. To test for interactive effects of sediment and nitrogen additions on marsh elevation dynamics, we conducted a controlled, full-factorial mesocosm experiment using brackish marsh sods exposed to one of three nitrate subsidies [+ 0.01, 0.021, 0.079 gL-1 KNO3] and one of five sediment additions [0, 30, 60, 90, 120 mm]. To examine effects on biological feedbacks to elevation, we measured elevation change and plant health throughout the growing season with a modified mini-surface elevation table (SET) and normalized difference vegetation index (NDVI) using a hyper spectrometer, respectively. Additionally, we quantified aboveground standing biomass (AGBM) and plant community composition both non-destructively and through a final biomass harvest.
Results/Conclusions
Sediment additions provided direct elevation capital to marsh sods, but subsequent patterns of elevation change and AGBM differed depending on the amount of sediment added (F4,45 = 4.85, p=0.002 and F4,45 = 4.51, p=0.004, respectively), regardless of nitrate amendments. All mesocosms experienced elevation declines following sediment inputs, presumably via compression of the original sod and compaction of the new sediment layer. After the first week, surface elevations stabilized in mesocosms amended with ≥ 60 mm sediment, never returning to pre-sediment levels. Yet, surface elevations continued to decline in mesocosms amended with ≤ 30 mm sediment, resulting in a net loss of elevation compared to pre-sediment conditions. Patterns of NDVI also stabilized after the first week, but AGBM was significantly higher at 30 mm than at 90 mm (p=0.02) and 120 mm (p=0.002) of added sediment. Collectively, these results may reflect a tradeoff, with aboveground plant production occurring at the expense of belowground production, thereby delaying biological contributions to elevation following sediment addition. Evaluating effects of nitrate and sediment subsidies on marsh elevation responses is timely, as sediment nourishment is increasingly utilized for restoration and as its delivery during storms becomes more frequent with climate change.