Increasing tidal inundation corresponds to rising porewater nutrient concentrations in a southeastern U.S. salt marsh

Abstract

Salt marshes are ecologically and economically important features of coastal environments that are vulnerable to sea level rise, the rate of which has accelerated in recent decades along the southeastern US Atlantic coast. Increased flooding frequency and duration across the marsh platform is predicted to impact vegetation community structure and overall marsh persistence, but the effect of changing inundation patterns on biogeochemical processes in marsh sediments remains largely unexplored. As part of a long-term monitoring effort to assess how marshes are responding to sea level rise in North Inlet estuary (South Carolina, USA), we collected data on porewater nutrient concentrations from a series of permanent monitoring plots across multiple transects spanning the marsh elevation gradient during the growing season from 2009 to 2019. Additionally, we calculated time inundated for each plot using local water level data and high-resolution elevation measurements to assess the change in time flooded at each plot. Our results indicate that both NH4 and PO4 nutrient concentrations have increased in most permanent plots over the 11-year study period and that nutrient concentrations are higher with increasing proximity to the creek. Spatial patterns in nutrient increases through time are coincident with considerable increases in tidal inundation observed over the marsh platform. Across plots located in the low marsh, porewater NH4 and PO4 concentrations have risen at average rates of 8.96 μM/year and 0.86 μM/year, respectively, and have reached rates as high as 27.25 μM/year and 3.13 μM/year. We suggest that increased inundation time due to rising sea level has altered biogeochemical conditions influencing nutrient availability in marsh porewater, resulting in increases that likely have relevance for larger scale nutrient cycles as well as marsh ecosystem stability and function.

Contrasting trends in water quality between adjacent ocean‐ and river‐dominated estuaries: Evidence for marsh porewaters as a source of nutrient enrichment?

“…North Inlet estuary (Krask et al 2022). Our results suggest that sea level rise can be a highly influential driver of water column NH 4 concentrations in smaller, ocean-dominated estuaries.…”

“…Drivers of mean NH 4 concentration at the annual scale differed between North Inlet estuary and Winyah Bay, supporting our hypothesis that mean sea level is the dominant force leading to increases in NH 4 and Chl a concentrations within North Inlet. Previously, we demonstrated a positive relationship between the inundation of the salt marsh platform due to sea level rise and concentrations of NH 4 within the interstitial porewaters of salt marsh sediments (Krask et al 2022). NH 4 is the dominant form of inorganic nitrogen found in salt marsh porewaters due to the highly reducing nature of the sediments, and porewater concentrations of NH 4 are often orders of magnitude higher than concentrations in adjacent tidal creeks (Morris 2000).…”

“…Sea level rise is driving higher water levels in estuaries across the southeast U.S. (Sweet et al 2022), including the North Inlet estuary (Krask et al 2022). Our results suggest that sea level rise can be a highly influential driver of water column NH 4 concentrations in smaller, ocean‐dominated estuaries.…”

“…We hypothesize that sea level rise associated with climate change has driven decadal‐scale increases in inorganic nutrient concentrations within the ocean‐dominated North Inlet estuary. We suggest that this is primarily due to complex interactions between salt marsh porewaters and tidal pumping associated with increasing inundation time (Krask et al 2022). Conversely, we propose that riverine input from the large Winyah Bay watershed exerts overwhelming control over nutrient concentrations in the Winyah Bay estuary, such that any changes resulting from sea level rise are comparatively minimal.…”

Vegetation Loss Following Vertical Drowning of Mississippi River Deltaic Wetlands Leads to Faster Microbial Decomposition and Decreases in Soil Carbon

“…Ammonium is the preferred inorganic nitrogen source for marsh plants like Spartina (Bowen et al., 2020; Mendelssohn, 1979) and porewater PO 4 3− is used as a source of phosphorus (Patrick & DeLaune, 1977). Therefore, the progressive increase in porewater NH 4 + and PO 4 3− in ponds is likely driven by ongoing release through iron reduction (PO 4 3− ) and organic matter decomposition (NH 4 + ) in the absence of corresponding plant uptake ( sensu Krask et al., 2022; Walker et al., 2022).…”