Ecological zonation of salt marsh macrophytes is strongly influenced by hydrologic factors, but these factors are poorly understood. We examined groundwater flow patterns through surficial sediments in two saltmarshes in the southeastern United States to quantify hydrologic differences between distinct ecological zones. Both sites included tall- or medium-form Spartina alterniflora near the creek bank; short-form Spartina alterniflora in the mid-marsh; salt flats and Salicornia virginica in the high marsh; and Juncus roemarianus in brackish-to-fresh areas adjacent to uplands. Both sites had relatively small, sandy uplands and similar stratigraphy consisting of marsh muds overlying a deeper sand layer. We found significant hydrologic differences between the four ecological zones. In the zones colonized by S. alterniflora, the vertical flow direction oscillated with semi-diurnal tides. Net flow (14-day average) through the tall S. alterniflora zones was downward, whereas the short S. alterniflora zones included significant periods of net upward groundwater flow. An examination of tidal efficiency at these sites suggested that the net flow patterns rather than tidal damping controlled the width of the tall S. alterniflora zone. In contrast to the S. alterniflora zones, hypersaline zones populated by S. virginica were characterized by sustained periods (days) of continuous upward flow of saline water during neap tides. The fresher zone populated by J. roemarianus showed physical flow patterns that were similar to the hypersaline zones, but the upwelling porewaters were fresh rather than saline. These flow patterns were influenced by the hydrogeologic framework of the marshes, particularly differences in hydraulic head between the upland water table and the tidal creeks. We observed increases in hydraulic head of approximately 40 cm from the creek to the upland in the sand layers below both marshes, which is consistent with previous observations that sandy aquifers below fine-grained marsh soils act as conduits for flow from uplands to tidal creeks. This hydrologic framework supports relatively good drainage near the creek, increased waterlogging in the mid-marsh, and the development of hypersalinity adjacent to the freshwater upland. These hydrologic differences in turn support distinct ecological zones.
Submarine groundwater discharge (SGD) varies significantly across time scales ranging from hours to years, but studies that allow quantitative comparisons between different time scales are few. Most of these studies have focused on beach settings, where the combined variations in fresh and saline SGD can be difficult to interpret. We calculated variations in saline SGD based on a 1 year record of hydraulic head in a salt marsh, where we could isolate variations in saline, tidally driven SGD. Observed SGD varied by an order of magnitude over the course of the year. Groundwater discharge was proportional to tidal amplitude and varied by at least a factor of 2 between spring and neap tides. Monthly average SGD was inversely proportional to average sea level; it increased by nearly a factor of 2 as sea level declined by 50 cm from late summer to late winter. This variation was far larger than that predicted by analytic models, owing to the flat topography and inundation of the marsh platform. The effect of short-term (days) variations in sea level associated with wind events and storms was small in comparison. SGD is probably proportional to tidal amplitude in nearly all coastal settings, including beaches. Seasonal variations in sea level may not affect the volume of SGD as significantly in coastal settings where the slope of the intertidal zone is relatively constant, but such variations have the potential to strongly affect the composition of SGD.
Current conceptual models for groundwater flow in beaches highlight an upper saline plume, which is separated from the lower salt wedge by a zone of brackish to fresh groundwater discharge. There is currently limited knowledge of what conditions allow an upper saline plume to exist and what factors control its formation. We used variable-density, saturated-unsaturated, transient groundwater flow models to investigate the configuration of the freshwater-saltwater interface in beaches with slopes varying from 0.1 to 0.01. We also varied hydraulic conductivity, dispersivity, tidal amplitude and inflow of fresh groundwater. The simulated salinity configuration of the freshwater-saltwater interfaces varied significantly. No upper saline plumes formed in any beach with hydraulic conductivities less than 10 m/d. The slope of the beach was also a significant control. Steeper beach faces allowed stronger upper saline plumes to develop. Median sediment grain size of the beach is strongly correlated to both beach slope and permeability, and therefore the development of an upper saline plume. Prior studies of groundwater flow and salinity in beaches have used a range of theoretical dispersivities and the appropriate values of dispersivity to be used to represent real beaches remains unclear. We found the upper saline plume to weaken with the use of larger values of dispersivity. Our results suggest that upper saline plumes do not form in all beaches and may be less common than previously considered.
Approximately 40% of the total global rate of nitrogen fixation is the result of human activities, and most of this anthropogenic nitrogen is used to fertilize agricultural fields. Approximately 23% of the applied agricultural nitrogen is delivered to the coastal zone, often reducing water quality and driving eutrophication. Nitrogen cycling in coastal sediments can mitigate eutrophication by removing bioavailable nitrogen. However, some of these processes generate nitrous oxide, a potent greenhouse gas, as a by‐product. Here we report the discovery of a nitrous oxide production hot spot in shallow barrier island sands. Nitrous oxide concentrations, production and consumption rates, vertical diffusion fluxes, and flux to the atmosphere were measured across triplicate depth profiles. Using a mass balance approach, rates of net nitrous oxide production were estimated to be 40 µmol m−2 d−1. This production was driven by a hot spot of nitrate consumption that removed bioavailable nitrogen from the coastal environment at a rate of 10 mmol m−2 d−1, a rate that is comparable with the highest rates of denitrification reported for coastal sediments.
Methane concentrations can be high in coastal groundwater, resulting in methane export driven by submarine groundwater discharge. However, the magnitude of this methane flux depends significantly on the rate of methanotrophy, the often overlooked process of microbial methane consumption that occurs within coastal aquifer sediments. Here we describe a zone of methanogenesis within the freshwater lens of a barrier island aquifer and investigate the methane source/sink behavior of the barrier island system as a whole. The median concentration of methane dissolved in fresh groundwater beneath the center of the island was 0.6 mM, supported by high rates of potential methanogenesis (22 mmol m-2 day-1). However, rates of microbial methane consumption were also elevated in surrounding sediments (18 mmol m-2 day-1). Groundwater flowing from the zone of methanogenesis to the point of discharge into the ocean had a long residence time within methanotrophic sediments (~195 days) such that the majority of the methane produced within the barrier island aquifer was likely consumed there.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.