Marsh-dominated Estuary Research Articles

Tryptophan-Like Fluorescence for Monitoring Fecal Contamination in a Marsh-Dominated Estuary

Tryptophan-Like Fluorescence for Monitoring Fecal Contamination in a Marsh-Dominated Estuary

Decadal-Scale Changes in Subtidal Nekton Assemblages in a Warm-Temperate Estuary

Coincidental long-term changes in estuarine nekton assemblages and environmental conditions are widely reported. In this study, from a warm-temperate, high salinity, salt marsh-dominated estuary in the southeastern USA, decreases in overall abundance, shifts in species and life stage composition, and changes in seasonal patterns of occurrence coincided with increased water temperature. Biweekly trawl collections in a subtidal creek were made during two 4-year periods separated by more than 30 years. Of the total 111 taxa in the North Inlet estuary, South Carolina, 64% (71) occurred during both the historic (1981–84) and recent (2013–16) periods. The top five species and their proportions of the total annual catches changed between periods. In the recent study period, near-bottom species (Lagodon rhomboides, Bairdiella chrysoura, Litopenaeus setiferus) increased, and pelagic species (Anchoa spp. and Lolliguncula brevis) decreased. The mean abundance of total nekton in the recent period was approximately 50% of historic abundance. Large, but temporary increases in nekton abundance occurred when salinity decreased after major storms. In the recent study period, shifts in the timing of peak abundances from spring to fall, the occurrence of juveniles during winter, and increased diversity suggested responses to significantly warmer winters and summers. Over the 30 year period, the subtidal nekton assemblage transitioned to a state of lower abundance and different composition. Future increases in water temperature, incidences of major storms, and modifications of estuarine habitats due to rising sea level could lead to additional changes in the fauna of warm-temperate estuaries.

Fate of cohesive sediments in a marsh-dominated estuary

Deposition of fine sediments on a marsh platform favors accretion that counteracts Sea Level Rise. However, it is difficult to assess the sediment trapping capacity of a marsh given the heterogeneity of sediment sources and the geometric complexity of the system, with a network of dendritic and meandering creeks dissecting the intertidal area. Here we use a numerical model to study the sediment trapping capacity of a marsh-dominated estuary, Plum Island Sound, USA, and its variations across the landscape. The results highlight the importance of the timing between sediment inputs and tidal phase and show that sediment discharged from tidal rivers deposit within the rivers themselves or in adjacent marshes. Most sediment is deposited in shallow tidal flats and channels and is unable to penetrate farther inside the marshes because of the limited water depths and velocities on the marsh platform. Trapping capacity of sediment in different intertidal subdomains decreases logarithmically with the ratio between advection length and the typical length of channels and tidal flats. Moreover, sediment deposition on the marsh decreases exponentially with distance from the channels and marsh edge. This decay rate is a function of settling velocity and the maximum value of water depth and velocity on the marsh platform.

Inorganic carbon and oxygen dynamics in a marsh-dominated estuary.

We conducted a free‐water mass balance‐based study to address the rate of metabolism and net carbon exchange for the tidal wetland and estuarine portion of the coastal ocean and the uncertainties associated with this approach were assessed. We measured open water diurnal O2 and dissolved inorganic carbon (DIC) dynamics seasonally in a salt marsh‐estuary in Georgia, U.S.A. with a focus on the marsh‐estuary linkage associated with tidal flooding. We observed that the overall estuarine system was a net source of CO2 to the atmosphere and coastal ocean and a net sink for oceanic and atmospheric O2. Rates of metabolism were extremely high, with respiration (43 mol m−2 yr−1) greatly exceeding gross primary production (28 mol m−2 yr−1), such that the overall system was net heterotrophic. Metabolism measured with DIC were higher than with O2, which we attribute to high rates of anaerobic respiration and reduced sulfur storage in salt marsh sediments, and we assume substantial levels of anoxygenic photosynthesis. We found gas exchange from a flooded marsh is substantial, accounting for about 28% of total O2 and CO2 air–water exchange. A significant percentage of the overall estuarine aquatic metabolism is attributable to metabolism of marsh organisms during inundation. Our study suggests not rely on oceanographic stoichiometry to convert from O2 to C based measurements when constructing C balances for the coastal ocean. We also suggest eddy covariance measurements of salt marsh net ecosystem exchange underestimate net ecosystem production as they do not account for lateral DIC exchange associated with marsh tidal inundation.

Seasonal variation in the metratranscriptomes of a Thaumarchaeota population from SE USA coastal waters

We used a combination of metatranscriptomic analyses and quantitative PCR (qPCR) to study seasonal changes in Thaumarchaeota populations from a salt marsh-dominated estuary. Surface waters (0.5 m depth) were sampled quarterly at Marsh Landing, Sapelo Island, GA, USA over a 3-year period. We found a mid-summer peak in Thaumarchaeota abundance measured by qPCR of either 16S rRNA or amoA genes in each of the 3 years. Thaumarchaeota were 100-1000-fold more abundant during the peak than at other times of the year, whereas the abundance of ammonia- and nitrite-oxidizing Bacteria varied <10-fold over the same period. Analysis of the microdiversity of several highly transcribed genes in 20 metatranscriptomes from a 1-year subset of these samples showed that the transcriptionally active population consisted of 2 or 3 dominant phylotypes that differed between successive summers. This shift appeared to have begun during the preceding winter and spring. Transcripts from the same genes dominated the Thaumarchaeota mRNA pool throughout the year, with genes encoding proteins believed to be involved in nitrogen uptake and oxidation, and two hypothetical proteins being the most abundant transcripts in all libraries. Analysis of individual genes over the seasonal cycle suggested that transcription was tied more closely to variation in growth rates than to seasonal changes in environmental conditions. Day-night differences in the relative abundance of transcripts for ribosomal proteins suggested diurnal variation in Thaumarchaeota growth.

The influence of tidal forcing on groundwater flow and nutrient exchange in a salt marsh-dominated estuary

Data from salt marshes in the U.S. Southeast show that long-term variations in mean water level (MWL) correlate strongly with salt marsh productivity and porewater salinity. Here we used numerical models of tidally-driven groundwater flow to assess the effect of variations in tidal amplitude and MWL on porewater exchange between salt marshes and tidal creeks. We modeled homogeneous and layered stratigraphy and compared flat and sloped topography for the marsh surface. Results are consistent with field observations and showed that increases in tidal amplitude increased groundwater flushing, particularly when increasing the tidal amplitude caused the marsh platform to be inundated at high tide. Increases in MWL caused groundwater flushing to increase if that rise caused greater areas of the marsh to be inundated at high tide. Once the marsh was fully inundated at high tide, further increases in MWL caused groundwater flushing to decrease. Results suggest that small increases in MWL associated with sea level rise could increase nutrient export significantly in marshes with elevations that are equilibrated near mean high water, but rising sea level could decrease the export of nutrients to, and thus fertility in, estuaries adjacent to marshes that are equilibrated lower in the tidal frame. Likewise, macrotidal estuaries are predicted to be subject to much larger groundwater and nutrient exchange than similar microtidal estuaries. We speculate that the early stages of rising relative sea level may significantly impact water quality in estuaries that are not river-dominated by raising the discharge of nutrients from coastal wetlands.

Tidal circulation and energy dissipation in a shallow, sinuous estuary

The tidal dynamics in a pristine, mesotidal (>2 m range), marsh-dominated estuary are examined using moored and moving vessel field observations. Analysis focuses on the structure of the M 2 tide that accounts for approximately 80% of the observed tidal energy, and indicates a transition in character from a near standing wave on the continental shelf to a more progressive wave within the estuary. A slight maximum in water level (WL) occurs in the estuary 10–20 km from the mouth. M 2 WL amplitude decreases at 0.015 m/km landward of this point, implying head of tide approximately 75 km from the mouth. In contrast, tidal currents in the main channel 25 km inland are twice those at the estuary mouth. Analysis suggests the tidal character is consistent with a strongly convergent estuarine geometry controlling the tidal response in the estuary. First harmonic (M 4) current amplitude follows the M 2 WL distribution, peaking at mid-estuary, whereas M 4 WL is greatest farther inland. The major axis current amplitude is strongly influenced by local bathymetry and topography. On most bends a momentum core shifts from the inside to outside of the bend moving seaward, similar to that seen in unidirectional river flow but with point bars shifted seaward of the bends. Dissipation rate estimates, based on changes in energy flux, are 0.18–1.65 W m−2 or 40–175 μW kg–1. A strong (0.1 m/s), depth-averaged residual flow is produced at the bends, which resembles flow around headlands, forming counter-rotating eddies that meet at the apex of the bends. A large sub-basin in the estuary exhibits remarkably different tidal characteristics and may be resonant at a harmonic of the M 2 tide.

Carbon dioxide degassing and inorganic carbon export from a marsh‐dominated estuary (the Duplin River): A marsh CO2 pump

We investigated the seasonal changes and the interactions of the CO2 system in a marsh‐dominated estuary, the Duplin River, on Sapelo Island, Georgia. Surface water pCO2, total alkalinity (TA), and total dissolved inorganic carbon (DIC) showed a pronounced seasonal progression in this system. The estuary released 256‐306 g C m‐2 (of water) yr‐1 of CO2 to the atmosphere and exported 109 g C m‐2 (of water) yr‐1 of DIC to adjacent coastal waters. There was a clear seasonal pattern in both fluxes with spring minima, and late summer and fall maxima. Release of inorganic carbon as a respiratory product from surrounding salt marshes is the primary process supporting these two carbon export terms. Concurrently, TA was exported from the Duplin River mainly in summer and fall as a result of anaerobic respiration in marshes. Based on data from the upper Duplin River, export of inorganic carbon from salt marshes was 156 g C m‐2 (of marsh) yr‐1. Extrapolation of this value to all southeastern U.S. salt marshes indicates that they cumulatively export 0.7 X 1012 g inorganic carbon annually to coastal waters. This export rivals that of riverine inorganic carbon flux. We propose that the CO2 fixation of marsh grasses and the subsequent export of inorganic and organic carbon is one major mechanism that causes the marsh‐influenced nearshore and offshore waters in the southeastern U.S. to be annual net sources of atmospheric CO2. We also suggest that this process provides an efficient and unique means for ocean carbon sequestration of atmospheric CO2.

Ecosystem Modulation of Dissolved Carbon Age in a Temperate Marsh-Dominated Estuary

We measured the concentrations and isotopic values (Δ14C and δ13C) of dissolved inorganic, dissolved organic, and particulate organic carbon (DIC, DOC, and POC, respectively) in the Parker River watershed and estuary in Massachusetts, USA, to determine the age of carbon (C) entering the estuary and how estuarine processing affects the quantity and apparent age of C transported to the Gulf of Maine. The watershed measurements indicated the transport of Δ14C-enriched modern DIC and DOC and variably aged POC from the watershed to the estuary. The transport of organic matter from the watershed was dominated by DOC transport, with POC making up less than 10% of the total. Surveys within the watershed aimed at determining which land-use type dominated the DOC export indicated that wetlands, although they made up only around 20% of the land use, could be responsible for approximately 75% of the DOC export. We therefore conclude that the wetland land uses of the Parker River watershed are exporting mainly Δ14C-enriched modern DOC. DIC isotopes indicate that the source of DIC in the Parker River watershed is dominated by the weathering of noncarbonate parent material by Δ14C-enriched carbon dioxide (CO2) originating from the respiration of young organic matter in soils. Transects in the estuary displayed net additions of all C species. For DOC and DIC, the export of this internally added DOC and DIC was approximately equal to the amount being exported from the watershed, showing the importance of focusing on estuaries when estimating the export of C to the coastal ocean. With respect to DIC, the total input is even larger when the atmospheric exchange of excess pCO2 is calculated. The Δ14C-DOC and Δ14C-DIC transects indicate that the internally added DOC and DIC is Δ14C-enriched modern material. The source of this material is the fringing marshes and estuarine phytoplankton, with the relative importance of these two sources changing over time. Taken together, the bulk C and Δ14C measurements show that the estuary is adding significant quantities of young DOC despite the presence of vast quantities of old marsh peat flanking the entire estuary. Furthermore, the DIC data indicate that Δ14C-enriched modern material is what is fueling the majority of heterotrophic respiration within the system.

Patterns of sedimentation in a salt marsh-dominated estuary.

in significantly higher growth rates for this organism. Furthermore, the observed seaward encroachment of P. australis is slowly pushing out D. spicata, the species that we found to have the highest nutritional quality (2). Given a continued shoreward migration of S. alterniflora due to sea-level rise and the seaward spread of P. australis, the overall food quality of marsh detritus for this invertebrate could decline. This suggests that changes in marsh grass species composition could affect higher trophic levels in this salt-marsh ecosystem. This research was supported by an NSF-REU fellowship through the Plum Island Ecosystem LTER to AMA.

Spring-Neap Tidal Contrasts and Nutrient Dynamics in a Marsh-Dominated Estuary

This contribution presents a new perspective on water chemistry and its relation to tidal hydrology in marsh-dominated estuaries. Results are derived from both field and modeling experiments. A heuristic model based on a tidally-averaged advection-dispersion equation is used in conjunction with source-sink terms (for benthic, marsh surface, and open-water exchanges) to make predictions of nutrient concentrations in the water column. Spring-neap tidal contrasts are associated with significant changes in water-column chemistry for a variety of nutrients sampled during the growing season in the Parker River estuary (Massachusetts). For ammonium, phosphate, nitrate plus nitrite, total dissolved N, and total dissolved P, concentrations are significantly lower during spring tides (marshes flooded) than during neap tides (marshes unflooded). Model results indicate that physical changes and open-water processing are insufficient to produce the observed effect, and that explicit biogeochemical processing on marsh surfaces is required. Field observations of changes in nutrient to nutrient ratios with the onset of marsh inundation also support this conclusion. As tides progress from the neap to spring condition, a “spectrum” of trajectories emerges in salinity-nutrient plots developed from both observational datasets and model output. Care must therefore be exercised in designing sampling programs for water chemistry in marsh-dominated ecosystems and in interpreting the resulting mixing diagrams.

Marsh Loss in Nanticoke Estuary, Chesapeake Bay

Marsh loss in the Nanticoke estuary dates from the 1920s. Since 1938 overall rate of loss has averaged 49.6 ha annually, with rates increasing down estuary. Most losses have occurred in submerged upland marshes; tidal freshwater marshes are stable. Interior ponding has been the primary mechanism of loss. Rising water levels are postulated to be the underlying force behind marsh losses in this estuary. R ECENT predictions of accelerated rates of sea-level rise in the next century because of anthropogenic greenhouse warming of the atmosphere have renewed concern about increased coastal erosion and land loss. The immediate threat is greatest for coastal marshes, where vertical accretion rates are outstripped by local sea-level rise. The ominous consequences faced by coastal marshes from a rapidly rising sea level are best exemplified in the Mississippi delta, where local rates of submergence, largely from subsidence, are approximately 1 cm annually. Particularly evident is the rapidity of the primary mechanism of marsh loss, which consists of interior-pond formation, coalescence, and enlargement. Elsewhere along the Gulf and Atlantic coasts of the United States, documentation of rates and processes of marsh loss is more sketchy. Most of the reported losses are local, involving creek-bank erosion, formation of rotten spots, and shoreline-cliff retreat. To date, extensive marsh loss from interior ponding has been reported only for the Blackwater Wildlife Refuge on the Eastern Shore of Maryland (Stevenson, Ward, and Kearney 1986), where approximately one-half of the upland marshes have been lost since 1938. The extent of interior ponding and other mechanisms of marsh loss in other marsh types of Chesapeake Bay is unknown. The research reported in this article was undertaken to assess the broad importance of interior ponding and other processes in marsh types of the Nanticoke River, a representative marsh-dominated estuary of Chesapeake Bay. The Nanticoke River is one of the largest estuarine tributaries on the Eastern Shore. The stream meanders almost