Henry N. McKellar

Patterns of nutrient loading in forested and urbanized coastal streams

As part of a larger investigation of the effects of coastal urbanization on estuaries, stream nutrient loading was examined over a range of hydrologic and seasonal conditions for an urbanized and a forested watershed (11 ha versus 37 ha). Despite the smaller size, the urbanized stream produced 72% greater annual streamflow volume (162 versus 94 × 103m3 · yr−1), and 66% greater annual sediment load than the forested stream (1796 versus 1082 kg · yr−1 ). This was due to the longer period of groundwater interception at the urbanized site (increased drainage), and the elevated sediment production resulting from deep excavation (2.1 versus 0.4 m), bank instability, and resuspension of sediment. Mean annual DOC concentration in the urbanized stream (13 mg C · l−1 ) was only half as concentrated as the forested stream (26 mg C · l−1 · yr−1). However, the annual DOC load between streams was within 10% (urban 2.25 · 103 versus forest 2.5 × 103kg · C · yr−1 ) due to the greater runoff volume at the urbanized stream. More than twice the amount of dissolved inorganic nitrogen (NOxN and NH4N) flowed out of the urbanized watershed than from the forested watershed (34 vs. 14 kg · N · yr−1 ). An even bigger difference between sites was observed with respect to the NOxN load. Greater runoff volume at the urbanized stream combined with higher mean annual concentration of NOxN ( 130 versus 43 μg N · l−1 · yr−1 ) resulted in 11 × more NOxN loading at the urbanized stream than the forested stream (18.0 versus 1.6 kg · N · yr−1). Near channel soil aeration brought on by deep excavation may have promoted more oxidized (and mobile) forms of mineral N. Transport patterns of the two forms of mineral nitrogen varied substantially between streams, with the urbanized site exhibiting a steady release of the NH4N and NOxN the effects of an extensive unsaturated soil. In contrast, nitrogen loading of NH4N and NOxN at forested Oyster Creek was more episodic, with spring pulses generating much of the load of NOxN (47%), and summer periods of high concentration resulting in most of the annual load of NH4N (32%).

MODELLING THE IMPACTS OF A RIVER DIVERSION ON BOTTOMLAND FOREST COMMUNITIES IN THE SANTEE RIVER FLOODPLAIN, SOUTH CAROLINA

Abstract To study the impact of an altered hydrologic regime on the growth and succession of a coastal forested floodplain in South Carolina, a bottomland hardwood succession model (FORFLO) was developed. Hydrologic parameters were used as major controls for seed germination, tree growth, and tree mortality. Individual species responses to these parameters were used to predict succession (species composition) on the simulated sites. Coupling the simulation model with a geographical information mapping system permitted rapid visual inspection of predicted bottomland community changes in the Santee River floodplain. The model predicted a loss of up to 97% of the existing bottomland forest. An alternative water release schedule may retain much of this area as cypress-tupelo forest.

Twenty More Years of Marsh and Estuarine Flux Studies: Revisiting Nixon (1980)

In 1980, Scott Nixon reviewed the role of salt marshes in estuarine and coastal productivity. His review was effectively a progress report on the testing of “The Outwelling Hypothesis” (Odum, 1980). Nixon (1980) signaled a crucial turning point in the direction of estuarine flux studies conducted since then. In this review we revisit Nixon (1980), focusing on research and thinking that has been guided by The Outwelling Concept in the last two decades. Since 1980, estuarine flux studies have been conducted at 41 different sites and presented in over 42 publications. More than a third of these were conducted in Europe, Africa, Australia, or Mexico. Our review of these studies highlighted several important advances. The first was evolution of a conceptual approach that decomposes the estuary-coastal ocean landscape into interacting subsystems (i.e., the coastal ocean, estuarine basins, and marsh). Most post-1980 flux studies have addressed interactions between these individual subsystems, often in an hierarchical sense. Over half of these quantified exchanges between marsh-dominated basins and the greater estuary-generally through a single, well-defined tidal channel. From these data, we found that tidal range, subsystem area, and distance to the ocean together explained 87% of the variability in total organic carbon (TOC) exchanges and 92% of the variability in total suspended solids (TSS) fluxes, with exports occurring at lower tidal ranges, areas, and distances. Tidal range explained 40% of the variability in nitrate + nitrite (NN) exchange (with uptake at ranges below about 1.2 m and export at greater tidal ranges) and 39% of available phosphorus (SRP) flux variation (with export at ranges below about 1.6 m). We were unable to extract similar relationships from whole-estuary exchange studies because so few exist. The geomorphological setting and degree of ecological maturity (analogous to geologic age) of a marsh or basin within an estuary are important controllers of ecological function, thus flux behavior. We applied concepts of community succession and ecosystem development to data from marsh-water column flux studies, and found that slope of flux vs. tidal height relationships was greater for younger marshes compared to all marshes, and much greater for younger marshes compared to older marshes. This change in slope often caused a shift in the inflection point that indicated the tidal range at which export shifted to import, or vice versa. These studies quantified surficial fluxes, though, and a number of post-1980 studies demonstrated the importance of other processes, including subsurface flow, subtidal advection, and the movement of nutrients and organic matter by animals (other chapters in this volume address these processes). Finally, a number of studies showed strong controls on fluxes by exogenous environmental forcing, and we reviewed several studies that used innovative budgeting and modeling of flux dynamics and ecological processes to incorporate these sources of variability. Since 1980 we have learned a great deal more about how estuarine wetlands interact with their estuaries, and of the value of establishing a conceptual framework and system boundaries. Estuarine ecologists have learned a great deal about outwelling as a concept although few flux studies have directly addressed the original Outwelling Hypothesis. We suggest that the question should not be “Is The Outwelling Hypothesis true?” but rather: 1) how are materials being exchanged between different subsystems in estuary-coastal ocean landscapes? 2) what are the mechanisms of this exchange? and 3) how do exogenous forcings control these patterns of exchange? Estuarine scientists are encouraged to view The Outwelling Hypothesis as a conceptual stimulus of ideas and not as a strict statistical hypothesis that must be proven or disproven.

The flume design — a methodology for evaluating material fluxes between a vegetated salt marsh and the adjacent tidal creek☆

An experimental flume is described which can be used as a tool to assess whether a vegetated marsh surface is a source or sink for nutrients via tidal inundation. An initial calibration study (two tidal cycles) was conducted to determine the optimum sampling design and aid in model development for flux calculations. A statistical analysis of the data showed a negligible concentration difference as a function of water depth for most of the constituents analyzed. This coupled with the low tidal velocities over the marsh surface (<1.5cm/s) suggested that a volumetric model was adequate for calculations of instantaneous discharge and nutrient flux through any station perpendicular to tidal flow. The resultant instantaneous mass flux calculations showed that water discharge was one of the dominant factors controlling the movement of material. A sine-cosine statistical model utilizing the main tidal periodicities was designed to: (1) model the instantaneous fluxes, (2) calculate the average net flux of suspended and dissolved materials, and (3) test the hypothesis that the average net flux equals zero versus a two-sided alternative using a standard regression t-test.

Nitrogen exchange between a southeastern USA salt marsh ecosystem and the coastal ocean

The salt marsh ecosystem at North Inlet, South Carolina, USA consistently exported dissolved inorganic nitrogen via tidal exchange with the coastal Atlantic Ocean. Concentrations centrations of NH4+and NO3-+NO2-displayed distinct tidal patterns with rising values during ebb flow. These patterns suggest the importance of biogeochemical processes in the flux of material from the salt marsh. NH4+export peaked during the summer (15 to 20 mg m-2 tide-1) during a net balance of tidal water exchange. Remineralization of NH4+within the salt marsh system appears to be contributing to the estimated annual net export of bout 4.7 g NH4+-N m-2 yr-1. NO3-+NO2-exports were higher in the fall and winter of 1979 (2 to 4 mg N m-2 tide-1). The winter export coincided with a considerable net export of water with no distinctive concentration patterns, suggesting a simple advective export. However, the fall peak of NO3-+NO2-export occurred during a period of net water balance in tidal exchange and an insignificant freshwater input from the western, forested boundary. During the summer and fall, tidal concentration patterns were particularly apparent, suggesting that nitrification within the salt marsh system was contributing to the estimated annual net export of ca 0.6 g NO3-+NO2--N m-2 yr-1.

Can urbanization limit iron availability to estuarine algae

Abstract Bioavailable forms of iron are highly unstable in oxygenated saline water, but one way in which iron bioavailability to algae can be enhanced is by chelation to dissolved organic matter (DOM). We hypothesized that urbanization-associated deforestation in Murrells Inlet, South Carolina caused a reduction of iron bioavailability to estuarine phytoplankton by decreasing the supply of forest-derived DOM (i.e., the iron chelation source). Bioassay experiments were conducted comparing the potential for iron depletion by phytoplankton in natural populations and Cylindrotheca closterium (Ehr.) Reimann et Lewin cultures, transferred to Murrells Inlet and North Inlet (an undeveloped estuary) water. Chelated iron addition to incubated natural populations transferred to Murrells Inlet water resulted in increased abundances of phototrophic microplankton (accounted for by Cylindrotheca ), nanoplankton, and picoplankton (dominated by Synechococcus spp.). In North Inlet water, iron enrichment to natural populations only enhanced Synechococcus growth, but this stimulation was much less than that in Murrells Inlet water. The effect of iron on Synechococcus growth in Murrells Inlet was striking (up to 34-fold greater abundance in iron-enriched treatment), suggesting that estuarine Synechococcus may be sensitive to iron stress. The results indicate that iron could be depleted much more readily in Murrells Inlet water, and suggest that iron availability to estuarine phytoplankton may be reduced by urbanization-related practices such as coastal forest clear-cutting.

A simulation model of estuarine subsystem coupling and carbon exchange with the sea. II. North Inlet model structure, output and validation

Abstract A first-stage verified model of carbon/energy flux through the North Inlet (South Carolina) marsh—estuarine ecosystem is presented. The time series output for model compartments and overall septem carbon flow are compared with observed data collected over the past five to ten years. Results indicate that the model is stable and can broadly reproduce some of the major trends of a salt marsh—estuarine system. Further avenues of research are suggested.

A sensitivity analysis of an ecosystem model of estuarine carbon flow

Abstract The North Inlet Marsh-Estuarine System Model (NIMES) is a 19-compartment real-time deterministic ecosystem simulation model of intrasystem carbon flow and exchange between an estuary and adjacent coastal water. A complete sensitivity analysis of this model with regard to POM, DOM and nekton annual exchange and annual system net productivity was completed and the functional relationship between these system behaviors and the perturbed parameters were determined by regression techniques. Simulated POM annual exchange between the estuary and the sea was largely controlled by offshore POM concentration, water column respiration and the gross productivity of the marsh and water column flora. Simulated DOM annual estuarine-oceanic exchange was most sensitive to perturbations in the gross productivity and biomass changes in marsh flora and water column microbial DOM uptake. Simulated nekton exchange reflected a sensitivity to migratory behavior and subtidal benthic biomass changes. System annual net productivity as simulated by the model showed a high sensitivity to all model processes which affected component primary production and respiration. From this sensitivity analysis, a scheme is developed to evaluate research needs for further model development for the North Inlet ecosystem.

A simulation of saltmarsh water column dynamics

Abstract This deterministic model of water column dynamics in saltmarsh tidal creeks focuses mainly on mechanisms of carbon and nitrogen cycling within the water. The model includes biotic interactions among phytoplankton, zooplankton, dissolved inorganic nitrogen, detritus and dissolved organics. In addition, the model examines the effects of tidal exchange with the nearshore coastal ocean. In simulation experiments, the model isolates the water column submodel from simulated vegetated marsh surface, oyster reefs, and subtidal benthos. By comparing simulation results with field data from the North Inlet saltmarsh (Georgetown County, SC) we examined the relative importance of water column processes in controlling seasonal changes observed in the tidal creeks. Results suggest that for the components with rapid biotic turnover rates (phytoplankton and dissolved inorganic nitrogen) much of the observed seasonal pattern is controlled by internal cycling within the water column (planktonic productivity, nutrient uptake and remineralization). For components with slower turnover rates (zooplankton, detritus, and dissolved organic matter) tidal exchange with the coastal ocean plays a larger role in controlling seasonal variability. The model accounts for less of the observed variability in detritus and dissolved organic matter, suggesting the seasonal importance of upland runoff and exchanges with other marsh subsystems in controlling concentrations in tidal creek.

Network Analysis of the North Inlet Salt Marsh Ecosystem

Salt marshes occur along the east coast of North America from the arctic marshes of Canada, through New England to the semi-tropical marshes of Florida (Teal & Teal, 1969). These systems reach their most extensive development behind the barrier beaches of South Carolina and Georgia. Considerable interest in the functioning of salt marsh ecosystems derives from their role in maintaining high biological productivity and in providing nutrient-rich nursery areas for valuable aquatic and wetland biota. The importance of salt marshes as a source of dissolved and particulate organic matter to the estuarine area and surrounding ocean was pointed out by Odum & de la Cruz (1967). Teal (1962) quantified the potential for large exports of organic matter from salt marshes to adjacent estuarine waters despite the high respiration rate of the dominant producer (Spartina alterniflora). On the other hand, Nixon (1980) reviewed the functioning of salt marshes and concluded that while many marshes do export carbon, it is doubtful that they make a significant difference to the production of finfish and shellfish in the adjacent waters. Similar conclusions were reached by Mann (1986). However, recent data from the extensive salt marshes of the Southeast US continue to indicate the importance of marsh-estuary exchanges in terms of organic export and trophic relationships (Kjerfve & McKellar 1980; Pomeroy & Wiegert 1981; Chrzanowski et al. 1982, 1983; Mitsch & Gosselink 1986).