IMPACTS OF URBANIZATION ON NUTRIENT CONCENTRATIONS IN SMALL SOUTHEASTERN COASTAL STREAMS1

ABSTRACT: Coastal watersheds in the southeastern United States are rapidly changing due to population growth and attendant increases in residential development, industry, and tourism related commerce. This research examined spatial and temporal patterns of nutrient concentrations in streams from 10 small watersheds (< 4 km²) that drain into Murrells Inlet (impacted) and North Inlet (pristine), two high salinity estuaries along the South Carolina coast. Monthly grab samples were collected during baseflow during 1999 and analyzed for total and dissolved inorganic and organic forms of nitrogen and phosphorus. Data were grouped into forested wetland creeks (representing predevelopment reference sites), urban creeks, and urban ponds. DON and NH4 concentrations were greater in forested streams than in urban streams. NO3 and TP concentrations were greatest in urban streams. Seasonally, concentrations were highest during summer for TN, NH4, DON, and TP, while NO3 concentrations were greatest during winter. Nutrient ratios clearly highlighted the reduction in organic nitrogen due to coastal development. Multiple regression models to predict instream nutrient concentrations from land use in Murrells Inlet suggest that effects are not significant (small r²). The findings indicate that broad land use/land cover classes cannot be used to predict nutrient concentrations in streams in the very small watersheds in our study areas.     

Dissolved organic nitrogen regulation in freshwaters

Dissolved organic nitrogen (DON) has been hypothesized to play a major role in N cycling in a variety of ecosystems. Our aim was to assess the seasonal and concentration relationships between dissolved organic carbon (DOC), DON, and NO3- within 102 streams and 16 lakes within catchments of differing complexity situated in Wales. Further, we aimed to assess whether patterns of land use, soil type, and vegetation gave consistent trends in DON and dissolved inorganic nitrogen (DIN) relationships over a diverse range of catchments. Our results reinforce that DON constitutes a significant component of the total dissolved N pool typically representing 40 to 50% of the total N in streams and lakes but sometimes representing greater than 85% of the total dissolved N. Generally, the levels of DON were inversely correlated with the concentration of DIN. In contrast to DIN concentrations, which showed distinct seasonality, DON showed no consistent seasonal trend. We hypothesize that this reflects differences in the bioavailability of these two N types. The amount of DON, DOC, and DIN was significantly related to soil type with higher DON export from Histosol-dominated catchments in comparison with Spodosol-dominated watersheds. Vegetation cover also had a significant effect on DON concentrations independent of soil type with a nearly twofold decrease in DON export from forested catchments in comparison with nonforested watersheds. Due to the diversity in catchment DON behavior, we speculate that this will limit the adoption of DON as a broad-scale indicator of catchment condition for use in monitoring and assessment programs.     

Estimating the Frequency and Extent of Overbank Flow of a Baseflow‐Dominated Stream Flowing through a Wetland

River floodplains provide critical habitat for a wide range of animal and plant species and reduce phosphorus and nitrogen loads in streams. It has been observed that baseflow‐dominated streams flowing through wetlands are commonly at or near bankfull and overflow their banks much more frequently than other streams. However, there is very little published quantitative support for this observation. The study focuses on a 1‐km reach of Black Earth Creek, a stream in the Midwestern United States (U.S.). We used one‐dimensional hydraulic modeling to estimate bankfull discharge at evenly spaced stream cross sections, and two‐dimensional modeling to quantitate the extent of wetland inundation as a function of discharge. We then used historical streamflow data from two U.S. Geological Survey gaging stations to quantitate the frequency of wetland inundation. For the with‐sediment case, the frequency of overbank conditions at the 38 cross sections in the wetland ranged from 3 to 85 days per year and averaged 43 days per year. Ten percent of the wetland was inundated for an average of 35 days per year. For the without‐sediment case, the frequency of overbank conditions ranged from 2.6 to 48 days per year and averaged 14 days per year. Also, 10% of the wetland was inundated for an average of 25 days per year. These unusually high rates of floodplain inundation are likely due in part to the very low stream gradient and shallow depths of overbank flow.     

FLUVIAL SEDIMENT STORAGE IN WETLANDS1

ABSTRACT: The important ecological and hydrological roles of wetlands are widely recognized, but the geomorphic functions of wetlands are also critical. Wetlands can be defined in geomorphic, as well as in hydrological or biological terms, and a geomorphic definition of wetlands is proposed. An analysis of fluvial sediment budget studies shows that wetlands typically serve as short-term sediment sinks or longer-term sediment storage sites. In ten study basins of various sizes, an estimated 14 to 58 percent of the total upland sediment production is stored in alluvial wetland or other aquatic environments. Of the sediment reaching streams, 29 to 93 percent is stored in alluvial wetland or channel environments. For basins of more than 100 km², more than 15 percent of total upland sediment production and more than 50 percent of sediment reaching streams is deposited in wetlands. The data underestimates the magnitude of wetland sediment storage due to the lack of data from large river systems. A theoretical analysis of river channel sediment delivery shows that wetland and aquatic sediment storage is inevitable in fluvial systems and systematically related to basin size. Results suggest that wetlands should be managed in the context of drainage basins, rather than as discrete, independent units.     

Hydrologic Controls on Nitrogen and Phosphorous Dynamics in Relict Oxbow Wetlands Adjacent to an Urban Restored Stream

Although wetlands are known to be sinks for nitrogen (N) and phosphorus (P), their function in urban watersheds remains unclear. We analyzed water and nitrate (NO₃^?) and phosphate (PO₄^3?) dynamics during precipitation events in two oxbow wetlands that were created during geomorphic stream restoration in Baltimore County, Maryland that varied in the nature and extent of connectivity to the adjacent stream. Oxbow 1 (Ox1) received 1.6‐4.2% and Oxbow 2 (Ox2) received 4.2‐7.4% of cumulative streamflow during storm events from subsurface seepage (Ox1) and surface flow (Ox2). The retention time of incoming stormwater ranged from 0.2 to 6.7 days in Ox1 and 1.8 to 4.3 days in Ox2. Retention rates in the wetlands ranged from 0.25 to 2.74 g N/m²/day in Ox1 and 0.29 to 1.94 g N/m²/day in Ox2. Percent retention of the NO₃^?‐N load that entered the wetlands during the storm events ranged from 64 to 87% and 23 to 26%, in Ox1 and Ox2, respectively. During all four storm events, Ox1 and Ox2 were a small net source of dissolved PO₄^3? to the adjacent stream (i.e., more P exited than entered the wetland), releasing P at a rate of 0.23‐20.83 mg P/m²/day and 3.43‐24.84 mg P/m²/day, respectively. N and P removal efficiency of the oxbows were regulated by hydrologic connectivity, hydraulic loading, and retention time. Incidental oxbow wetlands have potential to receive urban stream and storm flow and to be significant N sinks, but they may be sources of P in urban watersheds.     

NITROGEN AND PHOSPHORUS CONCENTRATIONS IN FOREST STREAMS OF THE UNITED STATES1

ABSTRACT: Seventy to eighty percent of the water flowing in rivers in the United States originates as precipitation in forests. This project developed a synoptic picture of the patterns in water chemistry for over 300 streams in small, forested watersheds across the United States. Nitrate (NO₃^?) concentrations averaged 0.31 mg N/L, with some streams averaging ten times this level. Nitrate concentrations tended to be higher in the northeastern United States in watersheds dominated by hardwood forests (especially hardwoods other than oaks) and in recently harvested watersheds. Concentrations of dissolved organic N (mean 0.32 mg N/L) were similar to those of NO₃～, whereas ammonium (NH₄⁺) concentrations were much lower (mean 0.05 mg N/L). Nitrate dominated the N loads of streams draining hardwood forests, whereas dissolved organic N dominated the streams in coniferous forests. Concentrations of inorganic phosphate were typically much lower (mean 12 mg P/L) than dissolved organic phosphate (mean 84 mg P/L). The frequencies of chemical concentrations in streams in small, forested watersheds showed more streams with higher NO₃^? concentrations than the streams used in national monitoring programs of larger, mostly forested watersheds. At a local scale, no trend in nitrate concentration with stream order or basin size was consistent across studies.     

Upland disturbance affects headwater stream nutrients and suspended sediments during baseflow and stormflow

Because catchment characteristics determine sediment and nutrient inputs to streams, upland disturbance can affect stream chemistry. Catchments at the Fort Benning Military Installation (near Columbus, Georgia) experience a range of upland disturbance intensities due to spatial variability in the intensity of military training. We used this disturbance gradient to investigate the effects of upland soil and vegetation disturbance on stream chemistry. During baseflow, mean total suspended sediment (TSS) concentration and mean inorganic suspended sediment (ISS) concentration increased with catchment disturbance intensity (TSS: R2= 0.7, p = 0.005, range = 4.0-10.1 mg L(-1); ISS: R2= 0.71, p = 0.004, range = 2.04-7.3 mg L(-1)); dissolved organic carbon (DOC) concentration (R2= 0.79, p = 0.001, range = 1.5-4.1 mg L(-1)) and soluble reactive phosphorus (SRP) concentration (R2= 0.75, p = 0.008, range = 1.9-6.2 microg L(-1)) decreased with increasing disturbance intensity; and ammonia (NH4+), nitrate (NO3-), and dissolved inorganic nitrogen (DIN) concentrations were unrelated to disturbance intensity. The increase in TSS and ISS during storms was positively correlated with disturbance (R2= 0.78 and 0.78, p = 0.01 and 0.01, respectively); mean maximum change in SRP during storms increased with disturbance (r = 0.7, p = 0.04); and mean maximum change in NO3- during storms was marginally correlated with disturbance (r = 0.58, p = 0.06). Soil characteristics were significant predictors of baseflow DOC, SRP, and Ca2+, but were not correlated with suspended sediment fractions, any nitrogen species, or pH. Despite the largely intact riparian zones of these headwater streams, upland soil and vegetation disturbances had clear effects on stream chemistry during baseflow and stormflow conditions.     

Linking Soil Erosion to Instream Dissolved Phosphorus Cycling and Periphyton Growth

Phosphorus (P) is a limiting nutrient in freshwater systems and when present in runoff from agricultural lands or urban centers may contribute to excessive periphyton growth. In this study, we examined the link between soil erosion and delivery of eroded soil to streams during flow events, and the impact of that freshly deposited soil on dissolved reactive P (DRP) concentrations and periphyton growth under baseflow conditions when the risk of stream eutrophication is greatest. A microcosm experiment was designed to simulate the release of P from soil which had been amended with different amounts of P fertilizer to overlying water during baseflow conditions. Unglazed tiles, inoculated for five days in a second order stream, were incubated for seven days in microcosms containing soil with eight levels of soil Mehlich‐3 plant available phosphorus (M3P) ranging from 20 to 679 mg/kg M3P. Microcosm DRP was monitored. Following incubation tiles were scraped and the periphyton analyzed for chlorophyll a. Microcosm DRP concentrations increased with increasing soil M3P and equilibrium phosphorus concentration (EPC₀). Relationships between M3P, EPC₀, and DRP were nonlinear and increases in soil M3P and/or DRP had a greater impact on biomass accumulation when these parameters were above threshold values of 30 mg/kg M3P and 0.125 mg/L DRP. Significantly, this ecological threshold corresponds to the agronomic thresholds above which increased soil M3P does not increase plant response.     

Evaluating stream water quality through land use analysis in two grassland catchments: impact of wetlands on stream nitrogen concentration

We evaluated the impacts of natural wetlands and various land uses on stream nitrogen concentration in two grassland-dominated catchments in eastern Hokkaido, Japan. Analyzing land use types in drainage basins, measuring denitrification potential of its soil, and water sampling in all seasons of 2003 were performed. Results showed a highly significant positive correlation between the concentration of stream NO3-N and the proportion of upland area in drainage basins in both catchments. The regression slope, which we assumed to reflect the impact on water quality, was 24% lower for the Akkeshi catchment (0.012 +/- 0.001) than for the Shibetsu catchment (0.016 +/- 0.001). In the Akkeshi catchment, there was a significant negative correlation between the proportion of wetlands in the drainage basins and stream NO3-N concentration. Stream dissolved organic nitrogen (DON) and carbon (DOC) concentrations were significantly higher in the Akkeshi catchment. Upland and urban land uses were strongly linked to increases in in-stream N concentrations in both catchments, whereas wetlands and forests tended to mitigate water quality degradation. The denitrification potential of the soils was highest in wetlands, medium in riparian forests, and lowest in grasslands; and was significant in wetlands and riparian forests in the Akkeshi catchment. The solubility of soil organic carbon (SOC) and soil moisture tended to determine the denitrification potential. These results indicate that the water environment within the catchments, which influences denitrification potential and soil organic matter content, could have caused the difference in stream water quality between the two catchments.     

Differences in Dissolved Organic Carbon and Nitrogen Responses to Storm‐Event and Ground‐Water Conditions in a Forested,Glaciated Watershed in Western New York1

Abstract: Differences in the storm‐event responses of dissolved organic carbon (DOC) and nitrogen (DON) in streamflow and ground water were evaluated for a glaciated forested watershed in western New York. Eight to ten storm events with varying rainfall amounts, intensities, and antecedent moisture conditions were studied for three catchments (1.6, 3.4, and 696 ha) over a three‐year period (2003‐2005). Concentrations of DOC in streamflow exiting the catchments were significantly higher for storm events following a dry period, whereas no similar response was observed for DON. Highest DON concentrations in streamflow were typically associated with storm events following wet antecedent moisture conditions. In addition to antecedent moisture conditions, DOC concentrations were also positively correlated with precipitation amounts, while DON did not reveal a consistent pattern. Streamwater and ground‐water concentrations of DOC during storm events were also strongly correlated with riparian ground‐water depths but a similar relationship was not observed for DON. Ground‐water DON concentrations were also more variable than DOC. We hypothesized that the differences in DOC and DON responses stemmed from the differences in catchment sources of these solutes. This study suggests that while DOC and DON are intrinsically linked as components of dissolved organic matter, their dynamics and exports from watersheds may be regulated by a different set of mechanisms and factors. Identifying these differences is critical for developing more reliable and robust models for transport of dissolved organic matter.     

Aquatic Plant and Dissolved Oxygen Changes in a Reference‐Condition Prairie Stream Subjected to Experimental Nutrient Enrichments

Effects of controlled nutrient additions on a prairie stream were studied using a before‐after‐control‐impact paired design. The site is in a reference condition with low soluble nitrate (NO₃) and phosphate (soluble reactive phosphorus [SRP]) in summer (3 μg NO₃‐N/L, 4 μg SRP/L). Nutrients were added to two reaches over the growing season at two levels (Low Dose — 39 μg NO₃‐N/L and 4.4 SRP/L; High Dose — 119 μg NO₃‐N/L and 15.6 μg SRP/L). Continuously measured dissolved oxygen (DO) and changes in aquatic flora were compared to an upstream Control. Enrichment led microalgae and filamentous algae to increase in density, areal coverage, and thickness, and the magnitude of the changes were largely concordant with dosing (more in the High Dose); algal growth also suppressed macrophytes in the High Dose. Enrichment caused significant increases in diel DO swings whose magnitudes were consistent with dosing level. In the High Dose, benthic algae flourished in the growing season and then senesced en masse in fall. The decomposing algae led DO to crash (ca. 0 mg/L on the bottom), but DO impacts were out‐of‐sync with peak algal growth and photosynthesis, which occurred weeks earlier. This finding provides a plausible explanation as to why high DO delta in streams impacts aquatic life even when concurrently measured DO is not low. When DO crashed, DO was longitudinally patchy, some areas having low DO near the bottom, others near saturation. Geomorphology and exposure to wind may have caused this pattern.     

Do Nutrients Limit Algal Periphyton in Small Blackwater Coastal Plain Streams?1

Abstract: We examine the potential for nutrient limitation of algal periphyton biomass in blackwater streams draining the Georgia coastal plain. Previous studies have investigated nutrient limitation of planktonic algae in large blackwater rivers, but virtually no scientific information exists regarding how algal periphyton respond to nutrients under different light conditions in smaller, low‐flow streams. We used a modification of the Matlock periphytometer (nutrient‐diffusing substrata) to determine if algal growth was nutrient limited and/or light limited at nine sites spanning a range of human impacts from relatively undisturbed forested basins to highly disturbed agricultural sites. We employed four treatments in both shaded and sunny conditions at each site: (1) control, (2) N (NO₃‐N), (3) P (PO₄‐P), and (4) N + P (NO₃‐N + PO₄‐P). Chlorophyll a response was measured on 10 replicate substrates per treatment, after 15 days of in situ exposure. Chlorophyll a values did not approach what have been defined as nuisance levels (i.e., 100‐200 mg/m²), even in response to nutrient enrichment in sunny conditions. For Georgia coastal plain streams, algal periphyton growth appears to be primarily light limited and can be secondarily nutrient limited (most commonly by P or N + P combined) in light gaps and/or open areas receiving sunlight.     

Nitrogen Subsidies from Hillslope Alder Stands to Streamside Wetlands and Headwater Streams,Kenai Peninsula,Alaska

We examined nitrogen transport and wetland primary production along hydrologic flow paths that link nitrogen‐fixing alder (Alnus spp.) stands to downslope wetlands and streams in the Kenai Lowlands, Alaska. We expected that nitrate concentrations in surface water and groundwater would be higher on flow paths below alder. We further expected that nitrate concentrations would be higher in surface water and groundwater at the base of short flow paths with alder and that streamside wetlands at the base of alder‐near flow paths would be less nitrogen limited than wetlands at the base of long flow paths with alder. Our results showed that groundwater nitrate‐N concentrations were significantly higher at alder‐near sites than at no‐alder sites, but did not differ significantly between alder‐far sites and no‐alder sites or between alder‐far sites and alder‐near sites. A survey of ¹⁵N stable isotope signatures in soils and foliage in alder‐near and no‐alder flow paths indicated the alder‐derived nitrogen evident in soils below alder is quickly integrated downslope. Additionally, there was a significant difference in the relative increase in plant biomass after nitrogen fertilization, with the greatest increase occurring in the no‐alder sites. This study demonstrates that streamside wetlands and streams are connected to the surrounding landscapes through hydrologic flow paths, and flow paths with alder stands are potential “hot spots” for nitrogen subsidies at the hillslope scale.     

Benthic Algal (Periphyton) Growth Rates in Response to Nitrogen and Phosphorus: Parameter Estimation for Water Quality Models

Nitrogen (N) and phosphorus (P) are significant pollutants that can stimulate nuisance blooms of algae. Water quality models (e.g., Water Quality Simulation Program, CE‐QUAL‐R1, CE‐QUAL‐ICM, QUAL2k) are valuable and widely used management tools for algal accrual due to excess nutrients in the presence of other limiting factors. These models utilize the Monod and Droop equations to associate algal growth rate with dissolved nutrient concentration and intracellular nutrient content. Having accurate parameter values is essential to model performance; however, published values for model parameterization are limited, particularly for benthic (periphyton) algae. We conducted a 10‐day mesocosm experiment and measured diatom‐dominated periphyton biomass accrual through time as chlorophyll a (chl a) and ash‐free dry mass (AFDM) in response to additions of N (range 5–11,995 µg nitrate as nitrogen [NO₃‐N]/L) and P (range 0.89–59.51 µg soluble reactive phosphorus/L). Resulting half‐saturation coefficients and growth rates are similar to other published values, but minimum nutrient quotas are higher than those previously reported. Saturation concentration for N ranged from 150 to 2,450 µg NO₃‐N/L based on chl a and from 8.5 to 60 µg NO₃‐N/L when based on AFDM. Similarly, the saturation concentration for P ranged from 12 to 29 µg‐P/L based on chl a, and from 2.5 to 6.1 µg‐P/L based on AFDM. These saturation concentrations provide an upper limit for streams where diatom growth can be expected to respond to nutrient levels and a benchmark for reducing nutrient concentrations to a point where benthic algal growth will be limited.     

ALGAL PRODUCTIVITY AND NITRATE ASSIMILATION IN AN EFFLUENT DOMINATED CONCRETE LINED STREAM1

This study examined algal productivity and nitrate assimilation in a 2.85 km reach of Cucamonga Creek, California, a concrete lined channel receiving treated municipal wastewater. Stream nitrate concentrations observed at two stations indicated nearly continuous loss throughout the diel study. Nitrate loss in the reach was approximately 11 mg/L/d or 1.0 g/m²/d as N, most of which occurred during daylight. The peak rate of nitrate loss (1.13 mg/l/hr) occurred just prior to an afternoon total CO₂ depletion. Gross primary productivity, as estimated by a model using the observed differences in dissolved oxygen between the two stations, was 228 mg/L/d, or 21 g/m²/d as O₂. The observed diel variations in productivity, nitrate loss, pH, dissolved oxygen, and CO₂ indicate that nitrate loss was primarily due to algal assimilation. The observed levels of productivity and nitrate assimilation were exceptionally high on a mass per volume basis compared to studies on other streams; these rates occurred because of the shallow stream depth. This study suggests that concrete‐lined channels can provide an important environmental service: lowering of nitrate concentrations similar to rates observed in biological treatment systems.     

Does Wetland Location Matter When Managing Wetlands for Watershed‐Scale Flood and Drought Resilience?

Wetland protection and restoration strategies that are designed to promote hydrologic resilience do not incorporate the location of wetlands relative to the main stream network. This is primarily attributed to the lack of knowledge on the effects of wetland location on wetland hydrologic function (e.g., flood and drought mitigation). Here, we combined a watershed‐scale, surface–subsurface, fully distributed, physically based hydrologic model with historical, existing, and lost (drained) wetland maps in the Nose Creek watershed in the Prairie Pothole Region of North America to (1) estimate the hydrologic functions of lost wetlands and (2) estimate the hydrologic functions of wetlands located at different distances from the main stream network. Modeling results showed wetland loss altered streamflow, decreasing baseflow and increasing stream peakflow during the period of the precipitation events that led to major flooding in the watershed and downstream cities. In addition, we found that wetlands closer to the main stream network played a disproportionately important role in attenuating peakflow, while wetland location was not important for regulating baseflow. The findings of this study provide information for watershed managers that can help to prioritize wetland restoration efforts for flood or drought risk mitigation.     

Wetland Flowpaths Mediate Nitrogen and Phosphorus Concentrations across the Upper Mississippi River Basin

Eutrophication, harmful algal blooms, and human health impacts are critical environmental challenges resulting from excess nitrogen and phosphorus in surface waters. Yet we have limited information regarding how wetland characteristics mediate water quality across watershed scales. We developed a large, novel set of spatial variables characterizing hydrological flowpaths from wetlands to streams, that is, “wetland hydrological transport variables,” to explore how wetlands statistically explain the variability in total nitrogen (TN) and total phosphorus (TP) concentrations across the Upper Mississippi River Basin (UMRB) in the United States. We found that wetland flowpath variables improved landscape-to-aquatic nutrient multilinear regression models (from R² = 0.89 to 0.91 for TN; R² = 0.53 to 0.84 for TP) and provided insights into potential processes governing how wetlands influence watershed-scale TN and TP concentrations. Specifically, flowpath variables describing flow-attenuating environments, for example, subsurface transport compared to overland flowpaths, were related to lower TN and TP concentrations. Frequent hydrological connections from wetlands to streams were also linked to low TP concentrations, which likely suggests a nutrient source limitation in some areas of the UMRB. Consideration of wetland flowpaths could inform management and conservation activities designed to reduce nutrient export to downstream waters.     

The Influence of Urban Density and Drainage Infrastructure on the Concentrations and Loads of Pollutants in Small Streams

Effective water quality management of streams in urbanized basins requires identification of the elements of urbanization that contribute most to pollutant concentrations and loads. Drainage connection (the proportion of impervious area directly connected to streams by pipes or lined drains) is proposed as a variable explaining variance in the generally weak relationships between pollutant concentrations and imperviousness. Fifteen small streams draining independent subbasins east of Melbourne, Australia, were sampled for a suite of water quality variables. Geometric mean concentrations of all variables were calculated separately for baseflow and storm events, and these, together with estimates of runoff derived from a rainfall-runoff model, were used to estimate mean annual loads. Patterns of concentrations among the streams were assessed against patterns of imperviousness, drainage connection, unsealed (unpaved) road density, elevation, longitude (all of which were intercorrelated), septic tank density, and basin area. Baseflow and storm event concentrations of dissolved organic carbon (DOC), filterable reactive phosphorus (FRP), total phosphorus (TP) and ammonium, along with electrical conductivity (EC), all increased with imperviousness and its correlates. Hierarchical partitioning showed that DOC, EC, FRP, and storm event TP were independently correlated with drainage connection more strongly than could be explained by chance. Neither pH nor total suspended solids concentrations were strongly correlated with any basin variable. Oxidized and total nitrogen concentrations were most strongly explained by septic tank density. Loads of all variables were strongly correlated with imperviousness and connection. Priority should be given to low-impact urban design, which primarily involves reducing drainage connection, to minimize urbanization-related pollutant impacts on streams.     

Nitrate Removal in a Restored Spring‐Fed Wetland,Pennsylvania, USA1

ABSTRACT:  In 2001, the 1.04‐ha Hornbaker wetland in south‐central Pennsylvania was restored by blocking an artificial drainage ditch to increase water storage and hydraulic retention time (HRT). A primary goal was to diminish downstream delivery of nitrate that enters the wetland from a limestone spring, its main source of inflow. Wetland inflow and outflow were monitored weekly for two years to assess nitrate flux, water temperature, pH, and specific conductivity. In Year 2, spring discharge was measured weekly to allow calculation of nitrate loads and hydraulic retention time. Surface runoff was confirmed to be a small fraction of wetland inflows via rainfall‐runoff modeling with TR‐55. The full dataset (n = 102) was screened to remove 13 weeks in which spring discharge constituted < 85% of total inflows because of high precipitation and surface runoff. Over two years (n = 89), mean nitrate‐nitrogen concentrations were 7.89 mg/l in inflow and 3.68 mg/l in outflow, with a mean nitrate removal of 4.19 mg/l. During Year 2 (n = 47), for which nitrate load data were available, the wetland removed an average of 2.32 kg N/day, 65% of the load. Nitrate removal was significantly correlated with HRT, water temperature, and the concentration of nitrate in inflow and was significantly greater during the growing season (5.36 mg/l, 64%) than during the non‐growing season (3.23 mg/l, 43%). This study indicates that hydrologic restoration of formerly drained wetlands can provide substantial water quality benefits and that the hydrologic characteristics of spring‐fed wetlands, in particular, support effective nitrogen removal.     

Natural and Anthropogenic Methane Emission from Coastal Wetlands of South India

For the first time, the methane emissions from diverse coastal wetlands of South India have been measured. Annual emission rates varied widely, ranging from 3.10 mg/m²/hr (Bay of Bengal) to 21.56 mg/m²/hr (Adyar River), based on nature of the perturbance to each of the ecosystems studied. Distinct seasonality in methane emission was noticed in an unpolluted ecosystem (mangrove: 7.38 mg/m²/hr) and over a twofold increase was evident in the ecosystem that was disturbed by human activities (21.56 mg/m²/hr). The wide ranges in estimate suggest that methanogenesis occurs by both natural and anthropogenic activities in these coastal wetlands. Several physical and chemical factors such as salinity, sulfate, oxygen, and organic matter content influenced methanogenesis to a large degree in each of these ecosystems in addition to individual responses to human-induced stress. For example, there was a clear negative correlation between oxygen availability (0.99), sulfate (0.98), and salinity (0.98) with CH₄ emission in the Adyar river ecosystem. Although similar results were obtained for the other wetland ecosystems, CH₄ emission was largely influenced by tidal fluctuations, resulting in a concomitant increase in methanogenesis with high sulfate concentrations. This study demonstrates that coastal wetlands are potentially significant sources of atmospheric methane and could be a greater source if anthropogenic perturbations continue at the current rate. RID=" ID=" &ast;Author to whom correspondence should be addressed.