Total Nutrient Load Research Articles

Simulation of base-flow and tile-flow for storm events in a subsurface drained watershed

Quantifying base-flow and tile-flow to agricultural drainage ditches is essential to understand groundwater flow and nutrient dynamics in a subsurface (tile) drained watershed. This can give an insight into the contributions of different flow components to the total nutrient loads in a stream. MODFLOW (Groundwater Vistas) was used to simulate (steady-state) base-flow and tile-flow for storm events in the subsurface drained Big Ditch watershed in Champaign County, IL, USA. A stream section of approximately 200 m in an agricultural drainage ditch was selected for this study. Two tile drains were draining into the stream section from both sides (north and south banks, respectively). Two cut-throat flumes were installed at the upstream and downstream ends of the stream section to do a mass balance of flow volumes to determine base-flow. Dataloggers and stilling basins were connected to the flumes and tile drains to measure flow rates. Twelve wells were installed on both sides of the channel section to measure groundwater level for model calibration. Three storm periods were used to calibrate the model and another three storm periods to validate it. The simulated mean hydraulic conductivity of the study site was 5.52E-04 m s−1. The mean conductance values of the tile drains flowing from north and south banks were 6.39E-04 m2 s−1 and 2.71E-03 m2 s−1, respectively. The simulated flow rates were within 20% of the measured rates. It may be concluded that the model simulated base-flow and tile-flow for the storm periods successfully.

Catchment scale modelling of point source and non-point source pollution loads using pollutant export coefficients determined from long-term in-stream monitoring data

This paper presents a framework to estimate pollutant export coefficients from commonly available in-stream water quality monitoring data and using these pollutant export coefficients for catchment scale modelling of pollution loadings. A case study is presented to illustrate the framework in the Fuji River catchment, Japan. The total pollution loading was estimated by constructing an empirical source contribution model. The estimated pollutant export coefficients were used to estimate non-point source pollution loadings and point source loadings were estimated using the data collected from major wastewater treatment plants within a catchment. The non-point source loadings, especially from forest and agriculture, were found to be the major contributor whereas the point source loadings were found to be a minor contributor of total organic matter and nutrient loadings in the study catchment. These results are encouraging especially given the pressing need to identify appropriate management practices to improve the water quality within the Fuji River catchment. It is recommended to investigate further the required number of water quality monitoring stations, sampling frequencies and sampling duration of water quality constituents to enhance the robustness and usefulness of the proposed methodology.

Land use scenarios for England and Wales: evaluation of management options to support ‘good ecological status’ in surface freshwaters

AbstractThis paper analyses historic records of agricultural land use and management for England and Wales from 1931 and 1991 and uses export coefficient modelling to hindcast the impact of these practices on the rates of diffuse nitrogen (N) and phosphorus (P) export to water bodies for each of the major geoclimatic regions of England and Wales. Key trends indicate the importance of animal agriculture as a contributor to the total diffuse agricultural nutrient loading on waters, and the need to bring these sources under control if conditions suitable for sustaining ‘Good Ecological Status’ under the Water Framework Directive are to be generated. The analysis highlights the importance of measuring changes in nutrient loading in relation to the catchment‐specific baseline state for different water bodies. The approach is also used to forecast the likely impact of broad regional scale scenarios on nutrient export to waters and highlights the need to take sensitive land out of production, introduce ceilings on fertilizer use and stocking densities, and controls on agricultural practice in higher risk areas where intensive agriculture is combined with a low intrinsic nutrient retention capacity, although the uncertainties associated with the modelling applied at this scale should be taken into account in the interpretation of model output. The paper advocates the need for a two‐tiered approach to nutrient management, combining broad regional policies with targeted management in high risk areas at the catchment and farm scale.

Estimation of nutrient load from urban diffuse sources: experiments with runoff sampling at pilot catchments of Lake Balaton, Hungary

About a quarter of the total nutrient loading of Lake Balaton (Hungary) originates from urban diffuse sources, mostly from direct shoreline watersheds. This load cannot be measured directly. Sampling of urban runoff can help improving load estimations. The dynamic processes characterizing the accumulation and washoff of contaminants suggest that randomly observed concentrations are likely under- or overestimated. The results of two recent pilot programs aimed towards achieving continuous measurement of nutrient load carried by urban runoff are introduced. Stations were implemented in two pilot catchments located on the shore of Lake Balaton. Storm event runoff was sampled automatically and manually. Discharge, precipitation and rainfall intensities were also recorded. Results proved that the more a specific pollutant is associated with solid particles, the more of its load comes from a few but large storm events, nevertheless the cumulative effect of small rainfall events is not negligible, either. Event mean concentrations of solid-related pollutants were found to be dependent on rainfall intensity. The derived empirical relationships for SS, TP and TN event mean concentrations were indeed found to be applicable for reducing the uncertainty of load estimations of these pollutants significantly, as compared to using long-time average (i.e. annual mean) concentration values.

Effects of Container Spacing Practice and Fertilizer Placement on Runoff from Overhead-Irrigated Sweet Viburnum

Abstract Information on how management practices affect runoff volume and nutrient content is needed to improve irrigation and fertilizer efficiency while minimizing environmental impacts. Runoff (leachate plus unintercepted irrigation and rain) was collected weekly for 20 weeks during production of sweet viburnum (Viburnum odoratissimum (L.) Ker-Gawl.) in trade #1 (2.7 liter) containers fertilized with 15 g (0.53 oz) of a resin-coated, controlled-release fertilizer Osmocote 18N–2.6P–10K (18–6–12), 8–9 month 21C (70F)] and overhead-irrigated with water at 1 cm/day (0.39 in). Treatments were a factorial arrangement of two container spacing practices [spaced at planting (SP) or spaced midseason (SM)] and two fertilizer placement methods [incorporated (INC) or surface-applied (SURF)]. Cumulative runoff volume averaged 1590 liters/m2 (3900 gal/100 ft2) or 66% of irrigation plus rain and was 9% higher for SP than SM. A 37% reduction in shoot dry weight of SP versus SM plants was attributed to heat stress in SP containers. SURF decreased N, P, and K leaching losses (mg/container) 42, 42, and 25%, respectively, at SP and 16, 25, and 4%, respectively, at SM. Nutrient leaching losses as a percent of applied were 11–18% for N, 7–13% for P, and 19–28% for K. Total nutrient loads in runoff were 4.6–11.1 g/m2 for N, 0.48–1.25 g/m2 for P, and 5.8–10.1 g/m2 for K with peak nutrient loss occurring during the first two weeks after planting.

Direct determination of total and fresh groundwater discharge and nutrient loads from a sandy beachface at low tide (Cape Henlopen, Delaware)

Total groundwater discharge from a sandy seepage site to the Delaware Bay at Cape Henlopen, Delaware, was determined together with the associated nutrient loads by trapping the discharge from a length of the shoreline in a tidal pond at low tide and measuring the discharge of the trapped water through a weir at steady state. Salinity was used to parse the total groundwater discharge from the beachface at low tide into a “recycled” estuarine component and a “new” fresh groundwater component. Based on 16 measurements over 18 months, average total discharge and average freshwater discharge at low tide were found to be 2.76 ± 1.08 and 0.87 ± 0.43 L min−1 m−1 of shoreline, respectively. The estuarine component of discharge varied with the maximum height of the immediately preceding high tide. Owing to the control of freshwater discharge by the average upland hydraulic gradient away from the beach, individual measurements of freshwater discharge at low tide represent a good estimate of this discharge component over time scales of hours to days. Based on the nutrient concentrations in the discharge waters, nutrient loads from the beach to the adjacent estuary at low tide were 148 ± 79, 7.5 ± 5.1, and 197 ± 101 µmol min−1 m−1 of shoreline for total dissolved N, P, and Si, respectively. Groundwater discharge and these nutrient fluxes contribute significantly to the unusually productive benthic communities that inhabit the seepage and nearby intertidal zones.

WATER QUALITY AND NUTRIENT LOADS IN THE ABUKUMA WATERSHED UNDER NORMAL FLOW CONDITION

This study investigated present status of water quality and nutrient loads in the Abukuma River Watershed under normal flow condition. The analysis of routinely measured data showed typical transversal patterns of BOD, TN and TP concentrations along the mainstream, which can be related to the agricultural, domestic and livestock pollutant sources. Water quality in tributaries showed strong correspondence to the land use. NO3-N is strongly related to the percentage of cropland, and BOD, NH4-N and TP to the percentage of built area in each sub-basin. The total nutrient and BOD loads from tributaries and wastewater treatment plants are greatly reduced in the main stream, probably because of the deposition and nutrient uptake enhanced by dams in the mainstream. Phosphorus in the mainstream is likely deposited by chemical and/or physical processes in the flow such as adsorption and flocculation.

Sediment nutrient flux in a coastal lake impacted by diverted Mississippi River water

The internal nutrient load from bottom sediment to the water column of a Louisiana Barataria Basin lake (Lake Cataouatche) receiving diverted Mississippi River water was determined. Dissolved reactive phosphorus (DRP), ammonium (NH4-N) and nitrate (NO3-N) flux from sediment to water column were measured. The DRP flux averaged−0.22 mg m−2 d−1 under aerobic water column conditions, as compared with that 3.29 mg m−2 d−1 under anaerobic conditions. The average NH4-N released under anaerobic conditions (1.42 mg m−2 d−1) was significantly greater than rates under aerobic conditions (−0.02 mg m−2 d−1), indicating a strong relationship between nutrient flux and oxygen availability in the water column. The average NO3-N flux was 2.13 mg m−2 d−1 under aerobic conditions as compared with−0.24 mg m−2 d−1 under anaerobic conditions in the sediment–water column. When the water column maintained under anaerobic conditions was switched to aerobic conditions, the DRP, NH4-N, and NO3-N concentrations in overlying water decreased rapidly over a short period of time. The mean annual internal DRP and NH4-N load from the sediment to the overlying water was estimated to be 69.26 and 29.9 tonnes (Mg) yr−1 respectively, which represents a significant portion of the total nutrient load to the Lake. Results demonstrate that the internal flux of nutrients from sediments can contribute a significant portion of the total nutrient load to the water column and should be considered in decisions on impact of nutrient in diverted Mississippi River on water quality of Barataria Basin.

Surface Irrigation of Dairy Farm Effluent, Part I: Nutrient and Bacterial Load

When handled separately from manures, dairy farm effluents (DE) are costly to manage because of their low nutrient content and large volume. In anticipation of using surface irrigation to lower the land application cost of DE, this projects investigated the impact, on DE characteristics, of different sources and storage systems. Milk house wastewaters were monitored on two farms using a 500l manhole intercepting these before entering the septic tank. Manure runoff, with and without milk house wastewater, was also characterised on six farms for 1 yr, and on two farms for three consecutive years, where each farm used a different management system. The results indicated that DE containing milk house wastewater, manure runoff or a mixture of both had a relatively low nutrient load, confirming that their application rate needed to range between 205 and 2050 m 3 ha � 1 , depending on the management system used. Stored along with solid manure, DE generally had a higher total solids (TS), nutrient loads and bacterial count, as compared to that drained away from the solid manure. Furthermore, for effluent drained away from solid manures, rainfall rather than snow occurring from October to May, inclusively, tended to increase their TS and nutrient load. The ratio of faecal coliforms to faecal streptococci (FC/FS) was greater than 1� 0 when milk house wastewaters were stored along with the manure runoff because milk house wastewaters increased the death rate for FS compared to FC.

Diffuse source apportionment of the Po river eutrophying load to the Adriatic sea: Assessment of Lombardy contribution to Po river nutrient load apportionment by means of an integrated modelling approach

The source apportionment of the annual nutrient load carried by the Po river to the Adriatic sea has been studied. An integrated modelling approach was applied to the Lombardy plain area, which covers about 34% of the Po river watershed area and accounts for about 50% of the point sources’ loads carried by the river. To extract all the information available from direct instream measurements, two different modelling tools were alternatively used. The source apportionment was investigated considering both dry and wet weather scenarios. In order to quantify the apportionment in dry-weather conditions, the Lombardy portion of the Po river basin was modelled by using the US-EPA QUAL2E model. Such a simulation allowed to assess a significant contribution (about 50% of the total dry-weather load) of a not rain-driven diffuse pollution component (i.e. groundwater, springs, lake emissaries). Moreover, to estimate the rain-driven surface runoff contribution to the instream total load, the Lombardy plain area was also modelled by means of the US-DA SWAT model. SWAT results indicate a runoff contribution to the Po river instream total load of about 10 000 t N yr −1 and 1300 t P yr −1 (i.e. approximately the 10–20% of the total annual Lombardy nutrient load). At the event scale (i.e. the single rainstorm event) the runoff contribution may rise up to 30–80% of the total instream load. Finally, the total annual nitrogen load at the Po basin closure was estimated for the period 1985–2001. Out of a total annual load of 140 000 t N yr −1, Lombardy accounts for 43% (point plus diffuse sources). The rain-driven diffuse sources constitute the 20% of the overall total load, the point sources account for 40%, whereas the remaining 40% is mainly constituted by “dry-weather diffuse sources” (i.e. groundwater, springs, lake emissaries).

Pigment composition and size distribution of phytoplankton in a confined Mediterranean salt marsh ecosystem

Pigment composition and size distribution of phytoplankton were analysed in a group of Mediterranean salt marshes, where hydrology is dominated by sudden inputs during sea storms, followed by long periods of confinement. These marshes are characterized by a low inorganic–organic nutrient ratio, and inorganic nitrogen is especially scarce due to denitrification. Nutrients were the main factor affecting phytoplankton biomass, while zooplankton grazing did not control either phytoplankton community composition, or their size distribution. The relative abundance of the different phytoplankton groups was analysed by correspondence analysis using the pigment composition measured by high-performance liquid chromatography (HPLC) and analysed with the CHEMTAX programme. In this analysis, phytoplankton pigment composition was correlated with two nutrient gradients. The first gradient was the ratio of nitrate–total nitrogen (TN), since the different phytoplankton groups were distributed according to their eco–physiological differences in nitrogen uptake. The second gradient was correlated with total nutrient loading. Biomass size distributions frequently showed a lack of intermediate sized nanophytoplankton (2.5–4 μm in diameter), and the importance of this lack of intermediate sizes correlated with dinoflagellate biomass. These results suggested that in confined environments, where nutrients are mainly in an organic form, dinoflagellates take advantage of their mixotrophy, by competing and grazing on smaller phytoplankters simultaneously.

Nutrient And Particulate Inputs Into The Mar Menor Lagoon (Se Spain) From An Intensive Agricultural Watershed

The Mar Menor is a Mediterranean coastal lagoon of high conservation interest, but highly threatened by non-point pollution derived from agricultural lands. This is the first comprehensive study that evaluates the inputs into the Mar Menor from a drainage channel and the Albujon wadi, the main watercourse, and their influence on the trophic state of the lagoon. Discharge variation during the study period was closely related to the precipitation pattern. Suspended sediments and particulate organic matter loads greatly increased with flash floods. Nitrate concentrations, too, increased after heavy autumn rains through washing of the nitrates accumulated in soils, although no significant correlations were found between nitrate concentrations and precipitation or discharge. The nitrate load depended on several factors including the intensity and frequency of precipitation, and the nitrates accumulated in soils as a result of fertilisation. Phosphate concentrations decreased with higher flows. The total input into the lagoon from the two discharges estimated for the period September 2002–October 2003 were 10,142 t.yr−1of suspended sediments, 389 t.yr−1 of particulate organic matter, 2,010 t.yr−1 of dissolved inorganic nitrogen (93 % as NO−3) and 178 t.yr−1 of soluble reactive phosphorus. The Albujon wadi exported about 80% of the N load and 70% of the P load. Higher flows contributed approximately 80% of the total discharge and nutrient loads, 99% of suspended sediment and 88% of the particulate organic matter. Mean suspended solids and nutrient concentrations in the lagoon followed a spatial pattern, decreasing with distance from the mouth of the Albujon wadi. Water nitrate and phosphate concentrations in the Mar Menor lagoon were lower than reference limits for eutrophic conditions, except after heavy rains. Nutrient concentrations were positively correlated with precipitation variables and particulate and nutrient inputs. Phosphorus was the most limiting nutrient in the Mar Menor, the DIN:SRP ratios found being higher than the Redfield ratio on all sampling dates. Phytoplankton bloom in the lagoon was only found next to the Albujon mouth during late summer and extending 5000 m into the lagoon in autumn as a consequence of large freshwater discharge caused by flash floods. Around 53.6% of the variation in chlorophyll a in the lagoon was explained by the NH4 and NO3 inputs and distance from the Albujon mouth. A good deal of P input into the lagoon is retained in the sediments, supporting a high biomass of the benthic macroalga Caulerpa prolifera in spring and summer.

Point and diffuse nutrient emissions and loads in the transboundary Danube River Basin. I. A modelling approach

Nutrient emissions by point and diffuse sources were estimated for the Danube River Basin on the basis of a harmonized and integrated database at the sub-catchment level by application of the MONERIS model. The estimation of nutrient emissions was carried out for the period 1998-2000 for 388 sub-catchments as well as the thirteen countries each responsible for more than 1 % of the total area of the Danube River Basin. A total of 756 kt a -1 nitrogen (N) and 68 kt a -1 phosphorus (P) are emitted by the different pathways. 77 % of N-emissions and 53 % of P-emissions were due to diffuse sources. The dominant pathway is groundwater for N-emissions and point source discharges for P-emissions with 48% and 47 %, respectively. The regionalized analysis facilitates the identification of spatial hot spots for the individual emission pathways. This analysis allows one to deduce specific measures for the reduction of nutrient emissions in the river system of the Danube and the Black Sea. Background nutrient emissions were calculated to approx. 6.5 kt a -1 P and 61 kt a -1 N. Consequently, the human impact is about 10 times higher than the background for P and 12 times higher for N, and 54 and 35% of the total N and P-emissions were due to agricultural activities. Urban settlements are responsible for 50% (P) and 22% (N) of the total emissions. The total nutrient loads of the Danube into the Black Sea were estimated to 22.5 kt a -1 for total P, 396 kt a -1 for dissolved inorganic N, and 468 kt a -1 for total N. The comparison of calculated and observed loads shows that the mean load deviation for the investigated sub-catchments of the Danube River Basin was 21 % for dissolved inorganic N (93 stations) and 32 % for P (65 stations).

Aquatic birds as bioindicators of trophic changes and ecosystem deterioration in the Mar Menor lagoon (SE Spain)

The Mar Menor is the largest coastal lagoon in the Western Mediterranean and it is an important site for wintering and breeding waterfowl. During recent decades several hydrological and land-use changes in the watershed have increasingly threatened the conservation of the lagoon due to the development of urban areas, tourism and agriculture. A dynamic system model has been developed at the watershed scale to estimate the annual load of nutrients reaching the Mar Menor-associated wetlands. At present, mean annual loadings of approximately 2000 tonnes of nitrogen and 60 tonnes of phosphorus are delivered to the lagoon. The simulation results emphasize the role of heavy rainfall events and floods in the formation of the total nutrient load. The composition of aquatic bird communities has been used to assess the nutrient impact on the lagoon food-web. The Great Crested Grebe is apparently the species most closely dependant on local trophic conditions. The related Black-necked Grebe, that dominates the waterbird community of the lagoon, plays a similar role, but its more opportunistic response to changes in food resources, reduces its indicator value. The abundance of the two species of grebes seems to closely track the nitrogen load curve, especially during the first phase of enrichment, suggesting the existence of a direct trophic relationship. In the following phase, jellyfish blooms coincide with the bird decline. Jellyfishes seem to have a buffering effect towards nutrients, determining a bottom up limitation to other trophic compartments. In recent years, this buffering capacity has probably been overloaded, favouring the growth of new food resources available to the grebes. Unlike grebes, Mergus serrator, a typical piscivorous bird, does not seem to be affected positively by eutrophication since it shows a long-term stability in numbers or even a slight decline. Since this suspected decline would parallel a long-term reduction of fish catches, the species could be regarded as a potential indicator of habitat deterioration.

A comparison of diatom phosphorus transfer functions and export coefficient models as tools for reconstructing lake nutrient histories

Summary1. We compared the baseline phosphorus (P) concentrations inferred by diatom‐P transfer functions and export coefficient models at 62 lakes in Great Britain to assess whether the techniques produce similar estimates of historical nutrient status.2. There was a strong linear relationship between the two sets of values over the whole total P (TP) gradient (2–200 μg TP L−1). However, a systematic bias was observed with the diatom model producing the higher values in 46 lakes (of which values differed by more than 10 μg TP L−1 in 21). The export coefficient model gave the higher values in 10 lakes (of which the values differed by more than 10 μg TP L−1 in only 4).3. The difference between baseline and present‐day TP concentrations was calculated to compare the extent of eutrophication inferred by the two sets of model output. There was generally poor agreement between the amounts of change estimated by the two approaches. The discrepancy in both the baseline values and the degree of change inferred by the models was greatest in the shallow and more productive sites.4. Both approaches were applied to two lakes in the English Lake District where long‐term P data exist, to assess how well the models track measured P concentrations since approximately 1850. There was good agreement between the pre‐enrichment TP concentrations generated by the models. The diatom model paralleled the steeper rise in maximum soluble reactive P (SRP) more closely than the gradual increase in annual mean TP in both lakes. The export coefficient model produced a closer fit to observed annual mean TP concentrations for both sites, tracking the changes in total external nutrient loading.5. A combined approach is recommended, with the diatom model employed to reflect the nature and timing of the in‐lake response to changes in nutrient loading, and the export coefficient model used to establish the origins and extent of changes in the external load and to assess potential reduction in loading under different management scenarios.6. However, caution must be exercised when applying these models to shallow lakes where the export coefficient model TP estimate will not include internal P loading from lake sediments and where the diatom TP inferences may over‐estimate TP concentrations because of the high abundance of benthic taxa, many of which are poor indicators of trophic state.

Macroalgal Population and Sustainability

Algal blooms, often associated with eutrophication, are common in temperate regions in coastal lagoons and estuarine areas as well as in open seas. However, because of recent changes in water flows and sediment and nutrient loads in most rivers, eutrophication phenomena are now occurring in areas previously non affected. One of its most widespread and easily recognized effects in a coastal marine ecosystem is the excessive growths and drifts of macroalgae, to such a degree that the biomass becomes a significant problem. The nutrients, notably from anthropogenic origin, constitute a significant factor in the appearance of the blooms in coastal waters. Besides a background originating from natural areas and precipitations, nutrients come from agriculture, sewage, industry, and aquaculture, in very various proportions following the local conditions. At first, the development of opportunist and tolerant seaweeds acts as a purifying system. Then, when the seaweed is stranded or the environmental conditions become unfavourable, the plants die and decompose. The degradation of the chemical equilibrium induces the break-down of the biological balance in the ecosystem. At this stage, a large biomass can become troublesome, resulting in an increase in the herbivore population, competition or toxicity towards flora and fauna, alteration of the sediment, recycling of nutrients and pollutants in the ecosystem, nuisance for local residents and reduction of tourism (beaches degradation, chironomids, odors). Sooner or later, the water deteriorates, which may have a toxic effect on fauna including the commercial fish stocks. Eutrophication problems have thus become a matter of major concern. Its management is neither a simple nor a cheap task. But, environmental and sustainability concerns may, in a not too distant future, override the lack of financial benefit. At any rate, underlayed action is required because in estuarine and potamologic milieus, potential land disposal arrangements are becoming rarer and costlier. Control of eutrophication can only be reached effectively by drastic reduction of the total nutrient load of an overloaded water system. In order to be successful, only an integrated approach, based on a water body's nutrient mass balance and taking into consideration specific geographical, climatological and ecological conditions, can be effective. Moreover, it may take many years before recovery of a eutrophicated ecosystem, because the sediments constitute a complementary source of nutrients for macroalgae. But, prevention will prevent the nutrients from being discharged in the lagoons or in the sea. It will concern the waste treatment, agricultural mode change and treatment of gaseous effluent from polluting human activities.

Impact of Climate Change on the Water Cycle and Nutrient Losses in a Finnish Catchment

Changes in climate, either long or short-term changes, can alter significantly the hydrological behavior of catchments. A statistical analysis of a thirty-four year time series of meteorological data collected in the Vantaanjoki watershed (Southern Finland) shows an increase in temperature and precipitation. The hydrological model SWAT was applied to the Vantaanjoki watershed in order to assess the impact of the measured transient climate change on the hydro-biogeochemical behavior of the catchment. The SWAT model was calibrated and validated for a period extending from 1965 to 1998. The model performance was evaluated comparing the measured and predicted time series for flow, suspended solids, total nitrogen and total phosphorus at the watershed outlet. The model was then run for the same period with climatic data where the observed increase in temperature and precipitation was removed using non-parametric techniques. It was shown that the observed climate change was responsible for the decrease of the snow cover and increase of winter runoff. On an annual basis, small increases were noted in nutrient losses, however, with significant seasonal differences. Globally, the observed climate change was responsible for an increased contribution of diffuse nutrient losses to the total nutrient load.

Nitrogen and Phosphorus Flux Rates from Sediment in the Lower St. Johns River Estuary

Internal cycling of nutrients from the sediment and water column can be an important contribution to the total nutrient load of an aquatic ecosystem. Our objective was to estimate the internal nutrient loading of the Lower St. Johns River (LSJR). Dissolved reactive phosphorus (DRP) and ammonium (NH(4)-N) flux from sediments were measured under aerobic and anaerobic water column conditions using intact cores, to estimate the overall contribution of the sediments to P and N loading to the LSJR. The DRP flux under aerobic water column conditions averaged 0.13 mg m(-2) d(-1), approximately 37 times lower than that under anaerobic conditions (4.77 mg m(-2) d(-1)). The average NH(4)-N released from the anaerobic cores (18.03 mg m(-2) d(-1)) was also significantly greater than in the aerobic cores for all sites and seasons, indicating the strong relationship between nutrient fluxes and oxygen availability in the water column. The mean annual internal DRP load was estimated to be 330 metric tons (Mg) yr(-1), 21% of the total P load to the river, while the mean annual internal load of NH(4)-N was determined to be 2066 Mg yr(-1), 28% of the total N load to the LSJR estuary. As water resource managers reduce external loading to the LSJR the frequency of anaerobic events should decline, thereby reducing nutrient fluxes from the sediment to the water column, reducing the internal loading of DRP and NH(4)-N. Results from this study demonstrate that the internal flux of nutrients from sediments may be a significant portion of the total load and should be accounted for in the total nutrient budget of the river for successful restoration.

A watershed perspective on nutrient enrichment, science, and policy in the Patuxent River, Maryland: 1960–2000

The Patuxent River, Maryland, is a nutrient-overenriched tributary of the Chesapeake Bay. Nutrient inputs from sewage outfalls and nonpoint sources (NPS) have grown substantially during the last four decades, and chlorophyll a levels have increased markedly with concomitant reductions in water quality and dissolved oxygen concentrations. The Patuxent has gained national attention because it was one of the first river basins in the U.S. for which basin-wide nutrient control standards were developed. These included a reduction in NPS inputs and a limit on both nitrogen (N) and phosphorus (P) loadings in sewage discharges intended to return the river to 1950s conditions. Full implementation of point source controls occurred by 1994, but population growth and land-use changes continue to increase total nutrient loadings to the river. The present paper provides the perspectives of scientists who participated in studies of the Patuxent River and its estuary over the last three decades, and who interacted with policy makers as decisions were made to develop a dual nutrient control strategy. Although nutrient control measures have not yet resulted in dramatic increases in water quality, we believe that without them, more extensive declines in water quality would have occurred. Future reductions will have to come from more effective NPS controls since future point source loadings will be difficult to further reduce with present technology. Changing land use will present a challenge to policy makers faced with sprawling population growth and accelerated deforestation.

Effect of fall-applied manure practices on runoff, sediment, and nutrient surface transport from silage corn in south coastal British Columbia

Runoff from manured cropland during the wet fall and winter season, when 70% of the annual rainfall occurs, is a surface water quality concern in the Lower Fraser Valley of British Columbia. This study compares different fall-manure application strategies on runoff and contaminant transport from silage corn (Zea mays) land. The treatments were (i) a control, which did not receive manure in the fall; (ii) manure broadcast in the fall on corn stubble; and (iii) manure broadcast in the fall on corn stubble with an established relay crop. Runoff, solids, and nutrients loads from natural precipitation were measured on replicated experimental plots (0.0125 ha) from 1996 to 1998. Fall-applied manure on 3–5% sloping silage cornland without a relay crop resulted in a high risk to surface water quality, due to high suspended solid loads of between 7 and 14 Mg ha-1 yr-1 and high nutrient transport with mean annual total Kjeldahl N (TKN) P, and K loads of 98, 21, and 63 kg ha-1, respectively. Compared with no relay crop, intercropping silage corn with a relay crop of Italian ryegrass (Lolium multiflorum) reduced the mean annual runoff and suspended solid load by 53 and 74%, respectively, TKN load by 56%, P load by 42%, K load by 31%, and Cu load by 57%. Even though total nutrient loads were lower with the relay crop treatment, all fall manure treatments including the relay crop resulted in nutrient loads above guidelines for the first three runoff events immediately following application. Key words: Runoff, suspended solids, nutrient loads, relay crop, cover crop, silage corn, manure