Agriculture and urban activities are major sources of phosphorus and nitrogen to aquatic ecosystems. Atmospheric deposition further contributes as a source of N. These nonpoint inputs of nutrients are difficult to measure and regulate because they derive from activities dispersed over wide areas of land and are variable in time due to effects of weather. In aquatic ecosystems, these nutrients cause diverse problems such as toxic algal blooms, loss of oxygen, fish kills, loss of biodiversity (including species important for commerce and recreation), loss of aquatic plant beds and coral reefs, and other problems. Nutrient enrichment seriously degrades aquatic ecosystems and impairs the use of water for drinking, industry, agriculture, recreation, and other purposes. Based on our review of the scientific literature, we are certain that (1) eutrophication is a widespread problem in rivers, lakes, estuaries, and coastal oceans, caused by overenrichment with P and N; (2) nonpoint pollution, a major source of P and N to surface waters of the United States, results primarily from agriculture and urban activity, including industry; (3) inputs of P and N to agriculture in the form of fertilizers exceed outputs in produce in the United States and many other nations; (4) nutrient flows to aquatic ecosystems are directly related to animal stocking densities, and under high livestock densities, manure production exceeds the needs of crops to which the manure is applied; (5) excess fertilization and manure production cause a P surplus to accumulate in soil, some of which is transported to aquatic ecosystems; and (6) excess fertilization and manure production on agricultural lands create surplus N, which is mobile in many soils and often leaches to downstream aquatic ecosystems, and which can also volatilize to the atmosphere, redepositing elsewhere and eventually reaching aquatic ecosystems. If current practices continue, nonpoint pollution of surface waters is virtually certain to increase in the future. Such an outcome is not inevitable, however, because a number of technologies, land use practices, and conservation measures are capable of decreasing the flow of nonpoint P and N into surface waters. From our review of the available scientific information, we are confident that: (1) nonpoint pollution of surface waters with P and N could be reduced by reducing surplus nutrient flows in agricultural systems and processes, reducing agricultural and urban runoff by diverse methods, and reducing N emissions from fossil fuel burning; and (2) eutrophication can be reversed by decreasing input rates of P and N to aquatic ecosystems, but rates of recovery are highly variable among water bodies. Often, the eutrophic state is persistent, and recovery is slow.

Phosphorus (P) is an essential element for all life forms. It is a mineral nutrient. Orthophosphate is the only form of P that autotrophs can assimilate. Extracellular enzymes hydrolyze organic forms of P to phosphate. Eutrophication is the overenrichment of receiving waters with mineral nutrients. The results are excessive production of autotrophs, especially algae and cyanobacteria. This high productivity leads to high bacterial populations and high respiration rates, leading to hypoxia or anoxia in poorly mixed bottom waters and at night in surface waters during calm, warm conditions. Low dissolved oxygen causes the loss of aquatic animals and release of many materials normally bound to bottom sediments including various forms of P. This release of P reinforces the eutrophication. Excessive concentrations of P is the most common cause of eutrophication in freshwater lakes, reservoirs, streams, and headwaters of estuarine systems. In the ocean, N becomes the key mineral nutrient controlling primary production. Estuaries and continental shelf waters are a transition zone, where excessive P and N create problems. It is best to measure and regulate total P inputs to whole aquatic ecosystems, but for an easy assay it is best to measure total P concentrations, including paniculate P, in surface waters or N/P atomic ratios in phytoplankton. S OCIETY normally wishes to maintain a reasonable level of productivity in our lakes, rivers, and estuaries and this requires the presence of modest levels of mineral nutrients. Historically, many of these water bodies have progressed from low productivity or oligotrophic settings to productive mesotrophic conditions to overenriched hypertrophic or eutrophic conditions. The results are often algal or cyanobacterial mats, anoxia, and fish kills leading to greatly reduced biodiversity (e.g., Carpenter et al., 1969; Jaworski, 1981; Likens, 1972). The relationships among P input, primary production n, dissolved oxygen, biodiversity and trophic status are shown conceptually in Fig. 1. From the human perspective it is desirable to prevent or minimize eutro

Abstract. Nutrient (C, N, and P) concentration changes were measured in surface runoff and shallow groundwater as they moved through a small agricultural (cropland) watershed located in Maryland. During the study period (March 1981 to March 1982, dramatic changes in water-borne nutrient loads occurred in the riparian forest of the watershed. From surface runoff waters that had transited :::::50 m of riparian forest, an estimated 4.1 Mg of particulates, II kg of particulate organic-N, 0.83 kg ofammonium-N, 2. 7 kg ofnitrate-N and 3.0 kg of total particulate-P per ha of riparian forest were removed during the study year. In addition, an estimated removal of 45 kg· ha-1 ·yr-' ofnitrate-N occurred in subsurface flow as it moved through the riparian zone.Nutrient uptake rates for the cropland and riparian forest were estimated. These systems were then compared with respect to their pathways of nutrient flow and ability to retain nutrients. The cropland appeared to retain fewer nutrients than the riparian forest and is thought to incur the majority of its nutrient losses in harvested crop. The dominant pathway of total-N loss from the riparian forest seemed to be subsurface flux. Total phosphorus loss from the riparian forest appeared almost evenly divided between surface and subsurface losses.Nutrient removals in the riparian forest are thought to be of ecological significance to receiving waters and indicate that coupling natural systems and managed habitats within a watershed may reduce diffuse-source pollution.

/ Maryland, Virginia, and Pennsylvania, USA, have agreed to reduce nutrient loadings to Chesapeake Bay by 40% by the year 2000. This requires control of nonpoint sources of nutrients, much of which comes from agriculture. Riparian forest buffer systems (RFBS) provide effective control of nonpoint source (NPS) pollution in some types of agricultural watersheds. Control of NPS pollution is dependent on the type of pollutant and the hydrologic connection between pollution sources, the RFBS, and the stream. Water quality improvements are most likely in areas of where most of the excess precipitation moves across, in, or near the root zone of the RFBS. In areas such as the Inner Coastal Plain and Piedmont watersheds with thin soils, RFBS should retain 50%-90% of the total loading of nitrate in shallow groundwater, sediment in surface runoff, and total N in both surface runoff and groundwater. Retention of phosphorus is generally much less. In regions with deeper soils and/or greater regional groundwater recharge (such as parts of the Piedmont and the Valley and Ridge), RFBS water quality improvements are probably much less. The expected levels of pollutant control by RFBS are identified for each of nine physiographic provinces of the Chesapeake Bay Watershed. Issues related to of establishment, sustainability, and management are also discussed.Research is sometimes applied to broad-scale environmental issues with inadequate knowledge or incomplete understanding. Public policies to encourage or require landscape management techniques such as riparian (streamside) management will often need to proceed with best professional judgment decisions based on incomplete understanding.Riparian forest buffer systems (RFBS) are streamside ecosystems managed for the enhancement of water quality through control of nonpoint source pollution (NPS) and protection of the stream environment. The use of riparian management zones is relatively well established as a best management practice (BMP) for water quality improvement in forestry practices (Comer-

Summary 1. Riparian structure and function were considered from a longitudinal perspective in order to identify multiscale couplings with adjacent ecosystems and to identify research needs. 2. We characterized functional zones (with respect to vegetation development in association with various biogeochemical processes) within geomorphological settings using a delineation based upon erosional, transitional and depositional properties. 3. Vegetation dynamics within the riparian corridor are clearly influenced substantially by hydrological disturbance regimes. In turn, we suggest that vegetation productivity and diversity may widely influence riverine biogeochemical processes, especially as related to the consequences of changing redox conditions occurring from upstream to downstream. 4. However, surface and groundwater linkages are the predominant controls of landscape connectivity within riparian systems. 5. The importance of riparian zones as sources and sinks of matter and energy was examined in context of structural and functional attributes, such as sequestering or cycling of nutrients in sediments, retention of water in vegetation, and retention, diffusion or dispersal of biota. 6. The consequences of interactions between different communities (e.g. animals and plants, micro‐organisms and plants) on biogeochemical processes are notably in need of research, especially with respect to control of landscape features. Multiscale approaches, coupling regional and local factors in all three spatial dimensions, are needed in order to understand more synthetically and to model biogeochemical and community processes within the river‐riparian‐upland landscape of catchments.

Abstract. During a 1-year period we measured discharges of water, suspended solids, and nutrients from 27 watersheds having differing proportions of cropland in the Piedmont and Coastal Plain provinces of the Chesapeake Bay drainage. Annual flow-weighted mean concentrations of nitrate and organic N and C in stream water correlated with the relative proportions of base flow and storm flow. As the proportion of base flow increased, the concentration of nitrate increased and the concentrations of organic N and C decreased. This suggests that discharge of nitrate is promoted by groundwater flow but discharges of organic N and C are promoted by surface runoff. Concentrations of N species also increased as the proportion of cropland increased. We developed a statistical model that predicts concentrations of N species from the proportions of cropland and base flow. P concentrations did not correlate with cropland or base flow but correlated with the concentration of suspended solids, which differed among watersheds. IntroductionOver the past few decades, rivefine discharges of plant nutrients have increased in response to enormous increases in anthropogenic inputs.

We measured annual discharges of water, sediments, and nutrients from 17 Chesapeake Bay watersheds with differing proportions of agricultural lands on the inner, central, and outer Coastal Plain. In all regions of the Coastal Plain, the flow-weighted mean concentrations of N species in watershed discharge increased as the proportion of cropland in the watershed increased. In contrast, the concentrations of P species did not correlate with any land use. Instead, P concentrations correlated with the concentration of suspended particles, which differed greatly among watersheds in different regions of the Coastal Plain. Consequently, the ratio of NIP in discharges differed widely among watersheds, potentially affecting N or P limitation of phytoplankton growth in the receiving waters. Concentrations of dissolved silicate, organic C, pH, and alkalinity in discharges did not differ greatly among watersheds or correlate with land use. Nitrogen discharge correlated with net anthropogenic inputs of N to the watershed, but usually less than one-third of the net anthropogenic inputs were discharged.GRICULTURAL activities can increase fluvial dis-

To investigate the ability of riparian forest to intercept nutrients leaving adjacent cropland, we examined changes in the chemistry of groundwater flowing from a corn (Zen mays L.) field through a riparian forest. This study provided a comparison to previous studies of a different forest. We sampled groundwater from a transect of wells, and used a Br-tracer to confirm that groundwater moved laterally along the transect through the forest. As groundwater flowed through the forest, NO; concentrations decreased from about 8 mgl L at the edge of the corn field to <0.4 mg/L halfway through the forest. Dissolved organic N and NH: increased by less than 0.1 mg/ L, and dissolved organic C did not change with distance. Sulfate remained constant with distance until midway through the forest, where it began to increase. Chloride concentration rose until midway through the forest, then fell. Values of pH increased from under 5 at the edge of the corn field to over 7 at the stream bank, perhaps as a result of the NO; consumption. Most of the change in NO; occurred abruptly at the edge of a floodplain within the forest. There the water table was closest to the surface and soil Eh below the water table was less than -90 mV. Such strongly reducing conditions may have promoted denitrification in the floodplain. In contrast, soil Eh on the adjacent hill slope was above 500 mV, too high to support denitrification. There were only slight seasonal changes in groundwater chemistry. We also studied the net annual accretion of sediment in the riparian forest by measuring changes in the elevation of the soil surface. There was little or no accretion in the forest, but along a path of overland storm flow there was net erosion. Thus, nutrient retention by this forest, in contrast with the forest we previously studied, was entirely a below ground process. Functional differences within sections of this forest and among different riparian forests suggest a need for research on the factors that control nutrient retention. SEVERAL STUDIES have shown that riparian forests can take up N, P, and sediment from discharges leaving adjacent croplands (e.g., Lowrance et al., 1984a,b;Peterjohn and Correll, 1984, 1986;Jacobs and Gilliam, 1985;Pinay and Decamps, 1988;Correll and Weller, 1989;Haycock and Pinay, 1993). This finding has important implications for land use management. Removal of NO; reduces the hazards of high NO3 concentrations in drinking water. Nitrate removal also raises the pH of the groundwater, thus reducing acidification of downstream aquatic ecosystems. Removal of N and P also reduces eutrophication of aquatic ecosystems. Removal of sediment from surface runoff reduces turbidity and sediment deposition downstream.Strong effects of riparian forests on agricultural runoff have consistently been reported for coastal plain forests with shallow underlying aquicludes. The aquicludes force cropland groundwater flowing toward the stream to move laterally through the near-surface layers of the riparian forest. Such forests can remov...