Elsevier

Ecological Engineering

Volume 14, Issue 4, April 2000, Pages 337-362
Ecological Engineering

The effects of forest on stream water quality in two coastal plain watersheds of the Chesapeake Bay

https://doi.org/10.1016/S0925-8574(99)00060-9Get rights and content

Abstract

Forest had varying effects on stream nutrients in two coastal plain basins of the Delmarva Peninsula, USA. In the Choptank basin, forest was strongly associated with low stream total nitrogen (TN) and nitrate (NO3) concentrations (r2∼0.70), and forest placement along first order streams was important in maintaining low stream nitrogen (N) concentrations (r2∼0.35). In addition, a multiple regression model explained ∼40% of the stream total phosphorus (TP) variance and indicated that forest directly adjacent to streams (0–100 m) acted as a TP source and forest further away (100–300 m) from streams acted as a TP sink. In contrast, stream nutrients in the nearby Chester basin demonstrated a strong relationship with soil hydrologic properties. Forest had no significant effect on stream N and P because the finer-textured soils, higher stream slopes, and higher runoff potential of the Chester basin appeared to result in less baseflow compared to that in the Choptank basin. This reduced the opportunity for forest to intercept N via plant uptake and denitrification in the high runoff potential soils of the Chester basin. The high percentage of stormflow (40%) coupled with high stream slopes resulted in high soil erosion potential, which may explain the higher TP stream concentrations measured in the Chester compared to that in the Choptank. Differences in the hydrologic pathway appear to explain the different effects of forest on water quality in these two basins.

Introduction

Chesapeake Bay is the largest estuary in North America. Studies conducted since the 1970s indicate that increases in the intensity of agriculture, population growth, and sewage discharge are causing the Bay to become eutrophic (EPA, 1983). Nutrient enrichment of streams and other anthropogenic disturbances within the Bay’s watershed have triggered a series of undesirable effects. Decline of water quality in Chesapeake Bay is indicated by increased phytoplankton biomass (Malone et al., 1988, Harding, 1994), increased sediment loads (Cooper and Brush, 1991), decline of submerged aquatic vegetation (Kemp et al., 1983, Dennison et al., 1993, Stevenson et al., 1993), and loss of commercially valuable fish and waterfowl due to habitat loss and over-harvesting (Officer et al., 1984, Lubbers et al., 1990).

A regional, multi-state program has been created to reduce nutrient inputs to Chesapeake Bay by 40% by the year 2000 (EPA, 1987). While improvements in wastewater treatment and a ban on P-containing detergents has resulted in reduction of point-source loads, non-point source N and P from agriculture continue to be the largest contributor of nutrients to the Bay: 39 and 49% of total N and P nutrient sources, respectively (Chesapeake Bay Program, 1995). Reduction of nonpoint source N and P from agricultural activities will require reduction strategies which are tailored to local environmental conditions, including the three major physiographic regions within the basin (coastal plain, piedmont, ridge and valley).

The difficulty of addressing diffuse N and P pollution from agricultural activities is compounded by the long residence time of groundwater. Studies conducted by the US Geological Survey (USGS) (Bohlke and Denver, 1995) on the Delmarva peninsula have demonstrated that current stream discharge originated from groundwater recharge more than 20 years ago. Since large increases in fertilizer application occurred during 1960–1980 (Bohlke and Denver, 1995), nutrient input to Delmarva streams has increased (Fisher et al., 1998) and is expected to continue to increase during the next two decades. Economic pressure on farmers to provide an inexpensive food supply for the growing Bay population will increase the intensity of agricultural land use, making the 40% nutrient reduction goal signed by the Chesapeake Bay Commission increasingly difficult to achieve by the year 2000.

The ability of riparian vegetation, particularly forest, to mediate non-point source pollution in agricultural landscapes has received much attention (Hill, 1996). Riparian forests are those which flank stream banks and usually intercept a significant fraction of N and P moving towards the stream from adjacent uplands. Riparian forests which have saturated soils for extended periods during the growing season are also referred to as riparian wetlands. Riparian forests positively impact water quality by: (1) acting as effective sediment traps; (2) consuming and storing nutrients by accreting biomass; (3) stimulating microbial assimilation of nutrients in forest soils and (4) providing an environment conducive to anaerobic microbial dissimilation of nitrate to nitrogen gas (denitrification) or ammonia. The ability of riparian vegetation to intercept nutrients and eroded sediment depends not only on the presence of vegetation but also on a microbial energy source, (electron donor), adequate temperatures, redox conditions, groundwater aquifer characteristics, and position of vegetation within the landscape. Reviews by Haycock et al. (1993), Osborne and Kovacic (1993), Gilliam (1994) and Hill (1996) offer excellent discussions on the water quality functions of riparian vegetation.

Water quality functions of riparian forests have been studied throughout the Atlantic coastal plain in landscapes on or similar to those of the Delmarva peninsula. A riparian forest studied by Jordan et al. (1993) effectively removed nitrate from an adjacent corn field. The groundwater contained nitrate concentrations of ∼8 mg l−1 at the edge of a corn field which was reduced to 0.4 mg l−1 halfway (20 m) into the forest. A riparian forest studied by Peterjohn and Correll (1984) demonstrated a 90% reduction in annual sediment load, an 80% reduction of nitrate in overland flow and an 85% reduction in groundwater nitrate originating from an adjacent agricultural field. Lowrance et al. (1984) calculated that riparian forest in Georgia’s Little River Watershed retained 68% of N and 30% of P received from adjacent cropland and precipitation.

In contrast to the large number of detailed studies on the role of nutrient interception by riparian forests, little attention has been given to the spatial distribution of these forests at the large watershed area. While some studies have stressed the importance of landscape position (Whigham et al., 1988, Brinson, 1993, Haycock et al., 1993), few have included this variable in water quality studies. In a nationwide study conducted by Omernik et al. (1981), positional effects of forest and agriculture did not improve a regression model’s ability to predict stream nutrient levels over the use of land cover’s total extent within a watershed. The lack of forest positional effects on stream nutrients may have resulted from the high variation attributed to regional watershed characteristics which influence hydrologic flow and stream chemistry (soil type, precipitation, geology). Stream nutrient concentrations in the Salt Fork basin, a central Illinois watershed with ≈90% of its land in agriculture, was found to be more influenced by proximity of urban areas than by extent or position of cropland (Osborne and Wiley, 1988). However, with only 10% forest in the basin, most of which was in the lower portion, it is not surprising that positional effects were small in this highly disturbed basin. Johnston et al. (1990) determined that proximity of herbaceous wetlands to streams in a nine-county region surrounding Minneapolis significantly influenced nitrate and dissolved P during base flow as well as ammonium, nitrate, and TP during storm flow. In Chesapeake Bay’s Rhode River estuary, Houlahan et al. (1992) showed that an implementation of best management practices on cropland directly adjacent to streams could reduce agricultural nutrient loadings by 90%.

We have also attempted to understand the role of forest in retention of agricultural nutrients. Water quality studies were conducted in two adjacent basins dominated by agriculture on the Atlantic coastal plain in the Chesapeake Bay watershed. In addition to the extent of forest cover in these watersheds, this study attempted to characterize the effects of spatial distribution of forest throughout these basins. We hypothesize that riparian forests in coastal plain watersheds effectively remove and/or retain anthropogenic nutrients from agricultural activities. Furthermore, we hypothesize that the spatial distribution of forest within a watershed has an important effect on local stream nutrients. We analyze forest and other landscape variables and their effects on N and P concentrations in streams throughout the Choptank and Chester watersheds.

Section snippets

Study site description

The Choptank and Chester basins are located on the Atlantic coastal plain on the western side of the Delmarva peninsula within the Chesapeake watershed (Fig. 1, Fig. 2). The basins span the border between the states of Maryland and Delaware, and land use is dominated by agriculture and forest. Most of the agriculture is cropland, with small amounts of pasture (∼5%), nurseries (∼2%) and feedlots (∼0.5%). The majority of the cropland in these two watersheds is occupied by a corn–soybean rotation (

Methods

In a cooperative effort with the US Fish and Wildlife Service, we sampled at 59 non-tidal subwatersheds within these two basins (Fig. 4). Avoiding tidally influenced areas was necessary to estimate local hydrology and restricted the sampling to portions of the basin far from estuarine areas. Monthly base- and stormflow for each subwatershed were estimated using GWLF (Haith and Shoemaker, 1987). The sampled subwatersheds of both the Choptank and Chester basins were sparsely populated and devoid

Climate

A comparison of air temperature, precipitation and stream discharge during the sampling periods and long-term averages was made in order to determine if sampling conditions reflected ‘typical’ conditions. Monthly air temperatures for the Chester and Choptank watersheds during the sampling periods followed long-term trends with little deviation (Fig. 3). During the sampling period in the Choptank, February was significantly cooler and March and November were significantly warmer compared to

Discussion

Many studies of coastal plain riparian forests have demonstrated N and P retention/removal capabilities (Hill, 1996). However, the ability of these forests to retain nutrients from agricultural uplands requires an efficient delivery of nutrients. When coastal plain watersheds are characterized by sandy soils, there appears to be a strong relationship between riparian forest cover and low stream nutrient export (Peterjohn and Correll, 1984, Jordan et al., 1993, Gilliam, 1994). However, a

Conclusions

Forest had a different effect on stream nutrients in the Choptank and Chester coastal plain basins. In the Choptank, forest cover was strongly associated with low TN and NO3 concentrations. Within first order streams, the conduits of water from terrestrial to aquatic systems, the presence of forested stream banks also had a strong relationship with low stream N. In addition, the amount of riparian wetlands and degree of ‘wetness’ was inversely correlated with stream N in the Choptank basin. In

Acknowledgements

This work was supported under US Fish and Wildlife Service cooperative agreement 14-48-0005-92-9036 with University of Maryland, Center for Environmental Science. Computer support was provided by the US Fish and Wildlife Service, Chesapeake Bay Field Office. Special thanks are due to Chris Victoria and Anne Gustafson for their efforts in stream sampling and GIS database compilation. We thank Ray Weil, Dick Weismiller, Karen Prestagaard and Mike Kearney for ideas and suggestions. We also thank

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