Donald E. Weller

Relating nutrient discharges from watersheds to land use and streamflow variability

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.

Human contributions to terrestrial nitrogen flux

This article assesses the sources and fates of anthropogenic fixed nitrogen. Anthropogenic nitrogen from most sources comes together in the common pathways of the agricultural food chain. The topics covered in this article include the following: inputs of newly fixed nitrogen; nitrogen flow in agriculture; calculating production of plant nitrogen; calculating consumption of nitrogen; harvests and grazing and the effect on nitrogen; nitrogen flow through livestock; net trade; nitrogen flows in regions of the United States; riverine discharges. 51 refs., 4 figs., 4 tabs.

Nutrient flux in a landscape: Effects of coastal land use and terrestrial community mosaic on nutrient transport to coastal waters

Long-term interdisciplinary studies of the Rhode River estuary and its watershed in the mid-Atlantic coastal plain of North America have measured fluxes of nitrogen and phosphorus fractions through the hydrologically-linked ecosystems of this landscape. These ecosystems are upland forest, cropland, and pasture; streamside riparian forests; floodplain swamps; tidal brackish marshes and mudflats; and an estuarine embayment. Croplands discharged far more nitrogen per hectare in runoff than did forests and pastures. However, riparian deciduous hardwood forest bordering the cropland removed over 80 percent of the nitrate and total phosphorus in overland flows and about 85 percent of the nitrate in shallow groundwater drainage from cropland. Nevertheless, nutrient discharges from riparian forests downslope from croplands still exceeded discharges from pastures and other forests. The atomic ratio of nitrogen to phosphorus discharged from the watersheds into the estuary was about 9 for total nutrients and 6 for inorganic nutrient fractions. Such a low N:P ratio would promote nitrogen rather than phosphorus limitation of phytoplankton growth in the estuary. Estuarine tidal marshes trapped particulate nutrients and released dissolved nutrients. Subtidal mudflats in the upper estuary trapped particulate P, released dissolved phosphate, and consumed nitrate. This resulted in a decrease in the ratio of dissolved inorganic N:P in the estuary. However, the upper estuary was a major sink for total phosphorus due to sediment accretion in the subtidal area. Bulk precipitation accounted for 31 percent of the total nongaseous nitrogen influx to the landscape, while farming accounted for 69 percent. Forty-six percent of the total non-gaseous nitrogen influx was removed as farm products, 53 percent either accumulated in the watershed or was lost in gaseous forms, and 1 percent entered the Rhode River. Of the total phosphorus influx to the landscape, 7 percent was from bulk precipitation and 93 percent was from farming. Forty-five percent of the total phosphorus influx was removed as farm products, 48 percent accumulated in the watershed, and 7 percent entered the Rhode River. These nitrogen and phosphorus discharges into the Rhode River, although a small fraction of total loadings to the watershed, were large enough to cause seriously overenriched conditions in the upper estuary.

Self-thinning exponent correlated with allometric measures of plant geometry

The —3/2 power rule of self—thinning, which describes the course of growth and mortality in crowded, even—aged plant stands, predicts that average mass is related to plant density by a power equation with exponent —3/2. The rule is widely accepted as an empirical generalization and quantitative rule or law. Simple geometric models of space occupation by growing plants yield a power equation, but the exponent can differ from —3/2 when realistic assumptions about the allometric growth of plants are considered. Because such deviations conflict with the empirical evidence for the —3/2 value as a law—like constant, the geometric model have not produced an accepted explanation and the thinning rule remains poorly understood. Recent studies have concluded that thinning exponents can deviate more widely from —3/2 than previously thought, motivating the present re—evaluation of the geometric explanation. I extend the simple models to predict the relationships of the thinning exponent to allometric exponents derived from commonly measured stand dimensions, such as height, average mass, average bole diameter at breast height (DBH), and average bole basal area. If the form and exponent of the thinning equation arise from the geometry of space filling, then thinning exponents should be systematically related to the exponents of allometric equations relating average height to average mass, average height to average DBH, and average height to average basal area. I also predict some values for the slopes and intercepts of regression lines relating thinning exponents to the allometric exponents. The predictions are verified by statistically comparing the thinning exponents and allometric exponents of self—thinning populations. The expected negative correlations are present and statistically significant (P ≤ .05), and the slopes and intercepts of linear regressions relating thinning exponents to allometric exponents are near the predicted values. These results support the hypothesis that the thinning equation arises from the geometry of space filling, but recognition that thinning exponents differ from —3/2 as predicted by simple geometric considerations weakens the case for a quantitative rule or law.

HEURISTIC MODELS FOR MATERIAL DISCHARGE FROM LANDSCAPES WITH RIPARIAN BUFFERS

For landscapes with riparian buffers, we develop and analyze models pre- dicting landscape discharge based on material release by an uphill source area, the spatial distribution of riparian buffer along a stream, and retention within the buffer. We model the buffer as a grid of cells, and each cell transmits a fixed fraction of the materials it receives. We consider the effects of variation in buffer width and buffer continuity, quantify the relative contributions of source elimination and buffer retention to total discharge reduction, and develop statistical relationships to simplify and generalize the models. Width variability reduces total buffer retention, increases the width needed to meet a management goal, and changes the importance of buffer retention relative to source elimination. Variable- width buffers are less efficient than uniform-width buffers because transport through areas of below-average buffer width (particularly gaps) dominates landscape discharge, especially for narrow buffers of highly retentive cells. Uniform-width models overestimate retention, so width variability should be considered when testing for buffer effects or designing buffers for water quality management. Adding riparian buffer to a landscape can decrease material discharge by increasing buffer retention and by eliminating source areas. Source elimination is more important in unretentive or wide buffers, while buffer retention dominates in narrow, retentive buffers. We summarize model results with simpler statistical relationships. For unretentive buffers, average width is the best predictor of landscape discharge, while the frequency of gaps was best for narrow, retentive buffers. Together, both predictors explain >90% of the variance in average landscape transmission for any value of buffer reten- tiveness. We relate our results to ecological theory, landscape-scale buffer effects, buffer management, and water quality models. We recommend more empirical studies of buffer width variability and its effects on material discharge. Landscape models should represent width variability and the nonlinear interactions between buffers and source areas.

Improved methods for quantifying potential nutrient interception by riparian buffers

Efforts to quantify the effects of riparian buffers on watershed nutrient discharges have been confounded by a commonly used analysis, which estimates buffer potential as the percentage of forest or wetland within a fixed distance of streams. Effective landscape metrics must instead be developed based on a clear conceptual model and quantified at the appropriate spatial scale. We develop new metrics for riparian buffers in two stages of increasing functional specificity to ask: (1) Which riparian metrics are more distinct from measures of whole watershed land cover? (2) Do functional riparian metrics provide different information than fixed-distance metrics? (3) How do these patterns vary within and among different physiographic settings? Using publicly available geographic data, we studied 503 watersheds in four different physiographic provinces of the Chesapeake Bay Drainage. In addition to traditional fixed-distance measures, we calculated mean buffer width, gap frequency, and measures of variation in buffer width using both “unconstrained” metrics and “flow-path” metrics constrained by surface topography. There were distinct patterns of relationship between watershed and near-stream land cover in each physiographic province and strong correlations with watershed land cover confounded fixed-distance metrics. Flow-path metrics were more independent of watershed land cover than either fixed-distance or unconstrained measures, but both functional metrics provided greater detail, interpretability, and flexibility than the fixed-distance approach. Potential applications of the new metrics include exploring the potential for land cover patterns to influence water quality, accounting for buffers in statistical nutrient models, quantifying spatial patterns for process-based modeling, and targeting management actions such as buffer restoration.

How novel is too novel? Stream community thresholds at exceptionally low levels of catchment urbanization

Novel physical and chemical conditions of many modern ecosystems increasingly diverge from the environments known to have existed at any time in the history of Earth. The loss of natural land to urbanization is one of the most prevalent drivers of novel environments in freshwaters. However, current understanding of aquatic community response to urbanization is based heavily upon aggregate indicators of community structure and linear or wedge-shaped community response models that challenge ecological community theory. We applied a new analytical method, threshold indicator taxa analysis (TITAN), to a stream biomonitoring data set from Maryland to explicitly evaluate linear community response models to urbanization that implicitly assume individual taxa decline or increase at incrementally different levels of urbanization. We used TITAN (1) to identify the location and magnitude of greatest change in the frequency and abundance of individual taxa and (2) to assess synchrony in the location of change points as evidence for stream community thresholds in response to percent impervious cover in catchments. We documented clear and synchronous threshold declines of 110 of 238 macroinvertebrate taxa in response to low levels of impervious cover. Approximately 80% of the declining taxa did so between 0.5% and 2% impervious cover, whereas the last 20% declined sporadically from 2% to 25% impervious cover. Synchrony of individual responses resulted in distinct community-level thresholds ranging from < or = 0.68% (mountains), 1.28% (piedmont), and 0.96% (coastal plain) impervious cover. Upper limits (95% confidence intervals) of community thresholds were < 2% cover in all regions. Within distinct physiographic classes, higher-gradient, smaller catchments required less impervious cover than lower gradient, larger catchments to elicit community thresholds. Relatively few taxa showed positive responses to increasing impervious cover, and those that did gradually increased in frequency and abundance, approximating a linear cumulative distribution. The sharp, synchronous declines of numerous taxa established a consistent threshold response at exceptionally low levels of catchment urbanization, and uncertainty regarding the estimation of impervious cover from satellite data was mitigated by several corroborating lines of evidence. We suggest that threshold responses of communities to urban and other novel environmental gradients may be more prevalent than currently recognized.

Sources of nutrient inputs to the Patuxent River estuary

We quantified annual nutrient inputs to the Patuxent River estuary from point and nonpoint sources and from direct atmospheric deposition. We also compared nonpoint source (NPS) discharges from Piedmont and Coastal Plain regions and from agricultural and developed lands. Using continuous automated-sampling, we measured discharges of water, nitrogen, phosphorus, organic carbon (C), and suspended solids from a total of 23 watersheds selected to represent various proportions of developed land and cropland in the Patuxent River basin and the neighboring Rhode River basin. The sampling period spanned two years that differed in annual precipitation by a factor of 1.7. Water discharge from the watershed to the Patuxent River estuary was 3.4 times higher in the wet year than in the dry year. Annual water discharges from the study watersheds increased as the proportion of developed land increased. As the proportion of cropland increased, there were increases in the annual flow-weighted mean concentrations of nitrate (NO3−), total nitrogen (TN), dissolved silicate (Si), total phosphate (TPO43−), total organic phosphorus (TOP), total P (TP), and total suspended solids (TSS) in NPS discharges. The effect of cropland on the concentrations of NO3− and TN was stronger for Piedmont watersheds than for Coastal Plain watersheds. As the proportion of developed land increased, there were increases in annual mean concentrations of NO3−, total ammonium (TNH4+), total organic N (TON), TN, total organic C (TOC), TPO43−, TOP, TP, and TSS and decreases in concentrations of Si. Annual mean concentrations of TON, TOC, forms of P, and TSS were highest in the wet year. Annual mean concentrations of NO3−, TNH4+, TN, and Si did not differ significantly between years. We directly measured NPS discharges from about half of the Patuxent River basin and estimated discharges from the other half of the basin using statistical models that related annual water flow and material concentrations to land cover and physiographic province. We compared NPS discharges to public data on point source (PS) discharges. We estimated direct atmospheric deposition of forms of N, P, and organic C to the Patuxent River estuary based on analysis of bulk deposition near the Rhode River. During the wet year, most of the total terrestrial and atmospheric inputs of forms of N and P came from NPS discharges. During the dry year, 53% of the TNH4+ input was from atmospheric deposition and 58% of the NO3− input was from PS discharges; NPS and PS discharges were about equally important in the total inputs of TN and TPO43−. During the entire 2-yr period, the Coastal Plain portion of the Patuxent basin delivered about 80% of the NPS water discharges to the estuary and delivered similar proportions of the NPS TNH4+, TN, TOP, and TSS. The Coastal Plain delivered greater proportions of the NPS TON, TOC, Si, and TP (89%, 90%, 93%, and 95%, respectively) than of water, and supplied nearly all of the NPS TPO43− (99%). The Piedmont delivered 33% of the NPS NO3− while delivering only 20% of the NPS water to the stuary. We used statistical models to infer the percentages of NPS discharges supplied by croplands, developed lands, and other lands. Although cropland covers only 10% of the Patuxent River basin, it was the most important source of most materials in NPS discharge, supplying about 84% of the total NPS discharge of NO3−; about three quarters of the TPO43−, TOP, TP, and TSS; and about half of the TNH4+ and TN. Compared to developed land, cropland supplied a significantly higher percentage of the NPS discharges of NO3−, TN, TPO43−, TOP, TP, and TSS, despite the fact development land covered 12% of the basin.

Effects of Land-use Change on Nutrient Discharges from the Patuxent River Watershed

We developed an empirical model integrating nonpoint source (NPS) runoff, point sources (PS), and reservoir management to predict watershed discharges of water, sediment, organic carbon, silicate, nitrogen, and phosphorus to the Patuxent River in Maryland. We estimated NPS discharges with linear models fit to measurements of weekly flow and 10 material concentrations from 22 study watersheds. The independent variables were the proportions of cropland and developed land, physiographic province (Coastal Plain or Piedmont), and time (week). All but one of the NPS models explained between 62% and 83% of the variability among concentration or flow measurements. Geographic factors (land cover and physiographic province) accounted for the explained variability in largely dissolved material concentrations (nitrate [NO3], silicate [Si], and total nitrogen [TN]), but the explained variability in flow and particulates (sediment and forms of phosphorus) was more strongly related to temporal variability or its interactions with land cover and province. Average concentrations of all materials increased with cropland proportion and also with developed land (except Si), but changes in cropland produced larger concentration shifts than equivalent changes in developed land proportion. Among land cover transitions, conversions between cropland and forest-grassland cause the greatest changes in material discharges, cropland and developed land conversions are intermediate, and developed land and forest-grassland conversions have the weakest effects. Changing land cover has stronger effects on NO3 and TN in the Piedmont than in the coastal Plain, but for all other materials, the effects of land-use change are greater in the Coastal Plain. We predicted the changes in nutrient load to the estuary under several alternate land cover configurations, including a state planning scenario that extrapolates current patterns of population growth and land development to the year 2020. In that scenario, declines in NPS discharges from reducing cropland are balanced by NPS discharge increases from developing an area almost six times larger than the lost cropland. When PS discharges are included, there are net increases in total water, total phosphorus, and TN discharges.

Transport of nitrogen and phosphorus from Rhode River watersheds during storm events

We studied storm discharges of nitrate and dissolved and particulate forms of ammonium, organic N, phosphate, and organic P from four adjacent small watersheds of differing land use on the Atlantic Coastal Plain in Maryland. We used V-notch weirs and automated storm samplers to measure discharges and collect samples at flow intervals during 76 storms. The watershed aquifers are perched on an impervious clay layer slightly above sea level, so that combined groundwater and surface water discharges were measured at the weirs. The concentrations of particulate forms of organic N (PON), organic P (POP), and inorganic P (PPi) increased up to 3 orders of magnitude during storm events and usually peaked prior to the peak water discharge, while concentrations of dissolved forms of organic N (DON), organic P (DOP), inorganic P (DPi), and nitrate did not change very much. Dissolved and particulate ammonium (DNH4 and PNH4, respectively) concentrations increased up to fivefold in storm events but remained low compared to other N forms. The watershed with the most cropland discharged the highest concentrations of total N, PON, DNH4, nitrate, POP, and PPi. A forested watershed discharged the highest concentrations of DON, PNH4, and DOP. The watershed with the most grazed land discharged the highest concentration of DPi. PON and POP were the dominant forms of N and P in storm discharges from all watersheds. Concentrations of nitrogen were higher in spring and summer storms than in winter storms, but phosphorus concentrations were much higher in the summer storms than in spring or winter. The concentrations of PPi, POP, PON, DNH4, and PNH4 increased significantly with peak water discharge among storms, while concentrations of DPi, DOP, DON, and nitrate were not correlated with peak discharge. The ratios of TN/TP and TIN/TIP declined significantly with peak water discharge among storms.