William H. Renwick

Budgets of soil erosion and deposition for sediments and sedimentary organic carbon across the conterminous United States

The fate of soil organic matter during erosion and sedimentation has been difficult to assess because of the large size and complex turnover characteristics of the soil carbon reservoir. It has been assumed that most of the carbon released during erosion is lost to oxidation. Budgets of bulk soil and soil organic carbon erosion and deposition suggest that the primary fates of eroded soil carbon across the conterminous United States are trapping in impoundments and other redeposition. The total amount of soil carbon eroded and redeposited across the United States is ∼0.04 Gt yr−1. Applying this revision to the U. S. carbon budget by Houghton et al. [1999] raises their net sequestration estimate by 20–47 %. If comparable rates of erosion and redeposition occur globally, net carbon sequestration would be ∼1 Gt yr−1.

Distribution and significance of small, artificial water bodies across the United States landscape

At least 2.6 million small, artificial water bodies dot the landscape of the conterminous United States; most are in the eastern half of the country. These features account for approximately 20% of the standing water area across the United States, and their impact on hydrology, sedimentology, geochemistry, and ecology is apparently large in proportion to their area. These features locally elevate evaporation, divert and delay downstream water flow, and modify groundwater interactions. They apparently intercept about as much eroded soil as larger, better-documented reservoirs. Estimated vertical accretion rates are much higher, hence, inferred sedimentary chemical reactions must be different in the small features than in larger ones. Finally, these features substantially alter the characteristics of aquatic habitats across the landscape.

Dissolved and particulate nutrient flux from three adjacent agricultural watersheds: A five-year study

Fluxes of dissolved and particulate nitrogen (N) and phosphorus(P) from three adjacent watersheds were quantified with ahigh-resolution sampling program over a five-year period. The watershedsvary by an order of magnitude in area (12,875, 7968 and 1206 ha), and inall three watersheds intensive agriculture comprises > 90% ofland. Annual fluxes of dissolved N and P per unit watershed area (exportcoefficients) varied ∼2X among watersheds, and patterns were notdirectly related to watershed size. Over the five-year period, meanannual flux of soluble reactive P (SRP) was 0.583 kg P ·ha−1 · yr−1 from the smallestwatershed and 0.295 kg P · ha−1 ·yr−1 from the intermediate-sized watershed, which hadthe lowest SRP flux. Mean annual flux of nitrate was 20.53 kg N ·ha−1 · yr−1 in the smallestwatershed and 44.77 kg N · ha−1 ·yr−1 in the intermediate-sized watershed, which had thehighest nitrate flux. As a consequence, the export ratio of dissolvedinorganic N to SRP varied from 80 (molar) in the smallest watershed to335 in the intermediate-sized watershed. Because most N was exported asnitrate, differences among watersheds in total N flux were similar tothose for nitrate. Hence, the total N:P export ratio was 42(molar) for the smallest watershed and 109 for the intermediate-sizedwatershed. In contrast, there were no clear differences among watershedsin the export coefficients of particulate N, P, or carbon, even though> 50% of total P was exported as particulate P in allwatersheds. All nutrient fractions were exported at higher rates in wetyears than in dry years, but precipitation-driven variability in exportcoefficients was greater for particulate fractions than for dissolvedfractions.Examination of hydrological regimes showed that, for all nutrientfractions, most export occurred during stormflow. However, theproportion of nitrate flux exported as baseflow was much greater thanthe proportion of SRP flux exported as baseflow, for all threewatersheds (25–37% of nitrate exported as baseflow vs.3–13% of SRP exported as baseflow). In addition, baseflowcomprised a greater proportion of total discharge in theintermediate-sized watershed (43.7% of total discharge) than theother two watersheds (29.3 and 30.1%). Thus, higher nitrateexport coefficients in the intermediate-sized watershed may haveresulted from the greater contribution of baseflow in this watershed.Other factors potentially contributing to higher nitrate exportcoefficients in this watershed may be a thicker layer of loess soils anda lower proportion of riparian forest than the other watersheds. Theamong-watershed variability in SRP concentrations and exportcoefficients remains largely unexplained, and might represent theminimum expected variation among similar agriculturalwatersheds.

Landscape change with agricultural intensification in a rural watershed, southwestern Ohio, U.S.A.

Specialized cash grain production, emergent in the midwestern United States during the post-WWII era, typifies the Upper Four Mile Creek watershed in southwestern Ohio. This style of agriculture intensifies cropland use, with consequent increases in soil erosion and stream sedimentation - a serious problem in the lower reservoir, Acton Lake. Agricultural statistics and aerial photographs compiled between 1934 and 1984 were used to quantify agricultural dynamics and landscape change in the watershed, including land-use apportionment, diversity, and the structural configuration of forest, woodland, and old-field/brushland patches and corridors. A questionnaire sent to all land owners in the basin documented farm-level characteristics and factors that influence management decisions. Crop diversity (H′) in Preble County, Ohio decreased from 1.42 in 1934 to 1.17 in 1982, as corn and soybeans dominated the landscape mosaic. Yields rose, but net profits were reduced by declining prices per bushel and increases in fertilizer and petroleum-based subsidies. Landuse diversity in the county also declined (H′ = 1.37 in 1934 tot 0.80 in 1982) in response to cropland expansion, whereas forest land in the watershed increased from 1605 to 2603 ha. Fragmentation declined and the landscape became polarized after 1956, with a concentration of agricultural patches in the upper watershed and forest-patch coalescence in stream gullies and state park land in the lower watershed. The questionnaire (~ 29% return) further supported, at the farm-level, observed regional trends toward expansion (farm coalescence and lease contracts) and specialization (conversion toward corn and soybeans). The most important factors influencing farm size and management were better equipment and family traditions. Thus, cultural and technological factors that operate at the farm-level, coupled with meso-scale variation in the physical conditions of a catchment basin, tend to influence landscape-level patterns more than regional socioeconomics and governmental policies.

FATES OF ERODED SOIL ORGANIC CARBON: MISSISSIPPI BASIN CASE STUDY

We have developed a mass balance analysis of organic carbon (OC) across the five major river subsystems of the Mississippi (MS) Basin (an area of 3.2 × 106 km2). This largely agricultural landscape undergoes a bulk soil erosion rate of ∼480 t·km−2·yr−1 (∼1500 × 106 t/yr, across the MS Basin), and a soil organic carbon (SOC) erosion rate of ∼7 t·km−2·yr−1 (∼22 × 106 t/yr). Erosion translocates upland SOC to alluvial deposits, water impoundments, and the ocean. Soil erosion is generally considered to be a net source of CO2 release to the atmosphere in global budgets. However, our results indicate that SOC erosion and relocation of soil apparently can reduce the net SOC oxidation rate of the original upland SOC while promoting net replacement of eroded SOC in upland soils that were eroded. Soil erosion at the MS Basin scale is, therefore, a net CO2 sink rather than a source.

Linking Landscapes and Food Webs: Effects of Omnivorous Fish and Watersheds on Reservoir Ecosystems

Abstract Ecologists increasingly recognize the need to understand how landscapes and food webs interact. Reservoir ecosystems are heavily subsidized by nutrients and detritus from surrounding watersheds, and often contain abundant populations of gizzard shad, an omnivorous fish that consumes plankton and detritus. Gizzard shad link terrestrial landscapes and pelagic reservoir food webs by consuming detritus, translocating nutrients from sediment detritus to the water column, and consuming zooplankton. The abundance of gizzard shad increases with watershed agriculturalization, most likely through a variety of mechanisms operating on larval and adult life stages. Gizzard shad have myriad effects on reservoirs, including impacts on nutrients, phytoplankton, zooplankton, and fish, and many of their effects vary with ecosystem productivity (i.e., watershed land use). Interactive feedbacks among watersheds, gizzard shad populations, and reservoir food webs operate to maintain dominance of gizzard shad in highly productive systems. Thus, effective stewardship of reservoir ecosystems must incorporate both watershed and food-web perspectives.

NUTRIENT CYCLING BY FISH SUPPORTS RELATIVELY MORE PRIMARY PRODUCTION AS LAKE PRODUCTIVITY INCREASES

Animals can be important in nutrient cycling in particular ecosystems, but few studies have examined how this importance varies along environmental gradients. In this study we quantified the nutrient cycling role of an abundant detritivorous fish species, the gizzard shad (Dorosoma cepedianum), in reservoir ecosystems along a gradient of ecosystem productivity. Gizzard shad feed mostly on sediment detritus and excrete sediment-derived nutrients into the water column, thereby mediating a cross-habitat translocation of nutrients to phytoplankton. We quantified nitrogen and phosphorus cycling (excretion) rates of gizzard shad, as well as nutrient demand by phytoplankton, in seven lakes over a four-year period (16 lake-years). The lakes span a gradient of watershed land use (the relative amounts of land used for agriculture vs. forest) and productivity. As the watersheds of these lakes became increasingly dominated by agricultural land, primary production rates, lake trophic state indicators (total phosphorus and chlorophyll concentrations), and nutrient flux through gizzard shad populations all increased. Nutrient cycling by gizzard shad supported a substantial proportion of primary production in these ecosystems, and this proportion increased as watershed agriculture (and ecosystem productivity) increased. In the four productive lakes with agricultural watersheds (>78% agricultural land), gizzard shad supported on average 51% of phytoplankton primary production (range 27-67%). In contrast, in the three relatively unproductive lakes in forested or mixed-land-use watersheds (>47% forest, <52% agricultural land), gizzard shad supported 18% of primary production (range 14-23%). Thus, along a gradient of forested to agricultural landscapes, both watershed nutrient inputs and nutrient translocation by gizzard shad increase, but our data indicate that the importance of nutrient translocation by gizzard shad increases more rapidly. Our results therefore support the hypothesis that watersheds and gizzard shad jointly regulate primary production in reservoir ecosystems.

Equilibrium, disequilibrium, and nonequilibrium landforms in the landscape

Abstract Landscapes are composed of a diversity of landforms that may be characterized as equilibrium, disequilibrium or nonequilibrium. Equilibrium is a constant relation between input and output or form, toward which a landform tends or around which it fluctuates in time. Because of recent environmental changes and long relaxation times, however, many landforms are not adjusted to present inputs. Disequilibrium landforms are those that tend toward equilibrium but have not had sufficient time to reach such a condition. Some landforms, called nonequilibrium, do not tend toward equilibrium even with relatively long periods of environmental stability, but rather undergo frequent and large change in form. Nonequilibrium may be caused by high-magnitude thresholds that cause low-frequency weather events to dominate form, positive feedbacks, and/or nonlinearities that result in deterministic chaos. Landscapes contain mixtures of equilibrium, disequilibrium and nonequilibrium landforms, and these types of landforms interact in geomorphic systems such as sediment cascades. The three types of behavior may coexist in the same landscape, and the outputs of such mixed geomorphic systems reflect the combined effects of equilibrium, disequilibrium, and nonequilibrium subsystems. This classification provides a framework for analysis and modeling of complex landscapes.

Water Quality Trends and Changing Agricultural Practices in a Midwest U.S. Watershed, 1994-2006

Sediment and nutrient concentrations in surface water in agricultural regions are strongly influenced by agricultural activities. In the Corn Belt, recent changes in farm management practices are likely to affect water quality, yet there are few data on these linkages at the landscape scale. We report on trends in concentrations of N as ammonium (NH(4)) and nitrate (NO(3)), soluble reactive phosphorus (SRP), and suspended sediment (SS) in three Corn Belt streams with drainage areas of 12 to 129 km(2) for 1994 through 2006. During this period, there has been an increase in conservation tillage, a decline in fertilizer use, and consolidation of animal feeding operations in our study watersheds and throughout the Corn Belt. We use an autoregressive moving average model to include the effects of discharge and season on concentrations, LOWESS plots, and analyses of changes in the relation between discharge and concentration. We found significant declines in mean monthly concentrations of NH(4) at all three streams over the 13-yr period, declines in SRP and SS in two of the three streams, and a decline in NO(3) in one stream. When trend coefficients are converted to percent per year and weighted by drainage, area changes in concentration are -8.5% for NH(4), -5.9% for SRP, -6.8% for SS, and -0.8% for NO(3). Trends in total N and P are strongly tied to trends in NO(3), SRP, and SS and indicate that total P is declining, whereas total N is not.

Nutrient and light limitation of reservoir phytoplankton in relation to storm-mediated pulses in stream discharge

We investigated the dynamics of nutrient and light limitation of phytoplank- ton in a reservoir ecosystem in relation to storm-mediated variation in stream dis- charge, and how dynamics differed at a shallow site near stream inflows versus one in deep water near the lake outflow. Storm-mediated discharge events reduced the sev- erity of nutrient limitation and increased the severity of light limitation, as predicted by a model of reservoir resource limitation developed by Kimmel et al. (1990). The sev- erity of nutrient limitation was negatively correlated with discharge to the lake; the correlation was strongest with discharge over the preceding 10-14 day period and wea- ker at shorter and longer time scales. However, discharge events also flushed phyto- plankton from the lake and enhanced light limitation, so it is not clear by which mech- anism(s) discharge events mediate phytoplankton resource limitation. Phytoplankton near stream inflows were less nutrient limited than phytoplankton at the lake outflow, consistent with predictions of the Kimmel et al. (1990) model. However, this was true even when streamflow was negligible, suggesting alternative mechanisms for reduced nutrient limitation near stream inflows. In contrast to predictions of the model, phyto- plankton were not more light limited near stream inflows than at the outflow; shallo- wer depth near inflows compensated for higher turbidity, in terms of the light climate experienced by phytoplankton. Our results show that the mechanisms by which dis- charge events mediate phytoplankton resource limitation are complex and require fur- ther study in reservoirs as well as other aquatic systems subject to a high degree of temporal variation in discharge.