We investigated how watershed land use (a gradient of agricultural vs. forested land) relates to phytoplankton primary production (PPr) and photosynthetic parameters in 12 reservoirs in Ohio and ...examined spatial variation in these parameters. Shallow sites near stream inflows had higher light attenuation, total phosphorus (TP), chlorophyll, nonvolatile suspended solids (NVSS), light-saturated photosynthesis (PmB), and volumetric PPr than deeper sites near dam outflows, but areal PPr and the initial slope of the photosynthesis-irradiance curve (αB) were not significantly different between sites. Mean mixed layer irradiance and the severity of light limitation did not differ between sites because shallower depths compensated for higher light attenuation at inflow sites. Watershed land use (percent agriculture) was only weakly (but significantly) related to mean annual PPr, TP, and chlorophyll, but there was a well-defined upper limit to the effect of land use on all three of these parameters. Multiple regression showed that inclusion of additional watershed factors (the ratio of watershed land area to reservoir volume and the ratio of cropland area to number of livestock) greatly increased the variance explained compared to land use alone. TP and chlorophyll were highly correlated with each other and with PPr. Comparison of our TP-chlorophyll, TP-PPr, and chlorophyll-PPr regressions with those of other studies suggests that reservoirs have lower PPr per unit TP than natural lakes, probably because of lower light intensity and higher concentrations of nonalgal P in reservoirs.
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.
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 (NH4) and nitrate (NO3), soluble reactive phosphorus (SRP), and suspended sediment (SS) in three Corn Belt streams with drainage areas of 12 to 129 km2 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 NH4 at all three streams over the 13-yr period, declines in SRP and SS in two of the three streams, and a decline in NO3 in one stream. When trend coefficients are converted to percent per year and weighted by drainage, area changes in concentration are -8.5% for NH4, -5.9% for SRP, -6.8% for SS, and -0.8% for NO3. Trends in total N and P are strongly tied to trends in NO3, SRP, and SS and indicate that total P is declining, whereas total N is not.
Aquatic ecosystem management requires knowledge of the links among landscape-level anthropogenic disturbances and aquatic ecosystem properties. With large catchment area to surface area ratios ...(CA:SA), reservoirs often receive substantial terrestrial subsidies and can be particularly sensitive to eutrophication. Reservoir numbers and attendant management problems are increasing, and tools are needed to categorize their eutrophication status. We analyzed a dataset of 109 reservoirs in Ohio (USA) in an effort to classify eutrophication status using landscape-level features and reservoir morphometry. These predictor variables were selected because they are relatively stable and easily measured. We employed regression tree analysis and used a composite eutrophication variable as our response variable. Our regression tree analysis accurately divided 67% of Ohio reservoirs into 4 eutrophication status groups using 3 predictor variables: percentage of catchment area composed of agriculture versus forest; maximum reservoir depth; and CA:SA. We can infer that reservoirs with catchments containing >71% forest will likely be oligotrophic to mesotrophic. For reservoirs with <71% catchment forest, trophic status is determined by the relative extent of catchment row crops and either CA:SA or maximum depth. We applied our regression tree to a subset of reservoirs in the Environmental Protection Agency's National Lakes Assessment (NLA; n = 339 reservoirs). With a few exceptions, we categorized NLA reservoirs by eutrophication status despite their broad geographical range across the contiguous USA. Our results show that a few easily measured, stable parameters can classify reservoir eutrophication status. Models like ours may be useful for broad-scale management decisions.
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.
Fluxes of dissolved and particulate nitrogen (N) and phosphorus (P) from three adjacent watersheds were quantified with a high-resolution sampling program over a five-year period. The watersheds vary ...by an order of magnitude in area (12,875, 7968 and 1206 ha), and in all three watersheds intensive agriculture comprises > 90% of land. Annual fluxes of dissolved N and P per unit watershed area (export coefficients) varied ∼2X among watersheds, and patterns were not directly related to watershed size. Over the five-year period, mean annual flux of soluble reactive P (SRP) was 0.583 kg P· ha-1· yr-1 from the smallest watershed and 0.295 kg P· ha-1· yr-1 from the intermediate-sized watershed, which had the lowest SRP flux. Mean annual flux of nitrate was 20.53 kg N· ha-1· yr-1 in the smallest watershed and 44.77 kg N· ha-1· yr-1 in the intermediate-sized watershed, which had the highest nitrate flux. As a consequence, the export ratio of dissolved inorganic N to SRP varied from 80 (molar) in the smallest watershed to 335 in the intermediate-sized watershed. Because most N was exported as nitrate, differences among watersheds in total N flux were similar to those for nitrate. Hence, the total N:P export ratio was 42 (molar) for the smallest watershed and 109 for the intermediate-sized watershed. In contrast, there were no clear differences among watersheds in the export coefficients of particulate N, P, or carbon, even though > 50% of total P was exported as particulate P in all watersheds. All nutrient fractions were exported at higher rates in wet years than in dry years, but precipitation-driven variability in export coefficients was greater for particulate fractions than for dissolved fractions. Examination of hydrological regimes showed that, for all nutrient fractions, most export occurred during stormflow. However, the proportion of nitrate flux exported as baseflow was much greater than the proportion of SRP flux exported as baseflow, for all three watersheds (25-37% of nitrate exported as baseflow vs. 3-13% of SRP exported as baseflow). In addition, baseflow comprised a greater proportion of total discharge in the intermediate-sized watershed (43.7% of total discharge) than the other two watersheds (29.3 and 30.1%). Thus, higher nitrate export coefficients in the intermediate-sized watershed may have resulted from the greater contribution of baseflow in this watershed. Other factors potentially contributing to higher nitrate export coefficients in this watershed may be a thicker layer of loess soils and a lower proportion of riparian forest than the other watersheds. The among-watershed variability in SRP concentrations and export coefficients remains largely unexplained, and might represent the minimum expected variation among similar agricultural watersheds.
High external nutrient loads to lakes are a primary cause of eutrophication. As such, management activities in the watersheds of heavily impacted systems are commonly implemented to reduce nutrient ...supply to lakes with high phytoplankton biomass. Although management efforts are often successful, resilience against the reversal of eutrophication may be maintained by a variety of different drivers of phytoplankton biomass. These may include internal sources of nutrients, and increases in light availability with declining sediment loads. We used a 21-yr dataset to examine the responses of Acton Lake, a eutrophic agriculturally impacted reservoir, to determine the relative strength of potential drivers of changes in phytoplankton biomass through time. We also identified sources of ecosystem resilience that allow the lake to remain eutrophic. Despite declining nutrient concentrations in inflow streams, chlorophyll increased in Acton Lake over approximately the first decade of our study, and remained relatively stable in the second decade. Time series modeling suggests two primary drivers of increased phytoplankton: (1) increased nutrient excretion by detritivorous fish (gizzard shad, Dorosoma cepedianum), and (2) declining sediment loads from the watershed that reduced phytoplankton light limitation. We also identify precipitation patterns and stream discharge as being possible sources of ecosystem resilience, contributing to high loads of nutrients even if concentrations in inflow streams have declined with better management. These results suggest a more comprehensive view of management efforts to reduce eutrophication may be necessary, incorporating interactions among internal nutrient sources, climate variation, and changes in light availability.
Concentrations of nutrients and sediments in streams reflect land use and climate patterns in the watersheds they drain and also have implications for downstream ecosystems. Here we present 15 years ...of high-resolution data on the concentrations of nutrients (ammonium, nitrate, and soluble reactive phosphorus) and suspended sediments in three streams that drain watersheds in Ohio, USA, dominated by row crop agriculture (mostly maize and soy). In addition, we present daily precipitation and stream discharge data over this 15-year period. Agricultural practices in these watersheds have changed over this period, including a pronounced increase in conservation tillage, a reduction in fertilizers, a reduction in the number of hogs, and improved manure management. The data set consists of over >42 000 nutrient and sediment concentrations (3198-3898 samples from each of three streams for four nutrient and sediment constituents). The data set is unique in terms of its combination of length and resolution (sampling frequency 6-8 hours during storms) as well as its utility in assessing how changes in agricultural practices affect the concentrations and export rates of nutrients and sediments. Data presented here have been used in several papers quantifying nutrient export from streams, nutrient loading to a downstream reservoir (Acton Lake), and how nutrient inputs modulate ecosystem function and food web dynamics in Acton Lake.
The concept of new and regenerated production has been used extensively in marine ecosystems but rarely in freshwaters. We assessed the relative importance of new and regenerated phosphorus (P) in ...sustaining phytoplankton production in Acton Lake, a eutrophic reservoir located in southwestern Ohio, USA. Sources of nutrients to the euphotic zone, including watershed loading, fluxes from sediments, and excretion by sediment-feeding fish (gizzard shad,
Dorosoma cepedianum
), were considered sources of new P input that support new primary production and were quantified over the course of a growing season. Regenerated production was estimated by the difference between new and total primary production. New production represented 32%-53% of total primary production, whereas regenerated production represented 47%-68% of total primary production. P excretion by gizzard shad supplied 45%-74% of new P and 24% of P required for total production. In summary, fluxes of P from the watershed and those from sediment-feeding fish need to be considered in strategies to reduce eutrophication in reservoir ecosystems.
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