titlegreat blue heron

Major Findings

Ecosystem subsidies and food web interactions

Watershed inputs and nutrient translocation by gizzard shad both represent important nutrient fluxes to the Acton Lake water column. Both of these nutrient sources can be considered "new" nutrient inputs to phytoplankton because they represent fluxes of nutrients from outside the water column (Vanni 2002). The relative importance of these two nutrient sources (watershed vs. fish) varies greatly over various temporal scales (Vanni et al. 2001, 2005, 2006a, 2011). In wet years, the watershed provides more soluble reactive P (SRP) than gizzard shad. However, in dry years P excretion by gizzard shad exceeds SRP inputs from the entire watershed averaged over the entire growing season (April-September) (Vanni et al. 2005) and this excretion can support a significant proportion of phytoplankton primary production (Vanni et al. 2006b). The relative importance of the two nutrient sources also varies over shorter time intervals, which are highly relevant to phytoplankton generation times. During storms (i.e. over a few days), watershed P inputs greatly exceed nutrient excretion by gizzard shad, but during intervening low-flow periods P excretion by shad exceeds SRP inputs from the watershed, even in wet years (Vanni et al. 2005). These shorter time scales are important because phytoplankton become P-limited within a week or so after storms (Vanni et al. 2006a). Other sources of new P to the euphotic zone such as direct release from sediments (including microbally-mediated P flux), entrainment from hypolimnetic water, and excretion by benthic invertebrates are usually much less important than watershed inputs or excretion by shad (DeVine and Vanni 2002; Nowlin et al. 2005; Domine et al. 2010; Vanni et al. 2011).

Subsidies of allochthonous sediment are also important in Acton Lake, and interact with nutrient subsidies at several time scales. Storms deliver large quantities of sediments, inducing a shift from nutrient- to light-limitation (Vanni et al. 2006a). Over longer time scales, sediment and dissolved nutrient subsidies can directly and indirectly increase detrital food resources for gizzard shad, thereby increasing shad abundance and the importance of nutrient translocation. Data from hydrogen isotopes (deuterium) suggest that terrestrial detritus sustains about 30% of gizzard shad production in Acton Lake (and between 20-50% in 10 other reservoirs) (Babler et al. 2011). However, among-lake gizzard shad biomass is negatively correlated with "allochthony" (i.e., reliance on terrestrial detritus), suggesting that phytodetritus is a better food resource (Babler et al. 2011).

Change in agricultural practices

Agricultural practices in Acton Lake’s watershed have changed markedly since the early 1990s, and these changes have induced declines in nutrient and detritus subsidies to the lake, and an apparent response of the lake ecosystem. The agricultural changes occurring in the Acton watershed reflect shifts occurring throughout the Midwest (and many other areas), driven by conservation and economics. The Acton Lake watershed exemplifies these changes, and provides an excellent model system in which to study linkages between changing agricultural management at the landscape scale and ecological responses in reservoirs. Three important changes are occurring: 1) a pronounced increase in conservation tillage (defined as practices that leave > 30% of the soil surface covered with crop residue; this includes no-till); 2) reduced fertilizer use; and 3) restructuring of the meat production industry.

tillage trends in Acton Lake's watershedConservation tillage has increased dramatically since the early 1990s in the Acton watershed, facilitated by economic incentives provided to farmers toward the goal of reducing soil erosion and subsequent sedimentation rates in Acton Lake. Conservation tillage promotes infiltration of rainfall into soil, reducing overland flow, soil erosion and thus nutrient losses via surface runoff. P fertilizer use (based on county-wide fertilizer deliveries) is also declining in the Acton watershed, reflecting state-wide patterns (Renwick et al. 2008) and probably those throughout the Midwest. Reduced fertilizer use probably results from both rising prices and innovations, such as precision agriculture, that allow farmers to better estimate fertilizer needs. A third type of change is a ~50% decline in the number of hogs since 1990 (Renwick et al. 2008). Corresponding to a widespread shift towards large mega-farms, many smaller farms with animals within the Acton watershed either closed down or shifted entirely to corn/soy production. Although large mega-farms (with thousands of animals) are common in some areas of the Midwest, this is not the case in the Acton watershed. The decline in hogs is accompanied by a decline in the formerly ubiquitous small feedlots that can be important sources of nutrients to streams.

Trends in nutrient and sediment subsides from inflow streams to Acton Lake

Flow-corrected concentrations of suspended sediments (SS) and soluble reactive P (SRP) declined significantly in Acton Lake inflow streams from 1994 through 2006, as determined by ARIMA models that statistically account for discharge (streamflow) and seasonality, LOWESS regression, and analysis of temporal trends in the residuals of discharge vs. concentration regressions (Renwick et al. 2008). Accounting for differential flow among streams, flow-standardized SS and SRP concentrations are declining by 6.9% and 5.7% per year for the Acton watershed as a whole, from 1994-2006. Particulate C, N and P concentrations, which are highly correlated with SS concentrations, also declined during this period. In contrast, nitrate (which comprises 80-90% of N loading in these streams) showed relatively weak or no decline during the same period (Renwick et al. 2008).

Trends in stream nutrients and sediments are consistent with changes in agricultural practices and the known effects of conservation tillage. Exports of sediments and associated P are affected more by conservation tillage than are nutrients associated with ground water (e.g., nitrate). For example, trends in Acton streams are similar to those in streams draining into Lake Erie during a period when conservation tillage increased by similar amounts (http://www.epa.gov/med/grosseile_site/indicators/maumee-p.html).

suspended sediment and total phosphorus loading to Acton LakeDeclining flow-adjusted concentrations do not necessarily equate to declines in total subsidy or loading rate (the mass of an element exported) to a downstream ecosystem. This is because loading rate is the product of discharge x concentration, and discharge is heavily influenced by precipitation-induced runoff. Thus, loading rate depends on precipitation as well as watershed management. The loading rate to an ecosystem is important, because a lake ecosystem is more likely to respond the total mass of a substance entering the ecosystem, rather than the concentrations in inflow streams. Loading rates (total subsidies) of suspended sediment (SS), SRP and total P (TP) to Acton Lake vary greatly from year to year and are highly correlated with stream discharge.

Trends in Acton Lake: Response to variable nutrient and sediment subsidies

Non-volatile suspended solids and chlorophyll trends in Acton Lake

In-lake concentrations of non-volatile (inorganic) suspended sediments (NVSS) have shown a general decline in Acton Lake over the years.

***All manuscripts referenced above can be accessed on the "Publications" tab***