ARE WATERSHED AND LACUSTRINE CONTROLS ON PLANKTONIC N₂FIXATION HIERARCHICALLY STRUCTURED

Abstract

N2 fixation can be an important source of N to limnetic ecosystems and can influence the structure of phytoplankton communities. However, watershed-scale conditions that favor N2 fixation in lakes and reservoirs have not been well studied. We measured N2 fixation and lacustrine variables monthly over a 19-month period in Waco Reservoir, Texas, USA, and linked these data with nutrient-loading estimates from a physically based watershed model. Readily available topographic, soil, land cover, effluent discharge, and climate data were used in the Soil and Water Assessment Tool (SWAT) to derive watershed nutrient-loading estimates. Categorical and regression tree (CART) analysis revealed that lacustrine and watershed correlates of N2 fixation were hierarchically structured. Lacustrine conditions showed greater predictive capability temporally. For instance, low NO3(-) concentration (<25 microg N/L) and high water temperatures (>27 degrees C) in the reservoir were correlated with the initiation of N2 fixation seasonally. When lacustrine conditions were favorable for N2 fixation, watershed conditions appeared to influence spatial patterns of N2 fixation within the reservoir. For example, spatially explicit patterns of N2 fixation were correlated with the ratio of N:P in nutrient loadings and the N loading rate, which were driven by anthropogenic activity in the watershed and periods of low stream flow, respectively. Although N2 fixation contributed <5% of the annual N load to the reservoir, 37% of the N load was derived from atmospheric N2 fixation during summertime when stream flow in the watershed was low. This study provides evidence that watershed anthropogenic activity can exert control on planktonic N2 fixation, but that temporality is controlled by lacustrine conditions. Furthermore, this study also supports suggestions that reduced inflows may increase the propensity of N2-fixing cyanobacterial blooms in receiving waters of anthropogenically modified landscapes.

Nutrient enrichment in tropical wetlands: shifts from autotrophic to heterotrophic nitrogen fixation

“…Data from freshwater wetlands, however, are rather limited (Bristow 1973;Tjepkema and Evans 1976;Scott et al 2008;Č erná et al 2009). …”

Hot spots and hot moments of planktonic nitrogen fixation in a eutrophic southern reservoir

“…Daily areal N 2 fixation rates (mg N/m 2 ·d) were estimated from the rates observed under varying light conditions as described by Scott et al (2008). Briefly, we subdivided the water column into a light saturated zone (upper 25% of z eu ), a low light zone (lower 75% of z eu ) and a dark zone.…”

“…The large pulse of N 2 fixation occurred in both years only during warmest summer temperatures and very low NO 3 -N concentrations. Scott et al (2008) reports the results of a CART analysis that identifies a threshold value of 27.5 C for initiation of conditions favoring high rates of N 2 fixation; however, warm temperatures are not sufficient for N 2 fixation because low ambient levels of available nitrogen are also necessary. Regression tree analyses determined that N 2 fixation is highest only at NO 3 -N levels below 0.035 mg/L, and the upper 90% confidence limit for the predictor was 0.065 mg/L.…”

“…These conditions may be common in southern reservoirs during summer periods with reduced river inflows. Scott et al (2008) and Forbes et al (2008) have recently demonstrated the hierarchical nature of key environmental factors regulating nitrogen fixation, including water temperature, nutrient concentrations, relative watershed size, seasonal hydrologic regime, and nutrient loading.…”

A combined watershed–water quality modeling analysis of the Lake Waco reservoir: II. Watershed and reservoir management options and outcomes

“…However, no nitrogen is added to the water from a mass balance basis. Significant nitrogen fixation occurs in the Lake Waco reservoir at the inlet of the North Bosque River related to high phosphorus inflows and temperature (Scott et al 2008, Doyle et al 2010. A TN:TP ratio target was set to discourage conditions that favored N 2 -fixing cyanobacteria because these are linked to taste and odor episodes.…”