Bridget R. Deemer

Reservoir Water-Level Drawdowns Accelerate and Amplify Methane Emission

Water-level fluctuations due to reservoir management could substantially affect the timing and magnitude of reservoir methane (CH4) fluxes to the atmosphere. However, effects of such fluctuations on CH4 emissions have received limited attention. Here we examine CH4 emission dynamics in six Pacific Northwest U.S. reservoirs of varying trophic status, morphometry, and management regimes. In these systems, we show that water-level drawdowns can, at least temporarily, greatly increase per-area reservoir CH4 fluxes to the atmosphere, and can account for more than 90% of annual reservoir CH4 flux in a period of just a few weeks. Reservoirs with higher epilimnetic [chlorophyll a] experienced larger increases in CH4 emission in response to drawdown (R2 = 0.84, p < 0.01), suggesting that eutrophication magnifies the effect of drawdown on CH4 emission. We show that drawdowns as small as 0.5 m can stimulate ebullition events. Given that drawdown events of this magnitude are quite common in reservoirs, our results suggest that this process must be considered in sampling strategies designed to characterize total CH4 fluxes from reservoirs. The extent to which (and the mechanisms by which) drawdowns short-circuit connections between methanogenesis and methanotrophy, thereby increasing net CH4 fluxes to the atmosphere, should be a focus of future work.

Elevated Nitrogen and Phosphorus Concentrations in Urbanizing Southwest Washington Streams

Abstract In southwest Washington, rapid population growth and associated land use change have resulted in elevated stream nutrient concentrations. To evaluate the extent and nature of human alterations to stream nutrient concentrations in this region, we compiled four water years of total phosphorus (TP) and dissolved inorganic nitrogen (DIN) data from two long-term monitoring programs. We also quantified watershed characteristics likely to affect aquatic nutrient loading, and tested for correlations between these characteristics and stream nutrient concentrations. Average nutrient concentrations in study streams were significantly elevated relative to EPA recommended nutrient criteria in all sites for DIN and in nine out of 14 sites for TP. Of the watershed characteristics investigated, percent “impervious” (+) and percent “forested” (-) were the best predictors of TP concentration (R2 = 0.41 and 0.64, respectively, + and — indicate the slope of the regression). Percent “developed” (+) and percent “forest and woody wetland” (-) were the best predictors of DIN concentration (R2 = 0.75 and 0.73, respectively). In urban streams, the mean dry season DIN concentration was significantly higher than the mean wet season DIN concentration, but this pattern was reversed in less urban watersheds. Urban streams also had significantly higher DIN than non-urban streams. The strong relationship between DIN and “developed land” suggests that as southwest Washingtons population continues to grow, targeted N management will become increasingly important. The strong negative relationship between “forest and woody wetland” and both TP and DIN concentration suggests that this land use type is particularly important in reducing stream nutrient loading.

Are elusive anaerobic pathways key methane sinks in eutrophic lakes and reservoirs

Collectively, freshwaters constitute a significant source of methane to the atmosphere, and both methane production and methane oxidation can strongly influence net emissions. Anaerobic methane oxidation (AOM) is recognized as a strong regulator of marine methane emissions and appreciation of AOM’s importance in freshwater is growing. In spite of this renewed interest, recent work and reactive-transport modeling results we present in this paper point to unresolved pathways for AOM. Comparison of recent observations from a eutrophic reservoir, Lacamas Lake, with predictions of a 1D steady-state model of water column methane dynamics indicates that high rates of methane oxidation measured via bottle assays cannot be explained with conventional electron acceptors (O2, NO2−, NO3−, SO42−, Mn4+, and Fe3+). Reactive-transport modeling suggests that solute oxidant concentrations at the thermocline would have to be around 10 times higher than observed to explain the measured methane consumption. Organic acids—a major constituent of organic matter—may account for part of this unexplained AOM given their abundance in eutrophic systems, although the details of these pathways remain elusive (e.g., which species are involved, seasonal renewal of reduced species, contribution of particulate versus dissolved phases). We point to several observations consistent with organic acid-mediated AOM, both in Lacamas Lake and in other systems. Nevertheless, direct evidence of this pathway is still lacking and testing for this remains an important direction for future work. To this end, we identify several new avenues of research that would help quantify the role of organic acid-mediated AOM relative to other electron acceptors.