Whole-Catchment Manipulations of Internal and External Loading Reveal the Sensitivity of a Century-Old Reservoir to Hypoxia

Abstract

Climate change is predicted to have widespread impacts on freshwater lake and reservoir nutrient budgets by altering both hypolimnetic hypoxia and runoff, which will in turn alter the magnitude of internal and external nutrient loads. To examine the effects of these potential climate scenarios on nitrogen (N) and phosphorus (P) budgets, we conducted a whole-catchment manipulation of hypolimnetic oxygen conditions and external loads to Falling Creek Reservoir (FCR), an old, eutrophic reservoir in a reforested catchment with a history of agricultural land use. Throughout 2 years of monitoring, internal N and P loading during hypoxic conditions dominated the hypolimnetic mass of nutrients in FCR, regardless of changes in external loading. FCR commonly functioned as a net sink of N and P, except during hypoxic conditions, when the reservoir was a net source of ammonium (NH þ 4 ) to downstream. We observed extremely high nitrate-nitrite (NO À 3 ÀNO À 2 ), soluble reactive P (SRP), total nitrogen (TN), and total phosphorus (TP) retention rates, indicating that the reservoir served as a sink for greater than 70% of NO À 3 ÀNO À 2 inputs and greater than 30% of SRP, TN, and TP inputs, on average. Our study is notable in the length of time since reforestation (>80 years) that a reservoir is still exhibiting high N and P internal loading during hypoxia, potentially as a result of the considerable store of accumulated nutrients in its sediment from historical agricultural runoff. Our whole-catchment manipulations highlight the importance of understanding how multiple aspects of global change, waterbody and catchment characteristics, and land use history will interact to alter nutrient budgets in the future.

The Magnitude and Drivers of Methane Ebullition and Diffusion Vary on a Longitudinal Gradient in a Small Freshwater Reservoir

“…The best fitting model for T1 ebullition exhibited positive associations with temperature and mean wind speed and a negative association with ∆ pressure . Using FCR's longitudinal gradient as a guide, T1 is upstream of the primary inflow to FCR and below a wetland (Gerling et al, ), which likely had a strong influence on the CH 4 ebullition dynamics at this site. Wetlands are known to emit large quantities of CH 4 via ebullition (reviewed in Whalen, ), and previous studies have found that the same physical variables in the best fitting model for T1 also drive ebullition rates in wetlands (Aben et al, ; Goodrich et al, 2011; Morin et al, ; Whalen, ).…”

“…Surprisingly, wind speed was not a significant predictor of CH 4 diffusion at any transect or at the whole‐reservoir scale, in contrast to earlier studies (Berg et al, 2017; Cole et al, ; Wanninkhof, ). However, FCR is a small, sheltered reservoir in a steep forested watershed that is protected from higher winds (Gerling et al, ). A similar spatiotemporal CH 4 emission study in naturally formed lakes also found that wind speed was not a significant predictor of CH 4 diffusion in two of the three lakes at the Skogaryd Research Catchment, Sweden (Natchimuthu et al, ).…”

“…We determined the inflow discharge into FCR using a weir ~150 m upstream from the major stream outlet into the reservoir. Discharge was determined every 15 min using an INW Aquistar PT2X pressure sensor (INW, Kirkland, WA, USA; Gerling et al, ; Carey, Gerling, et al, ; Carey, Lofton, et al, 2018). Details on the weir and pressure transducer are described by Carey, Gerling, et al () and Carey, Lofton, et al (2018).…”

Cross‐scale Perspectives: Integrating Long‐term and High‐frequency Data into Our Understanding of Communities and Ecosystems

“…Taking advantage of minute‐resolution oxygen sensors, an experimental hypolimnetic oxygenation system, and the ability to manipulate external loading from the upstream reservoir, Gerling showed that the reservoir was a net sink of N and P and thus provides an important ecosystem service in reducing nutrient loads to downstream ecosystems (Gerling et al. ).…”

“…Paralleling Williamson ' s talk, Alexandra Gerling (Virginia Tech) evaluated the importance of external nutrient loading vs. within-lake nutrient recycling-this time by chemical mechanisms rather than biological-for N and P budgets in Falling Creek Reservoir, Virginia (USA). Taking advantage of minute-resolution oxygen sensors, an experimental hypolimnetic oxygenation system, and the ability to manipulate external loading from the upstream reservoir, Gerling showed that the reservoir was a net sink of N and P and thus provides an important ecosystem service in reducing nutrient loads to downstream ecosystems (Gerling et al 2016 ).…”