Metalimnetic oxygen minimum zones (MOMs) commonly develop during the summer stratified period in freshwater reservoirs because of both natural processes and water quality management. While several previous studies have examined the causes of MOMs, much less is known about their effects, especially on reservoir biogeochemistry. MOMs create distinct redox gradients in the water column which may alter the magnitude and vertical distribution of dissolved methane (CH4) and carbon dioxide (CO2). The vertical distribution and diffusive efflux of CH4 and CO2 was monitored for two consecutive open-water seasons in a eutrophic reservoir that develops MOMS as a result of the operation of water quality engineering systems. During both summers, elevated concentrations of CH4 accumulated within the anoxic MOM, reaching a maximum of 120 mu M, and elevated concentrations of CO2 accumulated in the oxic hypolinin ion, reaching a maximum of 780 mu M. Interestingly, the largest observed diffusive CH4 effluxes occurred before fall turnover in both years, while peak diffusive CO2 effluxes occurred both before and during turnover. Our data indicate that MOMs can substantially change the vertical distribution of CH4 and CO2 in the water column in reservoirs, resulting in the accumulation of CH4 in the metalimnion (vs. at the sediments) and CO2 in the hypolimnion. (C) 2013 Elsevier B.V. All rights reserved.

Chaoborus spp. (midge larvae) live in the anoxic sediments and h-ypolimnia of freshwater lakes and reservoirs during the day and migrate to the surface waters at night to feed on plankton. It has recently been proposed that Chaoborus take up methane (CH4) from the sediments in their tracheal gas sacs, use this acquired buoyancy to ascend into the surface waters, and then release the CH4, thereby serving as a CH4 "pump" to the atmosphere. We tested this hypothesis using diel surveys and seasonal monitoring, as well as incubations of Chaoborus to measure CH, transport in their gas sacs at different depths and times in a eutrophic reservoir. We found that Chaoborus transported CH, from the hypolimnion to the lower epilimnion at dusk, but the overall rate of CH4 transport was minor, and incubations revealed substantial variability in CH4 transport over space and time. We calculated that Chaoborus transport similar to 0.1 mmol CH4 m(-2) yr(-1) to the epilimnion in our study reservoir, a very low proportion (<1%) of total CH4 diffusive flux during the summer stratified period. Our data further indicate that CH4 transport by Chaoborus is sensitive to water column mixing, Chaoborus density, and Chaoborus species identity.

Organic carbon (OC) mineralization in freshwaters is dependent on oxygen availability near the sediments, which controls whether OC inputs will be buried or respired. However, oxygen dynamics in waterbodies are changing globally due to land use and climate, and the consequences of variable oxygen conditions for OC burial are unknown. We manipulated hypolimnetic oxygen availability in a whole-reservoir experiment and used a mass balance OC model to quantify rates of OC burial. Throughout summer stratification, we observed that OC burial rates were tightly coupled to sediment oxygen concentrations: oxic conditions promoted the mineralization of "legacy" OC that had accumulated over years of sedimentation, resulting in negative OC burial. Moreover, our study demonstrates that fluctuating oxygen conditions can switch ecosystem-scale OC burial in a reservoir between positive and negative rates. Consequently, changing oxygen availability in freshwaters globally will likely have large implications for the role of these ecosystems as OC sinks.

Water column mixing can influence community composition of pelagic phytoplankton in lakes and reservoirs. Previous studies suggest that low mixing favors cyanobacteria, while increased mixing favors green algae and diatoms. However, this shift in community dominance is not consistently achieved when epilimnetic mixers are activated at the whole-ecosystem scale, possibly because phytoplankton community responses are mediated by mixing effects on other ecosystem processes. We conducted two epilimnetic mixing experiments in a small drinking water reservoir using a bubble-plume diffuser system. We measured physical, chemical, and biological variables before, during, and after mixing and compared the results to an unmixed reference reservoir. We observed significant increases in the biomass of cyanobacteria (from 0.8 +/- 0.2 to 2.4 +/- 1.1 g L-1, p = 0.008), cryptophytes (from 0.7 +/- 0.1 to 1.9 +/- 0.6 g L-1, p = 0.003), and green algae (from 3.8 to 4.4 g L-1, p = 0.15) after our first mixing event, likely due to increased total phosphorus from entrainment of upstream sediments. After the second mixing event, phytoplankton biomass did not change but phytoplankton community composition shifted from taxa with filamentous morphology to smaller, rounder taxa. Our results suggest that whole-ecosystem dynamics and phytoplankton morphological traits should be considered when predicting phytoplankton community responses to epilimnetic mixing.

Lakes and reservoirs worldwide are increasingly experiencing depletion of dissolved oxygen (anoxia) in their bottom waters (the hypolimnion) because of climate change and eutrophication, which is altering the dynamics of many freshwater ecological communities. Hypolimnetic anoxia may substantially alter the daily migration and distribution of zooplankton, the dominant grazers of phytoplankton in aquatic food webs. In waterbodies with oxic hypolimnia, zooplankton exhibit diel vertical migration (DVM), in which they migrate to the dark hypolimnion during the day to escape fish predation or ultraviolet (UV) radiation damage in the well-lit surface waters (the epilimnion). However, due to the physiologically stressful conditions of anoxic hypolimnia, we hypothesized that zooplankton may be forced to remain in the epilimnion during daylight, trading oxic stress for increased predation risk or UV radiation damage. To examine how anoxia impacts zooplankton vertical migration, distribution, biomass, and community composition over day-night periods, we conducted multiple diel sampling campaigns on reservoirs that spanned oxic, hypoxic, and anoxic hypolimnetic conditions. In addition, we sampled the same reservoirs fortnightly during the daytime to examine the vertical position of zooplankton throughout the summer stratified season. Under anoxic conditions, most zooplankton taxa were predominantly found in the epil-imnion during the day and night, did not exhibit DVM, and had lower seasonal biomass than in reservoirs with oxic hypolimnia. Only the phantom midge larva, Chaoborus spp., was consistently anoxia-tolerant. Consequently, our results suggest that hypolimnetic anoxia may alter zooplankton migration, biomass, and behavior, which may in turn exacerbate water quality degradation due to the critical role zooplankton play in freshwater ecosystems.

In the era of big data, ecologists are increasingly relying on computational approaches and tools to answer existing questions and pose new research questions. These include both software applications (e.g., simulation models, databases and machine learning algorithms) and hardware systems (e.g., wireless sensor networks, supercomputing, drones and satellites), motivating the need for greater collaboration between computer scientists and ecologists. Here, we outline some synergistic opportunities for scientists in both disciplines that can be gained by building collaborations between the computer science and ecology research communities, with a focus on the benefits to ecology specifically. We also identify past contributions of computer science to ecology, including high-frequency environmental sensor technology, advanced supercomputing capacity for ecological modeling, databases for long-term and high-frequency datasets, and software programs for ecological analyses, to anticipate future potential contributions. These examples highlight the power and potential for further integration of computer science technology and ideas into the ecological research community. Finally, we translate our own experiences working together as a team of computer scientists and ecologists over the past decade into a conceptual framework with recommendations for supporting productive collaborations at the interface of the two disciplines. We specifically focus on how to apply best practices of team science for bridging computer science and ecology, which we advocate will substantially benefit ecology long-term.

Munger ZW, Carey CC, Gerling AB, Doubek JP, Hamre KD, McClure RP, Schreiber ME. 2018. Oxygenation and hydrologic controls on iron and manganese mass budgets in a drinking-water reservoir. Lake Reserv Manage. 35:277-291. In seasonally stratified lakes and reservoirs, fluctuating hypolimnetic oxygen and hydrologic conditions in the watershed can influence the retention of metals and their exchange between the sediments and water column. In particular, iron (Fe) and manganese (Mn) cycling at the sediment-water interface can be dynamic in response to variability in the watershed and within the waterbody, which has substantial implications for drinking water quality. We calculated a mass budget for Fe and Mn in a shallow drinking-water reservoir over a 2-year period in which we manipulated the tributary inflow rate and dissolved oxygen (DO) concentrations in the hypolimnion at the reservoir scale. We found that the net Fe and Mn release from the sediments into the water column was suppressed during oxygenation; however, both metals continued to be released from the sediments, even during well-oxygenated conditions. Oxygenation in the hypolimnion had no effect on the net export of metals from the reservoir to downstream. Instead, the overall net export of Fe and Mn during the stratified period was influenced by hydrologic inflows. In summary, we found that manipulating hypolimnetic oxygenation had an important effect on the cycling of Fe and Mn within the hypolimnion, but that the net retention of metals in the reservoir was driven primarily by hydrology.

Cyanobacterial blooms are increasing in waterbodies worldwide because of anthropogenic forcing. Most blooms occur at the water's surface, but some cyanobacterial taxa, such as Planktothrix, are able to modify their buoyancy to access more favorable growing conditions in deeper waters. Here, we used in situ fluorometry to examine the vertical distribution and biomass of Planktothrix in a seasonally anoxic reservoir for 3 consecutive summers. We also collected depth profiles of photosynthetically active radiation, temperature, and nutrients to evaluate which environmental drivers were most important for predicting Planktothrix biomass. In all 3 summers, Planktothrix dominated the phytoplankton community, exhibiting a large (concentrations similar to 100 mu g/L), persistent (lasting similar to 100 d) bloom below the thermocline. The bloom consistently exhibited maximum biomass at or below the depth reached by 1% of surface light. Light availability probably was the most important factor driving the vertical distribution of the stratified Planktothrix bloom, and light, temperature, and nutrients together were strong predictors of cyanobacterial biomass in the hypolimnion, explaining 71 to 93% of the variation in biomass. Our data suggest that Planktothrix remained in the hypolimnion where nutrient availability was maximized, while progressing slightly upward in the water column through each summer in response to light limitation. Our findings demonstrate that Planktothrix can dominate in low light and anoxic conditions and can form persistent blooms that last for multiple months. As cyanobacterial blooms become more prevalent, monitoring cyanobacteria at the surface and at depth will become critically important in freshwater ecosystems.

In freshwater lakes and reservoirs, climate change and eutrophication are increasing the occurrence of low-dissolved oxygen concentrations (hypoxia), which has the potential to alter the variability of zooplankton seasonal dynamics. We sampled zooplankton and physical, chemical and biological variables (e.g., temperature, dissolved oxygen, and chlorophyll a) in four reservoirs during the summer stratified period for three consecutive years. The hypolimnion (bottom waters) of two reservoirs remained oxic throughout the entire stratified period, whereas the hypolimnion of the other two reservoirs became hypoxic during the stratified period. Biomass variability (measured as the coefficient of the variation of zooplankton biomass) and compositional variability (measured as the community composition of zooplankton) of crustacean zooplankton communities were similar throughout the summer in the oxic reservoirs; however, biomass variability and compositional variability significantly increased after the onset of hypoxia in the two seasonally-hypoxic reservoirs. The increase in biomass variability in the seasonally-hypoxic reservoirs was driven largely by an increase in the variability of copepod biomass, while the increase in compositional variability was driven by increased variability in the dominance (proportion of total crustacean zooplankton biomass) of copepod taxa. Our results suggest that hypoxia may increase the seasonal variability of crustacean zooplankton communities.

The development of low dissolved oxygen (DO) concentrations in the hypolimnion of drinking water reservoirs during thermal stratification can lead to the reduction of oxidized, insoluble iron (Fe) and manganese (Mn) in sediments to soluble forms, which are then released into the water column. As metals degrade drinking water quality, robust measurements of metal fluxes under changing oxygen conditions are critical for optimizing water treatment. In this study, we conducted benthic flux chamber experiments in summer 2018 to directly quantify Fe and Mn fluxes at the sediment-water interface under different DO and redox conditions of a eutrophic drinking water reservoir with an oxygenation system (Falling Creek Reservoir, Vinton, VA, USA). Throughout the experiments, we monitored DO, oxidation-reduction potential (ORP), water temperature, and pH in the chambers and compared the metal fluxes in the chambers with time-series of fluxes calculated using a hypolimnetic mass balance method. Our results showed that metal fluxes were highly variable during the monitoring period and were sensitive to redox conditions in the water column at the sediment-water interface. The time-series changes in fluxes and relationship to redox conditions are suggestive of "hot moments", short time periods of intense biogeochemical cycling. Although the metal concentrations and fluxes are specific to this site, the approaches for examining relationships between metals, oxygen concentrations and overall redox conditions can be applied by water utilities to improve water quality management of Fe and Mn. (C) 2020 Elsevier Ltd. All rights reserved.