Jonathan P. Doubek

Salting our freshwater lakes

Significance In lakes, chloride is a relatively benign ion at low concentrations but begins to have ecological impacts as concentrations rise into the 100s and 1,000s of mg L−1. In this study, we investigate long-term chloride trends in 371 freshwater lakes in North America. We find that in Midwest and Northeast North America, most urban lakes and rural lakes that are surrounded by >1% impervious land cover show increasing chloride trends. Expanding on this finding, thousands of lakes in these regions are at risk of long-term salinization. Keeping lakes “fresh” is critically important for protecting the ecosystem services freshwater lakes provide, such as drinking water, fisheries, recreation, irrigation, and aquatic habitat. The highest densities of lakes on Earth are in north temperate ecosystems, where increasing urbanization and associated chloride runoff can salinize freshwaters and threaten lake water quality and the many ecosystem services lakes provide. However, the extent to which lake salinity may be changing at broad spatial scales remains unknown, leading us to first identify spatial patterns and then investigate the drivers of these patterns. Significant decadal trends in lake salinization were identified using a dataset of long-term chloride concentrations from 371 North American lakes. Landscape and climate metrics calculated for each site demonstrated that impervious land cover was a strong predictor of chloride trends in Northeast and Midwest North American lakes. As little as 1% impervious land cover surrounding a lake increased the likelihood of long-term salinization. Considering that 27% of large lakes in the United States have >1% impervious land cover around their perimeters, the potential for steady and long-term salinization of these aquatic systems is high. This study predicts that many lakes will exceed the aquatic life threshold criterion for chronic chloride exposure (230 mg L−1), stipulated by the US Environmental Protection Agency (EPA), in the next 50 y if current trends continue.

Anthropogenic land use is associated with N-fixing cyanobacterial dominance in lakes across the continental United States

Cyanobacteria cause many water quality problems in lakes worldwide. Although many studies have examined factors that influence the structure of cyanobacterial communities, few have explicitly compared the effects of within-lake conditions (e.g., nutrient concentrations) and watershed parameters (e.g., land use) on a diverse array of cyanobacterial taxa. Here, we analyzed data from the 2007 U.S. Environmental Protection Agency’s National Lakes Assessment to quantify how lakeshore anthropogenic land use, nutrient concentrations and ratios, and surface water temperatures predict cyanobacterial biovolume and dominance in 236 naturally-formed lakes spanning the continental U.S. We observed that anthropogenic lakeshore land use was the best predictor for cyanobacterial dominance, whereas in-lake nitrogen (N) and phosphorus (P) concentrations were the best predictors for cyanobacterial biovolume. Our analyses suggest that anthropogenic land use may influence cyanobacterial dominance via means other than increased nutrient concentrations. The sum of agricultural and human-developed lakeshore land use was the best predictor of N-fixing cyanobacterial dominance, but there was no significant relationship between anthropogenic land use and non-N-fixing cyanobacterial dominance. In addition, we observed a total N:P ratio threshold for cyanobacterial dominance in the phytoplankton community (150:1) that was much higher than previously reported ratios. Consequently, management strategies to control cyanobacteria need to account for eco-physiological variation among different cyanobacterial taxa, and should consider nutrients and the other effects of land use to control overall lake cyanobacterial biovolume and cyanobacterial dominance, as the two cyanobacterial metrics may be sensitive to different drivers.

Effectiveness of hypolimnetic oxygenation for preventing accumulation of Fe and Mn in a drinking water reservoir

The accumulation of Fe and Mn in seasonally stratified drinking water reservoirs adversely impacts water quality. To control issues with Fe and Mn at the source, some drinking water utilities have deployed hypolimnetic oxygenation systems to create well-oxygenated conditions in the water column that are favorable for the oxidation, and thus removal, of Fe and Mn. However, in addition to being controlled by dissolved oxygen (DO), Fe and Mn concentrations are also influenced by pH and metal-oxidizing microorganisms. We studied the response of Fe and Mn concentrations to hypolimnetic oxygenation in a shallow drinking water reservoir in Vinton, Virginia, USA by sequentially activating and deactivating an oxygenation system over two summers. We found that maintaining well-oxygenated conditions effectively prevented the accumulation of soluble Fe in the hypolimnion. However, while the rate of Mn oxidation increased under well-oxygenated conditions, soluble Mn still accumulated in the slightly acidic to neutral (pH 5.6 to 7.5) hypolimnion. In parallel, we conducted laboratory incubation experiments, which showed that the presence of Mn-oxidizing microorganisms increased the rate of Mn oxidation in comparison with rates under oxic, abiotic conditions. Combined, our field and laboratory results demonstrate that increasing DO concentrations in the water column is important for stimulating the oxidation of Fe and Mn, but that the successful management of Mn is also tied to the activity of Mn-oxidizing organisms in the water column and favorable (neutral to alkaline) pH.

Long-term chloride concentrations in North American and European freshwater lakes

Anthropogenic sources of chloride in a lake catchment, including road salt, fertilizer, and wastewater, can elevate the chloride concentration in freshwater lakes above background levels. Rising chloride concentrations can impact lake ecology and ecosystem services such as fisheries and the use of lakes as drinking water sources. To analyze the spatial extent and magnitude of increasing chloride concentrations in freshwater lakes, we amassed a database of 529 lakes in Europe and North America that had greater than or equal to ten years of chloride data. For each lake, we calculated climate statistics of mean annual total precipitation and mean monthly air temperatures from gridded global datasets. We also quantified land cover metrics, including road density and impervious surface, in buffer zones of 100 to 1,500 m surrounding the perimeter of each lake. This database represents the largest global collection of lake chloride data. We hope that long-term water quality measurements in areas outside Europe and North America can be added to the database as they become available in the future.

Catchment, morphometric, and water quality characteristics differ between reservoirs and naturally formed lakes on a latitudinal gradient in the conterminous United States

Abstract Constructed lakes and impoundments (reservoirs) likely exhibit substantial differences in their physics, chemistry, and biology from naturally formed lakes; however, because reservoirs and natural lakes generally have different latitudinal distributions, less is known about quantitative, generalized differences between the 2 waterbody types. We compared a suite of limnological variables among 1033 reservoirs and natural lakes across multiple size classes on a latitudinal gradient in the conterminous United States. In general, reservoirs had significantly greater perimeters, catchment areas, and catchment area:surface area ratios than natural lakes. Interestingly, several lakeshore land use, morphometric, and water quality response variables exhibited significant interactions between waterbody type and latitude. Southern reservoirs were deeper and had higher proportions of forested land and less agriculture and developed land use in their lakeshore than southern natural lakes, whereas northern reservoirs were shallower and had less forest and more agriculture and developed land in their lakeshore than northern natural lakes. Following the waterbody depth and land use data, natural lakes also had greater total phosphorus (TP) concentrations and shallower Secchi disk depths at lower latitudes, whereas reservoirs had greater TP concentrations and shallower Secchi disk depths at higher latitudes. Overall, natural lakes were more eutrophic than reservoirs, having greater total nitrogen and chlorophyll a concentrations, regardless of latitude. Our findings indicate that many physical, chemical, and lakeshore land use characteristics of reservoirs and natural lakes vary on a latitudinal gradient, which has implications for the water quality, ecology, and management of these waterbodies.

In situ fluorometry reveals a persistent, perennial hypolimnetic cyanobacterial bloom in a seasonally anoxic reservoir

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 ~100 μg/L), persistent (lasting ~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.

Spatial variation in dinoflagellate recruitment along a reservoir ecosystem continuum

Physical and chemical gradients across ecosystems, such as stream-to-lake continua within human-made reservoirs, provide valuable opportunities to examine how organisms respond to changing environments. We quantified the rate of dinoflagellate recruitment across a small reservoir to test the hypothesis that organisms are controlled by different factors along a reservoir continuum. We predicted that recruitment would be tightly coupled with reservoir physics in the riverine zone and closely related to water chemistry in the lacustrine zone. For the dominant dinoflagellate genus in the reservoir, Peridinium, recruitment from the sediments accounted for a median of 16% of increases in pelagic cell abundance throughout the summer. As predicted, Peridinium recruitment rates at the riverine site were correlated with physical variables, while at the lacustrine site, recruitment rates were highly correlated with water chemistry (e.g. nutrient ratios and dissolved oxygen). Recruitment patterns of the second most common genus, Gymnodinium, were not correlated with environmental variables, though Gymnodinium’s much lower densities suggest that its dynamics were controlled by other factors. Our results reveal that the physical–biological coupling controlling algal recruitment, which can play a large role in pelagic population growth and bloom formation, can vary substantially on a spatial gradient within even a small reservoir.