ABSTRACTStudies of the environmental factors that lead to cyanotoxic blooms in tropical/subtropical reservoirs are limited. While technological advances for water treatment and remediation techniques can mitigate the effects of cyanobacterial blooms, the identification of initial drivers and tipping points for minimizing cyanotoxin concentrations could be more cost effective. We studied factors related to microcystin (MC) concentrations and estimated thresholds to limit MC using a water quality dataset from 6 subtropical Brazilian reservoirs. Reservoirs varied by water chemistry and toxin concentrations (MC yearly means <0.1–17 µg/L). Phytoplankton community composition in most reservoirs was dominated by cyanobacteria, and both nitrogen-fixing and non-fixing genera were common. We found positive correlations (p < 0.05, Spearman’s rank) between MC and nutrients (ρ = 0.48–0.74) and chlorophyll a (Chl-a, ρ = 0.65) and a negative correlation between MC and total nitrogen/total phosphorus (TN/TP, ρ = −0.49). While no association between MC and water temperature was observed, MC normalized to Chl-a values were positively related to higher temperatures and lower wind velocities, indicating warmer and more stable waters had a higher probability of toxic blooms. We found thresholds for MC ≥0.1 and ≥1 µg/L related to cyanobacterial cell counts of 20 060–136 165 cells/mL, respectively, and for turbidity (5–12 NTU) and TP (0.016–0.028 mg/L), respectively. We considered these MC concentrations (0.1 and 1 µg/L) to account for different levels of toxin production. The thresholds were different from those published for temperate ecosystems but not consistently lower or higher. TN/TP molar ratios favored toxin concentration when ≤121, suggesting that less phosphorus in relation to nitrogen is necessary to allow MC production in subtropical freshwaters. High phosphorus concentrations can increase concentration of cyanotoxins in subtropical reservoirs, and our data confirm that global warming could exacerbate problems associated with toxic algal blooms.

ABSTRACTWe numerically reproduced the spring riverine thermal bar in the area of the Selenga inflow into Lake Baikal using the nonhydrostatic 2.5D mathematical model. Our model took into account the diurnal variability of the heat fluxes and wind stress on the lake surface based on the atmospheric data from the Babushkin weather station archive during 1–30 May 2015. Propagation of the thermal bar is driven principally by the mechanical and thermal energy of the river inflow together with shortwave radiation while longwave radiation and latent and sensible heat fluxes make smaller contributions. Numerical modeling of the lake hydrodynamics demonstrated that the thermal bar propagation decelerated at night and that strong reverse motion of the thermal bar (toward the shore) was possible within some specific areas due to night cooling and opposing wind. These effects appear following an initial period in which the dynamics are dominated by the river inflow. As the distance from the mouth of the Selenga to the thermal bar increased, the impact of westerly winds on the reverse movement of the thermal bar was reinforced, although the influence of wind does not extend to the full depth of the lake.

The damming of rivers results in hydrological modifications that not only affect the aquatic ecosystem but also adjoining terrestrial systems. Thirteen dams commissioned along the Wujiang River have induced ecological problems, including decreased water turbidity and loss of biodiversity, which potentially influence ecosystem net primary production (NPP) and hence the sequestration, transformation, and storage of carbon. We used terrestrial NPP (NPPT) as a bioindicator to assess the impact of dams on carbon storage in the Wujiang catchment. MODIS satellite and meteorological data were used as inputs to the CASA model to calculate annual NPPT from 2000 to 2014. NPPT was calculated at the catchment and landscape scale to quantify the impact of dams on surrounding terrestrial ecosystems. Mean NPPT was calculated for concentric buffer zones covering a range of spatial extents (0–10 km) from the reservoir shoreline. We found a negligible impact from construction of a single dam on NPPT at the catchment scale. By contrast, the impact of dam construction was scale-dependent, with a stronger landscape-scale effect observed at short distances (i.e., 0–1 km) from the reservoir. Decreases in NPPT were mainly ascribed to the loss of vegetated land resulting from dam impoundment and subsequent urbanization of the surrounding area.

ABSTRACTThe δ15N of nitrate (NO3−) and particulate organic nitrogen (PON) was used to study the sources and fate of nitrogen in the impounded Maotiao River, southwest China. During months when the reservoirs thermally stratified, denitrification was the key process in the hypolimnion in Hongfeng and Baihua reservoirs. Based on the δ15N of PON and NO3−, PON in the epilimnion of reservoirs was dominated by NO3− assimilation during stratification. Most δ15N of PON was higher than that of NO3− in the reservoirs in October, indicating that PON in reservoirs was mainly derived from exogenous nitrogen input or denitrification rather than assimilation of NO3− in the epilimnion. Relationships between the molar carbon:nitrogen (C:N) ratio and δ15N-PON indicated the main sources of PON in reservoirs were from phytoplankton, produced in inflowing rivers and the reservoir under the joint influence of soil organic matter, denitrification, and exogenous nitrogen inputs. The δ15N in PON and NO3− increased gradually from upstream to downstream, by 8.21 ± 0.65‰ and 1.74 ± 3.66‰, respectively, suggesting an accumulative effect caused by the downstream waters. Overall, this study provides insight into the nitrogen biogeochemical cycling of river–reservoirs systems using stable nitrogen isotopes and C:N to specify the effects of river impoundment on nitrogen dynamics.

Weather-induced episodic mixing events in lake ecosystems are often unpredictable, and their impacts are therefore poorly known. The impacts can be short-lived, including changes in water temperature and stratification, but long-lasting effects on the lake’s biology may also occur. In this study we used automated water quality monitoring (AWQM) data from 8 boreal lakes to examine how the episodic weather-induced mixing events influenced thermal structure, hypolimnetic dissolved oxygen (DO), fluorometric chlorophyll estimates (Chl- a ), and lake metabolism and how these events varied in frequency and magnitude in lakes with different characteristics. Rise in wind speed alone had an effect on the lakes with the weakest thermal stability, but a decrease in air temperature together with strong wind induced mixing events in all lakes. The return period of these mixing events varied widely (from 20 to 92 d) and was dependent on the magnitude of change in weather. In lakes with strong stability, thermal structure and hypolimnetic DO concentration were only slightly affected. Weather-induced mixing in the upper water column diluted the surface water Chl- a repeatedly, whereas seasonal maximum occurred in late summer on each lake. Although Finnish lakes have been characterized with stable stratification during summer, we observed many substantial mixing events of relatively short return periods relevant to both chemical and biological properties of the lakes.

We applied coarse spectral analysis to more than 2 decades of daily near-surface water temperature (WT) measurements from Müggelsee, a shallow polymictic lake in Germany, to systematically characterize patterns in WT variability from daily to yearly temporal scales. Comparison of WT with local air temperature indicates that the WT variability patterns are likely attributable to both meteorological forcing and internal lake dynamics. We identified seasonal patterns of WT variability and showed that WT variability increases with increasing Schmidt stability, decreasing Lake number and decreasing ice cover duration, and is higher near the shore than in open water. We introduced the slope of WT spectra as an indicator for the degree of lake mixing to help explain the identified temporal and spatial scales of WT variability. The explanatory power of this indicator in other lakes with different mixing regimes remains to be established.

Stream grade control structures (GCSs) can have strong effects on stream habitat and communities. While most studies of GCSs have examined their impacts on fish and macroinvertebrate assemblages, few have focused on their impacts on benthic algae. Thus, this research studied effects of GCSs by comparing benthic algal communities in a forested stream section with those in a GCS section in a subtropical stream of central Taiwan. We included habitat and water quality surveys and an algal growth experiment with 3 treatments: forested, GCS, and shaded GCS. The GCS section had a greater stream wetted width, percentage of pool area, and algal biomass indicated by chlorophyll a, algal biovolume, and density than the forested section, whereas the forested section had a higher canopy cover, substratum size, and habitat diversity. Filamentous green algae were the dominant group in the GCS section, whereas diatoms were dominant in the forested section. Experimental results showed that GCSs strongly elevated algal biomass and changed community compositions. The shaded treatment significantly reduced >80% of the algal biomass in the GCS section, close to the reduction in the forested treatment. Considering all variables altered by the GCS, canopy cover might be the primary responsible factor. We suggest considerations on design and construction of GCSs.

At the La Grande Hydroelectric Complex (Québec, Canada), total mercury (THg) levels in fish were monitored from 1978 to 2012 in more than 37 000 fish comprising 5 species: lake whitefish (Coregonus clupeaformis), longnose sucker (Catostomus catostomus), northern pike (Esox lucius), walleye (Sander vitreus), and lake trout (Salvelinus namaycush). In reservoirs, concentrations of all species increased rapidly after impoundment, peaking after 4–11 yr in nonpiscivorous species and after 9–14 yr in piscivorous species, at levels 2–8 times higher than those measured in surrounding natural lakes. In fish of standardized length, maximum levels reached 0.33–0.72 μg g−1 in nonpiscivorous species and 1.65–4.66 μg g−1 in piscivorous species. Depending on the reservoir, the return to levels equivalent (p < 0.05) to those found in fish in surrounding natural lakes was completed after 10–20 yr for all nonpiscivorous species and after 20– 31 yr in most piscivorous species, if no additional flooding occurred. These results tend to confirm the findings of other authors suggesting that the following reservoir characteristics play a major role in determining the intensity and duration of after-impoundment THg increases in fish: flooded area, annual volume of water flowing through the reservoir, filling period, water temperature, and percentage of flooded area located in the drawdown zone.

Temporary pools are the most abundant insular aquatic habitats in the world and are ideal systems for studying ecological and evolutionary processes; however, they are poorly studied in many regions of the world in favor of larger, permanent systems like lakes and rivers. The Nebraska Sandhills, Nebraska, USA, contains thousands of groundwater-fed lakes and temporary pools, ranging from freshwater to hypersaline-alkaline. Much of the variation in water chemistry is associated with a longitudinal climate gradient where the climate is mesic in the eastern Sandhills and transitions to semiarid in the western Sandhills. Our objective was to conduct a survey of the temporary pools to learn more about their environmental and ecological characteristics and how they differ along the longitudinal climate gradient. We sampled 30 pools from 3 areas along the climate gradient from April to July 2013. Branchiopod crustaceans were used to measure ecological characteristics because they are adapted to live in temporary pools and are sensitive to environmental changes. The frequency and salinity of saline pools increased westward as aridity increased. Salinity was the primary environmental gradient structuring branchiopod communities. Freshwater alpha and gamma diversity declined westward, which may be due to the westward decline in freshwater habitat. The transition from a homogeneous freshwater-dominated landscape in the eastern Sandhills to a heterogeneous landscape consisting of fresh-, low-, and high-salinity waters in the western Sandhills makes this system ideal for studying ecological processes at local and regional scales.

The worldwide spread of bigheaded carp (Hypophthalmichthys spp.) is considered an ecological threat and problem in many countries. The aim of this study was to consider the likelihood of bigheaded carp reproduction in Lake Balaton and its catchment area (Hungary). The within-year trends of gonadosomatic index values show that female bigheaded carp increase ovary masses from spring until midsummer, after which a decline is typical. This trend may indicate bulk egg release, but the lack of a sharp drop in fecundity (number of eggs in the ovary) values during and after the potential spawning period suggests that a general and considerable ovary evacuation does not likely explain the declining gonadosomatic index values. Rather, it is more probable that female bigheaded carp resorb their eggs, supported by the presence of atretic oocytes in the ovaries after the potential spawning season. In turn, postovulatory follicles were present in the posterior ovary segments of 2 individuals, indicating that mature eggs were released by some females. The evidence of sporadic egg release does not necessarily imply that successful reproduction has occurred, however. The genetic analyses showed remarkable deviation from Hardy-Weinberg equilibrium within the stock, suggesting that bigheaded carp do not constitute a natural reproductive community in Lake Balaton. Thus, the present bigheaded carp stock in Lake Balaton likely derives primarily from external origin (i.e., from the surrounding fish ponds).