Water quality in a shallow eutrophic lake is unaffected by extensive thinning of planktivorous and benthivorous fish species.

Ambitious to fulfill the European Water Framework Directive obligations, the European governments support projects to rehabilitate lakes with poor water quality. However, most lake restorations having relied on biomanipulation by fish thinning have failed to improve or even maintain water quality. Previous attempts removed all target fish species simultaneously, thus making it impossible to assess the specific impact of each feeding group on water chemistry. Lake Bromme was selected for extensive, time-selective fish biomanipulation to improve water clarity and promote submerged macrophytes and piscivorous fish stocks over a three-year monitoring period. Thinning of adult benthivorous bream (Abramis brama) and tench (Tinca tinca) was conducted throughout year one while thinning in years two and three targeted planktivorous roach (Rutilus rutilus), juvenile bream, and small perch (Perca fluviatilis). Yearly fish surveys assessed changes in fish population structure and biomass. Water quality parameters were monitored continually, and the cover of submerged macrophytes was surveyed annually via sonar. We found no improvement in water clarity or reductions of nutrients, organic particles, chlorophyll concentrations, or watercolor, despite a 6-fold thinning of total estimated fish biomass, from 112 to 19 kg ha-1. Over the period, the macrophyte cover increased from 0.8 to 13.5 %, but no recruitment of large piscivorous fish (perch and pike (Esox lucius) > 10 cm) was detected. We found higher correlations of particle concentration and water clarity to water temperature than to wind speed, which indicates sediment particle resuspension by the remaining fish community (mostly carp Cyprinus carpio) that forage on benthos in shallow lakes. Further system-ecological research in Lake Bromme should evaluate whether thinning the stock of carp and increasing plant cover may improve water quality and test which optical properties sustain high water turbidity and prevent shallow, eutrophic lakes like Lake Bromme from responding to intense fish thinning.

Removal of chromophoric dissolved organic matter under combined photochemical and microbial degradation as a response to different irradiation intensities.

Throughout the freshwater continuum, Dissolved Organic Carbon (DOC) and the colored fraction, Chromophoric Dissolved Organic Material (CDOM), are continuously being added, removed, and transformed, resulting in changes in the chromophoricity and lability of organic matter over time. We examined, experimentally, the effect of increasing irradiation-intensities on the combined photochemical and microbial degradation of CDOM and DOC. This was done by using a simulated mixed water column: aged water from a humic lake was exposed to four irradiation-intensities - representing winter, early and late spring, and summer conditions (0.10, 0.16, 0.36, and 0.58 W/m2) - and compared with dark controls over 37 days. We found a linear relationship between CDOM degradation and irradiation-intensities up to 0.36 W/m2; the degradation rate saturated at higher intensities, both at specific wavelengths and for broader intervals. After 37 days at high irradiation-intensity, CDOM absorption of irradiation at 340 nm had been reduced by 41%; 48% of DOC had been removed and DOC degradation continued to increase. Aromaticity (SUVA254) declined significantly over 37 days at the two lowest but not at the two highest UV- intensities; levels in unexposed control water remained constant. Direct observations of the humic lake showed that CDOM absorption of irradiation (340 nm) declined by 27% from winter to summer. A model based on hydrological CDOM input and CDOM degradation calculated from field measurements of UV-radiation and experimental CDOM degradation with UV-exposure from sunlight accurately predicted the annual course as observed in the lake. With no external CDOM input, 92% of the CDOM could be degraded in a year. The results support the notion that combined photochemical and microbial CDOM degradation can be remarkably higher in lakes than previously thought and that humic lakes retain their color due to light absorption by ongoing CDOM input.

Century-long records reveal shifting challenges to seagrass recovery.

Global losses over the 20th century placed seagrass ecosystems among the most threatened ecosystems in the world, with eutrophication, and associated deterioration of the submarine light environment identified as the main driver. Growing appreciation of the ecological and societal benefits of healthy seagrass meadows has stimulated efforts to protect and restore them, largely focused on reducing nutrient input to coastal waters. Here we analyze a unique data set spanning 135 years on eelgrass (Zostera marina), the dominant seagrass of the northern hemisphere. We show that meadows in the Western Baltic Sea exhibited major declines relative to historic (1890-1910) reference due to the wasting disease in the 1930s followed by eutrophication peaking in the 1980s, but have only shown modest improvement despite major eutrophication mitigation, halving nitrogen input since the 1980s. Across the past century, we identified generally shallower colonization depths of eelgrass for a given submarine light penetration and, hence, increased apparent light requirements. This suggests that eelgrass recovery is limited by additional stressors. Our study indicates that bottom trawling and intense recent warming (0.5°C per decade, 1985-2018), which impact on deeper and shallower meadows, respectively, suppress eelgrass from fully recovering from eutrophication. Warming is most severe in shallow turbid waters, while clear-water areas offer eelgrass refugia from warming in deeper, cooler waters; but trawling can prevent eelgrass from reaching these refugia. Efforts to reduce nutrient input and thereby improve water clarity have been instrumental in avoiding a catastrophic loss of eelgrass ecosystems. However, local-scale future management must, in addition, reduce bottom trawling to facilitate eelgrass reaching deeper, cooler refugia, and increase resilience toward realized and further warming. Warming needs to be limited by meeting global climate change mitigation goals.

More is less: net gain in species richness, but biotic homogenization over 140 years.

While biodiversity loss continues globally, assessments of regional and local change over time have been equivocal. Here, we assess changes in plant species richness and beta diversity over 140 years at the level of regions within a country. Using 19th-century flora censuses for 14 Danish regions as a baseline, we overcome previous criticisms concerning short time series and neglect of completely altered habitats. We find that species composition has changed dramatically and directionally across all regions. Substantial species losses were more than offset by large gains, resulting in a net increase in species richness in all regions. The occupancy of initially widespread species increased, while initially rare species lost terrain. These changes were accompanied by strong biotic homogenization; i.e. regions are more similar now than they were 140 years ago. Species declining in Denmark were found to be in similar decline all over Northern Europe.

Sexual conflict and intrasexual polymorphism promote assortative mating and halt population differentiation.

Sexual conflict is thought to be an important evolutionary force in driving phenotypic diversification, population divergence, and speciation. However, empirical evidence is inconsistent with the generality that sexual conflict enhances population divergence. Here, we demonstrate an alternative evolutionary outcome in which sexual conflict plays a conservative role in maintaining male and female polymorphisms locally, rather than promoting population divergence. In diving beetles, female polymorphisms have evolved in response to male mating harassment and sexual conflict. We present the first empirical evidence that this female polymorphism is associated with (i) two distinct and sympatric male morphological mating clusters (morphs) and (ii) assortative mating between male and female morphs. Changes in mating traits in one sex led to a predictable change in the other sex which leads to predictable within-population evolutionary dynamics in male and female morph frequencies. Our results reveal that sexual conflict can lead to assortative mating between male offence and female defence traits, if a stable male and female mating polymorphisms are maintained. Stable male and female mating polymorphisms are an alternative outcome to an accelerating coevolutionary arms race driven by sexual conflict. Such stable polymorphisms challenge the common view of sexual conflict as an engine of rapid speciation via exaggerated coevolution between sexes.

Shallow plant-dominated lakes - extreme environmental variability, carbon cycling and ecological species challenges.

BACKGROUND: Submerged plants composed of charophytes (green algae) and angiosperms develop dense vegetation in small, shallow lakes and in littoral zones of large lakes. Many small, oligotrophic plant species have declined due to drainage and fertilization of lakes, while some tall, eutrophic species have increased. Although plant distribution has been thoroughly studied, the physiochemical dynamics and biological challenges in plant-dominated lakes have been grossly understudied, even though they may offer the key to species persistence. SCOPE: Small plant-dominated lakes function as natural field laboratories with eco-physiological processes in dense vegetation dictating extreme environmental variability, intensive photosynthesis and carbon cycling. Those processes can be quantified on a whole lake basis at high temporal resolution by continuously operating sensors for light, temperature, oxygen, etc. We explore this hitherto hidden world. CONCLUSIONS: Dense plant canopies attenuate light and wind-driven turbulence and generate separation between warm surface water and colder bottom waters. Daytime vertical stratification becomes particularly strong in dense charophyte vegetation, but stratification is a common feature in small, shallow lakes also without plants. Surface cooling at night induces mixing of the water column. Daytime stratification in plant stands may induce hypoxia or anoxia in dark bottom waters by respiration, while surface waters develop oxygen supersaturation by photosynthesis. Intensive photosynthesis and calcification in shallow charophyte lakes depletes dissolved inorganic carbon (DIC) in surface waters, whereas DIC is replenished by respiration and carbonate dissolution in bottom waters and returned to surface waters before sunrise. Extreme diel changes in temperature, DIC and oxygen in dense vegetation can induce extensive rhythmicity of photosynthesis and respiration and become a severe challenge to the survival of organisms. Large phosphorus pools are bound in plant tissue and carbonate precipitates. Future studies should test the importance of this phosphorus sink for ecosystem processes and competition between phytoplankton and plants.

Carbon limitation of lake productivity.

Phytoplankton productivity in lakes controls the rate of synthesis of organic matter that drives energy flow through the food webs and regulates the transparency and oxygen conditions in the water. Limitation of phytoplankton productivity and biomass by nutrients and light availability is an established paradigm for lake ecosystems, whereas invasion of atmospheric CO2 has been assumed to cover the high demands of dissolved inorganic carbon (DIC) during intense organic productivity. We challenge this paradigm, and show up to a 5-fold stimulation of phytoplankton productivity and biomass in outdoor mesocosms enriched with DIC, compared to mesocosms with lower DIC concentrations. High DIC supported phytoplankton productivity by direct algal uptake of bicarbonate, through the release of CO2 coupled to calcification and by inducing high pH that greatly enhances atmospheric CO2 invasion. Comparisons of 204 natural Danish lakes supported mesocosm experiments showing higher phytoplankton biomass and pH levels in hard water than soft water lakes for the same nutrient and light availabilities. The most productive lakes are nutrient-rich, hard water lakes that attain surface pHs of 10-11 and chemically enhance atmospheric CO2 uptake 10-15-fold. Our results will help understand natural variations of lake productivity along gradients in nutrients, DIC and pH.

Extreme diel dissolved oxygen and carbon cycles in shallow vegetated lakes.

A common perception in limnology is that shallow lakes are homogeneously mixed owing to their small water volume. However, this perception is largely gained by downscaling knowledge from large lakes to their smaller counterparts. Here we show that shallow vegetated lakes (less than 0.6 m), in fact, undergo recurring daytime stratification and nocturnal mixing accompanied by extreme chemical variations during summer. Dense submerged vegetation effectively attenuates light and turbulence generating separation between warm surface waters and much colder bottom waters. Photosynthesis in surface waters produces oxygen accumulation and CO2 depletion, whereas respiration in dark bottom waters causes anoxia and CO2 accumulation. High daytime pH in surface waters promotes precipitation of CaCO3 which is re-dissolved in bottom waters. Nocturnal convective mixing re-introduces oxygen into bottom waters for aerobic respiration and regenerated inorganic carbon into surface waters, which supports intense photosynthesis. Our results reconfigure the basic understanding of local environmental gradients in shallow lakes, one of the most abundant freshwater habitats globally.

Profound afternoon depression of ecosystem production and nighttime decline of respiration in a macrophyte-rich, shallow lake.

Small, shallow lakes with dense growth of submerged macrophytes are extremely abundant worldwide, but have remained grossly understudied although open water oxygen measurements should be suitable to determine diel fluctuations and test drivers of ecosystem metabolism during the day. We measured the temporal and spatial variability of environmental conditions as well as net ecosystem production (NEP) and respiration (R) in a small, shallow Swedish lake with dense charophyte stands by collecting data from oxygen-, pH-, temperature- and light-sensors across horizontal and vertical gradients during different periods between April and June in 3 years. We found reproducible diel oxygen patterns and daily metabolic rates. The charophyte canopy accounted for almost all primary production and respiration of the ecosystem. Two novel discoveries-profound afternoon depression of production and nighttime decline of respiration-occurred on virtually every day. Extensive increase of oxygen-, temperature- and pH-levels and depletion of dissolved inorganic carbon (DIC) and CO2 concentrations could account for maximum NEP-rates before noon and afternoon depression with low NEP-rates. Ecosystem respiration declined during the night to 24-70% of rates at sunset, probably because of depletion of respiratory substrates. Afternoon depression of photosynthesis should be widespread in numerous habitats with dense growth of macrophytes, periphyton, or phytoplankton implying that daily photosynthesis and growth are restricted and species with efficient DIC use may have an advantage.

Leaf gas films, underwater photosynthesis and plant species distributions in a flood gradient.

Traits for survival during flooding of terrestrial plants include stimulation or inhibition of shoot elongation, aerenchyma formation and efficient gas exchange. Leaf gas films form on superhydrophobic cuticles during submergence and enhance underwater gas exchange. The main hypothesis tested was that the presence of leaf gas films influences the distribution of plant species along a natural flood gradient. We conducted laboratory experiments and field observations on species distributed along a natural flood gradient. We measured presence or absence of leaf gas films and specific leaf area of 95 species. We also measured, gas film retention time during submergence and underwater net photosynthesis and dark respiration of 25 target species. The presence of a leaf gas film was inversely correlated to flood frequency and duration and reached a maximum value of 80% of the species in the rarely flooded locations. This relationship was primarily driven by grasses that all, independently of their field location along the flood gradient, possess gas films when submerged. Although the present study and earlier experiments have shown that leaf gas films enhance gas exchange of submerged plants, the ability of species to form leaf gas films did not show the hypothesized relationship with species composition along the flood gradient.

Ecophysiology of gelatinous Nostoc colonies: unprecedented slow growth and survival in resource-poor and harsh environments.

BACKGROUND: The cyanobacterial genus Nostoc includes several species forming centimetre-large gelatinous colonies in nutrient-poor freshwaters and harsh semi-terrestrial environments with extended drought or freezing. These Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts embedded in an extensive gelatinous matrix of polysaccharides and many other organic substances providing biological and environmental protection. Large colony size imposes constraints on the use of external resources and the gelatinous matrix represents extra costs and reduced growth rates. SCOPE: The objective of this review is to evaluate the mechanisms behind the low rates of growth and mortality, protection against environmental hazards and the persistence and longevity of gelatinous Nostoc colonies, and their ability to economize with highly limiting resources. CONCLUSIONS: Simple models predict the decline in uptake of dissolved inorganic carbon (DIC) and a decline in the growth rate of spherical freshwater colonies of N. pruniforme and N. zetterstedtii and sheet-like colonies of N. commune in response to a thicker diffusion boundary layer, lower external DIC concentration and higher organic carbon mass per surface area (CMA) of the colony. Measured growth rates of N. commune and N. pruniforme at high DIC availability comply with general empirical predictions of maximum growth rate (i.e. doubling time 10-14 d) as functions of CMA for marine macroalgae and as functions of tissue thickness for aquatic and terrestrial plants, while extremely low growth rates of N. zetterstedtii (i.e. doubling time 2-3 years) are 10-fold lower than model predictions, either because of very low ambient DIC and/or an extremely costly colony matrix. DIC uptake is limited by diffusion at low concentrations for all species, although they exhibit efficient HCO3(-) uptake, accumulation of respiratory DIC within the colonies and very low CO2 compensation points. Long light paths and light attenuation by structural substances in large Nostoc colonies cause lower quantum efficiency and assimilation number and higher light compensation points than in unicells and other aquatic macrophytes. Extremely low growth and mortality rates of N. zetterstedtii reflect stress-selected adaptation to nutrient- and DIC-poor temperate lakes, while N. pruniforme exhibits a mixed ruderal- and stress-selected strategy with slow growth and year-long survival prevailing in sub-Arctic lakes and faster growth and shorter longevity in temperate lakes. Nostoc commune and its close relative N. flagelliforme have a mixed stress-disturbance strategy not found among higher plants, with stress selection to limiting water and nutrients and disturbance selection in quiescent dry or frozen stages. Despite profound ecological differences between species, active growth of temperate specimens is mostly restricted to the same temperature range (0-35 °C; maximum at 25 °C). Future studies should aim to unravel the processes behind the extreme persistence and low metabolism of Nostoc species under ambient resource supply on sediment and soil surfaces.

Methane and carbon dioxide fluxes at high spatiotemporal resolution from a small temperate lake.

Lakes are hotspots for CH4 and CO2 effluxes, but their magnitude and underlying drivers are still uncertain due to high spatiotemporal variation within and between lakes. We measured CH4 and CO2 fluxes at high temporal (hourly) and spatial resolution (approx. 13 m) using 24 automatic floating chambers equipped with continuously recording sensors that enabled the determination of diffusive and ebullitive gas fluxes. Additionally, we measured potential drivers such as weather patterns, water temperature, and O2 above the sediment. During five days in autumn 2021, we conducted measurements at 88 sites in a small, shallow eutrophic Danish Lake. CH4 ebullition was intense (mean 54.8 µmol m-2 h-1) and showed pronounced spatiotemporal variation. Ebullition rates were highest in deeper, hypoxic water (5-7 m). Diffusive CH4 fluxes were 4-fold lower (mean 15.0 µmol m-2 h-1) and spatially less variable than ebullitive fluxes, and significantly lower above hard sediments and submerged macrophyte stands. CO2 concentration in surface waters was permanently supersaturated at the mid-lake station, and diffusive fluxes (mean 919 µmol m-2 h-1) tended to be higher from deeper waters and increased with wind speed. To obtain mean whole-lake fluxes within an uncertainty of 20 %, we estimated that 72 sites for CH4 ebullition, 39 sites for diffusive CH4 fluxes and 27 sites for diffusive CO2 fluxes would be required. Thus, accurate whole-lake quantification of the dominant ebullitive CH4 flux requires simultaneous operation of many automated floating chambers. High spatiotemporal variability challenges the identification of essential drivers and current methods for upscaling lake CH4 and CO2 fluxes. We successfully overcame this challenge by using automatic floating chambers, which offer continuous CH4 and CO2 flux measurements at high temporal resolution and, thus, are an improvement over existing approaches.

Tolerance of the widespread cyanobacterium Nostoc commune to extreme temperature variations (-269 to 105°C), pH and salt stress.

Nostoc commune is a widespread colonial cyanobacterium living on bare soils that alternate between frost and thaw, drought and inundation and very low and high temperatures. We collected N. commune from alternating wet and dry limestone pavements in Sweden and tested its photosynthesis and respiration at 20°C after exposure to variations in temperature (-269 to 105°C), pH (2-10) and NaCl (0.02-50 g NaCl kg(-1)). We found that dry field samples and rewetted specimens tolerated exposure beyond that experienced in natural environmental conditions: -269 to 70°C, pH 3-10 and 0-20 g NaCl kg(-1), with only a modest reduction of respiration, photosynthesis and active carbon uptake at 20°C. (14)CO(2) uptake from air declined markedly below zero and above 55°C, but remained positive. Specimens maintained a high metabolism with daily exposure to 6 h of rehydration and 18 h of desiccation at -18 and 20°C, but died at 40°C. The field temperature never exceeded the critical 40°C threshold during the wet periods, but it frequently exceeded this temperature during dry periods when N. commune is already dry and unaffected. We conclude that N. commune has an excellent tolerance to low temperatures, long-term desiccation and recurring cycles of desiccation and rewetting. These traits explain why it is the pioneer species in extremely harsh, nutrient-poor and alternating wet and dry environments.

Moss cushions facilitate water and nutrient supply for plant species on bare limestone pavements.

Dense moss cushions of different size are distributed across the bare limestone pavements on Øland, SE Sweden. Increasing cushion size is predicted to physically protect and improve performance and colonization by vascular plants. Therefore, we tested water balance, phosphorus supply, and species richness, and evaluated duration of plant activity during desiccation as a function of ground area, for a large collection of moss cushions. We found that lower evaporation and higher water storage contributed equally to extending the desiccation period with increasing cushion size. Evaporation rates declined by the -0.36 power of cushion diameter, and were not significantly different from -0.50 for the square root function previously predicted for the increasing thickness of the boundary layer, with greater linear dimensions for smooth flat objects at low wind velocities. Size dependence vanished under stagnant conditions. One moss species was added to the species pool for every nine-fold increase in cushion area. Vascular plants were absent from the smallest cushions, whereas one or two species, on average, appeared in 375- and 8,500-cm(2) cushions with water available for 6 and 10 days during desiccation. Phosphorus concentrations increased stepwise and four-fold from detritus to surface mosses and to vascular plants, and all three pools increased with cushion size. We conclude that cushion mosses and cushion size play a critical role in this resource-limited limestone environment by offering an oasis of improved water and nutrient supply to colonization and growth of plants.

Predicting water quality from geospatial lake, catchment, and buffer zone characteristics in temperate lowland lakes.

Lakes provide essential ecosystem services and strongly influence landscape nutrient and carbon cycling. Therefore, monitoring water quality is essential for the management of element transport, biodiversity, and public goods in lakes. We investigated the ability of machine learning models to predict eight important water quality variables (alkalinity, pH, total phosphorus, total nitrogen, chlorophyll a, Secchi depth, color, and pCO2) using monitoring data from 924 to 1054 lakes. The geospatial predictor variables comprise a wide range of potential drivers at the lake, buffer zone, and catchment level. We compared the performance of nine predictive models of varying complexity for each of the eight water quality variables. The best models (Random Forest and Support Vector Machine in six and two cases, respectively) generally performed well on the test set (R2 = 0.28-0.60). Models were then used to predict water quality for all 180,377 mapped Danish lakes. Additionally, we trained models to predict each water quality variable by using the predictions we had generated for the remaining seven variables. This improved model performance (R2 = 0.45-0.78). Overall, the uncovered relationships were in line with the findings of previous studies, e.g., total nitrogen was positively related to catchment agriculture and chlorophyll a, Secchi depth, and alkalinity were influenced by soil type and landscape history. Remarkably, buffer zone geomorphology (curvature, ruggedness, and elevation) had a strong influence on nutrients, chlorophyll a, and Secchi depth, e.g., curvature was positively related to nutrients and chlorophyll a and negatively to Secchi depth. Lake area was a strong predictor of multiple variables, especially its relationship with pH (positive), pCO2 (negative), and color (negative). Our analysis shows that the combination of machine learning methods and geospatial data can be used to predict lake water quality and improve national upscaling of predictions related to nutrient and carbon cycling.

Physiological Adaptation and Plant Distribution along a Steep Hydrological Gradient.

Plant species often separate strongly along steep environmental gradients. Our objective was to study how coupling between plant physiology and environmental conditions shapes vegetation characteristics along a distinct hydrological gradient. We therefore investigated species photosynthesis in air and under water within a limited area from dry-as-dust to complete submergence in a nutrient-poor limestone habitat on Öland's Alvar, Sweden. We found structural and physiological adaptations of species to endure water limitation at the dry end (e.g., moss cushions and CAM-metabolism) and diffusive carbon limitation (e.g., bicarbonate use) at the submerged end of the gradient. As anticipated, mean photosynthesis in air increased 18-fold from the species-poor assembly of cushion-mosses and Sedum CAM-species on mm-thin limestone pavements to the species-rich assembly of C-3 terrestrial plants in deeper and wetter soils. A GLM-model indicated that 90% of the variation in species richness could be explained by a positive effect of soil depth, a negative effect of the duration of water cover and their interaction. In water, mean photosynthesis was highest among aquatic species, low among Sedum species and cushion mosses, and negligible among C-3 terrestrial plants. While aquatic species dried out in air, drought-resistant small species were probably competitively excluded from the more suitable terrestrial habitats on deeper soils with moderate flooding by taller species of high photosynthetic capability. In conclusion, the clear distribution of species along the steep hydrological gradient reflects distinct structural and physiological adaptations, environmental filtering and interspecific competition.

Wind drives fast changes of light climate in a large, shallow re-established lake.

With ever greater frequency, wetlands and shallow lakes that had been diverted for agriculture are being re-established to reduce nutrient loss and greenhouse gas emission, as well as to increase biodiversity. Here, we investigate drivers of water column light attenuation (Kd) at multiple time scales and locations in Lake Fil, Denmark, during the first five years after its re-establishment in 2012. We found that Kd was generally high (overall mean: 3.4 m-1), with resuspended sediment particles and colored dissolved organic matter being the main contributors. Using daily time series of light attenuation recorded at four stations, we used a generalized additive model to analyze the influence of wind speed and direction on Kd. This model explained a high proportion of the variation (R2 = 0.62, RMSE = 0.74 m-1, and MAE = 0.55 m-1) and showed that higher wind speed increased Kd on the same day and, with smaller influence, on the next day. Furthermore, we found a significant influence of wind direction and an interaction between wind speed and wind direction, a combination that suggests that short-term variations in light climate depends on the interplay between wind direction and sources of particles. Wind from non-prevailing directions thus influence Kd more, as it can activate previously deposited particles. The maximum colonization depths of submerged vegetation occurred at ~2-6% of sub-surface light from 2014 to 2016 and peaked at 1.2 m in 2016. The fast, day-to-day variation of Kd in Lake Fil reveals the importance of wind on light climate and in turn biological elements such as phytoplankton and submerged macrophyte development in shallow lakes. The implications are essential for the prior planning and management of future lake re-establishment.

High sensitivity of Lobelia dortmanna to sediment oxygen depletion following organic enrichment.

• Lobelia dortmanna thrives in oligotrophic, softwater lakes thanks to O(2) and CO(2) exchange across roots and uptake of sediment nutrients. We hypothesize that low gas permeability of leaves constrains Lobelia to pristine habitats because plants go anoxic in the dark if O(2) vanishes from sediments. • We added organic matter to sediments and followed O(2) dynamics in plants and sediments using microelectrodes. To investigate plant stress, nutrient content and photosynthetic capacity of leaves were measured. • Small additions of organic matter triggered O(2) depletion and accumulation of NH(4)(+), Fe(2+) and CO(2) in sediments. O(2) in leaf lacunae fluctuated from above air saturation in the light to anoxia late in the dark in natural sediments, but organic enrichment prolonged anoxia because of higher O(2) consumption and restricted uptake from the water. Leaf N and P dropped below minimum thresholds for cell function in enriched sediments and was accompanied by critically low chlorophyll and photosynthesis. • We propose that anoxic stress restricts ATP formation and constrains transfer of nutrients to leaves. Brief anoxia in sediments and leaf lacunae late at night is a recurring summer phenomenon in Lobelia populations, but increased input of organic matter prolongs anoxia and reduces survival.

Large pools and fluxes of carbon, calcium and phosphorus in dense charophyte stands in ponds.

Bicarbonate and calcium set bounds on photosynthesis and degradation processes in calcareous freshwaters. Charophytic algae use bicarbonate in photosynthesis, and direct variable proportions to assimilate organic carbon and to precipitate calcium carbonate on their surfaces. To evaluate pools of organic carbon (Corg), carbonate carbon (Ccarbonate), and phosphorus (P) in dense charophyte vegetation, we studied apical and basal tissue and carbonate surface precipitates, as well as underlying sediments in ten calcareous ponds. We also quantified the release of calcium, bicarbonate and phosphate from charophyte shoots in dark experiments. We found that the Corg:Ccarbonate quotient in charophyte stands averaged 1.19 during spring and summer. The Corg:Ccarbonate quotient in the sediments formed by dead charophytes averaged 0.97 in accordance with some respiratory CO2 release without carbonate dissolution to bicarbonate. The molar quotient of carbon to calcium was close to 2.0 in sediment and pond water. In dark incubations, shoots subjected to calcium carbonate dissolution released bicarbonate and calcium with a molar quotient of 2:1; lowered pH (7.0-8.0) increased the release. Thus, the carbonate surface crust on living charophytes was not inert, as hitherto anticipated. Phosphate dark release occurred from basal shoots only, was unrelated to pH, and may have derived from organic decomposition, rather than from carbonate dissolution. Extensive phosphorus pools were associated with the charophyte stands (200-600 mg m-2) and had about 2/3 incorporated in alga tissue and 1/3 in carbonate crust. Overall, the biogeochemistry of carbon, calcium and phosphorus are closely linked in calcareous charophyte ponds. Carbonate dissolution from charophyte crusts at night and continuously from sediment might balance extensive carbonate precipitation during daytime photosynthesis. The substantial P-pool in charophyte stands may not derive from P-deprived water, but from P-rich sediment. Charophyte photosynthesis may still contribute to nutrient-poor conditions by forming carbonate-rich sediment of high P-binding capacity.

From drought to flood: Sudden carbon inflow causes whole-lake anoxia and massive fish kill in a large shallow lake.

Fish kills are a recurring phenomenon in hypereutrophic lakes. The effects of a sudden injection of anoxic bottom water into surface waters are well known, as well as the degradation of phytoplankton blooms and the release of phytoplankton toxins. However, in this study we report on a new, climate-related cause of fish kills in a shallow lake. We observed that a long period of drought in a hot summer followed by heavy rain resulted in a large input of labile organic matter. This was followed by a condition of whole-lake anoxia and fish kill in the lake basin receiving the input, while the second basin, immediately downstream, was left unaffected. To test the causal relationship between these events, an oxygen model calculated that respiration had increased by 230% following the organic input and caused whole-lake nocturnal anoxia for four days despite unaltered daytime photosynthesis. One year after the fish kill, roach and bream had migrated from the downstream lake basin and re-established dense populations, while large predatory perch and pike remained very few. This imbalance in the fish food webs may last for several years and in turn increase predation on zooplankton and release phytoplankton from grazing control. The prolonged effects of fish kills on fish and lake community structure demand further research, as weather-induced anoxia can be expected to become more common.