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.

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.

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.

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.