Lakes In Denmark Research Articles

Neotectonic deformations of the lakebeds in Esrum Sø, eastern Denmark, interpreted to indicate a Postglacial pull-apart basin.

Esrum Sø, with a marked elongation trending N–S, located in the glaciomorphological landscape of north-eastern Sjælland, is one of the biggest lakes in Denmark. The lake is shaped as a large ‘sink’ with steep sides and a maximum depth of c. 22 m. The present investigation and mapping of the structures of the lakebeds is based on the interpretation of 10 reflectors on 82 acquired high-resolution seismic sections (Chirp III) and correlation to the 8.5 m long Esrum Sø drill core in the northern part of Esrum Sø. A new formation, the Esrum Formation, with two members, the Fredensborg and Nødebo Members, is erected based on the geological information obtained from the drill core, and traced on the seismic sections over the major part of Esrum Sø. In the northern part of the lake, the seismic sections reveal a number of anti- and synform structures, which we group into five architectural features. The mapping of these structures show N–S trending vertical fractures, fault scarps and fold crests, which in places are bent towards a NW direction. In some, mainly southern, parts of the lake, gas stored in the gyttja (Nødebo Member) and close to the lake floor degrades the seismic signal significantly. The gas is ascribed to seepage and production close to the lake floor. The integration of the seismic interpretation and the Esrum Sø drill core data shows that the Young Baltic till constitutes the base of the depression where Esrum Sø is located. In Bølling to Preboreal time the Esrum Sø was a shallow freshwater lake with no tectonic activity (Fredensborg Member), and the major deformation and subsidence of the lake basin took place in Boreal to Atlantic time (Nødebo Member). Our interpretation favours a model of a Postglacial pull-apart basin related to extension in the wrench-fault tectonic zone aligned with the Sorgenfrei–Tornquist Zone.

Ecosystem changes in eight Danish lakes after zebra mussel invasion

AbstractInvasion of zebra mussels (Dreissena polymorpha) can have profound effects on lake ecosystems. We investigated the overall ecological effects after invasion and fast expansion of zebra mussels in eight lakes in Denmark belonging to the same watercourse systems but with different morphological and eutrophication characteristics. A few years after the invasion, most of the lakes had experienced profound changes in lake water quality, e.g. chlorophyll a had been reduced by 18–62% and Secchi depth had increased by 38–171%. Nutrient concentrations also decreased, total phosphorus to 37–64% and total nitrogen to 49–77% of pre-invasion concentrations. Seasonally, differences between both summer and winter phosphorus and nitrogen concentrations diminished, suggesting a change in internal nutrient cycling after the invasion. In most lakes, the abundance of submerged macrophytes rose substantially, and often coverage increased by a factor 10 or more, and the maximum colonisation depth increased by up to 5.4 m. Plant species numbers increased markedly in four of the six lakes with macrophyte data. The fish community changed towards higher dominance of potential piscivorous perch and an increase in their body size. The profound effects of zebra mussel invasion may affect lake management and how ecological quality should be interpreted.

Predicting lake bathymetry from the topography of the surrounding terrain using deep learning

AbstractLake morphometric features like surface area, volume, mean, and maximum depth are important predictors of many physical, biological, and ecological processes. Lake bathymetric maps that present the lake basin contours are thus an integral part of limnological investigations. Accurate but cumbersome traditional bathymetric surveys measure the depth using a lead line or echosounder. Recently, airborne bathymetric mapping using imagery or laser scanning has been attempted in shallow freshwater and coastal habitats. However, these methods depend on the ability of light to penetrate the water column, which can be problematic in eutrophic lakes and shallow lakes. To alleviate these issues, we developed and tested a deep learning model (based on the U‐net) using data from 153 lakes in Denmark to predict bathymetry using the topography of the surrounding terrain. The deep learning model performed much better (pixel‐wise mean absolute error: validation set = 1.75 and test set = 2.15 m) than baseline interpolation approaches (validation set = 3.12 m). In addition, the deep learning model generated more realistic bathymetry maps that did not suffer from interpolation artifacts. We find that the model performance improves slightly with increasing model size (number of trainable parameters) and the extent of the surrounding terrain. In addition, our pretraining procedure improved performance and reduced the time for model convergence. Because the model only relies on digital elevation data which are widely available, it can be fine‐tuned and used to predict lake bathymetry in other geographical regions.

Environmental predictors of lake fish diversity across gradients in lake age and spatial scale

Abstract Freshwater ecosystems have experienced a great loss of biodiversity in the recent century due to eutrophication and loss of habitats, particularly in agricultural lowlands. Some of these ecosystems have recently been re‐established, or entirely new lakes have been created to improve nutrient removal, biodiversity, and recreation. Fish have an important structuring role in lake ecosystems, but little is known about the temporal development of fish species richness or composition in new or re‐established lakes, especially in comparison with natural lakes. We investigated the influence of environmental variables and landscape features on fish species richness and fish assemblage in drainage basins and in 34 new (between 0 and 99 years) and 193 natural lakes in Denmark, using structural equation modelling. Fish species richness in drainage basins is influenced primarily by basin elevation, lake area, and salinity at the basin outlet; low‐salinity coastal regions are important migration pathways for many freshwater fish species, not just for anadromous or catadromous species. Land use does not appear to influence richness at the drainage basin scale, indicating that the influence of anthropogenic activities is minor or that significant effects thereof occurred a long time ago (decades to centuries). The drainage basin richness defines the apparent pool of species that may colonise lakes within the basin, but the actual number of species is particularly influenced by stream network connectivity. Disconnected lakes have fewer species, a deficit that is sustained over long time scales. In addition to the influence of stream connectivity, lake characteristics such as elevation, surface area, and water chemistry (alkalinity and pH) are also important predictors of fish species richness, whereas we did not find any effect of lake nutrient concentrations. The direct effect of land use was not evaluated. Ordination analysis shows that fish assemblage depends on both lake type (new/natural) and stream connectivity (connected/disconnected). Furthermore, species‐specific occurrences reveal some differences between new and natural lakes and indicate that stocking plays a role in new lakes. While new and natural lakes may initially differ in terms of both how many and which species they contain, they converge over time in a process that can be enhanced by improving stream connectivity.

Do Cross-Latitude and Local Studies Give Similar Predictions of Phytoplankton Responses to Warming? An Analysis of Monitoring Data from 504 Danish Lakes

Cross-latitude studies on lakes have a potential to predict how global warming may cause major changes in phytoplankton biomass and composition, e.g., the development of favourable conditions for cyanobacteria dominance. However, results from these studies may be influenced by biogeographical factors, and the conclusions may, therefore, not hold when considering local response patterns. We used monthly monitoring data from 504 lakes in Denmark—a small and homogeneous geographical region—to establish empirical relationships between key phytoplankton groups and a set of explanatory variables including total phosphorus (TP), total nitrogen (TN), lake mean depth (DEP) and water temperature (TEMP). All variables had strong effects on phytoplankton biomass and composition, but their contributions varied over the seasons, with TEMP being particularly important in June–October. We found dominance of cyanobacteria in terms of biomass and also an increase in dinophytes biomass at higher TEMP, while diatoms and chlorophytes became less important. In May, however, the TEMP effect on total phytoplankton biomass was negative, likely reflecting intensified zooplankton grazing. Our results suggest that biogeographical effects are of minor importance for the response patterns of phytoplankton to temperature and that substantial concentration reductions of TN and TP are needed in eutrophic lakes to counteract the effect of the climate change-induced increase in TEMP.

Turnover of lake sediments treated with sediment microbial fuel cells: a long-term study in a eutrophic lake

Sediment microbial fuel cells (SMFCs) have previously been successfully used to reduce phosphate release from the sediments of eutrophic lakes. In this study, we investigate the risk that SMFCs stimulate sediment decomposition with the unwanted side effect being the release of legacy pollutants stored in sediments. Electrode pairs (16 m2 each) were installed in a eutrophic lake in Denmark and the electricity production was monitored over more than a year at three electrode fields. Equations were derived that allow calculation of the substrate turnover by the SMFCs from the working potential, the open circuit potential, and the external resistance of the SMFCs. The resulting turnover data suggest that the decomposition of the sediment is only slightly expedited by the SMFCs, and that the decomposition process is not significantly stimulated by the type of SMFCs installed in the lake. The measured maximum power density with stainless steel electrodes in the lake sediment was 0.9 mW/m2, which was sufficient to reduce P outflux from sediment. At this power density, the decomposition half-life of the lake sediment (top 5 cm) is calculated to be 277 years, which is only about 10% of natural lake sediment decomposition half-lives. Higher power densities are not necessary for P fixation but inadvertently increase the risk that legacy pollutants buried in the sediment are released.

Trematode diversity reflecting the community structure of Danish freshwater systems: molecular clues

BackgroundDigenean trematodes are parasitic platyhelminths that use several hosts in their life cycles and are thereby embedded in various ecosystems affected by local environmental conditions. Their presence in a habitat will reflect the presence of different host species and, as such, they can serve as ecological indicators. Only limited information on the occurrence of trematodes and their link to other trophic levels in the Danish freshwater ecosystems is currently available.Therefore, the main aim of the present study was to increase our knowledge in this field.MethodsSnails were sampled from 21 freshwater lakes in Denmark, following which shedding procedures were performed, cercariae were recoved and the released parasites were identified using molecular tools (PCR and sequencing).ResultsA total of 5657 snail hosts belonging to ten species were identified, revealing a highly diverse parasite fauna comprising 22 trematode species. The overall trematode prevalence was 12.6%, but large variations occurred between host species. The snail host Lymnaea stagnalis showed the highest prevalence and also exhibited the highest diversity, accounting for 47.6% of the species richness.ConclusionsThis survey contributes updated information on parasite–host relations and compatibility and may assist in describing the ecological structure of the investigated Danish freshwater ecosystems.Graphical

Monitoring of saxitoxin production in lakes in Denmark by molecular, chromatographic and microscopic approaches

Diversity of phytoplankton in three eutrophic and two mesotrophic lakes in Denmark was determined by microscopic and HPLC pigment analyses to identify and quantify potential saxitoxin (STX) producing cyanobacteria. Potential dominant STX-producers were identified to the filamentous genera Dolichospermum, Cuspidothrix, Phormidium and Planktolyngbya. Presence of STX production was documented by extraction of five intracellular STXs that included (in declining concentration in the cyanobacteria) dc-neo-STX, neo-STX, dc-STX, STX and GTX. Total concentrations of the five STXs varied from 9 to 142 fg per potential STX producer, corresponding to 87 to 985 ng L−1 in the lakes. For molecular detection of the STX-producers, a quantitative PCR method was developed by design of a new robust primer set with broad coverage to target the sxtA gene that is common to all STX-producing cyanobacteria. After validation, copy numbers of the sxtA gene were determined to vary from about 104 (mesotrophic lakes) to 108 per mL (the most eutrophic lake). A moderate but significant correlation was observed between abundance of the sxtA copies and concentrations of the five intracellular STXs. The qPCR assay was found to be a rapid and robust procedure for quantification of STX producers. Saxitoxin and its analogs appeared not to cause health concerns in the lakes, but commercial fishing for pike perch in the most eutrophic lake should be monitored to test for food web accumulation of STXs.

Relationship between Polycyclic Aromatic Hydrocarbons in Sediments and Invertebrates of Natural and Artificial Stormwater Retention Ponds

Sediments and invertebrates were sampled from 9 stormwater retention ponds (SWRPs) and 11 natural, shallow lakes in Denmark. Samples were analyzed for 13 polycyclic aromatic hydrocarbons (PAHs). The SWRPs received urban and highway runoff from various types of drainage areas and the lakes were located in areas of various land uses. Comparing PAHs in the sediments of the SWRPs and the lakes, it was found that levels of total PAH were similar in the two aquatic systems, with median values of 0.94 and 0.63 mg·(kg·DM)−1 in sediments of SWRPs and lakes, respectively. However, the SWRP sediments tended to have higher concentrations of high-molecular-weight PAHs than the lakes. A similar pattern was seen for PAHs accumulated in invertebrates where the median of total PAH was 2.8 and 2.1 mg·(kg·DM)−1 for SWRPs and lakes, respectively. Principal component analysis on the PAH distribution in the sediments and invertebrates showed that ponds receiving highway runoff clustered with lakes in forests and farmland. The same was the case for some of the ponds receiving runoff from residential areas. Overall, results showed that sediment PAH levels in all SWRPs receiving runoff from highways were similar to the levels found in some of the investigated natural, shallow lakes, as were the sediment PAH levels from some of the residential SWRPs. Furthermore, there was no systematic trend that one type of water body exceeded environmental quality standards (EQS) values more often than others. Together this indicates that at least some SWRPs can sustain an invertebrate ecosystem without the organisms experiencing higher bioaccumulation of PAHs then what is the case in shallow lakes of the same region.

Early fish colonization and community development in a shallow re-established lake

The establishment of new lakes has gained momentum throughout Europe and North America as a way to remove nutrients, manage extreme rain events, establish recreational areas, and increase biodiversity. We compared freshwater fish colonization and community development of re-established Lake Fil, a 900-ha lake in Denmark, with fish communities in lakes in close proximity as well as those throughout the region. We found that fish migrated quickly into the lake from nearby refugia with fast initial colonization followed by saturation of the species pool. A total of 14 fish species migrated into the lake during the first five years. Over this period, the lake showed increasing similarity not only to the two directly connected lakes, but also to other lakes in the region. This could be seen in terms of species occurrence similarity, catch per unit of effort (CPUE) and biomass per unit of effort (BPUE). The first immigrant was the three-spined stickleback (Gasterosteus aculeatus), followed by roach (Rutilus rutilus) and pike (Esox lucius), which both came to dominate the fish biomass in the following years. However, the pike population showed no reproduction and perch (Perca fluviatilis) had low immigration and reproduction, pointing to a limited success, overall. One solution might be to stock Lake Fil and other new lakes with predatory fish, which might ensure a more balanced food web structure which positively influence the turbidity of the lake water and provide recreational value for anglers.

Decadal changes in zooplankton biomass, composition, and body mass in four shallow brackish lakes in Denmark subjected to varying degrees of eutrophication

ABSTRACT During the past century, many brackish shallow lakes worldwide have become eutrophic. How the zooplankton have responded to this development is not well elucidated. Here, we analysed the decadal changes (from 1999–2000 to 2017–2018) in zooplankton biomass, body mass, and potential top-down control on phytoplankton during summer in 4 Danish shallow brackish lakes (Lund Fjord, Han Vejle, Selbjerg, and Glombak) subjected to varying degrees of eutrophication. Significant reductions of zooplankton biomass, body mass, the ratio of large-sized cladoceran to total cladoceran biomass, and the ratio of zooplankton to phytoplankton biomass were observed in low to moderately vegetated lakes (Selbjerg and Glombak). However, in the macrophyte-dominated lake (Han Vejle), zooplankton biomass, body mass, and the contribution of large-sized cladocerans (Daphnia spp.) to total cladoceran biomass increased without a corresponding increase in the zooplankton to phytoplankton biomass ratio, which may be attributed to the refuge effect of submerged macrophytes. Using the pooled dataset, multivariate analysis indicated that total phosphorus concentrations and fish abundance were the main drivers of shifts in the zooplankton community and that zooplankton body mass was strongly negatively related to fish abundance. From a lake management perspective, our results suggest that eutrophication, through increased fish predation and reduced submerged vegetation abundance, has major effects on zooplankton communities in temperate coastal brackish shallow lakes, and a reduction in the zooplankton grazing pressure on phytoplankton is predicted if the eutrophication process continues.

To measure chlorophyll or phytoplankton biovolume: an aquatic conundrum with implications for the management of lakes

Canfield DE, Jr., Bachmann RW, Hoyer MV, Johansson LS, Søndergaard M, and Jeppesen E. 2018. To measure chlorophyl or phytoplankton biovolume: an aquatic conundrum with implications for the management of lakes. Lake Reserv Manage. 35:181–192.The log10-transformed relationship between measured phytoplankton biovolumes and chlorophyll concentrations, surrogates for algal biomass, was examined using 13,000-plus paired samples collected from lakes in Denmark (250), the continental United States (1835), and Florida (159). A positive (R2 = 0.57) relationship was found but predicted biovolumes had a 95% confidence interval of 11–912%. Regressing chlorophyll as opposed to phytoplankton biovolume against total phosphorus (TP) (R2 = 0.43 vs. R2 = 0.21), total nitrogen (TN) (R2 = 0.24 vs. R2 = 0.08), and Secchi disk (SD) (R2 = 0.60 vs. R2 = 0.39) yielded stronger relationships. Three algal groups (Cyanophyta, Chlorophyta, and Bacillariophyceae) contributed approximately 100% of the biovolume in some samples. For these groups and samples, biovolume increased significantly with chlorophyll (R2 = 0.35, R2 = 0.27, and R2 = 0.31, respectively) and TP (R2 = 0.18, R2 = 0.13, and R2 = 0.13) and decreased significantly with increases in SD (R2 = 0.29, R2 = 0.31, and R2 = 0.17). All empirical relationships had substantial confidence intervals. Contingency tables for variance within the independent (horizontal variance) and dependent (vertical variance) variables are presented, providing managers information on how much change is required to insure noticeable effects. If resources are limited, chlorophyll is recommended for monitoring long-term trends because it provides an estimate of biomass magnitude and has better relationships with nutrients and SD. Managers may integrate occasional biovolume measurements if concerned with Cyanophyta abundance, taste and odor production, or changing algal population dynamics.

Using hydrogeophysical methods to assess the feasibility of lake bank filtration

Lake Tissø is the fourth largest lake in Denmark with sufficient, continuous surface water input making lake bank filtration an optimal surface water extraction method provided that there are suitable shallow aquifers with a good hydraulic connection to the lake. The shallow subsurface under the lake was mapped by five waterborne Electrical Resistivity Tomography (ERT) and eight Ground Penetrating Radar (GPR) surveys to locate potentially suitable aquifers. Distributed Temperature Sensing (DTS) was used for the general characterisation of the hydraulic connection between the lake and aquifer. The waterborne ERT surveys showed several zones of coarser material along the shore extending up to five meters depth below the lake surface, while on-land ERT surveys confirmed that these zones extend to the lakeshore with similar thickness. Both waterborne ERT and GPR profiles agreed that at a selected field site the highest possibility of coarse sediments is 60–140 m from the lakeshore. Lakebed temperatures measured in December 2016, May and June 2017 all indicated potential groundwater discharge to the lake at approximately 135 m from the shoreline supporting the results of geophysical surveys showing coarser sediments in that area. A 2D numerical flow model of the area with a geological setup based on geophysical information and slugtest data, also showed upward groundwater discharge peaks at 60 and 140 m from the shoreline confirming the findings of the field surveys. Thus, this study shows that the combination of waterborne ERT and GPR surveys with DTS measurements is a fast and efficient way to assess the feasibility of lake bank filtration in these settings. This approach of combined hydrogeophysical methods is robust and has a potential to site lake bank filtration areas not only next to large rivers where coarse sediments are found in abundance, but even in different sedimentary environments where suitable layers for lake bank filtration are more difficult to identify.

Recovery of lake vegetation following reduced eutrophication and acidification

Abstract In recent decades, many aquatic ecosystems in Europe and North America have experienced reduced inputs of nutrients and acidifying substances because of improved sewage treatment and reduced emission of sulphur oxides. We evaluated the consequences of these efforts to changes in water chemistry, species richness and community composition of aquatic macrophytes in 56 lakes in Denmark around 1990 and again around 2010. Reductions in lake water concentrations of phosphorus and nitrogen were strongest in eutrophic and hypertrophic lakes, for example, lakes which had been heavily affected by domestic sewage. These changes translated into decreased algal biomass in the most eutrophied lakes. Oligo‐ and mesotrophic lakes did not change significantly in terms of nutrients or algal biomass. Water clarity increased across all lakes but not significantly in specific trophic lake groups. Alkalinity and pH increased significantly (up to 2 pH‐units) in low‐alkaline lakes, while well‐buffered high‐alkaline lakes (>0.5 meq/L) did not show any change. Macrophyte species richness per lake increased, on average, by 13% during the 20‐year study period. The increase was strongest in species preferring nutrient‐rich conditions and could be directly attributed to reductions in phytoplankton biomass in lakes of medium water clarity. The similarity among all lakes in terms of species composition increased over the study period. This development was closely related to higher average species richness and was mainly caused by recolonisation of lakes, recovering from past eutrophication, by relatively common species (e.g., Lemna trisulca, Sparganium emersum and Potamogeton berchtoldii). Higher pH in low‐alkaline lakes was accompanied by a shift from acid‐tolerant to more acid‐sensitive species. Our results demonstrate that investment in pollution control has been successful in terms of markedly improving water quality of lakes and, with a time lag, macrophyte species richness. Although relatively common species have spread across lakes and resulted in homogenised macrophyte communities, continued efforts to reduce pollution could ensure the survival of rare specialist species and perhaps even increase their abundance in the future.

Gravel pit lakes in Denmark: Chemical and biological state

Mining of gravel and sand for construction purposes is big business and gravel pit lakes have become increasingly common all over the world. In Denmark, hundreds of gravel pit lakes have been created during the past decades. We investigated the chemical and biological status of 33–52 gravel pit lakes and compared the results with data from similar-sized natural Danish lakes. The area of the lakes ranged from 0.2 to 13ha and their age from 0.5 to 26years. Generally, the gravel pit lakes were clear with low nutrient concentrations, the median concentrations of total phosphorus and total nitrogen being 0.023mg/l and 0.30mg/l compared with 0.115mg/l and 1.29mg/l, respectively, in natural lakes. Correspondingly, median chlorophyll a was 5μg/l in the gravel pit lakes and 36μg/l in the natural lakes. Submerged macrophytes were found in all gravel pit lakes, with particularly high cover in the shallow ones. Most gravel pit lakes were deeper than the natural lakes, which may restrict the area potentially to be covered by submerged macrophytes, with implications also for the biological quality of the lakes. Fish were found in most of the gravel pit lakes, roach (Rutilus rutilus), perch (Perca fluviatilis) and rudd (Scardinius erythrophalmus) being the most frequently observed species. Fish stocking was common and included also non-native species such as carp (Cyprinus carpio) and rainbow trout (Oncorchynchus mykiss). Compared with the natural lakes, fish species richness and catch per gillnet were overall lower in the gravel pit lakes. Groundwater-fed gravel pit lakes add importantly to the number of high-quality lakes in Denmark and with an optimised design and by avoiding negative side effects, they can be positive for both nature and society.

Remarkable richness of aquatic macrophytes in 3-years old re-established Lake Fil, Denmark

In August 2014, we discovered a surprisingly high species richness of aquatic macrophytes (33 species) in Lake Fil only two years after the large, shallow lake had been re-established following land reclamation and 60 years of intense agricultural use. In 2015, plant richness had increased even further (to 40 species). Some rare, oligotrophic species, which formerly were common in the historical lake, have re-established populations (e.g. Elatine hydropiper and Lythrum portula). However, a larger number of meso- and eutrophic species have colonized the re-established lake, changing the character of aquatic macrophyte flora as compared to the historical lake. The very rare Baldellia repens, considered extinct species in Denmark, has become re-established with at least 13 subpopulations in the new Lake Fil. Also two species recently discovered in Denmark (Callitriche obtusangula and Lemna turionifera) were found. Half of the species may have colonized the re-established lake from now submerged drainage canals and from an upstream lake, while the other half presumably have colonized from more distant lakes. The high richness of aquatic macrophytes in the re-established Lake Fil is matched by only a few natural lakes in Denmark. According to predictions of species richness from environmental conditions and colonized area in 56 natural Danish lakes, the observed richness in Lake Fil is two-fold the expected. We attribute the high plant richness to: shallow waters, large open areas on grazed or physically exposed lake shores and high likelihood of species recruitment from neighboring lakes via dense populations of migratory waterfowl. High plant richness was established despite low transparency due to high release of dissolved organic matter and particles from flooded soils. We conclude that suitable bathymetry, physical disturbance, grazing by cattle and waterfowl in the littoral zone and high dispersal capacity of propagules may ensure rapid establishment of a rich aquatic vegetation in new lakes, even when nutrient-rich farmland is the starting point.

Projecting the future ecological state of lakes in Denmark in a 6 degree warming scenario

4 Geological Survey of Denmark and Greenland (GEUS), Oster Voldgade 10, 1350 Copenhagen, Denmark ABSTRACT: Lakes are highly sensitive to climate change, and climate warming is known to induce eutrophication symptoms in temperate lakes. In Denmark, climate is projected to cause increased precipitation in winter and increased air temperatures throughout the year by the end of the 21st century. Looking further into the future, the warming trend is projected to continue and likely reach a 6°C increase around the 22nd century (relative to a baseline period of 1986�2005). In the present study, we evaluate the consequences of such extreme changes for temperate Danish lakes. We use a multifaceted modelling approach by combining an eco-hydrological model to estimate future water runoff and catchment nutrient exports with both mechanistic and empirical lake models, describing key biogeochemical indicators in lakes, in order to quantify the effects of future nutrient loads and air temperature on lake ecosystems. Our model projections for the future scenario suggest that annual water runoff will increase (46%), driving also increases in exports of nitrogen and phosphorus (13 and 64%, respectively). Both the mechanistic and empir- ical modelling approaches suggest that phytoplankton biomass will increase and that potentially toxin-producing cyanobacteria may become a dominant feature of the phytoplankton community from spring. Warming and increased nutrient loads also affect the food webs within the lakes in the direction of higher fish control of algae-grazing water fleas, further reinforcing eutrophication. To be able to mitigate these eutrophication effects, external nutrient loading to the lakes must be reduced considerably.

PH influences the importance of niche-related and neutral processes in lacustrine bacterioplankton assembly.

pH is an important factor that shapes the structure of bacterial communities. However, we have very limited information about the patterns and processes by which overall bacterioplankton communities assemble across wide pH gradients in natural freshwater lakes. Here, we used pyrosequencing to analyze the bacterioplankton communities in 25 discrete freshwater lakes in Denmark with pH levels ranging from 3.8 to 8.8. We found that pH was the key factor impacting lacustrine bacterioplankton community assembly. More acidic lakes imposed stronger environmental filtering, which decreased the richness and evenness of bacterioplankton operational taxonomic units (OTUs) and largely shifted community composition. Although environmental filtering was determined to be the most important determinant of bacterioplankton community assembly, the importance of neutral assembly processes must also be considered, notably in acidic lakes, where the species (OTU) diversity was low. We observed that the strong effect of environmental filtering in more acidic lakes was weakened by the enhanced relative importance of neutral community assembly, and bacterioplankton communities tended to be less phylogenetically clustered in more acidic lakes. In summary, we propose that pH is a major environmental determinant in freshwater lakes, regulating the relative importance and interplay between niche-related and neutral processes and shaping the patterns of freshwater lake bacterioplankton biodiversity.

Diagenesis of settling seston: identity and transformations of organic phosphorus

Solution (31)phosphorus NMR spectroscopy and sequential fractionation were used to follow diagenetic changes in phosphorus forms during decomposition of settling seston in Lake Nordborg, a shallow eutrophic lake in Denmark. In a decomposition experiment, seston released >60% of their total phosphorus during ~50 days incubation, although seston collected during summer contained more phosphorus and released it over a longer period compared to seston collected during spring. Seston decomposition increased concentrations of potentially bioavailable polyphosphate and phosphodiesters, but also promoted the formation of refractory phosphorus forms that might be buried permanently in the sediment. Combining these results with in situ measurements of phosphorus concentrations in lake water and sediment traps revealed that the release from settling seston plays only a minor role in the accumulation of phosphorus in the hypolimnion of Lake Nordborg.

Seasonal Dynamics of CO2 Flux Across the Surface of Shallow Temperate Lakes

We used data collected from 1989 to 2009 from 151 shallow (mean depth < 3 m) temperate lakes in Denmark to explore the influence of lake trophic status, surface area and catchment size on the seasonal dynamics of the air–water flux of CO2. Monthly CO2 fluxes were derived from measurements of acid neutralizing capacity (ANC), pH, ionic strength, temperature, and wind speed. CO2 fluxes exhibited large seasonal variability, in particular in oligo-mesotrophic lakes. Most of the lakes emitted CO2 during winter (median rates ranging 300–1,900 mg C m−2 day−1), and less CO2 during summer or, in the case of some of the highly eutrophic lakes, retained CO2 during summer. We found that seasonal CO2 fluxes were strongly negatively correlated with pH (r = −0.65, P < 0.01), which in turn was correlated with chlorophyll a concentrations (r = 0.48, P < 0.01). Our analysis suggests that lake trophic status (a proxy for pelagic production) interacts with the lake ANC to drive the seasonal dynamics of CO2 fluxes, largely by changing pH and thereby the equilibrium of the free CO2 and bicarbonate relation. Long-term observations from four lakes, which have all undergone a period of oligotrophication during the past two decades, provide further evidence that CO2 efflux generally increases as trophic status decreases, as a consequence of decreased pH. Across these four lakes, the annual average CO2 emission has increased by 32% during the past two decades, thus, demonstrating the strong link between lake trophic status and CO2 flux.