We used data on nutrients, chlorophyll
a
(Chla) and submerged macrophyte cover from up to 817 Danish lakes to elucidate seasonal variations in nitrogen (N) and phosphorus (P) concentrations and to ...study the impact of N or its role in combination with P. In both deep and shallow lakes, we found marked seasonality in the ratio between total N and total P (TN:TP) and in the inorganic concentrations of nitrogen (DIN), indicating that N more easily becomes a limiting nutrient as summer proceeds. TN:TP reached its lowest values of <7 (by mass) in August in 25% of the shallow lakes. Chla generally related more strongly to TP than to TN, but at high TP concentrations TN explained more of the variability in Chla than TP. Macrophyte cover tended to decrease at increasing TN when TP was between 0.1 and 0.4 mg/l. At macrophyte cover above 20%, Chla was considerably lower compared with lakes with low macrophyte cover. We conclude that P is of key importance for the ecological quality of Danish lakes but that increased N concentrations, particularly in shallow lakes with moderate to high TP, may have significantly adverse effects on lake water quality and ecological status in summer.
Fresh waters make a disproportionately large contribution to greenhouse gas (GHG) emissions, with shallow lakes being particular hot spots. Given their global prevalence, how GHG fluxes from shallow ...lakes are altered by climate change may have profound implications for the global carbon cycle. Empirical evidence for the temperature dependence of the processes controlling GHG production in natural systems is largely based on the correlation between seasonal temperature variation and seasonal change in GHG fluxes. However, ecosystem‐level GHG fluxes could be influenced by factors, which while varying seasonally with temperature are actually either indirectly related (e.g. primary producer biomass) or largely unrelated to temperature, for instance nutrient loading. Here, we present results from the longest running shallow‐lake mesocosm experiment which demonstrate that nutrient concentrations override temperature as a control of both the total and individual GHG flux. Furthermore, testing for temperature treatment effects at low and high nutrient levels separately showed only one, rather weak, positive effect of temperature (CH₄ flux at high nutrients). In contrast, at low nutrients, the CO₂ efflux was lower in the elevated temperature treatments, with no significant effect on CH₄ or N₂O fluxes. Further analysis identified possible indirect effects of temperature treatment. For example, at low nutrient levels, increased macrophyte abundance was associated with significantly reduced fluxes of both CH₄ and CO₂ for both total annual flux and monthly observation data. As macrophyte abundance was positively related to temperature treatment, this suggests the possibility of indirect temperature effects, via macrophyte abundance, on CH₄ and CO₂ flux. These findings indicate that fluxes of GHGs from shallow lakes may be controlled more by factors indirectly related to temperature, in this case nutrient concentration and the abundance of primary producers. Thus, at ecosystem scale, response to climate change may not follow predictions based on the temperature dependence of metabolic processes.
1. Eutrophication constitutes a serious threat to many European lakes and many approaches have been used during the past 20-30 years to improve lake water quality. Results from the various lake ...restoration initiatives are diverse and the long-term effects are not well described. 2. In this study we evaluated data from more than 70 restoration projects conducted mainly in shallow, eutrophic lakes in Denmark and the Netherlands. Special focus was given to the removal of zooplanktivorous and benthivorous fish, by far the most common internal lake measure. 3. In more than half of the biomanipulation projects, Secchi depth increased and chlorophyll a decreased to less than 50% within the first few years. In some of the shallow lakes, total phosphorus and total nitrogen levels decreased considerably, indicating an increased retention or loss by denitrification. The strongest effects seemed to be obtained 4-6 years after the start of fish removal. 4. The long-term effect of restoration initiatives can only be described for a few lakes, but data from biomanipulated lakes indicate a return to a turbid state within 10 years or less in most cases. One of reasons for the lack of long-term effects may be internal phosphorus loading from a mobile pool accumulated in the sediment. 5. Synthesis and applications. Lake restoration, and in particular fish removal in shallow eutrophic lakes, has been widely used in Denmark and the Netherlands, where it has had marked effects on lake water quality in many lakes. Long-term effects (> 8-10 years) are less obvious and a return to turbid conditions is often seen unless fish removal is repeated. Insufficient external loading reduction, internal phosphorus loading and absence of stable submerged macrophyte communities to stabilize the clear-water state are the most probable causes for this relapse to earlier conditions.
Since its inception, the theory of alternative equilibria in shallow lakes has evolved and been applied to an ever wider range of ecological and socioecological systems. The theory posits the ...existence of two alternative stable states or equilibria, which in shallow lakes are characterised by either clear water with abundant plants or turbid water where phytoplankton dominate. Here, we used data simulations and real-world data sets from Denmark and north-eastern USA (902 lakes in total) to examine the relationship between shallow lake phytoplankton biomass (chlorophyll-a) and nutrient concentrations across a range of timescales. The data simulations demonstrated that three diagnostic tests could reliably identify the presence or absence of alternative equilibria. The real-world data accorded with data simulations where alternative equilibria were absent. Crucially, it was only as the temporal scale of observation increased (>3 years) that a predictable linear relationship between nutrient concentration and chlorophyll-a was evident. Thus, when a longer term perspective is taken, the notion of alternative equilibria is not required to explain the response of chlorophyll-a to nutrient enrichment which questions the utility of the theory for explaining shallow lake response to, and recovery from, eutrophication.
1. We analysed submerged macrophyte communities from 300 Danish lakes to determine the efficacy of different species, maximum colonisation depth (Cmax) of plants as well as coverage and plant volume ...inhabited (PVI) as indicators of eutrophication. 2. Most species occurred at a wide range of phosphorus and chlorophyll a (Chla) concentrations, but some species of isoetids (Lobelia, Isoëtes) and Potamogeton (Potamogeton gramineus, Potamogeton alpinus and Potamogeton filiformis) were mainly found at low nutrient concentrations and hence may be considered as indicators of nutrient poor conditions. However, species typically found in nutrient-rich conditions, such as Elodea canadensis and Potamogeton pectinatus, were also found at total phosphorus (TP) <0.02 mg P L⁻¹ and Chla <5 μg L⁻¹ and therefore cannot be considered as reliable indicators of eutrophic conditions. 3. Submerged macrophyte coverage, PVI and the Cmax were negatively correlated with TP and Chla. However, variability among lakes was high and no clear thresholds were observed. At TP between 0.03 and 0.07 mg P L⁻¹ plant coverage in shallow lakes ranged from nearly 0 to 100%, whilst at concentrations between 0.10 and 0.20 mg P L⁻¹ only 29% of the lakes had coverage >10%. Cmax was found to be a useful indicator only in deep lakes with unvegetated areas in the deeper part, whereas the use of coverage was restricted to shallow lakes or shallow areas of deep lakes. 4. Overall, submerged macrophytes responded clearly to eutrophication, but the metrics investigated here showed no well-defined thresholds. We developed a simple index based on species richness, presence of indicator species, coverage and Cmax, which might be used to track major changes in macrophyte communities and for lake classification.
With the implementation of the EU Water Framework Directive (WFD), the member states have to classify the ecological status of surface waters following standardised procedures. It was a matter of ...some surprise to lake ecologists that zooplankton were not included as a biological quality element (BQE) despite their being considered to be an important and integrated component of the pelagic food web. To the best of our knowledge, the decision of omitting zooplankton is not wise, and it has resulted in the withdrawal of zooplankton from many so-far-solid monitoring programmes. Using examples from particularly Danish, Estonian, and the UK lakes, we show that zooplankton (sampled from the water and the sediment) have a strong indicator value, which cannot be covered by sampling fish and phytoplankton without a very comprehensive and costly effort. When selecting the right metrics, zooplankton are cost-efficient indicators of the trophic state and ecological quality of lakes. Moreover, they are important indicators of the success/failure of measures taken to bring the lakes to at least good ecological status. Therefore, we strongly recommend the EU to include zooplankton as a central BQE in the WFD assessments, and undertake similar regional calibration exercises to obtain relevant and robust metrics also for zooplankton as is being done at present in the cases of fish, phytoplankton, macrophytes and benthic invertebrates.
Climate change may have profound effects on phosphorus (P) transport in streams and on lake eutrophication. Phosphorus loading from land to streams is expected to increase in northern temperate ...coastal regions due to higher winter rainfall and to a decline in warm temperate and arid climates. Model results suggest a 3.3 to 16.5% increase within the next 100 yr in the P loading of Danish streams depending on soil type and region. In lakes, higher eutrophication can be expected, reinforced by temperature-mediated higher P release from the sediment. Furthermore, a shift in fish community structure toward small and abundant plankti-benthivorous fish enhances predator control of zooplankton, resulting in higher phytoplankton biomass. Data from Danish lakes indicate increased chlorophyll a and phytoplankton biomass, higher dominance of dinophytes and cyanobacteria (most notably of nitrogen fixing forms), but lower abundance of diatoms and chrysophytes, reduced size of copepods and cladocerans, and a tendency to reduced zooplankton biomass and zooplankton:phytoplankton biomass ratio when lakes warm. Higher P concentrations are also seen in warm arid lakes despite reduced external loading due to increased evapotranspiration and reduced inflow. Therefore, the critical loading for good ecological state in lakes has to be lowered in a future warmer climate. This calls for adaptation measures, which in the northern temperate zone should include improved P cycling in agriculture, reduced loading from point sources, and (re)-establishment of wetlands and riparian buffer zones. In the arid Southern Europe, restrictions on human use of water are also needed, not least on irrigation.
•Submerged macrophytes exhibited phosphorus stoichiometric homeostasis (HP).•HP value dictated submerged macrophyte community stability.•Eutrophication altered interspecific interactions among ...submerged macrophytes.•Decline of high-HP species reduced community stability.
According to stoichiometric homeostasis theory, eutrophication is expected to increase the dominance of submerged macrophytes with low homeostatic regulation coefficients (H) relative to those with high H values, ultimately reducing macrophyte community stability. However, empirical evidence supporting this hypothesis is limited. In this study, we conducted a three-year tracking survey (seven sampling events) at 81 locations across three regions of Erhai Lake. We assessed the H values of submerged macrophyte species, revealing significant H values for phosphorus (P) and strong associations of HP values (range: 1.58–2.94) with species and community stability. Moreover, in plots simultaneously containing the dominant high-HP species, Potamogeton maackianus, and its low-HP counterpart, Ceratophyllum demersum, we explored the relationships among eutrophication, interspecific interaction shifts, and community dynamics. As the environmental P concentration increased, the dominance of P. maackianus decreased, while that of C. demersum increased. This shift coincided with reductions in community HP and stability. Our study underpins the effectiveness of H values for forecasting interspecific interactions among submerged macrophytes, thereby clarifying how eutrophication contributes to the decline in stability of the submerged macrophyte community.
Display omitted
Mitigating nutrient losses from anthropogenic nonpoint sources is today of particular importance for improving the water quality of numerous freshwater lakes worldwide. Several empirical ...relationships between land use and in-lake water quality variables have been developed, but they are often weak, which can in part be attributed to lack of detailed information about land use activities or point sources. We examined a comprehensive data set comprising land use data, point-source information, and in-lake water quality for 414 Danish lakes. By excluding point-source-influenced lakes (
n
= 210), the strength in relationship (
R
2
) between in-lake total nitrogen (TN) and total phosphorus (TP) concentrations and the proportion of agricultural land use in the watershed increased markedly, from 10-12% to 39-42% for deep lakes and from 10-12% to 21-23% for shallow lakes, with the highest increase for TN. Relationships between TP and agricultural land use were even stronger for lakes with rivers in their watershed (55%) compared to lakes without (28%), indicating that rivers mediate a stronger linkage between landscape activity and lake water quality by providing a "delivery" mechanism for excess nutrients in the watershed. When examining the effect of different near-freshwater land zones in contrast to the entire watershed, relationships generally improved with size of zone (25, 50, 100, 200, and 400 m from the edge of lake and streams) but were by far strongest using the entire watershed. The proportion of agricultural land use in the entire watershed was best in explaining lake water quality, both relative to estimated nutrient surplus at agricultural field level and near-lake land use, which somewhat contrasts typical strategies of management policies that mainly target agricultural nutrient applications and implementation of near-water buffer zones. This study suggests that transport mechanisms within the whole catchment are important for the nutrient export to lakes. Hence, the whole watershed should be considered when managing nutrient loadings to lakes, and future policies should ideally target measures that reduce the proportion of cultivated land in the watershed to successfully improve lake water quality.
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.
Display omitted
•35 Danish gravel pit lakes with an area between 0.2 and 13ha were studied.•The lakes were nutrient poor and with clear water compared with natural lakes.•Submerged macrophytes were abundant and fish present in most gravel pit lakes.•Gravel pit lake may be a way to increase biodiversity and high quality lakes.
PDF
