Mercury content in lake sediments and suspended matter — temporal variation and relation to water chemistry

Twenty-five small, mainly oligotrophic, forest Swedish lakes were treated in different ways (e.g. liming) during 1987 in order to reduce the Hg levels in fish. The temporal variation of Hg concentration in sediments and water, and the extent that this variation can be related to normal lake characteristics and/or altered chemical conditions due to lake remedial measures was studied. Yearly mean alkalinity was increased from 0.07 to 0.18 meq l^–1 and pH from 6.0 to 6.5 due to the treatments. The Hg concentration, as well as the Hg:C ratio, in seston was reduced after treatment; the median Hg/C ratio in seston decreased from 1.6 g g^–1 during 1986 to 0.8 g g^–1 during 1988. In the water column the concentration of the reactive plus non-reactive Hg fraction (RIHg, detected after reduction with NaBH₄) showed a similar inter-year variation to the Hg content in seston, in contrast to the total Hg concentration in water column which was not decreased after the treatments. The increase in pH due to the remedial measures appeared to be the main cause of the decreased RIHg concentration, although the humic content (measured as colour) also explained a large part of the variation in RIHg concentration between the lakes. The decrease in the RIHg concentration in the lake water is the probable reason for the reduced Hg levels observed in the seston.     

Seasonal patterns of mercury species in water and plankton from softwater lakes in Northeastern Minnesota

Twelve softwater lakes in NE Minnesota were sampled in spring, summer, and fall of 1992 and 1993 for labile (unextracted) methyl-Hg, total (extracted) methyl-Hg, and total Hg in lake water and net plankton (300 µm). The lakes are small (5.6–56 ha), low productivity, headwater drainage or seepage lakes. They are acid-sensitive (ANC 200 µeq/L) but not low pH lakes (average pH 6.6). The lakes ranged in color from 8.5 to 70 PCU. Statistical analysis of the water chemistry variables and mercury species support the conclusion that these were a homogeneous set of lakes; therefore, seasonality of mercury forms was analyzed on combined (mean) data from the 12 lakes. Methyl-Hg in water declined throughout the growing season. Hg_T also declined sharply from spring to summer but increased again in the fall. In contrast to the methyl-Hg and Hg in water, concentrations in plankton were at the lowest levels in spring and rose to higher levels in summer. The mass of mercury in plankton increased from spring to fall, as did the methyl-Hg fraction, which increased from 20% of Hg_T in spring to 52% in autumn. Bioaccumulation factors (BAF) for methyl-Hg in net plankton increased over the growing season. Overall, log BAF for Hg_T in net plankton (wet wt.) was 4.45. Log BAF for methyl-Hg in plankton was 4.90 to 5.43 depending on the analytical form of methyl-Hg in water (labile or total). Seasonal patterns of methyl-Hg and Hg_T did not covary in water, but did covary in plankton. These results support the conclusion that measurement of Hg in water is not adequate in itself to determine the amount of bioavailable Hg (i.e., methyl-Hg) in a lake. Labile (unextracted) methyl-Hg could be a useful measurement of bioavailable Hg. Labile methyl-Hg exhibits the same seasonal patterns as total methyl-Hg, but does not require the extraction steps necessary for measuring total methyl-Hg.     

Factors influencing changes in mercury concentrations in lake water and yellow perch (<Emphasis Type="Italic">Perca flavescens</Emphasis>) in Adirondack lakes

Over the past 20+ years, fish with elevated concentrations of mercury (Hg) have been observed in remote lake districts, including the Adirondack region of New York. Across eastern North America studies have also reported a negative correlation between fish Hg concentration and lake pH. Recent controls in emissions of sulfur dioxide (SO₂) have resulted in some improvement in the acid–base status of acid-impacted surface waters including Adirondack lakes. In addition, there has been an apparent decrease in atmospheric Hg deposition. A synoptic survey of 25 lakes in the Adirondacks was conducted in 1992–1993 to analyze spatial patterns of Hg in the water column and yellow perch (Perca flavescens). The same cluster of 25 lakes was resurveyed in 2005–2006 to evaluate if changes in lake concentrations of Hg species or fish Hg have occurred. We observed a varied response of changes in water chemistry and fish Hg concentrations. In twelve of the resurveyed lakes the yellow perch had lower Hg concentrations, six lakes had yellow perch with higher Hg concentrations, and in seven lakes yellow perch Hg concentrations did not change significantly (α = 0.05). Four variables appear to influence the change in yellow perch Hg concentrations in the Adirondacks: watershed area, elevation, change in pH, and change in fish body condition. We hypothesize that as the acidity in lakes is attenuated, the lakes may become more productive and/or water quality conditions less stressful to fish leading to increasing fish body condition. As fish body condition improves, fish exhibit “growth dilution” of tissue contaminants leading to lower fish Hg concentrations.     

Food quality for Daphnia in humic and clear water lakes

1. Growth and reproduction of Daphnia fed lake seston were measured in two categories of meso‐ to eutrophic lakes differing with respect to terrestrial organic matter influence (humic and clear water lakes). The content of highly unsaturated fatty acids (HUFA), P and N, as well as the taxonomical composition of seston were analysed. 2. Seston HUFA and C : P ratios were similar between lake categories, whereas C : N ratios were lower in the clear water lakes in both spring and summer. Despite the similarity in HUFA and P content of seston, Daphnia growth rate, clutch size and the proportion of gravid females were, respectively, about 1.5, 3 and 6 times higher in the clear water lakes. 3. Differences in growth and reproduction were related to a combination of higher N content and good fatty acid quality of the seston in the clear water lakes. Relatively high biomass of edible algae, such as Rhodomonas sp. and Cryptomonas sp., in the clear water lakes, and differences in water pH likely contributed to the observed differences in Daphnia growth and reproduction between lake categories. Additionally, it is possible that Daphnia was energy limited in the humic lakes despite high particulate organic carbon (POC) concentrations, as the contribution of non‐algal and detrital C to the POC pool was high. 4. Our results suggest that dietary HUFA content has the potential to improve herbivore growth and reproduction if N and P are not limiting. N merits more attention in studies of zooplankton nutrition.     

Chernobyl-derived radiocaesium in fish as dependent on water quality and lake morphometry

¹³⁷Cs concentrations in perch Perca fluviatilis , pike Esox lucius and roach Rutilus rutilus obtained from lakes of different size and water quality in an area which received about 10–67 kBq m⁻²¹³⁷Cs, were compared with environmental data. Radiocaesium concentrations were highest in pike, and were about two to three times higher in the pike and perch than in the roach. The largest perch had about four times more ¹³⁷Cs than the smallest ones, but the activities in the pike and roach were independent of fish size. All of the water quality parameters examined correlated with ¹³⁷Cs concentrations in the fish, but the concentrations of ¹³⁷Cs in the bottom sediment did not. ^l37Cs fallout and water retention time in lakes showed a positive correlation with fish ¹³⁷Cs, while the size of the catchment area, phosphorus content of the water, water colour, pH and electrical conductivity showed a negative one. Thus there was more ¹³⁷Cs in the fish from the oligotrophic lakes than from the eutrophic ones, from the less humic lakes than from the more humic ones, from the more acid than from the less acid ones and from the lakes containing less potassium than from those containing more potassium. The depth, area and volume of the lake did not affect radiocaesium in the fish. Regression equations are presented for predicting ¹³⁷Cs in fish on the basis of environmental data. Fallout, total phosphorus and pH together had the highest influence in multiple regression models but were less significant in the roach than in the perch and pike.     

Regularities in Primary Production,Secchi Depth and Fish Yield and a New System to Define Trophic and Humic State Indices for Lake Ecosystems

General relationships between phytoplankton production, chlorophyll, total, dissolved and particulate phosphorus, Secchi depth, humic level, trophic level, fish production and latitude are described by regression equations using an extensive “Soviet” data base covering a wide domain of lake characteristics and a European data base. New systems for defining lake trophic and humic status are presented. The results may be used for more precise estimates of fundamental lake properties and for many practical issues of lake management, e.g., predictions of fish catch. We have used strict chlorophyll‐a concentrations for every trophic class and we have omitted Secchi depth from the trophic classes, since Secchi depth and other variables strongly related to water clarity (like suspended particulate matter and particulate organic carbon) depend on autochthonous production, allochthonous influences and resuspension. We have used the Secchi depth as a simple operational measure of the effective depth of the photic zone. It has also been shown that among these lakes there exist a very strong relationship between primary production and latitude. In fact, 74% of the variability among the lakes in mean summer primary production can be statistically related to variations in latitude. These data also show a strong relationship between primary production and fish yield, which can be used to address many fundamental issues in lake management, like “normal and abnormal fish production”.     

Visual conditions and habitat shape the coloration of the Eurasian perch (Perca fluviatilis L.): a trade‐off between camouflage and communication?

In theory, selection for effective camouflage (i.e. dull coloration) in fish should be strongest when the conditions for visual predation are most favourable, such as in structurally simple pelagic habitats. By contrast, in more sheltered (e.g. littoral) habitats, selection may favour effective intra‐specific communication (i.e. bright coloration) (at the expense of crypsis). Poor transparency, as in highly humic waters, should constrain colour adaptations. We investigated phenotypic variation in body coloration of Eurasian perch (Perca fluviatilis L.) in littoral and pelagic habitats of four humic boreal lakes. Perch from the most transparent lake had the lightest and less coloured belly and perch were more colourful in the littoral habitats than in the pelagic areas, with the pattern being clearest in the most transparent lake. In addition, perch in the most transparent lake exhibited sexual dichromatism, with males having a more colourful belly than the females, whereas no indications of sexual dichromatism were found in more humic lakes. Moreover, in the most transparent lake, the condition of fish correlated with bright belly coloration in the littoral, but with dull belly coloration in the pelagic habitat. The results obtained in the present study suggest that selection on perch coloration may differ between lakes as a result of visual properties of the water, and within lakes as a result of divergent selection for camouflage and communication in pelagic and littoral habitats. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 47–59.     

Relationships between morphometry,geographic location and water quality parameters of European lakes

I addressed the question how lake and catchment morphometry influences water chemistry and water quality over a large scale of European lakes, and developed the regression equations between most closely related morphometric and water quality indices. I analysed the data of 1,337 lakes included in the European Environment Agency (EEA) database, carrying out separate analyses for three basic lake types: large lakes (area ≥100 km², 138 lakes), shallow lakes (mean depth ≤3 m, 153 lakes) and large and shallow lakes (area ≥100 km² and mean depth ≤8 m, 35 lakes). The study revealed that in Europe, the lakes towards North are larger but shallower and have smaller catchment areas than the southern lakes; lakes at higher altitudes are deeper and smaller and have smaller catchment areas than the lowland lakes. Larger lakes have generally larger catchment areas and bigger volumes, and they are deeper than smaller lakes, but the relative depth decreases with increasing surface area. The lakes at higher latitudes have lower alkalinity, pH and conductivity, and also lower concentrations of nitrogen and phosphorus while the concentration of organic matter is higher. In the lakes at higher altitudes, the concentration of organic matter and nutrient contents are lower and water is more transparent than in lowland lakes. In larger lakes with larger catchment area, the alkalinity, pH, conductivity and the concentrations of nutrients and organic matter are generally higher than in smaller lakes with smaller catchments. If the lake is deep and/or its residence time is long, the water is more transparent and the concentrations of chlorophyll a, organic matter and nutrients are lower than in shallower lakes with shorter residence times. The larger the catchment area is with respect to lake depth, area and volume, the lower is the water transparency and the higher are the concentrations of the nutrients, organic matter and chlorophyll as well as pH, alkalinity and conductivity. The links between lake water quality and morphometry become stronger towards large and shallow lakes. Along the decreasing gradients of latitude, altitude and relative depth, the present phosphorus concentration and its deviation from the reference concentration increases.     

Scaling of Pelagic Metabolism to Size,Trophy and Forest Cover in Small Danish Lakes

ABSTRACT We tested whether pelagic light and nutrient availability, metabolism, organic pools and CO₂-supersaturation were related to lake size and surrounding forest cover in late summer–autumn measurements among 64 small (0.02–20 ha), shallow seepage lakes located in nutrient-rich, calcareous moraine soils in North Zealand, Denmark. We found a strong implicit scaling to lake size as light availability increased significantly with lake size while nutrient availability, phytoplankton biomass and dissolved organic matter declined. Forest lakes had significantly stronger net heterotrophic traits than open lakes as higher values were observed for light attenuation above and in the water, dissolved organic matter, pelagic community respiration (R) relative to maximum gross primary production (R/GPP) and CO₂-supersaturation. Total-phosphorus was the main predictor of phytoplankton biomass (Chl) despite a much weaker relationship than observed in previous studies of larger lakes. Maximum gross primary production increased with algal biomass and decreased with dissolved organic matter, whereas community respiration increased with dissolved organic matter and particularly with gross primary production. These results suggest that exogenous organic matter supplements primary production as an energy source to heterotrophs in these small lakes, and particularly so in forest lakes experiencing substantial shading from the forest and dissolved humic material. This suggestion is supported by 20–30-fold CO₂ supersaturation in the surface water of the smallest forest lakes and more than sixfold supersaturation in 75% of all measurements making these lakes among the most supersaturated temperate lakes examined so far.     

Seasonal fluctuations in macrophyte cover and water transparency of four brown-water lakes: implications for crustacean zooplankton in littoral and pelagic habitats

The dynamics of crustacean zooplankton in the littoral and pelagic zones of four forest lakes having variable water qualities (colour range 130–340 mg Pt l⁻¹, Secchi depth 70–160 cm) were studied. The biomass of zooplankton was higher in the littoral zone than in the pelagic zone only in the lake having the highest transparency. In the three other lakes, biomass was significantly higher in the pelagic zone than in the littoral zone. In the two lakes with highest transparency, the littoral biomass of cladocerans significantly followed the development of macrophyte vegetation, and cladoceran biomass reached the maximum value at the time of highest macrophyte coverage. In lakes with lowest transparency, littoral zooplankton biomass developed independently of macrophyte density and decreased when macrophyte beds were densest. The seasonal development of the littoral copepod biomass did not follow the development of macrophytes in any of the lakes. The mean size of cladocerans in the pelagic zone decreased with increasing Secchi depth of the lake, whereas in the littoral zone no such phenomenon was detected. Seasonally, when water transparency increased temporarily in two of the lakes, the mean size of cladocerans in the pelagic zone decreased steeply. For copepods, no relationship between water transparency and body size was observed. The results suggested that in humic lakes the importance of the littoral zone as a refuge decreases with decreasing transparency of the water and that low water transparency protects cladocerans from fish predation. All the observed between-lake differences could not be explained by fish predation, but were probably attributed to the presence of chaoborid larvae with variable densities. Feeding efficiency of chaoborids is not affected by visibility and thus they can obscure the relationship between water quality, fish density, and the structure of crustacean zooplankton assemblages. Handling editor: S. I. Dodson     

Availability of dissolved organic carbon for planktonic bacteria in oligotrophic lakes of differing humic content

Bacterioplankton from 10 oligotrophic lakes, representing a gradient from clearwater to polyhumic, were grown in dilution cultures of sterile filtered lake water. The bacterial biomass achieved in the stationary phase of the dilution cultures was positively correlated with the amount of both humic matter and dissolved organic carbon (DOC) in the lakes. About the same fraction of the total DOC pool was consumed in the dilution cultures of all lakes (average 9.5%, coefficient of variation (CV) 24%), with approximately the same growth efficiency (average 26%, CV 28%). Thus, humic lakes could support a higher bacterial biomass than clearwater lakes due to their larger DOC pools. The relevance of the results to planktonic food webs of humic and clearwater lakes is discussed.     

Dissolved organic carbon and fluorescence in Lake Hovsgol: factors reducing humic content of the lake water

 Lake Hovsgol is a large tectonic lake located in northern Mongolia, which has extremely transparent lake water. In our survey, the dissolved organic carbon of the lake water was 80–100 μM-C, and the fluorescence intensity in an excitation and emission matrix was very low. The brown color and high content of humic substances in river water flowing from a watershed consisting of grassland and forests rapidly declined in the coastal area of the lake. The decrease in humic content may be due not only to dilution by the lake water but also to flocculation and photobleaching. Among tectonic lakes in Asia, Lake Hovsgol would appear to have unique biological and hydrological features that reduce humic content and help to maintain water transparency. Received: June 25, 2002 / Accepted: January 10, 2003 Acknowledgments The authors acknowledge helpful discussion with Dr. J. Urabe. We thank Dr. T. Galbaatar, Mongolian Academy of Science, Mongolia, for his arrangements on the expeditions in 1999. We are also indebted to Mr. D. Hadbaatar, B. Ganbat, and the cruise staff of the R/V Suchbaatar for their assistance in the course of the study. This study was supported by Grant-in-Aid No. 09041159 and 13575034 for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Correspondence to:K. Hayakawa     

The growth of young pike in small Finnish lakes with different acidity-related water properties and fish species composition

Pike growth was generally slower in small Finnish lakes than has been reported from large lakes or brackish coastal waters. Growth was unrelated to pH, alkalinity, COD, or conductivity of the water, but length of 1–2-year-old pike correlated positively with water colour (i.e. humic content), nitrogen content, and phosphorus content. These water properties also correlated with each other. Growth of 1–2-year-old pike was negatively correlated with lake area in lakes smaller than 20 ha.
In lakes with a strong roach population or without ruffe the 0 group pike grew faster than in lakes without roach or with ruffe. The growth of pike aged 1–4 years in two very acid and practically fishless lakes did not differ clearly from the rest of the lakes, although the pike were forced to forage for abundant acid resistant invertebrates like dragonfly nymphs (Odonata) or Asellus aquatlcus .
The results of this work support earlier studies where the growth rate above all depends on water temperature and food availability. Acidity was not shown to affect the growth rate of pike.     

How light and nutrients affect the relationship between autotrophic and heterotrophic biomass in a tropical black water periphyton community

This study examines how nutrients and light affect the relationship between autotrophic biomass and non-autotrophic periphyton organic matter in a tropical black water lake biofilm community. We hypothesized that there is no positive correlation between autotrophic and non-autotrophic organic matter in the periphytic community of a black water humic lake, where non-algal components of periphyton can rely on carbon sources external to the periphyton matrix and where nutrient availability is low. Second, we sought to test our hypothesis that non-autotrophic periphyton organic matter will benefit from nutrient enhancement in a lake where the availability of DOC is high. We performed a field experiment using in situ lake mesocosms to manipulate nutrient concentrations and light availability in a 2 × 2 factorial design. Control treatments (no nutrient added) and nutrient treatments (N + P) were compared in different light conditions: high light (near surface water) and low light (near bottom). No positive correlation was found between autotrophic biomass and non-autotrophic periphyton organic matter, but a negative correlation was observed in high nutrient and light conditions. The low C:P and N:P ratios revealed that the non-autotrophic organic matter mostly comprised a heterotrophic microbial biofilm. High levels of light and nutrients together caused significant changes in periphyton community properties. The non-autotrophic periphyton organic matter was negatively affected by nutrient addition, whereas autotrophic biomass was positively affected, especially in high light conditions. Our results strongly suggest that non-autotrophic periphyton organic matter in a humic lake is primarily comprised of a bacterial biofilm that directly competes for nutrients with autotrophs in the periphytic community. We also observed no effect of nutrient addition on periphyton growing in light-limited conditions. These results suggest that heterotrophic periphytic organisms might experience carbon limitation despite the high availability, but usually low quality, of dissolved carbon in the water column of humic lakes.     

Lignin biomarkers as tracers of mercury sources in lakes water column

This study presents the role of specific terrigenous organic compounds as important vectors of mercury (Hg) transported from watersheds to lakes of the Canadian boreal forest. In order to differentiate the autochthonous from the allochthonous organic matter (OM), lignin derived biomarker signatures [Lambda, S/V, C/V, P/(V + S), 3,5-Bd/V and (Ad/Al)v] were used. Since lignin is exclusively produced by terrigenous plants, this approach can give a non equivocal picture of the watershed inputs to the lakes. Moreover, it allows a characterization of the source of OM and its state of degradation. The water column of six lakes from the Canadian Shield was sampled monthly between June and September 2005. Lake total dissolved Hg concentrations and Lambda were positively correlated, meaning that Hg and ligneous inputs are linked (dissolved OM r ² = 0.62, p < 0.0001; particulate OM r ² = 0.76, p < 0.0001). Ratios of P/(V + S) and 3,5-Bd/V from both dissolved OM and particulate OM of the water column suggest an inverse relationship between the progressive state of pedogenesis and maturation of the OM in soil before entering the lake, and the Hg concentrations in the water column. No relation was found between Hg levels in the lakes and the watershed flora composition—angiosperm versus gymnosperm or woody versus non-woody compounds. This study has significant implications for watershed management of ecosystems since limiting fresh terrestrial OM inputs should reduce Hg inputs to the aquatic systems. This is particularly the case for large-scale land-use impacts, such as deforestation, agriculture and urbanization, associated to large quantities of soil OM being transferred to aquatic systems.     

Phosphorus transformations in the epilimnion of humic lakes: biological uptake of phosphate

SUMMARY. 1. The hypothesis that dissolved humic material (DHM) stimulates bacterial involvement in phosphorus transformations and may thus lead to decreased accessibility of phosphorus to algae was investigated by studying three small forest lakes in southern Finland representing a wide range of concentrations of DHM. 2. Other chemical differences between the three lakes were slight, although the most humic lake exhibited higher concentrations of total phosphorus and of molybdate-reactive phosphorus. Bacterial biomass did not differ significantly between the lakes, but algal biomass was significantly lower at higher DHM concentrations. Consequently the ratio of algal biomass to bacterial biomass was significantly lower in the most humic lake. 3. Uptake of phosphorus from added ³³PO₄ was partitioned between algal and bacterial size fractions by differential filtration. No significant variation between lakes was found in the proportion of particulate ³³P recovered from the algal fraction. 4. Turnover times for phosphate were significantly longer in the most humic lake and also showed lower variability. In general turnover times were long in comparison with values reported from many other lakes. Only briefly in mid summmer did turnover times in two of the lakes shorten to values which would indicate that demand for phosphate was outstripping supply. 5. Short-term storage of samples from the most humic lake stimulated biological incorporation of ³³P, but additions of nitrogen and iron had little effect on phosphate uptake. 6. In these small forest lakes it is probable that no single nutrient consistently limits plankton development. Since no evidence was found that DHM shifts the balance of plankton phosphate uptake away from algae towards bacteria, the influence of DHM on phosphorus transformations may rather be through chemical regulation of free phosphate availability.     

Occurrence of Cryptophyceae and katablepharids in boreal lakes

One of the most important algal groups in Finnish lakes are the Cryptophyceae. Changes in the community structure of Cryptophyceae in a total of 22 lakes belonging to the Vuoksi river basin in eastern Finland were studied. The existence of lakes with water qualities varying from oligotrophic to eutrophic, often loaded by human activities, provides a good opportunity to study the effects of environmental variables on the occurrence and size variation of Cryptophyceae. In the Vuoksi river basin, the main soil type is moraine. Twelve of the lakes were large or moderately large and with clear, i.e. oligo-humic water, and one lake could be described as a small clear water lake. Eight large or moderately large lakes were humic, with a water colour number of 40–70 mg l^?1 Pt, including three lakes impacted by nutrient loads. One lake was naturally eutrophic, with a high water colour number of 100 mg l^?1 Pt, and was also impacted by municipal and pulping effluents. CCA-ordination analysis grouped the studied lakes into: (1) clear water lakes, (2) humic lakes and (3) the naturally eutrophic brown water lake. In the CCA-ordination analysis based on cell numbers small Cryptophyceae (Cryptomonadales), Rhodomonas lacustris and the katablepharid Katablepharis ovalis were grouped into the first axis, which was positively correlated with Secchi depth (r=0.58) and NO₃N - nitrogen (r=0.24) and negatively with Ptot (r=-0.69), PO₄P (r=-0.69) and water colour number (r=-0.66). In humic lakes, medium-sized Cryptophyceae were abundant. The naturally eutrophic lake was grouped into first axis, which is positively correlated with Ptot (r=0.69), PO₄P (r=0.69) and water colour number (r=0.66). The lake formed a distinct group with large Cryptophyceae. Only in this lake was the heterotrophic Katablepharis ovalis rather abundant. However, large-sized taxa dominated the biomass of the Cryptophyceae assemblage in all lake types excluding large clear water lakes, where Rhodomonas lacustris dominated and large Cryptophyceae co-dominated.     

Essential fatty acids and phosphorus in seston from lakes with contrasting terrestrial dissolved organic carbon content

1. It is often assumed that lakes highly influenced by terrestrial organic matter (TOM) have low zooplankton food quality because of elemental and/or biochemical deficiencies of the major particulate organic carbon pools. We used the biochemical [polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA) – 20:5ω3] and elemental (C : P ratio) composition of particulate matter (PM) as qualitative measures of potential zooplankton food in two categories of lakes of similar primary productivity, but with contrasting TOM influence (clear water versus humic lakes). 2. C : P ratios (atomic ratio) in PM were similar between lake categories and were above 400. The concentration (μg L⁻¹) and relative content (μg mg C⁻¹) of EPA, as well as the particulate organic carbon concentration, were higher in the humic lakes than in the clear‐water lakes. 3. Our results show high fatty acid quality of PM in the humic lakes. The differences in the biochemical quality of the potential zooplankton food between lake categories can be attributed to the differences in their phytoplankton communities. 4. High biochemical quality of the food can result in high efficiency of energy transfer in the food chain and stimulate production at higher trophic levels, assuming that zooplankton are able to ingest and digest the resource available.     

Acidification in southern Norway: seasonal variation of aluminium in lake waters

Total aluminium, pH and water colour were measured over two years in three lakes of different pH in an area undergoing anthropogenic acidification in southern Norway. Al content decreased in Ca-rich lakes near the coast, but also with increasing organic content of the lakes. Seasonal variation was pronounced in the most acidic lake. Al concentration seems virtually independent of pH, suggesting that it is in the form of organic complexes, at least in the two most coloured lakes. Precipitation processes affecting fish metabolism in the acidic lakes with high Al content may have caused the frequent lack of success in liming lakes in this area. Similarily, humic acidic lakes with organic complexed Al still carry considerable fish biomass.     

Relationship of trophic and chemical conditions to photobleaching of dissolved organic matter in lake ecosystems

Dissolved organic matter (DOM) is a major light-absorbing substance, responsible for much of the color in water bodies. When sunlight energy is absorbed by DOM, some color can be lost by the process of photobleaching. We measured rates of DOM photobleaching in thirty lakes that varied greatly in color, trophic status and ionic composition. Loss of color (measured as absorbance at 440 nm and expressed as absorption coefficients) was a first order function of sunlight dose, and rates were nearly identical for 0.2m- and GF/F-filtered samples suggesting that the process was predominantly abiotic. Photobleaching rates were rapid (color loss of 1–19% d^–1) and varied about seven-fold among lakes. Our method under-estimated the actual rate by 15–20% based on comparisons between the glass bottles we used in the survey and quartz containers. The large variation in photobleaching rates was examined in relation to lake trophy and chemical conditions. The best predictor of this variability was acid-neutralizing capacity (ANC) (r ²=0.94;p<0.001) such that photobleaching was most rapid in the most alkaline lakes. The relationship between ANC and photobleaching suggests that differences in ionic conditions among lakes may influence the solubility and configuration of humic and fulvic acids and hence their susceptibility to photobleaching.