Changing patterns of sediment accumulation in a small lake in Scania,southern Sweden

Bioassays with water of eutrophic Lake Tjeukemeer were carried out in the laboratory, and, in the lake itself, by placing plexiglass tubes (? 9 cm) firmly in the mud. In the laboratory nitrogen had a strong growth-promoting effect on the concentrations of chlorophyll a, N-cell and COD-cell. In the bioassays in situ nitrogen increased the concentration of chlorophyll a, but not those of cellular N and cellular COD. It is argued that chlorophyll a is not a good indicator for algal growth in bioassays and that the extrapolation of laboratory experiments — where nitrogen limited algal growth — to the field situation — where light limited algal growth — often leads to erroneous results, especially in shallow eutrophic lakes. The main reason is probably that in bioassays in the laboratory the nutrient supply — both from external loading and from sediments — is often different and that processes like denitrification occur in the lake, but not in the bioassays.     

Decomposition of Blue-Green Algal (Cyanobacterial) Blooms in Lake Mendota, Wisconsin

Decomposition of natural populations of Lake Mendota phytoplankton dominated by blue-green algae (cyanobacteria) was monitored by using oxygen uptake and disappearance of chlorophyll, algal volume (fluorescence microscopy), particulate protein, particulate organic carbon, and photosynthetic ability (¹⁴CO₂ up-take). In some experiments, decomposition of ¹⁴C-labeled axenic cultures of Anabaena sp. was also measured. In addition to decomposition, mineralization of inorganic nitrogen and phosphorus were followed in some experiments. Decomposition could be described as a first-order process, and the rate of decomposition was similar to that found by others using pure cultures of eucaryotic algae. Nitrogen and phosphorus never limited the decomposition process, even when the lake water was severely limited in soluble forms of these nutrients. This suggests that the bacteria responsible for decomposition can obtain all of their key nutrients for growth from the blue-green algal cells. Filtration of lake water through plankton netting that removed up to 90% of the algal biomass usually did not cause a similar decrease in oxygen demand, suggesting that most of the particulate organic matter used for respiration of the decomposing bacteria was in a small-particle fraction. Short-term oxygen demand correlated well with the particulate chlorophyll concentration of the sample, and a relationship was derived that could be used to predict community respiration of the lake from chlorophyll concentration. Kinetic analysis showed that not all analyzed components disappeared at the same rate during the decomposition process. The relative rates of decrease of the measured parameters were as follows: photosynthetic ability > algal volume > particulate chlorophyll > particulate protein. Decomposition of ¹⁴C-labeled Anabaena occurred at similar rates with aerobic epilimnetic water and with anaerobic sediment, but was considerably slower with anaerobic hypolimnetic water. Of the various genera present in the lake, Aphanizomenon and Anabaena were more sensitive to decomposition than was Microcystis. In addition to providing a general picture of the decomposition process, the present work relates to other work on sedimentation to provide a detailed picture of the fate of blue-green algal biomass in a eutrophic lake ecosystem.     

Factors that Control the Species Composition of Freshwater Phytoplankton,with Special Attention to Nutrient Concentrations

Variations of species composition and population size of planktonic algae were studied in relation to the different nutrient levels in a eutrophic pond and an oligotrophic lake. The results obtained were discussed in comparison with the changes in photosynthesis and chlorophyll of several algal populations cultured under different nutrient conditions. As based on unialgal culture experiments two types of algae (the Chlorella-type and the Melosira-type) could be distinguished with regard to variations of photosynthesis and chlorophyll in response to different nutrient levels. Distribution of the Chlorella-type algae may be confined to eutrophic waters, while the Melosira-type algae can be distributed widely in waters from oligotrophic to eutrophic.     

Phototrophic Picoplankton in Temperate Lakes: Seasonal Abundance and Importance along a Trophic Gradient

The seasonal development of autotrophic picoplankton was investigated in seven Danish lakes representing a eutrophication gradient. Highest cell abundance between 1.5 to 6 × 10⁵ cells ml⁻¹ were found in mid-summer. Minor peaks were observed in spring. In winter, densities were below 10³ ml⁻¹. The highest relative picoplankton contribution to total autotrophic biomass also occurred in mid-summer. In the eutrophic lakes and one humic lake the average seasonal contribution of picoplankton to total chlorophyll was below 1% increasing to 5-8% in the meso- and oligotrophic clear water lakes. During short periods the proportion of picoplankton did reach 25%. The higher relative importance of picoplankton in less productive lakes was not due to higher actual chlorophyll concentrations, but due to a much more pronounced response by larger algae at higher nutrient loading. Both cyanobacteria and eukaryote organisms were present as picoplankton. Only eukaryotes were found in one eutrophic lake and an acidic, humic lake. In the eutrophic lakes eukaryote picoplankton was dominant; both with respect to cell densities and biovolume, whereas cyanobacteria dominated the two meso-oligotrophic lakes. Autotrophic picoplankton were present in all lake types, however their importance seemed to be less in most eutrophic lakes than in less productive, meso-oligotrophic lakes.     

Uncoupling of chlorophyll and nutrients in lakes – possible reasons,expected consequences

Total phosphorus and total nitrogen explained a low percentage of summer chlorophyll variability in epilimnia of the Great Masurian Lakes. Division of the whole data set into two subgroups of lakes improved approximation of the chlorophyll nutrient relationship but revealed also functional differences between the lakes distinguished in that way. Chlorophyll in eutrophic lakes correlated well with nitrogen and phosphorus, that in mesotrophic lakes (those with summer chlorophyll <=22 mg m^–3 as calculated in the model) was related to none of the nutrients. Higher summer chlorophyll content in epilimnetic waters was accompanied by higher chl:PP and chl:PN ratios. Algal adaptation to poor light conditions in eutrophic lakes is postulated as a possible reason for that difference.Chlorophyll – nutrient relationships varied with the trophic status of lakes. Epilimnetic chlorophyll strictly followed phosphorus changes in eutrophic lakes but did not do so in mesotrophic ones. Detailed comparison of selected meso- and eutrophic lakes showed marked differences in the seasonal changes of chlorophyll and nutrient concentrations and in sedimentation rates, especially in spring. Nutrient limitation rather than zooplankton grazing is suggested as a possible mechanism of controlling algal abundance and the sequence of spring events in a eutrophic lake. It is hypothesised that phosphorus turnover in eutrophic lakes is dominated by seasonal vertical fluxes, while in mesotrophic lakes it is more conservative with consumption and regeneration restricted mostly to metalimnion. Possible consequences of such conclusion are discussed in the paper.     

Fish manipulation as a lake restoration tool in shallow,eutrophic temperate lakes 1: cross-analysis of three Danish case-studies

The use of fish manipulation as a tool for lake restoration in eutrophic lakes has been investigated since 1986 in three shallow, eutrophic Danish lakes. The lakes differ with respect to nutrient loading and nutrient levels (130–1000 μg P l⁻¹, 1–6 mg N l⁻¹). A 50% removal of planktivorous fish in the less eutrophic cyanobacteria-diatom dominated Lake V?ng caused marked changes in lower trophic levels, phosphorus concentration and transparency. Only minor changes occurred after a 78% removal of planktivorous fish in eutrophic cyanobacteria dominated Frederiksborg Castle Lake. In the hypertrophic, green algae dominated Lake S?byg?rd a low recruitment of all fish species and a 16% removal of fish biomass created substantial changes in trophic structure, but no decrease in phosphorus concentration. The different response pattern is interpreted as (1) a difference in density and persistence of bloomforming cyanobacteria caused by between-lake variations in nutrient levels and probably also mixing- and flushing rates, (2) a difference in specific loss rates through sedimentation of the algal community prevaling after the fish manipulation, (3) a decreased impact of planktivorous fish with increasing mean depth and (4) a lake specific difference in ability to create a self-increasing reduction in the phosphorus level in the lake water. This in turn seems related to the phosphorus loading.     

Escherichia coli Behavior in the Presence of Organic Matter Released by Algae Exposed to Water Treatment Chemicals

When exposed to oxidation, algae release dissolved organic matter with significant carbohydrate (52%) and biodegradable (55 to 74%) fractions. This study examined whether algal organic matter (AOM) added in drinking water can compromise water biological stability by supporting bacterial survival. Escherichia coli (1.3 × 10⁵ cells ml⁻¹) was inoculated in sterile dechlorinated tap water supplemented with various qualities of organic substrate, such as the organic matter coming from chlorinated algae, ozonated algae, and acetate (model molecule) to add 0.2 ± 0.1 mg of biodegradable dissolved organic carbon (BDOC) liter⁻¹. Despite equivalent levels of BDOC, E. coli behavior depended on the source of the added organic matter. The addition of AOM from chlorinated algae led to an E. coli growth equivalent to that in nonsupplemented tap water; the addition of AOM from ozonated algae allowed a 4- to 12-fold increase in E. coli proliferation compared to nonsupplemented tap water. Under our experimental conditions, 0.1 mg of algal BDOC was sufficient to support E. coli growth, whereas the 0.7 mg of BDOC liter⁻¹ initially present in drinking water and an additional 0.2 mg of BDOC acetate liter⁻¹ were not sufficient. Better maintenance of E. coli cultivability was also observed when AOM was added; cultivability was even increased after addition of AOM from ozonated algae. AOM, likely to be present in treatment plants during algal blooms, and thus potentially in the treated water may compromise water biological stability.     

Seasonal Changes in Chlorophyll-containing Picoplankton Populations of Ten Lakes in Northern England

Key features of photosynthetic picoplankton populations were compared during 1988 in ten lakes in northern England ranging from oligotrophic to slightly eutrophic; two of the three eutrophic lakes were shallow and lacked a thermocline. Measurements were made at 0.5 m depth of temperature, total chlorophyll a, chlorophyll-containing picoplankton cell density, mean picoplankton cell volume and percentage of phycoerythrin-rich cells in the total picoplankton population. All lakes showed maxima for total chlorophyll concentration and picoplankton cell density in mid- to late summer. The maximum value for picoplankton density ranged from 3.4 × 10³ (Esthwaite Water) to 1.3 × 10⁶ cells ml⁻¹ (Ennerdale Water). There was a significant negative relationship (p < 0.05) between log₁₀ of maximum picoplankton cell density and maximum total chlorophyll, the latter being taken as an indicator of lake trophic status. The ratio of maximum to minimum picoplankton density during the year in a particular lake ranged from 39 to 2360 and showed no obvious relationship to lake type. Overall, the seasonal range in picoplankton density was about one order of magnitude greater than the range in total chlorophyll a, but there were considerable differences between lakes. Phycoerythrin-rich picoplankton as a percentage of total picoplankton reached a maximum in summer in all lakes. Values were always very low (<5%) in the two shallow eutrophic lakes, but reached 97% and over in the four most oligotrophic lakes. In two of the oligotrophic lakes, Wast-water and Ennerdale Water, phycoerythrin-rich picoplankton was a major component of the summer phytoplankton biomass.     

Measurement and interpretation of sedimentary pigments

SUMMARY.

• 1 The factors that control the concentrations of pigments in lake sediments are examined using data from (i) transects across lake basins, (ii) surface samples from logographic and atrophic lakes, and (iii) two ²¹⁰Pb-dated short cores. Methods for the rapid non-chromatographic analysis of percent native chlorophyll and the blue-green algal pigments oscillaxanthin and myxoxanthophyll are described.
• 2 The concentrations of chlorophyll derivatives, total arytenoids, oscillaxanthin and myxoxanthophyll, and percent native chlorophyll, are higher under conditions along the transects favorable for preservation. Chlorophyll degrades at about the same rate as total carotenoids, and oscillaxanthin degrades at about the same rate as myxoxanthophyll. Therefore, both the ratio of chlorophyll to arytenoids (CD/TC) and the ratio of oscillaxanthin to myxoxanthophyll (OSC/MYX) are mostly determined by the quality of autochthonous production rather than by preservation conditions.
• 3 CD/TC values are higher in logographic lakes than in atrophic lakes, apparently as a result of differences in production of the two pigment types. Calculations show that CD/TC shifts in these two cores cannot be a record of the allochthonous-autochthonous Baltic in the systems. Instead, the CD/TC values shift with large changes in the kinds of plants dominating primary production.
• 4 The stratigraphy of percent native chlorophyll appears to record the hypohmnetic oxygen concentrations at the time of deposition.
• 5 No obvious reason exists why a eutrophic lake should produce more pigment per gram organic matter than should an oligotrophic lake. However, several correlated mechanisms tend to produce higher sedimentary pigment concentrations in eutrophic lakes. Most of these mechanisms, such as hypolimnetic oxygen consumption, indirectly translate greater primary productivity into greater sedimentary pigment concentration. Therefore, pigment concentrations are sometimes misleading, and pigment accumulation rate may be a better indicator of past primary production.

     

Production and chlorophyll concentration of epipelic and epilithic algae in fertilized and nonfertilized subarctic lakes

The production and chlorophyll concentration of epipelic and epilithic algae was measured during four years (1972–1975) in two shallow, Swedish subarctic lakes. One lake (Lake Hymenjaure) was fertilized with phosphorus or a combination of phosphorus and nitrogen while the other (Lake Stugsjön) served as a reference. The benthic algae in both lakes were dominated by Cyanophyceae of the same species during the whole investigation. The chlorophyll concentration of epipelic and epilithic algae was 100 and 20 mg·m^–2 respectively and fairly constant during the season. In 1974–1975 there was a significant increase in chlorophyll concentration of the benthic algae in Lake Hymenjaure, probably as a response to the poorer light climate in the lake due to a large phytoplankton development. The annual benthic production was 3.4–7.2 gC·m^–2 and it was not enhanced by the fertilization. Compared to the other primary producers (phytoplankton and macrophytes) the benthic algae constituted 70–83% of the total production in Lake Stugsjön. In Lake Hymenjaure, however, the importance of the benthic algae decreased from 50 to 22% of the total due to the great increase in phytoplankton production induced by the lake fertilization.     

Biomanipulation-induced reduction of sediment phosphorus release in a tropical shallow lake

Biomanipulation via fish regulation combined with submerged plant introduction is an effective measure to restore eutrophic shallow lakes. Improved water quality and clarity promote growth of benthic algae, which with submerged plants may limit sediment phosphorus (P) release, thereby reinforce lake recovery. Our study sought to evaluate the effect of such a biomanipulation on water quality, benthic algal development and sediment P release in a shallow, tropical lake by (1) comparing porewater and lake water quality, light intensity and benthic algal development in restored and unrestored sections; (2) conducting a ³²P radiotracer experiment to track P release from sediment cores sampled from both sections. The biomanipulation led to lower total P, total dissolved P, and soluble reactive P concentrations in lake water, lower phytoplankton biomass, and increased light intensity at sediment surface, stimulating benthic algal development. Moreover, sediment ³²P release was lower in the restored than unrestored section. Concurrently, dissolved oxygen levels in upper layers of the sediment cores were higher in the restored section. Our study indicates that the biomanipulation improved water quality and enhanced growth of benthic algae, thereby reducing sediment P release, which may be one of the main mechanisms to create successful restoration.     

The effect of temperature on growth characteristics and competitions of <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis> and <Emphasis Type="Italic">Oscillatoria mougeotii</Emphasis> in a shallow,eutrophic lake simulator system

Blue-green algal blooms formed by Microcystis and Oscillatoria often occur in shallow eutrophic lakes, such as Lake Taihu (China) and Lake Kasumigaura (Japan). Growth characteristics and competitions between Microcystis aeruginosa and Oscillatoria mougeotii were investigated using lake simulator systems (microcosms) at various temperatures. Oscillatoria was the superior competitor, which suppressed Microcystis, when temperature was <20°C, whereas the opposite phenomenon occurred at 30°C. Oscillatoria had a long exponential phase (20 day) and a low growth rate of 0.22 day⁻¹ and 0.20 day⁻¹ at 15°C and 20°C, respectively, whereas Microcystis had a shorter exponential phase (2–3 days) at 30°C and a higher growth rate (0.86 day⁻¹). Interactions between the algae were stronger and more complex in the lake simulator system than flask systems. Algal growth in the lake simulator system was susceptible to light attenuation and pH change, and algae biomasses were lower than those in flasks. The outcome of competition between Microcystis and Oscillatoria at different temperatures agrees with field observations of algal communities in Lake Taihu, indicating that temperature is a significant factor affecting competition between Microcystis and Oscillatoria in shallow, eutrophic lakes.     

Limitations of using extracellular alkaline phosphatase activities as a general indicator for describing P deficiency of phytoplankton in Chinese shallow lakes

Extracellular phosphatase can be produced by phytoplankton to utilize organic phosphorus under phosphorus (P) deficiency. However, there is a controversy about its use as an indicator of P deficiency in natural phytoplankton community inferred by such an “induction–repression” mechanism. Size-fractionation of alkaline phosphatase activity (APA), soluble reactive phosphorus (SRP) concentration, algal density, and composition were determined in six Chinese shallow lakes ranking in gradient of trophic status, where a positive relationship between SRP concentration and algal density was observed. Enzyme-labeled fluorescence (ELF) method was used to localize phosphatase on cell membrane of algae. The so-called algal APA that associated with coarser particle (>3.0 μm) accounted for the largest part of total APA. Within a lake with lower SRP concentration, the “induction–repression” mechanism held true. Contrastingly, both algal and total APA were positively related to SRP concentration based on the data across all study lakes with statistical significance, which may be explained firstly by algal composition. The lakes with higher SRP concentration were dominated by diatoms and green algae, while they easily produced extracellular phosphatases as evidenced by ELFA labeling. In parallel, the lakes with lower SRP concentration were dominated by cyanobacteria, while it was never ELFA-positive; secondly, ELFA-positive dots or structures suggested that, in lakes with higher trophic status, attached bacteria or heterotrophic microorganisms could substantially contribute to extracellular phosphatases for hydrolyzing organophosphoric compounds but probably utilizing the organic moiety as an organic carbon source. This process simultaneously produces inorganic P, leading to the co-occurrence of high phosphate concentration and APA. So, the contributor of APA are complex, which may produce extracellular phosphatase species-specific or not exclusively for P nutrient and consequently make it difficult to normalize APA with the exact biomass estimators. Therefore, it is not reasonable to use APA, normalized or not, as a general indicator for describing P deficiency of phytoplankton in shallow lakes especially eutrophic ones.     

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.     

Detrital aggregates in some Iowa lakes and reservoirs

Detrital aggregates in three eutrophic Iowa lakes and four eutrophic Iowa reservoirs were studied with light and scanning electron microscopy to determine if aggregate morphologies and concentrations were similar. Lake and reservoir aggregates were composed of organic and inorganic particles bound together in an organix matrix. Many of the inorganic particles were calcium carbonate. Obvious bacterial and fungal attachment to the aggregates was rare. Aggregate concentrations ranged from 4 to 274 million aggregates per liter. Aggregates smaller than 18 μm diameter dominated the hyperbolic size-frequency distribution of aggregates in all lakes. Reservoir and lake aggregate concentrations did not differ significantly, but mean aggregate concentrations were directly correlated to the mean chlorophyll a concentration of the lakes. These data strongly suggest that detrital aggregate concentrations are influenced by the trophic status of a lake. Journal Paper No. J-8705 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project 2051. Journal Paper No. J-8705 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project 2051.     

Algal Diet of Small-Bodied Crustacean Zooplankton in a Cyanobacteria-Dominated Eutrophic Lake

Small-bodied cladocerans and cyclopoid copepods are becoming increasingly dominant over large crustacean zooplankton in eutrophic waters where they often coexist with cyanobacterial blooms. However, relatively little is known about their algal diet preferences. We studied grazing selectivity of small crustaceans (the cyclopoid copepods Mesocyclops leuckarti, Thermocyclops oithonoides, Cyclops kolensis, and the cladocerans Daphnia cucullata, Chydorus sphaericus, Bosmina spp.) by liquid chromatographic analyses of phytoplankton marker pigments in the shallow, highly eutrophic Lake Võrtsjärv (Estonia) during a seasonal cycle. Copepods (mainly C. kolensis) preferably consumed cryptophytes (identified by the marker pigment alloxanthin in gut contents) during colder periods, while they preferred small non-filamentous diatoms and green algae (identified mainly by diatoxanthin and lutein, respectively) from May to September. All studied cladoceran species showed highest selectivity towards colonial cyanobacteria (identified by canthaxanthin). For small C. sphaericus, commonly occuring in the pelagic zone of eutrophic lakes, colonial cyanobacteria can be their major food source, supporting their coexistence with cyanobacterial blooms. Pigments characteristic of filamentous cyanobacteria and diatoms (zeaxanthin and fucoxanthin, respectively), algae dominating in Võrtsjärv, were also found in the grazers’ diet but were generally avoided by the crustaceans commonly dominating the zooplankton assemblage. Together these results suggest that the co-occurring small-bodied cyclopoid and cladoceran species have markedly different algal diets and that the cladocera represent the main trophic link transferring cyanobacterial carbon to the food web in a highly eutrophic lake.     

Summer fluctuations of microbial planktonic communities in a eutrophic lake--Cierva Point, Antarctica

A eutrophic lake at Cierva Point, Antarctic Peninsula was surveyedduring the summers of 1997 and 1998. Phytoplankton size fractions(micro-, nano- and picoplankton) were analysed, as well as theabundance of bacterioplankton and planktonic ciliates. No permanentvertical stratification was found owing to the shallowness ofthe lake. Both nutrient concentrations and chlorophyll a valuesindicated highly eutrophic conditions, which are a consequenceof a natural enrichment by seabirds. Significant differencesin temperature between the 1997 and 1998 seasons strongly influencedmost of the biological features. The phytoplankton communityshowed a high algal species-richness, with important contributionsof epilithic, cryobiontic and soil algae. The dominant algalgroup was Chlorophyta, mainly represented by Chlamydomonas aff.celerrima, followed by Chl. aff. braunii. Some replacement ofphytoplanktonic species took place in summer and was more evidentin 1998. Picophytoplankton reached high densities, similar tothose reported from other Antarctic lakes. Bacterioplanktonabundances were typical of eutrophic and hypereutrophic lakes.There was a positive correlation between bacterial and totalphytoplankton abundance. Ciliates reached some very high peaks,with higher figures than those reported for other Antarcticsystems with similar trophic status.     

Environmental factors influencing chlorophyll v. nutrient relationships in lakes

SUMMARY. 1. A model relating log chlorophyll a concentration to log epilimnetic total phosphorus (TP) concentration was re-examined based on: (a) comparative and temporal studies of four stratifying Wisconsin and other highly eutrophic temperate lakes; (b) comparative summer lake surveys from Iowa and Alberta.
2. Although P-limited, deeper lakes with long hydraulic residence times and low external and internal nutrient loading in summer had summer chlorophyll a yields below model predictions based on spring and summer epilimnetic TP concentrations.
3. For lakes with summer epilimnetic TP between 30 and 80 mg m⁻³, chlorophyll a concentrations exceeded model predictions based on summer TP. This relationship held even for Lake Delavan, Wisconsin, where the ratio of available N to P was unfavourably low during spring turnover, and where the trans-thermocline N:P flux ratio was sub-optimal for algal needs in early summer.
4. With increasing summer TP concentrations and/or increasing epilimnetic circulation depth (>5m), chlorophyll a concentrations fell below model predictions—independent of the potential for N-limitation. This plateauing in chlorophyll a response occurred at lower epilimnetic TP content (−2) in lakes with elevated non-algal light extinction coefficients. Using Tailing's algorithm for the'column compensation point' (algal photosynthesis = algal respiration over diel cycle), light limitation best explains this fall-off in chlorophyll a yield.
5. The failure of the Dillon & Rigler (1974) spring TP v . summer chlorophyll a model for these Wisconsin lakes is unrelated to N-limitation. Instead, it reflects internal adjustment in take TP in response to stratification and seasonal external P loading.     

The impact of substrate and lake trophy on the biomass and nutrient status of benthic algae

Algal biomass, C:N:P (carbon:nitrogen:phosphorus) ratios and APA (biomass specific alkaline phosphatase activity) were measured in benthic algal communities on living substrates (mussels and macrophytes) and on rocks and stones (epilithon) in three lakes of different trophy. Benthic algal communities on living substrates had lower C:N:P ratios than epilithon, whereas algal biomass was highest on rocks and stones. Benthic algal biomass increased with the trophic level of a lake despite an increase of C:N:P ratios in the benthic community. The differences in C:N:P ratios and algal biomass between lakes of different trophy were higher on inert substrates than on macrophytes and mussels, probably because algae on living substrates could compensate a poor nutrient supply from lake water with substrate nutrients. However, the substrate was not, as expected, the most important nutrient supply in the oligotrophic lake, but in the eutrophic lake. Therefore, differences between inert and living substrates in a single lake were highest in the eutrophic lake. APA values of the oligotrophic lake were very high especially for benthic algae on stones, indicating an ability of the community to take up nutrients from organic sources. In conclusion, living substrates were an important nutrient source for benthic algae and the importance of this nutrient supply did not decrease with increasing lake trophy.     

Strong dependence between phytoplankton and water chemistry in a large temperate lake: spatial and temporal perspective

In lakes, spatial and temporal variability of water chemistry and phytoplankton are characteristic phenomena although often difficult to link together. This motivated us to study their interplay in Lake Vanajanselkä, a eutrophic lake in Finland. We hypothesized that in summer spatial and temporal differences in phytoplankton and water chemistry can be extended in comparison to spring and autumn. Therefore, chlorophyll a and water chemistry was examined by six sampling campaigns with 15 sampling sites over the lake in May–October 2009–2010. In summer, chlorophyll, pH, and oxygen were horizontally and vertically unevenly distributed in the lake, and in the epilimnion pH and oxygen showed a distinct diurnal variability suggesting high photosynthesis during the day. Daily >1 pH unit difference between the sites and 2.5 pH unit difference between the epi- and hypolimnion were found. In agreement with pH and oxygen, NO₃-N and NH₄-N could be unevenly distributed in the epilimnion. In autumn no spatial differences were found, however. The results emphasized that algae and cyanobacteria were responsible, at least partly, for the variability in water chemistry in the surface layer, and short- and long-term gradients in space and time need to be considered when productive lakes are studied.