Summer depth selection in crustacean zooplankton in nutrient‐poor boreal lakes is affected by recent residential development

1. Strong vertical gradients in light, water temperature, oxygen, algal concentration and predator encounters during summer in stratified lakes may influence patterns of depth selection in crustacean zooplankton, especially Daphnia species. 2. To test how crustacean depth selection varies among lakes along a gradient of catchment disturbance by recent residential development and land use change, we calculated the weighted mean depth distribution of the biomass of crustaceans by day and night in eight nutrient‐poor boreal lakes. 3. Generally, the greatest biomass of crustaceans was located at the metalimnion or at the lower boundary of the euphotic zone during thermal stratification in July. The crustacean zooplankton avoided warm surface layers and tended to stay in colder deep waters by both day and night. They also remained at greater depths in lakes with a more extensive euphotic zone. 4. There was some evidence of upward nocturnal migrations of large Daphnia and copepods in some lakes, and one case of downward migration in a lake inhabited by chaoborid larvae. 5. Multivariate regression trees (MRT) were used to cluster crustaceans and Daphnia species in homogeneous groups based on lake natural and disturbance factors. For crustaceans, the depth of the euphotic zone, the sampling depth (epilimnion, metalimnion and hypolimnion), time (day or night) of sampling and the biomass of chlorophyll a were the main driving factors. For Daphnia species, the drainage area, the sampling depth, the cleared land surface area within the catchment and the concentration of total dissolved phosphorus were the main factors.     

CHLOROPHYLL A VERSUS ADENOSINE TRIPHOSPHATE AS ALGAL BIOMASS INDICATORS IN LAKES1

The relation between phyloplankton chlorophyll a and adenosine triphosphate (ATP) was investigated during a yearly cycle in Lake Tahoe, California–Nevada, and a variety of North American and New Zealand lakes of ranging trophic state. Since cellular concentrations of ATP have been shown as acceptable indicators of live biomass among natural microbial populations, the ratio of chlorophyll a to cellular ATP reveals the extent of pigment production per unit biomass under diverse environmental conditions. In general, mixed systems, i.e., epilimnion of most lakes sampled, showed good consistency of cellular chlorophyll a:ATP ratios. Under thermally stratified conditions at Lake Tahoe extensive modifications of the ratio occurred, leading to a 10-fold increase in the ratio at the bottom of the euphotic zone by late summer. Thus, the applicability of chlorophyll a as a quantitative algal biomass indicator is greatly restricted in lakes having thermally stratified euphotic zones.     

Macrozooplankton and the persistence of the deep chlorophyll maximum in a stratified lake

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Mixing patterns and deep chlorophyll a maxima in an eutrophic tropical lake in western Mexico

In many temperate oligotrophic lakes, algal accumulations can form below the mixing zone. However, Deep Chlorophyll Maxima (DCM) have also been found in some eutrophic, tropical lakes and in this paper we aim to identify if they are recurrent features in these kinds of lakes and to recognize the factors that favor their formation. We analyzed 5 years of thermal stratification, water quality, and chlorophyll a concentrations in a tropical eutrophic lake in Central Mexico. Thermal stratification patterns were characteristic of warm monomictic lakes. Full water column deoxygenation during winter mixing was recorded in 3 of the analyzed years, and an increase of ~ 1 °C in the hypolimnion was detected between 2011 and 2015. DCM were detected in 4 out of the 5 studied years, at the top of the hypolimnion when the water column was stratified (spring–summer). This study is the first report of recurrent DCM formation in the northern limit of the Neotropics. It confirms that high light penetration is a necessary condition for DCM. Stratified nutrients with epilimnetic P depletion are also factors favoring DCM formation.

     

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.     

Secchi disk — chlorophyll relationships in a lake with highly variable phytoplankton biomass

In meseutrophic Lake Constance mean euphotic phytoplankton chlorophyll concentrations vary about 100-fold over the year. Concomitant fluctuations in euphotic depth (Z_eu) and Secchi depth (Z_s) are related to each other in a non-linear fashion that as a rough approximation can be expressed by Z_eu 5 Z_s.Secchi depth is to a great extent a function of beam attenuation of light which depends on the inherent optical properties of the water and is highly sensitive to light scattering from particles. Euphotic depth, by contrast, is a function of the vertical light attenuation coefficient which also depends on absorption and scattering, but is less sensitive to the latter than beam attenuation. Algal cells both absorb and scatter light and therefore influence Secchi depth and euphotic depth, however, in different fashions.Whenever the lake is clear due to scarce phytoplankton, scattering is small and beam attenuation only exceeds vertical light attenuation by a relatively small factor. As a consequence, the ratio of euphotic depth to Secchi depth is small (1.5–2.5). When the lake is turbid due to high algal density, enhanced scattering from algal cells and detrital particles causes beam attenuation to rise more than vertical light attenuation, thus leading to high ratios of euphotic depth to Secchi depth (3–5). The relatively close relationships between Secchi depth and chlorophyll in Lake Constance are due to (1) high influence of chlorophyll concentration on water transparency, (2) co-variation of phytoplankton and other suspended particles, and (3) limited variation of cellular chlorophyll contents.     

The Formation and Dynamics of Deep Bacteriochlorophyll Maximum in the Temperate and Partly Meromictic Lake Verevi

Vertical distribution of phytoplankton and the formation of deep chlorophyll maximum (DCM) in the metalimnion of a small stratified and partly meromictic temperate lake was studied in 1999 and 2000. During summer DCM usually occurred on the borderline of H₂S and oxygen-containing waters. At the depths where the bacteriochlorophyll (Bchl) maxima were observed, the sulphide concentration was usually relatively low compared to the bottom layers, where its concentration reached as high as possible saturation level. In April 2000, DCM was formed at the depth of 3.5 m, and lowered thereafter slowly to 6.5 m by October. The concentration of Bchl d reached the highest values (over 1000 μg l⁻¹) just before the water column was mixed up in autumn. In December and April Bchl d was detectable only near the bottom of the lake. The concentration of chlorophyll a yielded by the spectrophotometric phaeopigment corrected method and by HPLC (high pressure liquid chromatography), fit rather well in the upper layers. In deeper water layers chlorophyll a concentration (Chl a) measured by spectrophotometry was overestimated about 47 times if compared to HPLC values because of the high Bchl d in that layer. In most cases vertical profiles of primary production (PP) did not coincide with the vertical distribution of the pigment content; the maximum values of PP were found in the epilimnion. In some cases PP had notably high values also at the depth of DCM. In the upper layers Chl a usually did not exceeded 20 μg l⁻¹ in spring and 10 μg l⁻¹ in summer_. The moderately high Chl a in the epilimnion in spring was significantly reduced after the formation of thermocline most probably because of the establishment of the nutrient limitation in epilimnion. Decreasing Chl a concentration in the epilimnion led to increased water transparency and better light conditions for photosynthetic bacteria in metalimnion.     

Chemical limnology of two artificial lakes used for both stormwater management and recreation

The aims of this study were to document the mainly chemical behaviour of two linked artificial lakes used for both stormwater management and recreation in the new town of Craigavon. Further, the understanding of their behaviour should help in their management and the design of other similar lakes.The lake mean total phosphorus (73 µg P l^–1), nitrate (0.50 mg N l^–1) and chlorophyll a (25 µg l^–1) concentrations, Secchi depth (1.2 m) and the estimated total phosphorus loading (1.98 g m^–2 a^–1) all classify the main lake as eutrophic. An important source of the phosphorus load on the lakes is the urban area of Craigavon (52% of the total load). The interrelationships between total phosphorus, chlorophyll a and Secchi depth in the main lake are similar to those in natural ones. In addition, the lake follows the total phosphorus load — trophic state relationships (lake total phosphorus and chlorophyll a concentrations and Secchi depth) found to apply elsewhere. These two points indicate that the artificial lakes in Craigavon behave similarly to natural ones.     

Vertical distribution of primary production and phytoplankton in two small lakes with different humus concentration in southern Finland

Vertical distribution of primary production and phytoplankton was studied in a polyhumic brownwater lake and in an oligo-mesohumic lake. During summer both lakes were thermally, chemically and biologically stratified. In the brownwater lake primary production was restricted to the uppermost layer of 1–1.5 m of epilimnion. In the oligo-mesohumic lake noticeable primary production was detected down to depths of 2–3 m. The ice-free period primary production was about 20% higher in the oligo-mesohumic lake, though occasionally the surface production was 2–3 times higher in the brownwater lake. Epilimnetic total phosphorus and total nitrogen concentrations were higher in the brownwater lake, while nitrate-nitrite, ammonium and phosphate concentrations were very low in both lakes.
Phytoplankton was confined to the uppermost productive layer in the brownwater lake. In the oligo-mesohumic lake phytoplankton was distributed more evenly, though the mean maximum biomass was at the depth of 3–4 m. Below the oxic water layer biomass decreased abruptly in both lakes. In the oligo-mesohumic lake chlorophyll concentration was extremely high (max. 320 mg chl a m⁻³) in the anoxic hypolimnion, due to green sulphus bacteria.
Flagellated chlorophytes and thrysophytes dominated in the brownwater lake; in spring Chlamydomonas species, followed by Mallomonas caudata . In the oligo-mesohumic lake small coccal green algae, such as Oocystis, Scenedesmus and Westella -like species, dominated in mid-summer, and chrysophytes and cryptomonads in autumn.     

Responses of a Maritime Antarctic lake to a catastrophic draining event under a climate change scenario

The limnological features of Lake Boeckella, the main water body of Esperanza/Hope Bay (Antarctic Peninsula), were evaluated over a 16-year period, under a climate change context evidenced by the increasing air temperature trend reported for this region for the last 50 years. We analyzed the physicochemical and phytoplankton data of the lake obtained from 1991 to 2007 during the austral summers. At the beginning of January 2001, a sudden water level drop (~3 m) occurred in Lake Boeckella as a consequence of an extremely high water discharge to the sea. This was triggered by the progressive thawing of the permafrost in the basin of the system. After this disturbance, nutrients, conductivity, chlorophyll a (Chl a) and picoplankton density showed strong peaks. The pre-draining and post-draining periods showed significant differences for most of the limnological variables analyzed. Secchi disk depth significantly decreased throughout the study period, resulting in a thinner euphotic layer. Chrysophyceae and Volvocales dominated the >2 μm phytoplankton fraction in the lake, but from 2004 onwards, other small-sized eukaryotic algae (3–5 μm) also became very abundant. Autotrophic picoplankton showed a significant peak during the summer when the water level decreased. A shift in their composition was observed through the study period: in 1998, picocyanobacteria were numerically dominant; from 2002 onwards, picoeukaryotes increased and became dominant in 2004. This study suggests that climate change may trigger the thawing of the permafrost in the catchments of Maritime Antarctic lakes, leading to catastrophic draining events, which favor natural eutrophication processes.     

Hypolimnetic phosphorus retrieval by diel vertical migrations of lake phytoplankton

SUMMARY. I. Movement of ³³P from hypolimnion to epilimnion in a small, dystrophic lake was investigated using small-diameter experimental tubes enclosing thermally stratified water columns. This approach was made possible by the extremely sharp stratification found in such lakes, in which the euphotic zone closely coincides with the epilimnion.
2. The vertical distribution of inorganic phosphorus in the lake showed a sharp increase across the thermocline so that enhanced concentrations were available to phytoplankton just below the thermocline. Inorganic nitrogen concentrations did not show such a marked relation to thermal stratification.
3. One abundant motile alga ( Cryptomonas marssonii ) showed striking and regular vertical migrations in the lake, moving below the thermocline at night and returning to the surface waters in early morning. These migrations took cells across a 10°C temperature gradient. Non-motile phytoplankton showed constant vertical distributions.
4. In the experimental tubes an upward movement of phosphorus took place from hypolimnion to epilimnion which was only attributable to transport by phytoplankton cells undertaking active vertical migrations. No equivalent movement of phosphorus occurred in control tubes from which algae were absent.
5. The possible significance of such nutrient retrieval is discussed with reference to plankton phosphorus budgets and competition between phytoplankton species.     

Epilimnetic and hypolimnetic symptoms of eutrophication in Great Mazurian Lakes, Poland

SUMMARY. An increase in the trophic state and productivity of a lake is not necessarily accompanied by a decrease in epilimnetic Secchi disc transparency. In smaller lakes with sharp thermoclines the symptoms of eutrophication may be more evident as a decrease in transparency in the epilimnion but, in larger lakes with poorly defined thermoclines, symptoms of eutrophication may be more evident in the hypolimnion as shown by an increase in the rate of hypolimnetic oxygen consumption.     

Evaluating short-term effects of omnivorous fish removal on water quality and zooplankton at a subtropical lake

We evaluated a biomanipulation program to test for short-term changes in water quality (chlorophyll a, Secchi depth, total phosphorus) and macrozooplankton biomass following partial removal of omnivorous gizzard shad Dorosoma cepedianum. The removal occurred at a eutrophic subtropical lake, and responses were compared to an unmanipulated control lake using a before-after-control-impact paired series analysis. The removal reduced the biomass of large (>300 mm) gizzard shad by 75% over 2 years via a subsidized commercial gill net fishery. However, the total population biomass of gizzard shad was reduced by approximately 32% from an average pre-manipulation biomass of 224 kg ha⁻¹ due to the size selectivity of the gear, which did not effectively capture small fish (<300 mm). No significant short-term changes in chlorophyll a concentration, Secchi depth, total phosphorus concentration or macrozooplankton biomass were detected following biomanipulation. The partial removal may have fallen short of the biomass reduction required to cause ecosystem responses. Our results suggest that moderate omnivore removals (i.e., <40% biomass reduction) will have little short-term benefits to these lakes, and future manipulations should use a less size-selective gear to achieve a larger total biomass reduction.     

Effects of re‐oligotrophication and climate change on lake thermal structure

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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”.     

Fitness optimization of Daphnia in a trade-off between food and temperature

In thermally stratified lakes with a deep chlorophyll maximum (DCM), Daphnia face a trade-off between food availability and optimum development temperatures. We hypothesize that Daphnia optimize their fitness by allocating the time spent in the different vertical habitats depending on the distribution of algal resources and the temperature gradient. We used the plankton towers (large indoor mesocosms) to study the vertical distribution of a population of Daphnia hyalina×galeata in three different temperature gradients with a DCM. Additionally, we determined the fitness of Daphnia in the epilimnion and hypolimnion by transferring water from these layers into flow-through systems where we raised Daphnia and assessed their juvenile growth rate as a measure of fitness. The fitness distribution was correlated with the vertical distribution. The vertical distribution most likely reflected the proportions of time Daphnia allocated to dwelling in the two vertical habitats.     

Prediction of Secchi disc depths in Florida lakes: Impact of algal biomass and organic color

A model for the prediction of Secchi disc depths in Florida lakes was developed and tested using data from 205 lakes. A statistical analysis showed that the best estimate of lake Secchi disc depths could be obtained by In (SD) = 2.01 ? 0.370 In (Chla) ? 0.278 In (C) where SD is Secchi disc depth (m), Chla is the chlorophyll a concentration (mg/m³) and C is the organic color concentration (mg/l as Pt). The model yields unbiased estimates of lake Secchi disc depths over a wide range of algal and organic color concentrations and has a 95% confidence interval of 47 to 224% of the calculated Secchi disc depth. Other published Secchi-Chlorophyll models are less precise but can be used almost equally well. This indicates organic color concentrations do not affect lake Secchi disc depths as much as algal levels. Further reductions in the remaining error term, however, might be accomplished by including a variable for suspended inorganic sediment.     

Growth and survival of Daphnia in epilimnetic and metalimnetic water from oligotrophic lakes: the effects of food and temperature

SUMMARY 1. In oligotrophic lakes, phytoplankton and bacteria growing in the deep chlorophyll maximum in the cool metalimnion of lakes often dominate biomass and production, but the importance of this source of food for zooplankton is unknown.
2. During much of the day, Daphnia rosea in two mountain lakes inhabited deep chlorophyll layers where food availability was at least equal to that in the epilimnion.
3. To determine the importance of the two strata (epilimnion and metalimnion) for Daphnia , we used a cross-classified factorial experiment to measure how epilimnetic and metalimnetic food and temperature (10 and 16 °C) influenced survival, growth and reproduction.
4. Daphnia survived and grew better when fed seston from the epilimnion of one lake, although chlorophyll, particulate nitrogen and particulate carbon were 2–2.5 times greater in the metalimnion.
5. Temperature had no significant influence on Daphnia survival or growth. Similar results were obtained with food from the second lake, with Daphnia surviving and reproducing better when provided with epilimnetic, rather than metalimnetic food, although the quantities of chlorophyll and carbon in the two strata were similar.
6. Food quality, rather than quantity or temperature, appeared to be the most important determinant influencing survival, growth and reproduction, and the greater food quantity in the metalimnia was not used effectively by the Daphnia .     

Modelling the contribution of deep chlorophyll maxima to annual primary production in the North Sea

Seasonally stratified areas in temperate shelf seas are usually characterized by a strong spring bloom, followed by limited production within the surface mixed layer as nutrients are depleted in the post-bloom period. The bottom mixed layer remains nutrient-rich, due to regeneration processes. When the thermocline is within the euphotic zone, primary production at thermocline depth is possible as organisms can access nutrients from the bottom layer. Ship-based observations indicate that production within these deep chlorophyll maxima (DCM) can form a substantial part of the annual production, but this process is not captured by satellite observations. The spatial and temporal behaviour of DCM-related production is therefore less well known. To study the extent of DCM occurrence we applied a fully three dimensional, hydro-biogeochemical model to the North Sea. The simulation was used to quantify sub-surface production in space and time to determine the importance of production within the DCM layer with respect to annual and spring bloom primary production in the North Sea. The results showed that locally, in thermally stratified areas in the North Sea, up to 30% of annual net primary production occurred below 15 m depth. In a 21-year average, sub-surface production contributed 10% to the total annual net primary production in the entire North Sea. These values should be interpreted as lower estimates, as the model was unable to represent observed hot-spots of very high chlorophyll a concentrations at depth. The importance of DCM-related production may increase under future climate scenarios when the length of the stratified period may increase.     

The influence of calcium on the chlorophyll–phosphorus relationship and lake Secchi depths

The basic aim of this study was to analyse the influence of calcium on the Chl–TP relationship and to apply the findings to improve dynamic (mechanistically-based) modelling of phosphorus and lake eutrophication. We have analysed long-term data from 73 lakes. The influences of calcium found in these statistical analyses have been integrated into a dynamic foodweb model, the LakeWeb-model, which also includes a mass-balance model for phosphorus. Differences in the model outcome between simulations without and with considerations to the role of calcium are discussed. We can conclude that calcium is an important factor influencing both the Chl–TP relationship and Secchi depths in mesotrophic and eutrophic lakes. Our results also indicate that lakes with long-term median Ca-concentration between 10–30mg/l function as hardwater lakes. The results also stress the importance of taking a holistic view of lakes since the bedrock, soils and land-use activities in the catchment influence the calcium concentration in lakes and therefore the phosphorus cycle, water clarity and the productivity of a given lake. The predictive power of the Chl–TP regression increases markedly if hardwater lakes are omitted from the model domain. For lake foodweb and mass-balance modelling, we show that the inclusion of the presented calcium moderator clearly improved the predictions of lake TP-concentrations in water and sediments, chlorophyll and Secchi depths in Lake Erken, a hardwater lake in Sweden.