Vertical distribution and diel migration of flagellated phytoplankton in a small humic lake

The vertical distributions and migrations are described of the most abundant flagellated phytoplankton species from the summer community of a small forest lake in southern Finland. The lake showed a steep and stable thermal stratification with a shallow oxygenated epilimnion. Horizontal variation of phytoplankton distribution within the lake was tested on two scales and found to be statistically significant only in the case of Mallomonas reginae. The vertical distribution of flagellated phytoplankton was assessed by reference to the distribution of a non-motile, neutrally buoyant species Ankyra judayi. Statistically significant, active vertical positioning was demonstrated for all the flagellates examined with the exception of Spiniferomonas bourrellyi. Diel vertical migrations were apparent for all species showing active positioning and the pattern of an evening descent and a morning ascent was ubiquitous. The extent and timing of diel migrations varied between species. The most extensive migrations were by Cryptomonas marssonii which crossed a temperature gradient of 14 °C and penetrated far into the anoxic hypolimnion. Several categories of competitive advantage can be gained by species undertaking such diel vertical migrations.     

Migrations of haemoglobin-rich Daphnia longispina in a small,steeply stratified,humic lake with an anoxic hypolimnion

Migrations of Daphnia longispina were studied in a small humic lake with an exceptionally shallow oxic epilimnion. Horizontal distributions showed clear avoidance of the shoreline, which might be explained by the lower density of predators (Chaoborus sp. and Notonecta sp.) in the central parts of the lake. In early summer all size classes of D. longispina exhibited upward nocturnal vertical migration, descending to the upper hypolimnion in daytime. Later in summer, when the nocturnally migrating Chaoborus sp. had grown large enough to graze on small Daphnia, the latter seemed to shift towards twilight migration. However, large Daphnia individuals showed no synchronized migration; rather their bimodal vertical distributions suggested asynchronous vertical migration. Large individuals showed a particular tendency to concentrate near to the oxycline, close to the dense phytoplankton and bacteria populations in the upper part of the anoxic hypolimnion. According to vertical trap experiments, large D. longispina visited the anoxic hypolimnion and might harvest its abundant food resources. The high haemoglobin content of large individuals seems a specific adaptation to allow access to low oxygen water and hence to maximize grazing potential, in both epi- and hypolimnion, and minimize predation pressure. By staying predominantly in cooler water near the oxycline, Daphnia might also minimize its energy consumption to adjust to low food availability while sustaining a sufficiently high population density to exploit those unpredictable short periods with abundant food which are common in small headwater lakes. It is suggested that migrations of zooplankton are a complex behavioural adaptation which may not be explained by any single factor. In humic lakes with shallow stratification, vertical migrations seem to offer particularly high potential advantages, because of the short distances between dramatically different environments in the water column. In further studies more emphasis should be placed on migrations of individuals rather than populations, and migrations should be considered as a dynamic part of the structure and function of the whole planktonic ecosystem.     

Food web modelling on the structure and functioning of a Mediterranean lentic system

A number of modelling results suggested thermocline shifts as a consequence of global climate change in stratifying lakes. Abundance and composition of the phytoplankton assemblage is strongly affected by the stratification patterns, and therefore, change in the thermocline position might have a substantial effect on this community or even on the whole lake ecosystem. In this study, thermocline depths in large mesocosms installed in Lake Stechlin (Germany) were deepened by 2 meters and phytoplankton changes were analysed by comparing changes to untreated mesocosms. Higher amounts of SRP were registered in the hypolimnion of treatment mesocosms than in the controls, and there were no differences in the epilimnion. Small but significant changes were observed on the phytoplankton community composition related to the effect of deepening the thermocline; however, it was weaker than the yearly successional changes. The most remarkable differences were caused by Planktothrix rubescens and by chlorophytes. P. rubescens became strongly dominant at the end of the experiment in the mesocosms, and in the open lake as well. The results of the experiment cannot clearly support the proliferation of cyanobacteria in general; however, the deepened thermocline can modify the behaviour of some species, as was observed in case of P. rubescens.     

Chemical and thermal stratification in lakes

An index that shows chemical stratification strength [IC-i; i = water quality item such as chlorophyll-a (Chl.a) and soluble phosphorus (SP)] was proposed and compared with one of thermal stratification strength indices, Schmidt’s stability index (SSI), in Shiozu Bay and Lake Biwa, Japan. The proposed indices of IC-i can be easily calculated with at least one set of each water quality data in both the epilimnion and the hypolimnion. The SSI was shown to be consistent with the traditional thermocline index of thermocline strength index (TSI), but SSI is used as the stability index of the whole lake, whereas TSI is used as the stability index near the thermocline. Analyses showed that chemical stratification strength is determined largely by thermal stratification strength. Totally different characteristics of IC-Chl.a and IC-phosphate (PO₄) at high SSI in the main North Basin of Lake Biwa and in Shiozu Bay were possibly due to the difference in their volumes and hydrodynamic conditions. The proposed index and relationships are especially useful to roughly determine thermal and chemical stratification when only few water quality data are available.     

Temporal and vertical dynamics of phytoplankton net growth in Castle Lake, California

The impact of nutrient additions, zooplankton grazing and light intensity on phytoplankton net growth with depth and season was studied with five microcosm experiments in meso-oligotrophic, subalpine Castle Lake, California, during the period of summer stratification in June-September 1994. The incubations (4 day) were performed at 5 m intervals from the surface to the bottom using natural phytoplankton and zooplankton assemblages, with enrichments of phosphorus and nitrogen. The phytoplankton community was only limited by nutrients in the upper 5 m (epilimnion), as indicated by change in chlorophyll concentration. Nutrient enrichments had the greatest effect on the phytoplankton net growth in June and July. High light inhibited the phytoplankton net growth at the surface. Low light intensities limited phytoplankton at 20 m and below, and at the end of the growing season already around 10-15 m. A deep chlorophyll maximum in the hypolimnion in June-August was not limited by either light or nutrients. The results showed variation in grazers' impact on phytoplankton. These results suggest the importance of nutrient limitation only in the epilimnion with light inhibition at the surface, light limitation in the hypolimnion, and varying impact of zooplankton grazing in influencing the development of the phytoplankton in Castle Lake.      

Statistical quantification of the effect of thermal stratification on patterns of dispersion in a freshwater zooplankton community

The vertical distribution of crustacean zooplankton species was examined during 2000 in Windermere, Cumbria. Patterns of dispersion were evaluated quantitatively using two different approaches. Firstly, Morisita’s index was used to test whether patterns of dispersion differed significantly from a state of randomness and, secondly, the relative distribution of zooplankton individuals between the epilimnion and hypolimnion was investigated, for a series of standardised vertical profiles of organism density. All six of the dominant species of planktonic crustaceans showed aggregated patterns of dispersion throughout the year. For most species, patterns of dispersion were affected by the onset and breakdown of thermal stratification in the lake. The degree of aggregation in the vertical plane, measured using Morisita’s index, increased when the lake became thermally stratified. Furthermore, for most species, there was a positive association between the degree of vertical differentiation in abundance across the thermocline, and the degree of temperature differentiation in the stratified water column. The results of the present analysis provide quantitative evidence for the phenomenon known as ‘zooplankton stratification’ and for temporal variation in patterns of zooplankton dispersion.     

Asynchronous vertical migrations of zooplankton in stratified lakes

Asynchronous vertical migrations of calanoid copepods Arctodiaptomus salinus were studied in two stratified lakes in the south of Siberia using the method of two-section enclosures. It was found that the presence of a pronounced thermocline and a depth maximum of phytoplankton (Lake Shira) contributes to the appearance of intensive individual migrations of copepods between areas of the epi- and hypolimnion.     

Nitrogen limitation of phytoplankton in a Spanish karst lake with a deep chlorophyll maximum: a nutrient enrichment bioassay approach

An in vitro nutrient addition bioassay was performed to testthe relative inorganic nitrogen (N) and phosphorus (P) limitationof phytoplankton in a Spanish karst lake (El Tejo) during thelast part of the stratification period, when nutrient limitationis most pronounced. Nutrient deficiency was tested in samplesfrom three different layers of the lake: the epilimnion, metalimnionand oxic hypolimnion. Nitrogen additions, either without orcombined with P, increased phytoplankton growth in all threestrata, compared with controls or P treatments. This showedthat N was the nutrient limiting phytoplankton growth in latesummer–early fall. Since both hypolimnetic diffusion andgroundwater fluxes of N-rich waters into the lake are much reducedduring summer, N becomes the limiting nutrient as stratificationadvances. We suggest that in this Mediterranean area with lowatmospheric deposition of anthropogenic N and in lakes relativelyfree of surface run-off, nutrient supply by atmospheric depositionmight be a key factor in controlling nutrient deficiency forphytoplankton growth.     

The vertical distribution of some ciliated Protozoa in the plankton of a eutrophic pond during summer stratification

The vertical distribution of some ciliated Protozoa in the plankton of a pond in north-west England was investigated during August 1971. At this time, when the pond was stratified with an oxygen dificient hypolimnion, ciliates were counted at 10-cm depth intervals every 5 h over 25 h. The most common species (Loxodes magnus and L. striatus) were confined to the hypolimnion; there was no diurnal migration into the epilimnion. Earlier work had shown that Loxodes species require oxygen; it is therefore possible that these ciliates, which inhabited the oxygen dificient hypolimnion, migratedvertically, from time to time, to an oxygen supply at the boundary with the well-oxygenated epilimnion. To test this, Loxodes populations were confined in cellophane tubese both in the hypolimnion (at 3 m) and epilimnion (0.5 m) for 12 and 24 h (earlier trials had shown that the tubes were not markedly toxic). The ciliates died at both depths, and in a further experiment when Loxodes were confined at 3 m and 0.5 m and sampled at 5-h intervals up to 25 h it was found that they survived longer in the hypolimnion. It is suggested that ciliatees confined at 3 m died because they were unable to migrate vertically to an oxygen supply, while those at 0.5 m died because some other adverse factor was operating in the eiplimnion. Laboratory experiments showed that Loxodes died inn water in which phytoplankton photosynthesis took place and it is suggested that side effects of photosynthesis in the epilimnion (e.g. a rise in pH) caused the death of ciliates exposed at 0.5 m.     

Lakes of south-western Uganda

Lake Bunyonyi was formed about 18,000 years B.P. by a volcanic eruption which blocked a steep-sided valley. Earlier this century fish were introduced into the lake but recently there have been mass fish deaths. Although the thermocline was inconspicuous, density profiles showed the lake to the stratified and chemical evidence suggested stratification was for many years. The water was clear with little plankton and the epilimnioncontained low concentrations of dissolved mineral ions. The hypolimnion was anoxic and had a relatively high mineral ion concentration. The mass fish deaths were attributed to a violent shallow mixing, probably caused by wind, but mixing was insufficient to remineralize the epilimnion.     

The effect of environmental factors on estimated viable and total populations of planktonic bacteria in lakes and experimental enclosures

SUMMARY. The difference between the results of viable and total counting procedures for bacteria are exemplified by vertical profiles from a deep and a shallow lake and from seasonal changes in the epilimnion and hypolimnion of a shallow eutrophic lake. The viable count was, on average, 0.25% of the total count, the greatest difference being noted in the anoxic hypolimnion, probably due to the inadequacy of the viable counting procedure for the isolation of bacteria from such samples. There was a general trend for the more nutrient-rich waters to support larger bacterial populations but such observations did not provide any further information on the factors responsible for the population changes observed. Seasonal fluctuations in the counts are studied and the qualitative and quantitative changes resulting from artificial enclosure of water are discussed. Not all the temporal changes could be explained and short-term changes resulting from nutrient additions to the experimental enclosures were not always reproducible. Horizontal variability was examined, found to be significant and could play an important role where water movement and turbulence is considerable. Results from six sites sampled between 1969 and 1974, representing total and viable bacterial population estimates and a total of eighteen independent or regressor variables were then subjected to principal components analysis. Results taken from the whole water column showed the overwhelming effect of the process of stratification on the bacterial population accounting for 30%-60% of its variability. Secondary components representing algal productivity could account for 10% to 20% of the variability. Many of the chosen regressor variables were acting as measures of the same phenomenon without providing significant information on what affected the bacterial population. To overcome this problem results from the hypolimnion and epilimnion were analysed separately. The analysis demonstrated the importance of seasonal changes in nutrient concentrations in the epilimnion and the development of anoxic conditions in the hypolimnion. Algal biomass, phosphate concentration and the interaction of pH and ammonia appeared to be important. It was concluded that most of the variability in the bacterial population estimates could have been explained by five of the regressor variables and that the factors most likely to provide more information would include some measure of predation and lake retention time.     

Advantages from diel vertical migration can explain the dominance of Gonyostomum semen (Raphidophyceae) in a small, steeply-stratified humic lake

In late summer, a large flagellated alga, Gonyostomum semen(Raphidophyceae), constituted most of the phytoplankton biomassin a small steeply-stratified humic lake. Its diel verticalmigration (DVM) was very distinct and extended at night intothe anoxic hypolimnion. After midsummer, the depletion of hypolimneticoxygen led to a gradual release of soluble reactive phosphorus(SRP) from the sediment, but one month later, irrespective ofcontinuing stratification, the concentrations again returnedto undetectable levels down to the bottom. As this coincidedwith the rapid increase in G.semen population, the latter wasprobably responsible for the depletion of SRP. The flux of SRPfrom the sediment to the epilimnion was virtually interrupted,making non-migrating phytoplankton dependent only on regeneratedand inflowing inorganic phosphorus. Besides nutrient availability,DVM also benefited G.semen in the reduction of metabolic andgrazing losses. In this lake, the remarkable multiple advantagesof DVM probably explain the dominance of the large G.semen inthe late summer phytoplankton biomass over much smaller algae.     

Whitemere, a lake that defies some conventions about nutrients

1. Whitemere, a kettle-hole lake in north-west England (z_m,14 m, area 22.5 ha) has extremely high maximal concentrations (around 1 mg L⁻¹) of total phosphorus (TP) and soluble reactive phosphorus (SRP), and comparatively low maximal concentrations of dissolved inorganic nitrogen (<0.5 mg N L⁻¹). Bioassays indicate that its phytoplankton is nitrogen limited, and it has surface blue-green algal blooms in summer. Palaeoecological investigations have shown that high populations of cyanophytes occurred more than 6000 years ago, even before human settlement of the catchment.
2. The reasons for this combination of features are not immediately clear. This paper attempts to distinguish between two hypotheses: that the high phosphorus concentrations arise from input of phosphate-rich groundwater; and that the high phosphorus concentrations arise from concentration and recycling mechanisms within the lake.
3. The lake is entirely fed by ground water, direct rainfall and surface sheet flow. The concentrations of phosphorus in the ground water are much lower than in the lake water. The lake is stratified and has high phosphorus concentrations in the anaerobic hypolimnion in summer as a result of substantial release from the sediment. Vertical turbulence in the relatively weakly stratified water column may transfer substantial amounts to the epilimnion.There is also substantial phosphorus release from the sediments under the aerobic epilimnion.
4. There is little outflow of water and phosphorus. Most of the phosphorus is returned to the sediment under winter isothermal conditions, to be released again the following summer. The lake has probably maintained these mechanisms for millennia. Recent eutrophication is likely to have been driven by nitrogen inputs from greater intensification of agriculture in the catchment.     

Lake Chisholm,a polyhumic forest lake in Tasmania

Lake Chisholm is a polyhumic, warm monomictic forest lake in western Tasmania. Its large relative depth and sheltering forest result in nine months stratification. The high humic content is a dominant feature, producing a sharp, shallow thermocline, a shallow euphotic depth (< 1 m) and an underwater light climate dominated by red wavelenghts. The hypolimnion is anoxic and sulphide-laden and even in winter circulation is sluggish. For much of the year the lake resembles a biogenically meromictic lake, and though there is only slight chemical enrichment of the hypolimnion there is nonetheless considerable vertical structure in the water column. Chromophyte flagellates are the dominant algae, a few species often forming monospecific blooms in a sporadic manner. Lake Chisholm is seen as an oceanic, mid-latitude counterpoint to dimictic, polyhumic, flagellate haunts in Scandinavia.     

Hypolimnetic phosphorus and nitrogen dynamics in a small,eutrophic lake with a seasonally anoxic hypolimnion

In situ estimates of sediment nutrient flux are necessary to understand seasonal variations in internal loading in lakes. We investigated the sources and sinks of nutrients in the hypolimnion of a small (0.33 km²), relatively shallow (18 m max. depth), eutrophic lake (Lake Okaro, New Zealand) in order to determine changes in sediment nutrient fluxes resulting from a whole lake sediment capping trial using a modified zeolite phosphorus inactivation agent (Z2G1). Sediment nutrient fluxes in the hypolimnion were estimated as the residual term in a nutrient budget model that accounted for mineralisation of organic nutrients, nutrient uptake by phytoplankton and mixing, nitrification, adsorption/desorption and diffusion of dissolved nutrients at the thermocline. Of the total hypolimnetic phosphate and ammonium fluxes during one period of seasonal stratification (2007–08), up to 60 and 50%, respectively, were derived from the bottom sediments, 18 and 24% were due to mineralisation of organic species, 36 and 28% were due to phytoplankton uptake and 9 and 6% were from diffusion across the thermocline. Adsorption/desorption of phosphate to suspended solids and nitrification were of minor (<8%) importance to the total fluxes. Any reduction in sediment nutrient release by Z2G1 was small compared with both the total sediment nutrient flux and the sum of other hypolimnetic fluxes. Uneven sediment coverage of Z2G1 may have been responsible for the limited effect of the sediment capping layer formed by Z2G1.     

Epilimnetic nutrient loading by metalimnetic erosion and resultant algal responses in Lake Waramaug,Connecticut

Phosphorus regeneration from lake sediments, and subsequent migration to trophogenic surface water, significantly contributes to the lake nutrient budgets and algal bloom conditions in some lake types. Decomposition of organic matter in deep water and sediments results in the accumulation of regenerated nutrients, alternate electron acceptors (reduced products of anaerobic respiration = COD), carbon dioxide, and depletion of dissolved oxygen (electron acceptor in aerobic respiration). Thermal stratification creates spatial segregation of trophogenic and tropholytic environments in the lake, resulting in gradients between sediments, hypolimnion, and the epilimnion. Exchange of oxygen, nutrients, and reduced alternate electron acceptors between the hypolimnion and epilimnion affects the productivity of a lake. Secchi depth, temperature, and dissolved oxygen profiles were determined twice each week from May 1980 to October 1980 at each of five lake stations. Nutrient concentration profiles, including total soluble and total phosphorus, ammonium-N, nitrate, soluble Kjeldahl, and total Kjeldahl nitrogen were determined twice each month. Epilimnetic algal samples were collected twice each week using Kemmerer and water column ‘straw’ amplers. Cell counts of total, green, bluegreen, and diatom algae groups were made. Three methods were used to describe hypolimnetic-epilimnetic exchange, including coefficients of eddy diffusion (based on lake heat budget), a graphical method of defining thermocline location, and relative thermal resistance to mixing (RTRM, based on density differences). All three methods yeilded comparable estimates of net seasonal transport. The graphical and RTRM methods described events occurring at shorter intervals (greater resolution). We find general agreement between the three methods of describing hypolimnetic-epilimnetic transport. The frequency of sampling resulted in increased resolution of thermal profiles (in time), allowing accurate estimation of short-term nutrient flux into epilimnetic waters. An algal bloom event occurred 5 to 12 days following erosion of the top of the metalimnion to below the aerobic-anaerobic interface. The lag time to peak algal concentration, following such events, decreased through the summer (June = 12 days, September = 5 days)     

Seasonal changes in the chemistry and biology of a meromictic lake (Big Soda Lake,Nevada, U.S.A.)

Big Soda Lake is an alkaline, saline lake with a permanent chemocline at 34.5 m and a mixolimnion that undergoes seasonal changes in temperature structure. During the period of thermal stratification, from summer through fall, the epilimnion has low concentrations of dissolved inorganic nutrients (N, Si) and CH₄, and low biomass of phytoplankton (chlorophyll a ca. 1 mgm ^-3). Dissolved oxygen disappears near the compensation depth for algal photosynthesis (ca. 20 m). Surface water is transparent so that light is present in the anoxic hypolimnion, and a dense plate of purple sulfur photosynthetic bacteria (Ectothiorhodospira vacuolata) is present just below 20 m (Bchl a ca. 200 mgm^-3). Concentrations of N H₄ ⁺, Si, and CH₄ are higher in the hypolimnion than in the epilimnion. As the mixolimnion becomes isothermal in winter, oxygen is mixed down to 28 m. Nutrients (NH₄ ⁺, Si) and CH₄ are released from the hypolimnion and mix to the surface, and a diatom bloom develops in the upper 20 m (chlorophyll a > 40 mgm^-3). The deeper mixing of oxygen and enhanced light attenuation by phytoplankton uncouple the anoxic zone and photic zone, and the plate of photosynthetic bacteria disappears (Bchl a ca.10mgm^-3). Hence, seasonal changes in temperature distribution and mixing create conditions such that the primary producer community is alternately dominated by phytoplankton and photosynthetic bacteria: the phytoplankton may be nutrient-limited during periods of stratification and the photosynthetic bacteria are light-limited during periods of mixing.     

Exploitation of a deep-water algal maximum by Daphnia: a stable-isotope tracer study

The exploitation of a deep algal maximum by Daphnia in the absence of fish predation was studied in large indoor mesocosms. Facing the dilemma of low food but high temperature in the epilimnion vs. high food but low temperature in the hypolimnion, Daphnia distribute above and below the thermocline in order to optimise their fitness. Labelling hypolimnetic algae with ¹⁵N revealed that the vertical distribution of Daphnia is dynamic, i.e., all individuals traverse the thermocline and allocate a certain proportion of their time to feeding in the cold water. The overall energy gain from the deep-water algal maximum is lower than from the same algal concentration in the epilimnion due to the low temperature and the limited time an individual spends in the hypolimnion. The results provide mechanistic support for the hypothesis that Daphnia chose their habitat according to an Ideal Free Distribution with Costs model.     

Certain biological effects of thermocline shifts

Summary Under the influence of variable climatic conditions in the spring and due to the peculiar shape of its basin, Lake West Okoboji, Iowa, showed considerable variations in mid-summer temperatures from year to year. The lake either became stratified early in June or stayed mixed until some time in August. When the thermocline formed early between 10–12 m., as usual, it left a deep hypolimnion of relatively small volume. In such years, yellow perch and other fish had weak diel movements from 12 to 5 m. and back. When, in other years, the epilimnion was deep and ample oxygen extended to 20 m, the same fish showed more pronounced on- and off-shore migrations and spent part of the day at the 20 m. contour where they foraged on the tendipedid larvae which reached greatest abundance there. In years of early stratification, however, when the midge larvae were fewer and showed some concentration between 10 and 12 m., they were not a preferred item in the diet of the perch.The behavior of the fish is discussed with physiological limitations in mind and their ability to contract an oxygen debt as well as the adjustment of swim bladder volume are considered as critical factors. Some speculation is made about the possible role of density-dependent water movements on the distribution of midge larvae; the concentration zones of Tendipes larvae coincided closely with the depth to which the wind-driven circulation extended and where water density shifted most pronouncedly.Formerly Iowa State Teachers College and Iowa Lakeside Laboratory     

Taxonomic structure of phytoplankton assemblages in Crater Lake, Oregon, U.S.A.

SUMMARY. 1. A taxonomic analysis of 171 phytoplankton samples obtained from Crater Luke, Oregon, between 1985 and 1987 revealed 132 taxa in the upper 250 m of the water column. The greatest temporal variation in taxonomic structure occurred between 40 and 80 m below the water surface, a depth range which corresponded to the zone of maximum primary production.
2. Phytoplankton cell biovolume in the upper 20 m of the water column was relatively high during the summer months, a period when Nitzschia gracilis was dominant in the epilimnion. However, 72% or more of the cell biovolume between 0 and 200 m was distributed below 20 m and, during the winter and spring months, 61% was found below 80 m.
3. Cluster analysis identified a sparse, temporally ubiquitous flora which was modified to various degrees when environmental conditions became favourable for the growth of a few dominant taxa. These surges ot dominance by individual taxa accounted for 2 to 5-fold increases in cell biovolume and generated a pronounced taxonomic discontinuity between the floras in the epilimnion and hypolimnion.
4. While the taxonomic structure of the phytoplankton in the epilimnion corresponded closely with the structure found in a 1978–80 study, the flora below the metalimnion was more diverse and less predictable in species composition than the pattern reported in the earlier study.