Limnology of a desertic terminal lake,Walker Lake,Nevada, U.S.A.

Walker Lake, a large terminal lake in west-central Nevada, is rapidly desiccating and becoming more saline. From 1937 to 1977, total dissolved solids increased from 5 650 to 10 650 mg l^–1. Ionically, the lake is a chlorocarbonate one; however, as desiccation continues it should eventually have about equal amounts of chloride, sulfate and carbonate. With regard to algal nutrients, the lake appears to be nitrogen limited. The phytoplankton is currently dominated by Nodularia spumigena; at times this produces noxious blooms. The zooplankton community is depauperate and composed of three species. The fish fauna is limited to three endemic fishes, with tui chub, Gila bicolor, dominant. Lahontan cutthroat trout, Salmo clarki henshawi, is restricted during summer to a narrow mid-water zone low enough in temperature and high enough in dissolved oxygen to meet physiological requirements.Dr. Koch passed away in October 1983.     

Phytoplankton ecology of Loch Ard,Scotland

The results of the ecological investigation discussed in this paper represent the first seasonal study of the phytoplankton to be carried out in Loch Ard. This body of water is a typical warm monomictic lake, with water temperatures during the period of study ranging from 18.6 to 4.4 °C and oxygen saturation from 68 to 112%. Stratification lasted from May to November. Phosphate was generally very low, being almost undetectable for about six months and not exceeding 0.125 µg.at.P 1^–1 even at the winter maximum. Dissolved silica showed a regular seasonal pattern ranging from 9 to 21 µg.at.Si 1^–1. Nitrate, like silicate, never showed signs of complete exhaustion, with a minimum of 4.0 and a maximum of 21.0 µg.at.N 1^–1. The pH remained on the acid side of neutrality while the alkalinity never exceeded 7 mg CaCO₃/1.From an examination of net phytoplankton Loch Ard could be described as diatom-desmid in nature. Quantitatively the population was dominated by diatoms(Cyclotella, Asterionella andMelosira) till May whence a mixed population of blue-green algae, desmids and dinoflagellates persisted till late September.     

On the algal ecology of the lowland Iraqi marshes

An account is given of the algal vegetation in the near boat channels in the central area of the lowland Iraqi marshes. Sampling was carried out in February and September. The water here may be characterized as relatively turbid, slightly saline, highly calcareous and probably typically mesotrophic. Diatoms dominated the phytoplankton, many of the species being ones which are at least as characteristic of benthic communities. Periphyton was more conspicuous in February than September, with filamentous green algae especially common in February. Blue-green algae were more common in September; they frequently formed a felt with several different species and irregular deposition of calcite. Heterocystous blue-green algae were frequent in the periphyton but apparently not so in the plankton. A well-developed laminated community of blue-green algae and purple phototrophic bacteria occurred on the wooden hull of a boat.     

Dynamics and ecological significance of daily internal load of phosphorus in shallow Lake Balaton, Hungary

1. As supported by field data, turbidity recorded by light scattering sensors could reliably be converted into concentration of suspended particulate matter (SPM) and coefficient of vertical light attenuation (K_d) in Lake Balaton. 2. Autocorrelation analysis revealed that proper determination of SPM concentration and K_d required daily sampling. To approximate daily rate of resuspension, 15 min or more frequent measurements were needed. Thus, routine monitoring provides very little insight into environmental variability of shallow lakes as habitats for phytoplankton. 3. The internal P load was estimated from daily rate of resuspension and P desorption capacity of sediments. The latter was assumed to be proportionate to the potentially mobile inorganic P content of SPM. A comparison with net primary production and nutrient status of phytoplankton showed that the proposed method of estimating time series of internal P load captured seasonal trends. 4. The daily rate of resuspension was high whereas that of internal P load was low in Lake Balaton relative to other shallow lakes. The latter reflects favourable behaviour of the calcite‐rich sediments. As a consequence, carrying capacity of Basin 1 of Lake Balaton was P‐determined. 5. The timing of external and internal loads was radically different. While the former showed mostly seasonal changes, large pulses characterised the latter. As a consequence, internal load may supply more P to phytoplankton growth during the critical summer months than external load. However, the relative importance of these sources may show substantial interannual variability. 6. Large resuspension events often followed each other during periods of 10–15 days. It has been shown that disturbances in this frequency range are of key importance in maintaining the diversity of phytoplankton. We propose that resuspension can be perceived not only as a disturbance factor but also as a factor that periodically relaxes nutrient stress. The former feature may dominate the instantaneous effect, whereas the latter may determine the persistent effect of resuspension on succession of phytoplankton.     

Biogenic silica record in the sediments of Little Round Lake,Ontario

The biogenic silica (BSi) record has been determined in the sediments of Little Round Lake, Ontario in order to review its postglacial development and study the relationship between BSi and diatoms. BSi concentrations in the sediment stratigraphy were found to correspond for the most part to the trophic history of Little Round Lake. Calculation of accumulation rates for BSi improved the correspondence of the BSi profile to the trophic history. Thus, BSi is a valuable paleoindicator when concentration and flux profiles are considered concurrently. Regression analysis of BSi and diatoms revealed that the concentration of BSi in the sediments was not a simple function of diatom numbers or biovolume, but that factors such as a correction for the rate of dissolution and abundance of chrysophycean scales and cysts were also important.     

Chemistry and plant ecology of zinc-rich wastes dominated by blue-green algae

An account is given of the chemistry and vegetation of wet areas below the Elvins tailings pile in the Old Lead Belt of Missouri. Zinc in particular is present at highly elevated levels in the water (5.9–21.0 mg l^–1) and most sediments. The two most widespread plants are a blue-green alga,Plectonema gracillimum, and a moss,Dicranella sp. In wetter areas the latter exists predominantly as protonema.Plectonema and the protonema typically grow in very close association, although situations do occur where only one or the other is present. For instance, algal flocs in two permanent ponds consisted largely ofPlectonema and unicellular blue-green algae, with a few eukaryotic algae, but no protonema. Other dominants in the area areEleocharis sp.,Scirpus americanus andTypha latifolia. One seepage was studied in more detail; it showed marked changes in both chemistry and vegetation on passing from source to a site 15 m downstream. Aqueous Zn dropped from 21 to 8 mg l^–1. ThePlectonema-Dicranella protonema community predominated in the upper part, developing a laminated structure up to 3 cm thick (justifying recognition as a stromatolite). Further downstream a white crust including much hydrozincite and with a very high overall zinc content (c. 370 mg g^–1 Zn) was deposited over the green layer, making it endolithic in position, though probably not in origin. Still further downstream the green layer disappeared, leaving only a hard white crust. A discussion is included of the extent to which observed variations in the zinc content of water and sediments below the tailings pile are influenced by living and dead plants. An apparent diel cycle in aqueous zinc at some sites is of particular interest, as it was not matched by changes in Ca or Cd.     

Investigation of a toxic water-bloom of Microcystis aeruginosa (Cyanophyceae) in Lake Akersvatn,Norway

During the summer and fall of 1984 and 1985, the eutrophic Lake Akersvatn in south-eastern Norway, used as reserve drinking water reservoir, was found to produce heavy water-blooms of the colonial blue-green alga Microcystis aeruginosa. Samples of the water-bloom were found to be toxic using the mouse bioassay. No toxin was found free in the water as detected by HPLC and mouse bioassay. The toxic cells (minimum lethal dose 8–15 mg/kg body weight in mice) and purified toxin (minimum lethal dose 50 μg/kg body weight in mice) showed signs of poisoning in laboratory rats and mice identical to that of other hepatotoxin-producing M. aeruginosa blooms and strains reported from other parts of the world. The toxin has chemical properties similar to the cyclic heptapeptide reported for a South African M. aeruginosa toxin. The toxin from Lake Akersvatn bloom material has a molecular weight of 994. The toxic bloom of M. aeruginosa persisted from August to November in 1984 and reappeared in July of 1985. While water from Lake Akersvatn was not used for municipal water supply during this period, the presence of toxic blue-green algae in a drinking water reservoir indicates the need to develop monitoring and detection methods for toxic cells and toxin(s).     

Nutrient control of phytoplankton production in Lake Naivasha,Kenya

Lake Naivasha, a shallow tropical lake in Kenya's Rift Valley, has an unstable water column and is moderately eutrophic. Nutrient (bottom-up) control of primary production is more important than grazing (top-down) control. Experimental nutrient enrichment was used to investigate bottom-up control in more detail. Minor nutrients were not found to be limiting, whilst nitrogen was more limiting than phosphorus with an algal preference for ammonium over nitrate. Sediments form a phosphorus sink but there is hypolimnetic release from the one area showing regular temporary stratification. This indicates that the rate of primary production in the water column could double if conditions change to allow lake-wide nutrient release from sediments. Both external and recycled nutrient regeneration are important.     

Effects of nitrogen,phosphorus and carbon enrichment on planktonic and periphytic algae in a softwater,oligotrophic lake in Florida,USA

The objective of this study was to investigate nutrient limitation of algal abundance in Anderson-Cue Lake, a softwater clear oligotrophic lake in north-central Florida. Nutrient diffusing clay pots and cylindrical enclosures were used in the field to test effects of different combinations of nitrogen, phosphorus, silica, and carbon on algal standing crop and composition of periphytic and planktonic algae, respectively. Effects of nutrient enrichment on periphytic algae were examined in two studies conducted 31 May – 8 July and 10 June – 15 July 1991. Nutrient effects on planktonic algae were examined in one study from 13 June – 1 July 1991. Planktonic and periphytic algal biovolume was significantly higher (p<0.05) when nitrogen and carbon were added in combination than with treatments without nitrogen, carbon, or nitrogen and carbon. Treatments with nitrogen and carbon combined resulted in lower algal diversity and dominance by coccoid green algae andScenedesmus. Results indicate that carbon and nitrogen can be limiting factors to algal growth in Anderson-Cue Lake and possibly other lakes of similar water quality.     

Impact of summer warming on the thermal characteristics of a polymictic lake and consequences for oxygen, nutrients and phytoplankton

1. The impact of long thermal stratification events on some key properties in a polymictic lake was studied by determining the mixing regime of Müggelsee, Germany, using water temperature profiles taken hourly over 4 years. The period included two exceptional summer heatwaves. 2. Long thermal stratification events lasted from about 1 week to 2 months, and exhibited a high variability in thermocline depth and stratification intensity within and between events. 3. During stratification events, hypolimnetic oxygen concentrations strongly decreased while hypolimnetic SRP accumulation increased, depending on the duration and intensity of stratification and on hypolimnetic water temperature. 4. The impact of stratification on the functional phytoplankton composition increased with increasing stratification duration, but was rather different for the heatwaves. 5. Stratification events were followed by strong nutrient pulses into the euphotic zone and intense phytoplankton growth, particularly after the heatwaves. Hence, the influence of the climate extremes counteracted effects of reduced external nutrient loading.     

Effects of lake mixing with an axial flow pump on water chemistry and phytoplankton

This paper describes the effect of total lake mixing with an axial flow (Garton) pump on the limnology and phytoplankton of two Oklahoma lakes.The Garton pump destratified Ham's Lake (40 ha) in 3 days. Except for one small isolated basin, Ham's Lake remained completely destratified for the rest of the summer. Algal biomass declined, numbers of species of green algae increased, but numbers of species of blue-green algae did not decrease as expected. After destratification, pH remained high (> 8), carbonate alkalinity was observed and reactive phosphate was undetectable.An axial flow pump increased the heat content of Arbuckle Lake (951 ha) and caused the lake to destratify about one month earlier than usual. Increasing the heat content of the lake did not affect the concentration of most water quality parameters or the biomass of algae.     

Phytoplankton community structure and succession in the water column of Lake Naivasha,Kenya: a shallow tropical lake

Lake Naivasha is a shallow freshwater lake in the Rift Valley of Kenya. Since the 1980s, when the lake showed a seasonal shift between diatom and cyanobacterial dominance it has become moderately eutrophic. Its algal assemblage is now dominated by a persistent Aulacoseira italica population both numerically and in terms of contribution to overall primary production. Algal and cyanobacterial counts were used to derive Simpson's diversity, succession rate and total community succession, focusing on the 10 most numerically abundant taxa. 170 species were identified, 43 of which were in common with the 143 found in 1979–80, before the increase in trophic state. Most diatoms are indicators of moderate to high nutrient conditions. There is little horizontal or vertical variation in successional processes throughout the lake and although the absolute abundance of cells varies widely, proportional composition is relatively stable. In Crescent Island lagoon, the only regularly stratified site, hypolimnetic succession rates are lower than those in the epilimnion. Overall, community composition is controlled by mixing (and hence light regime) and nutrient availability. With `endless summer' conditions and full mixing, there is a successional pattern of `muted seasonality' adapted to physical instability and environmental stability.     

Nitrogen Fixation During Blooms of Nodularia in Coastal Waters and Backwaters of the Arkona Sea (Baltic Sea) in 1974

Nitrogen fixation was investigated by means of the acetylene reduction method during the development of a water bloom of Nodularia in coastal waters of the Baltic Sea west of the island of Hiddensee and in backwaters showing different degrees of eutrophication. Depending on plankton density, the values found varied greatly. The maximum of nitrogen fixation values found in extremely dense water blooms under special conditions (Baltic Sea, 2250 μg N₂/l · h; Kleiner Jasmunder Bodden, 374 μg N₂/l · h) are up to 10³ times higher than from other parts of the Baltic Sea or from inland waters. The average nitrogenase activity determined for coastal water populations of the Baltic Sea is 2.15 pg N₂/heterocyst · h and that of the inmost backwaters 0.77 pg N₂/heterocyst · h. The relationship between N₂-fixation and nutrient content in water is discussed.     

Phosphorus and Nitrogen Budgets of Barton Broad and Predicted Effects of a Reduction in Nutrient Loading on Phytoplankton Biomass in Barton,Sutton and Stalham Broads,Norfolk, United Kingdom

Barton, Sutton and Stalham Broads are shallow, man-made lakes formed in mediaeval times when peat excavations were flooded. Recently, a once diverse submerged macrophyte flora has, in most broads, been replaced by large phytoplankton populations. This change has been attributed to increased nutrient loadings. The seasonal cycles of chlorophyll α, nitrogen and phosphorus in the three broads are described and the inputs of nitrogen and phosphorus to Barton Broad are budgeted. A reduction in the phosphorus loading is recommended as the best method of restricting phytoplankton populations in these broads. An equation relating phosphorus loading, flushing rate, mean depth and retention coefficient to mean standing phosphorus concentration is used to predict the reduction in phosphorus loading required to limit phytoplankton populations and permit the re-establishment of submerged macrophytes.     

Experimental evidence of the dynamic effect of free-floating plants on phytoplankton ecology

1. There is an increasing concern to understand the role of free-floating plants (FFP) on the functioning and structure of shallow lakes, particularly the mechanisms by which their dominance is self-stabilizing and how they may outcompete phytoplankton.
2. In a field experiment with mesocosms, we simulated three commonly encountered scenarios in warm temperate shallow lakes: FFP dominance, FFP fluctuation and FFP absence. We explored the effects of several key processes, triggered by FFP dynamics, on the composition, diversity and production of phytoplankton, and on the physicochemical conditions.
3. The effects of persistent floating mats on light interception triggered a complex response by the phytoplankton: species were lost and biomass was low, yet high diversity and productivity potential were maintained. A contrasting phytoplankton response characterized mesocosms lacking FFP, where light was sufficient but nitrogen was limiting. Fluctuating FFP cover brought periodic shifts between these two limiting resources for the phytoplankton, which most probably explain the maintenance of richness, diversity and production in these shallow lakes.
4. These results support the recently proposed framework of alternative stable states, providing experimental field evidence of the mechanisms resulting from the shifts between floating plants and phytoplankton dominance.     

Seasonal dynamics of phytoplankton and phytobenthos,and associated chemical interactions,in a shallow upland lake (Malham Tarn,northern England)

Seasonal changes of phytoplankton were followed over 3 years (1985–87) in a shallow, unstratified and calcareous upland lake.The phytoplankton was of low to moderate abundance and generally dominated by phytoflagellates. Seasonality involved a winter minimum of abundance, a spring maximum of diatoms, and often brief increases in summer that included blue-greens, especially the colonial Gloeotrichia echinulata. Some components were of benthic origin. Seasonal growth of the main component of the phytobenthos, Chara globularisvar. virgata, caused a regular summer depletion in lake water of Ca²⁺ and HCO₃ ^- (alkalinity) by associated CaCO₃ deposition, and a more extreme (and unusual) depletion of K⁺. Chemical analysis of Chara biomass and of underlying sediments indicated a large benthic nutrient stock, much surpassing that represented by the phytoplankton. Growth in this biomass, and the magnitude of water-borne inputs, influenced the removals of Ca²⁺, K⁺ and inorganic N. The phytoplankton was probably limited by a low-P medium, to which co-precipitation of phosphate with CaCO₃ may have contributed. A vernal depletion of Si was probably limiting to diatom growth, and appeared to be mainly induced by benthic rather than planktonic diatoms. Examples of long-term change in composition of the phytoplankton and phytobenthos are noted and discussed in relation to the interaction of these components, nutrient enrichment, and possible alternative stable states.     

Phytoplankton periodicity of the Waitaki Lakes,New Zealand

The Waitaki River system in the South Island of New Zealand includes three large glacially-formed headwater lakes, Tekapo, Pukaki and Ohau, which drain into the manmade Lake Benmore. Phytoplankton periodicity was followed from December 1975 to January 1980 as part of a study investigating possible changes in these lakes as a consequence of hydroelectric development. The phytoplankton was highly dominated by diatoms, e.g., Diatoma elongatum, Cyclotella stelligera, Asterionella formosa, and Synedra acus, but in lakes Ohau and Benmore populations of green algae occasionally developed. In all four lakes seasonal phytoplankton periodicity was observed with maximum biomass in spring and summer. In Lake Tekapo, the first lake in the chain, maximum biomass did not exceed 300 mg m^–3, but in the very turbid Lake Pukaki the maximum summer biomass ranged between 300 and 800 mg m^–3. In Lake Ohau, the least turbid lake, maximum biomass was around 1 000 mg m^–3. In the newly created Lake Benmore periodicity was less evident and summer maxima reached over 1 500 mg m^–3. The phytoplankton periodicity in these lakes is greatly influenced by seasonal patterns of turbidity from inflowing glacial silt.     

The role of periphytes in the shift between macrophyte and phytoplankton dominated systems in a shallow, eutrophic lake (Lake Taihu, China)

Based on experiments of periphyte response to different trophic levels and their impact on macrophyte production, it was found that the periphyte biomass increased with the nutrient concentrations. Increased trophic level and periphyte biomass resulted in decreased macrophyte photosynthesis. It was suggested that the periphytes could cause resilience and hysteresis in the system shifts between macrophyte and phytoplankton domination. Other factors, such as fish farming, storm induced waves and mechanical destruction, and high water levels could be the perturbations during the system shifts, but these are not the key factors. Instead, the nutrient loading and periphyte abundance could determine the shift in lake ecosystem between macrophyte and phytoplankton domination. This finding could theoretically elucidate the mechanism of ecosystem shifts between macrophyte and phytoplankton domination.     

Role of phosphorus and nitrogen for bacteria and phytopankton development in a large shallow lake

Nutrient (P and N) enrichment experiments in small enclosures (20 l) were carried out to determine P and/or N limitation of bacterioplankton in Lake Võrtsjärv. The specific interest of the study was to test if it is possible to detect nutrient `physiological' or growth (rate) limitation of bacterioplankton and competition for nutrients (N and P) with phytoplankton in generally nutrient rich lake. Thymidine and leucine incorporation; leucine aminopeptidase, -D-glucosidase and alkaline phosphatase activity, total count of bacteria, chlorophyll a concentration and primary production as well as the concentrations of different chemical forms of N and P were followed during 4–5 days of the experiment. To address the question of the interactions between nutrients, bacterio- and phytoplankton, experimental and seasonal data sets were included in the analyses. Phosphorus (P) had a positive effect on bacterioplankton in enclosure experiments in June 1997; no effects of nutrients were found in September 1996, while in May 1996, P affected mainly the phytoplankton. On the seasonal scale, the development of bacterioplankton was connected to primary production, total phosphorus and temperature. In enrichment experiments, bacterioplankton was mainly related with primary productivity but the possible importance of bacterial grazers could be presumed. Thus, no evidence was found for nutrient growth limitation and/or competition for N and/or P, rather bacterioplankton depended on organic food supply originating from phytoplankton.