Stephanodiscus Astraea Research Articles

Autumn Diatom Variation in Lakes Along Altitude Gradient in the Jiuzhaigou National Park, Sichuan, China*

The Jiuzhaigou National Park in Sichuan,China is also a famous world natural heritage site.The water is the soul of the National Park and the wetland consists of 118 mountain lakes.In order to evaluate the status of the wetland,and to put forward rational advices for the wetland protection,a series of studies on the wetland were conducted.Diatom was used to monitor water quality because of its great sensitiveness to environmental changes.Based on the studies of 17 sampling sites in lakes from 2168 m to 3060 m in a gradient altitude in October ～ December 2007,113 taxa of diatoms(including species and varieties) were observed.The predominant genera in most sampling sites were Fragilaria,Navicula,Cymbella and Gomphonema,but the dominant Cyclotella only in the Long Lake.Two eutrophication-indicating species,Melosira granulata(Ehr.) Ralfs and Stephanodiscus astraea(Ehr.) Grun.,were found for the first time in the lakes of Jiuzhaigou in traveling season,and they disappeared in winter,indicating that human activities had impact on the lakes of Jiuzhaigou,and the ecological system had a strong ability to be recovered by itself.The result of statistical analysis showed that diatom cell density and altitude represented a positive relation as a result of co-effect of environmental factors.The Cairns Sequential Comparison Index(SCI),a simplified method to estimate the relative differences in biological diversity,was high except for the Long Lake,and it had negative relation with altitude and cell density,both resulted from interspecies-competition.But the SCI could not always reflect the quality of the lake water in Jiuzhaigou.Fig 2,Tab 6,Ref 25

Getting a seat at the table

The phytoplankton of an inshore area of Georgian Bay, Lake Huron, affected by artificial inputs of nutrients in treated sewage effluents from local municipalities, was studied during the ice-free periods of 1973 and 1974. The data are treated as background information prior to implementation of a sewage phosphorus removal programme in 1974-75.Ice-free period mean biomass of total phytoplankton ranged from 1.6 mm3/l in Sturgeon Bay to 6.3 mm3/l in Penetang Bay.Diatoms (Bacillariophyceae) were most important throughout the study area being represented mainly by Stephanodiscus astraea, S. invisitatus, Fragilaria crotonensis, F. capucina, Asterionella formosa, Tabel-laria fenestrata and Rhizosolenia longiseta. The cryptomonad Cryptomonas erosa was very abundant in Penetang Bay and comprised between 20 and 60% of total biomass throughout most of the ice-free periods of both years.Based on relationships between average total phosphorus concentration, total phytoplankton density and species composition in other Great Lakes waters and inland lakes of Ontario, a 50% reduction in total phytoplankton biomass is predicted for Penetang Bay following reduced loading of total phosphorus. Additionally, certain Chrysophyceae may become more important in Penetang Bay in the future.

The influence of irradiance, photoperiod and temperature on the growth kinetics of three planktonic diatoms

The influence of irradiance, photoperiod and temperature was determined for the growth kinetics of the diatoms Aulacoseira subarctica, Stephanodiscus astraea and Stephanodiscus hantzschii and the results compared with those of cyanobacteria. Irradiance and photoperiod relationships were qualitatively similar to those for cyanobacteria in that: (1) growth rate (K) was proportionally greater under short photoperiods, with ratios of K under continuous light to K under 3:21 light:dark (LD) cycles of 1·50, 1·80 and 2·96 for A. subarctica, S. astraea and S. hantzschii respectively; (2) at subsaturating irradiances, K was proportional to irradiance and independent of temperature with a negligible predicted maintenance growth rate requirement. Apparent growth efficiencies (GE) at subsaturating irradiances were 0·26±0·03, 0·42±0·03 and 0·50±0·03 divisions mol-1m2 for A. subarctica, S. astraea and S. hantzschii with the values for Stephanodiscus species comparable to values for Oscillatoria species. Under a 3:21 LD cycle at 4 °C, light-saturated growth rates were 0·066±0·004, 0·197±0·033 and 0·285±0·018 divisions day-1 for A. subarctica, S. astraea and S. hantzschii. S. hantzschii growth rate at 4 °C exceeded maximum Oscillatoria growth rates at 23 °C and the S. astraea growth rate at 4 °C was equivalent to O. agardhii growth rate at 20 °C. Temperature increases above 4 °C gave Q10 values between 4 °C and 12 °C of 3·68, 2·39 and 1·92 for A. subarctica, S. astraea and S. hantzschii, but higher temperatures resulted in minor increases in K. S. astraea growth rate peaked at 16 °C, declining sharply at higher temperatures. February to March in situ growth rates in Lough Neagh, mean temperature 4·3 °C, showed that the A. subarctica in situ K of 0·058 divisions day-1 was close to the laboratory K at 4 °C, but that S. astraea in situ K of 0·101 divisions day-1 was lower than the laboratory K at 4 °C.

OPINION Endless summer: internal loading processes dominate nutrient cycling in tropical lakes

SUMMARY. 1. Fossil diatom assemblages deposited in more than a dozen African lakes roughly 9500 years BP were dominated by a single planktonic species, Stephanodiscus astraea (Ehrcnb.) Grun. (although realistically this is likely to be a species complex). These diatoms flourished when lake‐levels were maximal. Data are included from many of (he large African lakes, and others extending from Lake Abhé0, Ethiopia, to Lake Cheshi, Zambia.2. Because the ecological physiology of Stephanodiscus species is well known one can predict the nutrient regime that must have existed when Stephanodiscus bloomed. Owing to competition for resources Stephano‐discus species dominate when the supply ratio of silicon to phosphorus (in moles) in the epilimnion is relatively low (Si:P∼1). Consequently, lakes dominated by S. astraea are often hypereutrophic.3. We propose a series of hypotheses to explain why tropical lakes have decreasing Si:P ratios as lake‐levels increase, primarily owing to internal P‐loading processes in the epilimnia. These observations appear to contradict present conceptions of the fundamental relationships governing nutrient loadings to and within lakes. Tropical lakes appear to have had increasing epilimnetic phosphorus loading as lake‐levels increased. In contrast, large, deep lakes in the temperate zone are usually oligotrophic, with high Si:P ratios.4. Our major conclusion is that regeneration rates are greater than removal rates for phosphorus in tropical lakes as compared to temperate lakes, especially where epilimnelic mixing exceeds 50 m. Biological control of the elemental cycles dominate in tropical lakes, whereas nutrient cycles in temperate lakes are dominated by physical processes for a large part of the year. This results in major differences in the fundamental mechanisms of nutrient regeneration and their relationships to morphometric features of lakes in the two regions.

Natural community bioassays to determine the abiotic factors that control phytoplankton growth and succession

SUMMARY. 1. The successional patterns of the dominant phytoplankton species in Lake Maarsseveen (The Netherlands) were very similar in 1980 and 1981. In December/January the diatoms Stephanodiscus hantzschii Grun., Stephanodiscus astraea (Ehr.) Grun. and Asterionella formosa Hass. dominated the algal community (A. formosa had several further population increases during the year). Fragilaria crotonensis Kitt. came to dominance in March/April, followed by the chrysophyte Dinobryon divergens Imhof and the diatom Cyclotella comta (Ehr.) Kütz in May/June. A second appearance of D. divergens was observed in July/August, followed in 1980 by F. crotonensis and a third small increase of D. divergens. In both years S. astraea and S. hantzschii started to grow again in November/December. Cryptophyceae and Chlorophyta were present throughout the year, but did not show a distinct succession.2. Natural community bioassays, performed under natural light and temperature conditions in a newly developed outdoor bioassay apparatus, showed that phosphate was the major nutrient limiting the growth rate of the phytoplankton. From January till June, during the decline in phosphorus concentration, the diatoms became successively phosphate limited in the sequence: S. hantzschii, S. astraea, F. crotonensis, A. formosa and C. comta. Light limitation was probably the major cause of the relatively late start of F. crotonensis in early spring.3. D. divergens, increasing after the diatoms from June till September, was stimulated by the addition of a chelator (EDTA). The chelator might stimulate the formation of trace metal species favouring their uptake (e.g. iron).4. The patterns of succession of the diatoms observed from January till June and from July till December were to a large extent symmetrical. The controlling factors followed opposite trends: declining phosphorus concentrations with increasing irradiance from winter till spring and increasing phosphorus concentrations with decreasing irradiance from summer till late winter.

The diatom stratigraphy and palaeoecology of lake Ohrid, Yugoslavia

An 8.85 m core (No. 9) was taken from Lake Ohrid, Yugoslavia, in August 1973 at a depth of 210 m, approximately 6 km offshore of Gorcia, Yugoslavia. The core was sampled at 10-cm intervals and analyzed for moisture, organic and CaCO 3 content, and diatoms. There is a slight but steady decrease of moisture with depth, probably due to sediment compaction. The organic content showed little variation throughout the core ( x ̄ = 2.73 ± 0.39% of total weight). The CaCO 3 content from 885−110 cm depth also showed little change ( x ̄ = 2.32 ± 0.55% of total weight) but increased in the top meter of the core (100−0 cm, x ̄ = 11.03 ± 8.86% of total weight). Throughout the core there is an inverse relationship between organic and CaCO 3 content, particularly in the top meter. Extrapolation to the bottom of the core using linear regression based on 14C-dating yields a date of ca. 30,000 years B.P. Results indicate that Lake Ohrid has remained an oligotrophic lake throughout the period represented by Core 9. Diatom analysis shows one persistent group of diatoms throughout the core. This group consists of Cyclotella fottii, C. hustedtii, and C. hustedtii var.1 and is referred to as the Cyclotella complex. Between 885−650 cm (ca. 30,000−20,000 yr B.P.) there is a sharp increase in numbers and a slight increase in diversity of species. This may correspond with the beginning of the warmer, more humid middle Pleniglacial. Between 650−100 cm (ca. 20,000−10,000 yr B.P.) the Cyclotella complex appears usually alone in small number. There are a few other scattered species in this section, mainly Opephora martyi and Fragilaria pinnata. This section of the core probably corresponds with the cold upper Pleniglacial of the Würm (Weichselian). Between 100−0 cm (ca. 10,000−0 yr B.P.) there is the largest increase in number and diversity of species, particularly above 60 cm. The Cyclotella complex, C. ocellata, Cyclotella sp.1, Stephanodiscus astraea var. intermedia fo.11 and Stephanodiscus sp.1 are dominant. This is the only section of the core in which Stephanodiscus species occur, and probably corresponds with the Holocene.

The effect of fungal parasitism on the succession of diatoms in Lake Maarsseveen I (The Netherlands)

SUMMARY. During late winter and early spring, diatoms are predominant in Lake Maarsseveen I and show a succession in blooming. To elucidate the role of parasitism in this succession, samples were taken once a week in 1978, 1979and 1980 and examined for fungal parasites, further, factors which presumably influenced the phytoplankton growth. including temperature and chemical compounds, were studied. Parasites were found on Asterionella formosa Hass., Stephanodiscus astraea (Ehr) Grun. and Cyclotella comta (Ehr) Kütz. The chytrid fungus Zygorhizidium planktonicumCanter was parasitic tin Asterionella formosa. This was the only case where high infection percentages were registered. The developmental phases of Zygorhizidium plunktonicum on Asterionella formosa were studied. The fungus was capable of affecting the spring increase and bringing about a premature end to the bloom. Only when the fungus was temporarily inhibited in its parasitic activity (this phenomenon coincided with periods of frost) was A. formosa able to reach a high abundance. Severe parasitism on A. formosa favoured the development of other diatoms (mainly of Fragilaria crotoriensis Kitt., Stephanodiscus hantzschii Grun. and Stephanodiscus astraea).Host‐parasite interrelationship studies performed in the laboratory showed that temperature can be an important environmental factor associated with epidemics of Zygorhizidium plankiomcum on Asterionella formosa. At very low temepratures (1.5. 1) the fungal activity was inhibited while Asterionella fortnosa still grew well. However, at 5, 10 and 18 the fungus manifested a high infection rate and was able to overtake Asterionella.

Inter-species variability in the consumption of algae by oligochaetes

The importance of algae in the diet of the oligochaetesArcteonais lomondi, Uncinais uncinata andLimnodrilus claparedeianus was determined from May 1978 to April 1979 in two lakes located in northern Canada. The lakes were small (130–157 ha), shallow, (<15 m) and mesotrophic. During summer, algae accounted for up to 57–75% by volume of ingested matter inA. lomondi andU. uncinata depending on lake whereas in winter the corresponding range was 10–22%. Dominant algae found in the gut included the diatomsSurirella robusta, Diploneis puella, Amphora ovalis, Stephanodiscus astraea, Fragilaria construens andAchnanthes minutissima. Although the same taxa were ingested byL. claparedeianus, algae always represented <5% by volume of the gut contents. This possibly reflects subsurface feeding in an area where algae were rare. The length distribution of ingested diatoms, ranging from 9 to 250 μm, was similar regardless of species in both lakes. Hence, there was no partitioning of algal food resources among oligochaetes.

Silica Budgets and the Ecology of Planktonic Diatoms in an Unstratified Lake (Lough Neagh, N. Ireland)

AbstractRiver inputs of dissolved silica into a shallow eutrophic lake (Lough Neagh, N Ireland) are quantified. Exports of silica are high (3.6 to 9.6 tonnes dissolved SiO2 km−2y−1), but the spring concentration of dissolved silica in the lake depends on release of silica from the sediments. Two planktonic diatoms Melosira italica subarctica O. MÜLL, and Stephanodiscus astraea (EHR.) GRUN. dominate the spring flora. Their periodicity is described over ten years and their usage of silica and phosphorus is examined. Data are presented on the resuspension of cells from the sediment, the growth rates of the diatoms in the lake, the incidence of parasites and changes in the filament length of M. italica subarctica.

Seasonal Distribution of Phytoplankton in Yellowknife Bay, Great Slave Lake

AbstractThe factors influencing the seasonal distribution of phytoplankton were determined between July 1975 and December 1977 in Yellowknife Bay, Great Slave Lake. The initiation of the spring bloom occurred during April of each year in response to changing light conditions and despite the 0 °C water temperatures. Although the plankton showed some fluctuations in density (100–400 mg/m3) during the summer, there was no vernal decrease in the size of the populations. The end of the growing season was characterized by a sharp drop in numbers near the end of October. Multiple regression analysis indicated that changes in the standing crop of the plankton during the summer were primarily related to temperature (r = 0.60) and the concentrations of NO3–N (r=–0.74) and SiO2 (r=–0.82). Changing light conditions probably had little effect on the population during the growing season.Chlamydomonas lapponica was dominant during April of each year. While the growth of this species caused a reduction in the concentrations of total phosphorus and NO3–N, the level of SiO2 remained constant and consequently diatoms (Asterionella formosa, Stephanodiscus astraea, Diatoma tenue var. elongatum and Melosira islandica) waxed rapidly during May. By the first of June, nutrient concentrations were low, thereby permitting the development of Dinobryon bavaricum and Dinobryon cylindricum. The extremely rapid decrease in the densities of common species during the fall was probably due to the rapid decline in temperature and light levels and the formation of ice on the bay.

Paleolimnology of a Late Pleistocene-Holocene Crater Lake from the Golan Heights, Eastern Mediterranean

ABSTRACT A deep sediment core from Birket Ram, a Late Pleistocene-Holocene crater lake in the Golan Heights, was examined. The sediments rest on volcanic rocks at 81 m depth, and represent entire lake history. The core consists of 2 major zones: A lower zone (81-42.5 m) composed principally of diatomite and an upper zone consisting mainly of fine grained, clastic sediments. The diatoms in the diatomite zone include exclusively centric, planktonic forms, indicating a rather deep, quiet, stratified, slightly alkaline lake, eutrophic (Stephanodiscus astraea) for most of the time, with some oligotrophic (Cyclotella ocellata) intervals. Most of the diatomite is varved, with couplets of an average thickness of 0.6 mm, consisting of dark diatom laminae alternating with calcite laminae. The major clay mineral in the elastic zone is a nontronitic smectite similar to the clay alteration product of some pyroclastic layers found intercalated amidst the diatomite zone. Microgranular (6-63 µm) siderite is present in the 32.5-21 m section of the zone, close to an organic C enrichment at 21-20.5 m. Mainly pennate, benthic diatom forms were identified in this zone. This composition is interpreted as indicating a shallow, at times marshy phase of the lake. Acidophilous diatom species at 42.5-32.5 m depth suggest a neutral or even acidic water body introducing the shallow water phase. From a radiocarbon dating at 24.5 m, giving 29.300 + 4000 Y.B.P., an average sedimentation rate of 0.8 mm/y for the elastic sediments was calculated. The formation of the lake is assumed to ave taken place about 100.000 Y.B.P., the transition from the deep into the shallow phase, possibly indicating a climatic shift, about 50.000 Y.B.P.

Seasonal size changes in certain diatoms and their possible significance

Cell sizes of two species of diatoms, Stephanodiscus astraea and Asterionella formosa, were determined in algal populations in four water bodies at various times of the year. Changes in the average cell diameter or length were observed throughout the growing seasons. The possible effects of these cell size changes are discussed in relation to parameters such as rate of sinking, nutrient absorption, temperature, viscosity and turbulence.

The spring development of phytoplankton in Lake Erken

SUMMARY. In Lake Erken there is a regular phytoplankton peak at ice‐breakup in the spring. Pechlaner (1970) studied the spring outburst of 1960. We carried out a similar study in 1973, when the climatic factors were completely different, with only 10 cm of ice and almost no snow. Thus, due to light penetration, there was a high phytoplankton biomass under the ice as early as March. Stephanodiscus astraea (Ehr.) Grun. and Asterionella formosa Hass. dominated the biomass during the spring development in 1973 instead of the normally dominating S. hantzschii v. pusillus Grun.The nutrient decrease was slower in 1973 than in 1960. A comparison of carbon assimilation and the decrease of nitrate and phosphate indicate sufficient nutrient supply in 1973. In 1960, the population grew logarithmically until the nutrients became limiting. In 1973, the average biomass was greater, but the mean probable primary production was only half that of 1960. From the activity coefficients for the 2 years it is also clear that, in 1960, S. hantzschii v. pusillus was highly productive during its short but intensive growth period. Its minute size and high surface/volume ratio enabled it to take up nutrients rapidly and efficiently. The chlorophyll content of S. astraea in 1973 was only half as much as that of S. hantzschii v. pusillus in 1960. Since the light conditions under the ice in March 1973 were more unfavourable than during the exponential phase of 1960, production was much lower.

Phytoplankton of an Inshore Area of Georgian Bay, Lake Huron, Prior to Reductions in Phosphorus Loading

The phytoplankton of an inshore area of Georgian Bay, Lake Huron, affected by artificial inputs of nutrients in treated sewage effluents from local municipalities, was studied during the ice-free periods of 1973 and 1974. The data are treated as background information prior to implementation of a sewage phosphorus removal programme in 1974-75. Ice-free period mean biomass of total phytoplankton ranged from 1.6 mm 3/l in Sturgeon Bay to 6.3 mm 3/l in Penetang Bay. Diatoms (Bacillariophyceae) were most important throughout the study area being represented mainly by Stephanodiscus astraea, S. invisitatus, Fragilaria crotonensis, F. capucina, Asterionella formosa, Tabel-laria fenestrata and Rhizosolenia longiseta. The cryptomonad Cryptomonas erosa was very abundant in Penetang Bay and comprised between 20 and 60% of total biomass throughout most of the ice-free periods of both years. Based on relationships between average total phosphorus concentration, total phytoplankton density and species composition in other Great Lakes waters and inland lakes of Ontario, a 50% reduction in total phytoplankton biomass is predicted for Penetang Bay following reduced loading of total phosphorus. Additionally, certain Chrysophyceae may become more important in Penetang Bay in the future.

Copper dispersal in a water-supply reservoir

Granular (“diamond”) copper sulfate was applied for algal control to the surface of Hoover Reservoir, Franklin Co., Ohio. Soluble and particulate Cu 2+ concentrations were measured by atomic absorption spectrophotometry at several depths for up to 96 hr after application. The soluble Cu 2+ concentration decreased to near baseline values in 2 or 6 hr when 0.2 or 0.4 g Cu 2+/m 2 was applied was applied respectively. Most of the copper sulfate dissolved in the first 1.75 m of the water column and only 2% of the total copper sulfate reached a depth of at least 4.5 m. A concentration of 0.4 g Cu 2+/m 2 controlled a diatom bloom consisting primarily of Melosira spp., Stephanodiscus astraea var. minutula and Asterionella formosa. The largest decrease in dry weight and algal cell numbers was observed 24 hr after copper sulfate treatment.

Factors influencing phytoplankton growth and succession in Farmoor Reservoir

SummaryWeekly observations were made at the Thames Water Authority's Farmoor reservoir during the development of the spring bloom of phytoplankters from November 1973 to March 1974. Counts were made of the phytoplankters, their fungal parasites and the zooplankters. Analyses were made of those chemical parameters which it was thought might exert an influence on phytoplankton growth. Physical and meteorological measurements were also made.The development and decline of four phytoplankton populations were observed. The growth of Asterionella formosa Hass. was interrupted in December when 43·5% of the cells were parastiized by Zygorhizidium affluens Canter. Stephanodiscus astraea (Ehr.) Grün. and Stephanodiscus hantzschii Grün. were not similarly affected. All three diatoms were apparently limited finally by silica depletion. The desmid, Closterium peracerosum Gay. became 66% parasitized by an undescribed chytrid and declined rapidly.Light, turbulence, zooplankton grazing and nutrients other than silica appeared to be unimportant.

Das Sediment des Kummerower Sees. Untersuchungen des Chemismus und der Diatomeenflora

The Sediment of Lake Kummerow. Investigations on the Chemism and the Diatom FloraThe paper deals with the distribution of chemical parameters and of diatoms in the deposits of the eutrophic Lake Kummerow (GDR), produced during about the last 3500 years. The sediment consists of a calcareous gyttja with a very low content of sand and clay. The results indicate a relatively constant trophic state of the lake in former times and an increased production within the last five centuries. Stephanodiscus astraea and its variety minutula is by far the most constant diatom species; in the uppermost parts of the sediment small sized planctonic taxa are increasing, in the main Stephanodiscus hantzschii var. pusillus, Melosira granulata var. angustissima, Asterionella formosa and Synedra acus var. angustissima and var. radians.

The recent diatom history of Loch Leven, Kinross

SummaryDiatoms in the top metre of lake sediment have been studied for indications or recent changes in the environment of Loch Leven. The diatoms indicate a eutrophic, alkaline lake with Melosira italica ssp. subarctica, Tabellaria Flocculosa and Stephanodiscus astraea the dominant taxa at the base of the 1‐m section. There is then an appeance of Melosira ambigua and Stephanodiscus hantzschii suggesting a richer environment. Post A.D. 1816 changes in taxa suggest possible correlation with the lowering of lake level in 1830 and the dominance of Diatoma elongatum, Cyclotella pseudostelligera, Stephanodiscus astraea var. minutula, S. tenuis, S. hantzschii, and Synedra rumpens and var. soctica in zone 4 agrees with present‐day plankton records.

THE DIATOM FLORA OF THE HOXNIAN DEPOSITS AT MARKS TEY, ESSEX

SummarySamples from cores taken in the central and marginal areas of Hoxnian lacustrine beds at Marks Tey have been analysed for diatoms. Diatoms are extremely scarce in samples from the marginal areas. Non‐planktonic diatoms dominated the early parts of the interglacial. The change to planktonic dominance took place during the early‐temperate zone HoII. Stephanodiscus astraea var. minutula was the leading planktonic diatom. No diatoms have been found in deposits of post‐temperate age (zone HoIV), nor in the succeeding glacial deposits (Gipping Early‐glacial zone). In the Lowestoftian Late‐glacial deposits diatoms are scarce and corroded.

An investigation of the distribution in time and space of the algae of a British water reservoir

i. The results of a continuous investigation of the algal flora of a water reservoir over a period of two years are given. The flora includes a littoral community (chiefly represented by Cladophora glomerata), the phytoplankton, and a number of bottom-living species. ii. The plankton includes a considerable number of species which are typical of the pelagic plankton of lakes, but there are in addition a number of species which are essentially littoral in their usual occurrence (e.g. Pediastrum Boryanum, Scenedesmus spp., Nitzschia acicularis). Many phytoplankton species are derived from the supply water of the River Thames (e.g. Staurastrum paradoxum, Stephanodiscus astraea, Fragilaria spp., Microcystis aeruginosa), but other important forms are lacking in the river and must have been introduced from other sources (e.g. Quadrigula closterioides, Coelastrum microporum, Anabaena spp., Aphanizomenon flos aquae). iii. The only perennial forms in the plankton were Cryptomonas ovata, Pediastrum Boryanum, Fragilaria crotonensis and Asterionella formosa. Carteria globosa, Staurastrum paradoxum, Anabaena spp. and Aphanizomenon flos aquae produced spores, but the majority formed no obvious resting stages and no doubt persisted as occasional individuals. iv. Evidence is given for the view that Scenedesmus spp., Coelastrum microporum, Fragilaria capucina, Nitzschia acicularis and Anabaena spp. persisted from one period of occurrence to the other in the silt. The same may be true of Carteria globosa, the species of Pediastrum, Oocystis and Closterium, Planktosphaeria gelatinosa, Staurastrum paradoxum, Melosira granulata var. angustissima, Fragilaria crotonensis, Synedra acus, Microcystis aeruginosa and Aphanizomenon flos aquae, but the evidence is not so satisfactory. v. The plankton differed very markedly in the two years, in 1937 including numerous Chlorophyceae, in 1938 consisting largely of diatoms and Myxophyceae. These differences can be correlated with the closure of the inlet to the reservoir in March 1937 which resulted in marked changes in the chemical components of the water. vi. The periodicticy shown by the algal flora can, in part, be correlated with changes in the amount of light, and the concentrations of nitrogen, phosphate, and silica in the water. In winter diatoms were conspicuous, although the dominant species were different in the three winters. The diatoms attained their maximum growth in spring, the important species in 1937 being Synedra acus and Nitzschia acicularis, in 1938 Stephanodiscus astraea and Asterionella formosa. In the summer of 1937 Chlorophyceae were conspicuous and were followed by Myxophyceae, but in 1938 the former were rare, and the Myxophyceaen phase succeeded immediately upon that of the pelagic diatoms. During the summer the nitrate content was negligible, while phosphates and silica increased. The autumn phase, characterised by the presence of a few diatoms and desmids, accompanied an increase first of ammoniacal nitrogen and later of nitrate nitrogen. vii. The Chlorophyceae (apart from desmids) of the plankton appear to require higher nitrogen and lower prosphate contents than do the diatoms, while the blue-green algae attain their chief development when the concentration of phosphate is high and the amount of inorganic nitrogen negligible.