Organic Carbon Flux to a Large Salt Lake: Pyramid Lake,Nevada, USA

Energy flux to a large, deep, salt lake from phytoplankton, periphyton and macrophyte primary production as well as fluvial transport and wind-transported terrestrial vegetation and dust were quantified. Average areal phytoplankton net photosynthesis was 511 mg C m⁻² d⁻¹. Highest rates were during water-blooms of the bluegreen alga, Nodularia spumigena. Although areal daily net photosynthesis by periphyton in Pyramid Lake was comparable to other salt lakes, annual carbon influx by periphyton was small due to the lake's graben morphology and moderate euphotic depth (mean, 11.9 m). Macrophytes were uncommon and, therefore a minor source of energy. Truckee River is the only major fluvial discharge to Pyramid Lake and dissolved organic carbon was the principal organic carbon fraction in river water. Large upstream water diversions coupled with several drought years resulted in an average fluvial organic carbon load of only 7.3 g Cm⁻²y⁻¹ or 4% of median phytoplankton net photosynthesis. Tumbleweeds were the most common terrestrial plant material observed in Pyramid Lake comprising a maximum projected importance of 6% of total annual carbon input. Windborne dust represented < .1% of annual carbon input. Phytoplankton primary production is the predominant energy source to Pyramid Lake, accounting for over 80% of annual carbon influx. The relative magnitude of autochthonous and allochthonous vectors to the annual carbon budget of this desert salt lake are comparable to those of the few other large lakes for which detailed energy input budgets have been calculated.     

Long-term phytoplankton changes in a shallow hypertrophic lake,Albufera of Valencia (Spain)

A long-term phytoplankton study was carried out in the Albufera of Valencia, a shallow hypertrophic lake (surface area 21 km², mean depth 1 m, total inorganic nitrogen load 155 g m^-2 y^-1, total inorganic phosphate load 15 g m^-2 y^-1) from 1980 to 1988. The lake functions as a reservoir for the surrounding rice cultivation. From 1940's to 1988, its phytoplankton assemblage has been altered from a mesotrophic to a hypertrophic character, as consequence of the increasing pollution. For 1980–88, annual variations in the phytoplankton were less pronounced than seasonal changes. The hypertrophic and morphometric features of the lake favoured the stability of the phytoplankton assemblage and chlorophyll a levels during the study period. Seasonal and horizontal distribution of the total phytoplankton abundance and biomass were highly influenced by the hydrological cycle of the lagoon. Compared with other shallow nutrient rich lakes, the Albufera of Valencia is similar to the shallow hypertrophic lakes of the Netherlands.     

Lake Muzahi,Rwanda: limnological features and phytoplankton production

Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y⁻¹ in the fifties vs 30 T y⁻¹ in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 μS cm⁻¹ at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NH _inf4 ^sup+ -N can be high. Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m⁻² in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O₂ m⁻² d⁻¹ with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d⁻¹ for a mixed layer of 4 m) In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha⁻¹ y⁻¹, the yield estimated from primary production ranges between 46 and 64 kg ha⁻¹ y⁻¹. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.     

Secondary production of crustacean zooplankton and biomass of major rotifer species in Lake Bosumtwi/Bosomtwe,Ghana, West Africa

Studies have shown a strong linkage between zooplankton and fisheries' potential in tropical lakes. High zooplankton production provides the basis for fish production, but knowledge of zooplankton production dynamics in African lakes is extremely limited. Crustacean zooplankton production and the biomass of dominant rotifers in Lake Bosumtwi were assessed over a 2‐year period. The crustaceans comprised an endemic and extremely abundant cyclopoid copepod, Mesocyclops bosumtwii and the cladoceran Moina micrura. Mean standing stock of the crustaceans was 429 mg dw m^?3, whilst annual production averaged 2.1 g dw m^?3 y^?1. Production doubled from 1.4 g dw m^?3 y^?1 in 2005 to 2.8 g dw m^?3 y^?1 in 2006. Copepods accounted for 98.5% of crustacean production. The biomass of the dominant rotifers Brachionus calyciflorus and Hexarthra intermedia was less than 1% of total zooplankton biomass. Daily turnover rate and turnover time of the crustaceans was 0.19 day^?1 and 6.2 days respectively. Crustacean production yielded no statistical relationship with phytoplankton biomass. Production was well within the range of tropical lakes. Peak crustacean production synchronized maximum rainfall, lake mixing and phytoplankton production. Most importantly, no one year's set of dynamics can be used to characterize zooplankton production in the lake.     

Assessments of primary and bacterial production in three large mountain lakes in Alberta,Western Canada

Preliminary studies on production by phytoplankton and bacteria in three large mountain lakes in Alberta, Canada (two in Waterton Lakes National Park and one in Jasper National Park) were concluded mainly through the use of the ¹⁴C technique. The main experiments were conducted in August, 1974, and some were repeated in August, 1975. Net primary production rates varied little from 1974 to 1975, even though there were drastic changes in the phytoplankton composition. Production in the largest lake (max. depth 135 m; mean phytoplankton production 206.5 mgC.m⁻².d⁻¹) was approximately twice that for each of the two smaller lakes (max. depths 19 and 27 m: average phytoplankton production 109 mgC.m⁻².d⁻¹). Bacterial production estimates averaged 3.8 times those for the phytoplankton production, after a proportionalety large error in the dark-uptake technique was subtracted. High production rates in the largest lake are probably due to enrichment. Bacterial production rates are comparable to those in smaller oligotrophic lakes in Europe.     

Whole-lake food-web manipulation as a means to study community interactions in a small ecosystem

Whole-lake food-web manipulation was carried out in the hypertrophic Lake Zwemlust (The Netherlands), with the aim of studying the effects on the lake's trophic status and to gain an insight into complex interactions among lake communities. Before manipulation this small (1.5 ha) and shallow (1.5 m) lake was characterized byMicrocystis blooms in summer and high chlorophyll-a concentrations were common (ca. 250 μg 1⁻¹). In March 1987 the planktivorous and benthivorous fish species in the lake were completely removed (ca. 1000 kg ha⁻¹), a new simple fish community (pike and rudd) was introduced and artificial refuges were created. The effects of this manipulation on the light climate, nutrient concentrations, phytoplankton, zooplankton, fish, macrophytes, and macrofauna were monitored during 1987, 1988 and 1989. Community interactions were investigated in phytoplankton bioassays and zooplankton grazing experiments. After the manipulation, despite the still high P and N loads to the lake (ca. 2.2 g P m⁻² y⁻¹ andca. 5.3 g N m⁻² y⁻¹), the phytoplankton density was low (Chl-a<5μg l⁻¹), due to control by large-sized zooplankton in spring and N-limitation in summer and autumn. A marked increase in the abundance of macrophytes and filamentous green algae in 1988 and 1989, as well as N loss due to denitrification, contributed to the N limitation of the phytoplankton. Before manipulation no submerged macro-vegetation was present but in 1988, the second year after manipulation, about 50% of the lake bottom was covered by macrophytes increasing to 80% in 1989. This led to substantial accumulation of both N and P, namely 76% and 73% respectively of the total nutrients in the lake in particulate matter. Undesirable features of the increase in macrophytes were: 1) direct nuisance to swimmers; and, 2) the large scale development of snails, especiallyL. peregra, which may harbour the parasite causing ‘swimmers' itch’. But harvesting of only about 3% of the total macrophyte biomass from the swimmers' area, twice a year, reduced the nuisance for swimmers without adversely affecting the water clarity.     

Phytoplankton Primary Productivity and Population Efficiency Studies in a Prairie-Parkland Lake Near Edmonton,Alberta, Canada

The standing crop and primary productivity of a small eutrophic, prairie-parkland lake were measured. In general, both standing crops and primary productivity were large, 29.4 and 73.09 mg chlorophyll a m⁻³ and m⁻² and 78.71 and 196.77 mg C hr ⁻¹m⁻³ and m⁻² respectively. Productivity decreased with increasing depth, therefore, decreasing light intensity. Relations between productivity and chlorophyll a content, productivity and light intensity, phytoplankton productivity efficiency and light intensity, productivity and water temperature were investigated, as was the photosynthetic index. Experiments designed to determine the photosynthetic capacity of the phytoplankton distinguished between actively growing and senescent populations. The latter were present during the winter ice cover.     

Primary productivity and trophic status of a Kashmir Himalayan Lake

The paper deals with general limnology, phytoplankton production and trophic status of a Himalayan lake of Kashmir. The primary production as measured by in situ ¹⁴C istope technique ranged from 36 to 906 mgC.m^–2 d^–1 with characteristic seasonal variation. The photosynthetic efficiency was from 0.02 to 0.48% during the same period. The yearly (1975–1976) production was estimated at 134 gC.m^–2 m^–2, which shows that the lake has not yet reached a high level of eutrophication. The waterbody has been classified as mesotrophic.Formed part of thesis for which Ph. D. was awarded to MAK by Kashmir University, India     

Population dynamics,production and angling catch of brown trout,Salmo trutta,in a mature upland reservoir in mid-Wales

Synopsis The brown trout in Llyn Frongoch, a mature upland reservoir, and its nursery stream was sampled during 1983. The stream stock consisted largely of the 1983 and 1982 year classes, with fish reaching mean lengths of 7.0 and 11.6 cm at one and two years of age. The size and biomass of the stream stock at the beginning of 1983 and 1984 were estimated to be 120 and 125 (1.20 and 1.25 fish m^–2) and 1.41 and 0.69 kg (14.1 g m^–2 and 6.9 g m^–2) respectively. Annual stream production ranged from an estimated minimum of 2.49 kg (24.9 g m^–2) to an estimated maximum of 4.59 kg (45.9 g m^–2). Both downstream and upstream movements of 0+ juveniles were recorded. The adult spawning stock was estimated at 79 males and 32 females, a sex ratio of 2.5:1, with most spawners belonging to the 1980 yearclass. The average size of the lake stock over the year was estimated to be 1 650 (229 fish ha^–1) or 250.8 kg (34.8 kg ha^–1). The 1980 yearclass was predominant; there were few fish older than five years. Seasonal variations in netting catches suggested movements to and from the littoral region. Growth in the lake was moderately fast, with fish reaching mean lengths of 21.7 and 27.2 cm by three and four years of age. Fish entering the lake after one year appeared to grow faster than fish which remained in the stream for two years. Annual production in the lake was estimated at 136.7 kg (19.0 kg ha^–1). The total angling catch for the season was estimated to be 62.6 kg (8.7 kg ha^–1).     

A 2-year experimental study on nutrient and predator influences on food web constituents in a shallow lake of north-west Spain

1. A 2‐year study was carried out on the roles of nutrients and fish in determining the plankton communities of a shallow lake in north‐west Spain. Outcomes were different each year depending on the initial conditions, especially of macrophyte biomass. In 1998 estimated initial ‘per cent water volume inhabited’ (PVI) by submerged macrophytes was about 35%. Phytoplankton biomass estimated as chlorophyll a was strongly controlled by fish, whereas effects of nutrient enrichment were not significant. In 1999 estimated PVI was 80%, no fish effect was observed on phytoplankton biomass, but nutrients had significant effects. Water temperatures were higher in 1998 than in 1999. 2. In the 1998 experiment, cladoceran populations were controlled by fish and cyanobacteria were the dominant phytoplankton group. There were no differences between effects of low (4 g fresh mass m^?2) and high (20 g fresh mass m^?2) fish density on total zooplankton biomass, but zooplankton biomass was higher in the absence of fish. With the high plant density in 1999, fish failed to control any group of the zooplankton community. 3. Total biovolume of phytoplankton strongly decreased with increased nutrient concentrations in 1998, although chlorophyll a concentrations did not significantly change. At higher nutrient concentrations, flagellate algae became more abundant with likely growth rates that could have overcompensated cladoceran feeding rates. This change in phytoplankton community composition may have been because of increases in the DIN : SRP ratio. Both chlorophyll a concentration and total phytoplankton biovolume increased significantly with nutrients in the 1999 experiment. 4. A strong decline of submerged macrophytes was observed in both years as nutrients increased, resulting in shading by periphyton. This shading effect could account for the plant decline despite lower water turbidity at the very high nutrient levels in 1998.     

Macrophyte-related shifts in the nitrogen and phosphorus contents of the different trophic levels in a biomanipulated shallow lake

Lake Zwemlust, a small highly eutrophic lake, was biomanipulated without reducing the external nutrient loading, and the effects were studied for four years. In this paper we pay special attention to the shifts in relative distribution of nitrogen and phosphorus in the different trophic levels and to the changes in growth limitation of the autotrophs.Despite of the high external nutrient loads to the lake (ca 2.4 g P m^–2 y^–1 and 9.6 g N m^–2 y^–1), the effects of biomanipulation on the lake ecosystem were pronounced. Before biomanipulation no submerged vegetation was present in the lake and P and N were stored in the phytoplankton (44% N, 47% P), fish (33% N, 9% P) and in dissolved forms (23% N, 44% P). P and N contents in sediments were not determined. In the spring and summer following the biomanipulation (1987), zooplankton grazing controlled the phytoplankton biomass and about 90% of N and P were present in dissolved form in the water. From 1988 onwards submerged macrophyte stands continue to thrive, reducing the ammonium and nitrate concentrations in the water below detection levels. In July 1989 storage of N and P in the macrophytes reached 86% and 80%, respectively. Elodea nuttallii (Planchon) St.John, the dominant species in 1988 and 1989, acted as sink both for N and P during spring and early summer, withdrawing up to ca 60% of its N and P content from the sediment. At the end of the year only part of the N and P from the decayed macrophytes (ca 30% of N and 60% of P) was recovered in the water phase of the ecosystem (chiefly in dissolved forms). The rest remained in the sediment, although some N may have been released from the lake by denitrification.In summer 1990 only 30% of the N and P was found in the macrophytes (dominant species Ceratophyllum demersum L.), while ca 30% of N and P was again stored in phytoplankton and fish.     

Characterization of Brackish Coastal Waters of Different Trophic Levels by Means of Phytoplankton Biomass and Primary Production

Investigations on phytoplankton have been carried out in Baltic coastal waters, which are widely separated from the sea. Corresponding to their distance to the sea they differ in their trophic level. Annual phytoplankton production and a mean biovolume of 100-200 g C · m⁻² and 2-10 mm⁻³ · 1⁻¹ respectively were found for the mesotrophic level, of 200-400 g C · m⁻² and 10-20 mm³ · 1⁻¹ for the eutrophic level, and of 400-600 g C · m⁻² and over 20 mm³ · 1⁻¹ for the polytrophic level. In the course of nearly two decades of investigation a slight increase of phytoplankton biovolume, but no increase of primary production was recorded. Some changes in species composition are to be recognized in Greifswalder Bodden, if old literature is compared with recent data. In the Darss-Zingst boddens a shift from Cyanobacteria dominance towards Chlorophyceae dominance has appeared for at least 5 years. This was on account of the disappearance of Gomphosphaeria pusilla, which is thought to be an indicator species of the mesotrophic level.     

Sedimentary Organic Carbon and Nitrogen Sequestration Across a Vertical Gradient on a Temperate Wetland Seascape Including Salt Marshes,Seagrass Meadows and Rhizophytic Macroalgae Beds

Coastal wetlands are key in regulating coastal carbon and nitrogen dynamics and contribute significantly to climate change mitigation and anthropogenic nutrient reduction. We investigated organic carbon (OC) and total nitrogen (TN) stocks and burial rates at four adjacent vegetated coastal habitats across the seascape elevation gradient of Cádiz Bay (South Spain), including one species of salt marsh, two of seagrasses, and a macroalgae. OC and TN stocks in the upper 1 m sediment layer were higher at the subtidal seagrass Cymodocea nodosa (72.3 Mg OC ha⁻¹, 8.6 Mg TN ha⁻¹) followed by the upper intertidal salt marsh Sporobolus maritimus (66.5 Mg OC ha⁻¹, 5.9 Mg TN ha⁻¹), the subtidal rhizophytic macroalgae Caulerpa prolifera (62.2 Mg OC ha⁻¹, 7.2 Mg TN ha⁻¹), and the lower intertidal seagrass Zostera noltei (52.8 Mg OC ha⁻¹, 5.2 Mg TN ha⁻¹). The sedimentation rates increased from lower to higher elevation, from the intertidal salt marsh (0.24 g cm⁻² y⁻¹) to the subtidal macroalgae (0.12 g cm⁻² y⁻¹). The organic carbon burial rate was highest at the intertidal salt marsh (91 ± 31 g OC m⁻² y⁻¹), followed by the intertidal seagrass, (44 ± 15 g OC m⁻² y⁻¹), the subtidal seagrass (39 ± 6 g OC m⁻² y⁻¹), and the subtidal macroalgae (28 ± 4 g OC m⁻² y⁻¹). Total nitrogen burial rates were similar among the three lower vegetation types, ranging from 5 ± 2 to 3 ± 1 g TN m⁻² y⁻¹, and peaked at S. maritimus salt marsh with 7 ± 1 g TN m⁻² y⁻¹. The contribution of allochthonous sources to the sedimentary organic matter decreased with elevation, from 72% in C. prolifera to 33% at S. maritimus. Our results highlight the need of using habitat-specific OC and TN stocks and burial rates to improve our ability to predict OC and TN sequestration capacity of vegetated coastal habitats at the seascape level. We also demonstrated that the stocks and burial rates in C. prolifera habitats were within the range of well-accepted blue carbon ecosystems such as seagrass meadows and salt marshes.

     

Phytoplankton and Nutrients in the River Strymon,Greece

Phytoplankton species composition, biomass, diversity, nutrients and chlorophyll a were studied at monthly intervals from December 1991 to December 1992 in a selected area of the river Strymon. SRP ranged from 53 to 182 μg⁻¹ l⁻¹ and DIN from 265 to 850 μg⁻¹ I⁻¹. Nutrient values do not indicate strong anthropogenic effects. Chlorophyll α ranged from 1.0 to 35.3 μg⁻¹ I⁻¹ and followed the temporal distribution of total phytoplankton biomass. Phytoplankton biomass exhibited maxima in winter – spring and summer (6.8 g m⁻³ in December 1991, 4.8 g m⁻³ in April 1992 and 9.3 g m⁻³ in August 1992) composed mainly of diatoms, chlorphytes, cyanophytes and dinophytes. Nanoplankton was the most important component of phytoplankton biomass (69.5%) revealing increased values in winter and early spring. Phytoplankton diversity ranged from 0.8 to 3.2. The hydrological conditions in the river Strymon seem to be appropriate for the algae to reproduce themselves in the running water and so, to develop as a true potamoplankton. However, significant populations of phytoplankton must have been carried out from the Kerkini reservoir, situated at the north of the sampling station. The phytoplankton species composition and their periodicity in the river resemble those of typical, large, lowland and nutrient – rich rivers of Europe.     

Did the loss of phytoplanktivorous fish contribute to algal blooms in the Mwanza Gulf of Lake Victoria?

Possible causes of the increased algal blooms in Lake Victoria in the 1980s have been disputed by several authors; some suggested a top-down effect by the introduced Nile perch, whereas others suggested a bottom-up effect due to eutrophication. In this article the potential impact is established of grazing by fish on phytoplankton densities, before the Nile perch upsurge and the concomitant algal blooms in the Mwanza Gulf. The biomass and trophic composition of fish in the sublittoral area of the Mwanza Gulf were calculated based on catch data from bottom trawls, and from gill nets covering the whole water column. Estimates of phytoplankton production in the same area were made from Secchi values and chlorophyll concentrations. The total phytoplankton intake by fish was estimated at 230 mg DW m⁻² day⁻¹. The daily gross production ranged from 6,200 to 7,100 mg DW m⁻² day⁻¹ and the net production from 1,900 to 2,200 mg DW m⁻² day⁻¹. Thus, losses of phytoplankton through grazing by fish were about 3–4% of daily gross and 10–12% of daily net phytoplankton production. As a consequence it is unlikely that the phytoplankton blooms in the second half of the 1980s were due to a top-down effect caused by a strong decline in phytoplankton grazing by fish.     

Composition and Productivity of the Benthic Macroinvertebrate Community of a Subtropical Reservoir

Physical-chemical conditions, phytoplankton productivity, community structure and productivity of the macroinvertebrate benthic community were determined during 1976–77 in a subtropical reservoir. Physical-chemical results revealed high nitrate and phosphate concentrations with highest values in the riverine segment. Large phytoplankton populations were present during most of the year. Phytoplankton productivity was high, producing an annual mean of 87 mg C · m⁻³ · h⁻¹ (12 hours light day). High turbidity in the riverine segment limited phytoplankton productivity during winter and spring. Macrobenthos was dominated by chironomids (Chironomus, Procladius, Coelotanypus and Tanypus) and oligochaetes (Limnodrilus). The annual mean benthic population was estimated at 1,626 · m⁻² with a mean dry weight of 0.66 g · m⁻². Mean benthic species diversity was 1.80. A lacustrine-riverine community gradient was revealed. Benthic productivity was 6.8 g · m⁻² · yr⁻¹ (dry weight) with a P: B ratio of 10. A low correlation was observed between benthic and phytoplankton productivity, and between phytoplankton standing crop and benthic macroinvertebrate numbers throughout the reservoir. Algal food supplies had little impact on the benthic community which was composed predominately of species which fed mostly on organic detritus. Stressful conditions caused by low dissolved oxygen concentrations probably inhibited development of the benthic community throughout the reservoir during summer months, while high sedimentation rates limited development in the head waters.     

The Seasonal Dynamics and Trophic Relations of the Plankton Components in Lake Peipsi (Peipus)

The seasonal dynamics of the biomass and production of phyto-, zoo- and bacterioplankton was investigated during the vegetation periods (from May to November) in 1985 and 1986 in the pelagial of the large eutrophic lake Peipsi (Estonia). The average values of productions per vegetation period for the investigation years were as follows: phytoplanktion − 203.5 gC · m⁻²; bacterioplankton − 37.9 gC · m⁻²; filter-feeding zooplankton − 20.6 gC · m⁻² and predatory zooplankton − 1.5 gC · m⁻². The herbivorous zooplankton production constituted 10.1% of primary production. This ratio indicates a direct relationship between zoo- and phytoplankton in the food chain — filtrators are feeding mostly on living algae and the detrital food chain seems of little importance. The dominance of large forms (Melosira sp., Aphanothece saxicola), in the phytoplankton during the major part of the vegetation period is assumed to be a result of high grazing pressure on small algae. Zooplankton grazing was investigated in situ in a specially constructed twin bathometer. Experimental measurements revealed, that zooplanktion presence in the experimental vessel actually stimulated the phytoplankton growth in many cases — the negative grazing values have been registered. That could be caused by the stimulation effect of nutrients (N, P), excreted by the concentrated zooplankton in the grazing chamber, which led to an increase of the nongrazed phytoplankton production. Bacteria have satisfied the zooplankton food requirements on average by 11%. Grazing on bacteria increased, when grazing on phytoplankton was somehow disturbed.     

Restoration of Botshol (The Netherlands) by reduction of external nutrient load: The effects on physico-chemical conditions,plankton and sessile diatoms

The nature reserve Botshol (Utrecht, The Netherlands), consisting of two shallow lakes, ditches and reedland, originated from excavation of peat by man in the 17th century. Up to 1960 Botshol was a clear-water Charophyte lake system. Since the sixties water quality deteriorated and phytoplankton concentrations increased, while the number and dispersion of Chara species decreased. Several restoration measures were attempted to restablish a Charophyte-dominated ecosystem. This paper reports the promising results of this restoration experiment and mentions some complications that arose in restoring the reserve to a less fertile state. The restoration measures have resulted in a sixfold reduction of the external phosphorus load, from 0.6 to 0.1 g m^–2.y^–1, and in a significant reduction of phosphorus levels at all locations. Moreover, the light climate improved and the phyto- and zooplankton compositions changed considerably Unexpectedly, a bloom ofPrymnesium parvum and a fish kill were observed during the last three months of 1990. Despite this fish kill the restoration of the lake is successful so far.     

Contemporary state of the zooplankton in Lake Peipsi

L. Peipsi is one of the richest fish lakes in Europe. Planktivorous smelt dominates in the fish fauna. The abundance of zooplankton fluctuates between 43 600–2241 500 ind m^–3, with the average 974 000 ind m^–3, biomass ranges from 0,09–3,69 g m^–3, with the average 1,86 g m^–3. Since the 1960s the abundance of rotifers has risen considerably while the mean zooplankter weight (B/N) has decreased from 0.005 mg to 0.004 mg. Zooplankton production (herbivores 20.6, predators 1.8, whole zooplankton community 22.4 g C m^–2 per period between May and October) can be considered high. Predatory zooplankton eats on an average 50% of the production of herbivorous zooplankton; about 50% of the whole zooplankton production (P_{Filt + Pred}) reaches fishes. The production of herbivorous zooplankton constitutes 10.1% of primary production. This ratio indicates a direct relationship between zoo- and phytoplankton in the food chain; the detrital food chain seems of little importance. About 6% of phytoplankton energy reaches fishes. The transformation of energy in the food web is efficient. On the basis of zooplankton L. Peipsi can be considered a moderately eutrophic or meso-eutrophic lake.     

Primary Production Characteristics of a Himalayan Lake in Kashmir

Lake Manasbal is the only valley lake in the region with a true thermal stratification during summer. Seasonal and annual rates of carbon fixation in the water column by plankton populations were related to physical and chemical parameters on specific days. The temporal and spatial changes showed the strong influence of nutrients on the primary production. No significant dependence of carbon uptake on either phytoplankton density or biomass was determined. The overall production pattern during the study period was definitely bimodal, with one minor peak in spring (719.3 mgC m⁻² day⁻¹) and a major one in summer (1496.2 mgC m⁻² day⁻¹ in June). The study also revealed that nannoplankton was responsible for 69% of the primary production.