Spring phytoplankton of 54 small lakes in southern Finland

The abundance and species composition of phytoplankton communities were studied rapidly following the spring ice-melt in 54 small Finnish lakes that form a unique mosaic of water bodies. Phytoplankton biomass and cell density varied among the study lakes with a factor 100 between the lowest and highest values. Highest biomass and densities of phytoplankton characterized small ( < 0.05 km²) lakes with moderate or high water colour (> 80 mg Pt l^–1). In contrast, biomass was low in clear-water lakes and lakes where water throughflow was strong. Typically one species dominated most phytoplankton communities, and usually comprised up to about 45% of the total phytoplankton biomass. Two-thirds of the 103 taxa observed were Chrysophyceans and Chlorophyceans. The most common taxa wereChlamydomonas spp. (Chlorophyceae) andCryptomonas ovata (Cryptophyceae).     

Seasonality of phytoplankton in some South Indian lakes

The landscape of South India is dotted with innumerable man-made lakes. They differ vastly in age, physiography, water flow characteristics, chemistry and trophic state, yet maintain a phytoplankton overwhelmingly dominated (43–93%) by blue-green algae; the subdominants are diatoms and/or Chlorococcales and euglenoids. The blue-greens apparently reach them from soils which are known to harbour a rich blue-green flora and several species in common with limnoplankton.South Indian lakes resemble some tropical counterparts in sustaining dense phytoplankton populations all the year round and temperate dimictic ones in showing two annual growth peaks that usually occur in summer (February–May) and the post-monsoon period (October–November), in synchrony with rise in temperature. In the chemically more oligotrophic lakes, the peaks are constituted by Raphidiopsis mediterranea Skuja, Navicula cryptocephala Kütz., Melosira granulata (Ehr.) Ralfs, and others and in hypereutrophic lakes by Microcystis aeruginosa Kütz., Synechocystis aquatilis Sauv., Oscillatoria spp., Burkillia coronata West & West and Euglena acus Ehr. The bimodal seasonality in abundance of phytoplankton reflects in chlorophyll and biomass concentrations although these are not in strict synchrony with each other. At the maxima chlorophyll a and over-dry biomass may rise to 8.5 mg l^–1 and 204 mg l^–1 respectively in highly productive waters. The highest rate of carbon assimilation recorded in such phases is 10.6 g C m^–3 d^–1.     

Interactions between phytoplankton,zooplankton and fish in a shallow,hypertrophic lake: a study of phytoplankton collapses in Lake Søbygård,Denmark

Since 1983 severe phytoplankton collapses have occurred 1–4 times every summer in the shallow and hypertrophic Lake Søbygård, which is recovering after a ten-fold decrease of the external phosphorus loading in 1982. In July 1985, for example, chlorophyll a changed from 650 µg l^–1 to about 12 µg 1^–1 within 3–5 days. Simultaneously, oxygen concentration dropped from 20–25 mg O₂l^–1 to less than 1 mg O₂l^–1, and pH decreased from 10.7 to 8.9. Less than 10 days later the phytoplankton biomass had fully recovered. During all phytoplankton collapses the density of filter-feeding zooplankton increased markedly, and a clear-water period followed. Due to marked changes in age structure of the fish stock, different zooplankton species were responsible for the density increase in different years, and consequently different collapse patterns and frequencies were observed.The sudden increase in density of filter-feeding zooplankton from a generally low summer level to extremely high levels during algae collapses, which occurred three times from July 1984 to June 1986, could neither be explained by changes in regulation from below (food) nor from above (predation). The density increase was found after a period with high N/P ratios in phytoplankton or nitrate depletion in the lake. During that period phytoplankton biomass, primary production and thus pH decreased, the latter from 10.8–11.0 to 10.5. We hypothesize that direct or indirect effects of high pH are important in controlling the filter-feeding zooplankton in this hypertrophic lake. Secondarily, this situation affects the trophic interactions in the lake water and the net internal loading of nutrients. Consequently, not only a high content of planktivorous fish but also a high pH may promote uncoupling of the grazing food-web in highly eutrophic shallow lakes, and thereby enhance eutrophication.A tentative model is presented for the occurrence of collapses, and their pattern in hypertrophic lakes with various fish densities.     

Role of planktonic bacteria in productivity and cycling of organic matter in the Eastern Pacific Ocean

Total number, biomass, production, and respiration of bacterioplankton were measured in oligotrophic, mesotrophic and eutrophic waters of the Eastern Pacific. Total number of bacteria in the upper mixed layer and in the upper thermocline boundary layers varied from 30–60.10³ ml^-1 in oligotrophic waters to 100–400.10³ ml^-1 in mesotrophic waters of fronts and divergences, and to 1–2,5.10⁶ ml^-1 in eutrophic waters of coastal upwellings. Wet biomass varied from 5–10 mg l^-1 in oligotrophic waters, to 50–200 mg l^-1 in mesotrophic waters, and to 1–2 g m^-3 in eutrophic waters. Below the layer of maximum temperature gradient i.e. below 35–50 m, bacterioplankton density decreased 5–10 times. P/B coefficients per day were highest in the oligotrophic surface water ( 1), and lowest in the eutrophic ones (0.2–0.4). In mesotrophic waters they were intermediate (0.4–1.0). the stock of labile organic matter (LOM) accessible to microbial action varied from 0.3 to 1.6 mg Cl^-1. Its highest value occurred in the upwelling area. The stock of LOM does not noticeably decrease from the euphotic zone to a depth of 2 000 m. Its turnover time varied from 5 to 45 days in surface waters, and 30–50 years in deep oceanic waters. The role of bacterioplankton in productivity and in cycling of organic matter in surface — and deep oceanic waters is discussed.     

Autotroph:herbivore biomass ratios; carbon deficits judged from plankton data

A survey on phytoplankton:zooplankton biomass ratios was performed in 342 Norwegian lakes, covering a wide range in lake size and productivity (total phosphorus: 3–246 g l^–1), but with most localities being oligo- to mesotrophic. Mean phytoplankton biomass was 88 g C l^–1, yet with the majority below 50 g C l^–1and a median of 25 g C l^–1. Total zooplankton biomass displayed a mean and median of 37 and 26 g C l^–1, respectively. Cladocerans were by far the dominant group, making up a median of almost 60% of total zooplankton biomass. Total zooplankton biomass as well as that of major aggregated metazoan taxa (cladocerans, calanoid copepods, cyclopoid copepods and rotifers) all showed a positive, but weak correlation with total phytoplankton biomass. These weak correlations suggest that algal biomass per se is a poor predictor of zooplankton biomass. An average phyto-:zooplankton biomass ratio (C:C) of 2.8 (SD±4.7) was found. 30% of the lakes had a phyto-:zooplankton biomass ratio below unity. While there was no correlation between the phyto-:zooplankton biomass ratio with increasing productivity in terms of P concentration, there was a higher biomass ratio in lakes with high fish predation pressure. The low ratio of phyto-:zooplankton biomass suggest major requirements from non-algal sources of C in the zooplankton diet. The need for dietary subsidizing is also supported by the fact that more than 75% of the lakes had algal biomass less than the estimated threshold for net positive growth of zooplankton, although it should be kept in mind that a high share of picoplankton would imply an underestimation of autotroph biomass in these lakes. Since the C-deficiency apparently is most pronounced in oligotrophic systems, it contradicts the view that the detritus pathways plays a predominant role in highly productive systems only, but while the source of detritus probably is mostly of autochthonous origin in eutrophic lakes, allochthonous detritus will be more important in oligotrophic systems.     

Picoplankton photosynthesis and diurnal variations in photosynthesis-irradiance relationship in a eutrophic and meso-oligotrophic lake

The abundance and relative importance of autotrophic picoplankton were investigated in two lakes of different trophic status. In the eutrophic lake, measurements of primary production were performed on water samples in situ and in a light incubator three times during the day whereas for the oligotrophic lake, only one measurement of primary production was performed on water samples in the incubator. Dark-carbon losses of phytoplankton from Lake Loosdrecht were investigated in time series. Cell numbers of autotrophic picoplankton in eutrophic Lake Loosdrecht (3.2 × 10⁴ cells ml^–1) were lower than in meso-oligotrophic Lake Maarsseveen (9.8 and 11.4 × 10⁴ cells ml^–1 at the surface and bottom respectively). In the phytoplankton of both lakes the ratio of picoplankton production increased with decreasing light intensity. In Lake Loosdrecht depth-integrated contribution of picoplankton to total photosynthesis was less than 4%. The P-I-relationship showed diurnal variations in light saturated photosynthesis, while light limited carbon uptake remained constant during the day. Dark carbon losses from short-term labelled phytoplankton during the first 12 hours of the night period accounted for 10–25% of material fixed during the preceeding light period.     

Primary production of phytoplankton and standing crops of zooplankton and zoobenthos in hypertrophic Lake Teganuma

Primary production of phytoplankton and standing crops of zooplankton and zoobenthos were intensively surveyed in Lake Teganuma during May 1983–April 1984. The annual mean chlorophyll a concentrations were as high as 304 µg · l^–1–383 µg · l^–1. The daily gross primary production of phytoplankton was high throughout the year. The peak production rate was recorded in August and September, when blue-green algae bloomed. The annual gross primary production was estimated as 1450 g C · m^–2 · y^–1, extremely high as compared with other temperate eutrophic lakes. Zooplankton was predominantly composed of rotifers. The annual mean standing crop of zooplankton was 0.182 g C · m^–2 around the middle between the inlets and the outlet and was lower than in most other temperate eutrophic lakes. Zoobenthos was mostly composed of Oligochaeta and chironomids. The annual mean standing crop of zoobenthos ranged from 0.052 g C · m^–2 to 0.265 g C · m^–2, the lowest values among temperate eutrophic lakes, which is in contrast to the high primary production.     

Establishment of Orthosiphon stamineus Cell Suspension Culture for Cell Growth

Callus of Orthosiphon stamineus could be induced successfully from petiole, leaf and stem tissues but not roots when cultured on MS medium containing different concentration of NAA (0–4.0 mg l^–1) and 2,4-D (0–2.0 mg l^–1). Highest fresh weight callus production was obtained from leaf explants and those with best friability were obtained on MS medium plus 1.0 mg l^–1 2,4-D plus 1.0 mg l^–1 NAA. Cell suspension cultures were established from these cultures. The appropriate cell inoculum size for the best cell growth was 0.75 g of cells in 20 ml culture medium. Cell suspension culture using MS medium supplemented with 1.0 mg l^–1 2,4-D promoted the best cell growth with maximum biomass of 8.609 g fresh weight and 0.309 g dry weight 24 days after inoculation. Cells that grew in MS medium supplemented with 1.0 mg l^–1 2,4-D reached the stationary growth phase in 15 days as compared to the cells that grew in MS medium supplemented with 1.0 mg l^–1 2,4-D + 1.0 mg l^–1 NAA reached the stationary phase in 24 days. MS medium supplemented with 1.0 mg l^–1 2,4-D was considered as the maintenance medium for maintaining the optimum cell growth of O. stamineus in the cell suspension cultures with 2-week interval subculture.     

Feeding and control of blue-green algal blooms by tilapia (Oreochromis Niloticus)

Outbreak of blue-green algal blooms, with associated unsightly scum and unpleasant odor, occurs frequently in eutrophic lakes. We conducted feeding experiments to study ingestion and digestion of Microcystis aeruginosa by tilapia (Oreochromis niloticus) under laboratory conditions and field testing to reduce Microcystis blooms by stocking tilapia in Lake Yuehu and other eutrophic waters in Ningbo, China between 2000 and 2003. Our results show that tilapia was capable of ingesting and digesting a large quantity of Microcystis. Digestion efficiency ranged from 58.6 to 78.1% at water temperature of 25 °C. Ingestion rate increased with increasing fish weight and water temperature. Intensive blooms occurred in Lake Yuehu in both 1999 and 2000. The lake was stocked with silver carp (Hypophthalmichthys molitrix), bighead (Aristichthys nobilis) and a freshwater mussel (Hyriopsis cumingii) at a total biomass of 9.8 g m⁻³ in early 2001, and tilapia at 3–5 g m⁻³ in April of 2002. From June to October, average phytoplankton density decreased from 897.6×10⁶ cells l⁻¹ in 2000 to 291.7×10⁶ cells l⁻¹ in 2001 and 183.0×10⁶ cells l⁻¹ in 2002. Compared to 2000, the annual average phytoplankton biomass in 2001 and 2002 decreased by 48.6% and 63.8%, respectively. The blue-green algal biomass which made up 70% of the total phytoplankton biomass in 2000 was reduced to 22.1% in 2001 and 11.2% in 2002. Meanwhile, Secchi depth increased from 20–50 cm to 55–137 cm during the same time period. Similar results were observed in some other eutrophic waters. For example, algal bloom disappeared about 20 days after tilapia fingerlings were stocked (8–15 g m⁻³) to a pond in Zhenhai Park. Chlorophyll a concentration and phytoplankton production declined dramatically whereas water transparency increased substantially. However, the impacts of tilapia on nitrogen and phosphorus dynamics in natural lakes need further investigation. Our studies revealed that stocking tilapia is an effective way to control algal blooms in eutrophic waters, especially in lakes where nutrient loading cannot be reduced sufficiently, and where grazing by zooplankton cannot control phytoplankton production effectively.     

Phytoplankton dynamics and structure: a comparative analysis in natural and man-made water bodies of different trophic state

Previous investigations on Sicilian man made lakes suggested that physical factors, along with the specific morphology and hydrology of the water body, are important in selecting phytoplankton species. In particular, the variations of the z _mix/z _eu ratio due to the operational procedure to which reservoirs are generally subject were recognised as a trigger allowing the assemblage shift. To investigate if these variations may be considered analogous to those occurring in natural lakes as trophic state and phytoplankton biomass increase, causing a transparency decrease and a contraction of the euphotic depth, phytoplankton were collected in two natural water bodies, one mesotrophic (Lake Biviere di Cesarò) the other eutrophic (Lake Soprano), and compared with those collected in two reservoirs with analogous trophic characteristics (Lake Rosamarina, mesotrophic and Lake Arancio, eutrophic). Particular attention was paid to the dynamics of two key groups: Cyanophytes and chlorophytes. In all four water bodies, transparency mainly depended on chlorophyll level. Annual average value of phytoplankton biomass in the mesotrophic environments was below 2.0 mg l^–1, whereas in the eutrophic systems it was well above 10 mg l^–1. All water bodies showed the presence of cyanophytes (e.g. Anabaena spp., Anabaenopsis spp., Microcystis spp., Planktothrix spp.) and chlorophytes (e.g. Chlamydomonas spp., Botryococcus spp., Oocystis spp., Scenedesmus spp., Pediastrum spp.), but their relative proportions and body size dimensions were different. In particular, small colonial chlorophytes and large-colony forming cyanophytes were most common in the most eutrophic water bodies, whereas larger colonies of green algae in those with a lower trophic state. The results showed that, under the same climatic conditions, autogenic (increase of biomass, decrease in light penetration and euphotic depth) and allogenic (use of the stored waters, anticipated breaking of the thermocline, increase of the mixing depth) processes may shift the structure of phytoplankton assemblage in the same direction even though the quantity of biomass remains linked to nutrient availability.     

Ecological Significance of Nitrogen Cycling by Tubificid Communities in Shallow Eutrophic Lakes of the Danube Delta

Importance of tubificid populations on nitrogen cycle in two categories of shallow eutrophic lakes in the Danube Delta was quantitatively assessed for the 1992-1993 period. The structure of the primary producers in the studied lakes was used to discriminate between the two categories:(i) lakes dominated by macrophytes (A₁) and (ii) lakes dominated by phytoplankton (A₂). In both categories tubificid worms represented important fraction of the entire benthic community (35 and 32%, respectively, as number of individuals). They influence the sediment-water exchange of nutrients. The main processes involved are excretion of nutrients and their continuous release from sediments by molecular diffusion or through channels created by bioturbation. Inorganic nitrogen released from bottom sediments may regulate nitrogen load in the water body and thus, phytoplankton production. In 1992-1993, nitrogen stocks in tubificid biomass accounted for 5.3% in A₁ lakes and 15.6% in A₂ lakes of the amount stocked in phytoplankton, and only for 1.2 and 2.9% respectively, of the nitrogen load in water body. Nitrogen excretion rates ranged between 60.52 and 153.74 mg N m^–2 year^–1, and release rates from sediments between 378.26 and 960.87 mg N m^–2 year^–1, the lowest values being recorded for A₂ category. Differences are related to tubificid biomass, structure and abundance of primary producers and to nutrient load in different ecosystems. Ratios between release rate of inorganic nitrogen by tubificid worms and sedimentation rate of organic nitrogen in the two categories of lakes were 8.3 and 6.4% respectively. Contribution of nitrogen released daily from sediments to the dissolved inorganic nitrogen load in the water column was less than 0.5%. However, in A₁ and A₂ lakes, the released nitrogen had a potential to sustain 24.74 and 8.01%, respectively, of the annual phytoplankton production. These values suggest the significance of tubificids in keeping the eutrophication process at a high level, especially during the periods when nitrogen is the main limiting factor for phytoplankton production.     

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

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

Density and biomass of Gastrotricha in sediments of different types of standing waters

The results of an investigation of the density and biomass of Gastrotricha in freshwater lakes and in small fertile and shallow water bodies in eastern Poland are presented. The density varied from 495.0 to 2600.0 thousand indiv. m^–2 and was affected by water fertility much less than expected. The highest biomass value, 517.9 mg fresh weight m^–2, was obtained for one of the fertile water bodies. For lakes, these values were from 200.0 mg m^–2 in a dystrophic to 80.0 in an a-mesotrophic lake. In the latter the biomass decreased gradually from 319.8 mg fresh weight m^–2 in littoral to 65.0 mg in the profundal zone.     

Protozooplankton in acidic mining lakes with special respect to ciliates

The planktonic protozoa, especially ciliates, were analysed in five mining lakes of various pH and acidity values in the Lusatian and mid-German mining area. Heliozoa were the main protozoan component in the very low pH (<2.9) lakes. In the lakes with pH >2.9, the ciliate community consisted of Hypotrichida, Prostomatida and Peritrichida. The species diversity of the ciliate community was reduced and typical representatives of plankton ciliates were lacking. During periods when populations were at their peak, the cell numbers and biomass of ciliates were comparable to those neutral lakes, reaching maximum values of 30 000 cells l^–1 and 0.3 mg l^–1. Cell numbers and biomass peaks appeared without a clear seasonal pattern but in some cases a correlation to phytoplankton dynamics was recognizable.     

Direct measurements of nanomolar nitrate uptake by the marine diatom Phaeodactylum tricornutum (Bohlin). Implications for studies of oligotrophic ecosystems

A new automated procedure for nanomolar nitrate analysis was applied to the study of uptake of low nitrate concentrations (< 100 ngat l^–1) by phytoplankton. The precision of this analytical method (± 3 ngat l^–1) made it possible to monitor the absorption of very low quantities of nitrate over short term periods by a low cell-density culture of the marine diatom Phaeodactylum tricornutum, where the levels of particulate nitrogen and chlorophyll were equivalent to those found in oligotrophic areas (0.5 µgat N l^–1 and 0.4 µg l^–1 respectively). By continuous monitoring of nitrate disappearance from the culture medium, we are able to describe accurately the transient uptake responses of the diatom after a spike addition of trace quantities of nitrate and thus to provide new information on the still largely unknown small-scale phenomenon of pulsed nitrate supply in the upper layer of stratified oceans and rapid uptake of these nitrate patches by phytoplankton.The results show that a N-limited culture of Phaeodactylum tricornutum is immediately capable of taking up trace quantities of nitrate (< 100 ngat l^–1) at high rates (0.10–0.14 h^–1) . These initial rates are one order of magnitude higher than the theoretical rates calculated from the Michaelis-Menten equation and are close to the level of V _max (0.15 h^–1) obtained when cells are exposed to saturating nitrate concentrations. This rapid initial uptake would be a considerable advantage in oligotrophic areas where nanomolar nitrate supply is thought to be episodic. The present results suggest that phytoplankton evolve adaptations to utilize the available nitrate at the spatial and temporal scales at which it occurs. On the other hand, we can consider this physiological adaptation as evidence of nitrate pulses in the field which would invalidate the steady-state approach to the oligotrophic ecosystems.     

Phytoplankton and physical-chemical features of Tavropos Reservoir,Greece

Phytoplankton biomass values in Tavropos Reservoir, ranging from 92 to 1071 mg m^–3, are within a range characteristic of oligotrophic waters. The seasonal sequence of biomass shows three annual peaks, differing from the monoacmic pattern seen in oligotrophic lakes. This sequence was profoundly affected by changes in water withdrawal and inflow rates. Diatoms, cryptophytes, chrysophytes and dinoflagellates, in that order, were the major constituents of the reservoir phytoplankton. The succession, from diatoms and chrysophytes in late winter-spring, to centric diatoms in late spring-summer and again to diatom-chrysophytes in late autumn was similar to that in oligotrophic lakes.     

Primary productivity and trophic status of Lakes Sorell and Crescent,Tasmania

The primary productivity of two turbid, shallow lakes on the Tasmanian Central Plateau was determined by the C¹⁴ technique from half-light day incubations in situ. Graphical integration of depth-rate curves gave estimates of areal day rates of production and of annual rates.The 2 lakes are closely adjacent and very similar physically and chemically, but have very different phytoplankton populations. Lake Crescent has ten times the standing crop biomass of Lake Sorell but its greater turbidity restricts light penetration, and production per unit of surface per day and per year is only 2.6 times that of Sorell.With day rates of 25-(44)-93 mgCm^–2 and annual production of 16.9 gCm^–2 Lake Sorell could be regarded as oligotrophic. Consideration of standing crop biomass and morphometry however indicates oligo-mesotrophy. Lake Crescent with day rates of 35-(115)-250 mgCm^–2 and annual production of 45 gCm^–2 is moderately eutrophic.Incubations in constant light demonstrated considerable variation in production rates in different parts of Lake Crescent.     

Structural and grazing responses of zooplankton community to biomanipulation of some Dutch water bodies

Structure and grazing activities of crustacean zooplankton were compared in five lakes undergoing manipulation with several unmanipulated eutrophic (shallow) and mesotrophic (deep) lakes in The Netherlands. The biomanipulated lakes had lesser number of species and their abundance, both of rotifers and crustaceans, and had much larger mean animal size (3–11 μg C ind.⁻¹) than in the unmanipulated eutrophic lakes (0.65 μG C ind.⁻¹). WhereasD. hyalina (=D. galeata) andD. cucullata generally co-occurred in the unmanipulated lakes, in the manipulated lakes bothD. hyalina and other large-bodied daphnids,D. magna,D. pulex (=D. pulicaria), were the important grazers. In the biomanipulated lakes an increase in the individual crustacean size and of zooplankton mass were reflected in a decrease in seston concentration, higher Secchi-disc depth and a marked decrease in the share in phytoplankton biovolume of cyanobacteria. Biomass relationship between seston (150 μm) and zooplankton indicated a Monod type relationship, with an initial part of the curve in which the zooplankton responds linearly to the seston increase up to aboutca. 2 mg C l⁻¹, followed by a saturation of zooplankton mass (0.39 mg C l⁻¹) at 3–4 mg C l⁻¹ seston, and an inhibitory effect on zooplankton mass at seston levels>4 mg C l⁻¹. This latter is related to predominance in the seston of cyanobacteria. In the biomanipulated lakes, the zooplankton grazing rates often exceeded 100% d⁻¹, during the spring, and food levels generally dropped to <0.5 mg C l⁻¹. The computed specific clearance rate (SCR) of zooplankton of 1.9 l mg⁻¹ Zoop C is well within the range of SCR values (1.7–2.2 l mg⁻¹ Zoop C) from deep and mesotrophic waters, but about an order of magnitude higher than in the eutrophic lakes, with the food levels 10-fold higher. For 25% d⁻¹ clearance of lake seston between 35 and 60 ind. l⁻¹ are needed in the biomanipulated lakes against 1200–1300 ind. l⁻¹ in eutrophic lakes. Similarly, about 10 to 15 times more crustacean grazers are required to eliminate the daily primary production in the eutrophic lakes than in the biomanipulated lakes. These numbers are inversely related to the differences in animal size. The corresponding biomass values of zooplankton needed to clear the daily primary production in the eutrophic waters were 0.1–0.2 mg C l⁻¹ in the biomanipulated lakes, but about 0.45 mg C l⁻¹ in the unmanipulated eutrophic waters. Only if the water was kept persistently clear by zooplankton was there a balanced seston budget between the inputvia primary production and elimination by zooplankton. Mostly, however, the input exceeded the assimilatory removal by zooplankton, such that the estimated seston loss could be attributed to sedimentation and mineralization.     

Estimation of reference conditions for phytoplankton in a naturally eutrophic shallow lake

Classification of waters using biological quality elements and determination of the degree of deviation from reference levels is a key issue in the Water Framework Directive of EU. Lakes in reference conditions with sufficient biological data are available for several boreal lake types with the exception of naturally eutrophic lakes. An empirical approach is one alternative for estimating the reference conditions of such lakes. We used the water transparency of the naturally eutrophic Lake Tuusulanjärvi recorded in August in the early 1910s to estimate reference values for phytoplankton biomass and chlorophyll a concentrations. Three phytoplankton samples during August 2000–2001 corresponded to the estimated reference values for total biomass (<5.6 mg l^?1) and chlorophyll a (<28 μg l^?1), as did the simultaneous Secchi depths. The phytoplankton assemblage in these samples with 24 eutrophy indicators (17% of the total taxa number) corresponded in general the species list from the early 1900s, which as such could be regarded as reference assemblage. Furthermore, in August 2000, 3 years after intensive fish removal a prominent decrease in cyanobacterial biomass and toxin concentration was observed. The costs of the measures and studies in Lake Tuusulanjärvi during 1989–2003 have been approximately 2.5 million euros.     

Seasonal change in the proportion of bacterial and phytoplankton production along a salinity gradient in a shallow estuary

We intended to evaluate the relative contribution of primary production versus allochthonous carbon in the production of bacterial biomass in a mesotrophic estuary. Different spatial and temporal ranges were observed in the values of bacterioplankton biomass (31–273 g C l^–1) and production (0.1–16.0 g C l^–1 h^–1, 1.5–36.8 mg C m^–2 h^–1) as well as in phytoplankton abundance (50–1700 g C l^–1) and primary production (0.1–512.9 g C l^–1 h^–1, 1.5–512.9 mg C m^–2 h^–1). Bacterial specific growth rate (0.10–1.68 d^–1) during the year did not fluctuate as much as phytoplankton specific growth rate (0.02–0.74 d^–1). Along the salinity gradient and towards the inner estuary, bacterio- and phytoplankton biomass and production increased steadily both in the warm and cold seasons. The maximum geographical increase observed in these variables was 12 times more for the bacterial community and 8 times more for the phytoplankton community. The warm to cold season ratios of the biological variables varied geographically and according to these variables. The increase at the warm season achieved its maximum in the biomass production, particularly in the marine zone and at high tide (20 and 112 times higher in bacterial and phytoplankton production, respectively). The seasonal variation in specific growth rate was most noticeable in phytoplankton, with seasonal ratios of 3–26. The bacterial community of the marine zone responded positively – generating seasonal ratios of 1–13 in bacterial specific growth rate – to the strong warm season increment in phytoplankton growth rate in this zone. In the brackish water zone where even during the warm season allochthonous carbon accounted for 41% (on average) of the bacterial carbon demand, the seasonal ratio of bacterial specific growth rate varied from about 1 to 2. During the warm season, an average of 21% of the primary production was potentially sufficient to support the whole bacterial production. During the cold months, however, the total primary production would be either required or even insufficient to support bacterial production. The estuary turned then into a mostly heterotrophic system. However, the calculated annual production of biomass by bacterio- and phytoplankton in the whole ecosystem showed that auto- and heterotrophic production was balanced in this estuary.