Filamentous green algae inhibit phytoplankton with enhanced effects when lakes get warmer

1. Filamentous green algae (FGA) may represent an alternative state in high‐nutrient shallow temperate lakes. Furthermore, a clear water state is sometimes associated with the dominance of FGA; however, the mechanisms involved remain uncertain. 2. We hypothesised that FGA may promote a clear water state by directly suppressing phytoplankton growth, mostly via the release of allelochemicals, and that this interaction may be affected by temperature. 3. We examined the relationships between FGA, phytoplanktonic chlorophyll a concentrations and zooplankton in a series of mesocosms (2.8 m³) mimicking enriched shallow ponds now and in a future warmer climate (0 and c. 5 °C above ambient temperatures). We then tested the potential allelopathic effects of FGA (Cladophora sp. and Spirogyra sp.) on phytoplankton using several short‐term microcosms and laboratory experiments. 4. Mesocosms with FGA evidenced lower phytoplanktonic chlorophyll a concentrations than those without. Zooplankton and zooplankton : phytoplankton biomass ratios did not differ between mesocosms with and without FGA, suggesting that grazing was not responsible for the negative effects on phytoplanktonic biomass (chlorophyll a). 5. Our field microcosm experiments demonstrated that FGA strongly suppressed the growth of natural phytoplankton at non‐limiting nutrient conditions and regardless of phytoplankton initial concentrations or micronutrients addition. Furthermore, we found that the negative effect of FGA on phytoplankton growth increased up to 49% under high incubation temperatures. The experiment performed using FGA filtrates confirmed that the inhibitory effect of FGA on phytoplankton may be attributed to allelochemicals. 6. Our results suggest that FGA control of phytoplankton growth may be an important mechanism for stabilising clear water in shallow temperate lakes dominated by FGA and that FGA may play a larger role when lakes get warmer.     

EFFECTS OF NUTRIENT ENRICHMENT ON NATURAL POPULATIONS OF THE BROWN TIDE PHYTOPLANKTON AUREOCOCCUS ANOPHAGEFFERENS (CHRYSOPHYCEAE)1

The brown tide picoalga Aureococcus anophagefferens Hargraves et Sieburth was present in approximately equal numbers in 12 large scale (13,000 L) mesocosms at the start of a nutrient addition experiment in June 1985. Increases in abundance in untreated systems mimicked the pattern of bloom development in Narragansett Bay, Rhode Island, the seawater source for the experiment. Aureococcus increased to maximal values of 2.6 × 10⁹ cells. L^?1 and persisted at high numbers (10⁸ cells·L^?1) for 7–8 weeks. In nutrient addition tanks, the picoalgae bloomed briefly (1–3 weeks) but rapidly declined to the usual level (～10⁷ cells·L^?1) for eukaryotic algae in Narragansett Bay. The decline in picoalgae abundance was followed by an increase in total diatoms in all nutrient treated tanks. Mean picoalgae abundance in the mesocosms and the bay was significantly (P < 0.05) and inversely correlated (r =–0.93) with mean concentration of dissolved inorganic nitrogen. The persistence of the brown tide species in control mesocosms and Narragansett Bay appears related to its ability to grow at very low concentrations of dissolved inorganic nitrogen, levels previously shown to limit diatom growth.     

Experimental measurement of resource competition between planktonic microalgae and macroalgae (seaweeds) in mesocosms simulating the San Francisco Bay-Estuary,California

Planktonic algae are not abundant in the brackish waters of San Francisco Bay-estuary (mean chlorophyll a 5 µg 1^–1), despite the high level of nutrients usually present due to the input of treated sewage from 3 million people. Macroalgae (seaweeds) are sometimes locally abundant in the Bay. Phytoplankton are abundant (chlorophyll a > 50 µg 1^–1) and seaweeds uncommon in the almost freshwater Delta and upper estuary despite lower nutrient levels. Direct competition between these algal groups could explain the observed distributions.Given the size of the algae, large containers were needed for the determination of possible resource competition. Experiments were carried out in flow-through mesocosms (analog tanks) of 3 m³ volume. The macroalgae Ulva lactuca or Gigartina exasperata and a diatom-dominated phytoplankton, all from San Francisco Bay, were grown separately and together and with and without treated sewage effluent or other artificial nutrient additions. When grown alone phytoplankton and macroalgae were greatly stimulated by wastewater addition to unmodified baywater. The phytoplankton grew much more slowly in the presence of natural densities of Ulva. Allelochemical effects were tested for but not demonstrated.Resource competition for inorganic nitrogen was determined to be the probable cause of the depression of phytoplankton by Ulva. At its rapid growth rates in the flow-through mesocosms (up to 14% day^–1) this macroalga can reduce inorganic nitrogen to low levels. Ulva has a greater affinity (lower KS) for nitrogen than do some of the plankton of the Bay. Ulva may outcompete phytoplankton by reducing nitrogen to levels below those capable of supporting phytoplankton growth. Other macroalgae such as Gigartina and Enteromorpha need to be studied to determine if they also can depress phytoplankton growth by resource competition.     

Eutrophication of the salt valley reservoirs, 1968–73 I. The effects of eutrophication on standing crop and composition of phytoplankton

A study was carried out over a period of six years to determine the effects of eutrophication upon standing crop and composition of the phytoplankton in four recently constructed flood control reservoirs in Nebraska. Water samples collected weekly during June, July, and August from 1968–73 were analyzed for chlorophyll a, phytoplankton composition, and phytoplankton abundance. Total volume of phytoplankton was calculated from appropriate dimensions and formulae. Inorganic turbidity in one reservoir was an important factor regulating the size and composition of the phytoplankton standing crop. In that reservoir diatoms were the most important component of the phytoplankton community during those years in which inorganic turbidity was greatest. When inorganic turbidity declined, blue-green algae became dominant.In the clear-water reservoirs chlorophyll a, phytoplankton number, and phytoplankton volume were significantly correlated with reservoir age, with the oldest containing chlorophyll a concentrations up to 247 mg/m³ and mean phytoplankton volumes up to 329 mm³/l, values sufficient to place it in the hypereutrophic category. The reservoirs had 2–6 times more chlorophyll a present at the end of the study than at the beginning, with the increase being greatest in the newest reservoir. Phytoplankton volume was significantly correlated with chlorophyll a in all the reservoirs.Blue-green algae quickly became established as community dominants in the reservoirs, making up over 80 percent of the phytoplankton volume in the newest reservoir by the second year of its existence. In the other clear-water reservoirs, blue-greens usually constituted over 95 percent of the total phytoplankton volume in summer. three genera, Microcystis, Aphanizomenon, and Anabaena, were responsible for virtually the entire standing crop of blue-greens in all the reservoirs.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

Microbial herbivory on the brown tide alga, Aureococcus anophagefferens: results from natural ecosystems, mesocosms and laboratory experiments

Experiments were conducted with natural plankton assemblages from two areas in Great South Bay (GSB) and the Peconic Bays Estuary System, NY, to compare the rates of growth and pelagic grazing mortality of Aureococcus anophagefferens with co-occurring phytoplankton. We hypothesized that A. anophagefferens would experience low mortality rates by microbial herbivores (relative to feeding pressure on other algae) thus providing it with a competitive advantage within the phytoplankton community. In fact, substantial rates of mortality were observed in nearly every experiment in our study. However, mortality rates of A. anophagefferens were less than intrinsic growth rates of the alga during late spring and early summer in Great South Bay, resulting in positive net growth rates for the alga during that period. This timing coincided with the development of a brown tide in this estuary. Similarly, growth rates of the alga also exceeded mortality rates during bloom development in natural plankton assemblages from the Peconic Bays Estuary System held in mesocosms. In contrast to the situation for A. anophagefferens, growth rates of the total phytoplankton assemblage, and another common picoplanktonic phytoplankter (Synechococcus spp.), were frequently less than their respective mortality rates. Mortality rates of A. anophagefferens in both systems were similar to growth rates of the alga during later stages of the bloom. Laboratory studies confirmed that species of phagotrophic protists that consume A. anophagefferens (at least in culture) are present during brown tides but preference for or against the alga appears to be species-specific among phagotrophic protists. We conclude that two scenarios may explain our results: (1) protistan species capable of consuming the brown tide alga were present at low abundances during bloom initiation and thus not able to respond rapidly to increases in the intrinsic growth rate of the alga, or (2) the brown tide alga produced substance(s) that inhibited or retarded protistan grazing activities during the period of bloom initiation. The latter scenario seems less likely given that significant mortality of A. anophagefferens was measured during our field study and mesocosm experiment. However, even a minor reduction in mortality rate due to feeding selectivity among herbivores might result in a mismatch between growth and grazing of A. anophagefferens that could give rise to significant net population growth of this HAB species. Either scenario infers an important role for trophic interactions within the plankton as a factor explaining the development of brown tides in natural ecosystems.     

Field and Laboratory Studies on Nile Phytoplankton in Egypt. II. Phytoplankton

Variations in chlorophyll a concentration and in qualitative and quantitative counts of Nile phytoplankton were followed at Assiut from September 1980 to September 1982. Chlorophyll a concentrations usually correlated well with phytoplankton density. The total phytoplankton exhibited higher counts in spring and summer than in autumn and winter. While diatoms exhibited the highest counts, green algae contributed more genera to the phytoplankton community.     

The introduced bivalve <Emphasis Type="Italic">Limnoperna fortunei</Emphasis> boosts <Emphasis Type="Italic">Microcystis</Emphasis> growth in Salto Grande reservoir (Argentina): evidence from mesocosm experiments

In order to assess the effects of the introduced bivalve Limnoperna fortunei on water-column properties of Salto Grande reservoir, experiments were conducted using six 400 L mesocosms: 2 with 100 mussels, 2 with 300 mussels, and 2 controls (without mussels). At 0, 1, 2, 3, 7, 14, 21, 28, and 35 days we measured nutrient and chlorophyll a concentrations, counted and identified the phytoplankton, and estimated the density, size, and number of cells of the colonies of Microcystis spp. Cumulative periphyton growth and total accumulated sediments were assessed in all enclosures at the end of the experiment. Throughout the experiment, in the controls ammonia and phosphates dropped to near zero, whereas in the mesocosms with L. fortunei they increased two- to tenfold. Nitrates decreased in all mesocosms. In the presence of the mussel, chlorophyll a and algal cells dropped until day 3 increasing thereafter, whereas in the controls they increased from day 0. Periphyton growth and sediment accumulation were significantly higher in the mesocosms with mussels that in the controls. Cell density, proportion of colonial cells and colony size of Microcystis spp. increased in all enclosures, but these increases were dramatically (and very significantly) higher in enclosures with 100 and, especially, with 300 mussels, than in the controls. Our results indicate that L. fortunei modifies nutrient concentrations and proportions, and promotes aggregation of solitary Microcystis spp. cells into colonies; both these effects can favor blooms of this often noxious cyanobacteria.     

Response of cyanobacteria and phytoplankton abundance to warming,extreme rainfall events and nutrient enrichment

Cyanobacterial blooms are an increasing threat to water quality and global water security caused by the nutrient enrichment of freshwaters. There is also a broad consensus that blooms are increasing with global warming, but the impacts of other concomitant environmental changes, such as an increase in extreme rainfall events, may affect this response. One of the potential effects of high rainfall events on phytoplankton communities is greater loss of biomass through hydraulic flushing. Here we used a shallow lake mesocosm experiment to test the combined effects of: warming (ambient vs. +4°C increase), high rainfall (flushing) events (no events vs. seasonal events) and nutrient loading (eutrophic vs. hypertrophic) on total phytoplankton chlorophyll‐a and cyanobacterial abundance and composition. Our hypotheses were that: (a) total phytoplankton and cyanobacterial abundance would be higher in heated mesocosms; (b) the stimulatory effects of warming on cyanobacterial abundance would be enhanced in higher nutrient mesocosms, resulting in a synergistic interaction; (c) the recovery of biomass from flushing induced losses would be quicker in heated and nutrient‐enriched treatments, and during the growing season. The results supported the first and, in part, the third hypotheses: total phytoplankton and cyanobacterial abundance increased in heated mesocosms with an increase in common bloom‐forming taxa—Microcystis spp. and Dolichospermum spp. Recovery from flushing was slowest in the winter, but unaffected by warming or higher nutrient loading. Contrary to the second hypothesis, an antagonistic interaction between warming and nutrient enrichment was detected for both cyanobacteria and chlorophyll‐a demonstrating that ecological surprises can occur, dependent on the environmental context. While this study highlights the clear need to mitigate against global warming, oversimplification of global change effects on cyanobacteria should be avoided; stressor gradients and seasonal effects should be considered as important factors shaping the response.     

Acidity status and phytoplankton species richness,standing crop,and community composition in Adirondack,New York,U.S.A. lakes

The mid-summer phytoplankton communities of more than 100 Adirondack lakes ranging in pH from 4.0 to 7.2 were characterized in relation to 25 physical-chemical parameters. Phytoplankton species richness declined significantly with increasing acidity. Acidic lakes (pH < 5.0) averaged fewer than 20 species while more circumneutral waters (pH > 6.5) averaged more than 33 species. Phytoplankton abundance was not significantly correlated with any of the measured physical-chemical parameters, but standing crop parameters, i.e., chlorophyll a and phytoplankton biovolume, did correlate significantly with several parameters. Midsummer standing crop correlated best with total phosphorus concentration but acidity status affected the standing crop-phosphorus relationship. Circumneutral waters of low phosphorus content, i.e. < 10 µg·1^–1 TP, averaged 3.62 µg·1^–1 chlorophyll a whereas acidic lakes of the same phosphorus content averaged only 1.96 µg·1^–1 chlorophyll a. The midsummer chlorophyll content of lakes of high phosphorus content, i.e. > 10 µg·1^–1 TP, was not significantly affected by acidity status.Adirondack phytoplankton community composition changes with increasing acidity. The numbers of species in midsummer collections within all major taxonomic groups of algae are reduced with increasing acidity. The midsummer phytoplankton communities of acidic Adirondack lakes can generally be characterized into four broad types; 1) the depauperate clear water acid lake assemblage dominated by dinoflagellates, 2) the more diverse oligotrophic acid lake community dominated by cryptomonads, green algae, and chrysophytes, 3) the productive acid lake assemblage dominated by green algae, and 4) the chrysophyte dominated community. The major phytoplankton community types of acid lakes are associated with different levels of nutrients, aluminum concentrations, and humic influences.     

The use of dialysis culture in a study of chlorophyll budget in a brackish lagoon,Japan

Growth rates of the entire phytoplankton community of a brackish lagoon in northeastern Japan were estimated by measuring increasing chlorophyll a content in dialysis bags during the summer and early autumn of 1986. The chlorophyll a contents of lagoon water fluctuated between 20 and 200 mg m^–3. At lower densities of phytoplankton (20–50 mg chl. a m^–3), growth rates (the rate of increase of chlorophyll a) exceeded 1 turnover per day, while at higher densities (more than 50 mg chl. a m^–3), the growth rate decreased rapidly. Tidal exchanges of chlorophyll a showed net exports of chlorophyll a from the lagoon to adjacent waters. The exchange rate of chlorophyll a was estimated to be 0.65 d^–1. At about 140 mg m^–3 of chlorophyll a concentration, the increase of chlorophyll in the lagoon water compensated for tidal export. Only a small proportion of primary production was consumed by zooplankton in the lagoon. There were also net exports of ammonium and phosphate from the lagoon. Nutrient flux from sediment exceeded the phytoplankton requirement and was the major source of the ammonium and phosphate exports from the lagoon. The low inorganic N/P atom supply ratio in the lagoon suggests that nitrogen is a major nutrient limiting phytoplankton growth.     

The influence of water level on macrophyte growth and trophic interactions in eutrophic Mediterranean shallow lakes: a mesocosm experiment with and without fish

1. Water‐level fluctuations are typical of lakes located in the semi‐arid Mediterranean region, which is characterised by warm rainy winters and hot dry summers. Ongoing climate change may exacerbate fluctuations and lead to more severe episodes of drought, so information on the effects of water level on the functioning of lake ecosystems in such regions is crucial. 2. In eutrophic Lake Eymir, Turkey, we conducted a 4‐month (summer) field experiment using cylindrical 0.8‐m‐ (low‐water‐level) and 1.6‐m‐deep (high‐water‐level) mesocosms (kept open to the sediment and atmosphere). Fish (tench, Tinca tinca, and bleak, Alburnus escherichii) were added to half of the mesocosms, while the rest were kept fishless. Ten shoots of Potamogeton pectinatus were transplanted to each mesocosm. 3. Sampling for physicochemical variables, chlorophyll a (chl‐a), zooplankton and per cent plant volume inhabited (PVI%) by macrophytes was conducted weekly during the first 5 weeks, and subsequently biweekly. Macrophytes were harvested on the last sampling date. During the course of the experiment, the water level decreased by 0.41 ± 0.06 m. 4. Throughout the experiment, fish affected zooplankton abundance (?), nutrient concentrations (+), chl‐a (+) and water clarity (?) most strongly in the low‐water‐level mesocosms and the zooplankton community shifted towards dominance of small‐sized forms. The fishless mesocosms had a higher zooplankton/phytoplankton ratio, suggesting higher grazing. 5. Greatest macrophyte growth was observed in the low‐water‐level fishless mesocosms. However, despite high nutrient concentrations and low water clarity, macrophytes were also abundant in the fish mesocosms and particularly increased following a water‐level decrease from midsummer onwards. Macrophyte growth was poor in the high‐water‐level mesocosms, even in the fishless ones with high water clarity. This was ascribed to extensive periphyton development reducing light availability for the macrophytes. 6. Our results indicate that a reduction in water level during summer may help maintain the growth of macrophytes in Mediterranean eutrophic shallow lakes, despite a strong negative effect of fish predation on water clarity. It is therefore probable that an expected negative effect of global climate change on water clarity because of eutrophication and enhanced top‐down control of fish may be, at least partly, counteracted by reduced water level, provided that physical disturbance is not severe.     

The influence of floodplain habitat on the quantity and quality of riverine phytoplankton carbon produced during the flood season in San Francisco Estuary

Primary productivity, community respiration, chlorophyll a concentration, phytoplankton species composition, and environmental factors were compared in the Yolo Bypass floodplain and adjacent Sacramento River in order to determine if passage of Sacramento River through floodplain habitat enhanced the quantity and quality of phytoplankton carbon available to the aquatic food web and how primary productivity and phytoplankton species composition in these habitats were affected by environmental conditions during the flood season. Greater net primary productivity of Sacramento River water in the floodplain than the main river channel was associated with more frequent autotrophy and a higher P:R ratio, chlorophyll a concentration, and phytoplankton growth efficiency (α^B). Total irradiance and water temperature in the euphotic zone were positively correlated with net primary productivity in winter and early spring but negatively correlated with net primary productivity in the late spring and early summer in the floodplain. In contrast, net primary productivity was correlated with chlorophyll a concentration and streamflow in the Sacramento River. The flood pulse cycle was important for floodplain production because it facilitated the accumulation of chlorophyll a and wide diameter diatom and green algal cells during the drain phase. High chlorophyll a concentration and diatom and green algal biomass enabled the floodplain to export 14–37% of the combined floodplain plus river load of total, diatom and green algal biomass and wide diameter cells to the estuary downstream, even though it had only 3% of the river streamflow. The study suggested the quantity and quality of riverine phytoplankton biomass available to the aquatic food web could be enhanced by passing river water through a floodplain during the flood season.     

Pheopigment dynamics,zooplankton grazing rates and the autumnal ammonium peak in a Mediterranean lagoon

The dilution technique was used to estimate chlorophyll and pheopigment, net and gross production as well as zooplankton grazing over a 12-month period in a coastal lagoon in Southern France. Chlorophyll a (Cha) based gross growth rates of phytoplankton ranged from undetectable in February to 2.6 day⁻¹ in June, corresponding to 3.8 divisions per day. Cha-based grazing rates ranged from undetectable in February to 1.1 d⁻¹ in June. The seasonal growth pattern of picoplankton was similar to that of the whole community, with a peak in July, corresponding to four divisions per day. Grazing processes represented from 20 to 150% of the phytoplankton daily growth, and the grazing pressure was stronger on small phytoplankton cells than on larger cells. Gross growth rates of phytoplankton were related to zooplankton grazing rates, and both were related to water temperature. Mesozooplankton which escaped sampling or oysters had to be also invoked as additional sinks for the primary production. In the fall, pheopigment concentrations greater than chlorophyll concentrations coincided with high ammonium levels in the water column. Pheopigment a production rates were highly correlated to chlorophyll -based microzooplankton grazing rates. The pheopigment a to chlorophyll a ratio was correlated with ammonium concentrations and could be used an index of the balance between ammonium supply (degradation) and demand (uptake by phytoplankton). In addition, pheopigment degradation rates in absence of grazing could be related to irradiance, indicating photo-degradation of these compounds.     

Phytoplankton structure and dynamics in a volcanic lake in Deception Island (South Shetland Islands,Antarctica)

This work constitutes the first floristic and ecological analysis of the phytoplankton community of a volcanic freshwater lake in Deception Island (62°57′S, 60°38′W, South Shetland Islands, Antarctica). The main limnological features and phytoplankton size fractions were analyzed. Samples were taken during the austral summer of 2002 at two opposite sites. According to ANOVA results performed with abiotic variables, no significant differences between sites were found. The phytoplankton community showed low algal species richness, with an important contribution of the tychoplanktonic taxa. In terms of species number, Bacillariophyceae was the dominant class. Autotrophic picoplankton registered the highest densities from the second sampling date onwards. Nanophytoplankton was represented by unidentified chrysophycean organisms, which showed different distribution patterns between sites. The net phytoplankton abundance remained low during the sampling period and was strongly correlated with chlorophyll a concentration. Both nutrient concentrations and chlorophyll a values indicated oligotrophic conditions.     

Effects of mixing on the pelagic food web in shallow lakes

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<Emphasis Type="Italic">In situ</Emphasis> Quantification of Phytoplankton in Reservoirs Using a Submersible Spectrofluorometer

A submersible in situ spectrofluorometer, which permits the differentiation of four algal groups (green algae, diatoms, cryptophytes and cyanobacteria), was used for phytoplankton monitoring in five reservoirs with varying levels of eutrophication and composition of their phytoplankton communities. Data obtained in situ were compared to standard laboratory methods for phytoplankton quantification; concentration of chlorophyll a and microscopy analysis. A high correlation (r = 0.95, n = 96) between chlorophyll a levels using different methods was found in all types of phytoplankton community. Taxonomic analyses and cell counts were closely related to the ratio of algal classes measured by the in situ spectrofluorometer. The submersible device used in the study measures in a continuous mode, which is advantageous in comparison with discrete sampling. This method appears to be a good tool for water quality management and can be used in the detection of natural horizontal and vertical variability in phytoplankton communities or for the early detection of cyanobacterial blooms. The device used in this study is recommended as a screening tool that enables more effective sampling that can be focused on the localities and depths where changes in phytoplankton composition occur.     

Effect of desiccation on mosquito oviposition site selection in Mediterranean temporary habitats

1. Hydroperiod duration has been identified as the main factor determining the faunal composition and structure of aquatic communities in temporary habitats. It is hypothesised that desiccation will positively affect mosquito oviposition habitat selection during the post-drought period due to the lack of antagonists. 2. An experiment was carried out in outdoor mesocosms to assess whether desiccation events have post-drought effects on community richness and mosquito oviposition. Three different treatments were randomly assigned to the mesocosms: (i) eight mesocosms were left dried for a week and then reflooded; (ii) eight mesocosms stayed dry for 4 weeks before they were reflooded; (iii) eight mesocosms were maintained at a constant water volume of 30 litres during the entire experiment as controls. Mosquito oviposition and invertebrate community richness were monitored in every mesocosm, along with environmental parameters [water temperature, pH, conductivity, dissolved oxygen, total suspended solids (TSS) and chlorophyll a concentration]. 3. Post-drought mosquito oviposition and larval abundance were higher in the short-drought and long-drought pools than in the control. Desiccation negatively affected the biomass of the filter feeder invertebrates in both desiccation treatments. Chlorophyll a concentrations were higher in the long-drought pools than in controls. The negative impact of desiccation on zooplankton led to a post-drought increase in algae, associated with an increase in mosquito oviposition. 4. Despite immediate negative effect on mosquitoes, pulsed disturbances can benefit mosquitoes as they favour oviposition during the post-disturbance recovery period due to a lower abundance of mosquito antagonists and higher food resources for their offspring.     

Picophytoplankton biomass, community structure and productivity in the Great Astrolabe Lagoon, Fiji

 Phytoplankton biomass, community structure and productivity of the Great Astrolabe lagoon and surrounding ocean were studied using measurements of chlorophyll concentration and carbon uptake. The contribution of picophytoplankton to biomass, productivity and community structure was estimated by size fractionation, ¹⁴C-incubation and flow cytometry analysis. Picoplankton red fluorescence was demonstrated to be a proxy for chlorophyll <3 μm. Consequently, the percentage contribution to chl a<3 μm from each picoplankton group could be calculated using regression estimated values of ψ _i (fg chl a per unit of red fluorescence). In the lagoon, average chlorophyll concentration was 0.8 mg m^-3 with 45% of phytoplankton <3 μm. Primary production reached 1.3 g C m^-2 day^-1 with 53% due to phytoplankton <3 μm. Synechococcus was the most abundant group at all stations, followed by Prochlorococcus and picoeukaryotes. At all stations, Prochlorococcus represented less than 4% of the chl a <3 μm, Synechococcus between 85 and 95%, and Picoeukaryotes between 5 and 10%. In the upper 40 m of surrounding oceanic waters, phytoplankton biomass was dominated by the >3 μm size fraction. In deeper water, the <1 μm size fraction dominated. Prochlorococcus was the most abundant picoplankton group and their contributions to the chlorophyll a<3 μm were close to that of the picoeukaryotes (50% each). Accepted: 27 May 1999     

Bioassay for phosphate demand in phytoplankton from acidified lakes: Lake Njupfatet,an example of phosphate deficiency induced by liming

A bioassay was developed, involving steady-state ATP level determinations, for estimation of phosphate demand and deficiency in natural phytoplankton communities. The studies were performed on phytoplankton from the moderately acidified Lake Njupfatet in central Sweden before and after liming. Phytoplankton samples from in situ enclosure experiments with low-dose enrichments of nitrate and phosphate and removal of large (> 100 µm) zooplankton and from the lake water were collected. The phytoplankton were concentrated by through-flow centrifugation and post-cultured in the laboratory with or without the addition of phosphate. A relative increase in the ATP:chlorophyll a ratio after the phosphate treatment as compared to samples without phosphate enrichment was found to be a highly reproducible indicator of phosphate deficiency in the natural phytoplankton population. In contrast, the absolute ATP:chlorophyll a ratio varied substantially between different sampling occasions. No phosphate deficiency was detected in phytoplankton from the acidic lake or from fertilized in situ enclosures. However, phytoplankton from in situ enclosures without added nutrients showed evidence of phosphate limitation after 21 days incubation. Also, the phytoplankton community developed a significant phosphate deficiency the summer after lake liming. The results from the ATP analyses are compared with chemical data of the lake water, phytoplankton community structure and phosphatase activities in the lake before and after liming. The average total biomass of phytoplankton and the average Tot-P measured during May to September decreased with appr. 30% after liming while Tot-N was essentially unaffected and the phosphatase activities increased by 1000–2000%.