Summer Phytoplankton Biomass Research Articles

Changes in the Phytoplankton Community of Lake Donghu Since the 1980s

ABSTRACT The phytoplankton of Lake Donghu was compared between two periods (i.e., 1982–83 and 1999–2001). During 1982–1983, Lake Donghu was characterized by cyanobacterial blooms, very high summer phytoplankton biomass, and a spring clear-water phase. However, during 1999–2001, the lake was dominated by dinophytes and diatoms and had a relatively low phytoplankton biomass and a very turbid state. Increased stocking of silver carp and bighead carp during the interim years eliminated cyanobacterial blooms and large-bodied daphnids from the lake, creating favorable conditions for growth of the small-sized algae.

A filamentous green algae-dominated temperate shallow lake: Variations on the theme of clear-water stable states?

Shallow lakes can have two alternative stable states over wide range of nutrient concentrations: a macrophyte-dominated clear-water state and a phytoplankton-dominated turbid-water state. Little Budworth Pool, Cheshire, UK is a small (surface area 3 ha), shallow (max. depth < 1m), clear-water lake with very high nitrate concentrations (annual mean 9.2 mg NO 3 -N 1 -1 ), relatively low phytoplankton growth (annual mean 13.5 μg chlorophyll-a 1 -1 ) and high metaphytic filamentous green algal abundance (FGA, mainly Spirogyra), but without any submerged plant-beds. To understand the mechanism limiting phytoplankton biomass in summer, the lake was sampled every two weeks for two years (November 2001-October 2003) for water chemistry, phytoplankton, zooplankton, benthic macroinvertebrates, FGA and benthic diatoms. Phytoplankton growth was mainly dominated by small species like Cyclotella spp., Chroomonas acuta and Cryptomonas spp., whereas among the zooplankton Cladocera showed high abundance of both small and large species (Bosmina longirostris and Daphnia longispina) especially in summer and autumn. FGA attained highest biomass in the warmer months and showed negative correlations with nitrate-N and total nitrogen concentrations suggesting nutrient removal ability of these plants, but also possible increased denitrification in summer. While phytoplankton growth was negatively correlated with FGA biomass, cladoceran densities were positively related to the latter. A comparison among macrophyte-dominated lakes and FGA-dominated lakes showed similarities among them regarding water clarity, nutrient concentrations, phytoplankton growth and species composition, zooplankton assemblage and abundance, but not for abundance of benthic macroinvertebrates. We suggest that in a lake lacking submerged plants, abundance of FGA may act as a buffer against a forward switch to phytoplankton dominance, thus maintaining a clear-water state. The control of phytoplankton growth was primarily not through FGA removing nutrients from the water because sufficient available nutrients were recorded in water during summer. Rather it was by intense grazing by cladocerans ostensibly supported in refuges provided by FGA.

Long-term variability of vertical chlorophyll a and nitrate profiles in the open Black Sea: eutrophication and climate change

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 294:95-107 (2005) - doi:10.3354/meps294095 Long-term variability of vertical chlorophyll a and nitrate profiles in the open Black Sea: eutrophication and climate change Oleg A. Yunev1,*, Snejana Moncheva2, Jacob Carstensen3,4 1Institute of Biology of the Southern Seas, National Academy of the Sciences of Ukraine, Nakhimov Av. 2, Sevastopol 99011, Ukraine2Institute of Oceanology, Bulgarian Academy of Sciences, Varna 9000, Bulgaria3National Environmental Research Institute, Roskilde 4000, Denmark4Joint Research Centre, Ispra 21020, Italy *Email: yunev@sevcom.net ABSTRACT: The objective of this study was to investigate long-term trends in the subsurface summer phytoplankton community and their underlying mechanisms in relation to nutrient enrichment of the open Black Sea. We analysed parameters describing the summer chlorophyll a (chl a) and annual mean nitrate depth distribution in terms of their maximum concentration and its depth, the mean nitrate gradient for the layer between the 2 maxima, and Secchi depth. Investigating the entire period from 1964 to 1996 did not reveal any direct relationship between the nitrate and phytoplankton levels. The nitrate level in the open Black Sea increased until 1984/85, corresponding with general increases in anthropogenic riverine nutrient inputs to the sea. Thus, the average maximum nitrate concentration and the nitrate gradient for all depth profiles increased between 1969 and 1984 and then remained constant. Despite the early increase in nitrate characteristics, the maximum in the chl a profile remained unaltered from 1964 to 1984. After the mid-1980s, the magnitude of the chl a profile gradually increased to 1991 and peaked with an exceptionally high concentration in 1992. In this same period there were only moderate fluctuations in the nitrate profile characteristics. The change of the summer phytoplankton biomass levels since 1984/85 coincided with an intensification of the Rim Current and the associated transport of nitrate to the euphotic zone. Prior to 1984/85 strong stratification and a deep halocline resulted in nitrate accumulation only, whereas uplifting of the halocline, combined with intensified advective currents, increased the transport of nitrate to the euphotic zone during the summer. This resulted in an associated increase in phytoplankton production and biomass in the following period. While this study shows that the open Black Sea, like the coastal regions, has been impacted by eutrophication, the response is different from the coastal region due to the different physical characteristics of the open waters. KEY WORDS: Chlorophyll profile · Nitrate profile · Secchi depth · Open Black Sea · Long-term variability · Eutrophication Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 294. Online publication date: June 09, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Changes to the phytoplankton assemblage of Lake Kinneret after decades of a predictable, repetitive pattern

Summary1. Phytoplankton abundance and species composition in Lake Kinneret, Israel, have been monitored at weekly or fortnightly intervals since 1969. This paper summarises the resulting 34‐year phytoplankton record with a focus on the last 13 years of new data, and reassesses an earlier conclusion that the lake phytoplankton shows remarkable stability despite a wide range of external pressures.2. The Kinneret phytoplankton record can be split into two major periods. The first, from 1969 till 1993, was a period of distinct stability expressed by a typical annual pattern revolving around a spring bloom of the dinoflagellate Peridinium gatunense that repeated each year. The second period, starting around 1994 and ongoing, is characterised by the loss of the previously predictable annual pattern, with both ‘bloom years’ and ‘no‐bloom years’.3. In the second period, deviations from the previous annual pattern include: the absence of the prevailing spring P. gatunense blooms in some years and increased variability in the magnitude of the bloom in others; intensification of winter Aulacoseira granulata blooms; higher summer phytoplankton biomass with replacement of mostly nanoplanktonic, palatable forms by less palatable forms; new appearance and establishment of toxin‐producing, nitrogen fixing cyanobacteria in summer; increase in the absolute biomass and percentage contribution of cyanobacteria to total biomass; and fungal epidemics attacking P. gatunense.4. The 34‐year record serves to validate Schindler's (1987) assessment that phytoplankton species composition will respond to increased anthropogenic stress before bulk ecosystem parameters.

Interactions among epilimnetic phosphorus, phytoplankton biomass and bacterioplankton metabolism in lakes of varying submerged macrophyte cover

The effect of submerged macrophytes on interactions among epilimnetic phosphorus, phytoplankton, and heterotrophic bacterioplankton has been acknowledged, but remains poorly understood. Here, we test the hypotheses that the mean summer phytoplankton biomass (chlorophyll a): phosphorus ratios decrease with increased macrophyte cover in a series of nine lakes. Further, we test that both planktonic respiration and bacterioplankton production increase with respect to phytoplankton biomass along the same gradient of increasing macrophyte cover. Increased macrophyte cover was associated with a lower fraction of particulate phosphorus in epilimnia, with total particulate phosphorus declining from over 80% of total phosphorus in a macrophyte free lake to less than 50% in a macrophyte rich lake. Phytoplankton biomass (chlorophyll a) too was lower in macrophyte dominated lakes, despite relatively high levels of total dissolved phosphorus. Planktonic respiration and bacterioplankton production were higher in macrophyte rich lakes than would be expected from phytoplankton biomass alone, pointing to a subsidy of bacterioplankton metabolism by macrophyte beds at the whole lake scale. The results suggest that the classical view of pelagic interactions, which proposes phosphorus determines phytoplankton abundance, which in turn determines bacterial abundance through the production of organic carbon, becomes less relevant as macrophyte cover increases.

Spatial and temporal heterogeneity in succession of cyanobacterial blooms in a Spanish reservoir

We describe here the limnological characteristics of the Santillana Reservoir (Spain) and study the phytoplankton distribution during a typical growing season (July to November). The reservoir is a eutrophic stratified waterbody in central part of Spain, in which the average phytoplankton biomass in summer is higher than 20 ug chlorophyll a 1-1. Maximum phytoplankton biomass in 1999 occurred between the end of summer and the beginning of autumn (September-October). Cyanobacteria, that were generally prevalent in phytoplankton community, clearly dominated during the peak of phytoplankton biomass. Microcystis wesenbergii was dominant amongst the cyanobacteria and represented almost 100 % of the biomass at the time of maximum phytoplankton value. The importance of environmental variables triggering cyanobacterial bloom events is discussed. Our results indicate that neither temperature nor N/P ratio are related to the bloom development. However, high nutrient concentra­ tion (ammonium in particular) and stability of the watercolumn seems to coincide with the maximum abundance of Microcystis „ wesenbergii.

Biological and chemical factors influencing shallow lake eutrophication: a long-term study

The focus of eutrophication research has tended to be upon short-term and experimental studies. However, given the range of factors that can influence eutrophication dynamics, and that these matter over a range of time scales, some discrete, some continuous, eutrophication dynamics may only be fully investigated when long-term, time-series data are available. The present study aims to evaluate the interacting effects of abiotic processes and biotic dynamics in explaining variations of phytoplankton biomass in a eutrophic shallow lake, Barton Broad (Norfolk, UK) using a long-term data set. Multivariate statistical analysis shows that the inter-relationships between phytoplankton variability, nutrient and grazing factors were highly sensitive to seasonal periodicity. In spring phytoplankton biomass was related to phosphorus, nitrogen and silicon. In summer phytoplankton biomass was associated with phosphorus, nitrogen and zooplankton. In autumn phytoplankton was related to phosphorus, nitrogen, silicon and zooplankton. In winter, no significant relationship could be established between phytoplankton and environmental variables. This paper improves our understanding of the governing role of nitrogen, phosphorus, silicon and zooplankton upon phytoplankton variability, and hence, improves management methods for eutrophic lakes.

Temporal and spatial patterns in the surface-water biomass of phytoplankton in the North Water

Abstract Temperature, salinity, and in vivo fluorescence of surface seawater in the North Water were recorded continuously, using a CTD+fluorometer, in August 1997, April–July 1998 and August–October 1999. The phytoplankton bloom started in the polynya on the Greenland side in April. In April and May, high phytoplankton biomass coincided with saline water on the Greenland side, while biomass was low on the Ellesmere Island (Canada) side where a deep mixed layer prevailed. High phytoplankton biomass extended over the whole polynya in June, when surface temperature increased due to solar heating and salinity decreased due to freshwater input. The initiation of the bloom was about 2 months earlier on the Greenland than the Canadian side. In July and August, phytoplankton biomass became low in the southern survey area, indicating that the phytoplankton bloom had ended. In September, relatively saline and warm water occurred in the southeastern part of the study area where, consistent with the change in water properties, high concentrations of chlorophyll a were observed again. These results imply that both the earlier start of the algal bloom in spring and the eventual increase in phytoplankton biomass in summer contribute to the high annual primary production along the Greenland side, thus influencing the structure and biological productivity of the entire North Water ecosystem.

Prediction of Lake Baikal ecosystem behaviour using an ecosystem disturbance model†

AbstractThis paper combines predictions of the effects of anthropogenic impacts on the plankton of Lake Baikal with models of ecosystem disturbance. Increases in mineralization, non‐toxic organic matter, nutrients, phenolic compounds, oil products and heavy metals were simulated. Significantly higher sensitivity of the community below the ice to external influences was demonstrated compared to the summer–autumn community, when there was no ice layer. Models of the distribution of aquatic pollutants demonstrate the occurrence of deviations from ecosystem state parameters in the bottom layer under the influence of pollutant input with precipitation and the distribution of perturbations over greater (up to 500 km) distances. Simulation of pollutant input at present levels shows that Lake Baikal is already perturbed. This is indicated by increases in bacterial and summer phytoplankton biomass and nutrient concentration, and by fluctuations in the zooplankton biomass.

Advantages from diel vertical migration can explain the dominance of Gonyostomum semen (Raphidophyceae) in a small, steeply-stratified humic lake

In late summer, a large flagellated alga, Gonyostomum semen (Raphidophyceae), consti- tuted most of the phytoplankton biomass in a small steeply-stratified humic lake. Its diel vertical migration (DVM) was very distinct and extended at night into the anoxic hypolimnion. After midsummer, the depletion of hypolimnetic oxygen led to a gradual release of soluble reactive phos- phorus (SRP) from the sediment, but one month later, irrespective of continuing stratification, the concentrations again returned to undetectable levels down to the bottom. As this coincided with the rapid increase in G.semen population, the latter was probably responsible for the depletion of SRP. The flux of SRP from the sediment to the epilimnion was virtually interrupted, making non-migrating phytoplankton dependent only on regenerated and inflowing inorganic phosphorus. Besides nutrient availability, DVM also benefited G.semen in the reduction of metabolic and grazing losses. In this lake, the remarkable multiple advantages of DVM probably explain the dominance of the large G.semen in the late summer phytoplankton biomass over much smaller algae.

Seasonal stages of phytoplankton community structure and sinking loss in the Gulf of Riga

In this paper, we present the biomass, species composition and sinking losses of phytoplankton and heterotrophic flagellates obtained during three seasonal stages (May 1995; June–July 1994; August 1993) from the Gulf of Riga — a eutrophied, semi-enclosed area in the Baltic Sea. The Gulf was characterised by intensive dinoflagellate (mainly Peridiniella catenata) dominated spring bloom (2700–7600 μg l −1 wet weight) while the diatom Thalassiosira baltica contributed most (80–90%) to the settling phytoplankton biomass (up to 6.5 g m −2 day −1). The mineral nutrients were abundant during the bloom and it is suggested that the differential sedimentation of species was caused by physical factors (thermal stratification of the water column). The phytoplankton biomass in summer (780–2600 μg l −1) was dominated by high abundance of filamentous cyanobacterium Aphanizomenon flos-aquae, autotrophic nanoflagellates and picoplankton. The primary vertical flux of phytoplankton (<1 g m −2 day −1) was mainly due to non-motile species and aggregate-bound picoplankton and A. flos-aquae. Below the pycnocline, resuspended dormant diatom populations caused a significant (up to 1 g m −2 day −1) secondary flux. Large heterotrophic dinoflagellates ( Gyrodinium/Gymnodinium) were abundant (up to 500 μg l −1) in the middle layers and associated to ammonium regeneration. The early autumn stage was a post cyanobacterial bloom situation with relatively low phytoplankton abundance (330–860 μg l −1) dominated by A. flos-aquae and nanoflagellates. Also, the sedimentation of phytoplankton was lowest (up to 340 mg m −2 day −1). Our results indicate high seasonal differences in the phytoplankton community structure and sedimentation. However, the high within-season temporal variability overrules the variability between different areas of the southern part of the Gulf.

Seasonal succession of phytoplankton and zooplankton along a trophic gradient in a eutrophic lake - implications for food web management

The water quality and seasonal dynamics of phytoplankton and zooplankton in Lake Hiidenvesi were explored in order to evaluate the possibilites to improve the water quality by mass removal of fish. The four sampling stations were situated in basins representing a transition from hypertrophy to mesotrophy and maximum depths from 3 m to 33 m. The maximal phytoplankton biomasses ranged from 5 to 33 mg l-1 and the maximal zooplankton biomasses from 160 to 520 μg C l-1. Cyanophytes dominated the phytoplankton biomass in summer and the copepod biomass was higher than that of cladocerans at all the stations. No spring peak in the zooplankton biomass occurred and the average size of daphnids was below 1 mm. At present, the top-down control of phytoplankton biomass is thus ineffective. However, the water quality and morphometry of L. Hiidenvesi seem favourable for biomanipulation. The nutrient concentrations and external loading fall in the limits that facilitate successful restoration and the morphometry of the lake is favourable for the recolonization by submerged macrophytes. However, since stations 2 and 3 receive their nutrient loading mainly from station 1, restoration of station 1 may be crucial for the whole lake. This may not be achieved through food web management alone, but requires a reduction of the nutrient flow from River Vihtijoki.

The Optical Water Quality of Cannonsville Reservoir: Spatial and Temporal Patterns, and the Relative Roles of Phytoplankton and Inorganic Tripton

ABSTRACT Longitudinal and temporal distributions of optical properties, and the relative role of selected light attenuating constituents in regulating light penetration, in Cannonsville Reservoir, NY, in 1995 are documented. The analysis is supported by field measurements of light penetration; die angular distribution of irradiance (reflectance); and the spectral quality of penetrating light; laboratory measurements of turbidity, chlorophyll, gelbstoff, and microscopy-based individual particle size and elemental chemistry, and calculations of absorption and scattering coefficients and their components. Longitudinal gradients in attenuating components, and therefore optical properties, prevailed along the main axis of the reservoir; light attenuation and the relative role of scattering decreased with the approach to the dam. Large temporal variations in optical characteristics occurred. These dynamics were mostly controlled by variations in phytoplankton biomass in spring and early summer, when the reservoir was full, but resuspended inorganic tripton (non-phytoplankton particles; suspensoids) became the regulating component as the reservoir was drawn down. Quantitative optical frameworks (models) are applied to simulate light penetration conditions that would have prevailed for scenarios of reductions in phytoplankton biomass and/or inorganic tripton.

Changes in Phytoplankton Size-class Abundance and Species Composition Coinciding with Changes in Water Chemistry and Zooplankton Community Structure of Lake Michigan, 1983 to 1992

Phytoplankton and zooplankton were collected at offshore sites of Lake Michigan during 37 cruises in the spring and summer from 1983 to 1992. For the period, 39 common phytoplankton species accounted for 96.0% of the total abundance and 85.6% of the biomass. Over the 10-year study, the lake-wide average spring and summer phytoplankton biomass in the pelagic waters ranged from 0.27 to 1.2 g/m 3 (mean ± S.E. = 0.54 ± 0.03 g/m 3) and phytoplankton abundance ranged from 5,132 to 39,780 cells/mL (mean ± S.E. = 18,291 ± 822 cells/mL). Mesotrophic diatoms accounted for 47.2% of the total phytoplankton biomass. The lack of a trend in the ratio of mesotrophic to eutrophic diatom indicator species suggested that no change in trophic status of the pelagic region occurred during the 1983 to 92 period; that is, the water quality of the offshore of Lake Michigan did not change. A year-to-year shift in dominance from one mesotrophic diatom species to another was evident. Over the 10-year period, flagellates accounted for 52.2% of the summer phytoplankton biomass. The data support the concept of a shift in summer species composition away from blue-green algae dominance to flagellates prior to 1982. However after 1987, the relative importance of the > 70 μm size class increased to over 21% of the phytoplankton abundance in the spring and over 55% in the summer. In particular, the colonial blue-greens, Anacystis and Aphanothece, became substantially more prevalent during the summer, while the flagellates, Chromulina and Ochromonas, decreased in abundance. The changes in the relative abundance of phytoplankton size classes and species composition were examined to determine if they correlated with either the top-down mediated changes in the zooplankton community during the 1980s and 1990s or with any bottom-up variability in nutrient chemistry. Canonical correspondence analysis (CCA) suggested a significant portion of the variability of the spring and summer phytoplankton size classes from 1983–1992 (88.0% and 99.2%, respectively) was explained by environmental axis 1. Primary determinants of relative abundance of phytoplankton size class included nutrients (silica, total phosphorus, and N:P ratio) and abundance of some species of zooplankton. Specifically, Diaptomus minutus, D. ashlandi, D. sicilis, and Daphnia galeata mendotae were inversely correlated with the 0 to 10 μm size class, which we interpret as a grazing effect on phytoplankton size structure, and positively correlated with an increase in abundance of the > 70 μm size class of phytoplankton. CCA suggested that top-down and bottom-up effects were affecting phytoplankton size composition and abundance simultaneously.

Summer Phytoplankton Primary Production and Biomass in An Alpine Lake, Mondsee (Austria)

In this study, phytoplankton primary production, phytoplankton biomass, light and temperature were investigated in depth profiles during late summer in Mondsee, Austria. During the research period, the highest and the lowest primary production was estimated at 29.4 and 0.2 mg C/m3.h at 2 and 12 m depth, respectively. The maximum rates of production occurred at low light transmission on the 5th of September. Integral primary production varied from 100.7 to 154.6 mg C/m2.h. Phytoplankton biovolumes increased towards the end of the experimental period. Chlorophyll a varied from 0.56 to 3.65 mg/m3. The depth profiles of primary production showed a similarity to phytoplankton biomass. During the research period, the composition of phytoplankton ranged from Cryptophyceae to Cyanophyceae by the dominance of Microcystis aeruginosa. Dinophyceae, Chlorophyceae and Chrysophyceae were usually low in number while Bacillariophyceae species increased during the research period.

Water Quality Improvements in Liverpool Docks: The Role of Filter Feeders in Algal and Nutrient Dynamics

Abstract. The redevelopment of disused docks around Liverpool into areas suitable for shops, offices, housing and recreational watersports has necessitated the maintenance of good water quality in these areas. The main source of water to the docks is the River Mersey, which contains high concentrations of nutrients, encouraging algal blooms ‐ including potentially nuisance species ‐ in the enclosed dock basins. Since 1988 an extensive monitoring programme has been run sampling the hydrography, plankton and benthos of the docks. During this period there has been an improvement in water quality, including a reduction in summer phytoplankton biomass and anoxic bottom water. There are strong indications that phytoplankton biomass has been severely reduced by the filter feeding of Mytifus edulis. Changes in the composition of the benthic community since 1988 suggest that the large population of M. edulis is now suffering from limited recruitment. The long‐term development and stability of the system is discussed with reference to community succession and its link to ecosystem functioning.

Some oceanographic features of northeastern Atlantic waters west of Ireland

Physical, chemical and biological observations were made in the oceanic and shelf waters of the northeast Atlantic to the west and northwest of Ireland in the spring and summer of 1990–1993. A predominant feature on the shelf to the west of Ireland was a thermohaline front separating Irish coastal water from oceanic waters further offshore. Over the shelf, temperature/salinity data showed a core of water freshening and cooling as it progressed northwards up the Rockall Channel. A consistent feature observed in spring was the existence of a wedge of warm, high salinity water flowing through the Porcupine Seabight onto the eastern side of the Porcupine Bank. In spring, surface dissolved oxygen and nutrient data were inversely related suggesting biological control of their distributions at this time of year. Small interannual variations were observed suggesting differences in the timing of the spring phytoplankton bloom between years. A brief description of the spring and summer phytoplankton biomass and species composition is presented.

Build-up and decline of summer phytoplankton biomass in the eastern Weddell Sea, Antarctica

The seasonal development and decline of phytoplankton was investigated in the eastern Weddell Sea during summer and fall 1991. During the first half of the study (15 Jan–13 Feb) in an area off Vestkapp, favourable irradiance/mixing regimes initiated net phytoplankton growth in ice-free waters on the shelf and in stretches of open water over the partially ice-covered deep ocean. Chi a concentrations in the upper water column were moderate (0.2–0.8 μg l−1), but significantly above winter values. Later in the season (16 Feb–11 March), a phytoplankton bloom with surface Chl a concentrations ranging from 1.6–2.3 μg l−1 was encountered in an area further to the east. We suggest that the upper water column must have been stratified in this region for time scales of weeks to faciliate bloom development. Bacterial biomass and productivity generally paralleled the seasonal development of the phytoplankton. Nitrate concentrations in the upper mixed layer were substantially lower than would be expected from the existing phytoplankton standing stock, suggesting that heterotrophic consumption of organic matter by bacteria and zooplankton removed a large fraction of the primary production. The shallow seasonal pycnocline was eventually eroded by the passage of a storm, resulting in a homogeneous distribution of phytoplankton biomass over the entire water column, followed by sedimentation and deposition of phytodetritus on the sea floor. After the storm induced destratification, bacterial productivity was particularly high, amounting to more than half of the primary production (range: 10%–120%) in the upper water column. Subsequently, phytoplankton biomass in the upper water column decreased to values <1 μg Chl a l−1. The combination of low incident irradiances and incessant deep mixing prevented the phytoplankton biomass to increase again. During the last week of the investigation, extensive new-ice formation was observed. A major fraction of the residual surface plankton was incorporated into new sea ice, thus terminating the pelagic growth season of the phytoplankton in the eastern Weddell Sea.

The first decade of oligotrophication of Lake Constance : II. The response of phytoplankton taxonomic composition.

In Lake Constance, after several decades of cutrophication, a decrease in phosphorus loading over the last decade has lead to a partial recovery from eutrophication. Here we analyse the shift in the taxonomic composition of phytoplankton during the first decade of oligotrophication in Lake Constance. During the 1980s, spring total P concentrations decreased from ca. 130 to less than 50 μ·l-1. This decrease was reflected by an approximately proportional decrease in summer phytoplankton biomass while spring phytoplankton biomass seemed unresponsive. Major taxonomic changes occured during both growth seasons. In spring, the proportion of diatoms, green algae and Chrysophyta increased while the proportion of Cryptophyta decreased. The summer trend was very different: the relative importance of diatoms decreased and Cryptophyta and Chrysophyta increased, while Chlorophyta reached their peak around 1985. These trends are also analysed at the genus level. Comparison with taxonomic trends during the eutrophication period shows the expected reversals in most cases. Comparison with other lakes shows general similarities, with the notable exception that Planktothrix rubescens has never been important in Lake Constance. The increase of diatoms during spring is attributed to their improved competitive performance with increasing Si:P ratios. Their decrease during summer is explained by the increasing silicate removal from the epilimnion by increasing spring populations.

Trophic interactions and direct physical effects control phytoplankton biomass and production in an estuary

The recent invasion of San Francisco Bay by the suspension-feeding clam Potamocorbula amurensis has provided an opportunity to document the ecological consequences of a major biological disturbance. Previous work over the last two decades has shown that phytoplankton biomass in the upper estuary is low (2–3 mg Chl a m−3) during seasonal periods of high river flow and short residence time, and it is usually high (peak > 30 mg Chl a m−3) during the summer‒autumn seasons of low river flow and long residence time. However since P. amurensis became widespread and abundant in 1987, the summer phytoplankton biomass maximum has disappeared, presumably because of increased grazing pressure by this newly introduced species. For the period 1977–1990, mean estimated primary production was only 39 g C m 2 yr−1 during years when bivalve suspension feeders were abundant (> 2,000 m−2), compared to 106 g C m−2 yr−1 when bivalves were absent or present in low numbers. These observations support the hypothesis that seasonal and interannual fluctuations in estuarine phytoplankton biomass and primary production can be regulated jointly by direct physical effects (e.g. river-driven transport) and trophic interactions (episodes of enhanced grazing pressure by immigrant populations of benthic suspension feeders).