Influence of sea ice on the composition of the spring phytoplankton bloom in the northern Baltic Sea

During the late winter and spring of 1994, the influence of sea ice on phytoplankton succession in the water was studied at a coastal station in the northern Baltic Sea. Ice cores were taken together with water samples from the underlying water and analysed for algal composition, chlorophyll a and nutrients. Sediment traps were placed under the ice and near the bottom, and the sedimented material was analysed for algal composition. The highest concentration of ice algae (4.1 mmol C m⁻²) was found shortly before ice break-up in the middle of April, coincidental with the onset of an under-ice phytoplankton bloom. The ice algae were dominated by the diatoms Chaetoceros wighamii Brightwell, Melosira arctica (Ehrenberg) Dickie and Nitzschia frigida Grunow. Under the ice the diatom Achnanthes taeniata Grunow and the dinoflagellate Peridiniella catenata (Levander) Balech were dominant. Calculations of sinking rates and residence times of the dominant ice algal species in the photic water column indicated that only one ice algal species (Chaetoceros wighamii) had a seeding effect on the water column: this diatom dominated the spring phytoplankton bloom in the water together with Achnanthes taeniata and Peridiniella catenata. Received: 9 May 1997 / Accepted: 15 February 1998     

The Effect of Artificial Destratification on Phytoplankton in a Reservoir

The effects of artificial destratification on limnological conditions and on phytoplankton were surveyed for 6 years (1995-2000) in Lake Dalbang (South Korea), a water supply reservoir receiving nutrients from agricultural non-point sources. In order to reduce odor problems caused by cyanobacterial blooms, six aerators were installed in 1996 and operated regularly during the warm season. Aeration destratified the water column of the reservoir and produced homogeneous physical and chemical parameters. The maximum surface temperature in summer decreased from 28.9 °C before aeration to 20.0-26.4 °C after aeration, whereas the maximum hypolimnetic temperature increased from 8.0 to 17.0-23.7 °C. Despite these changes, surface water concentrations of total phosphorus (TP) and chlorophyll a(CHLA) and their seasonal patterns did not change with destratification. Phosphorus loading was concentrated in heavy rain events during the summer monsoon, and TP and CHLA reached maximal concentrations in late summer after the monsoon. Because the hypolimnion was never anoxic prior to aeration, internal loading did not seem to be substantial. Cyanobacteria were the dominant phytoplankton in summer before aeration, but diatoms replaced them after operation of the aerator. Cyanobacteria blooms were eliminated. In contrast, total algal biomass in the water column (as CHLA integrated over depth) increased from 190 mg m^–2 in 1995 to 1150, 300, 170, and 355 mg m^–2 in 1997, 1998, 1999, and 2000, respectively. The increased ratio of mixing depth to euphotic depth to 2.5 may have resulted in a net reduction in the amount of underwater irradiance experienced by phytoplankton cells, and this may have favored the switch to diatom dominance. Furthermore, the mixing may have allowed diatoms to flourish in summer by lowering their settling loss that would be critical in stratified water columns. In conclusion, the destratification in this reservoir was effective in preventing cyanobacteria blooms, but not in reducing the total algal standing crop.     

Effects of light and nutrients on seasonal phytoplankton succession in a temperate eutrophic coastal lagoon

Rodeo Lagoon, a low-salinity coastal lagoon in the Golden Gate National Recreation Area, California, United States, has been identified as an important ecosystem due to the presence of the endangered goby (Eucyclogobius newberri). Despite low anthropogenic impacts, the lagoon exhibits eutrophic conditions and supports annual episodes of very high phytoplankton biomass. Weekly assessments (February–December 2007) of phytoplankton indicated diatoms, Nodularia spumigena, Chaetoceros muelleri var. muelleri, flagellated protozoa, a mixed assemblage, and Microcystis aeruginosa dominated the algal community in successive waves. Phytoplankton succession was significantly correlated (r ² = 0.37, p < 0.001) with averaged daily irradiance (max = 29.7 kW m⁻² d⁻¹), water column light attenuation (max = 14 m⁻¹), and orthophosphate and dissolved inorganic carbon concentrations (max = 1.5 and 2920 μM, respectively). Negative effects of phytoplankton growth and decay included excessive ammonia concentrations (exceeded EPA guidelines on 77% of sampling days), hypoxia (<3 mg l⁻¹ dissolved oxygen), and introduction of several microcystins, all in the latter half of the year. Our one-year study suggests that this coastal lagoon is a highly seasonal system with strong feedbacks between phytoplankton and geochemical processes.     

The Relationship between Phytoplankton Diversity and Community Function in a Coastal Lagoon

The decrease of biodiversity related to the phenomena of global climate change is stimulating the scientific community towards a better understanding of the relationships between biodiversity and ecosystem functioning. In ecosystems where marked biodiversity changes occur at seasonal time scales, it is easier to relate them with ecosystem functioning. The objective of this work is to analyse the relationship between phytoplankton diversity and primary production in St. André coastal lagoon – SW Portugal. This lagoon is artificially opened to the sea every year in early spring, exhibiting a shift from a marine dominated to a low salinity ecosystem in winter. Data on salinity, temperature, nutrients, phytoplankton species composition, chlorophyll a (Chl a) concentration and primary production were analysed over a year. Modelling studies based on production-irradiance curves were also conducted. A total of 19 taxa were identified among diatoms, dinoflagellates and euglenophyceans, the less abundant group. Lowest diversities (Shannon–Wiener index) were observed just before the opening to the sea. Results show a negative correlation (p<0.05) between diversity and chlorophyll a (Chl a) concentration (0.2–40.3 mg Chl a m⁻³). Higher Chl a values corresponded to periods when the community was dominated by the dinoflagellate Prorocentrum minimum (>90% of cell abundance) and production was maximal (up to 234.8 mg C m⁻³ h⁻¹). Maximal photosynthetic rates (P_max) (2.0–22.5 mg C mg Chl a⁻¹ h⁻¹) were higher under lower Chl a concentrations. The results of this work suggest that decreases in diversity are associated with increases in biomass and production, whereas increases correspond to opposite trends. It is suggested that these trends, contrary to those observed in terrestrial and in some benthic ecosystems, may be a result of low habitat diversity in the water column and resulting competitive pressure. The occurrence of the highest photosynthetic rates when Chl a is low, under some of the highest diversities, suggests a more efficient use of irradiance under low biomass–high diversity conditions. Results suggest that this increased efficiency is not explained by potential reductions in nutrient limitation and intraspecific competition under lower biomasses and may be a result of niche complementarity.     

Winter-time ecology in the Bothnian Bay, Baltic Sea: nutrients and algae in fast ice

Landfast ice algal communities were studied in the strongly riverine-influenced northernmost part of the Baltic Sea, the Bothnian Bay, during the winter-spring transition of 2004. The under-ice river plume, detected by its low salinity and elevated nutrient concentrations, was observed only at the station closest to the river mouth. The bottommost ice layer at this station was formed from the plume water (brine volume 0.71%). This was reflected by the low flagellate-dominated (93%) algal biomass in the bottom layer, which was one-fifth of the diatom-dominated (74%) surface-layer biomass of 88 μg C l⁻¹. Our results indicate that habitable space plays a controlling role for ice algae in the Bothnian Bay fast ice. Similarly to the water column in the Bothnian Bay, average dissolved inorganic N:P-ratios in the ice were high, varying between 12 and 265. The integrated chlorophyll a (0.1–2.2 mg m⁻²) and algal biomass in the ice (1–31 mg C m⁻²) correlated significantly (Spearman ρ = 0.79), with the highest values being measured close to the river mouth in March and during the melt season in April. Flagellates <20 μm generally dominated in both the ice and water columns in February–March. In April the main ice-algal biomass was composed of Melosira arctica and unidentified pennate diatoms, while in the water column Achnanthes taeniata, Scrippsiella hangoei and flagellates dominated. The photosynthetic efficiency (0.003–0.013 (μg C [μg chl a ⁻¹] h⁻¹)(μE m⁻²s⁻¹)⁻¹) and maximum capacity (0.18–1.11 μg C [μg chl a ⁻¹] h⁻¹) could not always be linked to the algal composition, but in the case of a clear diatom dominance, pennate species showed to be more dark-adapted than centric diatoms.     

Primary production and pigments in three low alkalinity connected reservoirs receiving mine wastes

A comparative study of primary production and pigments, conducted from April 1981 to April 1982, in a chain of three low alkalinity reservoirs situated in north of Portugal revealed significant lower values (159 mg C m⁻² d⁻¹ and 19.8 mg m⁻² as chlorophyll a, growing season means) in the reservoir receiving wastes rich in copper from a tungsten copper pyrite mine. This contrasts specially with the situation observed in the upstream reservoir which presents the highest values (409 mg C m⁻² d⁻¹ and 55.2 mg m⁻²) recorded in the system. Eventual effects of the mine discharge on phytoplankton biomass and photosynthesis such as high turbidity and copper toxicity are discussed. Results point out to be the presence of copper responsible for the alterations in the phytoplankton community. The system exhibits similar photosynthetic capacity which indicates an adaption to the relatively high copper concentrations in water and sediments. Experiments carried out in the largest unpolluted reservoir show that the loss of carbon through respiration is of major importance and indicate the phosphorus to be one of the factors limiting phytoplankton productivity.     

Different photochemical responses of phytoplankters from the large shallow Taihu Lake of subtropical China in relation to light and mixing

The maximum quantum yield of photosystem II was estimated from variable chlorophyll a fluorescence in samples of phytoplankton collected from the Taihu Lake in China to determine the responses of different phytoplankters to irradiance and vertical mixing. Meteorological and environmental variables were also monitored synchronously. The maximum quantum yield of three phytoplankton groups: cyanobacteria, chlorophytes, and diatoms/dinoflagellates, showed a similar diurnal change pattern. F _v/F _m decreased with a significant depth-dependent variation as irradiance increased during the morning and increased as irradiance declined in the afternoon. Furthermore, the rates of F _v/F _m depression were dependent upon the photon flux density, whereas the rates of recovery of F _v/F _m were dependent upon the historical photon density. Moreover, photoinhibition affected the instantaneous growth rates of phytoplankton. Although at noon cyanobacteria had a higher photoinhibition value (up to 41%) than chlorophytes (32%) and diatoms/dinoflagellates (34%) at the surface, no significant difference in diurnal growth rates among the three phytoplankton groups were observed indicating that cyanobacteria could photoacclimate better than chlorophytes and diatoms/dinoflagellates. In addition, cyanobacteria had a higher nonphotochemical quenching value than chlorophytes and diatoms/dinoflagellates at the surface at noon, which indicated that cyanobacteria were better at dissipating excess energy. The ratios of enclosed bottle samples F _v/F _m to free lake samples F _v/F _m showed different responses for the three phytoplankton groups to irradiance and vertical mixing when wind speed was approximately constant at about 3.0 m s⁻¹. When wind speed was lower than 3.0 m s⁻¹, cyanobacteria accumulated mainly at the surface and 0.3 m, because of their positive buoyancy, where diurnal growth rates of phytoplankton were relatively higher than those at 0.6 m and 0.9 m. Chlorophytes were homogenized completely by vertical mixing, while diatoms/dinoflagellates avoided active high irradiance by moving downward at noon, and then upward again when irradiance decreased. These results explain the dominance of cyanobacteria in Taihu Lake. Handling editor: L. Naselli-Flores     

Winter phytoplankton communities in different depths of three mesotrophic lakes (Łęczna-Włodawa Lakeland, Eastern Poland)

Phytoplankton samples were collected from three mesotrophic lakes: Piaseczno, Rogóźno and Krasne during winter seasons (from January to March). The samples were analyzed for species analysis and abundance of planktonic algae in relation to different depths of water column (0–7 m). Selected water physical-chemical parameters were also measured. Abundance of phytoplankton depended strongly on the thickness of snow and ice cover or mixing conditions. The maximal phytoplankton total number reached about 5 × 10⁶ ind. L⁻¹ beneath the clear ice in the Krasne Lake, minimal numbers were recorded under the thick snow and ice layers in the Piaseczno Lake (2 × 10³ ind. L⁻¹). The winter phytoplankton communities were dominated by flagellates principally cryptomonads (Cryptomonas spp. Rhodomonas minuta), euglenophytes (Trachelomonas volvocina, T. volvocinopsis), dinoflagellates (Peridinium bipes, Gymnodinium helveticum) and chrysophytes (Mallomonas elongata, M. akrokomos, Dinobryon sociale) or non-motile small species of blue-green algae (e.g. Rhabdoderma lineare, Limnothrix redekei), diatoms (Stephanodiscus spp., Asterionella formosa), and green algae (e.g. Scenedesmus spp., Monoraphidium spp.). Phytoplankton abundance and structure showed differentiation during the winter season and along the water column as well.     

Seasonal variation of phytoplankton community structure and nitrogen uptake regime in the Indian Sector of the Southern Ocean

This study investigates the dynamics of phytoplankton communities and nitrogen uptake in the Indian sector of the Southern Ocean during spring and summer. The study area is oligotrophic (Chl a stocks <50 mg m⁻²); nevertheless, a large spatial variation of phytoplankton biomass and community structure was observed. During both seasons the phytoplankton community in the seasonal ice zone showed higher biomasses and was mainly composed of large diatom cells. However, in the permanently open ocean zone the community had low biomass and was chiefly composed of nano- and picoflagellates. In the polar front zone, although biomass was higher, the community structure was similar to the open ocean zone. The results suggest that the variation in phytoplankton community structure on a larger scale resonates with gradients in water column stability and nutrient distribution. However, significant changes in biomass and nutrient stocks but little change in community structure were observed. Absolute nitrogen uptake rates were generally low, but their seasonal variations were highly significant. During spring the communities displayed high specific nitrate uptake (mean rate = 0.0048 h⁻¹), and diatoms (in the seasonal ice zone) as well as nano- and picoflagellates (in the permanently open ocean zone and polar front zone) were mainly based on new production (mean ƒ-ratio = 0.69). The transition to summer was accompanied by a significant reduction in nitrate uptake rate (0.0048 h⁻¹ → 0.0011 h⁻¹) and a shift from predominantly new to regenerated production (ƒ-ratio 0.69 → 0.39). Ammonium played a major role in the seasonal dynamics of phytoplankton nutrition. The results emphasize that, despite a large contrast in community structure, the seasonal dynamics of the nitrogen uptake regime and phytoplankton community structure in all three subsystems were similar. Additionally, this study supports our previous conclusion that the seasonal shift in nitrogen uptake regime can occur with, as well as without, marked changes in community structure. Received: 2 December 1997 / Accepted: 20 April 1998     

Phytoplankton biomass in the sub-Antarctic area of the Straits of Magellan (53°S), Chile during spring-summer 1997/1998

In order to provide a better understanding of the dynamics of phytoplankton in the coastal regions of high latitudes, a study was carried out to estimate the dynamics of carbon biomass of autotrophic and heterotrophic algal groups over the austral spring-summer 1997/1998 period. At a fixed station located in the central basin (Paso Ancho) of the Straits of Magellan (53°S), surface water samples were collected at least once a week from September 1997 (early spring) to March 1998 (late summer). Quantitative analysis of biomass of phytoplankton was estimated from geometric volumes, using non-linear equations, and converted to biomass. The pattern of chlorophyll a showed a strong temporal variability, with maximum values (mean 2.8 mg m⁻³) at the austral spring phytoplankton increase or bloom (October/November) and minimum values during early spring (September: <0.5 mg m⁻³) and summer (January/March: 0.5–1.0 mg m⁻³). During the spring bloom, diatoms made up to 90% of the total phytoplankton carbon (0.01–189 μg l⁻¹), followed by a maximum of thecate dinoflagellates (0.08–34 μg l⁻¹), and sporadic high biomass of phytoflagellates during summer. Heterotrophic algal groups such as Gymnodinium and Gyrodinium spp. dominated (70%, in the 5- to 25-μm size range) shortly before the main diatom bloom, and small peaks were observed within spring and early summer periods (0–0.4 μg l⁻¹). Phytoflagellates dominated earlier (spring) with higher carbon biomass (8 μg l⁻¹) and post-bloom periods (summer) when carbon biomass ranged between 1 and 4 μg l⁻¹. Accepted: 6 September 2000     

Phytoplankton and primary production in clear-vegetated,inorganic-turbid,and algal-turbid shallow lakes from the pampa plain (Argentina)

Shallow lakes often alternate between two possible states: one clear with submerged macrophytes, and another one turbid, dominated by phytoplankton. A third type of shallow lakes, the inorganic turbid, result from high contents of suspended inorganic material, and is characterized by low phytoplankton biomass and macrophytes absence. In our survey, the structure and photosynthetic properties (based on ¹⁴C method) of phytoplankton were related to environmental conditions in these three types of lakes in the Pampa Plain. The underwater light climate was characterized. Clear-vegetated lakes were more transparent (K _d 4.5–7.7 m⁻¹), had high DOC concentrations (>45 mg l⁻¹), low phytoplankton Chl a (1.6–2.7 μg l⁻¹) dominated by nanoflagellates. Phytoplankton productivity and photosynthetic efficiency (α ~ 0.03 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²) were relatively low. Inorganic-turbid lakes showed highest K _d values (59.8–61.4 m⁻¹), lowest phytoplankton densities (dominated by Bacillariophyta), and Chl a ranged from 14.6 to 18.3 μg l⁻¹. Phytoplankton-turbid lakes showed, in general, high K _d (4.9–58.5 m⁻¹) due to their high phytoplankton abundances. These lakes exhibited the highest Chl a values (14.2–125.7 μg l⁻¹), and the highest productivities and efficiencies (maximum 0.56 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²). Autotrophic picoplankton abundance, dominated by ficocianine-rich picocyanobacteria, differed among the shallow lakes independently of their type (0.086 × 10⁵–41.7 × 10⁵ cells ml⁻¹). This article provides a complete characterization of phytoplankton structure (all size fractions), and primary production of the three types of lakes from the Pampa Plain, one of the richest areas in shallow lakes from South America. Handling editor: J. Padisak     

A case of thermal pollution limited primary productivity in a southwestern U.S.A. Reservoir

North Lake, a small (330 ha. surface area) southwestern U.S.A. cooling water reservoir was found to contain less phytoplankton production (104.0 mg C m⁻³ day⁻¹), lower annual mean total organic carbon (3.7 mg l⁻¹) and phytoplankton standing crops (0.9 ml m⁻³) than other local area reservoirs. Concentrations of inorganic P and N were at or below test detection limits during the study year 1973–1974.In situ ¹⁴C non-filtration primary productivity techniques demonstrated significant (≃13 percent) stimulation of planktonic primary productivity due to power plant entrainment. Optical counts showed no destruction of entrained phytoplankters. Populations of Cyanophyta were never dominant, although they frequently bloom in most other local reservoirs. Thermal loading at North Lake is thought to ultimately depress phytoplankton primary production and standing crop by causing nutrient limitation.     

Phytoplankton biomass,P-I relationships and primary production in the Weddell Sea,Antarctica, during the austral autumn

An investigation into the changing phytoplankton biomass and total water column production during autumn sea ice formation in the eastern Weddell Sea, Antarctica showed reduced biomass concentrations and extremely low daily primary production. Mean chlorophyll-a concentration for the entire study period was extremely low, 0.15±0.01 mg.m⁻³ with a maximum of 0.35 mg.m⁻³ found along the first transect to the east of the grid. Areas of low biomass were identified as those either associated with heavy grazing or with deep mixing and corresponding low light levels. In most cases phytoplankton in the <20-μm size classes dominated. Integrated biomass to 100 m ranged from 7.1 to 28.0 mg.m⁻² and correlated positively with surface chlorophyll-a concentrations. Mean P^Bmax (photosynthetic capacity) and α^B (initial slope of the photosynthesis-irradiance curve) were 1.25±0.19 mgC. mgChla ⁻¹.h⁻¹ and 0.042±0.009 mgC.mgChla ⁻¹.h⁻¹.(μmol.m⁻².s⁻¹)⁻¹ respectively. The mean index of photoadaptation,I _k, was 32.2±4.0 μmol.m⁻².s⁻¹ and photoinhibition was found in all cases. Primary production was integrated to the critical depth (Z _cr) at each production station and ranged from 15.6 to 41.5 mgC.m⁻².d⁻¹. It appears that, other than grazing intensity, the relationship between the critical depth and the mixing depth (Z _mix) is an important factor as, ultimately, light availability due both to the late season and growing sea ice cover severely limits production during the austral autumn.     

Biofiltering efficiency, uptake and assimilation rates of Ulva clathrata (Roth) J. Agardh (Clorophyceae) cultivated in shrimp aquaculture waste water

The growth, biofiltering efficiency and uptake rates of Ulva clathrata were studied in a series of outdoor tanks, receiving waste water directly from a shrimp (Litopenaeus vannamei) aquaculture pond, under constant aeration and two different water regimes: (1) continuous flow, with 1 volume exchange a day (VE day^-1) and (2) static regime, with 1 VE after 4 days. Water temperature, salinity, pH, dissolved inorganic nitrogen (DIN), phosphate (PO₄), chlorophyll-a (chl-a), total suspended solids (TSS), macroalgal biomass (fresh weight) and tissue nutrient assimilation were monitored over 12 days. Ulva clathrata was highly efficient in removing the main inorganic nutrients from effluent water, stripping 70–82% of the total ammonium nitrogen (TAN) and 50% PO₄ within 15 h. Reductions in control tanks were much lower (Tukey HSD, P < 0.05). After 3 days, the mean uptake rates by the seaweed biomass under continuous flow were 3.09 mg DIN g DW day⁻¹ (383 mg DIN m⁻² day⁻¹) and 0.13 mg PO₄ g DW day⁻¹ (99 mg PO₄ m⁻² day⁻¹), being significantly higher than in the static regime (Tukey HSD, P < 0.05). The chl-a decreased in seaweed tanks, suggesting that U. clathrata inhibited phytoplankton growth. Correlations between the cumulative values of DIN removed from the water and total nitrogen assimilated into the seaweed biomass (r = 0.7 and 0.8, P < 0.05), suggest that nutrient removal by U. clathrata dominated over other processes such as phytoplankton and bacterial assimilation, ammonia volatilization and nutrient precipitation.     

Nutrient ratios and phytoplankton community structure in the large, shallow, eutrophic, subtropical Lakes Okeechobee (Florida, USA) and Taihu (China)

Analysis of ten- and four-year datasets for the large, shallow, subtropical, and eutrophic Lakes Okeechobee (USA) and Taihu (China), respectively, suggest that resource-ratio explanations for cyanobacteria dominance may not apply to these two lakes. Datasets were examined to identify relationships between nutrient ratios [total nitrogen (TN):total phosphorus (TP) and ammonium (NH₄ ⁺):oxidized N (NO _x )] and phytoplankton community structure (as proportions of cyanobacteria and diatoms to total phytoplankton biomass). Datasets were pooled by sampling month, averaged lake-wide, and analyzed with linear regression. In Okeechobee, the cyanobacteria proportion increased and the diatom proportion decreased with increasing TN:TP. In Taihu, cyanobacteria decreased with increasing TN:TP, but the opposite trend observed for diatoms was marginally significant. Okeechobee cyanobacteria increased and diatoms decreased with increasing NH₄ ⁺:NO _x , but no significant relationships between phytoplankton and NH₄ ⁺:NO _x were observed in Taihu. Both lakes had significant relationships between phytoplankton community structure and total nutrients, but these relationships were the opposite of those expected. Relationships between phytoplankton community structure and water quality parameters from the previous month resulted in improved relationships, suggesting a predictive capability. Statistical analysis of the entire datasets (not pooled) supported these and additional relationships with other parameters, including temperature and water clarity.     

Comparison of productivity and phytoplankton in a warm (Kongsfjorden) and a cold (Hornsund) Spitsbergen fjord in mid-summer 2002

Kongsfjorden and Hornsund are two glacial fjords without sills on the West Spitsbergen coast. Both sites are under the influence of relatively warm Atlantic-derived water, although Hornsund is more influenced by cold water from the Barents Sea. In this study, we compared the impacts of cold Arctic and warmer Atlantic waters on the pelagic ecosystems of Kongsfjorden and Hornsund. Both fjords were strongly influenced by Atlantic-derived waters during summer (2002). Diatoms were the most substantial contributors to phytoplankton biomass, especially in outer basins of both fjords, whereas the second most important contributors were autotrophic dinoflagellates in Kongsfjorden and nanoflagellates in Hornsund. Total phytoplankton biomass was highest in Hornsund. Primary production rates were an order of magnitude lower in Kongsfjorden than in Hornsund, and increased from inner to outer fjord (from 2.47 to 4.48 mg C m⁻² h⁻¹ in Kongsfjorden and from 14.00 to 86.65 mg C m⁻² h⁻¹ in Hornsund). Chlorophyll-a concentration was also substantially lower in Kongsfjorden. Zooplankton was dominated by omnivorous species in Kongsfjorden and herbivorous in Hornsund. Observed differences between the fjords may originate from (1) advection of different waters into the fjords; (2) differences in freshwater runoff and turbidity, and (3) timing of the phytoplankton bloom. Climate warming will likely increase the Atlantic water influence, and result in reduced production of diatoms and increase in flagellates.     

Seasonal abundance and feeding patterns of copepods Temora longicornis, Centropages hamatus and Acartia spp. in the White Sea (66��N)

We have studied the seasonal dynamics of abundance and feeding characteristics of three species of calanoid copepods (Acartia spp., Centropages hamatus and Temora longicornis) in the White Sea from the surface water layer (0–10 m), in order to assess their role in the pelagic food web and to determine the major factors governing their population dynamics during the productive season. These species dominated in the upper water layer (0–10 m) from June through September, producing up to 3 generations per year. Data on the food spectra revealed all species to be omnivorous; but some inter- and intraspecific differences were observed. Generally, copepods consumed diatoms, dinoflagellates and microzooplankton. The omnivory index ‘UC’ (i.e., fatty acid unsaturation coefficient) varied from 0.2 to 0.6, which implied ingestion of phytoplankton. The different degree of selectivity on the same food items by the studied species was observed, and therefore, successful surviving strategy with minimal overlapping could be assumed. In total, the populations of the three studied copepod species grazed up to 2.15 g C m⁻² day⁻¹ and released up to 0.68 g C m⁻² day⁻¹ in faecal pellets. They consumed up to 50% of particulate organic carbon, or up to 85% of phytoplankton standing stock (in terms of Chl. a), and thus played a significant role in the transformation of particulate organic matter. Seasonal changes in abundance of the studied species depended mostly on water temperature in the early summer, but were also affected by food availability (Chl. a concentration) during the productive season.     

The recovery of a very shallow eutrophic lake, 20 years after the control of effluent derived phosphorus

1. Monitoring at fortnightly to monthly intervals of a very shallow, lowland lake over 24 years has enabled the time course of recovery from nutrient enrichment to be investigated after high external P loading of the lake (>10 g P m^?2 year^?1) was reduced between 1977 and 1980. 2. The lake showed a relatively rapid response during the spring and early summer, with a reduction in phytoplankton biomass occurring after 5 years when soluble reactive phosphorus concentration was <10 μg L^?1. 3. However, during the later summer the response was delayed for 15 years because of sustained remobilisation of phosphorus from the sediment. The greater water clarity in spring and a gradual shift from planktonic to benthic algal growth may be related to the reduction in internal loading after 15 years. 4. Changes in the phytoplankton community composition were also observed. Centric diatoms became less dominant in the spring, and the summer cyanobacteria populations originally dominated by non‐heterocystous species (Limnothrix/Planktothrix spp.) almost disappeared. Heterocystous species (Anabaena spp. and Aphanizomenon flos‐aquae) were slower to decline, but after 20 years the phytoplankton community was no longer dominated by cyanobacteria. 5. There were no substantial changes in food web structure following re‐oligotrophication. Total zooplankton biomass decreased but body size of Daphnia hyalina, the largest zooplankton species in the lake, remained unchanged, suggesting that the fish population remained dominated by planktivorous species. 6. Macrophyte growth was still largely absent after 20 years, although during the spring water clarity may have become sufficient for macrophytes to re‐establish.     

Drift-ice and under-ice water communities in the Gulf of Bothnia (Baltic Sea)

The algal, protozoan and metazoan communities within different drift-ice types (newly formed, pancake and rafted ice) and in under-ice water were studied in the Gulf of Bothnia in March 2006. In ice, diatoms together with unidentified flagellates dominated the algal biomass (226 ± 154 μg ww l⁻¹) and rotifers the metazoan and protozoan biomass (32 ± 25 μg ww l⁻¹). The under-ice water communities were dominated by flagellates and ciliates, which resulted in lower biomasses (97 ± 25 and 21 ± 14 μg ww l⁻¹, respectively). The under-ice water and newly formed ice separated from all other samples to their own cluster in hierarchical cluster analysis. The most important discriminating factors, according to discriminant analysis, were chlorophyll-a, phosphate and silicate. The under-ice water/newly formed ice cluster was characterized by high nutrient and low chlorophyll-a values, while the opposite held true for the ice cluster. Increasing trends in chlorophyll-a concentration and biomass were observed with increasing ice thickness. Within the thick ice columns (>40 cm), the highest chlorophyll-a concentrations (6.6–22.2 μg l⁻¹) were in the bottom layers indicating photoacclimation of the sympagic community. The ice algal biomass showed additional peaks in the centric diatom-dominated surface layers coinciding with the highest photosynthetic efficiencies [0.019–0.032 μg C (μg Chl-a ⁻¹ h⁻¹) (μE m⁻² s⁻¹)⁻¹] and maximum photosynthetic capacities [0.43-1.29 μg C (μg Chl-a ⁻¹ h⁻¹)]. Rafting and snow-ice formation, determined from thin sections and stable oxygen isotopic composition, strongly influenced the physical, chemical and biological properties of the ice. Snow-ice formation provided the surface layers with nutrients and possibly habitable space, which seemed to have favored centric diatoms in our study.     

Phytoplankton biomass and production during late austral spring (1991) and summer (1993) in the Bransfield Strait

Phytoplankton biomass and productivity were measured during two cruises in the Bransfield Strait in December 1991 (D91) and January/February 1993 (J93). Strong seasonal variability in productivity values was observed due to differences in the physiological response of phytoplankton. However, although the photosynthetic capacity of phytoplankton was markedly lower in D91 [P _m ^B =0.61 ± 0.25 mg C (mg Chla)⁻¹ h⁻¹] than in J93 [P _m ^B =2.18 ± 0.91 mg C (mg Chla)⁻¹ h⁻¹], average water column chlorophyll values in different areas of the strait were approximately similar in D91 (49–78 mg Chla m⁻²) and J93 (22–76 mg Chla m⁻²). The spatial distribution of chlorophyll was patchy and generally associated with the influence of the different water masses that meet together in the Bransfield Strait. No correlation was found between the mixed layer depth and either the integrated chlorophyll or the productivity. Our results suggest that major phytoplankton blooms in the Bransfield Strait are advected from the nearby Gerlache Strait or Bellingshausen Sea following the main eastward surface currents. Accepted: 5 July 1998