Seasonal variations of microphytoplankton assemblages and environmental variables in the coastal zone of Bozcaada Island in the Aegean Sea (NE Mediterranean Sea)

Seasonal changes in the microphytoplankton assemblages were examined in the coastal zone of Bozcaada Island with regard to some major physical and chemical variables. Samples were collected from May 2000 to December 2001 at four stations. A total of 108 dinoflagellates, 102 diatoms, 1 chrysophycean, 3 dictyochophycean, and 1 prasinophycean species were identified and quantified during the study period. Diatoms and dinoflagellates were the most important in terms of species number and abundance. The maximum values of total microphytoplankton were observed at 0.5 m depth (46.2 × 10³ cells l⁻¹ at st. 3) in May as this was the month when the diatom Pseudo-nitzschia pungens bloomed. Chlorophyll (chl) a concentration ranged between 0.08 (August) and 0.78 μg l⁻¹ (February). May was another important month in which chlorophyll a increased (0.41–0.47 μg l⁻¹). Species diversity values (H′_log2) ranged from 1.66 bits (June, 20 m) to 4.11 bits (November, 0.5 m). The increase was attributed to a more balanced distribution of abundance among species. The amounts of nitrate + nitrite (0.6−3.7 μg-at N l⁻¹), phosphate (0.2−0.6 μg-at P l⁻¹) and silicate (0.7−2.5 μg-at Si l⁻¹) were recorded on each sampling occasion. Nutrient concentrations and chl a values of the research area were found to be poorer than those of the many other coastal areas in the northeastern Mediterranean. The mean atomic ratio of nitrogen to phosphorus varied from 1.3 (June) to 12.9 (February). This ratio was lower than the Redfield ratio of 16 for ocean phytoplankton, and phytoplankton was potentially limited by nitrogen for most of the months. The result of this study confirms and emphasizes the oligotrophic nature of the eastern Mediterranean.     

Epipelic and pelagic primary production in Alaskan Arctic lakes of varying depth

We compared on eight dates during the ice-free period physicochemical properties and rates of phytoplankton and epipelic primary production in six arctic lakes dominated by soft bottom substrate. Lakes were classified as shallow ( < 2.5 m), intermediate in depth (2.5 m <  < 4.5 m), and deep ( > 4.5 m), with each depth category represented by two lakes. Although shallow lakes circulated freely and intermediate and deep lakes stratified thermally for the entire summer, dissolved oxygen concentrations were always >70% of saturation values. Soluble reactive phosphorus and dissolved inorganic nitrogen (DIN = NO₃ ⁻–N + NH₄ ⁺–N) were consistently below the detection limit (0.05 μmol l⁻¹) in five lakes. However, one lake shallow lake (GTH 99) periodically showed elevated values of DIN (17 μmol l⁻¹), total-P (0.29 μmol l⁻¹), and total-N (33 μmol l⁻¹), suggesting wind-generated sediment resuspension. Due to increased nutrient availability or entrainment of microphytobenthos, GTH 99 showed the highest average volume-based values of phytoplankton chlorophyll a (chl a) and primary production, which for the six lakes ranged from 1.0 to 2.9 μg l⁻¹ and 0.7–3.8 μmol C l⁻¹ day⁻¹. Overall, however, increased resulted in increased area-based values of phytoplankton chl a and primary production, with mean values for the three lake classes ranging from 3.6 to 6.1 mg chl a m⁻² and 3.2–5.8 mmol C m⁻² day⁻¹. Average values of epipelic chl a ranged from 131 to 549 mg m⁻² for the three depth classes, but levels were not significantly different due to high spatial variability. However, average epipelic primary production was significantly higher in shallow lakes (12.2 mmol C m⁻² day⁻¹) than in intermediate and deep lakes (3.4 and 2.4 mmol C m⁻² day⁻¹). Total primary production (6.7–15.4 mmol C m⁻² day⁻¹) and percent contribution of the epipelon (31–66%) were inversely related to mean depth, such that values for both variables were significantly higher in shallow lakes than in intermediate or deep lakes. Handling editor: L. Naselli-Flores     

A study of the diatom-dominated microplankton summer assemblages in coastal waters from Terre Adélie to the Mertz Glacier, East Antarctica (139°E–145°E)

In January 2004 the microplankton community from the coastal waters of Terre Adélie and Georges V Land (139°E–145°E) was studied. Results showed a diatom-dominated bloom with chlorophyll a levels averaging 0.64 μg l⁻¹ at 5 m depth (range 0.21–1.57 μg l⁻¹). Three geographic assemblages of diatoms were identified, based on principal diatom taxa abundances. The stratified waters near the Mertz Glacier presented highest phytoplankton biomasses (0.28–1.57 μg Chl a l⁻¹ at 5 m) and diatom abundances (6,507–70,274 cells l⁻¹ at 5 m), but low diversity, dominated by Fragilariopsis spp. Lower biomasses (0.38–0.94 μg Chl a l⁻¹ at 5 m) and abundances (394–9,058 cells l⁻¹ at 5 m) were observed in the mixed waters around the Astrolabe Glacier with a diverse diatom community characterised by larger species Corethron pennatum and Rhizosolenia spp. Finally an intermediate zone between them over the shallower shelf waters of the Adélie Bank represented by Chaetoceros criophilus, where biomasses (0.21–0.35 μg Chl a l⁻¹ at 5 m) and abundances (1,190–5,431 cells l⁻¹ at 5 m) were lowest, coinciding with the presence of abundant herbivorous zooplankton.     

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     

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.     

Physical and chemical limnology of a wind-swept tropical highland reservoir

Valle de Bravo (VB) is a tropical reservoir located (19°21′30″ N, 100°11′00″ W) in the highlands of Mexico. The reservoir is daily swept by strong (7.4 m s⁻¹ mean speed) diurnal (12:00–19:00 h) winds that blow along its two main arms. As expected from its fetch (6.9 km) and its depth (21.1 m mean), the reservoir behaves as a warm monomictic water body. During 2001, VB was stratified from February to October, and well mixed from November to January. Its mean temperature was 19.9°C; the maximum found was 23.8°C in the epilimnion, while a minimum of 17.8°C was registered during mixing. VB exhibited a thermal regime similar to other water bodies of the Mexican tropical highlands, except for a steady increase of its hypolimnetic temperature during stratification, which is attributed to entrainment of epilimnetic water into the hypolimnion. During stratification, the hypolimnion was anoxic, while the whole water column remained under-saturated (60%) during mixing. The flushing time is 2.2 years. Mineralization and total alkalinity are low, which allows strong changes in pH. Ammonia remained low (2.4 μmol l⁻¹ mean) in the epilimnion, but reached up to 60 μmol l⁻¹ in the hypolimnion. Soluble reactive phosphorous had a mean of 0.28 μmol l⁻¹ in the epilimnion and a mean of 1.25 μmol l⁻¹ in the hypolimnion. Nitrate exhibited maxima (up to 21 μmol l⁻¹) during mixing, and also in the metalimnion (2 μmol l⁻¹) during stratification. Low dissolved inorganic nitrogen indicated nitrogen limitation during stratification. Eutrophication is an emerging problem in VB, where cyanobacteria dominate during stratification. At VB chlorophyll a is low during mixing (mean of 9 μg l⁻¹), and high during stratification (mean 21 μg l⁻¹), when blooms (up to 88 μg l⁻¹) are frequent. This pattern is similar to that found in other eutrophic tropical water bodies. We propose that in VB the wind regime causes vertical displacements of the thermocline (0.58–1.10 m hr⁻¹) and boundary mixing, enhancing the productivity during the stratification period in this tropical reservoir.     

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     

Microbial processes in a high-latitude fjord (Kongsfjorden, Svalbard): II. Ciliates and dinoflagellates

The composition and ecological role of ciliates and dinoflagellates were investigated at one station in Kongsfjorden, Svalbard, during six consecutive field campaigns between March and December 2006. Total ciliate and dinoflagellate abundance mirrored the seasonal progression of phytoplankton, peaking with 5.8 × 10⁴ cells l⁻¹ in April at an average chlorophyll a concentration of 10 μg l⁻¹. Dinoflagellates were more abundant than ciliates, dominated by small athecates. Among ciliates, aloricate oligotrichs dominated the assemblage. A large fraction (>60%) of ciliates and dinoflagellates contained chloroplasts in spring and summer. The biomass of the purely heterotrophic fraction of the ciliate and dinoflagellate community (protozooplankton) was with 14 μg C l⁻¹ highest in conjunction with the phytoplankton spring bloom in April. Growth experiments revealed similar specific growth rates for heterotrophic ciliates and dinoflagellates (<0–0.8 d⁻¹). Food availability may have controlled the protozooplankton assemblage in winter, while copepods may have exerted a strong control during the post-bloom period. Calculations of the potential grazing rates of the protozooplankton indicated its ability to control or heavily impact the phytoplankton stocks at most times. The results show that ciliates and dinoflagellates were an important component of the pelagic food web in Kongsfjorden and need to be taken into account when discussing the fate of phytoplankton and biogeochemical cycling in Arctic marine ecosystems.     

The specific inherent optical properties of three sub-tropical and tropical water reservoirs in Queensland, Australia

The underwater light climate, which is a major influence on the ecology of aquatic systems, is affected by the absorption and scattering processes that take place within the water column. Knowledge of the specific inherent optical properties (SIOPs) of water quality parameters and their spatial variation is essential for the modelling of underwater light fields and remote sensing applications. We measured the SIOPs and water quality parameter concentrations of three large inland water impoundments in Queensland, Australia. The measurements ranged from 0.9 to 42.7 μg l⁻¹ for chlorophyll a concentration, 0.9–170.4 mg l⁻¹ for tripton concentration, 0.36–1.59 m⁻¹ for a _CDOM(440) and 0.15–2.5 m for Secchi depth. The SIOP measurements showed that there is sufficient intra-impoundment variation in the specific absorption and specific scattering of phytoplankton and tripton to require a well distributed network of measurement stations to fully characterise the SIOPs of the optical water quality parameters. While significantly different SIOP sets were measured for each of the study sites the measurements were consistent with published values in other inland waters. The multiple measurement stations were allocated into optical domains as a necessary step to parameterise a semi-analytical inversion remote sensing algorithm. This article also addresses the paucity of published global inland water SIOP sets by contributing Australian SIOP sets to allow international and national comparison.     

Photosynthetic property and primary production of phytoplankton in sublittoral sand bank area in the Seto Inland Sea,Japan

We investigated the photosynthesis–light intensity (P–I) relationships of phytoplankton collected from a sublittoral sand bank in the Seto Inland Sea, Japan, under different temperature conditions. In spite of low chlorophyll a concentration (<3 mg m⁻³), phytoplankton had considerably high photosynthetic potential (>10 mg C (mg chl a)⁻¹ h⁻¹) in the study area. Based on the P–I relationships, we conducted numerical simulation of areal primary production using published data on water temperature, chlorophyll a concentration, and irradiance. The areal primary production ranged between 159 and 187 g C m⁻² year⁻¹. This production was within the range of typical values reported previously in deeper areas of the Seto Inland Sea. The productivity in the sand bank area was discussed in relation to water current, allochthonous resource input, and fisheries.     

Composition and significance of picophytoplankton in Antarctic waters

Filter fractionated picophytoplankton from Antarctic coastal waters (summer 2001) represented only 7–33% of total phytoplankton, even though total stocks were low (average Chl a = 0.32 μg l⁻¹, range = 0.13–1.03 μg l⁻¹). Though all cells passed a 2 μm filter, electron microscopy revealed most cells were over 2 μm, principally Parmales, Phaeocystis sp., and small diatoms. CHEMTAX analysis of HPLC pigment data suggested type 8 haptophytes (e.g. Phaeocystis sp. plus Parmales and pelagophytes) contributed 7–58% of picoplanktonic chlorophyll a, type 6 haptophytes (e.g. coccolithophorids) 18–59%, diatoms 0–18% (mostly type 2 diatoms, e.g. Pseudonitzschia sp., 0–15%), prasinophytes 0–17%, with cell fragments of cryptophytes 0–40%, and dinoflagellates 0–11%. Only stocks of type 8 haptophytes and prasinophytes differed significantly due to successional changes. Zeaxanthin concentrations exceeded estimates from previous cyanobacterial counts and may derive from non-photosynthetic bacteria.     

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.     

Reference conditions for phytoplankton at Danish Water Framework Directive intercalibration sites

Phytoplankton is one of the biological quality elements included in the EU Water Framework Directive (WFD). Classification of water quality according to the WFD is based on the deviation of the present conditions from reference conditions. Given the lack of data from pristine conditions, this study used approximately 100-year-old measurements of Secchi depths from Danish waters in combination with relationships between Secchi depth and chlorophyll a (as a proxy for phytoplankton biomass) obtained from recent monitoring to calculate ‘historical’ or reference chlorophyll a (Chl-a) concentrations. Historical Secchi depth data were available for 9 out of the 11 Danish WFD intercalibration sites. At eight of the sites, reference summer (May–September) Chl-a concentrations were in the range 0.7–1.2 μg l⁻¹. At one site, west of Bornholm in the western Baltic Sea, historical Secchi depth measurements date back to only the late 1950s corresponding to a calculated Chl-a concentration of 1.3 μg l⁻¹. This value cannot be considered representative of reference conditions. Guest editors: J. H. Andersen & D. J. Conley Eutrophication in Coastal Ecosystems: Selected papers from the Second International Symposium on Research and Management of Eutrophication in Coastal Ecosystems, 20–23 June 2006, Nyborg, Denmark     

Extracellular phosphatases produced by phytoplankton and other sources in shallow eutrophic lakes (Wuhan, China): taxon-specific versus bulk activity

Extracellular phosphatases are an important part of the phosphorus cycle in aquatic environments. Phosphatase activity (PA) in plankton was studied in seven subtropical shallow lakes of different exploitation management and trophic status in the urban area of Wuhan City. Bulk PA was rather high (range 1.1–11 μmol l⁻¹ h⁻¹), although concentrations of soluble reactive phosphorus (SRP) were also high (range 27 μg P l⁻¹ to ~1.5 mg P l⁻¹) in all lakes. Cell-associated extracellular PA in phytoplankton was detected using the fluorescence-labelled enzyme activity technique. Phytoplankton species partly contributed to the bulk PA. We found explicit differences in the presence of cell-associated phosphatase within the main phytoplankton groups; species belonging to Chlorophyta and Dinophyta were regularly phosphatase-positive, while Cyanophyta and Bacillariophyceae were phosphatase-negative in all but one case. Furthermore, there is a certain potential of extracellular phosphatases produced by heterotrophic nanoflagellates in most of the lakes. This new finding compromises the ‘traditional’ interpretation of bulk phosphatase data as being due to overall phytoplankton or bacterial P regeneration.     

Efficient production of ectoine using ectoine-excreting strain

Halophilic bacteria strain Halomonas salina DSM 5928 was found to excrete ectoine, suggesting its potential in the development of a new method of ectoine production. We performed HPLC and LC–MS analyses that showed that Halomonas salina DSM 5928 excreted ectoine under constant extracellular osmolarity. Medium adopting monosodium glutamate as a sole source of carbon and nitrogen was beneficial for ectoine synthesis. The total concentration of ectoine was not affected by NaCl concentration in the range 0.5–2 mol l⁻¹. The total concentration of ectoine and productivity in a 10-l fermentor with 0.5 mol l⁻¹ NaCl were 6.9 g l⁻¹ and 7.9 g l⁻¹ d⁻¹, respectively. These findings show that Halomonas salina DSM 5928 efficiently produces ectoine at relatively low NaCl concentration. This research also indicates the potential application of free or immobilized cells for continuous culture to produce ectoine.     

Thresholds and Stability of Alternative Regimes in Shallow Prairie–Parkland Lakes of Central North America

Numerous studies have demonstrated alternative regimes in shallow lake ecosystems around the world, with one state dominated by submerged macrophytes and the other by phytoplankton. However, the stability of each regime, and thresholds at which lakes shift to the alternative regime, are poorly known. We used a cross-sectional analysis of 72 shallow lakes located in prairie and parkland areas of Minnesota, USA, during 2005 and 2006 to assess the occurrence of alternative regimes and shifts between them. Cluster analysis revealed two distinct groups of lakes characterized not only by different macrophyte abundance and chlorophyll a levels but also by different total phosphorus–chlorophyll a relationships. Thirty-nine lakes were macrophyte- and 23 lakes phytoplankton-dominated in both years, whereas 10 sites shifted sharply between those regimes. We failed to detect a universal shifting threshold in terms of chlorophyll a or total phosphorus. However, 95% of the lakes with chlorophyll a concentrations less than 22 μg l⁻¹ were in a clear-water regime, whereas 95% of the lakes with chlorophyll a higher than 31 μg l⁻¹ were in a turbid regime. Total phosphorus less than 62 μg l⁻¹ was an accurate predictor of lakes in a stable clear-water regime, whereas a large change in biomass of planktivores and benthivores between years was the only variable weakly related to regime shifts. Our results support the theoretical prediction that regime thresholds vary among lakes. We recommend that lake managers focus on improving resilience of clear regimes in shallow lakes by reducing nutrient loading, rather than attempting to identify and manage complex triggers of regime shifts. Author contributions KDZ, MAH, BRH, and MLK all contributed to the design of the study, performed the research, analyzed data, and helped write the article.     

Factors controlling the seasonal development and distribution of the phytoplankton community in the lowland course of a large river in Northern Italy (River Adige)

The principal environmental factors influencing the seasonal dynamics of phytoplankton were examined from September 1997 to July 1998 in three stations along a 26-km stretch of the lowland course of River Adige (northeast Italy). Nutrient concentrations did not appear to be limiting for the phytoplankton growth. Annual minimum concentrations of reactive and total phosphorus, and dissolved inorganic nitrogen were 22 μg P l⁻¹, 63 μg P l⁻¹ and 0.9 mg N l⁻¹, respectively. The most critical forcing factors were physical variables, mainly water discharge and other variables related to hydrology, i.e. suspended solids and turbidity, which acted negatively and synchronously by diluting phytoplankton cells and decreasing light availability. Higher algal biomass was recorded in early spring, in conditions of lower flow velocity and increasing water temperature. In late spring and summer, higher water discharge caused a decrease in phytoplankton biomass. Conversely, low algal biomass in late autumn and winter, during low discharge, was mainly related to low water temperatures and shorter photoperiod. Physical constraints had a significant and measurable effect not only on the development of total biomass, but also on the temporal dynamics of the phytoplankton community. Abiotic and biotic variables showed a comparable temporal development in the three sampling stations. The small number of instances of spatial differences in phytoplankton abundance during the period of lower flow velocity were related to the increasing importance of biological processes and accumulation of phytoplankton biomass.     

Phytoplankton production after the collapse of the Larsen A Ice Shelf,Antarctica

Part of the Larsen A Ice Shelf (64°15′S to 74°15′S) collapsed during January 1995. A first oceanographic and biological data set from the newly free waters was obtained during December 1996. Typical shelf waters with temperatures near and below the freezing point were found. A nutrient-rich water mass (max: PO₄ ³⁻ 1.80 μmol L⁻¹ and NO₃ ⁻ 27.64 μmol L⁻¹) was found between 70 and 200 m depth. Chlorophyll-a (Chl-a) values (max 14.24 μg L⁻¹) were high; surface oxygen saturation ranged between 86 and 148%. Diatoms of the genera Nitzschia and Navicula and the prymnesiophyte Phaeocystis sp. were the most abundant taxa found. Mean daily primary production (Pc) estimated from nutrient consumption was 14.80 ± 0.17 mgC m⁻³ day⁻¹. Pc was significantly correlated with total diatom abundance and Chl-a. Calculated ΔpCO₂ (difference of the CO₂ partial pressure between surface seawater and the atmosphere) was –30.5 μatm, which could have contributed to a net CO₂ flux from the atmosphere to the sea and suggests the area has been a CO₂ sink during the studied period. High phytoplankton biomass and production values were found in this freshly open area, suggesting its importance for biological CO₂ pumping.     

Biogeochemical conditions and ice algal photosynthetic parameters in Weddell Sea ice during early spring

Physical, biogeochemical and photosynthetic parameters were measured in sea ice brine and ice core bottom samples in the north-western Weddell Sea during early spring 2006. Sea ice brines collected from sackholes were characterised by cold temperatures (range −7.4 to −3.8°C), high salinities (range 61.4–118.0), and partly elevated dissolved oxygen concentrations (range 159–413 μmol kg⁻¹) when compared to surface seawater. Nitrate (range 0.5–76.3 μmol kg⁻¹), dissolved inorganic phosphate (range 0.2–7.0 μmol kg⁻¹) and silicic acid (range 74–285 μmol kg⁻¹) concentrations in sea ice brines were depleted when compared to surface seawater. In contrast, NH₄ ⁺ (range 0.3–23.0 μmol kg⁻¹) and dissolved organic carbon (range 140–707 μmol kg⁻¹) were enriched in the sea ice brines. Ice core bottom samples exhibited moderate temperatures and brine salinities, but high algal biomass (4.9–435.5 μg Chl a l⁻¹ brine) and silicic acid depletion. Pulse amplitude modulated fluorometry was used for the determination of the photosynthetic parameters F _v/F _m, α, rETR_max and E _k. The maximum quantum yield of photosystem II, F _v/F _m, ranged from 0.101 to 0.500 (average 0.284 ± 0.132) and 0.235 to 0.595 (average 0.368 ± 0.127) in the sea ice internal and bottom communities, respectively. The fluorometric measurements indicated medium ice algal photosynthetic activity both in the internal and bottom communities of the sea ice. An observed lack of correlation between biogeochemical and photosynthetic parameters was most likely due to temporally and spatially decoupled physical and biological processes in the sea ice brine channel system, and was also influenced by the temporal and spatial resolution of applied sampling techniques.     

Microphytobenthos of Arctic Kongsfjorden (Svalbard, Norway): biomass and potential primary production along the shore line

During summer 2007, Arctic microphytobenthic potential primary production was measured at several stations around the coastline of Kongsfjorden (Svalbard, Norway) at ≤5 m water depth and at two stations at five different water depths (5, 10, 15, 20, 30 m). Oxygen planar optode sensor spots were used ex situ to determine oxygen exchange in the overlying water of intact sediment cores under controlled light (ca. 100 μmol photons m⁻² s⁻¹) and temperature (2–4°C) conditions. Patches of microalgae (mainly diatoms) covering sandy sediments at water depths down to 30 m showed high biomass of up to 317 mg chl a m⁻². In spite of increasing water depth, no significant trend in “photoautotrophic active biomass” (chl a, ratio living/dead cells, cell sizes) and, thus, in primary production was measured at both stations. All sites from ≤5 to 30 m water depth exhibited variable rates of net production from −19 to +40 mg O₂ m⁻² h⁻¹ (−168 to +360 mg C m⁻² day⁻¹) and gross production of about 2–62 mg O₂ m⁻² h⁻¹ (17–554 mg C m⁻² day⁻¹), which is comparable to other polar as well as temperate regions. No relation between photoautotrophic biomass and gross/net production values was found. Microphytobenthos demonstrated significant rates of primary production that is comparable to pelagic production of Kongsfjorden and, hence, emphasised the importance as C source for the zoobenthos.