Inorganic carbon uptake by phytoplankton in vineyard sound,Massachusetts. I. Measurements of the photosynthesis-irradiance response of winter and early-spring assemblages

The photosynthesis-irradiance dependence of natural phytoplankton assemblages from surface waters of Vineyard Sound, Massachusetts, was investigated over a several month period during late winter —early spring, 1982, when water temperatures were ? 0.5 to 8.5°C. Maximal photosynthetic rates not only were consistently observed between 7–15% I₀, but were substantially higher than previously reported rates for cold-water assemblages, averaging 10–20 μg C · μg Chl a^?1 · h^?1. At higher light intensities photo-inhibition was severe and developed within minutes to tens of minutes of the start of the experiment. Several lines of evidence, however, suggest that photoinhibition in situ may be much less that that measured in incubations of > 30 min duration; residence time of phytoplankton at surface light intensities is sufficiently short, and adaptation of photosynthetic capacity appears to have occurred to approximately the mean depth of the water column. These results further highlight the importance of establishing the time-dependent photosynthetic responses of phytoplankton and the relationship to the physical mixing regime in estimating primary productivity.     

Phytoplankton ecology in the Southern Benguela current. I. Biochemical composition

Investigations on phytoplankton communities in a nearshore region off the Cape Peninsula revealed three types of upwelled water. During active upwelling temperatures were < 10 °C and concentrations of inorganic nutrients were high (Type 1). Maturing upwelled water was characterized by temperatures > 10°C and nitrate concentrations varying between 2 and 15 μg-at. NO₃-N · 1^?1 (Type 2), while aged upwelled water (Type 3) contained low concentrations of nitrate (<2 μg-at. NO₃-N · 1^?1) at temperatures > 10°C. During the summer of 1978–1979 diatoms dominated the communities from October to January but microflagellates were dominant in February and March. In both types of community, low concentrations of ATP, chlorophyll a, protein and carbohydrate were measured in Type 1 water with protein/carbohydrate ratios being > 1. In Type 2 water concentrations of chlorophyll a, ATP and protein were high and the protein/carbohydrate ratio was > 1. Concentrations of chlorophyll a and ATP remained high in Type 3 water but the protein/carbohydrate ratio decreased to < 1 due to an increase in the concentration of acid-soluble glucan. It was concluded that the communities were in an active phase of growth in Type 1 and Type 2 water when adequate nutrients were available, but were in a slow-growing phase in Type 3 water when nitrate concentrations were low. Correlation coefficients, simple linear regressions and stepwise multiple regressions between biochemical and environmental variables confirmed that nitrate was the nutrient most closely related to the biochemical composition of phytoplankton. Using linear regression equations of biochemical variables on glucan it was estimated that chlorophyll a existed in a ratio of ≈ 1: 1 between living phytoplankton and bacteria/detritus, while the percentage of ATP was high in the phytoplankton component of Type 1 water but low in that of Type 2 water. The percentage of protein in detritus was greater than in living phytoplankton, and the carbohydrate content of living phytoplankton increased as the upwelled water matured from Type 1 and Type 2 to Type 3.     

Winter condition of marine plankton populations in Saanich Inlet,B.C., Canada. II. Micro-zooplankton

The abundance of micro-zooplankton (< 200 μm in size) was monitored weekly in Saanich Inlet throughout the winter, 1975–1976. Ciliate protozoans and metazoan nauplii were the major groups, with the former largely distributed in the top 5 m of the water column and the latter above 15 m. Average biomass throughout the winter was 3.1 mgC/m² for ciliates above 5 m and 5.5 mgC/m² for nauplii above 25 m, which made up 3.2 and 5.7% of the average phytoplankton biomass, respectively. They could consume over 30% of the phytoplankton production. Mesodinium sp. was an another abundant ciliate, but possibly a phyto-ciliate, with an average biomass of 8.2 mgC/m² above 5 m. Major energy flow at lower trophic levels during the winter was characterized by a ‘nanoflagellate → micro-zooplankton’ system which was comparable with the ‘centric diatoms → macro-zooplankton’ system occuring commonly at the other seasons in the inlet.     

SHADE ADAPTED BENTHIC DIATOMS BENEATH ANTARCTIC SEA ICE1

A dense community of shade adapted microalgae dominated by the diatom Trachyneis aspera is associated with a siliceous sponge spicule mat in McMurdo Sound, Antarctica. Diatoms at a depth of 20 to 30 m were found attached to spicule surfaces and in the interstitial water between spicules. Ambient irradiance was less than 0.6 μE · m^?2· s^?1 due to light attenuation by surface snow, sea ice, ice algae, and the water column. Photosynthesis-irradiance relationships determined by the uptake of NaH¹⁴CO₃ revealed that benthic diatoms beneath annual sea ice were light-saturated at only 11 μE·m^?2·s^?1, putting them among the most shade adapted microalgae reported. Unlike most shade adapted microalgae, however, they were not photoinhibited even at irradiances of 300 μE·m^?2·s^?1. Although in situ primary production by benthic diatoms was low, it may provide a source of fixed carbon to the abundant benthic invertebrates when phytoplankton or ice algal carbon is unavailable.     

Long‐term phytoplankton community changes in a deep subalpine lake: responses to nutrient availability and climatic fluctuations

1. In natural lakes, modifications in the species composition and abundance of phytoplankton communities may ultimately be responses to changes in nutrient availability and climatic fluctuations. Phytoplankton and associated environmental factors were collected at monthly intervals from the beginning of the 1990s to 2007 in the large subalpine Lake Garda (z_max = 350 m, V = 49 × 10⁹ m³). In this study period, the lake showed a slight and continuous increase of total phosphorus (TP) in the water column, up to concentrations of 18–20 μg P L^?1. This increase represented the last stage of a long‐term process of enrichment documented since the 1970s, when concentrations of TP were below or around 10 μg P L^?1. 2. At the community level, annual phytoplankton cycles underwent a unidirectional and slow shift mainly due to changes in the species more affected by the nutrient enrichment of the lake. After a first and long period of dominance by conjugatophytes (Mougeotia) and diatoms (Fragilaria), phytoplankton biomass in recent years was sustained by cyanobacteria (Planktothrix). Other important modifications in the development of phytoplankton were superimposed on this pattern due to the effects of annual climate fluctuations principally mediated by the deep mixing events at spring overturn and, secondarily, by temperature and thermal stability of the water column during the growing season. 3. Interannual variations in the stability and temperature of the water column appeared to influence the development of a few subdominant flagellates (dinophytes and cryptophytes). Nevertheless, the major impact of climate on phytoplankton was indirect, and mediated through the effects of winter climatic conditions on deep mixing dynamics. Winter climatic fluctuations proved to be a key element in a linked chain of causal factors including cooling of hypolimnetic waters, deep vertical mixing and epilimnetic nutrient replenishment. The process of fertilisation was measurable both for TP and dissolved inorganic nitrogen, although only the first had a large effect, reinforcing the seasonal growth of a few dominant groups. The degree of nutrient replenishment further increased the spring development of large diatoms and the increase of Planktothrix in summer and autumn. 4. Currently, changes in nutrient concentrations have the greatest effect on the phytoplankton community, while direct effects due to the interannual variations in the thermal regime are of secondary importance compared with the indirect effects mediated through deep water mixing and spring fertilisation. Overall, the results demonstrate that the consequences of climatic fluctuations and climate warming on phytoplankton communities need to be studied at different levels of complexity and integration, from the direct effects of temperature and thermal regime, to the indirect effects mediated by the physiographic characteristics of water bodies.     

Phytoplankton structure in the White Sea after summer bloom: Spatial variability in relation to hydrophysical conditions

The species composition and phytoplankton biomass, concentrations of chlorophyll “a” (Chl) and nutrients, concurrent hydrophysical conditions were studied in the south part of the White Sea in July 10–15, 2012 during chlorophyll “a” decrease after summer peak. The water column stability varied, the concentration of dissolved silicon in upper mixed layer was closed to the range favorable for diatoms with exception of areas of intensive tide mixing and areas influenced by waters of Severnaya Dvina River. In surface layer the dinoflagellates dominated excepting of areas with intensive tide mixing where diatoms prevailed. Diatoms provided major contribution to biomass in different stations above, in and under pycnocline and in deep waters out of photic zone. Structural analysis has revealed three phytoplankton communities that corresponded to different depths: communities of photic zone, intermediate and deep layers. Extension of layers inhabited by different communities depended on water column stability and on genesis of water masses. Integrated values of phytoplankton biomass and Chl varied from 250 to 1188 mg С/m², and from 22 to 51 mg/m², correspondently.     

Physical-Biological Coupling in the Western South China Sea: The Response of Phytoplankton Community to a Mesoscale Cyclonic Eddy

It is widely recognized that the mesoscale eddies play an important part in the biogeochemical cycle in ocean ecosystem, especially in the oligotrophic tropical zones. So here a heterogeneous cyclonic eddy in its flourishing stage was detected using remote sensing and in situ biogeochemical observation in the western South China Sea (SCS) in early September, 2007. The high-performance liquid chromatography method was used to identify the photosynthetic pigments. And the CHEMical TAXonomy (CHEMTAX) was applied to calculate the contribution of nine phytoplankton groups to the total chlorophyll a (TChl a) biomass. The deep chlorophyll a maximum layer (DCML) was raised to form a dome structure in the eddy center while there was no distinct enhancement for TChl a biomass. The integrated TChl a concentration in the upper 100 m water column was also constant from the eddy center to the surrounding water outside the eddy. However the TChl a biomass in the surface layer (at 5 m) in the eddy center was promoted 2.6-fold compared to the biomass outside the eddy (p < 0.001). Thus, the slight enhancement of TChl a biomass of euphotic zone integration within the eddy was mainly from the phytoplankton in the upper mixed zone rather than the DCML. The phytoplankton community was primarily contributed by diatoms, prasinophytes, and Synechococcus at the DCML within the eddy, while less was contributed by haptophytes_8 and Prochlorococcus. The TChl a biomass for most of the phytoplankton groups increased at the surface layer in the eddy center under the effect of nutrient pumping. The doming isopycnal within the eddy supplied nutrients gently into the upper mixing layer, and there was remarkable enhancement in phytoplankton biomass at the surface layer with 10.5% TChl a biomass of water column in eddy center and 3.7% at reference stations. So the slight increasing in the water column integrated phytoplankton biomass might be attributed to the stimulated phytoplankton biomass at the surface layer.     

PHOTOINHIBITION OF TEMPERATE LAKE PHYTOPLANKTON BY NEAR-SURFACE IRRADIANCE: EVIDENCE FROM VERTICAL PROFILES AND FIELD EXPERIMENTS1

To evaluate the in situ occurrence of phytoplankton photoinhibition, the light-mediated depression of chlorophyll in vivo fluorescence (IVF) and of the cellular fluorescence capacity (CFC) of phytoplankton was determined in three southeastern United States reservoirs. Vertical profiles of a fluorescence depression index (FDI) and of the CFC for reservoir phytoplankton showed that near-surface photoinhibition of fluorescence properties occurred in association with high surface irradiance and weak vertical mixing of the water column. To characterize the time scales of photochemical and photosynthetic responses to and recovery from exposure to supraoptimal light intensity, phytoplankton IVF responses and ¹⁴C-fixation rates were measured infield experiments. Phytoplankton chlorophyll IVF, CFC, and photosynthetic ¹⁴C fixation were rapidly (20–40 min) depressed when reservoir phytoplankton were exposed to surface irradiances (1700–2000 μE·m^?2·s^?1). Light-mediated increases in the FDI declined rapidly (20–40 min) to pre-exposure levels during a subsequent low-light (75–200 μE·m^?2·s^?1) period, but CFC and ¹⁴C fixation recovered more slowly (>40 min). Exposure of reservoir phytoplankton to a light-intensity gradient revealed both intensity and time thresholds for IVF and CFC depression. Phytoplankton photochemical responses to bright light operate on time scales that, in conjunction with vertical mixing, should limit the occurrence of photoinhibition to extreme irradiance environments. Our results support the hypothesis that the photoinhibition of phytoplankton productivity occurs less commonly than is indicated by fixed-depth incubation measurements.     

The interaction of components controlling net phytoplankton photosynthesis in a well-mixed lake (Lough Neagh, Northern Ireland)

The homogeneous distribution of the phytoplankton in a shallow (mean depth 8·6 m) unstratified lake, L. Neagh, Northern Ireland, facilitated the study of the interaction of components controlling gross photosynthesis per unit area. These included the photosynthetic capacity, the phytoplankton content of the euphotic zone, and a logarithmic function describing the effective radiation input. These factors were analysed for two sites, the open lake and Kinnego Bay, which respectively had standing crops of up to 90 and 300 mg chlorophyll a m^?3 and maximum daily rates of gross integral photosynthesis of 11·7 and 15·6 g O₂ m^?2 day^?1. Values are reduced by the high contribution to light attenuation by non-algal sources, which increases at low standing crops particularly in winter, when values of integral photosynthesis decrease to 0·5 g O₂ m^?2 day^?1. This relative change is the result of self-shading behaviour of the phytoplankton altering the crop content of the euphotic zone at different population densities. Changes in the irradiance function, incorporating day length, are largely responsible for the changes in daily rates of integral gross photosynthesis; as daily irradiance is also a determinant of water temperature, it exerts further influence through the photosynthetic capacity which was strongly correlated with temperature. Much of the gain in gross photosynthesis resulting from higher photosynthetic capacity may not be reflected in a higher net column photosynthesis, because of the greater proportional rise in respiration with temperature. The balance in the water column between respiration losses and photosynthetic input may frequently alter since the ratio of illuminated to dark zones is between 1/4 to 1/5 in the open lake, and small shifts in any of the controlling features may result in conditions unfavourable for growth. This is analysed especially for the increase of diatoms in spring, when small modifications of the underwater light field can delay growth.     

EFFECTS OF IRRADIANCE AND GRAZING ON LOTIC ALGAL ASSEMBLAGES1

A laboratory experiment was conducted for 75 days to examine how irradiance levels and grazing influence algal biomass and community structure. Twelve laboratory streams were used for experimental analyses, with four channels exposed to one of three irradiance levels (15, 100, or 400 μE·m^?2·s^?1). Three of the four stream at each light level were stocked with the snail Juga silicula (250·m^?2), leaving one stream at each light level without snails. Grazed stream exposed to low light levels developed low amounts of algal biomass (<2 g AFDW·m^?2) and were dominated by adnately attached diatoms. Mean algal biomass increased over time in the grazed streams exposed to intermediate light; by day 75, these streams were characterized by moderate algal biomasses (30-40 g AFDW·m^?2) and filamentous chlorophytes. Algal assemblages in high light, grazed channels had high levels of biomass at day 43 (70 g AFDW·m^?2) that declined to 30 g AFDW·m^?2at day 75 and were dominated by chlorophytes. Among ungrazed streams, algal biomass at day 75 was relatively low in the low light streams (<7g AFDW·m^?2) and was dominated by adnately attached diatoms. Ungrazed streams exposed to intermediate and high light levels had moderate biomasses (23 and 19 g AFDW·m^?2, respectively) and were dominated by chlorophytes and large diatoms. Grazing appeared both to delay and alter successional trajectories of algal assemblages, with alterations most noticeable during early seral stages at intermediate and high light levels. Grazing had the least effect on successional trajectories at low light.     

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.     

Artificial destratification of a small tropical reservoir: effects upon the phytoplankton

Seasonal changes in the phytoplankton community of a small tropical reservoir were monitored over a four year period comprising of an initial two seasonal cycles during which the water column stratified strongly for extended periods each year, and two further seasonal cycles after installation of a mechanical aeration system to induce artificial destratification. In the unmanaged reservoir, the concentration of chlorophyll a at 0.5 m reached maximum values (on one occasion > 90 mg m⁻³) when the water column was stratified and the epilimnion was very shallow (ca 2 m depth). The hypolimnion at this time was anoxic (less than 2% oxygen saturation) and had a high concentration of bacteriochlorophyll (100–200 mg m⁻³). The phytoplankton community of the unmanaged reservoir was generally dominated by cyanobacteria (Cylindrospermopsis raciborskii, Anabaena tenericaulis) during the warmer months of the year (November–March) (but replaced by chlorophyta, dinophyceae and euglenophyceae after periods of intense rain) and by bacillariophyceae (Synedra ulna var. chaseana, S. tenera) during the cooler, dry months. In the artificially destratified reservoir (8 h aeration day⁻¹), the phytoplankton community was largely dominated by diatoms except after depletion of the silica content of the water column which caused diatoms to be replaced by cyanobacteria (dominated by A. tenericaulis) and a range of chlorophytes. The changing pattern of stratification and circulation of the water column in the unmanaged reservoir caused repeated disruption of the established phytoplankton assemblage with peaks of high biomass associated with transient cyanobacterial blooms. Continuous aeration and the consequent increase in the ratio mixed: euphotic depth provided conditions suitable for dominance of the phytoplankton by diatoms, as long as silica was available, and resulted in average chlorophyll levels higher than in the unmanaged reservoir (120 ± 10 v. 64 ± 9 mg m⁻²). Hierarchical fusion analysis based on the biomass of species differentiated the phytoplankton samples into cluster groups that could be related primarily to stratification or mixing of the water column.     

Effects of simulated upwelling and oligotrophy on chemostat-grown natural marine phytoplankton assemblages

Natural phytoplankton assemblages from the Scripps Pier were grown in two chemostats under conditions that simulated two rates of upwelling followed by oligotrophic conditions. At a moderate upwelling rate (D = 0.3·day^?1) centric diatoms were selected, while at a low rate (D = 0.1·day^?1) a mixture of species dominated. Pumping of low-nutrient water (oligotrophy) resulted in a mixture of species at both rates. Upwelling at a high rate decreased diversity of the crop as compared with the low rate or oligotrophy. These results are compared with those of others who have subjected natural assemblages to continuous culture.     

Effects of mixing and silica enrichment on phytoplankton seasonal succession

During the summer of 1983, cryptophytes, diatoms, cyanophytes, and the dinoflagellate, Ceratium hirundinella were most prominant among the phytoplankton of Eau Galle Reservoir. In the open water, cryptophytes and diatoms peaked in the spring, cyanophytes were most successful in the early summer, and Ceratium was dominant from mid-July until early August. In general, the sequence of events corresponded quite closely to the model of seasonal succession developed by the Plankton Ecology Group of the International Society of Limnology. To a large extent, the same pattern held in four experimental water columns. Departures from the model involved the roles of specific nutrients in diatom and cyanophyte periodicity. Diatoms began to yield to cyanophytes in late spring despite intermittent mixing and silica enrichment. Although capable of buoyancy regulation and thus well adapted to stable water columns, cyanophytes had greater increases in biomass in mixed columns, and in those columns, were most successful during a period of intermittent mixing. Cyanophyte success varied inversely with TN : TP ratios during the period of intermittent mixing, but not subsequently. By mid-July, Ceratium dominated the phytoplankton of every column except that of a mixed column in which conditions favored cyanophytes and large diatom species.     

Yellow water in La Jolla Bay,California, July 1980. I. A bloom of the dinoflagellate, Gymnodinium flavum Kofoid & Swezy

During the latter half of July, 1980, a bloom of Gymnodinium flavum Kofoid & Swezy caused water discoloration in La Jolla Bay, California. This naked dinoflagellate dominated the phytoplankton, numeri- cally and by volume, and was found in concentrations as high as 6.15 × 10³ cells · ml^?1. It was most abundant near the surface above a shallow (5–10 m), sharp thermocline and a nitracline at 10 m. Near the end of July, the depth of maximum phytoplankton abundance descended and water discoloration was no longer visible at the surface even though areal concentrations of G. flavum did not decrease. Concurrent with changes in the vertical distribution of the phytoplankton, warm, nutrient-depleted water moved into the area and nitrate availability in the upper 20 m of the water column was drastically reduced. Measure- ments of the chemical composition of the phytoplankton do not, however, indicate progressive nutrient stress during the period of environmental change. We conclude that shoaling of the thermocline and nitracline associated with apparent upwelling were conducive to development of the bloom and that advection of warmer water from offshore led to disappearance of yellow surface water from the bay.     

UPTAKE AND RELEASE OF NITROGEN BY THE MACROALGAE GRACILARIA VERMICULOPHYLLA (RHODOPHYTA)1

Macroalgae, often the dominant primary producers in shallow estuaries, can be important regulators of nitrogen (N) cycling. Like phytoplankton, actively growing macroalgae release N to the water column; yet little is known about the quantity or nature of this release. Using ¹⁵N labeling in laboratory and field experiments, we estimated the quantity of N released relative to assimilation and gross uptake by Gracilaria vermiculophylla (Ohmi) Papenfuss (Rhodophyta, Gracilariales), a non‐native macroalgae. Field experiments were carried out in Hog Island Bay, a shallow back‐barrier lagoon on the Virginia coast where G. vermiculophylla makes up 85%–90% of the biomass. There was good agreement between laboratory and field measurements of N uptake and release. Daily N assimilation in field experiments (32.3±7.2 μ mol N·g dw^?1·d^?1) was correlated with seasonal and local N availability. The average rate of N release across all sites and dates (65.8±11.6 μ mol N·g dw^?1·d^?1) was 67% of gross daily uptake, and also varied among sites and seasons (range=33%–99%). Release was highest when growth rates and nutrient availability were low, possibly due to senescence during these periods. During summer biomass peaks, estimated N release from macroalgal mats was as high as 17 mmol N·m^?2·d^?1. Our results suggest that most estimates of macroalgal N uptake severely underestimate gross N uptake and that N is taken up, transformed, and released to the water column on short time scales (minutes–hours).     

First records of primary producers of epiglacial and supraglacial lakes in western Dronning Maud Land, Antarctica

Epiglacial and supraglacial lakes are characteristic lake types in Antarctica, and regardless of their mostly seasonal existence and ultraoligotrophy, some lakes have a relatively diverse microbial community. The results of water chemistry and phytoplankton, based on basic limnological methods, from five epiglacial and two supraglacial seasonal lakes are presented from western Dronning Maud Land, an area where only physical studies have been previously carried out. Electric conductivity varied mostly between 0.1 and 10 mS m^?1 (25 °C), phosphorus concentration was <5 mg m^?3, and nitrogen concentration was <300 mg m^?3 except in some shore areas, and water pH ranged from 6 to 11. Low phytoplankton biomasses (in most cases <10 mg m^?3) supported the ultraoligotrophic status of the lakes. Phytoplankton was found from both types of lakes, but less was found from supraglacial lakes. The charophyte Mesotaenium cf. berggrenii dominated the supraglacial lakes, while cyanoprokaryotes such as Gloeocapsopsis cf. magma, Planktothrix prolifica/rubescens, Nostoc cf. sphaericum, Cyanothece sp. and Phormidium sp. dominated the biomass in some epiglacial lakes. Chrysophytes (e.g. Pseudopedinella-type flagellates) were observed in both types of lakes, and they were occasionally dominant. The green alga Botryococcus braunii, some diatoms (Cyclotella sp., Diatoma tenuis, Luticola muticopsis), and non-planktonic microalgal colonies visible to the eye (incl. the cyanoprokaryote Nostoc commune) were also found. Signs of a living ecosystem with a food web were observed in one epiglacial lake, but not elsewhere, which indicates extreme circumstances in the Antarctic seasonal lakes. Altogether, only some 25 taxa were discovered.     

Partitioning of river metabolism identifies phytoplankton as a major contributor in the regulated Murray River (Australia)

1. River metabolism was measured over an annual cycle at three sites distributed along a 1000 km length of the lowland Murray River, Australia. 2. Whole system metabolism was measured using water column changes in dissolved oxygen concentrations while planktonic and benthic metabolism were partitioned using light‐dark bottles and benthic chambers. 3. Annual gross primary production (GPP) ranged from 775 to 1126 g O₂ m^?2 year^?1 which in comparison with rivers of similar physical characteristics is moderately productive. 4. Community respiration (CR) ranged from 872 to 1284 g O₂ m^?2 year^?1 so that annual net ecosystem production (NEP) was near zero, suggesting photosynthesis and respiration were balanced and that allochthonous organic carbon played a minor role in fuelling metabolism. 5. Planktonic rates of gross photosynthesis and respiration were similar to those of the total channel, indicating that plankton were responsible for much of the observed metabolism. 6. Respiration rates correlated with phytoplankton standing crop (estimated as the sum of GPP plus the chlorophyll concentration in carbon units), yielding a specific respiration rate of ?1.1 g O₂ g C^?1 day^?1. The respiration rate was equivalent to 19% of the maximum rate of phytoplankton photosynthesis, which is typical of diatoms. 7. The daily GPP per unit phytoplankton biomass correlated with the mean irradiance of the water column giving a constant carbon specific photon fixation rate of 0.35 gO₂ g Chl a^?1 day^?1 per μmole photons m^?2 s^?1 (ca. 0.08 per mole photons m^?2 on a carbon basis) indicating that light availability determined daily primary production. 8. Annual phytoplankton net production (NP) estimates at two sites indicated 25 and 36 g C m^?2 year^?1 were available to support riverine food webs, equivalent to 6% and 11% of annual GPP. 9. Metabolised organic carbon was predominantly derived from phytoplankton and was fully utilised, suggesting that food‐web production was restricted by the energy supply.     

Distribution and development of under-ice phytoplankton in 90-m deep water column of Lake Päijänne (Finland) during spring convection

Distribution and development of phytoplankton were studied in the deep and large Lake Päijänne from mid-winter until the disappearance of ice. Diatoms were an important part of the phytoplankton assemblage and, with cryptophytes and chrysophytes, made up 50–80% of the phytoplankton biomass. In mid-winter, chlorophyll a and phytoplankton biomass were uniformly distributed over the whole water column down to a depth of 90 m. Thus, most of the phytoplankton was in virtual darkness and there was negligible growth. Only motile cryptophytes were concentrated in the layers below the ice and were rare in deep water. After the disappearance of snow, convection developed, but at first cryptophytes were able to resist mixing. When convection turned from penetrative to predominantly horizontal, all phytoplankton were generally uniformly distributed in the water column. In spite of the full under-ice overturn with low average availability of light, the phytoplankton biomass doubled in April. The growth of cryptophytes was higher than that of diatoms, suggesting that motile species gained an advantage by being able to maintain themselves in the upper, illuminated layers. The results show that knowledge of the basic physical framework is essential for interpretation of under-ice phytoplankton results.     

Nutrient biogeochemistry in the water column (N,P, Si) and porewater (N) of sandy sediment of the Scheldt estuary (SW-Netherlands)

The Scheldt river drains a densely populated and industrialized area in northern France, western Belgium and the south-west Netherlands. Mineralization of the high organic load carried by the river leads to oxygen depletion in the water column and high concentrations of dissolved nitrogen and phosphorus compounds. Upon estuarine mixing, dissolved oxygen concentrations are gradually restored due to reaeration and dilution with sea water. The longitudinal redox gradient present in the Scheldt estuary strongly affects the geochemistry of nutrients. Dissolved nutrients in the water column and dissolved nitrogen species in sediment porewaters were determined for a typical summer and winter situation. Water column concentration-salinity plots showed conservative behaviour of dissolved Si during winter. During summer (and spring) dissolved Si may be completely removed from solution due to uptake by diatoms. The geochemistry of phosphorus was governed by inorganic and biological processes. The behaviour of nitrogen was controlled by denitrification in the anoxic fluvial estuary, followed by nitrification in the upper estuary (prior to oxygen regeneration). In addition, nitrogen was taken up during phytoplankton blooms in the lower estuary. Dissolved inorganic nitrogen species in porewaters from the upper 20 cm of sediments were obtained from a subtidal site in the middle of the lower estuary. Dissolved nutrient concentrations were low in the upper 10–15 cm of the sandy and organic poor (<1% POC) sediments mainly as a result of strong sediment mixing. The porewater profiles of ammonium and nitrate were evaluated quantitatively, using a one-dimensional steady-state diagenetic model. This coupled ammonium-nitrate model showed ammonification of organic matter to be restricted to the upper 4 to 7 cm of the sediments. Total nitrification ranged from 3.7–18.1 mmol m^?2 d^?1, converting all ammonium produced by ammonification. The net balance between nitrification and denitrification depended on the season. Nitrate was released from the sediments during winter but is taken up from the water column during summer. These results are in good agreement with data obtained from the independently calibrated water column model for the Scheldt Estuary (VAN GILSet al., 1993).