Primary production of phytoplankton in Lake Valencia (Venezuela)

Lake Valencia is heavily polluted by waste water of domestic, agricultural and industrial origin. The high organic load may have produced important changes in the limnological properties. Cyanobacteria dominated in numbers and biomass (over 90% throughout the year). Chlorophyll-a content averaged 37.7 + 15 μg · 1⁻¹. Maximum concentrations of 50–80 μg · 1⁻¹ were found near the inflows affected by organically polluted affluents. There has been a 50% reduction in the euphotic zone in only 13 years. The maximum rate of gross photosynthesis per hour at light saturation was determined within the uppermost 1-meter layer. The highest value was 16,290 mg O₂ · m⁻³ · h⁻¹. Lake Valencia is among the most productive lakes in the world, with areal net photosynthesis averaging 7.5 g C · m⁻² · d⁻¹.     

Colony growth of Streptomyces coelicolor A3(2) under conditions of continuous nutrient supply

Colony growth of Streptomyces coelicolor A3(2) was studied on a cellophane membrane beneath which was passed a continuous supply of liquid medium. Colony development and differentiation occurred normally but hyphal extension rates and colony radial growth rates were reduced and branch formation was increased in comparison with colonies grown on the same medium solidified with agar. These changes are thought to result from continuous removal of staling compounds which would otherwise suppress branching at the colony margin. Glucose concentrations in the range of 0–1 g · l⁻¹ had little effect on radial growth and branching except at a concentration of 1 g glucose · l⁻¹, at which branching at the colony margin was suppressed. This concentration of glucose did not permit continued growth on solid medium.     

Gas exchange characteristics of Gulf of Mexico coastal marsh macrophytes

Gas exchange characteristics of three major Louisiana Mississippi River deltaic plain marsh species, Spartina patens (Ait.) Muhl., Spartina altemiflora Lois., and Panicum hemitomon Shult., was studied under controlled environment conditions. The optimum temperature for maximum photosynthesis was ≈ 36 °C for S. patens, 27 °C for S. alterniflora, and 28 °C for rP. hemitomon. Net photosynthesis rates at optimum temperature averaged 20.1 μmol · m^t-2 · s^t-1 in S. patens, 22.8 μmol · m⁻² · s⁻¹ in S. alterniflora, and 11.4 μmol · m⁻² · s⁻¹ in P. hemitomon. Photosynthetic light saturation occurred ≈720, 530, and 750 μmol · m⁻² · s⁻¹ in S. patens, S. alterniflora, and P. hemitomon, respectively. Only S. patens had a midday depression of stomatal conductance, but net photosynthesis was not reduced by the depression. Maximum stomatal conductances were 285 mmol · m⁻² · s⁻¹ in S. patens, 238 mmol · m⁻² · s⁻¹ in S. alterniflora, and 335 mmol · m⁻² · s⁻¹ in P. hemitomon. In contract, net photosynthesis values were lower in P. hemitomon compared with the Spartina species, indicating a greater degree of water use efficiency of photosynthesis for both Spartina species.     

The gills as an important uptake route for the essential nutrient iron in freshwater rainbow trout Oncorhynchus mykiss

Short-term (3h) acquisition of iron (16 nmol ⁵⁹FeCl₃ l⁻¹) from oxic, alkaline fresh water was assessed in rainbow trout Oncorhynchus mykiss in the presence or absence of a range of iron chelators, all of which had differing binding affinities for ferric iron [100 μmol l⁻¹ of desferrioxamine (DFO), Log₁₀K₁ 32·5; citric acid Log₁₀K₁ 11·9; nitrilotriacetic acid (NTA) Log₁₀K₁ 15·9, CP20 and CP94 (Log₁₀K₁ > 30), as well as humic acid (HA), Log₁₀K₁ 5·04, 5 mg l⁻¹]. In the absence of chelators (control conditions) O. mykiss acquired iron from the water under laboratory lights (wavelength range of the lights 440–650 nm, peak intensity 548–626 nm) via the gill. In these conditions iron uptake onto the gill had a maximum transport capacity (J_max) of 11·2 pmol Fe g⁻¹ h⁻¹ (gill organ mass) and a K_m of 21·3 nmol Fe l⁻¹ h⁻¹. Furthermore, there were two components to iron accumulation into the carcass of these fish, a slow rate of aqueous iron uptake at low concentrations (6–24 nmol Fe l⁻¹), followed by a faster rate of uptake at higher iron concentrations (48–96 nmol Fe l⁻¹), suggesting that the rate-limiting step of iron uptake at low iron concentrations is the apical entry step. O. mykiss also acquired iron in the presence of HA, although the majority of the other chelators prevented iron uptake. Ultraviolet light (354 nm) treatment of Fe-DFO increased iron bioavailability. Results suggest that rainbow trout are able to access either the predicted very low concentrations (picomolar) of ferrous iron present in fresh water or the ferric oxide complexes present in oxic environments. The iron uptake rate measured (0·75 pmol g⁻¹ h⁻¹) would be sufficient to provide a substantial proportion (c. 85%) of the daily iron requirements of growing salmonid fry.     

Effect of light intensity and eye development on prey capture by larval striped bass Morone saxatilis

The efficacy of visual and non-visual feeding among pelagic striped bass Morone saxatilis larvae adapted to a turbid estuary was determined in the laboratory in clear water. Capture of Artemia salina (density 100 l⁻¹) was significantly affected by the interaction between age of larvae (range: 8–25 days post-hatch, dph) and light intensity (range: 0–10·6 μmol s⁻¹ m⁻² at the water surface). Visual feeding by larvae aged 9–11 dph was highest in dim light (0·086–0·79 μmol s⁻¹ m⁻²), with fish capturing up to 5 prey larva⁻¹ h⁻¹. As the larvae grew, prey capture in brighter light improved, associated with an increasing proportion of twin cone photoreceptors and improving ability of the retina to light- and dark-adapt. By age >22 dph, mean prey capture was greatest at highest light intensities (0·79 and 10·6 μmol s⁻¹ m⁻²) exceeding 100 prey larva⁻¹ h⁻¹. Incidence of feeding larvae generally improved as the larvae grew, reaching >80% in all light intensities from 16 dph onwards. The lower threshold for visual feeding, between 0·0084 and 0·03 μmol s⁻¹ m⁻², remained constant as the larvae grew, despite an increasing density of rod photoreceptors. Below this threshold, non-visual feeding was evident at a low rate (<6 prey larva⁻¹ h⁻¹) that was independent of larval age.     

Influence of the concentrations of d-xylose and yeast extract on ethanol production by Pachysolen tannophilus

To determine the most favorable conditions for the production of ethanol by Pachysolen tannophilus, this yeast was grown in batch cultures with various initial concentrations of two of the constituents of the culture medium: d-xylose (s_o), ranging from 1 g·l⁻¹ to 200 g·l⁻¹, and yeast extract (l_o), ranging from 0 g·l⁻¹ to 8 g·l⁻¹. The most favorable conditions proved to be initial concentrations of S_o=25 g·l⁻¹ and l_o=4 g·l⁻¹, which gave a maximum specific growth rate of 0.26 h⁻¹, biomass productivity of 0.023 g·l⁻¹·h⁻¹, overall biomass yield of 0.094 g·g⁻¹, specific xylose-uptake rate (q_s) of 0.3 g·g⁻¹·h⁻¹ (for t=50 h), specific ethanol-production rate (q_E) of 0.065 g·g⁻¹·h⁻¹ and overall ethanol yield of 0.34 g·g⁻¹; q_s values decreased after the exponential growth phase while q_E remained practically constant.     

Ethanol fermentation by an efficient strain,NRRL B-4286, of Zymomonas mobilis

We studied the effect of the initial substrate concentration over the range of 100–250 g·l⁻¹ on the fermentation kinetics in batch cultures of Zymomonas mobilis NRRL B-4286 on glucose, fructose, and sucrose, using an adapted initial inoculum. With increasing concentrations of substrate, parameters related to growth were more rapidly and strongly affected than those related to ethanol production. This strain produced 94.0 g·l⁻¹, 76.9 g·l⁻¹, and 66.5 g·l⁻¹ of ethanol at glucose, fructose, and sucrose concentrations of 200 g·l⁻¹, respectively, more than the amount produced by the efficient strain ZM4 (NRRL B-14023).     

Dietary-induced suppression of pre-ovulatory progesterone concentrations in superovulated ewes impairs the subsequent in vivo and in vitro development of their ova

In the first of three experiments, eight ovariectomised Greyface ewes primed with exogenous progesterone were used to provide quantitative data on the effects of two contrasting feeding levels (0.3 vs. 1.4 × maintenance) on plasma progesterone concentrations. Over the 9 day study period, mean (± SEM) daily progesterone concentrations were 4.3 ± 0.13 and 3.3 ± 0.17 μg l⁻¹ for the low and high feeding regimens, respectively (P = 0.06), indicating that high feed intake suppressed circulating progesterone levels. The second experiment examined the effect in superovulated Finn-Dorset ewes of a diet supplying either 0.6 (Group L, n = 8) or 2.3 (Group H, n = 8) times their daily energy needs for maintenance, from 1 day before introduction of exogenous progesterone to the time of insemination, on plasma progesterone concentrations and the viability of ova recovered 4 days after insemination. Mean (± SEM) plasma progesterone concentrations were 4.5 ± 0.17 μg l⁻¹ and 2.8 ± 0.16 μg l⁻¹ for L and H ewes, respectively, during the 12 day priming period (P < 0.001). Eight hours after progesterone withdrawal, levels had fallen to 0.9 ± 0.06 μg l⁻¹ and 0.8 ± 0.07 μg l⁻¹, respectively, then rose to 17.8 ± 3.01 μg l⁻¹ and 12.9 ± 2.50 μg l⁻¹ (P > 0.10) at ovum collection. Intervals (mean ± SEM) to oestrous onset (14.5 ± 0.38 h) and the luteinising hormone (LH) surge (27.1 ± 0.98 h) were unaffected by feed intake. Mean (± SEM) ovulation rates (8.1 ± 1.57 vs. 7.8 ± 1.10) and numbers of ova recovered (5.0 ± 1.39 vs. 4.8 ± 1.11) were also similar for each group. However, the proportions of ova considered viable (over 32 cells) at recovery were 0.53 and 0.22 for L and H groups, respectively (P < 0.005). Following 72 h culture (Tissue Culture Medium-199 (M199) + 10% foetal calf serum (FCS)), 0.55 and 0.27, respectively, had developed to blastocysts (P < 0.025). Of ova assessed as viable at recovery, similar proportions (0.86 vs. 0.75) from L and H treatments developed to blastocysts, with corresponding nuclei counts (mean ± SEM) of 55 ± 5.2 and 55 ± 13.2. The third experiment used 12 superovulated Greyface ewes, each offered a different feed level within the range 0.6–2.5 × maintenance, to determine the nature of the relationship between feeding level, pre-ovulatory progesterone concentrations and ovum development at Day 2 following insemination and subsequently during 7 day co-culture (M199 + FCS). Increases in feeding level were accompanied by linear decreases in plasma progesterone (r² = 0.79, P < 0.001), the interval to oestrous onset (r² = 0.52, P < 0.01) and timing of the LH surge (r² = 0.32, P < 0.06). Although undetectable at ovum collection, and somewhat equivocal after 4 day culture, high feeding levels prior to ovulation reduced the proportion of ova (0.16 vs. 0.58) developing to or beyond the expanding blastocyst stage after 7 day culture. Quantitative indices of cell division and protein synthesis confirmed this. In conclusion, excessive feeding during follicular recruitment and oocyte maturation in superovulated ewes imparts a legacy of embryonic loss and developmental retardation.     

Temporal variability in egg production rates of Acartia tonsa Dana in Long Island Sound

The relationship between week-by-week variations in the in situ egg production rates of Acartia tonsa Dana and changes in chlorophyll concentration in several size fractions was investigated by incubating adult females in natural sea water for 24-h periods. Our results indicate that the egg production of A. tonsa in Long Island Sound was better related to the 10 μm chlorophyll size fraction than to the total chlorophyll concentration. The < 10 μm size fraction comprised the greatest percentage of the chlorophyll during July and August when the water column was stratified. Egg production rates were lowest (8.7 eggs · female⁻¹ · day⁻¹) in early August when less than 0.5 μg chlorophyll 1 ⁻¹ was observed in the 10 μm chlorophyll a size fraction. Following destratification in late August, the “fall” diatom bloom occurred and egg production rates increased to the maximum observed rate of 56.6 eggs · female⁻¹ · day⁻¹. At this time, the concentration of the 10μm chlorophyll size fraction was 5.5 μg 1⁻¹. Maximum egg production rates were observed at chlorophyll concentrations as low as 0.8 μg 1⁻¹ in the 10 μm size fraction.     

Influence of amino acid supplements on the metabolism of Clostridium acetobutylicum

The effects of organic nitrogen on the metabolism of Clostridium acetobutylicum were investigated in batch fermentations. For this study, amino acids were added to a chemically defined medium in groups from the same biosynthetic pathways. In all cases the addition of amino acids shifted the solvent ratio to higher butanol production at the expense of that of acetone (except for the glutamic acid group) and ethanol (except for histidine). Highest biomass production was obtained from media containing aromatic amino acids and histidine (4.57 g · l⁻¹ and 5.4 g · l⁻¹, respectively). However, the solvent production (ca. 20 g · l⁻¹) and the solvent yield (ca. 33%) in both cases, were similar to those obtained from the synthetic medium. Lower values were obtained from fermentations carried out with other families of amino acids. The strongest inhibition of cell growth (1.13 g · l⁻¹) which related to the lowest solvent production (3.15 g · l⁻¹) was observed on a medium complemented with amino acids of the pyruvic acid group. During the second phase of fermentation, amino acids-complemented media caused a less efficient remetabolization of acetic and butyric acids. Highest production of acids was obtained with the aspartic acid group (7.4 g · l⁻¹). These observations suggest that amino acids can be used as a competitive nitrogen source and also modify the level of enzyme activities involved in acid and solvent production.     

THE MECHANISM OF DAYLENGTH PERCEPTION IN THE RED ALGA ACROSYMPHYTON PURPURIFERUM1

The red alga Acrosymphyton purpuriferum (J. Ag.) Sjöst. (Dumontiaceae) is a short day plant in the formation of its tetrasporangia. Tetrasporogenesis was not inhibited by 1 h night-breaks when given at any time during the long (16 h) dark period (tested at 2 h intervals). However, tetrasporogenesis was inhibited when short (8 h) main photoperiods were extended beyond the critical daylength with supplementary light periods (8 h) at an irradiance below photosynthetic compensation. The threshold irradiance for inhibition of tetrasporogenesis was far lower when supplementary light periods preceded the main photoperiod than when they followed it (<0.05 μmol·m⁻²·s⁻¹ vs. 3 μmol·m⁻²·s⁻¹). The threshold level also depended on the irradiance given during the main photoperiod and was higher after a main photoperiod in bright light than after one in dim light (threshold at 3 μmol·m⁻²·s⁻¹ after a main photoperiod at ca. 65 μmol·m⁻²·s⁻¹ vs. threshold at <0.5 μmol·m⁻²·s⁻¹ after a main photoperiod at ca. 35 μmol·m⁻²·s⁻¹). The spectral dependence of the response was investigated in day-extensions (supplementary light period (8 h) after main photoperiod (8 h) at 48 μmol·m⁻²·s⁻¹) with narrow band coloured light. Blue light (λ= 420 nm) was most effective, with 50% inhibition at a quantum-dose of 2.3 mmol·m⁻². However, yellow (λ= 563 nm) and red light (λ= 600 nm; λ= 670 nm) also caused some inhibition, with ca. 30% of the effectiveness of blue light. Only far-red light (λ= 710 nm; λ= 730 nm) was relatively ineffective with no significant inhibition of tetrasporogenesis at quantum-doses of up to 20 mmol·m⁻².     

Cultivation of Lemna gibba under desert conditions. I: Twelve months of continuous cultivation in open ponds

Duckweed, Lemna gibba, was grown in 12 m² shallow ponds in the Negev desert, during 12 months of continuous cultivation, beginning April 1984. Average monthly growth rates varied with the season of the year. The lowest daily yield, 2·6±0·4 g dry weight m⁻² day⁻¹, was obtained during January. Highest daily yields, 7·9±2·6 g dry weight m⁻² day⁻¹ and 7·0±1·2 g dry weight m⁻² day⁻¹, were obtained during September and May. A 35% decline of the yield was seen during midsummer (July), 4·8±1·2 g dry weight m⁻² day⁻¹. The average rate for the year was 5·15±1·7 g dry weight m⁻² day⁻¹. The protein content of the plants ranged from 30 to 38% per unit dry weight.Growth performance is discussed in relation to the prevailing climatic conditions.     

Improved performance of a hybrid design over an anaerobic filter for the treatment of dairy industry wastewater at laboratory scale

A combined system designed by converting the flow mixing chamber of an anaerobic filter into an UASB resulted in an increased efficiency of removal of organic matter of 92% and in a gas production of 4.64 l·l⁻¹·d⁻¹, at the highest organic loading rate tested compared with that of the unmodified anaerobic filter. Both reactors were tested using dairy industry wastewater at identical operating conditions at 30°C and organic loading rate between 1 to 8 g COD·l⁻¹·d⁻¹.     

Biomass yield and nutrient removal by water hyacinth (Eichhornia crassipes) as influenced by harvesting frequency

The effects of harvesting frequency on productivity, nutrient storage and uptake, and detritus accumulation by water hyacinth (Eichhornia crassipes /Mart/ Solms) cultured outdoors in nutrient-enriched waters were evaluated for a period of 13 months. Significant differences in hyacinth standing crop and productivity were measured with harvesting regimes of 1, 3 (harvest at maximum density) and 21 harvests over a 13-month period. The average plant standing crop decreased from 65 to 20 kg (fresh wt) m⁻² for systems with 1 and 21 harvests, respectively. Total harvested plant biomass was 67 kg (fresh wt) m⁻², 110 kg (fresh wt) m⁻² and 162 kg (fresh wt) m⁻² for 1, 3 and 21 harvests, respectively. The mean net productivity increased from 7·7 to 16·5 and 24·5 g (dry wt) m⁻² day⁻¹ for 1, 3 and 21 harvests, respectively. Nutrient storage in water hyacinth biomass (live, dead and detrital) at the end of the study decreased from 93 to 46 and 30 g N m⁻², and from 20 to 12 and 5 g P m⁻², for 1, 3 and 21 harvests, respectively. For the system with one harvest, 46% of the stored N and 25% of the stored P were recovered in dedrital tissue at the bottom of the tank. For the systtem with 21 harvests, only 11% of the stored N and 15% of the stored P were recovered in detrital tissue at the bottom of the tank. Ammonium-N and soluble reactive P concentrations in the water column were significantly higher for the treatment with one harvest compared to the treatments with 3 and 21 harvests.     

Influence of planting geometry on photosynthetically active radiation interception and dry matter production relationships in pearl millet

A field experiment was conducted at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) Center, Patancheru, India to study photosynthetically active radiation (PAR) interception and dry matter production relationships in pearl millet (Pennisetum americanum (L.) Leeke). Two pearl millet genotypes, BJ 104 (G₁) and ICH 226 (G₂) were sown at three planting geometries obtained by using combinations of row and plant spacings (S₁: 37·5 cm × 26·6 cm; S₂: 75·0 cm × 13·3 cm; S₃: 150·0 cm × 6·6 cm) such that plant population was constant at 100 000 ha⁻¹ in all treatments. Cumulative intercepted PAR was maximum (330 MJ m⁻²) in G₂S₂ and minimum (268 MJ m⁻²) in G₁S₃. Conversion efficiency values ranged from 1·87 g MJ⁻¹ in G₁S₂ to 2·32 g MJ⁻¹ in G₂S₃. Final above-ground dry matter followed the pattern of cumulative intercepted PAR and maximum dry matter (7·22 Mg ha⁻¹) was produced by G₂S₂ while G₁S₃ produced minimum dry matter (4·97 Mg ha⁻¹).     

Inorganic carbon uptake by phytoplankton in Vineyard Sound,Massachusetts. II. Comparative primary productivity and nutritional status of winter and summer assemblages

Using time-course, natural-light incubations, we assessed the rate of carbon uptake at a range of light intensities, the effect of supplemental additions of nitrogen (as NH₄⁺ or urea) on light and dark carbon uptake, and the rates of uptake of NH₄⁺ and urea by phytoplankton from Vineyard Sound, Massachusetts from February through August 1982. During the winter, photoinhibition was severe, becoming manifested shortly after the start of an incubation, whereas during the summer, there was little to no evidence of photoinhibition during the first several hours after the start of an incubation. At light levels which were neither photoinhibiting nor light limiting, rates of carbon uptake normalized per liter were high and approximately equal during winter and summer (22–23 μg C·l^?1 · h^?1), and low during spring (<10 μgC·l^?1· h^?1). In contrast, on a chlorophyll a basis, rates of carbon fixation were as high during spring (15–20μg C·μg Chl a^?1·h^?1), when concentrations of chlorophyll a were at the yearly minimum (<0.5 μg · l^?1) as during the summer, when chlorophyll a concentrations were substantially higher (0.8–1.3 μg · l^?1). Highest rates of NH₄⁺ and urea uptake were observed during summer, and at no time of the year was there evidence for severe nitrogen deficiency, although moderate nitrogen nutritional stress was apparent during the summer months.     

Influence of pH on methane and sulfide production from methanol

To investigate the influence of pH on methane and sulfide production, continuous cultures were done using a bio-reactor packed with pumice stone. Sulfate (1 g SO₄²⁻·l⁻¹) in a methanol defined medium (10 g·l⁻¹) was almost completely reduced to sulfide at pHs between 7.0 and 7.5 in methane fermentation, but at pHs between 6.2 and 6.8, sulfate reduction to sulfide was suppressed up to 40%. In addition, methane fermentation was not inhibited by 10 g sulfate·l⁻¹.     

Effect of heavy metals (Cu,Cd and Pb) on aspartate and alanine aminotransferase in Ruditapes philippinarum (Mollusca: Bivalvia)

The accumulation of cadmium, copper and lead and their effects on aspartate and alanine aminotransferases in digestive gland, gills, foot and soft body in the clam Ruditapes philippinarum were examined. The animals were exposed to different concentrations: Cd (200–600 μg·l⁻¹), Pb (350–700 μg·l⁻¹) and Cu (10–20 μg·l⁻¹) for 7 days. The highest concentrations were found in digestive gland for cadmium and copper, and in gills for lead, and the lowest values were observed in the foot. Aspartate aminotransferase activity (AST), in general, was not inhibited by cadmium, lead or copper during the exposure. Only in clams exposed to cadmium (600 μg·l⁻¹, 7 days) and copper (20 μg·l⁻¹, 5 days) were observed significant differences (P<0.05) in foot and gills, respectively, with respect to control. In the case of alanine aminotransferase activity (ALT), significant differences were observed for cadmium and lead in treated animals with respect to control. With regard to copper, a decrease in ALT was observed in gills and foot exposed to 20 μg·l⁻¹. A significant correlation (P<0.05) was observed between ALT and metal accumulation for cadmium, copper and lead in gills. In the case of soft body, only cadmium and lead showed a significant correlation. In summary, R. philippinarum can be considered a bioindicator species for cadmium and lead accumulation and ALT could be useful as biomarker of sublethal stress for these metals in soft tissues and gills. Only gills can be considered an adequate target tissue for copper.     

Characterization of Brackish Coastal Waters of Different Trophic Levels by Means of Phytoplankton Biomass and Primary Production

Investigations on phytoplankton have been carried out in Baltic coastal waters, which are widely separated from the sea. Corresponding to their distance to the sea they differ in their trophic level. Annual phytoplankton production and a mean biovolume of 100-200 g C · m⁻² and 2-10 mm⁻³ · 1⁻¹ respectively were found for the mesotrophic level, of 200-400 g C · m⁻² and 10-20 mm³ · 1⁻¹ for the eutrophic level, and of 400-600 g C · m⁻² and over 20 mm³ · 1⁻¹ for the polytrophic level. In the course of nearly two decades of investigation a slight increase of phytoplankton biovolume, but no increase of primary production was recorded. Some changes in species composition are to be recognized in Greifswalder Bodden, if old literature is compared with recent data. In the Darss-Zingst boddens a shift from Cyanobacteria dominance towards Chlorophyceae dominance has appeared for at least 5 years. This was on account of the disappearance of Gomphosphaeria pusilla, which is thought to be an indicator species of the mesotrophic level.     

PHOTOSYNTHESIS IN AN EXTREME SHADE ENVIRONMENT: BENTHIC MICROBIAL MATS FROM LAKE HOARE, A PERMANENTLY ICE-COVERED ANTARCTIC LAKE

We investigated the composition of benthic microbial mats in permanently ice-covered Lake Hoare, Antarctica, and their irradiance vs. photosynthetic oxygen exchange relationships. Mats could be subdivided into three distinct depth zones: a seasonally ice-free “moat” zone and two under-ice zones. The upper under-ice zone extended from below the 3.5 m thick ice to approximately 13 m and the lower from below 13 m to 22 m. Moat mats were acclimated to the high irradiance they experienced during summer. They contained photoprotective pigments, predominantly those characteristic of cyanobacteria, and had high compensation and saturating irradiances (E_c and E_k) of 75 and 130 μmol photons·m⁻²·s⁻¹, respectively. The moat mats used light inefficiently. The upper under-ice community contained both cyanobacteria and diatoms. Within this zone, biomass (as pigments) increased with increasing depth, reaching a maximum at 10 m. Phycoerythrin was abundant in this zone, with shade acclimation and efficiency of utilization of incident light increasing with depth to a maximum of 0.06 mol C fixed·mol⁻¹ incident photons under light-limiting conditions. Precipitation of inorganic carbon as calcite was associated with this community, representing up to 50% of the carbon sequestered into the sediment. The lower under-ice zone was characterized by a decline in pigment concentrations with depth and an increasing prevalence of diatoms. Photosynthesis in this community was highly shade acclimated and efficient, with E_c and E_k below 0.5 μmol·m⁻²·s⁻¹ and 2 μmol·m⁻²·s⁻¹, respectively, and maximum yields of 0.04 mol C fixed·mol⁻¹ incident quanta. Carbon uptake in situ by both under-ice and moat mats was estimated at up to 100 and 140 mg·m⁻²·day⁻¹, based on the photosynthesis–irradiance curves, incident irradiance, and light attenuation by ice and the water column.