Abundance and photosynthetic activity of ice algae in Resolute Passage in the Canadian high Arctic were measured in relation to in situ irradiance throughout the main growth season in 1985 and 1986. A simple model was used to calculate in situ production rates and the theoretical maximum (light limited) size of crops and production rates. Both the observed and the maximum possible crop sizes and production rates varied directly with irradiance over the natural range of snow cover, and crops attained the theoretical maximum imposed by self-shading (77 to 225 mg m-' chlorophyll a under thin snow cover) in both years. Calculated in situ production of ice algae under thin snow cover (S to 23 gC m-2 yr-l) could equal or exceed typical values for Arctic plankton. Comparison against observed biomass accumulation in the ice indicated that as much as 65 % of the production could be exported from the ice during the growth season. Where light was artificially increased by maintaining snow-free areas, observed crops were much less than the theoretical maximum despite the abscence of photosynthetic photoinhibition. Crops reported from some other arctic sites were also much less than their corresponding theoretical maxima. Low irradiance often limits ice algal growth, but our results suggest that losses associated with excessive irradiance and with grazing by amphipods at near-shore sites are additional factors determining algal abundance and production.
We measured simultaneously the size-distribution of oxygen consumption, inorganic and orgamc dissolved carbon assimilation, and the standing stocks of phytoplankton and bacteria, to develop simple carbon budgets and "C exchange models for the microplankton communities of a large conhnental shelf system. In spring samples (dominated by large-celled dlatoms) community respiration consumed 33 to 100 % or more of the gross primary production (GPP) measured by the 0,-light-anddark-bottle technique and was apparently due entirely to microheterotrophs. In fall samples (dominated by flagellates) community respiration consumed 100 % or more of the measured GPP, and appeared on average to be due equally to autotrophs and heterotrophs. Pnmary production, estimated by 14C-CO2 assimilation into particulate matter, often greatly underestimated GPP and community metabolism measured by the 0, technique, with apparent photosynthetic quotients (PQ) of about 5.0 in the spring samples. Modelling of the tracer dynamics suggested large rates of DOC release (> particulate production) from a small and rapidly cycling pool w i t h the autotrophs as the cause of the large apparent PQ's. The large share of primary production consumed by small microheterotrophs ( c 5 or l p) and frequent lack of positive net community production suggested a low efficiency for the transfer of microheterotrophic production to larger or.;a~isn?s.
ABSTRACT. Synthesis rates of macromolecular classes (protein, polysaccharide, neutral lipid, glycolipid and phospholipid) were measured using 14C incorporation to assess the physiological condition of phytoplankton during a spring bloon~ in the northeastern Sargasso Sea. The relative allocation of labelled photosynthate to polysaccharide increased with incubation irradiance, and declined overnight; opposite trends were found in the protein fraction. The rate of protein synthesis at night was a small fraction (0 to 23 '%I) of the day-time rate. Incubation irradiance and overnight metabolism had relatively little effect on total or class-specific lipid synthesis. lnorganic nutrients decreased to below analytical detection during the sampling period, with coincident declines in chlorophyll a concentrations. The relative allocation of photosynthate to protein was much reduced in the nutrient-depleted stations later on in the bloom, while the allocation to polysaccharide was increased. Total lipid synthesis was only slightly higher in the later stations, but the proportion of neutral lipid was significantly increased. The changes In macrornolecular labelling patterns were consistent with a physiological response to nutrient depletion by the phytoplankton, and a reduction of 50 % or more in specific growth rates.
Studies of the utilization of inorganic nitrogen (NO3-, NH,+) by sea-ice algal communities were conducted durinq 2 field seasons in Barrow Strait, Northwest Territories (NWTI, Canada. Results showed a significant temporal shift from NO3--dominated metabolism during the early stages of algal biomass accumulat~on under the ice to NHa+-dominated metabolism later on when biomass was in decline. Volume-based uptake rates of both nitrogen compounds were 2 to 3 orders of magnitude higher (1 to 80 pm01 N 1-' h-') than rates typical for coastal plankton populations but so were biomass levels (4 to 18 mg chlorophyll a I-') and interstitial nitrogen concentrations (NO3-:4 to 123 &m01 N I-', NH4+ : 4 to 40 pm01 N 1-'). Ammonium was utilized preferentially as is generally the case in planktonic systems. Despite high concentrations, however, NH4+ apparently had little inhibitory effect on the activity of the NO3-assimilatory enzyme, nitrate reductase (NR), at least during the early stages of ice-algal growth.Complementary physiological experiments camed out during this same period showed (1) concentration-dependent nitrogen uptake kinetics (K,) for these communities were similar to values seen in coastal plankton. (2) no apparent light-dependence of NO3-or NH,+ uptake was evident in short-term experiments. (3) organic nitrogen (urea, amino acids) may represent a significant portion of the sea-ice communities' nitrogen nutrition. (4) an important component of the metabolism of NH,+ and amino acids may be mediated by prokaryotic microorganisms. Our results, along with several other indirect lines of evidence, support the contention that these sea-ice communities are not nitrogen-limited.
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