Osmund Holm-Hansen

Observations of modeling of the Antartic phytoplankton crop in relation to mixing depth

The multi-disciplinary program RACER (Research on Antarctic Coastal Ecosystem Rates) conducted eight surveys of a 69-station grid in a 100 × 250 km area in the southwestern Bransfield Strait from December 1986 to March 1987. Mean phytoplankton crop size in the upper 50 m during December, January, February and March was 291, 176, 58 and 50 mg Chl α m−2, respectively, and was inversely proportional to the increasing mean depth of the upper mixed layer (UML) (15, 17, 26 and 30 m, respectively). Massive mid-summer phytoplankton blooms (> 10 mg Chl α + phaeo m−3) were persistent nearshore where we observed shallow UMLs ( 20 m) with small density gradients (Δσt 0.05–0.20 from 0 to 75 m). Proximity to stabilizing meltwater and protection from intense Antarctic storm activity appear to be essential for the development of persistent massive blooms. A model of Antarctic phytoplankton growth based on mixing depth and pigment-specific light attenuation and in situ photosynthesis-irradiance relationships indicates that the depth of the UML (ZUML) can be used to predict the upper limit of the phytoplankton crop size. Observed phytoplankton biomass for diverse Southern Ocean ecosystems is discussed in relation to the mean light level of the UML, growth and loss rates of Antarctic phytoplankton, and the depth and duration of stratification required before a bloom ensues. Assuming nutrients do not limit the crop size, a best-fit to observations indicates specific loss rates must be approximately 0.3–0.35 day−1 and massive blooms occur only if ZUML <25 m. The grazing component of this predicted loss rate is higher than previously estimated. We conclude that grazing rates are greater than previously reported, or vertical flux rates of nutrients limit massive blooms.

Spatial and temporal distribution of phytoplankton and primary production in the western Bransfield Strait region

Abstract Studies on phytoplankton were one component of the multi-disciplinary RACER program which had 69 stations within a 100 × 250-km rectangle in the southwestern Bransfield Strait and contiguous waters. Data were acquired during eight cruises between December 1986 and March 1987. All deep stations north of the continental shelf break were low in phytoplankton biomass ( 10 μM inorganic nitrogen) that nutrient depletion is not likely to have caused the rapid decline of the phytoplankton bloom. Grazing, sinking and advection all appear to be important mechanisms of massive bloom decline. Phytoplankton populations appeared to be low-light adapted, as they showed low Pmax values (1.1 mg C mg Chl a−1 h−1), low saturating ligh values (Ik ∼ 18 μEins m−2 s−1), high initial slope [α = 0.06 (mg C mg Chl a−1h−1/μEin m−2 s−1)] and a compensation point for net light-activated fixation of CO2 of ≈ 1.0 μEin m−2 s−1.

Bio-optical properties of Antarctic Peninsula waters: differentiation from temperate ocean models

Abstract An extensive biological and optical data set was collected during a 4 month cruise as part of the Research on Antarctic Coastal Ecosystem Rates (RACER) program conducted in coastal waters of the Antarctic Peninsula and adjacent open ocean waters of Drake Passage. Chlorophyll plus phaeopigment (Chl + Phaeo) concentration in the upper mixed layer ranged 2 orders of magnitude from 0.5 to 50 mg Chl + Phaeo m −3 during the study. The large variations in pigment corresponds to variations in the beam attenuation coefficient at 660 nm ( c t ranging from 0.5 to >2.5 m −1 and in the diffuse attenuation coefficient ( k d ) for 441 nm ranging from 0.04 to >1.0 m −1 . Chl + Phaeo specific particulate beam attenuation and spectral absorption coefficients suggest that detrital contributions are relatively low and that pigment package effects are relatively important compared to low latitude observations. The combination of these effects causes low pigment specific absorption and scattering. This regional differentiation in particulate optical properties has a significant effect on models of the relationship between Chl + Phaeo and spectral values of k d and upwelled radiance ( L u ). Implications of these effects for modeling light propagation through the water column and for remote sensing of phytoplankton pigments are discussed.

Photoacclimation of antarctic marine diatoms to solar ultraviolet radiation

Abstract The present study was carried out at Palmer Station (64.7 ° S, 64.1 ° W), Antarctica, during the austral spring-summer of the years 1993 and 1994. Two centric diatom species ( Thalassiosira sp. and Corethron criophilum Castracane) and two pennate species ( Pseudonitzschia sp. and Fragilariopsis cylindrus (Grunow) Krieger) were isolated from natural phytoplankton assemblages and exposed to solar radiation to study long term (more than 1 week) photoacclimation to ultraviolet radiation (UVR). At the beginning of the experiments, three of the cultures had relatively low concentrations of UV-absorbing compounds (i.e., mycosporine-like amino acids) and photosynthetic rates were significantly inhibited by UVR. At the end of the experiments (8–12 days), however, the two centric diatom species had high contents of mycosporine-like amino acids (MAAs) and did not show any significant differences in photosynthetic rates when exposed to either UVR + PAR or just to PAR. The synthesis of MAAs was slightly less when samples were exposed only to PAR than when exposed to UVR in addition to PAR. The rates of synthesis of MAAs, relative to phytoplankton carbon, for the two centric diatoms were 0.001 and 0.008 μg MAAs · (μg C) −1 · day −1 for shinorine and porphyra-334, respectively. The concentrations of MAAs in Pseudonitzschia sp., and Fragilariopsis cylindrus at the end of the experiments were much lower (less than one tenth) than that in the centric diatoms and the cultures were still inhibited by UVR. In the pennate diatoms MAAs increased in concentration as a response only to UVR and not to PAR. The loss rates of MAAs in Thalassiosira sp. after transferring the culture from high (1200 μE · m −2 · s −1 ) to low irradiance (250 μE · m −2 · s −1 ) were 0.0002 and 0.0023 μg MAAs · (μg C) −1 · day −1 for shinorine and porphyra-334, respectively. These results provide further evidence that MAA compounds are synthesized in response to high light conditions and that they do decrease the photoinhibitory effects of UVR.

Chemical composition of Skeletonema costatum (Grev.) Cleve and Pavlova (Monochrysis) lutheri (Droop) Green as a function of nitrate-, phosphate-, and iron-limited growth.

The variation in chemical composition of Skeletonema costatum (Grev.) Cleve and Pavlova lutheri (Droop) Green was studied in batch cultures with N-, P-, and Fe-deficient media under continuous light. In vivo fluorescence, chlorophyll a, ATP, cellular nitrogen, carbon, and phosphorus, and cell density were measured. The exponential growth rate was 2.9–3.5 doubl, day−1 for Skeletonema and 1.2–1.9 doubl, day−1 for Pavlova, with the lower rates pertaining to cultures in Fe-deficient media. In exponentially growing cultures nitrogen and carbon per cell increased with an increasing N/P ratio of the media, whereas chlorophyll a, ATP, and organic phosphorus per cell remained relatively constant. In Fe-deficient cells nitrogen and carbon per cell was high, as for cells in media with a high N/P ratio. The total variation in exponential cells was contained within a factor of three. Larger differences in chemical composition were found between exponentially growing cells and nutrient deficient cells. For all types of nutrient deficiency the chl/N and chl/C ratios decreased when cells went from the exponential phase to the starvation phase. The ratio between in vivo fluorescence and chlorophyll a increased 4–5 fold. The CN ratio (atomic) of both species increased from 5–7.5 for exponentially growing cells up to 35 during N-deficiency and up to 13 during P-deficiency. For cells in Fedeficient medium values were scattered in the 7–11 range. ATP and organic phosphorus followed each other closely and had values which were about three times higher in Skeletonema than in Pavlova when using any other parameter as a frame of reference for identical media and growth stages. In N-deficient media the NATP ratio was virtually the same for exponentially growing cells and starved cells. The CATP ratio reached high levels in nutrient deficient cells, with a maximum of 9000 for P-deficient cells of Pavlova. Both species had a minimum value of polyphosphates corresponding to ≈ 10 % of the total phosphorus. In P-rich media Pavlova had storage polyphosphates up to 70 % of the total P. Storage polyphosphates were not observed in Skeletonema. With the set of factors used it is possible to identify growth stages and type of nutrient deficiency for a given culture. It is indicated that, with certain limitations, this approach may be applicable in field studies.

Phytoplankton standing crop, primary productivity, and near-surface nitrogenous nutrient fields in the Ross Sea, Antarctica

Abstract In austral summer 1978 a study was made of the phytoplankton standing crop, primary productivity, and nutrient chemistry of the waters along a north-south transect between New Zealand and the Ross Ice Shelf and along two east-west transects in the Ross Sea. The area was characterized by a high degree of spatial biological variability, with pronounced differences in phytoplankton biomass, primary productivity, and near-surface nitrate and ammonium fields between the northern, central, and southern regions of the Ross Sea. An extensive bloom of the colonial flagelate Phaeocystis pouchetiii (Prymnesiophyceae) was encountered along the barrier edge of the Ross Ice Shelf and extended to depths of 100 to 150 m at some stations. Chlorophyll a throughout the euphotic zone along the ice shelf barrier edge averaged > 1 mg m−3, and primary productivity locally reached nearly 1 g C m−2 day−1 there. In contrast, both chlorophyll a concentration and rates of 14C uptake averaged two- to four-fold less some 200 km offshore of the Ross Ice Shelf, where P. pouchetii generally was rare. Where P. pouchetii was abundant, more than 25% of the water column primary production occurred at depths below the 1% light penetration level. Near-surface nitrate concentrations were about a third lower at the stations where P. pouchetii was abundant than at those with few or no P. pouchetii. Regionally there was a strong negative correlation between NH4+ and chlorophyll a, suggesting that the biogenic uptake of NH4+ by the phytoplankton controlled the distribution of NH4+ within the euphotic zone.

PHOTOINHIBITION OF PHOTOSYNTHESIS IN NATURAL WATERS

Abstract— A quantitative analysis of the wavelength‐dependent influence of solar irradiance on natural phytoplankton photosynthesis has been made. The effect on productivity due to several different UV radiation regimes has been measured. In the course of this analysis, it has been shown that the biological weighting function for photoinhibition of chloroplasts (Jones and Kok, 1966) allows the calculation of a biologically effective dose which is consistent with the measured photoinhibition in natural phytoplankton populations. The ecological implications of a change in available UV radiation, possibly due to anthropogenic altering of the ozone layer, are explored and it is found that the present static bottle l4C technique of measuring in situ phytoplankton productivity does not lend itself to assessing accurately the potential ecological consequences of possible increased MUV (middle ultraviolet radiation in the 280–340 nm region) on phytoplankton populations. A small change in MUV has a relatively minor effect on photoinhibition dose rates whereas it has a large potential effect on DNA dose rates.

Methodology and measurement of adenylate energy charge ratios in environmental samples

A method for measuring ATP, ADP and AMP levels in environmental samples was devised, and applied to seawater and bacterial cell extracts. This procedure is specifically designed for measuring the extremely low concentrations of total adenine nucleotides ([AT]=[ATP]+[ADP]+[AMP]) that are apt to occur in most natural ecosystems (i.e., ≤10 ng AT ml-1 of sample extract). Although the current assay methodology can be used with purified firefly luciferase reagents, it has been suitably modified to accept crude luciferase preparations as well. ATP, ADP and AMP levels have been measured, and the corresponding energy charge (EC) ratios determined for seawater samples collected off the Southern California coast. The EC ratios ranged from 0.50 to 0.89, with peak values corresponding to the subsurface maxima in ATP and chlorophyll a concentrations, and the minimum values corresponding to the deepest water sampled (1500 m). The measurement of adenylate energy charge ratios in environmental samples can be a useful indicator of mean community metabolic activity and potential for cell growth.

Ultraviolet Radiation and Its Effects on Organisms in Aquatic Environments

The problem of trying to determine the effect of solar ultraviolet radiation (UVR) on aquatic organisms is much more difficult than that of assessing the impact of UVR on terrestrial plants. The major reasons for this are that spectral irradiance changes dramatically with depth in the water column and that most aquatic organisms will be moving up and down in the upper water column, either through active motility processes or by physical mixing processes. It is thus not possible to determine the effect of UVR on planktonic organisms with any degree of certainty; the best one can do is to determine the effects under a wide variety of experimental techniques, and to estimate the potential damage to organisms when they are under completely natural conditions.

Effect of temperature on rate of photosynthesis in Antarctic phytoplankton

SummaryThe rate of photosynthesis of marine antarctic phytoplankton (western Scotia Sea and Bransfield Strait) was determined as a function of temperature, from ambient (-0.8°C to 1.0°C) to 28°C. Photosynthetic rates, based on radiocarbon incorporation during half-day incubations, were increased by as much as 2x with temperatures up to 7°C; at higher temperatures the rates decreased rapidly, so that at 28°C the rates were only 3% of that at ambient temperatures. In antarctic surface waters during the austral summer the rate of photosynthesis by phytoplankton thus is limited by thermodynamic effects on metabolic reactions, in spite of high nutrient concentrations and saturating light levels. The observed rates were in agreement with thermodynamic models of the dependence of phytoplankton growth rate on temperature.