Photosynthetic oxygen production and community respiration in the Indian sector of the Antarctic Ocean during austral summer

Photosynthetic oxygen production by phytoplankton and community respiration in the Indian sector of the Antarctic Ocean were estimated from changes in oxygen concentrations in light and dark bottles. Gross production varied between 0.1 and 5.1 µmol O₂ l^-1 day^-1. In the same water, community respiration (the sum of oxygen consumption by heterotrophs and phytoplankton) was 0.4-3.6 µmol O₂ l^-1 day^-1, which accounted for 47-343% of the gross production. Algal and heterotrophic respirations were distinguished using some assumptions. These estimates showed that heterotrophic respiration accounted for most of the community respiration (70-91% depending upon the assumptions), indicating that heterotrophic respiration plays an important role in the mineralization of phytoplankton production in the surveyed sea area. Gross production rate correlated with chlorophyll a concentration, showing that the photosynthetic production rate of oxygen depends on the abundance of phytoplankton. Moreover, there was a significant relationship between gross production and community respiration rates. These regression equations suggested that negative net production occurred under the usually low concentration of chlorophyll observed in the Indian sector of the Antarctic Ocean. Hence, the net exchange of carbon dioxide due to biological processes through the sea surface seemed to be not as large as expected in the Antarctic Ocean, although the number of data were limited at this stage.     

Primary production, light and vertical mixing in Potter Cove, a shallow bay in the maritime Antarctic

Phytoplankton photosynthesis was measured during spring-summer 1991-1992 in the inner and outer part of the shallow Potter Cove, King George Island. Strong winds characterise this area. Wind-induced turbulent mixing was quantified by means of the root-mean square expected vertical displacement depth of cells in the water column, Z_t. The light attenuation coefficient was used as a measure of the influence of the large amount of terrigenous particles usually present in the water column; 1% light penetration ranged between 30 and 9 m, and between 30 and 15 m for the inner and outer cove, respectively. Obvious differences between photosynthetic capacity [P*_max; averages 2.6 and 0.6 µg C (µg chlorophyll-a)^-1 h^-1] and photosynthetic efficiency {!*; 0.073 and 0.0018 µg C (µg chlorophyll-a)^-1 h^-1 [(µmol m^-2 s^-1)^-1]} values were obtained for both sites during low mixing conditions (Z_t from 10 to 20 m), while no differences were found for high mixing situations (Z_t>20 m). This suggests different photoacclimation of phytoplankton responses, induced by modifications of the light field, which in turn are controlled by physical forcing. Our results suggest that although in experimental work P*_max can be high, wind-induced mixing and low irradiance will prevent profuse phytoplankton development in the area.     

Factors influencing the short-term variation in phytoplankton composition and biomass in coral reef waters

The short-term temporal dynamics of phytoplankton composition was compared among coral reef waters, the adjacent ocean and polluted harbour water from July until October along the south-western coast of Curaçao, southern Caribbean. Temporal variations in phytoplankton pigment 'fingerprints' (zeaxanthin, chlorophyll b, 19'-hexanoyloxyfucoxanthin, fucoxanthin, 19'-butanoyloxyfucoxanthin, chlorophyll c₂ and c₃ relative to chlorophyll a) in the ocean were also observed in waters overlying the reef. However, with respect to specific pigments and algal-size distribution, the algal composition in reef waters was usually slightly different from that in the oceanic water. Phytoplankton biomass (chlorophyll a) was either higher or lower than in the oceanic water. The relative amount of fucoxanthin and peridinin was usually higher, and the relative and absolute amount of zeaxanthin was significantly lower than in oceanic water. Zeaxanthin-containing Synechococci were significantly reduced in reef water. Average algal cell size increased from the open water to the reef and the harbour entrance. Large centric diatoms (>20 m Ø) were better represented in reef than in oceanic water. In reef-overlying waters, the nitrate and nitrite concentrations were higher than in oceanic water. In front of the town, anthropogenic eutrophication (sewage discharge and ground water seepage) resulted in higher NH₄, NO₃ and PO₄ concentrations than at other reef stations. This concurred with significantly enhanced phytoplankton biomass (chlorophyll a), chlorophyll c₂ and peridinin amounts at Town Reef compared with the other reef stations. Polluted harbour water usually showed the highest phytoplankton biomass of all stations, dominated by diatoms and dinoflagellates. Conditions in reef waters and harbour water promoted the occurrence and the relative abundance of diatoms and dinoflagellates. Harbour water did not influence the phytoplankton composition and biomass at reef stations situated >5 km away from the harbour entrance. We conclude that phytoplankton undergoes a shift in algal composition during transit over the reef. The dominant processes appear to be selective removal of zeaxanthin-containing Synechococcus (by the reef benthos) and (relative) increase in diatoms and dinoflagellates. The difference in the phytoplankton composition between reef and oceanic waters tends to increase with decreasing dilution of reef water with ocean water.     

Lactate dehydrogenase from the Antarctic eelpout, Lycodichthys dearborni

As part of our studies to examine the molecular basis of cold-adaptation, we have determined the kinetic properties, thermal stability and deduced amino acid sequence of the enzyme lactate dehydrogenase (LDH) from an Antarctic zoarcid fish, Lycodichthys dearborni. Unlike Antarctic notothenioid fish which are endemic to the Southern Ocean, zoarcid fish are cosmopolitan and have a substantially longer evolutionary history as a sub-order. The A₄-LDH isoform was isolated and purified from the white muscle of L. dearborni. The kinetic parameters K_m^PYR and k_cat were determined at temperatures from 0 to 25°C. K_m^PYR was substantially higher at low temperatures than those from Antarctic and temperate notothenioid fish, whereas k_cat at these temperatures was essentially the same as those of the other fish LDH in this study. The sequence of L. dearborni A₄-LDH was determined from cDNA derived from white muscle RNA and found to be similar to, but distinct from, the A₄-LDH sequences of Antarctic notothenioid fish. Molecular modelling based on the structure of the A₄-LDH from Pagothenia borchgrevinki suggested that three conservative amino acid changes within the core of the protein that are not directly part of the active site but which might nonetheless influence the active site, may be important in cold-adaptation in L. dearborni A₄-LDH, and that several other changes on the surface of the protein might also play a role in cold-adaptation.     

Variation of particulate carbohydrate pools over time and depth in a diatom-dominated plankton community at the Antarctic Polar Front

Carbohydrate dynamics were studied in an algal community dominated by the diatom Fragilariopsis kerguelensis at the Antarctic Polar Front during austral autumn 1999. Water-extractable mono- and polysaccharide concentrations from the particulate fraction were measured at six depths in the upper 100 m at seven stations along a N-S transect (20°E, 48-50°S) during 2 consecutive days. In addition, field populations were incubated on deck for 18 h at four different light intensities. Polysaccharide concentrations varied between 2.7 and 13.6 wg/l, monosaccharide concentrations between 2.5 and 8.9 wg/l. Near the surface (0-60 m), a diel pattern was observed in the polysaccharide concentration when normalised to the chlorophyll a or the monosaccharide concentration. The deck incubations supported the hypothesis that this pattern resulted from the diurnal accumulation and nocturnal consumption of reserve glucan. More polysaccharides accumulated at high light intensities than at low light intensities, in accordance with the observed decrease in normalised polysaccharide concentration with depth. In addition, the lower concentrations at depth might be explained by consumption: polysaccharides that were accumulated during time spent near the surface were subsequently respired when cells were transported to deeper and dimmer water layers. The variation in carbohydrate pools over time and depth described here must be considered of ecological relevance to phytoplankton in the Southern Ocean subjected to extended periods of darkness (hours to days) due to vertical mixing and advection.     

Reproductive activity of two dominant Antarctic copepod species, Metridia gerlachei and Ctenocalanus citer, in late autumn in the eastern Bellingshausen Sea

The main reproductive period of herbivorous Antarctic copepods is coupled to the phytoplankton bloom in spring, while omnivorous or carnivorous species apparently reproduce year round. However, our knowledge of the reproduction during autumn and winter is limited. Therefore this study during a cruise with RV "Polarstern" aimed to measure reproductive activity in two dominant copepod species, Metridia gerlachei and Ctenocalanus citer, in late autumn (April/May). For this purpose, gonad development stages were determined from both species, taken with Bongo net in the eastern Bellingshausen Sea (Antarctic Peninsula region), and related to the ambient feeding conditions represented as chlorophyll a concentration. In addition, experiments were conducted with M. gerlachei to study the influence of feeding and starvation on gonad development. Gonad maturity of M. gerlachei differed considerably between stations and decreased over time. The correlation of chlorophyll a and the proportion of mature females, though significant, was weak and highly variable. In experiments, the gonads of feeding females were in better condition than those of their starving counterparts, showing that reproductive activity in M. gerlachei was related to ambient feeding conditions. Mature females of C. citer were found at each station (17-67%). The gonad stage composition was rather similar at all stations, showing no clear temporal and spatial trend. In this species, the proportion of mature females was not related to the chlorophyll a concentration, indicating either that the ambient phytoplankton stock was sufficient to fuel maturation or that other food sources were used. Our study shows that both species are reproducing during the austral autumn, indicating that their reproduction is partially decoupled from the spring phytoplankton bloom.     

Escape performance in the sub-Antarctic notothenioid fish Eleginops maclovinus

Fast-start performance associated with escape behaviour was investigated in the sub-Antarctic notothenioid Eleginops maclovinus from the Beagle Channel, Tierra del Fuego, Argentina (mean winter water temperature 4°C, mean summer water temperature 10°C). Fish acclimated to 8.5°C for 2 months were filmed at 2, 4, 6, 8 and 10°C. Escape responses were temperature dependent over the range of temperatures tested. Maximum length-specific velocity ([^(V)]_max )(\hat V_{\max } ) , maximum length-specific acceleration (Â_max) and inertial power output (P_iner) increased significantly with temperature. Q₁₀ values for [^(V)]_max\hat V_{\max } , Â_max and P_iner were 1.90, 3.27 and 8.90, respectively. Non-dimensional curvature of the spine ([^(c)])(\hat c) also varied significantly with temperature, but was higher at low temperatures. The values of [^(c)]\hat c were threefold lower than previously reported for Antarctic notothenioids and similar to the values for temperate species. The results indicate that the high values of [^(c)]\hat c observed during escape behaviour in Antarctic notothenioids are not a universal feature of the suborder. A greater flexion of the body during fast starts is therefore a promising candidate for a specialised feature of behaviour linked to low-temperature performance.     

Reproductive response of the copepod Rhincalanus gigas to an iron-induced phytoplankton bloom in the Southern Ocean

The reproductive response of Rhincalanus gigas to the build up of a phytoplankton bloom in the Southern Ocean was studied during the European iron fertilization experiment (EIFEX). Egg production experiments were conducted over a period of approximately 5 weeks during development of a diatom dominated bloom. R. gigas showed a clear response to increasing chlorophyll a concentrations and the total egg production of the R. gigas population was highest just after the peak of the bloom at day 29 after fertilization. The average peak production was 50 eggs female⁻¹ day⁻¹. The percentage of egg producing females increased from about 0 to 90% during the course of the experiment. Accordingly, the maturation of the gonads reflected the positive response towards enhanced chlorophyll a concentrations. The fast reproductive response indicate that R. gigas was food limited during the period of this study in the Antarctic Polar Front region (APF).     

<Emphasis Type="Italic">Campylonotus arntzianus</Emphasis>, a new species of the Campylonotidae (Crustacea: Decapoda: Caridea) from the Scotia Sea (Antarctica)

Two specimens of Campylonotus arntzianus sp. nov. were caught in the Antarctic Scotia Sea off Saunders Island (57°40.31'S, 26°27.81'W) using an Agassiz trawl at one station (depth: 475-589 m). The new species described here is the fifth representative of the monogeneric family Campylonotidae, and the first of the family south of the Antarctic Convergence. Campylonotus arntzianus sp. nov. is a shrimp of about 5 cm in total length. Due to similarities in adult morphology, C. arntzianus sp. nov. seems to be closely related to C. capensis, a deep-sea species from the Southern Atlantic Ocean. A simple key for the species identification of the Campylonotidae is provided.     

Hierarchical habitat selection by barren-ground grizzly bears in the central Canadian Arctic

Using resource selection functions, we examined habitat selection patterns of barren-ground grizzly bears (Ursus arctos) in the central Canadian Arctic among and within home ranges. There was no difference between the sexes with regard to habitat selection patterns at the home range level (Wilks' 5, approx. F_11,11=1.27, P=0.37). Bear home ranges contain more esker habitat, tussock/hummock successional tundra, lichen veneer, birch seep, and tall shrub riparian areas relative to the proportional availability of habitats in the study area. We observed differences in habitat selection within home ranges among levels of sex/reproductive status (Wilks' 5, approx. F_20,412=3.32, P<0.001) and by season (Wilks' 5, approx. F_30,605=2.71, P<0.001). Eskers and tall shrub riparian zones were the habitats most preferred by bears throughout the year. Tussock/hummock successional tundra was also favored by males at varying times during the year and lichen veneers were favored in spring and autumn by most bears. Females with cubs tended to avoid the highest ranked habitat for males throughout the year. This pattern of habitat selection was not observed for females without accompanying young. Results of this study underline the importance of scale dependence in habitat selection. Failure to view habitat selection as a hierarchical process may result in a narrow and possibly misleading notion of habitat selection patterns.     

An evaluation of the uniform stress hypothesis based on stem geometry in selected North American conifers

The uniform stress hypothesis of stem formation was evaluated by comparing stem taper of Abies balsamea, Abies lasiocarpa, Picea rubens, Pinus contorta, Pinus elliottii, Pinus palustris, Pinus ponderosa, Pinus taeda, and Pseudotsuga menziesii to the taper expected if stems develop to uniformly distribute bending stress. The comparison was conducted by regressing stem diameter at height h (D_h) against bending moment at h (M_h) using the model D_h=J (M_h)^' where J and ' are fitted coefficients, and testing for '=0.333, the hypothesized value. Twelve curves were fitted with the model. Seven of the fitted values of ' were significantly different from 0.333, but eight of the values were within ᆞ% of 0.333 and eleven values were within ᆣ% of 0.333. Where the fitted value of ' was >15% of 0.333, residuals were biased with height. Fit by relative height, values of ' were within ᆞ% of 0.333 for large portions of these stems. While most of the fitted values of ' support the uniform-stress hypothesis, the values of ' for Pseudotsuga menziesii trees clearly did not. Many of the fitted values of J were inversely related to the modulus of elasticity (E) of green wood reported for these species. With the exception of Pseudotsuga menziesii, growing conditions appeared to account for extraordinary values of J. Increases in J with stem height corresponded with reported decreases in E with height. The covariance between J and E suggests some regulation of bending curvature by adjustments in cross-sectional area. These results suggest that stems taper to maintain a uniform bending curvature and that when E is relatively constant within and among stems, diameter along the stem or across stems can be predicted from bending moment using a simple power function.     

Phytoplankton community composition and photosynthetic physiology in the Australian sector of the Southern Ocean during the austral summer of 2010/2011

Phytoplankton population dynamics play an important role in biogeochemical cycles in the Southern Ocean during austral summer. However, the relationship between phytoplankton community composition and primary productivity remains elusive in this region. We investigated the community composition and photosynthetic physiology of surface phytoplankton assemblages in the Australian sector of the Southern Ocean from December 2010 to January 2011. There were significant latitudinal variations in hydrographic and biological parameters along 110°E and 140°E. Surface (5 m) chlorophyll a (chl a) concentrations measured with high-performance liquid chromatography varied between 0.18 and 0.99 mg m^?3. The diatom contribution to the surface chl a biomass increased in the south, as estimated with algal chemotaxonomic pigment markers, while the contributions of haptophytes and chlorophytes decreased. In our photosynthesis–irradiance (P–E) curve experiment, the maximum photosynthetic rate normalized to chl a ( \(P_{ \hbox{max} }^{*}\) ), initial slope (α ^*), the maximum quantum yield of carbon fixation (Φ _{c max}), and the photoinhibition index (β ^*) were higher in the region where diatoms contributed >50 % to the chl a biomass. In addition, there were statistically significant correlations between the diatom contribution to the chl a biomass and the P–E parameters. These results suggested that the changes in the phytoplankton community composition, primarily in diatoms, could strongly affect photosynthetic physiology in the Australian sector of the Southern Ocean.     

Water availability and carbon isotope discrimination in conifers

The stable C isotope composition ('¹³C) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and '¹³C. A significant component of the variation in '¹³C was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km^-1 altitude, r²=0.49, and in foliage, 1.91‰ km^-1, r²=0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO₂ partial pressure into the leaf (P_a-P_i) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in P_a-P_i of stemwood (r²=0.45) and foliage (r²=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on '¹³C probably dominated the effect of water availability. We also examined the relationship between plant drought stress (O) and '¹³C within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha^-1, O and '¹³C were less (more negative) than at 250 stems ha^-1 indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater '¹³C of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage - attributes that increase rates of photosynthesis, reduce P_i and increase '¹³C. '¹³C was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the '¹³C-O relationship varied between thinning treatments. Within treatments, the relationship between '¹³C and O was strong (0.38<r²<0.58). We conclude that '¹³C may well be a useful indicator of water availability or drought stress, but only in seasonally dry climates (P/E<1) and where variation in other environmental factors can be accounted for.     

Mesozooplankton community structure and grazing impact in the Polar Frontal Zone of the south Indian Ocean during austral autumn 2002

Mesozooplankton community structure and grazing impact were investigated at 13 stations in the Polar Frontal Zone during the second Marion Offshore Variability Ecosystem Study (MOVES II), conducted during April 2002. Total integrated chl- a biomass ranged between 11.17 and 28.34 mg chl- a m^-2 and was always dominated by nano- and picophytoplankton (<20 µm). Throughout the study, small copepods, mainly Oithona similis and Ctenocalanus vanus, numerically dominated the mesozooplankton community, composing up to 85% (range 30-85%) of the total abundance. Grazing activity of the four most abundant copepods ( O. similis, C. vanus, Calanus simillimus and Clausocalanus spp.), constituting up to 93% of total mesozooplankton abundance, was investigated using the gut fluorescence technique. Results of gut fluorescence analyses indicated that Calanus simillimus and Clausocalanus spp. exhibited diel variability in gut pigments with maximum values recorded at night. In contrast, O. similis and Ctenocalanus vanus did not demonstrate diel variation in gut pigment contents. Ingestion rates of the four copepods ranged from 23.23 to 1462,02 ng(pigm.)ind^-1day^-1. The combined grazing impact of the four copepods ranged between 1 and 36% of the phytoplankton standing stock per day, with the highest daily impact occurring at stations occupied in the vicinity of the Antarctic Polar Front (~35.86% at station 23). Among the copepods, O. similis and Ctenocalanus vanus represented the most important consumers of phytoplankton biomass, collectively responsible for up to 89% (range 15-89%) of the total daily grazing impact. Carbon specific ingestion rates of the copepods varied between 42 and 320% body carbon per day.     

A phytoplankton absorption-based primary productivity model for remote sensing in the Southern Ocean

Recent global environmental changes such as an increase in sea surface temperature (SST) are likely to impact primary productivity of phytoplankton in the Southern Ocean. However, models to estimate net primary production using satellite data use SST and uncertain estimation of chlorophyll a (chl-a) concentration. A primary productivity model for satellite ocean color data from the Southern Ocean, which is based on the light absorption coefficient of phytoplankton to reduce uncertainties of sea surface chl-a estimations and bias in optimal values of chl-a normalized productivity derived from SST, has been developed. The new model was able to estimate net primary productivity in the water column (PP _eu) without dependency on temperature when in the range of −2 to 25°C, and it explained 51% of the observed variability in PP _eu with a root mean square error (RMSE) of 0.15. Application of the model revealed that the SST dependent model has overestimated PP _eu in warmer waters around the Subtropical Front, and underestimated PP _eu in colder waters poleward of the Sub-Antarctic Front. This absorption-based primary productivity model contributes to a study of the relationship among spatio-temporal variations in the physical environment, and biogeochemical cycles in the Southern Ocean.     

Gas exchange and chlorophyll fluorescence responses of <Emphasis Type="Italic">Pinus radiata</Emphasis> D. Don seedlings during and after several storage regimes and their effects on post-planting survival

Post-storage gas exchange parameters like CO₂ assimilation, stomatal conductance, transpiration, water use efficiency and intercellular CO₂ concentrations, together with several chlorophyll a fluorescence parameters: F_o, F_v, F_v/F_m, F_m/F_o and F_v/F_o were examined in radiata pine (Pinus radiata D. Don) seedlings that were stored for 1, 8 or 15 days at 4° or 10°C with or without soil around the roots. Results were analysed in relation to post-storage water potential and electrolyte leakage in order to forecast their vitality (root growth potential) following cold storage, and post-planting survival potential under optimal conditions. During storage at 4° and 10°C, photosynthesis was reduced, being more pronounced in bare-root seedlings than in seedlings with soil around the roots. The depletion of CO₂ assimilation seemed not to be solely a stomatal effect as effects on chloroplasts contributed to this photosynthetic inhibition. Thus, the fall in the ratios F_v/F_m, F_v/F_o and F_m/F_o indicated photochemical apparatus damage during storage. Photosynthetic rate was positively correlated with the root growth index and new root length showing that new root growth is dependent primarily on current photosynthesis. Pre-planting exposure of bare-root radiata pine seedlings to temperatures of 10°C for more than 24 h during transportation or storage is not recommended.     

Optical fractionation of chlorophyll and primary production for coastal waters of the Southern Ocean

Our objective was to quantify the potential variability in remotely sensed chlorophyll a (Chl a) and primary productivity in coastal waters of the Southern Ocean. From data collected throughout the springs/summers of 1991–1994, we calculated the proportion of water column Chl a and primary productivity within the upper optical attenuation length (K ⁻¹ _par) and the satellite-weighted depth. The temporal variability was resolved every 2–3 days and was observed to be greater within years than between years. Three-year averages (n=223) revealed that 10.2 ± 3.6% of total Chl a and 14.8 ± 6.5% of production occurred within satellite-weighted depth in predominantly Case I waters. The average values were twice as high within K ⁻¹ _par, 24.1 ± 8% of total Chl a and 34 ± 9% of production respectively. Masked in these long-term averages are very large changes occurring on short time scales of seasonal blooms. We observed that the patterns of Chl a vertical distribution within blooms are also subject to taxonomic influence and dependent upon the physiological state of the phytoplankton. Highest proportions of water column Chl a in the first optical depth were measured during the rapid onset of surface cryptophyte blooms each year, i.e. 50% within K ⁻¹ _par and 30% above the satellite-weighted depth. Lowest fractions, 6% and 2% of biomass within K ⁻¹ _par and satellite-weighted depth respectively, were associated with peak bloom conditions independent of taxonomy. Our analyses suggest that satellite-dependent models of Chl a and subsequent chlorophyll-dependent primary production will be challenging to develop for the near-shore Southern Ocean, especially given the potentially high natural variability in the vertical distribution of Chl a driven by physical forcing, the photoadaptive abilities of polar phytoplankton, and taxonomic influences. Accepted: 27 August 1999     

Acclimation of photosynthesis and pigments to seasonally changing light conditions in the endemic Antarctic red macroalga Palmaria decipiens

The influence of seasonally fluctuating Antarctic daylengths on the photosynthetic apparatus of Palmaria decipiens was studied in culture experiments. Maximal photosynthetic activity (ETRmax) and maximal quantum yield (Fv/Fm), measured by in vivo chlorophyll fluorescence, and concentrations of pigments (phycobiliproteins and Chl a) were determined monthly. Fv/Fm remained constantly high between 0.62 and 0.67 during mid-autumn, winter and spring. ETRmax and pigment contents increased continuously in mid-autumn and winter and were highest in spring. A positive correlation between pigments and ETRmax was found. In summer, ETRmax, Fv/Fm and pigment levels decreased to their lowest values. P. decipiens acclimated by increasing phycobilisome (PBS) number and changing PBS structure, probably changing rod length and rod number. The data show that P. decipiens is efficiently adapted to the short period of favourable light conditions in the field. A photoperiodic control of pigment synthesis triggered by daylength is suggested.     

Pigment distribution in the Pacific region of the Southern Ocean (autumn 1995)

Phytoplankton distribution patterns are still largely unknown for the Pacific region of the Southern Ocean. Pigment distributions were determined by HPLC on 40-m samples collected from the mixed layer during the ANTXII/4 cruise in March–May 1995 aboard RV “Polarstern”. A transect was covered (90°W, from 51°S to 70°S), crossing the Subantarctic Front in the north, the Polar Front, and the Southern Polar Front in the south. Coinciding with high concentrations of silicate, diatoms dominated in the Antarctic waters south of the Polar Front. North of the Polar Front, silicate concentrations dropped to values less than 10 μM. In this area flagellates (Prymnesiophyceae and green algae) were the dominant phytoplankton group. Nutrient depletion of the surface waters near the Southern Polar Front indicated formerly enhanced productivity. These findings confirmed previous observations by the British Sterna expedition, which described locally elevated chlorophyll a biomass near the southern boundary of the Southern Polar Front. We propose a role for supply of bioavailable iron via the front, and emphasise the importance of frontal systems for phytoplankton productivity in the Southern Ocean. Received: 11 June 1997 / Accepted: 16 November 1997