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A reformulation of Sverdrup’s critical-depth calculation, using recent optical and physiological information, is developed and applied to data from the Southern Ocean. Comparisons between calculated critical depths (Zc) and mixed-layer depths (Zm) indicate that both the marginal ice zone and the open waters of the Antarctic Circumpolar Current provide favorable irradiance-mixing regimes for the initiation and early development of phytoplankton blooms in summer (i.e. Zc > Zm when phytoplankton biomass is low and the water clear; that when ice-edge blooms develop, Zc shoals to depths about equal to Zm implying that phytoplankton standing stocks in ice-edge blooms may be self-limiting as a result of reduced penetration of irradiance; and that the highest chlorophyll levels that can be sustained in summer in open waters not stabilized by meltwater are ∼ 1.0 µg liter‒1 in the Weddell and Scotia Seas and may be less in areas that experience stronger winds.

Polar regions are poorly understood components of global biogeochemical cycles. This paper discusses the influences of nitrate and ammonium concentrations on nitrate uptake (and hence new production), particularly with regard to data collected within marginal ice zones in the Arctic and Antarctic. Subsurface ammonium maxima in waters over 150 m are frequently encountered in the Arctic and occasionally in the Antarctic. Such maxima result from the heterotrophic remineralization of organic matter, and because stratified environments occur more frequently in the Arctic, significant concentrations of ammonium accumulate as a result of lower diffusive losses. Causal agents (bacteria vs. Zooplankton) may also be different in the Arctic from those in the Antarctic. Elevated ammonium concentrations significantly reduce nitrate uptake, and it is suggested that this nutrient interaction may play a significant role in controlling new production, particularly in open water regions. The new production of the Southern Ocean is estimated, and it is suggested that the production and flux from the surface layer is significant relative to the world's oceans.

During late spring, 1987, observations were made of nitrate and ammonium uptake in two regions of the Greenland Sea, the Arctic Front and the Polar Front. In the area of the Arctic Front, mixed layers were relatively deep (generally below 100m), and the 1% isolume averaged 35 m. Ambient nitrate concentrations were always greater than 6 μM, whereas ammonium levels were always less than 0.6 μM. Surface nitrate and ammonium specific uptake rates averages 4.4 and 2.3×10−3 h−1, respectively. The Polar Front generally coincided spatially with the location of the ice edge, and vertical mixed layers were shallow (pycnocline depth ranged from 8–14 m), and the 1 % isolume averaged 37 m. Nitrate concentrations were somewhat lower than in the Arctic Front, but remained above 3 μM at all times. Ammonium levels reached 1.2 μM. Nitrate and ammonium specific uptake rates at the surface averaged 4.8×10−3 and 10×10−3 h−1, respectively. Integrated water column f-ratios for the Arctic and Polar Front regions averaged 0.63 and 0.31, and the ammonium relative preference indices at all depths within each study area were always greater than 8, indicating that ammonium remained the preferred nitrogen source for phytoplankton. New production in the two regions was approximately equal, but the Polar Front had a substantially greater amount of regenerated production, and hence total production as well. Irradiance (and not nutrient concentration) seems to be the most important environmental factor in controlling nitrogen uptake. The spatial variability observed within the Greenland Sea suggest that inclusion of this region in global carbon models will require increased spatial resolution of both the models and the data included.

We investigated diet composition and diversity and the diet overlap between species within the seabird community of the Scotia-Weddell Confluence region, Antarctica, during spring, autumn and winter. Seasonal changes in diet characteristics and overlap among species were viewed in the context of changes in prey availability as a function of vertical distribution in the water column. The latter was in turn affected by where phytoplankton and associated grazers were located. When primary productivity was confined to the pack ice during winter, thus bringing grazers and their predators closer to the surface, micronekton were much more vulnerable to avian predators and the latter were able to select prey on the basis of energetic value. As a consequence the upper food web was much less diffuse than during the other seasons when prey occurred throughout the upper portion of the water column and predators had to be opportunistic. The mobility of micronekton and nekton bring some structure to the upper portion of marine food webs, thereby introducing seasonal variability of trophic relationships within Antarctic food webs.

Sea ice in the Southern Ocean is a major controlling factor on phytoplankton productivity and growth, but the relationship is modified by regional differences in atmospheric and oceanographic conditions. We used the phytoplankton biomass (binned at 7-day intervals), PAR and cloud cover data from SeaWiFS, ice concentrations data from SSM/I and AMSR-E, and sea-surface temperature data from AVHRR, in combination with a vertically integrated model to estimate primary productivity throughout the Southern Ocean (south of 60"s). We also selected six areas within the Southern Ocean and analyzed the variability of the primary productivity and trends through time, as well as the relationship of sea ice to productivity. We found substantial interannual variability in productivity from 1997 - 2005 in all regions of the Southern Ocean, and this variability appeared to be driven in large part by ice dynamics. The most productive regions of Antarctic waters were the continental shelves, which showed the earliest growth, the maximum biomass, and the greatest areal specific productivity. In contrast, no large, sustained blooms occurred in waters of greater depth (> 1,000 m). We suggest that this is due to the slightly greater mixed layer depths found in waters off the continental shelf, and that the interactive effects of iron and irradiance (that is, increased iron requirements in low irradiance environments) result in the limitation of phytoplankton biomass over large regions of the Southern Ocean.

It is well established that an increase in iron supply causes an increase in total oceanic primary production in many regions, but the physiological mechanism driving the observed increases has not been clearly identified. The Southern Ocean iron enrichment experiment, an iron fertilization experiment in the waters closest to Antarctica, resulted in a 9-fold increase in chlorophyll (Chl) concentration and a 5-fold increase in integrated primary production. Upon iron addition, the maximum quantum yield of photosynthesis (φ m ) rapidly doubled, from 0.011 to 0.025 mol C·mol quanta −1 . Paradoxically, this increase in light-limited productivity was not accompanied by a significant increase in light-saturated productivity ( P max b ). P max b , maximum Chl normalized productivity, was 1.34 mg C·mg Chl −1 ·h −1 outside and 1.49 mg C·mg Chl −1 ·h −1 inside the iron-enriched patch. The importance of φ m as compared with P max b in controlling the biological response to iron addition has vast implications for understanding the ecological response to iron. We show that an iron-driven increase in φ m is the proximate physiological mechanism affected by iron addition and can account for most of the increases in primary production. The relative importance of φ m over P max b in this iron-fertilized bloom highlights the limitations of often-used primary productivity algorithms that are driven by estimates of P max b but largely ignore variability in φ m and light-limited productivity. To use primary productivity models that include variability in iron supply in prediction or forecasting, the variability of light-limited productivity must be resolved.

The haptophyte Phaeocystis antarctica G. Karst. is a dominant phytoplankton species in the Ross Sea, Antarctica, and exists as solitary cells and mucilaginous colonies that differ by several orders of magnitude in size. Recent studies with Phaeocystis globosa suggest that colony formation and enlargement are defense mechanisms against small grazers. To test if a similar grazer-induced morphological response exists in P. antarctica, we conducted incubation experiments during the austral summer using natural P. antarctica and zooplankton assemblages. Dialysis bags that allowed exchange of dissolved chemicals were used to separate P. antarctica and zooplankton during incubations. Geometric mean colony size decreased by 35% in the control, but increased by 30% in the presence of grazers (even without physical contact) over the 15 d incubation. The estimated colonial-to-solitary cell carbon ratio was significantly higher in the grazing treatment. These results suggest that P. antarctica colonies would grow larger in the presence of indigenous zooplankton and skew the carbon partitioning significantly toward the colonial phase. While these observations show that the colony size of P. antarctica was affected by a chemical signal related to grazers, the detailed nature and ecological significance of this signal remain unknown.

Observations of microbial distributions and metabolic activities were made during early austral autumn, 1986, in the marginal ice zone of the Weddell Sea east of the Antarctic peninsula. Bacterioplankton had distributions and activities similar to those of phytoplankton within the marginal ice zone. Bacterial production averaged 19 ± 10 mg C m−2 d−1 in regions partially covered with ice, and in open waters mean rates were 119 ± 69 mg C m−2 d−1. Pronounced productivity maxima were present seaward of the ice edge. Although biomass and productivity maxima for algae and bacteria were largely superimposed spatially, there was no obvious relationship between these biological features and the density field. Rates of primary production in early autumn, adjacent to a nearly stationary ice edge, were much lower than rates observed in spring when the ice edge was retreating. However, bacterial production rates were comparable during both seasons. During autumn secondary production by bacterioplankton in the upper 100 m often equaled or exceeded primary productivity; integrated bacterial production in open water areas averaged 76% of primary production. Mean bacterial growth rates in ice-covered and open water regions were 0.33 and 0.58 d−1, respectively, or from 5 to 11 times those of phytoplankton. Rates and ratios of macromolecular synthesis by bacterioplankton were comparable with previous temperature and polar studies. Several lines of evidence indicate that this ice edge bloom was dominated by heterotrophic processes.

Rates of primary production, N03- uptake, and NH,l uptake by phytoplankton were measured in the marginal ice zone of the Weddell Sea in austral spring 1983 and autumn 1986. In spring Chl concentrations averaged 3 pg liter-‘, primary productivity 490 mg C m-2 d-l, and surface phytoplankton growth rates 0.30 doublings d-l. In autumn these rates were all much lower, averaging 0.14 pg liter- l, 126 mg C m-2 d-l, and 0.14 doublings d-l. During both seasons NH,+ was consistently the preferred source of N, but because of the much greater availability of N03- in the euphotic zone (21-28 PM NO,- vs. -0.4 yM NH,+ during both seasons), N03- uptake rates generally equaled or exceeded those of NH, +. Vertically integrated&ratios (the ratio of NO3- uptake to the sum of NO,- plus NH,+ uptake) averaged 0.52 (range from 0.35 to 0.70) in spring and 0.72 (range from 0.60 to 0.84) in autumn. In 1983 we observed sharply elevated relative preference indices for NH,+ whenever the ambient NH,+ concentration was ~0.3 PM, which we interpret as evidence that Antarctic phytoplankton growing at NH.,+ concentrations CO.3 PM have the ability to increase their rate of NH,+ uptake rapidly in response to increased availability. We estimate the new production of the marginal ice zone to be at least 49 g C m-2 yr-1 for the NovemberMarch period. This value supports previous findings that the ice edge is the main site of production in the Southern Ocean and that it is likely to dominate the annual cycle of organic matter flux from the euphotic zone.

T H E EXPLORERS and naturalists who first visited Antarctica were struck by the large numbers of whales, seals and seabirds in such a harsh environment (summarized in Everson, 1977). Scientific expeditions in the twentieth century confirmed these observations, and also established that an even greater abundance of many different trophic levels occurred at the ice edge. Hart (1942) noted that phytoplankton were more abundant near the ice edge mad suggested that there was a distinct ice-edge flora, and Marr (1962) noted that krill (Euphausia superba) were concentrated at the interface between pack ice and open water. Routh (1949) observed that seabirds were very abundant at the ice edge and suggested that the enhancement was due to the high plankton concentrations found in the area. All of these observations suggested that the ice edge played a potentially important role in the ecology of the southem ocean. In the early 1980s it became evident that the ice edge in Antarctic waters could have a major impact on the region's trophic dynamics as well as on the global cycles of carbon dioxide and other biogenic elements (e.g., Kukla and Gavin, 1981 ). Sea ice covers about 7% of the total ocean surface and hence significantly reduces the amount of solar radiation absorbed at the surface, restricts the transfer of heat between the ocean and atmosphere, and influences global oceanic and atmospheric circulation (Zwalley et al., 1983a). Because the seasonal cycle of pack ice (ice which had formed the previous winter from surface cooling of seawater) in the Antarctic uncovers ca. 16 x 106 km 2 each year, the effects of ice on the entire southern ocean

[1] Satellite imagery shows that there was substantial variability in the sea ice extent in the Ross Sea during 2001–2003. Much of this variability is thought to be due to several large icebergs that moved through the area during that period. The effects of these changes in sea ice on circulation and water mass distributions are investigated with a numerical general circulation model. It would be difficult to simulate the highly variable sea ice from 2001 to 2003 with a dynamic sea ice model since much of the variability was due to the floating icebergs. Here, sea ice concentration is specified from satellite observations. To examine the effects of changes in sea ice due to iceberg C-19, simulations were performed using either climatological ice concentrations or the observed ice for that period. The heat balance around the Ross Sea Polynya (RSP) shows that the dominant term in the surface heat budget is the net exchange with the atmosphere, but advection of oceanic warm water is also important. The area average annual basal melt rate beneath the Ross Ice Shelf is reduced by 12% in the observed sea ice simulation. The observed sea ice simulation also creates more High-Salinity Shelf Water. Another simulation was performed with observed sea ice and a fixed iceberg representing B-15A. There is reduced advection of warm surface water during summer from the RSP into McMurdo Sound due to B-15A, but a much stronger reduction is due to the late opening of the RSP in early 2003 because of C-19.

[1] The continental shelf of the Ross Sea, Antarctica, is a unique region within the Southern Ocean. Phytoplankton growth is believed to be seasonally limited, first in austral spring by irradiance, and then in summer by biologically available iron. It also is historically known to have taxonomically distinct regimes: the south-central portion is dominated by Phaeocystis antarctica and to the west diatoms are abundant. We measured photochemical yield to interpret the health of the phytoplankton assemblage from 2001–2004 and interfaced these measurements with satellite remote sensing of pigments. The bloom of 2001–2002 was similar in both temporal and spatial distributions to the climatological mean of the Ross Sea, with a peak in biomass being observed in mid-December within the Ross Sea polynyas; Fv/Fm values averaged 0.43. We found high (0.50–0.65) Fv/Fm for most of the seasonal phytoplankton bloom for 2002–2003, suggesting that it was not seasonally iron limited. An unusual, large bloom occurred during 2003–2004, with an initial bloom of P. antarctica during austral spring followed by an extensive diatom bloom in summer that may have been enhanced by an intrusion of modified circumpolar deep water. On the basis of an analysis of the historical SeaWiFS records, accumulation of phytoplankton biomass in February may occur approximately every 2–4 years, potentially being a significant source of carbon on the continental shelf.

The continental shelf of the Ross Sea is one of the Antarctic's most intensively studied regions. We review the available data on the region's physical characteristics (currents and ice concentrations) and their spatial variations, as well as components of the neritic food web, including lower and middle levels (phytoplankton, zooplankton, krill, fishes), the upper trophic levels (seals, penguins, pelagic birds, whales) and benthic fauna. A hypothetical food web is presented. Biotic interactions, such as the role of Euphausia crystallorophias and Pleuragramma antarcticum as grazers of lower levels and food for higher trophic levels, are suggested as being critical. The neritic food web contrasts dramatically with others in the Antarctic that appear to be structured around the keystone species Euphausia superba . Similarly, we suggest that benthic–pelagic coupling is stronger in the Ross Sea than in most other Antarctic regions. We also highlight many of the unknowns within the food web, and discuss the impacts of a changing Ross Sea habitat on the ecosystem.

Phytoplankton assemblages in polynyas are strongly impacted by the unique environment of those systems, and their growth and accumulation is always greater within a polynya than under heavy ice. The extent of this enhancement is dependent on the physical conditions of the polynya—the duration of the polynya's existence, the distribution of ice and snow, and the physical circulation within it. We review the polynyas in both Arctic and Antarctic waters that have been intensively studied and compare them with respect to biomass, daily productivity, chemical and physical constraints, annual productivity, export, and effects on food webs and the local biogeochemical cycles. We conclude that the most productive polynyas (the North Water polynya and the Ross Sea polynya) have remarkably similar short-term productivity, and the annual productivity and seasonal timing of both are also similar. However, the two have strong dissimilarities in modes of control and export. The ecological consequences of enhanced production within a polynya are also investigated, and appear to vary among polynyas. We suggest that the differences among polynyas within polar systems reflect the differences in large-scale physical forcing that exist across the Arctic and Antarctic, and that generalizations among polynyas need to encompass this variability.

Historical Approaches to the Assessment of Primary Productivity Modern Approaches to the Assessment of Primary Productivity Nutrient Limitation of Primary Productivity Keywords: algae; benthic-associated primary productivity; marine environment; phytoplankton; primary productivity

The role of iron in regulating the flux of carbon through the surface layer of the ocean has become increasingly apparent during the past 15 years. Before that time, the analytical challenges of measuring trace (parts per trillion) iron concentrations from iron ships using gear suspended on an iron wire precluded oceanographers from making accurate measurements. Laboratory experiments were invariably conducted with samples that were seriously contaminated with elevated iron concentrations. We now recognize, through greatly improved methodologies, that iron is a key regulator of phytoplankton primary production throughout the ocean. Small changes in iron concentration may produce large variations in the export of particulate organic carbon from the ocean's sunlit surface layer into deep-sea sediments. These variations in carbon export may occur over glacial/interglacial cycles at a scale sufficient to influence the flux of carbon dioxide from the atmosphere to the ocean. Such processes have been hypothesized to be an important driver of the changes in atmospheric carbon dioxide concentration that have been recorded in ice cores over the past 400,000 years.

The United States Southern Ocean Joint Global Ocean Flux Study (JGOFS), also known as AESOPS (Antarctic Environment and Southern Ocean Process Study), focused on two distinct regions. The first was the Ross-Sea continental shelf, where a series of six cruises collected a variety of data from October 1996 through February 1998. The second area was the southwest Pacific sector of the Southern Ocean, spanning the Antarctic Circumpolar Current (ACC) at ∼170°W. Data were collected within this region during five cruises from September 1996 through March 1998, as well as during selected transits between New Zealand and the Ross Sea. The first results of these cruses are described in this issue. The Ross-Sea investigation extensively sampled the area along 76°30′S to elucidate the temporal patterns and processes that contribute to making this one of the Antarctic's most productive seas. Hydrographic distributions confirm that stratification is initiated early in October within the polynya, generating an environment that is favorable for phytoplankton growth. Significant spatial variations in mixed-layer depths, the timing of the onset of stratification, and the strength of the stratification existed throughout the growing season. Nutrient concentrations reflected phytoplankton uptake, and reached their seasonal minimal in early February. Chlorophyll concentrations were maximal in early January, whereas productivity was maximal in late November, which reflects the temporal uncoupling between growth and biomass accumulation in the region. Independent estimates of biogenic export suggest that majority of the flux occurred in late summer and was strongly uncoupled from phytoplankton growth. The ACC region exhibited seasonal changes that in some cases were greater than those observed in the Ross Sea. Sea ice covered much of the region south of the Polar Front in winter, and retreated rapidly in late spring and early summer. Mixed layers throughout the region shoaled in summer due to surface heating, while the addition of freshwater from melting sea ice enhanced stratification in the Seasonal Ice Zone, creating conditions favorable for phytoplankton growth. For example, silicic acid concentrations decreased from initial values as high as 65 to less than 2 μM within approximately 100 km (from 65.7 to 64.8°S). Fluorescence values, however, showed less than a two-fold variation over the same distance. The vertical flux of carbon in the Polar Front area is substantial, and marked variations in the composition of exported material exited over the region. The results provide a means whereby the controls of phytoplankton growth and organic matter flux and remineralization can be analyzed in great detail. Additional results of the AESOPS project are discussed.

THE Greenland Sea is particularly important to the world ocean circulation, and potentially to carbon dioxide exchange between the ocean and atmosphere, because it is an area of surface convergence and deep-water formation1–3. Previous investigations indicate that biological productivity is low4,5 in this area, especially in waters remote from the ice edge. During April and early May 1989, however, we observed the development of a massive bloom of the colonial prymnesiophyte Phaeocystis pouchetii across much of the Greenland Sea. From measurements of the rate of removal of nitrate from surface waters, we calculate that the average regional new production was about 40 g C m−2 during the 35-day period of our observations. This rate of new production is approximately equal to that observed in other hyperproductive polar regions, such as the Bering Sea and the Bransfield Strait. Because Phaeocystis blooms seem to be frequent and widespread in polar oceans4,6, our results suggest that the Greenland Sea may be a larger sink of atmospheric carbon dioxide than has been previously thought.

SeaWiFS estimates of surface chlorophyll concentrations are reported for the region of the U.S. JGOFS study in the Southern Ocean (∼ 170°W, 60°S). Elevated chlorophyll was observed at the Southern Ocean fronts, near the edge of the seasonal ice sheet, and above the Pacific-Antarctic Ridge. The elevated chlorophyll levels associated with the Pacific-Antarctic Ridge are surprising since even the crest of the ridge is at depths > 2000 m. This elevated phytoplankton biomass is likely the result of mesoscale physical-biological interactions where the Antarctic Circumpolar Current (ACC) encounters the ridge. Four cruises surveyed this region between October 1997 and March 1998, as part of the U.S. JGOFS. Satellite-derived chlorophyll concentrations were compared with in situ extracted chlorophyll measurements from these cruises. There was good agreement (r² of 0.72, from a linear regression of shipboard vs. satellite chlorophyll), although SeaWiFS underestimated chlorophyll concentrations relative to the ship data.

During the multidisciplinary ‘NEW92’ cruise of the United States Coast Guard Cutter (USCGC) Polar Sea to the recurrent Northeast Water (NEW) Polynya (77–81°N, 6–17°W; July–August 1992), total dissolved inorganic carbon and total alkalinity in the water column were measured with high precision to determine the quantitative impact of biological processes on the regional air-sea flux of carbon. Biological processes depleted the total inorganic carbon of summer surface waters by up to 2 mol C m−2 or about 3%. On a regional basis this depletion correlated with depth-integrated values of chlorophyll a, particulate organic carbon, and the inorganic nitrogen deficit. Replacement of this carbon through exchange with the atmosphere was stalled owing to the low wind speeds during the month of the cruise, although model calculations indicate that the depletion could be replenished by a few weeks of strong winds before ice forms in the autumn. These measurements and observations allowed formulation of a new hypothesis whereby seasonally ice-covered regions like the NEW Polynya promote a unique biologically and physically mediated “rectification” of the typical (ice free, low latitude) seasonal cycle of air-sea CO2 flux. The resulting carbon sink is consistent with other productivity estimates and represents an export of biologically cycled carbon either to local sediments or offshore. If this scenario is representative of seasonally ice-covered Arctic shelves, then the rectification process could provide a small, negative feedback to excess atmospheric CO2.

We are among the scientists objecting to the eco-certification of Ross Sea Antarctic toothfish (Dissostichus mawsoni), as described by E. Stokstad in his News Focus story "Behind the eco-label, a debate over Antarctic toothfish" (24 September, p. 1596). The public perceives a certification by the Marine Stewardship Council (MSC) to mean an environmentally friendly fishery, not one characterized by the "dearth of key data" as indicated in the article.

Conducting Antarctic research is challenging. Not only is the environment difficult, given low temperatures, high winds, and ice cover, but also the availability of sampling platforms is limited. The U.S. National Science Foundation has only two vessels that can routinely operate in ice-covered waters, the research vessel/icebreaker N.B. Palmer and the Antarctic research support vessel L.M. Gould. The restricted availability of vessels makes research operations difficult to schedule, especially during austral spring and summer. Traditional sampling methods (those confined to ships and moorings) have provided insights into distributions and processes in the Southern Ocean, but new research questions require new sampling means.

The patterns of particulate matter distribution (chlorophyll, phytoplankton pigmentation, biogenic silica, particulate carbon, and nitrogen, bacterial and phytoplankton abundance, and optical particle concentration) in the Northeast Water Polynya off the northern coast of Greenland are described, and their relationships to environmental conditions (hydrography, nutrients, and ice cover) are analyzed. Particulate matter concentrations were substantial in the euphotic zone, with the largest fraction consisting of phytoplankton material. Most of this phytoplanktonic material was diatomaceous during the study period (late summer). Surface chlorophyll concentrations were inversely correlated with nitrate concentrations, which were often undetectable in the surface layer, suggesting that phytoplankton standing stocks were controlled by nitrogen availability. The particulate carbon to particulate nitrogen ratio was unusual, in that elevated (approximately nine by weight) mean values were observed near the surface and reduced values (near 6 by weight) occurred at the base of the euphotic zone. These elevated C/N ratios apparently resulted from adaptation to low nutrient conditions. Other elemental ratios (particulate carbon to chlorophyll, biogenic silica to particulate carbon) appear similar to other Arctic systems. Bacterial numbers were greatest in waters with intermediate ice concentrations but in all cases contributed a small fraction (approximately 3%) of organic carbon in the particulate pool. Though variable in spatial distribution, the polynya was a source of substantial quantities of particles; resolving the fate of this material requires further analysis of these data in conjunction with rate measurements and coupled physical-biological models.

The primary productivity and rates of new production were assessed in the Northeast Water Polynya during July–August 1992 by using 14C and 15N tracer techniques. The average rate of primary production for the entire region, including an approximate correction for the effects of ice cover, was 0.21 g C m−2 d−1, with the maximum observed rate being slightly over 1.1 g C m−2 d−1. Rates of new production (uncorrected for effects of ice cover) averaged 3.08 mmol N m−2 d−1. Surface f ratios (the ratio between nitrate uptake and the total inorganic nitrate uptake) averaged 0.63, and depth-integrated f ratios equaled 0.69. It appeared that the water column in general was bilayered, in that f ratios were reduced near the surface in low (less than 2 μM) nitrate waters and increased deeper in the euphotic zone. Ammonium concentrations and uptake were consistently low throughout the region. Growth rates were modest, with surface growth rates averaging 0.33 per day. A simple relationship among the attenuation of solar radiation by sea ice (and its associated snow cover and microbial community), the percentage of open water, ice thickness, water column attenuation, and phytoplankton photosynthesis is described. The relationship is nonlinear, but, in general, nutrient-replete water column photosynthesis are largely a function of the percentage of open water rather than ice thickness. Both irradiance and nutrient fields exert strong influences on phytoplankton productivity and ultimately result in a mosaic of biomass within the polynya.

The biomass, taxonomy, size, and photosynthesis-irradiance responses of phytoplankton in the high Arctic (79°N) were investigated in March–April 1987 in the Fram Strait marginal ice zone. Chlorophyll a concentrations in open water during this period were extremely low, with surface values averaging 0.022 μg L−1. Vertical mixed layers were deep, ranging from 40 to 200 m, and surface incident irradiance was low (average of 44 μumol photons m−2 s−1 at local noon). The biomass of the microplankton was dominated by three groups: autotrophic flagellates (less than 10 μm), heterotrophic flagellates (less than 10 μm), and heterotrophic dinoflagellates (from 10 to 35 μm ). Detrital carbon contributed the largest fraction (on average 95%) to the particulate carbon pool. Photosynthesis-irradiance responses indicated that the resident phytoplankton populations were adapted to low irradiance levels, as would be expected after the long periods of darkness encountered during the Arctic winter. Estimates of primary productivity during the March–April period indicate that very little net carbon fixation was occurring (averaging 1.84 mg C m−2 d−1 with a maximum of 3.3 mg C m−2 d−1). Some small increases in surface chlorophyll concentrations (relative to those at the base of the mixed layer) were noted at a few stations, and hence the onset of spring growth may have just begun. However, in view of the biomass and productivity observed during this period, the accumulation of substantial phytoplankton standing stocks, which are often observed at the ice edge, cannot occur until much later in the growing season at this northerly location.

Large areas of elevated phytoplanton pigment concentration were observed using CZCS satellite data in the Weddell Sea marginal ice zone and adjacent regions during the austral summer-autumn transition. Pigment values higher in the south than in the north indicate that, besides the irradiance, there are other factors controlling spatial distributions of the phytoplankton pigment concentrations at this time. Low pigment concentrations (as in observations near the Greenwich meridian) may result from turbulent oceanic and atmospheric forcings during the early stage of sea ice development. The data indicate that the phytoplankton blooms are not only characteristic of the spring/summer period, but also extend into the austral autumn and may contribute significantly to regional productivity.

The effects of environmental variables, particularly irradiance, on the sinking rates of phytoplankton were investigated using cultures of Chaetoceros gracilis Schutt and C. flexuosum Mangin in laboratory experiments; these data were compared with results from assemblages in the open ocean and marginal ice zone of the Greenland Sea. In culture experiments both the irradiance under which the diatom was grown and culture growth rate were positively correlated with sinking rates. Sinking rates (ψ) in the Greenland Sea were smallest when determined from chlorophyll (mean ψchl= 0.14 m · d−1) and biogenic silica (ψsi= 0.14 m · d−1) and greatest when determined from particulate carbon (ψc= 0.55 m · d−1) and nitrogen (ψN= 0.64 m · d−1). Field measurements indicated that variations in sinking may be associated with changes in irradiance and nitrate concentrations. Because these factors do not directly affect water density, they must be inducing physiological changes in the cell which affect buoyancy. Although a direct response to a single environmental variable was not always evident, sinking rates were positively correlated with growth rates in the marginal ice zone, further indicating a connection to physiological processes. Estimats of carbon flux at stations with vertically mixed euphotic zones indicated that approximately 30% of the daily primary production sank from the euphotic zone in the form of small particulates. Calculated carbon flux tended to increase with primary productivity.

The release of dissolved organic carbon (DOC) by eelgrass (Zostera marina) and its epiphytic community was examined in a shallow estuarine system near Beaufort, North Carolina. Excretion by eelgrass alone, by epiphytes alone, and by heavily epiphytized eelgrass was determined by radiocarbon techniques using carbon 14. The percent of total photosynthate excreted was small in all cases: 0.9 percent for heavily colonized plants, 1.5 percent for Zostera alone, and 2.0 percent for the epiphytic community. The amount released by the eelgrass and epiphytes was proportional to the total amount of carbon fixed. Excretion rates in the dark were much lower than in the light. The rate of excretion increased after the plant underwent desiccation. The annual primary production and DOC excretion by phytoplankton, cordgrass (Spartina alterniflora), and the eelgrass community was estimated: 47 percent of the total annual primary production and 14 percent of the total excreted material was contributed by eelgrass and its epiphytes. Thus the eelgrass and epiphytes can play an important role in carbon cycling in estuarine systems.

Measurements of chlorophyll, particulate carbon, and biogenic silica concentrations near a receding ice edge off the coast of Victoria Land, Antarctica, indicated the presence of a dense phytoplankton bloom. The bloom extended 250 kilometers from the ice edge and was restricted to waters where the melting of ice had resulted in reduced salinity. The region involved was one of enhanced vertical stability, which may have favored phytoplankton growth, accumulation, or both. Epontic algae released from melting ice may have served as an inoculum for the bloom. Ratios of organic carbon to chlorophyll and biogenic silica to carbon were unusually high, resulting in high biogenic silica concentrations despite only moderately high chlorophyll levels.

Previous estimates of the marginal ice zone's quantitative contribution to biogeochemical cycles and annual productivity in the Southern Ocean may be conservative because of assumptions that phytoplankton blooms are associated only with actively retreating ice edges. Observations during March 1986, near an almost stationary ice edge in the northwestern Weddell Sea, revealed very low geostrophic currents, no appreciable horizontal gradients in temperature or salinity and no significant net melting or freezing in the ice-edge region. Vertical stratification within the upper 50 m was evident throughout the study area, and resulted primarily from prior melting of pack ice. In contrast with previous observations in marginal ice zones, the distribution of phytoplankton biomass showed little correlation with the meltwater field; here, significant horizontal biomass gradients occurred in an area where vertical stability was almost uniform laterally and both elevated biomass and diminished nutrient levels extended well below the pycnocline. Absolute levels of chlorophyll were modest (generally 10, 1.5 and 3.5 μmol l−1, respectively) were similar to those found previously during a spring phytoplankton bloom at the Weddell-Scotia Sea ice edge. The mean mole ratio of biogenic silica to organic carbon within the particle assemblage was 0.44, which is very high for surface seawater and about three times higher than that typically found in pure diatom cultures. Therefore, despite relatively low chlorophyll levels, the ice edge remained a localized maximum in biogenic particulate matter at least through March and this material was unusually rich in silica. Combined, these physical and biological data show that elevated phytoplankton biomass in the ice-edge zone can persist well past the time when net melting stops and the ice edge becomes stationary.