Open-water Blooms Research Articles

Spring phytoplankton bloom phenology during recent climate warming on the Bering Sea shelf

High-latitude ecosystems commonly experience large phytoplankton blooms in spring, which provide basal resources for a range of grazers including zooplankton, benthic consumers and fishes. Variation of the timing and intensity of the spring phytoplankton bloom influences the degree of spatial and temporal overlap with consuming organisms. In the Bering Sea, blooms occur in association with ice retreat or as pelagic open-water blooms. In the last few years, the Bering Sea shelf experienced unprecedented and widespread warming. Understanding how those climatic changes subsequently influenced phytoplankton bloom dynamics is critical for evaluating Bering Sea food web responses. We estimate spring bloom timing and type (ice-associated, open-water) across the Bering Sea shelf using a combination of data from the long-running oceanographic moorings on the eastern shelf (M2, M4, M5, M8) and satellite ocean color data from 1998 to 2022. We assess 1) if the Bering Sea shelf experienced noticeable changes in spring bloom timing or type in the last two decades, 2) whether bloom phenology was accentuated by the recent warm period (2018–2019), and 3) what influences do winds and sea surface temperatures have on spring bloom timing and where are these variables influential? Our spatial analyses reinforce the conclusion that ice retreat is the dominant forcing factor of bloom timing for the Bering Sea shelf with some influence of wind for open-water blooms. Overall, bloom timing has not shifted seasonally with climate warming in the last two decades for most of the Bering Sea shelf, except for nearshore areas and mainly in the northern Bering Sea. In warm years when ice retreats early prior to the last week of March, blooms form in open waters and bloom timing on the middle and outer shelf is delayed when wind mixing is prevalent in ice-free springs. In recent years, open-water blooms were more widespread than previously experienced, and even occurred in the northern Bering Sea during 2018–2019. A progression to more open water blooms in a future warmer climate will influence the availability of basal resources for pelagic and benthic consumers.

Monitoring Sargassum Inundation on Beaches and Nearshore Waters Using PlanetScope/Dove Observations

<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Sargassum</i> beaching events have been reported in recent years around the Caribbean Sea and FL, USA, causing numerous environmental and economic problems. Satellite remote sensing has been widely used to monitor <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Sargassum</i> blooms in open waters, yet due to either coarse spatial resolution or low-revisit frequency, it is difficult to provide timely information on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Sargassum</i> inundation from traditional satellite instruments. In this study, we demonstrate the capacity of 3-m resolution daily Dove imagery in monitoring <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Sargassum</i> beaching events on Miami beach (FL, USA) and Cancun beach (Mexico). A U-net deep learning (DL) computer model is developed to extract <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Sargassum</i> features from Dove imagery over beaches and nearshore waters. Application of the model to Dove image sequences between May and August 2019 shows two major inundation events on both Miami beach and Cancun beach, consistent with local reports. With the availability of 3-m resolution PlanetScope/Dove and PlanetScope/SuperDove data around the globe, the findings suggest that it is possible to monitor dynamic inundation events of not only <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Sargassum</i> but also other macroalgae in many other regions.

Distinctive spring phytoplankton bloom in the Bering Strait in 2018: A year of historically minimum sea ice extent

Under climate change, the Bering–Chukchi continental shelves have experienced drastic sea ice loss, breaking the minimum sea ice extent record in 2018. In this paper, we report a distinctive spring phytoplankton bloom in the Bering Strait based on year-round mooring observations during 2016–2018. A near-bottom fluorescence sensor mounted on a mooring and satellite observations indicate that, despite the early sea ice retreat in spring of 2018 (mid-April), the spring phytoplankton bloom was not observed for more than 20 days after sea ice retreat. In contrast, in 2017 when sea ice retreat did not occur as early as it did in 2018 (early May), the bloom and sea ice retreat occurred in tighter synchrony. Mooring (2016–2018) and satellite observations (2003–2011, 2013–2018) suggest that an open-water bloom rather than an ice-edge bloom occurred in the Bering Strait in the spring of 2018. Early sea ice retreat apparently provided unfavorable conditions for an ice-edge bloom due to weak sunlight and winter-like weather. Mid-winter sea ice loss was promoted by unusual southerly winds, bringing in warm water from the south. For the Bering Strait, the end of April is a reference date for sea ice retreat that distinguishes between an open-water bloom and an ice-edge bloom, which is approximately one month later than that in the southeastern Bering Sea.

Culturable Bacterial Flora Associated with the Dinoflagellate Green Noctiluca miliaris During Active and Declining Bloom Phases in the Northern Arabian Sea

A massive algal bloom of the dinoflagellate Noctiluca miliaris (green) was located in the Northern Arabian Sea by IRS-P4-2 (OCM-II) for microbiological studies, during two consecutive cruises of February-March 2009. Culturable bacterial load during bloom were ≈ 2-3-fold higher in comparison to non-bloom waters and ranged from 3.20 × 10(5) to 6.84 × 10(5) cfu ml(-1). An analysis of the dominant heterotrophs associated with Noctiluca bloom resulted in phylogenetic and a detailed metabolic characterization of 70 bacterial isolates from an overlapping active and declining bloom phase location near north-central Arabian Sea. The active phase flora was dominated by Gram-positive forms (70.59 %), a majority of which belonged to Bacillus (35.29 %) of Firmicutes. As the bloom declined, Gram-negative forms (61.11 %) emerged dominant, and these belonged to a diverse γ-proteobacterial population consisting of Shewanella (16.67 %) and equal fractions of a Cobetia-Pseudomonas-Psychrobacter-Halomonas population (36.11 %). A Unifrac-based principal coordinate analysis of partial 16S rDNA sequences showed significant differences among the active and declining phase flora and also with reported endocytic flora of Noctiluca (red). A nonparametric multidimensional scaling (NMDS) of antibiogram helped differentiation among closely related strains. The organic matter synthesized by N. miliaris appears to be quickly utilized and remineralized as seen from the high efficiency of isolates to metabolize various complex and simple C/N substrates such as carbohydrates, proteins/amino acids, lipids, sulfide production from organic matter, and solubilize phosphates. The ability of a large fraction of these strains (50-41.67 %) to further aerobically denitrify indicates their potential for nitrogen removal from these high-organic microniches of the Noctiluca bloom in the Arabian Sea, also known for high denitrification activity. The results indicate that culturable euphotic bacterial associates of Noctiluca are likely to play a critical role in the biogeochemical ramifications of these unique seasonally emerging tropical open-water blooms of the Northern Arabian Sea.

Sea ice impacts on spring bloom dynamics and net primary production in the Eastern Bering Sea

In the Eastern Bering Sea, changes in sea ice have been implicated in recent major upper‐trophic level shifts. However, the underlying relationships between sea ice and primary producers have not been well tested. Here, we combine data from multiple satellite platforms, reanalysis model results and biophysical moorings to explore the dynamics of spring and summer primary production in relation to sea ice conditions. In the northern Bering Sea, sea ice consistently retreated in late spring, leading to ice‐edge phytoplankton blooms in cold (0–1 °C) waters. However, in the southeastern Bering Sea, sea ice retreat was far more irregular. Although this did not significantly alter bloom timing, late retreat led to blooms at the ice‐edge while early retreat led to blooms in open waters that were warmer (≤5.4 °C) and &gt;70% more productive. Early sea ice retreat also led to higher productivity in summer, likely due to weaker thermal stratification. Overall, annual net primary production during warm years of early sea ice retreat was enhanced by 40–50% compared to years with late sea ice retreat in the southeastern Bering Sea. These findings suggest the potential for future sea ice loss to enhance overall carrying capacity of the southeastern Bering Sea ecosystem. Consistently warm blooms in the future may also channel more energy flow toward the pelagic, rather than benthic, environment. To date, however, neither sea ice extent nor the timing of its retreat have undergone long‐term changes in the Eastern Bering Sea.

Early spawning of Antarctic krill in the Scotia Sea is fuelled by “superfluous” feeding on non-ice associated phytoplankton blooms

The spawning success of Antarctic krill ( Euphausia superba) is generally assumed to depend on substantial winter sea ice extent, as ice biota can serve as a food source during winter/spring and the seasonal ice melt conditions the upper water column for extensive phytoplankton blooms. However, direct observations during spring are rare. Here we studied krill body condition and maturity stage in relation to feeding (i.e. stomach fullness, diet, absorption of individual fatty acids and defecation rate) across the Scotia Sea in November 2006. The phytoplankton concentrations were low at the marginal ice zone (MIZ) in the southern Scotia Sea (Stn. 1, 2, and 3), high in open waters of the Southern Antarctic Circumpolar Current Front (SACCF) in the central Scotia Sea (Stn. 5), and moderate further north (Stn. 6 and 7). Krill had low lipid reserves (∼6.5% of dry mass, DM), low mass:length ratios (∼1.7 mg DM mm −1), and small digestive glands (∼7% of total DM) near the ice edge. The stomachs contained lithogenic particles, diatom debris, and bacterial fatty acids, but low proportions of diatom-indicating fatty acids, which suggest that these krill were feeding on detritus rather than on fresh ice algae. In the SACCF, krill had higher lipid reserves (∼10% of DM), high mass:length ratios (∼2.2 mg DM mm −1), and large digestive glands (∼16% of total DM). Stomach content and tissue composition indicate feeding on diatoms. In the north, moderate food concentrations co-occurred with low lipid reserves in krill, and moderate mass:length ratios and digestive gland sizes. Only in the phytoplankton bloom in the SACCF had the mating season already started and some females were about to spawn. Based on the way krill processed their food at the different stations, we indicate two mechanisms that can lead to fast regeneration of body reserves and oocyte maturation in E. superba. One is “superfluous” feeding at high food concentrations, which maximises the overall nutrient gain. The other is a preferential absorption of crucial fatty acids: 20:5( n−3) and 22:6( n−3) when regaining body condition after the winter, and 14:0 and 16:1( n−7) during accumulation of oocyte yolk. Multi-year time series of phytoplankton distribution show that an early spring bloom, as found in 2006, is not unusual for the central Scotia Sea and coincides with high krill larval concentrations in this area. In conclusion, the Scotia Sea is a main spawning ground of Antarctic krill and this is linked to phytoplankton blooms in open waters of the SACCF rather than to ice edge blooms.

Observations of sea ice interannual variations and spring bloom occurrences at the Japanese scallop farming area in the Okhotsk Sea using satellite imageries

The dynamics of ice formation and phytoplankton bloom development in the coastal region of the Okhotsk Sea, Hokkaido, where the Japanese scallop, Mizuhopecten yessoensis, are cultured were investigated using seven years (1998–2004) satellite data from the Special Sensor Microwave/Imager (SSM/I) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS). The interannual variability of sea ice cover and timing of spring bloom occurrences were analyzed. Longer ice cover in 1999, 2001 and 2003 with the presence of ice until early April and shortened ice cover in 1998, 2000, 2002 and 2004 with the occurrence of ice until early March were recognized at this area. Variability in the timing of sea ice retreat and development of spring blooms at the scallop areas were observed. Progression of a single ice edge bloom showed higher Chl- a concentration compared to development of an initial ice edge bloom followed by a later open water bloom. Higher concentration of phytoplankton biomass was observed in the initial bloom when sea ice melting is delayed compared to when the sea ice leaves earlier. Wind events were also observed to affect the occurrences of spring bloom.

Removal of harmful algal cells ( Karenia brevis) and toxins from seawater culture by clay flocculation

Harmful algal blooms (HABs) occur worldwide causing serious threat to marine life, and to public health through seafood-borne illnesses and exposure to toxin-containing marine aerosol. This study was undertaken to assess the ability of phosphatic clay to remove the toxic dinoflagellate, Karenia brevis, and the potent neurotoxins (brevetoxins) produced by this species. Results showed that the addition of an aqueous slurry of 0.75 g (dry weight) clay to 3 l of K. brevis culture, containing 5×10 6 and 10×10 6 cells/l, removed 97±4% of brevetoxins from the water column within 4 h after the addition of clay. Clay flocculation of extra-cellular brevetoxins, released from cells ruptured (lyzed) by ultrasonication, removed 70±10% of the toxins. Addition of the chemical flocculant, polyaluminum chloride (PAC), removed all of the extra-cellular toxins. A 14 day study was undertaken to observe the fate of brevetoxins associated with clay flocculation of viable K. brevis cells. At 24 h following the clay addition, 90±18% of the toxins were removed from the water column, along with 85±4% of the cells. The toxin content of clay diminished from 208±13 μg at Day 1, to 121±21 μg at Day 14, indicating that the phosphatic clay retained about 58% of the toxins throughout the 14-day period. These studies showed the utility of natural clay as a means of reducing adverse effects from HABs, including removal of dissolved toxins, in the water column, although considerable work clearly remains before this approach can be used on natural blooms in open waters.

Climate change and control of the southeastern Bering Sea pelagic ecosystem

We propose a new hypothesis, the Oscillating Control Hypothesis (OCH), which predicts that pelagic ecosystem function in the southeastern Bering Sea will alternate between primarily bottom-up control in cold regimes and primarily top-down control in warm regimes. The timing of spring primary production is determined predominately by the timing of ice retreat. Late ice retreat (late March or later) leads to an early, ice-associated bloom in cold water (e.g., 1995, 1997, 1999), whereas no ice, or early ice retreat before mid-March, leads to an open-water bloom in May or June in warm water (e.g., 1996, 1998, 2000). Zooplankton populations are not closely coupled to the spring bloom, but are sensitive to water temperature. In years when the spring bloom occurs in cold water, low temperatures limit the production of zooplankton, the survival of larval/juvenile fish, and their recruitment into the populations of species of large piscivorous fish, such as walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus) and arrowtooth flounder (Atheresthes stomias). When continued over decadal scales, this will lead to bottom-up limitation and a decreased biomass of piscivorous fish. Alternatively, in periods when the bloom occurs in warm water, zooplankton populations should grow rapidly, providing plentiful prey for larval and juvenile fish. Abundant zooplankton will support strong recruitment of fish and will lead to abundant predatory fish that control forage fish, including, in the case of pollock, their own juveniles. Piscivorous marine birds and pinnipeds may achieve higher production of young and survival in cold regimes, when there is less competition from large piscivorous fish for coldwater forage fish such as capelin (Mallotus villosus). Piscivorous seabirds and pinnipeds also may be expected to have high productivity in periods of transition from cold regimes to warm regimes, when young of large predatory species of fish are numerous enough to provide forage. The OCH predicts that the ability of large predatory fish populations to sustain fishing pressure will vary between warm and cold regimes. The OCH points to the importance of the timing of ice retreat and water temperatures during the spring bloom for the productivity of zooplankton, and the degree and direction of coupling between zooplankton and forage fish. Forage fish (e.g., juvenile pollock, capelin, Pacific herring [Clupea pallasii]) are key prey for adult pollock and other apex predators. In the southeastern Bering Sea, important changes in the biota since the mid-1970s include a marked increase

Under‐ice blooms and source‐water odour in a nutrient‐poor reservoir: biological, ecological and applied perspectives

1. Algal taste and odour is usually associated with open water blooms and eutrophic systems. However, some algal species can produce high biomass under ice‐cover, even at low nutrient concentrations, that can impact water quality. This paper describes a winter odour outbreak in oligotrophic Glenmore Reservoir (Calgary, Alberta, Canada), the major algal species, volatile organic compounds (VOCs) and some treatment implications.2. Using sensory, chemical and microscope analyses, we monitored odour, algal biomass and taxa, bacteria and major nutrients. In a preliminary assessment of the effectiveness of standard water treatment with this type of algal biomass and odour, we used bench‐scale tests and sampled raw water from the Glenmore treatment plant at successive treatment stages.3. In the winter of 1999–2000 Glenmore ice‐cover was delayed, nutrients were characteristically low (TP &lt; ∼5 μg L–1), but organic carbon and bacteria were higher than in previous years.4. During this period there was an increase in algal biomass dominated by the mixotrophic chrysoflagellate Dinobryon divergens. Temporal dynamics of this species were inversely correlated with bacteria, and biomass declined following the establishment of ice‐cover, while depth profiles showed the highest abundance at subsurface layers. This suggested that the population outbreak was triggered by high bacteria abundance but depended on a minimum amount of light, consistent with in vitro studies of other mixotrophic chrysophytes.5. Other non‐bactiverous taxa were also numerous, notably Asterionella formosa, cryptomonads, dinoflagellates and the synurophyte Synura petersenii.6. Raw water odour was characteristically fishy, mainly caused by the VOCs 2,4,7‐decatrienal, 2,4‐heptadienal and 2,4‐decadienal. Based on algal population and VOC dynamics, these compounds were attributed to Dinobryon. Trace amounts of 2,6‐nonadienal (S. petersenii) and 1,3,5 and 1,3,6‐octatriene (A. formosa) were also detected. It was concluded that 2,4,7‐decatrienal was the major source of the raw water odour.7. Sensory and microscopic analyses of pre‐ and post‐treatment samples in the treatment plant indicated a complete removal of odour, but only a 30–60% removal of algal biomass and evident rupture of residual algal cells. Laboratory experiments showed that using standard treatment, chlorination rapidly oxidized 2,4,7‐decatrienal and 2,6‐nonadienal but had little effect on 2,4‐hepta‐ and decadienal.

Late-Holocene climatic change recorded in sediment cores from Ellis Fjord, eastern Antarctica

An analysis of diatom assemblages, Total Organic Carbon (TOC), carbon-nitrogen ratio, δ13C, biogenic silica and carboniate in two marine sediment cores from Ellis Fjord, eastern Anitarctica, reveals episodes of climatic change at 3600, 2500. 2000 and 1000 yr BP. Using the diatom data, principal component analysis, the fast-ice index (FI), benthic index (BI) and snow index (SI) (which are derived from the diatom data). a period of reduced temporal sea-ice cover is identified prior to 2500 BP. There was a large increase in sea-ice extent between 2500 and 2000 BP followed by a smaller decline. Tenmporal sea-ice extent after 2000 BP was still significantly greater than prior to 2500 BP. Both cores xvere taken from anoxic basins within the fjJord. It was found that the better preservationi allowed the recognition of taxa such as Clraetocelos vegetative cells, which are inot usually preserved. This enabled the distinction between ice-edge blooms, dominated by Fragilariopsis curta, and open-water blooms dominated by Chaeitoceros. Even though Watts Basin receives a significant summer meltwater input each year, it has not been possible to recognize this signal in the sedimiient core.

Omnivory and feeding selectivity in five copepod species during spring in the Bellingshausen Sea, Antarctica

Copepod grazing experiments were undertaken along a transect of five stations in the Bellingshausen Sea during austral spring. The stations spanned unproductive, nanoflagellate-dominated waters under pack ice to an open water bloom of large diatoms in the north. The feeding habits of five dominant species were compared by incubating them in natural sea water. Feeding rates on individual food taxa were calculated from optical cell counts. The larger species (Rhincalanus gigas, Calanus propinquus, and Calanoides acutus) tended to eat larger cells than did Oithona spp. and Metridia gerlachei, and cleared cells up to 100-200 $[$mu$]$m in length with increasing efficiency. Their clearance of longer cells remained high. Oithona spp. was the only species with high clearance rates on cells smaller than 10 $[$mu$]$m. Clearance rates on elongated cells were compared with those of shorter and fatter cells of similar volume. The longer cells were eaten at higher rates in 65% of comparisons, suggesting that cell elongation did not protect against ingestion by large copepods. Clearance rates of motile cells were compared with those of non-motile diatoms of similar size and shape. The copepods ranged widely, with Oithona spp. at one extreme, ingesting motile cells almost exclusively. M. gerlachei and C. propinquus showed a preference for motile taxa compared with similar sized diatoms. The fact that the motile fraction comprised an array of taxa (nanoflagellates, dinoflagellates, and ciliates), all with a tendency to be cleared more rapidly than diatoms by the above three species, suggests that they were using mechanoreception to detect prey. Oithona spp., M. gerlachei, and C. propinquus were actively feeding under the ice before the bloom, in low productivity waters characterized by a nanoflagellate community and a high inorganic detritus content. The selection for motile taxa persisted, by M. gerlachei at least, into the bloom, even though motile cells comprised only a small fraction of available carbon at that tim C. acutus and R. gigas by contrast, migrated to the surface layers to start feeding on the bloom. These two species appeared to have fed indiscriminately on both motile and non-motile taxa. The contribution of motile taxa to copepod diets depended on their contribution to the overall food source, the extent of motile/sessile selection, and also on their size and shape relative to the diatoms. Consequently the proportion of motile taxa in copepod diets ranged from 100% (Oithona spp. before the bloom) to 3% (R. gigas during the bloom).

Response of the copepod community to a spring bloom in the Bellingshausen Sea

During Austral spring 1992, R.R.S. James Clark Ross worked a five station transect in the Bellingshausen Sea. The transect spanned unproductive waters under solid pack ice to an open-water bloom in the north, three weeks later. This paper addresses the ontogenetic development of the copepod community, and from grazing experiments on females of five species investigates their trophic response to a spring bloom. Copepods dominated the mesozooplankton in both numbers and biomass. Their mean biomass in the top 600 m was low (0.85–1.5 mg drymass m −3), which is similar to other high latitude oceanic localities in the Southern Ocean. Almost all the major copepod species underwent an ontogenetic seasonal ascent, from mainly below 250 m under the ice to the top 250 m at the open-water bloom stations. Based on the timings of migration, feeding and reproduction, the species appeared to fall into two broad categories. Firstly, the pronounced seasonal migrants, Calanoides acutus and Rhincalanus gigas, ascended from below 250 m into the top 50 m to coincide with the bloom. The few individuals of C. acutus in the surface layers prior to the bloom do not appear to have been feeding. Reproduction of R. gigas was later than that of C. acutus. Production of the second group, namely Calanus propinquus, Oithona spp. and possibly Metridia gerlachei, appeared to have been less keyed to the bloom. Their seasonal migration was less, and individuals were actively feeding prior to the bloom, albeit at rates about half of those measured during the bloom. Mass specific feeding rates of the species in this group were greater than those of C. acutus and R. gigas. In the top 250 m, carbon:nitrogen ratios of C. propinquus and M. gerlachei were less than those of R. gigas and C. acutus, which suggests less reliance on depot lipids at this time of year. Despite the cold temperatures, the mass-specific feeding rates of the five species measured were broadly comparable to summer values from more northern regions of the Southern Ocean. However, the estimated grazing impact of the copepod community at all the stations was negligible, rising to a maximum of only 8.4% of daily primary production at the most northerly bloom station. These low values result from the very low numbers of copepods in the epipelagic, particularly under the ice. Of the copepods measured, grazing was mainly by the adult female population of Oithona spp. before the bloom, and appeared to be mainly by Oithona spp. and C. acutus during the bloom.

Physical and biological oceanographic interaction in the spring bloom at the Bering Sea marginal ice edge zone

At the edge of the melting sea ice pack in the Bering Sea in spring, physical, biological, and chemical oceanographic processes combine to generate a short‐lived, intense phytoplankton bloom that is associated with the retreating ice edge. The bloom begins a week or so before the first of May triggered by insolation and by the low‐salinity meltwater stratification in the presence of high nitrate concentrations (∼ &gt; 25 μM). Meltwater (salinity) stratification delineates ice edge blooms from open water blooms where temperature gradients generate the stratification. Five cross‐ice sections of temperature, salinity, σt, chlorophyll, and nitrate are presented as a time series from April 27 to May 5 illustrating the bloom. Evidence of two separate but concurrent blooms in the ice edge zone are presented. In addition, meteorological and oceanographic conditions were observed that should have been conducive to ice edge up welling. While significant ice and water movement occurred, upwelling was not observed. Finally, the Bering Sea ice edge spring bloom is compared with other ice edge systems in both hemispheres, showing that initial Bering Sea nitrate concentrations are among the highest observed but quickly become limiting owing to the rapid build up of phytoplankton populations. This primary production is not coupled to the pelagic Zooplankton because Zooplankton are largely absent on account of the cold temperatures. Observed maximum chlorophyll concentrations in the bloom are several times greater than those observed in other systems.

Oceanographic frontal structure and biological production at an ice edge

Marginal ice edge zones (MIZ) are unique frontal systems with air-ice-sea interfaces. Phytoplankton blooms, which occur along the edge of the melting ice pack in spring, are strongly related to the air-ice-sea interactive processes. In spring 1982, during a cruise to the Bering Sea ice pack, hydrographic sections, including standard biological oceanographic parameters, were collected across the MIZ showing such enhanced phytoplankton bloom populations in the ice edge. During this period the ice edge retreated at speeds of 6 to 38 cm s −1. Associated with the retreating ice edge were a faster moving upper layer oceanic front that kept pace with the retreating ice edge, and a nearly stationary deeper front. In the presence of light, the phytoplankton blooms are shown to be associated with, and primarily controlled by enhanced density stratification and frontal structure due to ice melt during the spring ice retreat. The ice melt water forms stratification that helps to maintain the phytoplankton within the photic zone. The ice edge blooms can be differentiated from open water blooms by the stratification mechanism; in MIZ blooms stratification is due to low salinity melt water as opposed to temperature derived stratification in most open water blooms. In addition, in the series of cross sections collected, a unique biophysical interaction was observed when the MIZ front moving north with the spring retreat, came in contact with a fixed shelf front forming a ‘dish’ shaped hydrographic structure within which a major phytoplankton bloom was observed. We suggest that upwelling from the tidally driven shelf front supplied nutrients to the surface waters extending the life of the bloom. Wind-driven ice edge upwelling was also observed but was difficult to distinguish from the shelf front circulation. In this same set of ice edge cross sections, a cold water mass was observed at the surface in the MIZ. This water mass was subsequently overridden by warmer water forming a cold tongue structure above the pycnocline and seaward of the shelf front. We suggest that this cold tongue was transient in nature, and illustrative of one mechanism by which the T-S characteristics of high latitude shelf waters are formed and altered.