Influences of upwelling and downwelling winds on red tide bloom dynamics in Monterey Bay,California

It has recently been shown that inner shelf waters of NE Monterey Bay, California function as an “extreme bloom incubator”, frequently developing dense “red tide” blooms that can rapidly spread. Located within the California Current upwelling system, this open bay is strongly influenced by oceanographic dynamics resulting from cycles of upwelling favorable winds and their relaxation and/or reversal. Different wind forcing causes influx of different water types that originate outside the bay: cold nutrient-rich waters during upwelling and warm nutrient-poor waters during relaxation. In this study, we examine how the bay's bloom incubation area can interact with highly variable circulation to cause red tide spreading, dispersal and retention. This examination of processes is supported by satellite, airborne and in situ observations of a major dinoflagellate bloom during August and September of 2004. Remote sensing of high spatial, temporal and spectral resolution shows that the bloom originated in the NE bay, where it was highly concentrated in a narrow band along a thermal front. Upwelling circulation rapidly spread part of the bloom, mixing cool waters of an upwelling filament with warm bloom source waters as they spread. Vertical migration of the dinoflagellate populations was mapped by autonomous underwater vehicle surveys through the spreading bloom. Following bloom expansion, a two-day wind reversal forced intrusion of warm offshore waters that dispersed much of the bloom. Upwelling winds then resumed, and the bloom was further dispersed by an influx of cold water. Throughout these oceanographic responses to changing winds, an intense bloom persisted in sheltered waters of the NE bay, where extreme blooms are most frequent and intense. Microscopic examination of surface phytoplankton samples from the central bay showed that spreading of the bloom from the NE bay and mixing with regional water masses resulted in significantly increased abundance of dinoflagellates and decreased abundance of diatoms. Similar dinoflagellate bloom incubation sites are indicated in other areas of the California Current system and other coastal upwelling systems. Through frequent bloom development and along-coast transports, relatively small incubation sites may significantly influence larger regions of the coastal marine ecosystems in which they reside.     

Mesoscale and submesoscale mechanisms behind asymmetric cooling and phytoplankton blooms induced by hurricanes: a comparison between an open ocean case and a continental shelf sea case

Right-side bias in both sea surface cooling and phytoplankton blooms is often observed in the wake of hurricanes in the Northern Hemisphere. This idealized hurricane modeling study uses a coupled biological-physical model to understand the underlying mechanisms behind hurricane-induced cooling and phytoplankton bloom asymmetry. Both a deep ocean case and a continental shelf sea case are considered and contrasted. Model analyses show that while right-side asymmetric mixing due to inertial oscillations and restratification from strong right-side recirculation cells contributes to bloom asymmetry in the open ocean, the well-mixed condition in the continental shelf sea inhibits formation of recirculation cells, and the convergence of water onto the shelf is a more important process for bloom asymmetry.     

A time-series study of the spring bloom at the Bering Sea ice edge I. Physical processes, chlorophyll and nutrient chemistry

An intense but short-lived phytoplankton bloom develops in the low-salinity melt waters at the edge of the Bering Sea ice as the ice melts and retreats each spring. In spring 1988 we followed the development of this bloom by sampling every 3 h while following a freely drifting drogue in the marginal ice-edge zone for two four-day periods. The first period (29 April–3 May) was at an early stage of the bloom while the second period (10–13 May) was at the peak of the bloom. Early in the bloom, the phytoplankton consumed all the nitrate (400 mmoles m⁻²) initially present in the surface water producing large accumulations of particulate carbon (>1000 mmoles C m⁻²). By the time of peak chlorophyll concentrations (35 mg M⁻³), nitrate concentrations had been depleted so that the sustained high productivity depended on either recycled or imported nutrients. After this point, there was little net additional accumulation of biomass. From these data plus cruise data from previous years, we find that the Bering Sea ice-edge bloom typically begins in the last week of April and appears to precede blooms in the adjacent ice-free waters by days to weeks. The variability in bloom onset observed over several years is not linked very closely to the large scale climatic variations found in this region, but rather appears to be related to local weather during the end of April and the first part of May, with calm, sunny weather being required to initiate the blooms.     

Wind and melt driven circulation in a marginal sea ice edge frontal system: a numerical model

A seasonal ice edge zone is a unique frontal system with an air-ice-sea interface. This paper is a report on the numerical results from a quasi-three dimensional, time dependent, non-linear numerical model of circulation at a continental shelf-seasonal ice edge zone. The purpose of the experiments is to model the hydrography and circulation, including upwelling, baroclinic geostrophic flow, and inertial oscillations, at the ice edge with emphasis on examining the driving forces of wind and melting ice. It is suggested that the non-linear acceleration terms and vertical density diffusion terms are negligible and that the horizontal density diffusion terms are of secondary importance within the time and space scales of the experiments. The vertical eddy viscosity terms are important in a spin-up time scale and for Ekman transport and a bottom Ekman layer. The effects of the horizontal eddy viscosity terms are observable (a long-ice jet is diffused away from the ice edge) by the end (72 h) of the model runs.Model results are compared with available oceanographic and meteorological data for verification. The observed and modeled features of melt water induced water column stability, frontal structure, and ice edge upwelling are briefly discussed relative to observed ice edge primary production. Because the model is relatively general in nature, it is readily applicable to other seasonal or marginal ice edge zones in either hemisphere.     

Production and transport of phytoplankton biomass over the continental shelf of the new york bight

Seasonal and event scale variations in the distribution and growth of phytoplankton in different hydrographic regions of the continental shelf are compared and evaluated in terms of floristic composition and the evolution of density and nutrient structure across the shelf. Annual cycles of phytoplankton biomass inshore of the 1000-m isobath are characterized by a March maximum and a July minimum. Cross-shelf biomass gradients usually increase in an offshore direction, a phenomenon that is most pronounced during March and April when biomass is high, diatoms dominate, and growth rate is light limited. This is a consequence of the combined effects of growth along the stratified side of the shelf-break front and offshore transport of biomass produced nearshore. We estimate that about 90% of the diatom biomass produced during the February to April bloom period (35% of annual production) is exported from shelf to slope water. Similar but less-pronounced gradients develop during summer due to the development of a chlorophyll maximum layer below the pycnocline where growth rate is also light limited. Production and loss are more tightly coupled under these conditions and about 9% of the biomass produced during May to October appears to be exported (5% of annual production). Export during the diatom bloom period is balanced mainly by nitrate inputs from the Gulf of Maine and adjacent slope water while summer export may be balanced by anthropogenic nitrogen input. The latter could be coupled with biomass export by ammonium remineralization and nitrification in the cold pool of the mid-shelf region. In general, export is greatest when diatoms dominate, growth is light limited, and biomass distributions are physically forced. Export is lowest when nanoplankton dominate, growth is nitrogen limited, and biomass distributions are controlled by grazing.The shelf-break front plays a key role, influencing patterns of phytoplankton growth, biomass distributions, and shelf export. During the diatom bloom period, the development of stratification in nutrient-rich offshore water between storm events results in high growth rates and biomass near the surface on the shelf side of the front. Under these conditions, biomass accumulates in the mid-shelf region on a time scale of days to weeks. Export occurs during wind events with net export from the shelf occurring on a time scale of weeks to months. Blooms also develop along the shelf side of the front during summer but below the pycnocline. Most of the summer export of biomass probably takes place here with accumulation and export occurring on a time scale of hours to days. While this export is small compared to export during the diatom bloom period, it may be critical to the prevention of anoxic events such as that of 1976.     

The COPAS’08 expedition to the Patagonian Shelf: Physical and environmental conditions during the 2008 coccolithophore bloom

Here we present observations of the hydrography of the Patagonian Shelf, shelf break and offshore waters, with reference to the environmental conditions present during the period of peak coccolithophore abundance. Analysis of a hydrographic dataset collected in December 2008 (austral spring/summer), as part of the Coccolithophores of the Patagonian Shelf (COPAS) research cruise, identified 5 distinct surface water masses in the region between 37°S and 55°S. These water masses, identified through salinity gradients, displayed varying mixed layer depths, macronutrient inventories and chlorophyll-a fluorescence. Subantarctic Shelf Water (SSW), located to the north of the Falkland Islands and extending north along the shelf break, was also host to a large coccolithophore bloom. The similarities between the distribution of calcite, as seen in remote sensing data, and SSW indicate that the coccolithophore bloom encountered conditions conducive to bloom development within this water mass. Analysis of chemical and environmental data also collected during the COPAS cruise revealed that many of the commonly cited conditions for coccolithophore bloom development were present within SSW (e.g. low N:P ratio, high N:Si ratio, shallow mixed layer depth). In the other water masses present on the Patagonian Shelf greater variability in these same parameters may explain the more diffuse concentration of calcite, and the smaller size of possible coccolithophore blooms. The distribution of SSW is strongly influenced by the latitudinal variation in shelf break frontal width, which varies from 20 to 200 km, and consequently strong hydrographic controls underlie the position of the coccolithophore bloom during austral summer.     

Seasonal upwelling on the Western and Southern Shelves of the Gulf of Mexico

An 8-year database of sea surface temperature (SST), 7 years of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color images, wind fields, and numerical model results are analyzed to identify regions and periods of coastal upwelling on the western and southern shelves of the Gulf of Mexico. On the seasonal scale, it is found that on the Tamaulipas, Veracruz, and southwestern Texas–Louisiana shelves there are upwelling favorable winds from April to August, when southeasterly winds are dominant and cold SST anomalies associated with upwelling are observed along their coasts. However, during summer, values of chlorophyll-a concentration are lower than those in autumn and winter, which are high due to advection of old bloom biological material from upstream. During winter, there is a cold front on the Tamaulipas shelf produced by advection of cold water from the Texas–Louisiana shelf and not due to upwelling. On the eastern Campeche Bank, persistent upwelling is observed due to favorable winds throughout the year with cold SST and large chlorophyll-a content along the inner shelf from May to September. On the Tamaulipas shelf, the summer upwelling delays the annual SST peak until September, while in most of the Gulf SST peaks in August. This difference is due to the end of the upwelling favorable wind conditions and the September seasonal current reversal.     

Abiotic controls of potentially harmful algal blooms in Santa Monica Bay,California

Despite the increasing occurrence of harmful phytoplankton blooms along the North American west coast, records of phytoplankton populations and related environmental conditions are uncommon. In this study, twice monthly measurements in the upper 50 m are used to assess physico-chemical conditions contributing to the growth of potentially harmful bloom taxa over two annual cycles (2004–2005) in the Santa Monica Bay, California. Results were compared to the predictions of the Intaglio model [Smayda, T.J., Reynolds, C.S., 2001. Community assembly in marine phytoplankton: application of recent models to harmful dinoflagellate blooms. Journal of Plankton Research 23, 447–461.] of phytoplankton community assembly. Potentially harmful taxa were present in every surface sample and were numerically dominant during the largest observed blooms, contributing up to 92% of the total phytoplankton abundance >5 μm. Large interannual variation was observed in the dominant taxa and bloom seasonality; Pseudo-nitzschia sp. dominated blooms in early 2004 (February and April), whereas Prorocentrum micans and Lingulodinium polyedrum blooms occurred in May and September of 2005, respectively. The Pseudo-nitzschia sp. blooms were associated with elevated nitrate, dissolved silicon and phosphate concentrations throughout the euphotic zone; the first bloom followed a strong upwelling and the second occurred during the onset of seasonal stratification. In contrast, the blooms of P. micans were associated with highly stratified, low nutrient waters. Multivariate analysis supports the roles of temperature, mixed-layer depth and nutrient concentrations as primary controls of bloom growth, following the conceptual Intaglio model. The strong presence of potentially harmful bloom species in the Santa Monica Bay during this study appears unusual in comparison to limited studies over the last several decades.     

Characteristics of seasonal and spatial variations of primary production over the southeastern Bering Sea shelf

In situ primary production data collected during 1978–1981 period and 1997–2000 period were combined to improve understanding of seasonal and spatial distribution of primary production in the southeastern Bering Sea. Mean daily primary production rates showed an apparent seasonal cycle with high rates in May and low rates in summer over the entire shelf of the southeastern Bering Sea except for oceanic region due to lack of data. There was also an increasing trend of primary production rates in the fall over the inner shelf and the middle shelf. There was a decreasing trend of primary production rates between late April and mid-May over the inner shelf while there was an abrupt increase between late April and mid-May over the middle shelf and the outer shelf. In the shelf break region, there was an increasing pattern in late May. These suggest that there was a gradual progression of the development of the spring phytoplankton bloom from the inner shelf toward the shelf break region. There was also a latitudinal variability of primary production rate over the middle shelf, probably due to either spatial variations of the seasonal advance and retreat of sea ice or horizontal advection of saline water in the bottom layer. Annual rates of primary production across the southeastern Bering Sea shelf were 121, 150, 145, 110, and 84 g C m⁻² yr⁻¹ in the inner shelf, the middle shelf, the outer shelf, the shelf break, and oceanic region, respectively. High annual rates of primary production over the inner shelf can be attributed to continuous summer production based on regenerated nitrogen and/or a continuous supply of nitrogen at the inner front region, and to fall production. There were some possibilities of underestimation of annual primary production over the entire shelf due to lack of measurement in early spring and fall, which may be more apparent over the shelf break and oceanic region than the inner shelf, the middle, and the outer shelf. This study suggests that the response of primary production by climate change in the southeastern Bering Sea shelf can be misunderstood without proper temporal and seasonal measurement.     

三峡水库支流小江富营养化模型构建及在水量调度控藻中的应用

三峡水库蓄水以来,支流小江呈富营养化加重的趋势,且多次暴发春季水华.水库蓄水以后支流流速变缓,水体滞留时间增加,是引发支流水华的主要因素之一.基于MIKE软件,建立小江调节坝下游至河口的二维水动力-富营养化模型,考虑碳、氮、磷3种元素在浮游植物有机体、死亡腐屑和无机盐中的循环转化,模拟小江河段的春季水华过程.分析小江生态调节坝的水量调节抑藻作用,即人为制造"洪水脉冲",增加短时间内的水流流速,对下游流场进行扰动以控制水华.计算结果表明,增大泄水量对调节坝下游的小江河段的春季藻华总体上具有一定的抑制作用.小江上游河段调度作用效果明显,下游高阳至入汇口河段调节作用较小,上游调节坝水力调度可以作为三峡水库支流水华应急治理措施之一.营养盐控制应该是控制支流水华的根本措施.     

Early spring phytoplankton blooms in the Gulf of Maine

Our observations of the distribution of early spring phytoplankton blooms (27 March to 5 April 1984) in the Gulf of Maine support previous contentions that bloom propagation begins when the depth averaged in situ solar irradiance within the upper mixed layer is about 40 ly d⁻¹. We found that prior to vernal warming and thermal stratification, the formation of an upper mixed layer in these waters appears to be determined in three ways: (1) by density stratification resulting from freshwater runoff immediately adjacent to the coast, (2) by intrusions of dense slope water and its subsequent doming to form a pycnocline in Jordan Basin, and (3) by local bathymetry.     

Microbial distribution and abundance in response to physical and biological processes on the continental shelf of southeastern U.S.A.

The distribution and abundance of bacteria and phytoplankton on the continental shelf of the southeastern United States were observed in relation to physical processes. Phytoplankton production was influenced by inputs of water of reduced salinity from the estuaries and by inputs of high salinity, low-temperature water from the west front of the Gulf Stream. The distribution of chlorophyll suggests that in each case production is influenced both by inputs of nutrients and by the enhanced vertical stability associated with the stratification of waters of different densities. The standing stock of bacteria on the inner shelf, 10⁶ ml^?1, is little changed by the influx of water of reduced salinity. On the outer shelf, where the usual standing stock of bacteria is 10⁵ ml^?1, the numbers increase to 10⁶ ml^?1 in and above intrusions of Gulf Stream water in which phytoplankton blooms have developed, suggesting that the bacteria respond to products of both phytoplankton and zooplankton production. Adenylate energy charge values in the waters of the southeastern shelf are variable and volatile. At times values of 0.7 to 0.8 are widespread over most of the shelf, while at other times values <0.6 are common, with localized patches of high values. Both autotroph-dominated and heterotroph-dominated microbial communities show these variations.     

Mechanisms leading to the frequent occurrences of hypoxia and a preliminary analysis of the associated acidification off the Changiiang estuary in summer

Based on literature data and shipboard observations,this study investigated the main environmental characteristics of the seafloor topography,current field,front,and upwelling that are closely related to hypoxia occurrence off the Changjiang estuary.The physical processes of the plume front and upwelling off the Changjiang estuary in summer were coupled.The vertical distribution pattern of the plume front was closely related to the upwelling.By reviewing and analyzing the historical summer hypoxia events off the Changjiang estuary,we statistically demonstrated the spatial structure of the horizontal distribution of the hypoxic zone and investigated the location of occurrence zone of the hypoxia.We found that the dissolved oxygen(DO)concentration on the inner continental shelf off the estuary showed a"V"shape in relation to station depth.Therefore,we noted that the hypoxic water on the inner continental shelf mostly occurred on the slopes with steep seafloor topography.Base on the current understanding of the hypoxic mechanisms off the Changjiang estuary,we analyzed the biogeochemical mechanisms that could cause the steep terrain off the Changjiang estuary to become the main areas prone to summer hypoxia and explained the internal relations between the location of the hypoxic zone on the slopes and the plume front and upwelling.The plume front and upwelling off the Changjiang estuary and their coupling were important driving forces of summer hypoxia.The continuous supply of nutrients affected by the interaction of the plume front extension of the Changjiang Diluted Water(CDW)and upwelling and the favorable light conditions were important mechanisms causing the phytoplankton blooms and benthic hypoxia off the Changjiang estuary in summer.By analyzing oxygen utilization,organic carbon mineralization,and nutrient regeneration in the hypoxic zone,we observed that the significant oxygen utilization process off the Changjiang estuary in summer also mainly occurred near the steep slopes with front and upwelling features and confirmed the apparent nutrient loss in the benthic hypoxic zone.Meanwhile,our analysis revealed that the sediment resuspension in the benthic boundary layer in the mud areas off the Changjiang estuary could also affect the oxygen utilization and mineralization of organic carbon and nutrient recycling and regeneration.This study also demonstrated that the steep terrain off the Changjiang estuary was the main location for summer acidification,and the coupling between the plume front and upwelling on the steep slopes was an important physical driving force inducing summer benthic acidification.Finally,we discussed issues to address in future studies of the hypoxic zone and water acidification off the Changjiang estuary.     

Mechanisms leading to the frequent occurrences of hypoxia and a preliminary analysis of the associated acidification off the Changjiang estuary in summer

Based on literature data and shipboard observations, this study investigated the main environmental characteristics of the seafloor topography, current field, front, and upwelling that are closely related to hypoxia occurrence off the Changjiang estuary. The physical processes of the plume front and upwelling off the Changjiang estuary in summer were coupled. The vertical distribution pattern of the plume front was closely related to the upwelling. By reviewing and analyzing the historical summer hypoxia events off the Changjiang estuary, we statistically demonstrated the spatial structure of the horizontal distribution of the hypoxic zone and investigated the location of occurrence zone of the hypoxia. We found that the dissolved oxygen (DO) concentration on the inner continental shelf off the estuary showed a “V” shape in relation to station depth. Therefore, we noted that the hypoxic water on the inner continental shelf mostly occurred on the slopes with steep seafloor topography. Base on the current understanding of the hypoxic mechanisms off the Changjiang estuary, we analyzed the biogeochemical mechanisms that could cause the steep terrain off the Changjiang estuary to become the main areas prone to summer hypoxia and explained the internal relations between the location of the hypoxic zone on the slopes and the plume front and upwelling. The plume front and upwelling off the Changjiang estuary and their coupling were important driving forces of summer hypoxia. The continuous supply of nutrients affected by the interaction of the plume front extension of the Changjiang Diluted Water (CDW) and upwelling and the favorable light conditions were important mechanisms causing the phytoplankton blooms and benthic hypoxia off the Changjiang estuary in summer. By analyzing oxygen utilization, organic carbon mineralization, and nutrient regeneration in the hypoxic zone, we observed that the significant oxygen utilization process off the Changjiang estuary in summer also mainly occurred near the steep slopes with front and upwelling features and confirmed the apparent nutrient loss in the benthic hypoxic zone. Meanwhile, our analysis revealed that the sediment resuspension in the benthic boundary layer in the mud areas off the Changjiang estuary could also affect the oxygen utilization and mineralization of organic carbon and nutrient recycling and regeneration. This study also demonstrated that the steep terrain off the Changjiang estuary was the main location for summer acidification, and the coupling between the plume front and upwelling on the steep slopes was an important physical driving force inducing summer benthic acidification. Finally, we discussed issues to address in future studies of the hypoxic zone and water acidification off the Changjiang estuary.     

An analytical model for ice-edge upwelling

Abstract

This paper presents an analytical, two-dimensional model of the wind-induced homogeneous circulation near the edge of an ice pack floating on the ocean surface. It is shown that a vertical shear layer arises under the ice edge, by which the wind-driven geostrophic motion in the open ocean is matched to the flow region underneath the ice. As in coastal upwelling models, this shear layer consists of a thin E ^1/2-layer inside a thicker E ^1/4-layer (E being the Ekman number). Under certain conditions the shear layer produces a vertical mass flux from the bottom to the surface Ekman layer. Near the surface this upwelling flux is concentrated in the narrow E ^1/2-layer. Comparison with observations of upwelling at the edge of a polar ice pack shows good agreement.     

Effect of upwelling on phytoplankton productivity of the outer southeastern United States continental shelf

Gulf Stream frontal disturbances cause nutrient-rich waters to frequently upwell and intrude onto the southeastern United States continental shelf between Cape Canaveral, Florida and Cape Hatteras, North Carolina. Phytoplankton response in upwelled waters was determined with three interdisciplinary studies conducted during April 1979 and 1980, and in summer 1978. The results show that when shelf waters are not stratified, upwelling causes productive phytoplankton (diatom) blooms on the outer shelf. Phytoplankton production averages about 2 g C m⁻² d⁻¹ during upwelling events, and ‘new’ production is 50% or more of the total. When shelf waters are stratified, upwelled waters penetrate well onto the shelf as a subsurface intrusion in which phytoplankton production averages about fives times higher than the nutrient-depleted overlying mixed layer. Phytoplankton within the intrusion deplete upwelled NO₃ in about 7 to 10 days, at which point no further net increase in phytoplankton biomass occurs.Current meter records show that upwelling occurs roughly 50% of the time on the outer shelf during November to April (shelf not stratified), and we estimate that seasonal primary production in upwelled waters is 175 g C m⁻² 6 months⁻¹ of which at least 50% is ‘new’ production. More than 90% of outer shelf primary and ‘new’ production occurs during upwelling and thus upwelling is the dominant process affecting primary productivity of the outer shelf. Our seasonal estimates of outer shelf primary and ‘new’ production are, respectively, three and ten times higher than previous estimates that did not account for upwelling.     

Frontal circulation induced by up-front and coastal downwelling winds

Two-dimensional (cross-shelf and depth) circulation by downwelling wind in the presence of a prograding front (with isopycnals that slope in the same direction as the topographic slope) over a continental shelf is studied using high-resolution numerical experiments. The physical process of interest is the cross-shelf circulation produced by northeasterly monsoon winds acting on the Kuroshio front over the East China Sea outer shelf and shelfbreak where upwelling is often observed. However, a general problem is posed and solved by idealized numerical and analytical models. It is shown that upwelling is produced shoreward of the front. The upwelling is maintained by (1) a surface bulge of negative vorticity at the head of the front; (2) bottom offshore convergence beneath the front; and (3) in the case of a surface front that is thin relative to water depth, also by upwelling due to the vorticity sheet under the front. The near-coast downwelling produces intense mixing due to both upright and slant-wise convection in regions of positive potential vorticity. The analytical model shows that the size and on-shore propagating speed of the bulge are determined by the wind and its shape is governed by a nonlinear advection–dispersion equation which yields unchanging wave-form solutions. Successive bulges can detach from the front under a steady wind. Vertical circulation cells develop under the propagating bulges despite a stable stratification. These cells can have important consequences to vertical exchanges of tracers and water masses.     

A numerical study of the effect of the marginal sea on coastal upwelling in a non-linear inertial model

Inertia theory and the finite element method are used to investigate the effect of marginal seas on coastal upwelling. In contrast to much previous research on wind-driven upwelling, this paper does not consider localized wind effects, but focuses instead on temperature stratification, the slope of the continental shelf, and the background flow field. Finite element method, which is both faster and more robust than finite difference method in solving problems with complex boundary conditions, was developed to solve the partial differential equations that govern coastal upwelling. Our results demonstrate that the environment of the marginal sea plays an important role in coastal upwelling. First, the background flow at the outer boundary is the main driving force of upwelling. As the background flow strengthens, the overall velocity of cross-shelf flow increases and the horizontal scale of the upwelling front widens, and this is accompanied by the movement of the upwelling front further offshore. Second, temperature stratification determines the direction of cross-shelf flows, with strong stratification favoring a narrow and intense upwelling zone. Third, the slope of the continental shelf plays an important role in controlling the intensity of upwelling and the height that upwelling may reach: the steeper the slope, the lower height of the upwelling. An additional phenomenon that should be noted is upwelling separation, which occurs even without a local wind force in the nonlinear model.     

Dimethylsulfide and Phaeocystis poucheti in the southeastern Bering Sea

Dimethylsulfide (DMS), a volatile excretion product of marine phytoplankton, was determined in the water column during the spring phytoplankton bloom on the southeastern Bering Sea shelf. In the same samples, a broad range of variables which characterize the biological processes in this region were measured. DMS was correlated with phytoplankton chlorophyll in the outer shelf and oceanic domains, but not in the middle shelf domain. A very strong correlation between the cell density of the haptophyte Phaeocystis poucheti and the DMS concentration in seawater was found, which suggests that this species accounts for most of the DMS present in the study region. We propose that in P. poucheti and certain other phytoplankton species the excretion of DMS is incidental to the release of acrylic acid which serves to inhibit bacterial attack upon the algae.