Altimeter-derived surface circulation in the large–scale NE Pacific Gyres. : Part 1. seasonal variability

The seasonal variability of sea surface height (SSH) and currents are defined by analysis of altimeter data in the NE Pacific Ocean over the region from Central America to the Alaska Gyre. The results help to clarify questions about the timing of seasonal maxima in the boundary currents. As explained below, the long-term temporal mean of the SSH values must be removed at each spatial point to remove the temporally invariant (and large) signal caused by the marine geoid. We refer to the resulting SSH values, which contain all of the temporal variations, as the ‘residual’ SSH. Our main findings are:

1. The maximum surface velocities around the boundaries of the cyclonic Alaska Gyre (the Alaska Current and the Alaska Stream) occur in winter, at the same time that the equatorward California Current is weakest or reversed (forming the poleward Davidson Current); the maximum surface velocities in the California Current occur in summer. These seasonal maxima are coincident with the large-scale atmospheric wind forcing over each region.

2. Most of the seasonal variability occurs as strong residuals in alongshore surface currents around the boundaries of the NE Pacific basin, directly connecting the boundaries of the subpolar gyre, the subtropical gyre and the Equatorial Current System.

3. Seasonal variability in the surface velocities of the eastward North Pacific Current (West Wind Drift) is weak in comparison to seasonal changes in the surface currents along the boundaries.

4. There is an initial appearance next to the coast and offshore migration of seasonal highs and lows in SSH, alongshore velocity and eddy kinetic energy (EKE) in the Alaska Gyre, similar to the previously-described seasonal offshore migration in the California Current.

5. The seasonal development of high SSH and poleward current residuals next to the coast appear first off Central America and mainland Mexico in May–June, prior to their appearance in the southern part of the California Current in July–August and their eventual spread around the entire basin in November–December. Similarly, low SSH and equatorward transport residuals appear first off Central America and Mexico in January–February before spreading farther north in spring and summer.

6. The maximum values of EKE occur when each of the boundary currents are maximum.

Horizontal flux of nutrients and plankton across and along the British Columbia continental margin

We report rate estimates for the horizontal transport of realized and potential “new” production across and along the Vancouver Island continental margin. Measurements consisted of three summer-season surveys (1993–1995) of water properties, chlorophyll and dissolved nutrient concentrations, zooplankton biomass and community composition. Sampling was done along paired 350-km station lines extending parallel to and approximately 25 km seaward of the shelf break. Horizontal transport of nutrients and plankton biomass was estimated from cross-products of concentration fields with cross-shore and alongshore geostrophic velocity fields and with space- and time-averaged estimates of Ekman volume transport. Because concentrations of nutrients and phytoplankton were low in the upper 30–50 m, their horizontal flux within the Ekman layer was relatively small (order 10% of geostrophic transport). Geostrophic transport was strongly localized and was correlated vertically with concentration gradients, and horizontally with eddies and meanders of the alongshore geostrophic currents. Net geostrophic transport was a small difference between larger localized seaward and shoreward components. Upper layer (0–50 m) transports of nutrients and phytoplankton biomass were of roughly similar magnitude. Both were much larger than transport of zooplankton biomass. Total cross-shore flux was a small fraction (<10%) of the estimated total productivity shoreward of the sampling lines. Direction and magnitude varied among survey periods, but for all 1990s surveys appear to have been weaker than in the mid-1980s, when summer-season averaged upwelling-favorable winds were stronger and the shelf-break current was faster.     

Statistical analysis of the distribution of fish eggs and larvae on the southeastern U.S. continental shelf with comments on oceanographic processes that may affect larval survival

Data collected in 1953 and 1954 by the U.S. Fish and Wildlife Service were statistically analyzed to ascertain where and when fish eggs and larvae are most abundant on the southeastern U.S. continental shelf. The results are related to some oceanographic processes that might affect the survival of larval fish. Along-shelf differences in numbers of eggs and larvae are minimal compared with differences that occur across the shelf. Highest numbers of fish larvae are found on the outer shelf during fall, winter and spring, but larvae are evenly distributed across the shelf in summer.Upwelling strongly influences the dynamics of plankton production on the outer shelf, and thus during most seasons of the year upwelling may be the most important process controlling the amount of food available to larval fish. During winter and spring, mean winds do not favor shoreward transport of larval fish from the outer shelf if the larvae are located in near-surface waters. Thus, during these seasons variability of winds on the ‘event’ time scale may be more important to onshore and offshore transport of larval fish than the mean strength and direction of monthly or seasonally averaged winds.     

Physical forcing of onshore transport of crab megalopae in the northern Portuguese upwelling system

Wind-driven circulation is a mechanism that may be involved in the shoreward transport of invertebrate larvae. This study investigated the temporal variability of the supply of crab megalopae to an estuary located in the northwest coast of Portugal, which is affected by seasonal upwelling, and related variations in supply to several upwelling indices. Megalopae of the European common shore crab Carcinus maenas (L.). were collected daily, using artificial settlement substrates, during a 3-month period, from March 15 to June 15, 2000. Two settlement episodes were detected, which were consistent with the variation in abundance and size of juvenile crabs. Variations in subtidal sea level and sea surface temperature anomaly were correlated with the strength of along-shore wind stress and consistent with upwelling theory. Contingency analyses showed positive significant associations between increasing intensity of southerly winds and megalopal supply, for time lags of 0–6 d, indicating that shoreward transport occurred during relaxation or reversal of upwelling favourable winds. Associations between megalopal supply and tidal range or the duration of nocturnal flood were not detected. Compared to other studies of the supply of crab megalopae to coastal habitats that used a similar methodology, very low numbers of megalopae were collected during this study. The paucity of C. maenas megalopae in the Ria de Aveiro is in agreement with the low densities of this larval stage in the plankton, and indicates that maintenance of the population is achieved despite a low supply of megalopae.     

The regional approach to the ocean, the ocean regions, and ocean regionalisation—a post-modern dilemma

Essentially, this paper aims at considering how the ocean regionalisation may be implemented focusing on the principle of sustainable development, on the subsequent criteria designed by the inter-governmental organisation framework, and on the approaches from the scientific literature. In this respect, a model is proposed, according to which two main stages are identified: (i) the stage of the modern approach to the ocean, which was operated by the modern society and was supported by the culture of modernity; (ii) the stage of the post-modern approach, which has been triggered by the converging inputs from the changes in society and nature. The watershed between these two stages may be located in the 1970s. The investigation may be carried out considering a triangular relationship between (i) the changing ocean reality (ontological dimension), (ii) the representation of this reality (semiological dimension), and (iii) the building up of signified, consisting in theories, meta-theories and values (epistemological and ethical dimensions). In this framework, special relevance is attributed to the interaction between science and policy.Moving from this basis, how ocean regionalisation had been conceived by oceanography, geography and law is considered focusing on the implications that have arisen in terms of ocean management. Analysis is essentially focused on three questions: (i) how much the conceptual implications of the approach to the ocean regional scale have been underestimated, and how ample the political consequences have been; (ii) why the political designs referring to this spatial scale of the ocean, which have been carrying out since the 1970s, have been marked by a lack of consistency of the legal framework with the prospect of operating sustainable management programmes; (iii) whether, and what kind of, discrepancy has solidified between the legal framework, provided by the 1982 UN Convention on the Law of the Sea (UNCLOS), and the ecosystem-oriented approach to the ocean, designed by the UN Conference on Environment and Development (UNCED).These considerations lead to identify three cardinal needs. First, the need to try lessons from the Regional Seas Programme of the United Nations Environment Programme (UNEP) by carrying out a critical analysis of the conceptual background and methodological endowment which it was based on, and of the subsequent political outcomes. Secondly, an increasing need for scientific approaches supported by the consideration of the ocean as a bi-modular system consisting in ecosystems and organisational patterns, being both modules subject to the impacts from global change and globalisation. Thirdly, a need to design and operate a more effective interaction between science and policy, and, as far as science in itself is concerned, the need to design a more epistemologically-sound interaction between natural and social disciplines.Moving from this discussion, it is proposed to distinguish the mere ocean area, where the organisational patterns have not yet created a real ocean system, from the ocean region, which differently has acquired the features of an ocean system. These two kinds of spaces may be found in the coastal milieu, extending up to the outer edge of the continental margin, in the deep-ocean, extending seawards from the continental margin, or they may extend across the continental margin and the deep-ocean. Where it is agreed that ocean reality may evolve on the basis of these two reference patterns, the following sequence of conceptual mises-au-point and statements may be considered.

• The ocean area—This kind of ocean space may solidify in those areas where the ocean is frequented and used in the traditional ways without benefiting from a well-designed organisational pattern. Human presence and resource uses have brought about spatial differentiation but not such a real cohesion which may be only achieved by adopting an organisational plan.

• The ocean region—This occurs only where an ocean area is endowed with such an organisational framework that allows the pursuit of clearly pre-determined objectives in terms of environmental, resource management, and economic development. This is the product of an extensive human interaction with the ocean ecosystem, and of a substantial political approach to the ocean milieu.

Where it is agreed that ocean reality may evolve on the basis of these two reference patterns, the following sequence of conceptual mises-au-point and statements may be considered:

• The ocean region and regional strategy—At the present time, ocean regions may be found only in quite limited parts of the ocean world.

• The final objective—Where the decision-making centres conform their programmes and actions to the principles and guidelines from UNCED, the objective of each ocean region should be the pursuit of sustainable development on the regional scale.

• Sustainable region—This occurs where the regional organisation is primarily based on the protection of the ecosystem integrity, where economic development operates through the optimisation of resource usage, and where social equity, including the access to the natural and cultural heritage of the ocean environment, is guaranteed.

• Ocean regionalisation—When an individual ocean is subject to the organisational forces that lead to the creation of regions, it can be stated that an ocean regionalisation has occurred.

• Global change—Ocean regionalisation should be viewed as one of the most important consequences of the global, environmental and social change that characterises the present phase of society.

• Globalisation—The setting up of a transport and communication global network, together with the associated establishment of global production and consume patterns, of market strategies and social behaviour, may be regarded as the cardinal set of socio-economic factors, which ocean regionalisation is going to increasingly depend on.

• Enlargement of the geographical approach—The development of ocean regions encourages to set up effective inter-disciplinary approaches, that primarily should focus on: (i) the consistency of the regional organisation with the regional objectives; (ii) the consistency of the ocean resource use with the protection of the ecosystem, primarily the safeguard of its biodiversity, productivity and resilience; and (iii) the configuration and functions of the decision-making system in guiding regional organisation.

• Ocean region and ecosystem—The most desirable conditions in terms of optimal ocean organisation on the regional scale occur where the spatial extent of the ocean, which is encompassed by an individual regional management programme, fully coincides with the spatial extent of an ocean ecosystem, or with a set of contiguous ecosystems.

• Decision-making systems—The more the co-operative process between decision-making systems operating in contiguous ocean regions develops, the greater the potential for a holistic political approach to the oceans becomes. The spatial consequences deriving from the interaction between the decision-making centres are of peculiar interest.

东海沿岸海区垂直环流及其温盐结构动力过程研究I．环流的基本特征

利用三维斜压流体动力学模型 ,通过对东海沿岸海区冬、夏季的斜压环流及其温盐结构的数值研究 ,揭示研究海区垂直环流及其温盐结构的动力过程及其成因。垂直环流的模拟结果表明 :冬季 ,沿岸海区的垂直环流以逆时针流动 ,近表层为向岸流 ,沿岸为下降流 ,近表层以下为离岸流 ,其在外海有明显的上升趋势 ,沿岸下降流自表层至底层逐渐由强变弱 ;夏季 ,沿岸海区的垂直环流以顺时针流动 ,近表层以下为向岸流 ,沿岸为上升流 ,近表层为离岸流 ,其在外海有明显的下降趋势 ,沿岸上升流自底层至表层逐渐由弱变强。就整个沿岸海区而论 ,冬季沿岸下降流和夏季沿岸上升流的强度都随着岸界地形坡度、风速及风向与岸线偏角的变化而变化。沿岸下降流形成的主要原因是由于冬季东北风与岸界地形的耦合效应及海区温盐分布不均匀所致 ,而沿岸上升流形成的主要原因则是由于夏季西南风与岸界地形的耦合效应及海区温盐分布不均匀所致。     

The response of a hydrofoil to wave orbital velocity fields

For study purposes, a simplified model of a hydrofoil craft is constructed with the assumption that it has:

• heave only motion;

• no surface proximity effect on the foil;

• no foil broaching.

It is then shown that a fully submerged hydrofoil, mounted at the bottom of rigid struts, can transmit large vertical force fluctuations to the hull, even in an idealized sinusoidal seaway because of the orbital velocity field in the water. But if the foil support struts are hinged, inclined aft and resiliently supported, so that the hydrofoil can swing about the strut's pivot in response to the changes in local water velocity, then the vertical accelerations transmitted to the hull are reduced. The more the strut is inclined to the vertical, the smaller are the accelerations transmitted to the hull. A hinged strut whose equilibrium angle (for 1 g) is 60° to the vertical can reduce the vertical accelerations by an order of magnitude. It also has two other practical advantages. The strut(s) and foil will ride up towards horizontal during the rare but inevitable impacts with large marine objects (such as whales or flotsam) and during groundings. And when they encounter a region of water moving rapidly downward (which can cause a conventional fully submerged hydrofoil to experience a violent hull impact on the water) they move in such a way as to maintain a roughly constant lift force on the hull, so that there is negligible hull motion in heave.     

Mooring optimization of floating platforms using a genetic algorithm

This paper presents a new procedure for the optimization of the mooring design of floating platforms, in which an automatic design sequence is also established. Regarding the optimization philosophy, the following aspects are dealt with:

• The optimization of the platform heading and its mooring pattern, taking into account the environmental force spreading;

• optimum line length or line tension for each mooring line, associated to the optimization of the mooring line materials and sizes.

Basically, the main goal of this paper is to introduce a new method, which will provide the quickest way to find the best mooring system, defined here as that which minimizes platform responses.A genetic algorithm (GA) is applied in this contribution, and this paper describes exactly the procedure of developing a GA code directed toward the solution of mooring design optimization problems. In order to prove the efficiency and the vast potential of the proposed algorithm as a design tool, sample moorings are analyzed for different environmental conditions and the final results, including the time required to run them, are presented.     

Video cathodoluminescence microscopy of diagenetic cements and its applications

Diagenetic studies of sedimentary rocks using cold cathodoluminescence (CL) microscopy have been severely limited because of the very low intensity of visible luminescent emissions, which required long photographic exposure times, and because of the difficulty in obtaining quantitative data from CL observations. The solution to this problem is to fit the microscope with a high-sensitivity digital colour video camera linked to a computer with image enhancement and image analysis software. The new technique described in this paper:

• produces digital CL images of consistent high quality, both quickly and cheaply;

• controls the capture and editing of CL images, to reveal detailed textural information even from minerals exhibiting low level luminescence such as quartz;

• acquires quantitative information on pore systems and the abundance of cement zones from CL images.

Examples are presented to demonstrate the high quality of images produced in this way and the range of uses to which the new technique can be applied. The ability to image exactly the same field of view in both plane polarised light and CL is a particular advantage. Image analysis techniques have been developed to give a statistical characterisation of both pore systems and cement phases that infill them. The abundance of contrasting cement zones seen in CL can be measured and the abundance of cement phases can be mapped across a reservoir. The history of porosity occlusion can be reconstructed quantitatively and integrated with burial history and hydrocarbon migration. Porosity can be measured accurately and, since the technique obtains data on pore geometry as well as abundance, the pore system can be characterised by pore size distributions and pseudo-capillary pressure curves. This also opens the possibility of estimating permeability from thin sections.     

CO2 outgassing off central Chile (31–30°S) and northern Chile (24–23°S) during austral summer 1997: the effect of wind intensity on the upwelling and ventilation of CO2-rich waters

The distribution of pH and alkalinity has been used to calculate the distribution of total inorganic carbon (TC) and fugacity of carbon dioxide (fCO₂) in the upper 200 m of the water column in coastal upwelling areas off northern Chile (23–24°S, near Antofagasta) and central Chile (30–31°S, near Coquimbo) during austral summer 1997. In these upwelling areas, colder surface waters were oxygen poor and strongly CO₂ supersaturated (100% near Antofagasta and 200% near Coquimbo), although below the pycnocline the CO₂ supersaturation invariably exceeded 200% in both areas. The larger surface CO₂ supersaturation and outgassing at 30°S were associated with stronger winds that promoted the upwelling of denser water (richer in CO₂) as well as a higher air–sea CO₂ transfer velocity. The consistent decrease in intensity of the southerly winds (as derived from NSCAT scatterometer data) from 30–31°S to 23–24°S suggests a corresponding decline in the intensity of the CO₂ outgassing due to upwelling. Additionally, we suggest here that the intensity of the local upwelling forcing (i.e. alongshore–equatorward winds) plays a role in determining the water mass composition and phytoplankton biomass of the coastal waters. Thus, while deep upwelling of salty and cold water resulted in high fCO₂ (up to 1000 μatm) and very low phytoplankton biomass (chlorophyll a concentration lower than 0.5 mg m⁻³), the shallow upwelling of less salty (e.g. salinity <34.5) and less CO₂-supersaturated water resulted in a higher phytoplankton biomass, which further reduced surface water fCO₂ by photosynthesis.     

Polyester mooring for the Mad Dog spar—design issues and other considerations

The Mad dog project will use a polyester mooring system on the drilling and production truss spar. This will be the first use of a permanent polyester mooring system on a Floating Production System (FPS) outside of Brazil and the first time polyester has been used on a spar. As such, there were many challenges, which include:

• Regulatory approval.

• Designing a mooring system which is dominated by current loadings.

• Largest polyester rope break load ever required.

• Rope design qualification and testing.

• Quality control and assurance.

• Inspection, Maintenance, Repair and Retirement (IMRR) of such a mooring system.

This paper will focus on the design issues, rope design, manufacturing, qualification and testing, and the IMRR plan that was developed to provide the assurance that the polyester mooring system could be safely operated over the 20-year field life.     

Inter-annual variability in larval supply to populations of three invertebrate taxa in the northern California Current

We investigated sources of inter-annual variability in larval supply to crab and sea urchin populations at Bodega Head and Point Reyes in northern California. During the spring and summer upwelling seasons of the years 1992 through 1997 we monitored the weekly settlement rates of nine species of crabs and two species of sea urchins. As observed in previous studies, daily values of alongshore windstress, temperature and salinity provided evidence for the poleward flow of relatively warm, low salinity water from south of Point Reyes, an apparent retention zone, during upwelling relaxation events. In years dominated by these events (1992, 1993, 1995 and 1996) we observed that alongshore windstress, temperature and salinity were coherent and temperature was significantly correlated with cancrid crab settlement. During these years the magnitude of cancrid crab settlement and the fraction of cancrid crabs relative to other crab species settling were high. Over four years of concurrent sampling there was consistently greater cancrid crab settlement at the Point Reyes site, within the retention zone, than at Bodega Head. Settlement of non-cancrid crabs (porcellanids, grapsids, pagurids and majids) was not as closely linked to intra-annual patterns of upwelling and relaxation, possibly due to the shorter seasonal availability of larvae allowing for the influence of fewer relaxation events. Settlement of this group among years was positively correlated with environmental indicators of strong seasonal upwelling; high salinity, Bakun upwelling index and low temperature. Sea urchin settlement events were observed in June and July of 1992, 1994 and 1997 during warming periods when salinity and temperature were increasing and alongshore windstress was low. Across the six years of the study, we found that cancrid crab larvae had a more even seasonal availability than larvae of non-cancrid species, which settled in greatest numbers during the early portion of the upwelling season. Sea urchins settled in greatest numbers during the later part of the upwelling season. Together these patterns demonstrate the taxon-specific way that inter-annual variability in larval supply is forced by the coincidence of larval availability with favorable physical transport mechanisms.     

Grazing impact of mesozooplankton in an upwelling region off northern California, 2000–2003

Mesozooplankton (>200 μm) grazing impact (% phytoplankton standing crop consumed d⁻¹) was determined by the gut fluorescence method during three springs (2000, 2001 and 2002) and two winters (2002 and 2003) in a coastal upwelling region off northern California. Wind events, in terms of both magnitude and duration, varied inter-annually and seasonally and included both upwelling-favorable and relaxation events. Grazing impact of mesozooplankton also varied inter-annually and seasonally, and was highest during June 2000 (mean=129% of standing crop d⁻¹), a prolonged period of wind “relaxation” and phytoplankton bloom. In contrast, mean grazing impact was lower during periods of stronger, more persistent winds, more active upwelling, greater cross-shelf transport, and lower chlorophyll concentration (25% and 38% in May–June 2001 and 2002, respectively). Wintertime conditions (January 2002 and 2003) were characterized by weakly upwelling or downwelling-favorable winds, low chlorophyll concentration, and lower mean mesozooplankton grazing impact (13% and 12%, respectively). The larger (>500 μm) size class contributed proportionally more to total mesozooplankton (>200 μm) grazing impact than the smaller (200–500 μm) size class during all sampling periods except spring 2002. These results suggest that mesozooplankton grazing impact is higher in spring than in winter, and that during the spring upwelling season, grazing is higher during periods of wind relaxation (weak upwelling) than during periods of stronger upwelling. Further, these results suggest an important role of mesozooplankton grazers on phytoplankton dynamics in the upwelling region off northern California.     

Settlement patterns,size and growth of puerulus and juvenile rock lobster Jasus lalandii at an oyster farm in Saldanha Bay,South Africa

Trends in the abundance, size and growth of newly settled pueruli and early juveniles of the rock lobster Jasus lalandii were determined from samples obtained from mesh bags used for oyster mariculture in Saldanha Bay, South Africa. Totals of 3 842 lobster pueruli and 10 158 juveniles were captured between August 2005 and June 2006, and distinct puerulus settlement events were identified in November and January. Juveniles also peaked during these months as well as in March. The size of newly settled pueruli increased significantly between October and March. The carapace length (CL) of pueruli and juveniles overlapped between 8.5 mm (smallest juvenile) and 10.4 mm (largest puerulus), and 50% of pueruli moulted into juveniles by 9.5–9.6 mm CL. There was a size progression in juveniles, with larger individuals in February and March reflecting an accumulation of juveniles from the earlier settlement events. The estimated growth increment from puerulus to first juvenile instar was 0.5–1.5 mm (4.9–16.1%), increasing to 1.4–2.5 mm (14.4–23.3%) for the second instar. Recruitment of J. lalandii pueruli occurred during periods when strong southerly winds prevailed, which results in a general offshore movement of surface waters along the West Coast and upwelling of cold nutrient-rich bottom waters. However, specific settlement events appeared to coincide with a change in wind direction, reduction in upwelling strength and increase in surface water temperatures, implying that larvae may take advantage of landward movements of surface waters during downwelling, or reversal of upwelling, to return them to shallow coastal waters.     

Impact of El Niño events and climate regime shift on living resources in the western North Pacific

Features of El Niño events and their biological impacts in the western North Pacific are reviewed, focusing on interactions between ENSO and the East Asian monsoon. Impacts of El Niño on the climate in the Far East become evident as ‘cool summers and warm winters’. Effects of climate regime shift on ENSO activities, western boundary currents and upper-ocean stratification, as well as their biological consequences are summarized. These have been:

1. In the western equatorial Pacific, an eastward extension of the warm pool associated with El Niño events induces an eastward shift of main fishing grounds of skip jack and big eye tunas.

2. The surface salinity front in the North Equatorial Current region retreats southward, associated with El Niño events. This leads to a southward shift of the spawning ground of Japanese eel, which is responsible for a reduction in the transport of the larval eels to the Kuroshio and Japanese coastal region, causing poor recruitment.

3. Intensification of winter cooling and vertical mixing associated with La Niña (El Niño) events in the northern subtropical region of the western (central) North Pacific reduces surface chlorophyll concentration levels and larval feeding condition for both Japanese sardines and the autumn cohort of Neon squid during winter–early spring. The semi-decadal scale calm winter that occurred during the early 1970s triggered the first sharp increase of sardine stock around Japan.

4. A remarkable weakening of southward intrusion of the Oyashio off the east coast of Japan during 1988–91, resulted in a decrease in chlorophyll concentrations and mesozooplankton biomass in late spring–early summer of the Kuroshio-Oyashio transition region. Changes occurred in the dominant species of small pelagic fish, through successive recruitment failures of Japanese sardine.

Off-shelf fluxes of labile materials by an upwelling filament in the NW Iberian Upwelling System

Daily changes in the concentrations of carbon and nitrogen species were monitored during the course of a Lagrangian drifter experiment in a recurrent upwelling filament south of Cape Finisterre (NW Iberian Upwelling System). A drifting buoy released at the southern edge of the upwelling centre generated by the Cape moved 60km southwestwards from 3 to 7 August 1998. Organic matter in the 50m deep study water mass (average 77±2 μM C) consisted of: 57μM C of dissolved organic matter (DOM) with a C/N molar ratio of 19±2; 6μM C of DOM with a C/N ratio of 9±2, and 14μM C of 50% DOM and 50% suspended organic matter (POM_susp) with a C/N ratio of 6.0±0.4.Net conversion of consumed inorganic salts into accumulated TOM=POM_susp+DOM was 40% for nitrogen and 30% for carbon. Since the parcel of water crossed the shelf-edge, these conversion efficiencies are equivalent to net horizontal export-ratio of 0.4 and 0.3 respectively. A second drifter was deployed in the offshore-end of the filament, and was displaced 20km west between 14 and 17 August 1998. Nitrate was exhausted in the surface water and no significant changes were observed in the variables measured during the course of the second experiment. Low C/N ratios (6.5±0.4) and rapid POM_susp/DOM inter-conversion in the 20 μM C excess observed in the study volume points to the persistence of the labile materials formed on the shelf during transport to the ocean. Our data demonstrate

a) the key role of upwelling filaments in off-shelf export of organic materials and

b) the major contribution of DOM to this horizontal export, a previously unaccounted amount.

The high nitrogen content of the materials exported make them attractive organic substrates for use by microbial populations in the adjacent oligotrophic ocean.     

Zooplankton distribution and cross-shelf transfer of carbon in an area of complex mesoscale circulation in the northern California Current

We conducted a research cruise in late summer (July–August) 2000 to study the effect of mesoscale circulation features on zooplankton distributions in the coastal upwelling ecosystem of the northern California Current. Our study area was in a region of complex coastline and bottom topography between Newport, Oregon (44.7°N), and Crescent City, California (41.9°N). Winds were generally strong and equatorward for >6 weeks prior to the cruise, resulting in the upwelling of cold, nutrient-rich water along the coast and an alongshore upwelling jet. In the northern part of the study area, the jet followed the bottom topography, creating a broad, retentive area nearshore over a submarine shelf bank (Heceta Bank, 44–44.4°N). In the south, a meander of the jet extended seaward off of Cape Blanco (42.8°N), resulting in the displacement of coastal water and the associated coastal taxa to >100 km off the continental shelf. Zooplankton biomass was high both over the submarine bank and offshore in the meander of the upwelling jet. We used velocities and standing stocks of plankton in the upper 100 m to estimate that 1×10⁶ m³ of water, containing an average zooplankton biomass of ～20 mg carbon m^?3, was transported seaward across the 2000-m isobath in the meandering jet each second. That flux equated to offshore transport of >900 metric tons of carbon each day, and 4–5×10⁴ tons over the 6–8 week lifetime of the circulation feature. Thus, mesoscale circulation can create disparate regions in which zooplankton populations are retained over the shelf and biomass can accumulate or, alternatively, in which high biomass is advected offshore to the oligotrophic deep sea.