Eastern Boundary Current Systems Research Articles

Decadal-scale Trends Across Several Marine Trophic Levels in the Southern Benguela Upwelling System off South Africa

Abstract Whereas zooplankton in most eastern boundary current systems (EBCSs) studied to date has declined in terms of biomass over the past 5 decades, it has increased 100-fold in terms of numerical abundance in the southern Benguela region, thereby undergoing a displacement in species composition through time. These long-term changes in coastal zooplankton off South Africa have previously been hypothesized to be mediated by both “bottom-up” and “top-down” control mechanisms, in contrast to other EBCSs and marine ecosystems in general which have traditionally been viewed to be structured from below. From a “bottom-up” perspective, the increase in zooplankton abundance is a long-term biological response to intensified coastal upwelling, common to EBCSs, and its associated processes of nutrient enrichment and increased phytoplankton production and biomass. On the other hand, such increase in zooplankton and the accompanying shift in its community structure are thought to result from a “cascading” trophic e...

Primary production and community respiration in the Humboldt Current System off Chile and associated oceanic areas

The high biological productivity of the Humboldt Current System (HCS) off Chile supports an annual fish catch of over 7 million t. The area is also important biogeochemically, because the outgassing of recently upwelled water is modulated by contrasting degrees of biological activity. However, very few field measurements of primary production and planktonic respiration have been undertaken within the Eastern Boundary Current (EBC) system off Chile. In this study an estimate of primary production (PP) and surface planktonic community respiration is presented from several research cruises in the HCS and adjacent oceanic areas. The highest production levels were found near the coast correlating closely with known upwelling areas. Both gross primary production (GPP) and community respiration (CR) showed important spatial and temporal fluctuations. The highest water column integrated GPP was measured in the southern and central fishing area (19.9 g C m -2 d -1 ) and off the Antofagasta upwelling ecosystem (9.3 g C m -2 d -1 ). The range of GPP agrees well with values reported for Peru (0.05 to 11.7 g C m -2 d -1 ). The Coquimbo upwelling system, despite being an area of persistent upwelling, showed lower production and community respiration values than the Antofagasta and Concepcion Shelf areas.The lowest surface PP values were measured within the oceanic region adjacent to the northern coastal upwelling zones (0.8 ± 0.5 μg C l -1 h -1 ), though slightly enhanced biological production was found within the Nazca Ridge (1.5 ± 2.1 μg C l -1 h -1 ) that separates the Chilean and Peruvian deep basins.

The influence of the Benguela upwelling system on Namibia's marine biodiversity

Namibia's marine environment falls within the Benguela system, an eastern boundary current upwelling system in the south eastern Atlantic Ocean. Conditions within much of this environment change continuously as a consequence of the upwelling of nutrient-rich water into the surface zone. In addition, irregular anomalies in temperature, oxygen concentration and salinity occur, particularly in shelf waters. These fluctuations, which are inherent in the functioning of the Benguela system, tend to favour the persistence of few, generalist species, while at the same time high productivity supports large abundances. This trend is evident in all the major marine habitats off Namibia, where diversity is often lower than in comparable habitats in the southern Benguela system off the west coast of South Africa. Namibia's marine environment is considered 'relatively pristine', although threats to biodiversity are posed by both natural and anthropogenic phenomena.

A numerical study of the effects of wind forcing on the Chile Current System

A high-resolution, multi-level, primitive equation ocean model is used to examine the response of the coastal region from 22.5°S to 35°S of the Chile Current System to both equatorward and climatological wind forcing. The results from both types of forcing show that an equatorward surface current, a poleward undercurrent, upwelling, meanders, filaments and eddies develop in response to the predominant equatorward wind forcing. When climatological wind forcing is used, an offshore branch of the equatorward surface current is also generated. These features are consistent with available observations of the Chile Current System. The model results support the hypothesis that wind forcing is an important mechanism for generating currents, eddies and filaments in the Chile eastern boundary current system and in other eastern boundary current regions which have predominantly equatorward wind forcing.

Currents in the deep ocean off Chile (30°S)

Results are reported from the first recording current meter observations in the eastern boundary current system off Chile. Currents at 100 m and 3400 m were observed for a 4–6 month period during the austral spring-summer-fall period of 1991–1992 at a deep ocean site 150 km off the Chilean coast and 70 km seaward of the axis of the Peru-Chile Trench. Results indicate energetic inertial oscillations, low eddy kinetic energy at 3400 m, southward flow, decreasing from austral spring to summer, at 100 m and 3400 m, and quite strong eastward flow, increasing from spring to summer, at 100 m. The latter result may help to explain satellite images, showing the zone of high biological production near the coast in the study region to be very narrow during the austral summer despite strong coastal upwelling in that season. The authors current observations, local geostrophic current profiles and abyssal potential temperature distributions indicate a five-layer structure for the alongshore flow in the eastern boundary current system off Chile: northward flow above 100 m, between about 400 m and 1700 m (maximum around 600–700 m, northward transport of Antarctic Intermediate Water) and below 3400 m; southward flow from above 100 m to about 400 m (maximum at 180 m, Peru-Chile Undercurrent) and from about 1700 m to below 3400. This proposed vertical structure is consistent with temperature, salinity, dissolved oxygen and nutrient distributions of the study area and bears resemblance to eastern boundary current simulations ( McCreary et al., 1987).

Upper trophic level predators indicate interannual negative and positive anomalies in the California Current food web

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 118:69-79 (1995) - doi:10.3354/meps118069 Upper trophic level predators indicate interannual negative and positive anomalies in the California Current food web Ainley, D. G., Sydeman, W. J., Norton, J. We used a 21 yr time series of productivity for 6 seabird species nesting in large numbers at the Farallon Islands, 40 km offshore of San Francisco, California, USA, to assess proximate and remote factors leading to variation in the food supplies available to these predators. The latter sampled prey throughout a 3200 km2 area. Depending on foraging ecology and reproductive capacity, some species were more sensitive to food web perturbation than others. A serious lack of food was indicated by negative reproductive anomalies during all warm-water events, some of which were classified as tropical El Niño and others which were not. Equally spectacular but positive anomalies occurred during years adjacent to the negative ones, particularly evident among the most sensitive species. Much of the annual variation, positive or negative, in seabird reproductive success was explained by variation in the Southern Oscillation and/or the Aleutian low pressure system, both of which affect sea-surface temperature and thermocline depth off California. Results indicate that perturbations in the marine food web of the California eastern boundary current system, as indicated by the availability of food to seabirds, are much more complex than is generally appreciated, and are not confined only to negative excursions from normalcy. ENSO is important, but other global atmosphere-ocean phenomena affect the California Current just as dramatically. California Current . El Niño . Food web . Interannual variability . La Niña . Seabirds . Aleutian low pressure system Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 118. Publication date: March 09, 1995 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 1995 Inter-Research.

Radiolarian microfauna in the northern California Current System: indicators of multiple processes controlling productivity

Abstract Radiolaria, as other plankton, appear to be highly tuned to specific oceanographic environments. Thus, in a transitional region such as the eastern North Pacific, where many different water masses are mixed, radiolaria provide very sensitive tracers of these water masses and the currents that carry them. We present the first two years of radiolarian results from the Multitracers sediment trap study across the northern California Current System. Three moorings, positioned along a transect at approximately 130, 280 and 650 km from the coast, sample a wide variety of oceanographic conditions both spatially and temporally. Selected species or species groups are presented along with hydrographic data from the region in order to demonstrate the basic trends in the radiolarian data and illustrate their relationships to fluctuations in their physical environment. Multiple linear regression is used to explore the relationship between radiolarian composition and the export of carbon from this system. The most important physical process controlling variability in the radiolarian composition along this transect is attributed to variability in the intensity of the southward-flowing California Current. The seasonality of the California Current is clearly reflected by changes in the composition of the radiolarian trap assemblages; very different species dominate this region in summer as compared to winter. In addition to seasonal trends, evidence in both the offshore and onshore environments suggests significant differences between years. This region appears to have been more strongly influenced by cold, subarctic water during the winter of 1988/1989 than during the previous year. The relationship between radiolarian species abundances and the flux of organic carbon strongly indicates that a number of different oceanographic processes contribute to enhanced productivity at these sites. This has important implications when making inferences from the geological record about past changes in the intensity of upwelling in this eastern boundary current system.

Seasonal variability of alongshore geostrophic velocity off central California

Seasonal variability of alongshore geostrophic velocity relative to 500 dbar is examined from 23 years of hydrographic data along two sections off central California (one off Point Sur and the other off Point Conception). The seasonal cycles are determined by least square fits of the gappy data records to harmonics with annual and semiannual periods. Attention is focused on variability over the outer continental slope. Geostrophic flow in the upper 100 m along both sections is coherent and predominantly annual with equatorward flow from February to September and poleward flow from October to January. The flow deeper than 100 m is distinctly different along the two sections. The nearshore deep flow is predominantly semiannual off Point Conception (notably different from surface flow) and poleward all year with maxima in December and June. This semiannual variability at depth is a prevalent feature of the California Current system. The nearshore deep flow off Point Sur is unusual in that it is predominantly annual with maximum poleward flow in December and weak equatorward flow from March to May. The seasonal variations over the continental slope compare favorably with existing models of eastern boundary current systems in two respects: (1) The surface flow leads the predominantly annual wind forcing in this region by about one month; and (2) the deep poleward counterflow is coherent with the local poleward barotropic pressure gradient at both locations, with a phase lag of approximately 2 months. However, contrary to the results of existing dynamical models, the semiannual poleward pressure gradient and undercurrent off Point Conception appear to be unrelated to the wind forcing, which is predominantly annual in this region. These semiannual variations are highly coupled to the semiannual flow throughout the water column inside the Southern California Bight, suggesting that they may be topographically generated.

Phosphate regeneration from sediments of the Peru continental margin by dissolution of fish debris

Generally, oxidative regeneration of phosphate from anoxic sediments is by microbially mediated sulfate reduction processes. Stoichiometric modelling of such reactions takes into consideration varying proportions of ‘decomposable’ organically bound P to account for the ratios among nutrients in depth-concentration profiles of near-surface sediments. New results of interstitial water composition from sediments underlying the water masses influenced by coastal upwelling of the eastern boundary current system off Peru indicate that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound P. Dissolved interstitial phosphate from fish debris is revealed by an excess amount of phosphate over that predicted from a simple stoichiometric oxidative regeneration model and by anomalously high dissolved interstitial fluoride concentrations. Phosphate flux estimates based on diffusion from the sediment suggest that this mechanism may generate up to 10% of the nutrient pool in the waters of the Peru undercurrent. Partitioning of P among the two sources reveals further that fish debris phosphate is about four times more important than organically bound P in nutrient generation from sediments of the Peru continental margin. Not only does this mechanism of regeneration affect the nutrient cycling but may also control widespread phosphorite formation in this area.

The glacial eastern boundary current along the Atlantic Eurafrican continental margin

Abstract The paleoceanography of the Bay of Biscay and of the northeast Atlantic continental margin has been reconstructed for the last glacial maximum (18,000 years ago); it is compared to the modern interglacial situation. Planktonic foraminiferal assemblages in glacial and modern sediment samples have been used to infer glacial and interglacial water mass distributions and to estimate the winter and summer sea surface temperatures. Eighteen thousand years ago, the temperate surface water masses of the modern Bay of Biscay were replaced by polar water masses (3 to 4°C) during the winter, but temperature to transitional (8 to 10°C) ones invaded the center of the day during the summer. The water masses in the Bay of Biscay and above the Ibero-Moroccan continental margin have been characterized by the steep temperature gradients across the eastern boundary current system. The latter transported surface waters, which were up to 10dgC cooler than the water masses of the central North Atlantic gyre, southwards along the continental edge.