Upwelling Of Nutrient-rich Deep Water Research Articles

The importance of the dissolved organic matter pool for the carbon sequestration potential of artificial upwelling

In the face of climate change there is a need to reduce atmospheric CO2 concentrations. Artificial upwelling of nutrient-rich deep waters has been proposed as a method to enhance the biological carbon pump in oligotrophic oceanic regions in order to increase carbon sequestration. Here we examine the effect of different artificial upwelling intensities and modes (single pulse versus recurring pulses) on the dynamics of the dissolved organic matter pool (DOM). We introduced nutrient-rich deep water to large scale mesocosms (~44 m3) in the oligotrophic subtropical North Atlantic and found that artificial upwelling strongly increased DOM concentrations and changed its characteristics. The magnitude of the observed changes was related to the upwelling intensity: more intense treatments led to higher accumulation of dissolved organic carbon (>70 μM of excess DOC over ambient waters for the most intense) and to comparatively stronger changes in DOM characteristics (increased proportions of chromophoric DOM (CDOM) and humic-like fluorescent DOM), suggesting a transformation of the DOM pool at the molecular level. Moreover, the single upwelling pulse resulted in higher CDOM quantities with higher molecular weight than the recurring upwelling mode. Together, our results indicate that under artificial upwelling, large DOM pools may accumulate in the surface ocean without being remineralized in the short-term. Possible reasons for this persistence could be a combination of the molecular diversification of DOM due to microbial reworking, nutrient limitation and reduced metabolic capabilities of the prokaryotic communities within the mesocosms. Our study demonstrates the importance of the DOC pool when assessing the carbon sequestration potential of artificial upwelling.

Distribution and biomass of gelatinous zooplankton in relation to an oxygen minimum zone and a shallow seamount in the Eastern Tropical North Atlantic Ocean

Physical and topographic characteristics can structure pelagic habitats and affect the plankton community composition. For example, oxygen minimum zones (OMZs) are expected to lead to a habitat compression for species with a high oxygen demand, while upwelling of nutrient-rich deep water at seamounts can locally increase productivity, especially in oligotrophic oceanic waters. Here we investigate the response of the gelatinous zooplankton (GZ) assemblage and biomass to differing oxygen conditions and to a seamount in the Eastern Tropical North Atlantic (ETNA) around the Cape Verde archipelago. A total of 16 GZ taxa (>1100 specimens) were found in the upper 1000 m with distinct species-specific differences, such as the absence of deep-living species Atolla wyvillei and Periphylla periphylla above the shallow seamount summit. Statistical analyses considering the most prominent groups, present at all stations, namely Beroe spp., hydromedusae (including Zygocanna vagans, Halicreas minimum, Colobonema sericeum, Solmissus spp.) and total GZ, showed a strong positive correlation of abundance with temperature for all groups, whereas oxygen had a weak negative correlation only with abundances of Beroe spp. and hydromedusae. To account for size differences between species, we established length-weight regressions and investigated total GZ biomass changes in relation to physical (OMZ) and topographic characteristics. The highest GZ biomass was observed at depths of lowest oxygen concentrations and deepest depth strata at the southeastern flank of the seamount and at two stations south of the Cape Verde archipelago. Our data suggest that, irrespective of their patchy distribution, GZ organisms are ubiquitous food web members of the ETNA, and their habitat includes waters of low oxygen content.

Multiple controls on the paleoenvironment of the early Cambrian black shale-chert in the northwest Tarim Basin, NW China: Trace element, iron speciation and Mo isotopic evidence

Research on early Cambrian sedimentary successions of different continental margins are important in better understanding the redox heterogeneity of the early Cambrian ocean, and further the evolutionary relationship between the early animals and their environment. In the present study, we carried out trace element, iron speciation, and Mo isotope analysis of the basal Cambrian Yurtus black shales and cherts in the inner ramp Shiairike section of the northwest Tarim Basin. Our integrated data, combined with previously published data of correlative units in this region, indicate strong basin restriction during deposition in the inner ramp area, with more severe restriction for the upper part of the studied unit. Our investigation supports strong upwelling of nutrient-rich deep water and high primary productivity, with predominately anoxic bottom waters during deposition of the rocks. The anoxia in bottom waters and preservation of organic carbon were likely maintained by both high primary productivity and bottom water stagnancy. Despite the overall anoxic bottom water condition, intermittent oxic intervals may have existed; and the variabilities in redox conditions and related Mn and/or Fe cycling in combination with low and variable [H2S]aq in bottom waters may have attributed to the highly variable δ98Mo values in the studied unit. Furthermore, a high δ98Mo value of 2.14‰ documented in the lower part of the unit is indicative of high δ98Mo of contemporary seawater, pointing to a relatively oxygenated ocean with limited seafloor anoxia.

Nitrogen loss processes in response to upwelling in a Peruvian coastal setting dominated by denitrification – a mesocosm approach

Abstract. Upwelling of nutrient-rich deep waters make eastern boundary upwelling systems (EBUSs), such as the Humboldt Current system, hot spots of marine productivity. Associated settling of organic matter to depth and consecutive aerobic decomposition results in large subsurface water volumes being oxygen depleted. Under these circumstances, organic matter remineralisation can continue via denitrification, which represents a major loss pathway for bioavailable nitrogen. Additionally, anaerobic ammonium oxidation can remove significant amounts of nitrogen in these areas. Here we assess the interplay of suboxic water upwelling and nitrogen cycling in a manipulative offshore mesocosm experiment. Measured denitrification rates in incubations with water from the oxygen-depleted bottom layer of the mesocosms (via 15N label incubations) mostly ranged between 5.5 and 20 nmol N2 L−1 h−1 (interquartile range), reaching up to 80 nmol N2 L−1 h−1. However, actual in situ rates in the mesocosms, estimated via Michaelis–Menten kinetic scaling, did most likely not exceed 0.2–4.2 nmol N2 L−1 h−1 (interquartile range) due to substrate limitation. In the surrounding Pacific, measured denitrification rates were similar, although indications of substrate limitation were detected only once. In contrast, anammox (anaerobic ammonium oxidation) made only a minor contribution to the overall nitrogen loss when encountered in both the mesocosms and the Pacific Ocean. This was potentially related to organic matter C / N stoichiometry and/or process-specific oxygen and hydrogen sulfide sensitivities. Over the first 38 d of the experiment, total nitrogen loss calculated from in situ rates of denitrification and anammox was comparable to estimates from a full nitrogen budget in the mesocosms and ranged between ∼ 1 and 5.5 µmol N L−1. This represents up to ∼ 20 % of the initially bioavailable inorganic and organic nitrogen standing stocks. Interestingly, this loss is comparable to the total amount of particulate organic nitrogen that was exported into the sediment traps at the bottom of the mesocosms at about 20 m depth. Altogether, this suggests that a significant portion, if not the majority of nitrogen that could be exported to depth, is already lost, i.e. converted to N2 in a relatively shallow layer of the surface ocean, provided that there are oxygen-deficient conditions like those during coastal upwelling in our study. Published data for primary productivity and nitrogen loss in all EBUSs reinforce such conclusion.

The mid-Tournaisian (Early Carboniferous) anoxic event in the Laurussian shelf basin (Poland): An integrative approach

A wide range of proxies (organic and inorganic geochemistry, gamma-ray spectrometry, microfacies, framboidal pyrite analysis) were applied in order to decipher changes in depositional conditions during the mid-Tournaisian anoxic event, referred to as the Lower Alum Shale Event (LASE) in the Polish part of the Laurussia basin. The LASE part of the section, unlike older Tournaisian deposits, is characterised by high concentrations of isorenieratane, palaeorenieratane, and their diagenetic derivatives, the occurrence of 2-methyl-3-iso-butyl-maleimides, and high values of the sterane-to-hopane ratio. The predominance of tiny pyrite framboids, inorganic data such as low values of the Th/U and high values of the Corg/P and V/Cr ratios, and generally higher enrichments of redox-sensitive metals such as U, V, Mo, Pb, and Zn and depletion in Mn, all suggest anoxic/euxinic conditions on the sea-bottom. The results of our investigations, especially Co, Mn, Cd, and Mo distribution patterns, demonstrate that enhanced primary productivity in the water column was responsible for a high degree of organic carbon burial rather than preservation in restricted conditions on the seafloor. This elevated productivity may have been generated by the upwelling of nutrient-rich deep water, an increase in volcanic activity and volcanogenic oceanic fertilisation, or both. Volcanically driven mid-Tournaisian climatic changes (warming), as manifested by high Zr concentrations and Hg anomalies, may have been responsible for the deglaciation of Gondwana resulting in the global crenulata transgression.The global rise in sea level and high level of organic productivity were responsible for the development of anoxic conditions and the halt of carbonate production, as reflected by drastic changes of facies not only in the sections investigated, but in many other parts of the world as well.

Metabolic Responses of Subtropical Microplankton After a Simulated Deep-Water Upwelling Event Suggest a Possible Dominance of Mixotrophy Under Increasing CO2 Levels

In the autumn of 2014, nine large mesocosms were deployed in the oligotrophic subtropical North-Atlantic coastal waters off Gran Canaria (Spain). Their deployment was designed to address the acidification effects of CO2 levels from 400 to 1,400 μatm, on a plankton community experiencing upwelling of nutrient-rich deep water. Among other parameters, chlorophyll a (chl-a), potential respiration (Φ), and biomass in terms of particulate protein (B) were measured in the microplankton community (0.7–50.0 μm) during an oligotrophic phase (Phase I), a phytoplankton-bloom phase (Phase II), and a post-bloom phase (Phase III). Here, we explore the use of the Φ/chl-a ratio in monitoring shifts in the microplankton community composition and its metabolism. Φ/chl-a values below 2.5 μL O2 h−1 (μg chl-a)−1 indicated a community dominated by photoautotrophs. When Φ/chl-a ranged higher, between 2.5 and 7.0 μL O2 h−1 (μg chl-a)−1, it indicated a mixed community of phytoplankton, microzooplankton and heterotrophic prokaryotes. When Φ/chl-a rose above 7.0 μL O2 h−1 (μg chl-a)−1, it indicated a community where microzooplankton proliferated (>10.0 μL O2 h−1 (μg chl-a)−1), because heterotrophic dinoflagellates bloomed. The first derivative of B, as a function of time (dB/dt), indicates the rate of protein build-up when positive and the rate of protein loss, when negative. It revealed that the maximum increase in particulate protein (biomass) occurred between 1 and 2 days before the chl-a peak. A day after this peak, the trough revealed the maximum net biomass loss. This analysis did not detect significant changes in particulate protein, neither in Phase I nor in Phase III. Integral analysis of Φ, chl-a and B, over the duration of each phase, for each mesocosm, reflected a positive relationship between Φ and pCO2 during Phase II [α = 230·10−5 μL O2 h−1 L−1 (μatm CO2)−1 (phase-day)−1, R2 = 0.30] and between chl-a and pCO2 during Phase III [α = 100·10−5 μg chl-a L−1 (μ atmCO2)−1 (phase-day)−1, R2 = 0.84]. At the end of Phase II, a harmful algal species (HAS), Vicicitus globosus, bloomed in the high pCO2 mesocosms. In these mesocosms, microzooplankton did not proliferate, and chl-a retention time in the water column increased. In these V. globosus-disrupted communities, the Φ/chl-a ratio [4.1 ± 1.5 μL O2 h−1 (μg chl-a)−1] was more similar to the Φ/chl-a ratio in a mixed plankton community than to a photoautotroph-dominated one.

Climatic and oceanic changes during the Middle-Late Ordovician transition in the Tarim Basin, NW China and implications for the Great Ordovician Biodiversification Event

The Middle-Late Ordovician transition (Darriwilian to Sandbian Age) witnessed a major pulse of the Great Ordovician Biodiversification Event (GOBE) and distinctive oceanic geochemical fluctuations, such as coeval negative C and Sr isotope excursions. In this study, investigations into geochemical variations, notably the Hg abundance (or Hg/TOC), have been carried upon the organic-rich black shale of the Middle-Upper Ordovician Saergan Formation to unravel the causes of this pulse. Based on these data, three phases were identified. Phase 1 (0 to 3 m) is characterized by rising Hg/TOC (up to 138 ppb/wt%) and Ti/Al values as well as high CIAcorr (corrected chemical index of alteration) values (68.9–72.3) with negligible enrichment of redox sensitive elements (RSE) and nutrient elements (e.g. U ≤ 5.2 ppm, V ≤ 153 ppm, Mo ≤ 1.8 ppm, P2O5 ≤ 0.2%), suggesting intensified volcanism, which could have emitted significant amounts of greenhouse gases, thereby leading to climate warming. In contrast, Phase 2 (3 to 11 m) is characterized by decreasing Hg/TOC and Ti/Al ratios, relatively low though slightly fluctuating CIAcorr values, generally depleted in RSE (except moderately enriched U up to 14.6 ppm) and increased P/Al and Ba/Al ratios, implying weakening volcanic activity and subsequent climate cooling and the potential for improved seawater ventilation as a result of oceanic upwelling. Phase 3 (Sandbian Age: 11–13 m) witnessed continuous decrease in Hg/TOC ratio, an increase in Ti/Al and CIAcorr values, fairly low values of RSE enrichment and P/Al and Ba/Al ratios, indicating recurrent climate warming, and the potential for slowed oceanic circulation and attenuated upwelling of nutrient-rich deep waters onto the shallow shelf. These changes could have diminished bioproductivity and organic output onto the seafloor. This study offers insights into volcanic-climatic-oceanic interactions during a major pulse of the GOBE around the Middle-Late Ordovician transition while black shales were extensively deposited.

Influence of Ocean Acidification and Deep Water Upwelling on Oligotrophic Plankton Communities in the Subtropical North Atlantic: Insights from an In situ Mesocosm Study

Oceanic uptake of anthropogenic carbon dioxide (CO2) causes pronounced shifts in marine carbonate chemistry and a decrease in seawater pH. Increasing evidence indicates that these changes—summarized by the term ocean acidification (OA)—can significantly affect marine food webs and biogeochemical cycles. However, current scientific knowledge is largely based on laboratory experiments with single species and artificial boundary conditions, whereas studies of natural plankton communities are still relatively rare. Moreover, the few existing community-level studies were mostly conducted in rather eutrophic environments, while less attention has been paid to oligotrophic systems such as the subtropical ocean gyres. Here we report from a recent in situ mesocosm experiment off the coast of Gran Canaria in the eastern subtropical North Atlantic, where we investigated the influence of OA on the ecology and biogeochemistry of plankton communities in oligotrophic waters under close-to-natural conditions. This paper is the first in this Research Topic of Frontiers in Marine Biogeochemistry and provides (1) a detailed overview of the experimental design and important events during our mesocosm campaign, and (2) first insights into the ecological responses of plankton communities to simulated OA over the course of the 62-day experiment. One particular scientific objective of our mesocosm experiment was to investigate how OA impacts might differ between oligotrophic conditions and phases of high biological productivity, which regularly occur in response to upwelling of nutrient-rich deep water in the study region. Therefore, we specifically developed a deep water collection system that allowed us to obtain ~85 m3 of seawater from ~650 m depth. Thereby, we replaced ~20% of each mesocosm's volume with deep water and successfully simulated a deep water upwelling event that induced a pronounced plankton bloom. Our study revealed significant effects of OA on the entire food web, leading to a restructuring of plankton communities that emerged during the oligotrophic phase, and was further amplified during the bloom that developed in response to deep water addition. Such CO2-related shifts in plankton community composition could have consequences for ecosystem productivity, biomass transfer to higher trophic levels, and biogeochemical element cycling of oligotrophic ocean regions.

Climatic-oceanic forcing on the organic accumulation across the shelf during the Early Cambrian (Age 2 through 3) in the mid-upper Yangtze Block, NE Guizhou, South China

The organic-rich sediments were widely deposited over the entire Yangtze Block during the Early Cambrian (late Nemakit-Daldynian to Botomian). In the mid-upper Yangtze region, northeastern Guizhou, South China, they comprise, in ascending order, the Niutitang, Jiumenchong and lower Bianmachong formations which are dominated by black shales except the middle one characterized by interbedded shales-limestones. Three third-order depositional sequences are identified in the two studied sections located on the upper slope to basin of the open shelf. The organic-rich sediments were mostly deposited notably during transgressions on the shallower upper slope-margin (TOC up to 25wt.%) where they are characterized by co-increases in C, P, Fe, and Ba concentrations, indicating the highest organic productivity and coupled C, P and Fe cycling there. In contrast, in the shelf basin, the concomitant organic-rich sediments yield lower organic (TOC <10wt.% mostly) and Ba abundances, and generally show antithetic relationship of TOC with P and Fe variations, indicative of a relatively low organic productivity and transient decoupled C, P and Fe cycling in the deeper basin. This spatial difference in C, P, Fe and Ba cycling as a whole thus indicate that the highest organic productivity wedge on the shallower shelf upper slope-margin was likely induced by the oceanic upwelling from the restricted basin where the P and Fe nutrients were transiently depleted without timely renewal. Meanwhile, multiple redox proxies, including V/Al, Ni/Al and Cr/Al ratios vs. TOC contents, together with V/(V+Ni) and V/Cr ratios, generally characterize a more severe euxinic water wedge localized on the shallower upper slope-margin of shelf where the organic production culminated, but a nonsulfidic (ferruginous) anoxic state in the basin due to the relatively low sulfate level. Meanwhile, episodic depletion of trace elements in the deeper basin during the higher organic productivity also reconciles the concomitantly enhanced upwelling from the somewhat restricted deep watermass likely silled by the underwater upland seaward. The coincidence of organic-rich intervals with more arid climate epoches, as shown by increased Ti/Al ratios indicative of increased aeolian fluxes, reconciles a subtropical arid climate of Yangtze Block within the north mid-low-latitude trade-wind zone during deposition (the Early Cambrian). As such, enhanced offshore currents driven by the trade winds could have further induced the upwelling of nutrient-rich deep waters along the shelf during the transgressions, although tended to wane onwards, leading to the tempo-spatial heterogeneities in organic production and redox state across the shelf sea.

Response of Mytilus edulis to enhanced phytoplankton availability by controlled upwelling in an oligotrophic fjord

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 518:139-152 (2015) - DOI: https://doi.org/10.3354/meps11036 Response of Mytilus edulis to enhanced phytoplankton availability by controlled upwelling in an oligotrophic fjord Tore Strohmeier1,*, Øivind Strand1, Marianne Alunno-Bruscia1,2, Arne Duinker3, Rune Rosland4, Jan Aure1, Svein R. Erga4, Lars J. Naustvoll5, Henrice M. Jansen1,6, Peter J. Cranford7 1Institute of Marine Research, PO Box 1870 Nordnes, 5817 Bergen, Norway 2Ifremer, UMR 6539, 11 Presqu’île du Vivier, 29840 Argenton-en-Landunvez, France 3National Institute of Nutrition and Seafood Research, PO Box 2029 Nordnes, 5817 Bergen, Norway 4Department of Biology, University of Bergen, PO Box 7803, 5020 Bergen, Norway 5Institute of Marine Research, Nye Flødevig veien 20, 4817 His, Norway 6Institute for Marine Resources and Ecosystem Studies (IMARES), PO Box 77, 4400 AB Yerseke, The Netherlands 7Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y4A2, Canada *Corresponding author: tore.strohmeier@imr.no ABSTRACT: The controlled upwelling of nutrient-rich deep water in oligotrophic coastal regions has been proposed as a means of increasing phytoplankton and, subsequently, bivalve aquaculture production. This was tested as part of a large-scale upwelling experiment in an oligotrophic environment (Lysefjord, Norway). The mean chlorophyll a concentration in the upwelling area was consistently higher than at the control site (mean ± SD: 3.3 ± 1.9 and 1.5 ± 0.6 mg chl a m-3, respectively) during the 4 mo of controlled upwelling. After 2 mo with upwelling, the dry flesh weight of 1 yr old and 2 yr old mussels was 95% and 24% higher, respectively, than that of the mussels at the control site. The 1 yr old mussels (Mytilus edulis) at the upwelling site achieved up to 2.4-fold higher dry flesh weight compared to the control. Reproductive output was also higher at the upwelling site and only there, spawning of 1 yr old mussels was detected. Standardized clearance and respiration rates showed maximum values during the most intense period of tissue growth. Average ingestion rates were 40% higher at the upwelling than at the control site. Tissue growth and clearance rates were not correlated with the measured seston parameters, suggesting that food acquisition was responsive to other exogenous parameters and/or to increased endogenous energy demands. It was concluded that the sustained upwelling of nutrient-rich deep water in an oligotrophic fjord can increase phytoplankton biomass, resulting in improved mussel growth performance and increased aquaculture production carrying capacity. Thus, controlled upwelling represents a simple but effective ecosystem engineering approach for enhancing human food production. KEY WORDS: Bivalve aquaculture · Production carrying capacity · Physiology · Fjord ecosystems · Shell growth · Tissue growth Full text in pdf format PreviousNextCite this article as: Strohmeier T, Strand Ø, Alunno-Bruscia M, Duinker A and others (2015) Response of Mytilus edulis to enhanced phytoplankton availability by controlled upwelling in an oligotrophic fjord. Mar Ecol Prog Ser 518:139-152. https://doi.org/10.3354/meps11036 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 518. Online publication date: January 07, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

High Arabian Sea productivity conditions during MIS 13 – odd monsoon event or intensified overturning circulation at the end of the Mid-Pleistocene transition?

Abstract. Marine isotope stage (MIS) 13 (~500 000 years ago) has been recognized as atypical in many paleoclimate records and, in particular, it has been connected to an exceptionally strong summer monsoon throughout the Northern Hemisphere. Here, we present a multi-proxy study of a sediment core taken from the Murray Ridge at an intermediate water depth in the northern Arabian Sea that covers the last 750 000 years. Our results indicate that primary productivity conditions were anomalously high during MIS 13 in the Arabian Sea and led to extreme carbonate dissolution and glauconitization in the deep-sea sediments. These observations could be explained by increased wind driven upwelling of nutrient-rich deep waters and, hence, by the occurrence of an exceptionally strong summer monsoon event during MIS 13, as it was suggested in earlier studies. However, ice core records from Antarctica demonstrate that atmospheric methane concentrations, which are linked to the extent of tropical wetlands, were relatively low during this period. This constitutes a strong argument against an extremely enhanced global monsoon circulation during MIS 13 which, moreover, is in contrast with results of transient climate modelling experiments. As an alternative solution for the aberrant conditions in the Arabian Sea record, we propose that the high primary productivity was probably related to the onset of an intensive meridional overturning circulation in the Atlantic Ocean at the end of the Mid-Pleistocene transition. This may have led to an increased supply of nutrient-rich deep waters into the Indian Ocean euphotic zone, thereby triggering the observed productivity maximum.

Decline of the marine ecosystem caused by a reduction in the Atlantic overturning circulation

Reorganizations of the Atlantic meridional overturning circulation were associated with large and abrupt climatic changes in the North Atlantic region during the last glacial period. Projections with climate models suggest that similar reorganizations may also occur in response to anthropogenic global warming. Here I use ensemble simulations with a coupled climate-ecosystem model of intermediate complexity to investigate the possible consequences of such disturbances to the marine ecosystem. In the simulations, a disruption of the Atlantic meridional overturning circulation leads to a collapse of the North Atlantic plankton stocks to less than half of their initial biomass, owing to rapid shoaling of winter mixed layers and their associated separation from the deep ocean nutrient reservoir. Globally integrated export production declines by more than 20 per cent owing to reduced upwelling of nutrient-rich deep water and gradual depletion of upper ocean nutrient concentrations. These model results are consistent with the available high-resolution palaeorecord, and suggest that global ocean productivity is sensitive to changes in the Atlantic meridional overturning circulation.

Water mass characteristics in the Cenomanian US Western Interior seaway as indicated by stable isotopes of calcareous organisms

Stable oxygen isotopes taken from taxon-specific planktic foraminifera are potential sources of water mass density proxy data and paleoclimate information. Material suitable for stable isotope analysis is uncommon in the middle to late Cenomanian strata of the Western Interior of the USA. However, well preserved material from 17 correlative horizons at five localities across SE Montana and NE Wyoming has provided an opportunity to obtain reliable δ 18O data from foraminiferal samples, fine-fraction carbonate (e.g. calcareous nannofossils) and inoceramid prisms. With the exception of inoceramids, which for a benthic organism have stable isotopic compositions resembling surface-dwelling calcareous organisms, these data indicate that the water column was vertically stratified and water masses were anomalously depleted in 18O. Within samples, planktic and benthic foraminifera display strong vertical gradients in δ 18O and δ 13C of several per mil. The vertical gradient of δ 18O ranges from more negative nearer the sea surface to less negative at the sea floor, with a gradient of about 4‰. This implies less saline/warmer waters near the surface with increasingly more saline/colder waters with depth. This isotopic distribution indicates a stably stratified water column at least seasonally. In addition, a persistent vertical carbon isotope gradient of 2–3‰ suggests a strong nutrient gradient that if mixed would result in high surface water fertility. Preservation of high amounts of organic carbon in the sediments corroborates this inference. Although the water column was stratified overall, the nutrients necessary to promote high productivity were clearly renewed at least seasonally. Nutrient renewal could have been achieved by (1) winter cooling of the upper layer allowing deeper seasonal mixing with nutrient-rich deep waters, (2) seasonal shallowing of the pycnocline into the photic zone, (3) entrainment and decomposition of organic matter at the previously proposed frontal convergence, (4) upwelling of nutrient-rich deep waters along the previously proposed oceanic front, or (5) mixing of ‘shelf’ waters with more oceanic waters across an oceanic front.

Late Neogene changes in diatom sedimentation in the North Pacific

During the late Neogene, North Pacific diatom sedimentation underwent major changes in response to high-latitude cooling and changes in surface and deep water circulation. At 9 Ma diatom mass accumulation rates (MARs) increased in the NW Pacific and off northeast Japan, possibly due to shoaling of the Isthmus of Panama, which lead to an enrichment of nutrients in North Pacific deep waters. During the latest Miocene, diatom MARs increased progressively off southem California (6.5 Ma), at high latitudes of the North Pacific (6.2 Ma), and off northeastern Japan (5.5 Ma), presumably in response to high latitude cooling. At about 4.5 Ma diatom sedimentation abruptly increased in the NW Pacific but declined off Japan and California, coincident with the onset of a prolonged period of high-latitude warmth. Enhanced upwelling of nutrient-rich deep waters in the NW Pacific probably stimulated diatom production there. A major step in high latitude cooling at 2.7 Ma caused a reversal of these mid Pliocene diatom sedimentation patterns. Upwelling of deep, nutrient-rich waters waned at higher latitudes, leading to a decline in diatom productivity; while wind-driven, coastal upwelling increased off southern California and stimulated diatom growth.

The Paleocene-Eocene transition in the Antarctic Indian Ocean: Inference from planktic foraminifera

Isotopic depth stratification and relative abundance studies of planktic foraminifera at ODP Site 738 reveal three major faunal turnovers during the latest Paleocene and early Eocene, reflecting the climatic and structural changes in the Antarctic surface ocean. Faunal Event 1 occurred near the Paleocene/Eocene boundary and is characterized by a faunal turnover in deep dwellers, decreased relative abundance in intermediate dwellers and increased relative abundance in surface dwellers. This event marks a temporary elimination of the vertical structure in the surface ocean over a period of more than 63,000 years that is apparently associated with the sudden shutdown of the “Antarctic Intermediate Water” production. The appearance of morozovellids before this event suggests that polar warming is the cause for the shutdown in the production of this water mass. At this time warm saline deep water may have formed at low latitudes. Faunal Event 2 occurred near the AP5a/AP5b Subzonal boundary and is characterized by a faunal turnover in deep dwellers with no apparent change in surface and intermediate dwellers. Increased individual size, wall-thickness and relative abundance in deep dwelling chiloguembelinids suggests the formation of a deep oxygen minima in the Antarctic Oceans during the maximum polar warming possibly as a result of upwelling of nutrient-rich deep water. Faunal Event 3 occurred in Subzone AP6 and is characterized by a faunal turnover in surface dwellers and a delayed diversification in deep dwellers. This event marks the onset of Antarctic cooling. A drastic decrease in the δ 13C/δ 18O values of the deep assemblage in Zone AP7 suggests an intensified thermocline and reduced upwelling following the polar cooling.

Transport and distribution of nutrients and chlorophyll- a by mesoscale eddies in the northeastern Mediterranean

The distribution of nutrient elements and chlorophyll- a in the Northern Levantine Basin (NLB) were investigated in some detail and are discussed together with the physical aspects of the region in the present study. The surface circulation pattern of the NLB was studied and the structure of the relatively large-scale Rhodes cyclonic gyre, which is located between Rhodes and Cyprus, was investigated for the same time period. The most important characteristics of this cyclonic gyre are the upwelling of nutrient-rich deep waters within the gyre and the reverse phenomenon at the peripheries. Anticyclonic circulation systems generally surround the Rhodes cyclonic gyre; the permanent ones are located in the southern part of the Rhodes gyre, in the Cilician Basin, and off İskenderun Bay. The vertical distribution of nutrients in the water column shows completely reversed trends and the nutrient gradient ranges between 300–400 m in the central parts of the anticyclonic systems. Thus downwelling processes also occur in the NLB and the formation of Levantine Intermediate Water (LIW) observed in the NLB matches the chemical data presented here. This special vertical and spatial distribution of nutrients affects the distribution of the phytoplankton population, as the patches of primary producers are aggregated in the central parts of the Rhodes gyre. This is confirmed by the chlorophyll- a data and the unexpected content of neuston net collection in the same region. The concentration of chlorophyll- a was relatively high at offshore stations, such as in the central parts of the cyclonic gyres, where zooplankton, small shrimp and fish larvae, etc., were observed in large quantities.

Influence of upwelling relaxation on dinoflagellates and shellfish toxicity in Ria de Vigo, Spain

Outbreaks of paralytic shellfish poisoning (PSP) along the north-west coast of Spain have become a serious threat to the extensive mussel farming industry in that region over the last decade. During the summer, high phytoplankton productivity is supported by the sustained upwelling of nutrient-rich deep water into the rias. An episode of PSP in the autumn of 1985 in Ria de Vigo coincided with the sudden appearance, rapid numerical increase, and dominance of two chain-forming dinoflagellates, Gymnodinium catenatum and Protogonyaulax affinis. Field data suggest that warm offshore surface water was transported into the ria as the summer upwelling ceased. This occurred when winds changed from northerly (upwelling favourable) to southerly or westerly (upwelling unfavourable); the injected water contained established populations of oceanic and neritic dinoflagellates. The simultaneous appearance and dominance of two dinoflagellates that form long chains leads us to speculate that the small-scale downwelling of water within the ria favoured efficient swimmers among the phytoplankton. These data not only implicate these two species as possible sources of the PSP toxins in local mussels, but they also suggest the feasibility of developing a bloom prediction capability for some dinoflagellate species based in part on an upwelling index that can indicate when offshore surface waters might be forced into the rias.