Nutrient-rich Deep Water Research Articles

Changes in the development of the winter-spring phytoplankton bloom in the Bay of Calvi (NW Mediterranean) over the last two decades: a response to changing climate?

The development of the winter-spring phytoplankton bloom was investigated in the Bay of Calvi (Corsica, Ligurian Sea, northwestern Mediterranean) in 1979, 1986, 1988, 1997 and 1998. A drastic reduction of phytoplankton biomass was evidenced over the last 2 decades, in relation to long- term changes in climatic and environmental conditions. Between 1979 and 1998, the monthly aver- aged chlorophyll a concentrations at 1 m decreased by about 80% during February, March and April. Simultaneously, major changes to hydrodynamic conditions include warmer water, overall decrease of salinity at 10 m depth, longer periods of bright sunshine and lower wind stress. The changes in environmental conditions were large enough to affect the vertical stability of the water column dur- ing the winter-spring period and to reduce nutrient replenishment of the surface layer prior to the usual period of phytoplankton growth. Until 1986, the main factor driving nutrient replenishment was the winter upward mixing of nutrient-rich deep waters, while the progressive reduction of mixing from 1988 induced nutrient limitation of surface waters in the last decade. The following hypotheses on changes in the development of the winter-spring phytoplankton bloom are made: (1) Until 1986, phytoplankton peaks took place in relatively high-nutrient waters and were diatom-dominated. (2) Between 1986 and 1988, decreasing Si availability led to Si limitation which caused a reduction in diatom abundance. This resulted in the disappearance of the diatom-dominated pulses and in lower phytoplankton biomass and was accompanied by a shift toward non-siliceous phytoplankton. (3) In 1988, 1997 and 1998, decreasing nitrate availability led to nitrate limitation, thus explaining the pro- gressive reduction in non-siliceous phytoplankton biomass. Other, associated changes in benthos assemblages and ichthyofauna are documented. The conclusions from the Bay of Calvi are extended to the whole western Corsican coast. This confirms that the Mediterranean reacts rapidly to external perturbations, which are driven by climate change in that particular area.

Physical controls on biogeochemical zonation in the Southern Ocean

The primary control on the N–S zonation of the Southern Ocean is the wind-induced transport of the Antarctic Circumpolar Current (ACC). The ACC divides the Southern Ocean into three major zones: the Subantarctic Zone (SAZ) north of the ACC; the ACC transport zone; and the zone south of the ACC (SACCZ). The zone of ACC transport is most often subdivided into two zones, the Polar Frontal Zone (PFZ) and the Antarctic Zone (AAZ), but it may be appropriate to define more subzones or indeed only one at some longitudes. To maintain geostrophic balance, isopycnals must slope upwards to the south across the ACC, thus raising nutrient-rich deep water closer to the surface as one goes polewards. In addition, silicate concentrations increase polewards along isopycnals because of diapycnic mixing with silicate-rich bottom water. Surface silicate concentrations therefore decrease northwards from high levels in the SACCZ to low levels in the SAZ. Within the SAZ and PFZ and even in the northern part of the AAZ, silicate levels may drop to limiting levels for siliceous phytoplankton production during summer. Nitrate concentrations also decrease northwards, but only become limiting in the Subtropical Zone north of the SAZ. The second circumpolar control is the changing balance of stratification, with temperature dominating near-surface stratification in the SAZ and salinity dominating further south because of fresh water input to the surface from melting ice. This results in circumpolar features such as the subsurface 2°C temperature minimum and the subduction of the salinity minimum of Antarctic Intermediate Water, which are often but not always associated with frontal jets and large transports. The transport of the ACC is dynamically constrained into narrow bands, the number and latitudinal location of which are controlled by the bathymetry and so vary with longitude. Thus it is not the fronts that are circumpolar, but the total ACC transport and scalar properties of the salinity and temperature fields. Evidence of summer silicate and nitrate uptake in all zones (SAZ, PFZ and AAZ) shows that there is productivity despite their high-nutrient low-chlorophyll status. Blooms covering large areas (say 400 km across) in the PFZ and AAZ are found in the vicinity of submarine plateaux, which suggest benthic iron fertilizatio

Derivation and validation of the seasonal thermal structure of Lake Malawi using multi-satellite AVHRR observations

Lake Malawi is the second largest lake in Africa by volume and an important regional source of food. Seasonal fluctuations in the primary production of the lake are principally controlled by the lake's thermal structure, which modulates the mixing of nutrient-rich deep water with that of the phytoplanktonrich near-surface layer. Satellites potentially offer an efficient, low cost method of providing information on the lakes thermal structure over the longer term via remote sensing observations of lake surface temperature. Here we investigate the accuracy of remotely sensed lake surface temperatures derived using data from the NOAA-11 AVHRR over a two-year period (1992-1993). Optimised triple window atmospheric correction algorithms are shown to provide an accuracy of around 0.5°C when compared to in situ

The Kirchrode I and II boreholes: technical details and evidence on tectonics, and the palaeoceanographic development during the Albian

The technical details of drilling and coring at the Kirchrode I and II sites are presented. At these sites, a sequence of claystones and marlstones from an Albian shelf basin was recovered. Constraints on the ages of the sediments in the two boreholes are provided by the occurrence of the inoceramid bivalve Actinoceramus sulcatus, the first appearance of which is used to define the Middle/Upper Albian boundary and by observed facies changes that can be correlated to the established lithostratigraphy. The cores from the two boreholes provide a rather complete, 285-m-long sequence of the Upper Albian, with a 155.5-m-long overlap. Analysis of the tectonic structures showed considerable shortening in the Middle and Lower Albian part of the sequence due to normal faulting. Of the Upper Albian, only the lowermost part is affected by faults. The increase in sedimentation rates of terrigenous detritus and of marine biogenic carbonate, which occurs in the basal part of the C. auritus Subzone, is interpreted to reflect a regional change to a more humid climate and regional tectonic movements (uplift of the Rhenish Bohemian massif, subsidence of the Lower Saxony basin intensified locally by halokinetic movements). The further increase in marine productivity in the latest Albian may be related to upwelling of more nutrient-rich deep water along submarine relief in this shelf sea. Identification of Milankovitch cyclicity documented by the fluctuating CaCO 3 contents of the sediments is used (i) to constrain the minimum time represented by the Upper Albian deposits, and (ii) to determine the duration of the sea level cycles (Cycle V: ≥1.6 Ma, Cycle VI: ≥2 Ma), and (iii) to establish the duration of the Late Albian ammonite subzones (e.g. Callihoplites auritus Subzone: 2.1 Ma). Average sedimentation rates determined from the identified 100-ka eccentricity cycles show a stepwise increase in sedimentation rates from 1–2 cm/1000 a in the Lower Albian dark claystones to 7–13 cm/1000 a in the late Late Albian. In addition to the general deepening trend through the Late Albian, two, nearly completely documented 3rd-order sea-level cycles in the Upper Albian of Kirchrode I were recognised, plus another one, cut short by faulting, at the base of the Upper Albian (documented in Kirchrode II). These global sea-level cycles were identified on the basis (a) of the sequence of the abundance maxima of selected benthos and plankton groups, (b) of trends in the fluctuations of the CaCO 3 content, and (c) of the abundance of glauconite. The transgression periods in this Upper Albian deep shelf-basin are characterised by intensified circulation. This intensified circulation is found to have affected first the surface-near waters, resulting e.g. in an increase in the abundance of immigrant plankton and nekton species from the Tethys. At a later stage the deep water was affected, supporting then an increased population of suspension-feeding benthos, and causing condensation and erosion in the sediment at the sea floor.

Cd in the western equatorial Pacific

Abstract Dissolved Cd was investigated in relation to PO4 in January 1999 at three stations in the western equatorial Pacific. The vertical distribution of Cd, as is typical with nutrients, was depleted in the surface water, while the concentration increased with depth to approximately 0.9 nM below 1000 m. At a depth of approximately 150 m, a thermocline developed, and surface and nutrient-rich deep water separated from each other. Cd was inserted into a Redfield-type stoichiometric equation as well as PO4. Preformed Cd and PO4 were shown as the intercept of the Cd or PO4 axis in the Cd, PO4-AOU (Apparent Oxygen Utilization) plot, and less than 0.04 nM and 0.5 μM for Cd and PO4, respectively, were estimated in the shallow layer (above 150 m). Preformed Cd and PO4 increased with depth below 200 m and reached approximately 0.7 nM and 1.2 μM for Cd and PO4, respectively, in deep water (below 1000 m). Above 150 m, the molecular ratios of regenerated Cd to consumed O2 were calculated from the Cd-AOU plot, and “1:1,220,000,” “1:1,180,000,” and “1:1,150,000” were obtained at the three stations. The ratio of regenerated PO4 to consumed O2 was adapted from the “Redfield Ratio” as “1:138,” so the ratios of regenerated Cd to PO4 were estimated as 0.113, 0.117, and 0.120 (nM/μM) above 150 m at the three stations. In the relationship between Cd and PO4, a clear deviation was observed where the concentration of PO4 was about 1.1 μM. Above 150 m, the concentration of PO4 was less than 1.1 μM, and the slopes of the regression line were 0.100, 0.127, and 0.156 (nMCd/μMphosphate). These values were quite similar to the regeneration ratio of Cd to PO4 at each station, which suggests that the regeneration processes might be a main factor that regulates the correlation of Cd and PO4 in this layer. Below 200 m, a relatively steep slope (average, 0.421) and obvious negative intercept in the Cd axis were shown, and the Cd/PO4 ratios increased gradually with depth to an almost constant value of 0.314 (nM/μM) below 1000 m, which is considered to be attributable to the apparent conservative mixing between water masses with an originally different Cd/PO4 ratio (different preformed values) in deep water and just below the thermocline.

Derivation and validation of the seasonal thermal structure of Lake Malawi using multi-satellite AVHRR observations

Lake Malawi is the second largest lake in Africa by volume and an important regional source of food. Seasonal fluctuations in the primary production of the lake are principally controlled by the lake's thermal structure, which modulates the mixing of nutrient-rich deep water with that of the phytoplanktonrich near-surface layer. Satellites potentially offer an efficient, low cost method of providing information on the lakes thermal structure over the longer term via remote sensing observations of lake surface temperature. Here we investigate the accuracy of remotely sensed lake surface temperatures derived using data from the NOAA-11 AVHRR over a two-year period (1992-1993). Optimised triple window atmospheric correction algorithms are shown to provide an accuracy of around 0.5°C when compared to in situ

Coccolithophore fluxes from sediment traps in the northeastern Arabian Sea off Pakistan

We investigated coccolithophore fluxes from four sediment traps located in the northeastern Arabian Sea at the continental slope of Pakistan. Two moorings, WPT and EPT (Western/isastern PAKOMIN Trap), consisting of one and two sediment traps were deployed at water depths of about 500 m and 1500 m. They collected settling particles from October 1993 to February 1994 and from May 1995 to February 1996 at intervals of 22 and 24 days, respectively. A longer sampling was prevented probably due to high sedimentation events which occluded the trap cones. Both areas were characterised by a strong seasonality in coccolith fluxes. High fluxes were recorded with up to 2000 × 10 6 coccoliths m –2 d –1 in the early SW (summer) monsoon and up to 5400 × 10 6 coccoliths m –2 d –1 in the early NE (winter) monsoon. Generally, coccosphere fluxes exhibited a similar trend but were some orders of magnitude lower than coccolith fluxes. Twenty-seven taxa were identified in this study, all well to moderately well preserved and with a small range of diversity. The species composition was dominated by Gephyrocapsa oceanica, Florisphaera profunda, and Emiliania huxleyi making up about 70% of the assemblages. During the SW (summer) monsoon high productivity in the surface waters of the eastern Arabian Sea may be linked to the upwelling system off Oman. Apparently, nutrient-rich surface water derived from the southwestern upwelling areas off Oman with high standing stocks were carried into this area by northeast flowing currents. In contrast, during the NE (winter) monsoon the locally observed high coccolithophore production is hypothesised to respond to local injections of nutrient-rich deep water into the surface water due to sea-surface cooling leading to convection. Consequently, fluxes during the winter monsoon were in the same range as during the summer monsoon.

Paleoceanographic significance of Late Paleocene dysaerobia at the shelf/slope break around New Zealand

A geochemical and micropaleontological study of the Waipawa Formation in New Zealand was undertaken to investigate its depositional setting and possible relationship to major changes in the marine environment. The organic-rich formation was deposited in the Late Paleocene, during a third-order, eustatic sea-level rise. Precise age control is lacking, but deposition can be placed between a climatic cooling at 59.1 Ma and a thermal maximum at 55.5 Ma, during the transition from cold to warm-saline deep-water circulation in the South Pacific. Deposition occurred around most of the land mass, generally near the shelf break, and microfaunal evidence suggests dysaerobic conditions. There appears to have been no contemporaneous organic-rich sedimentation on the shelf, so the widespread high phytoplanktonic productivity, which is reflected in positive excursions in kerogen δ 13C to −20‰, was probably caused by regional upwelling. However, the paleodistribution of the formation cannot be readily accounted for by wind-driven upwelling alone. There is strong positive correlation between organic content, kerogen δ 13C values and unusually high abundance of 24- n-propylcholestanes (20–55% of C 27–C 30 regular steranes). Surface-water conditions favoured a particular group of phytoplankton that preferentially synthesised the precursor C 30 sterols, and there is microfaunal evidence for warming. Carbonate oxygen isotopic data are also consistent with warming, but may be affected by diagenetic alteration. The Waipawa Formation may result from regional upwelling of warm, saline, oxygen-depleted, nutrient-rich deep water, in what appears to be an earlier and longer-term, but geographically more restricted, event than that responsible for the benthonic extinction event at 55.5 Ma.

Onset of permanent stratification in the subarctic Pacific Ocean

The surface waters of the modern subarctic Pacific Ocean are isolated from the nutrient-rich waters below by a steep vertical gradient in salinity (halocline), a feature which is a dominant control on upper-ocean stratification in polar environments1,2,3. The physical processes which maintain the halocline and, in turn, its physical, biological, and geochemical effects have long been subjects of intense inquiry3,4. The stratification of polar surface waters influences the exchange of CO2 between ocean and atmosphere5,6,7,8, so the history of the subarctic Pacific halocline may have played a role in past changes in atmospheric CO2 concentration. Here we report opal accumulation rates and nitrogen-isotope data from sediments in this region which indicate that the subarctic Pacific halocline developed abruptly 2.73 million years ago, coincident with the onset of extensive Northern Hemisphere glaciation. The halocline would have reduced the transport of nutrient-rich deep water into the euphotic zone, leading to a decrease in biological production but an increase in the fraction of nutrient stocks utilized. This increase in the efficiency of the ‘biological pump’ would have lowered the rate of CO2 evasion from ocean to atmosphere, potentially reducing atmospheric CO2 concentrations from the suggested higher level of the preceding mid-Pliocene warm interval9,10.

Hydrodynamic Performance of Wave-Driven Artificial Upwelling Device

Artificial upwelling and mixing is a new technology for enhancing open ocean mariculture using nutrient-rich deep ocean water. A wave-driven artificial upwelling device was developed based on mathematical modeling and hydraulic experiments. Results of the numerical modeling are in good agreement with hydraulic experiments. The mathematical model was applied to simulate the operation of a prototype device in typical Hawaiian waves. The prototype device consists of a buoy with a water chamber 4.0 m in diameter, a flow-controlling valve, and a long tail pipe 300 m in length and 1.2 m in diameter. This device can produce an upwelling flow of 0.62 m3/s in regular Hawaiian waves with a period of 12 s and a wave height of 1.9 m. Two analytical solutions to the simplified governing equations were also derived that provide a basis for the derivation of predictive equations, useful for preliminary engineering design and analysis.

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.

Late Quaternary δ13C gradients and carbonate accumulation in the western equatorial Atlantic

We investigated glacial–interglacial changes in vertical δ 13C gradients in the western equatorial Atlantic using the carbon isotopic composition of planktonic and benthonic foraminifera. Core top measurements show that the δ 13C difference between shallow-dwelling G. sacculifer and thermocline-dwelling G. truncatulinoides is an indicator of the vertical nutrient gradient in the upper water column. In the western equatorial Atlantic, the δ 13C differences between G. sacculifer and G. truncatulinoides are reduced during glacials and cold substages of interglacials, while the δ 13C differences between G. truncatulinoides and the benthonic species C. wuellerstorfi are increased. This indicates that nutrients were depleted in the thermocline and enriched in deep waters during cold substages. Covariance between the δ 13C records of G. truncatulinoides from the western equatorial Atlantic and C. wuellerstorfi from Caribbean intermediate water suggests that the upper part of the western equatorial Atlantic water column was largely composed of nutrient-poor central and intermediate waters of northern origin. This pattern might have been the result of a circulation mode in which subantarctic surface waters formed nutrient-rich deep waters rather than intermediate waters. Lower 230Th-normalized carbonate accumulation rates during cold substages imply that the decreased nutrient content of subsurface waters induced a lower primary productivity in the western equatorial Atlantic.

Cetacean distributions and oceanographic features near the Kerguelen Plateau

Cetacean surveys were conducted in the austral summer during World Ocean Circulation Experiment hydrolegs I8S and I9S in the southeastern Indian Ocean. The highest density of cetaceans occurred along the southern flank of the Kerguelen Plateau and northern edge of the Princess Elizabeth Trough, where the distribution of whales coincides with the mean positions of the Southern Front of the Antarctic Circumpolar Current (ACC) and southern water mass boundary (Southern Boundary) of the ACC. The topographic steerage of these features of the ACC around the southern edge of the Kerguelen Plateau brings shoaled, nutrient‐rich Upper Circumpolar Deep Water (UCDW) to higher latitude than in adjacent basins. The higher concentrations of cetaceans near the Kerguelen Plateau suggests that whales benefit from a cascade of trophic dynamics associated with the poleward extent of UCDW, the marginal ice edge zone, and the proximity of several oceanographic features: the high latitude penetration and close alignment of the Southern Front and Southern Boundary of the ACC; the unique poleward extent of the Southern Boundary as it is topographically steered around the plateau; the entrainment of macronutrients into the surface mixed layer near the Southern Boundary; isopycnal shoaling in cyclonic eddies; the presence of complex bathymetry; and the advection of a tongue of ice northward along the eastern side of the plateau. The association between these concurrent oceanographic features and whales suggests that the southeastern edge of the Kerguelen Plateau is a predictably productive foraging location for cetaceans and their prey.

Resultados paleoceanográficos deducidos a partir del contenido en diatomeas de los sedimentos siliceos miocenos («moronitas») en la Cuenca del Guadalquivir

During a part of the Miocene (Langhian middle-middle Tortonian) diatom bearing sediments, named «moronitas», were deposited in the Guadalquivir Basin. This sedimentation was the result of a highly productive coastal upwelling regime in the area. At this time, the oceanic circulation in the «nord-betic strait» was under stuarine conditions and the nutrients carne from deep Atlantic currents. The intensity of the circulation of deep water in oceans is usually related to global climatic changes. Coastal upwelling is not an effective process to bring up nutrient-rich deep waters to the surface. A complex hidrological process must occur to bring up deep water firstly to intermediate leveIs and then the nutrients may be upwelled from subsurface waters. It is necessary to take into account local factors related to the basin morphology and the changes that it has gone through in arder to place the ancient upwelling centers in the basin. The percentajes of the most common species ( Crucidenticula nicobarica + Crucidenticula punctata, Denticulopsis hustedtii, Thalassionema nitzschioides, Thalassionema nitzschioides var. parva, Thalassiothrix longissima + Thalassionema bacillaris ) and the biosilicic global content of the rocks has made possible the recognition of a change in the temperature of superficial waters and upwelling intensity in time. The interpretation of the A..P.C. graphics gives a paleoecological significance to the A.P.C. first two factors. In this way different productivity situations in response to different locations and intensity of the ancient upwelling centers in time have been identified.

Variability of water masses and of organic production-regeneration systems as related to eutrophic, mesotropic and oligotrophic conditions in the northeast Atlantic Ocean

In the eastern part of the tropical North Atlantic, the trade winds generate a permanent coastal upwelling system along West Africa. Eutrophic conditions resulting from the uplift of nutrient-rich deep waters are progressively dissipated westwards. The Eumeli 2 cruise (January–February 1991) realized in the framework of the France-JGOFS program, gave the opportunity to study the internal variability of the water column at three typical sites, eutrophic, mesotrophic and oligotrophic, located near 20°N of latitude. The hydrological survey at each site provides a description of the different water layers. The mixed surface layer of variable thickness at the three sites covers the Central Waters; North Atlantic Central Water and South Atlantic Central Water are competing along the Cape Verde Frontal Zone which crosses the mesotrophic and eutrophic zones. Underneath, the Antarctic Intermediate Water and the North Atlantic Deep Water correspond respectively to minimum and maximum salinities. The oxygen and carbon stable isotope compositions of these identified water masses bring complementary geochemical informations, such as the carbon cycling through production-regeneration. In surface and central waters, the ∂ 18O of water and salinities vary linearly with a slope of 0.46, typical of tropical oceanic areas. The major changes of ∂ 13C of ΣCO 2 occur in the upper 900 m. From oligotrophic to eutrophic conditions, the surficial ∂ 13C values decrease by about 1‰; the lowest ∂ 13C values which mark the level of maximum regeneration rate are reached very close to the surface at the eutrophic site (at about 50 to 100 m), and deeper at the mesotrophic and oligotrophic sites (respectively at about 200 m and 500 m). The NADW appears as a very homogeneous water layer, with almost constant ∂ 18O and ∂ 13C values at the three sites, implying that organic matter remineralization occurs at too slow rate at depth to introduce detectable amounts of 13C-depleted CO 2.

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.

The endo-upwelling concept: from geothermal convection to reef construction

Measurements of interstitial waters in deep wells (0–500 m) in Mururoa and in shallow bores (0–35 m) in Tikehau (French Polynesia) have shown high contents of dissolved nutrients and relatively low salinities, implying a deep oceanic origin. Studies of specific thermal fields within these atolls and carbonate platforms have led to models of deep oceanic water circulation by convection resulting from upward geothermal heat flow. Cold, low-salinity, low-pH, high-CO2, nutrient-rich deep oceanic water pervades the permeable atoll, where it loses density by heating related to geothermal heat flow, subsequently rising to seep out through the outer rim. New nutrients are thus continuously provided, enabling the reef-building community (algae+corals) to thrive in optimal conditions for photosynthesis and carbonate precipitation. This “geothermal endo-upwelling” concept is defined and its role in atoll trophic networks is emphasized as a necessary and sufficient process for reef net production and organic matter exportation. The endo-upwelled flow can then be viewed as a key factor for internal diagenesis, as early cementation of reef framework and dolomitization of deep limestone. Generalization of this interstitial, deep seawater circulation is discussed and linked to previously described thermal convection models in Florida or raised atolls.

Erratum to : Preliminary report on the first appearance of nutrient-rich deep water in inner zone of the Kuril Arc

Erratum to : Preliminary report on the first appearance of nutrient-rich deep water in inner zone of the Kuril Arc

Nutrient distribution in the upper 300 m of the eastern banda sea and northern Arafura sea during and after the upwelling season, August 1984 and February 1985

During the Indonesian-Dutch Snellius-II Expedition monsoonal differences in nutrient profiles were studied around the Outer Banda Arc (R.V. ‘Tyro’) and the Aru Islands (R.V. ‘Samudera’). During the cruises in the southeast monsoon (August 1984) coastal upwelling on the Arafura Shelf was most conspicuous. Aru Basin water from a depth of 100 to 150 m intruded eastwards over the bottom of the shelf up to depths of 40 to 50 m, both north and south of Aru. High chlorophyll concentrations in the Arafura Sea were evidently not due to river runoff but to enrichment of the upper, less saline layer by vertical mixing with the nutrient-rich deeper water. In the eastern Banda Sea and in the Aru Basin, patchy nutrient enrichment of the mixed layer occurred by doming and entrainment. At stations northwest of the Aru Islands and southwest of the Kai Islands nutrient profiles were shifted upwards into the photic zone. A minimum estimate of the mean daily new production based on vertical NO 3 and PO 4 fluxes during August 1984 came to 0.5 g C·m −2, compared to a net daily primary production of 1.9 g C·m −2. In the northwest monsoon (February 1985), nutrient concentrations in the upper 100 m were much lower. The mixed layer was nitrate-depleted with very low NO 2 and NH 4 values. Nutrient concentrations at isothermal surfaces of 18, 15 and 12°C (depth range 150 to 300 m) did not markedly differ between the upwelling and the oligotrophic season. During both cruises especially H 4SiO 4 concentrations were significantly higher in the Aru Basin than west of the Outer Banda Arc. Using silicate as tracer, it was calculated that the enrichment by Aru Basin water of some stations visited during the Indopac Expedition (1976) in the Molucca Sea also involves the 150 to 300 m water layer.

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.