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1.
The Patos–Mirim Lagoon system along the southern coast of Brazil is linked to the coastal ocean by a narrow mouth and by groundwater transport through a Holocene barrier. Although other groundwater systems are apparently active in this region, the hydraulic head of the lagoon, the largest in South America, drives groundwater transport to the coast. Water levels in wells placed in the barrier respond to changing water level in the lagoon. The wells also provide a measure of the nutrient concentrations of groundwater flowing toward the ocean. Additionally, temporary well points were used to obtain nutrient samples in groundwater on the beach face of the barrier. These samples revealed a subterranean freshwater–seawater mixing zone over a ca. 240 km shoreline. Previously published results of radium isotopic analyses of groundwater and of surface water from cross-shelf transects were used to estimate a water flux of submarine groundwater discharge (SGD) to nearshore surface waters of 8.5 × 107 m3/day. Using this SGD and the nutrient concentrations in different compartments, nutrient fluxes between groundwater and surface water were estimated. Fluxes were computed using both average and median reservoir (i.e. groundwater and surface water) nutrient concentrations. The SGD total dissolved inorganic nitrogen, phosphate and silicate fluxes (2.42, 0.52, 5.92 × 106 mol day− 1, respectively) may represent as much as 55% (total N) to 10% (Si) of the nutrient fluxes to the adjacent shelf environment. Assuming nitrogen limitation, SGD may be capable of supporting a production rate of ca. 3000 g C m2 year− 1in the nearshore surf zone in this region.  相似文献   

2.
In order to estimate submarine groundwater discharge (SGD) and SGD-driven nutrient fluxes, we measured the concentrations of nutrients, 224Ra, and 226Ra in seawater, river water, and coastal groundwater of Yeongil Bay (in the southeastern coast of Korea) in August 2004 and February 2005. The bottom sediments over the shallow areas of this bay are composed mainly of coarse sands. Large excess concentrations of 224Ra, 226Ra, and Si supplied from SGD were observed in August 2004, while these excess concentrations were not apparent in February 2005. Based on the mass balance for 224Ra, 226Ra, and Si, which showed conservative mixing behavior in seawater, SGD was estimated to be approximately 6 × 106 m3 day− 1 (seepage rate = 0.2 m day− 1) in shallow areas (< 9 m water depth) in August 2004, which is much higher than the SGD level typically found in other coastal regions worldwide. During the summer period, SGD-driven nutrients in this bay contributed approximately 98%, 12%, and 76% of the total inputs for dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicate (DSi), respectively. Our study implies that the ecosystem in this highly permeable bed coastal zone is influenced strongly by SGD during summer, while such influences are negligible in winter.  相似文献   

3.
There is increasing evidence that submarine groundwater discharge (SGD) in many areas represents a major source of dissolved chemical constituents to the coastal ocean. In Great South Bay, NY, previous studies have shown that the discharge of nutrients with SGD may cause harmful algal blooms. This study estimates SGD to Great South Bay during August 2006 by performing a mass balance for each of the dissolved Ra isotopes (224Ra, 223Ra, 228Ra, 226Ra). The budget indicates a major unknown source (between 30 and 60% of the total input) of Ra to the bay. This imbalance can be resolved by a flux of Ra-enriched groundwater on the order of 3.5–4.5 × 109 L d− 1, depending on the Ra isotope. The Ra-estimated SGD rates compare well with those previously estimated by models of flow that decreases exponentially away from shore. Compared to previous reports of fresh groundwater discharge to the bay, the Ra-estimated discharge must comprise approximately 90% recirculated seawater. The good agreement between Ra- and model-estimated flow rates indicates that the primary SGD endmember may be best sampled at shallow depths in the sediments a short distance bayward of the low tide line.  相似文献   

4.
Submarine groundwater discharge (SGD) is now recognized as an important pathway for water and chemical species fluxes to the coastal ocean. In order to determinate SGD to the Gulf of Lion (France), we measured the activities of 226Ra and 228Ra by thermal ionization mass spectrometry (TIMS) in coastal waters and in the deep aquifer waters of the Rhone deltaic plain after pre-concentration of radium by MnO2. Compared to conventional counting techniques, TIMS requires lower quantities of water for the analyses, and leads to higher analytical precision. Radium isotopes were thus measured on 0.25–2 L water samples containing as little as 20 fg of 226Ra and 0.2–0.4 fg of 228Ra with precision equal to 2%. We demonstrate that coastal surface waters samples are enriched in 226Ra and 228Ra compared to the samples further offshore. The high precision radium measurements display a small but significant 226Ra and 228Ra enrichment within a strip of circa 30 km from the coast. Radium activities decrease beyond this region, entrained in the northern current along the shelf break or controlled by eddy diffusion. The radium excess in the first 30 km cannot be accounted for by the river nor by the early diagenesis. The primary source of the radium enrichment must therefore be ascribed to the discharge of submarine groundwater. Using a mass-balance model, we estimated the advective fluxes of 226Ra and 228Ra through SGD to be 5.2 × 1010 and 21 × 1010 dpm/d respectively. The 226Ra activities measured in the groundwater from the Rhone deltaic plain aquifer are comparable to those from other coastal groundwater studies throughout the world. By contrast, 228Ra activities are higher by up to one order of magnitude. Taking those groundwater radium activities as typical of the submarine groundwater end-member, a minimum volume of 0.24–4.5 × 1010 l/d is required to support the excess radium isotopes on the inner shelf. This has to be compared with the average rivers water runoff of 15.4 × 1010 l/d during the study period (1.6 to 29% of the river flow).  相似文献   

5.
The four naturally-occurring radium isotopes (223Ra, 224Ra, 226Ra and 228Ra) were used to estimate the submarine groundwater discharge (SGD) in the Isola La Cura marsh area in the northern Venice Lagoon (Italy). By determining the radium contributors to the study area (river, coastal ocean and sediments) the radium excess in the lagoon water was quantified through a mass balance model. This radium excess is attributed to a submarine groundwater discharge source and represents the most important input of radium. Possible endmembers were considered from analysis of groundwater samples (subtidal and marsh piezometers, marsh wells and seepage meters) that were enriched in Ra by one to two orders of magnitude relative to surface waters. In particular, a permeable layer at 80 cm depth in the surrounding marsh is considered to be representative of the most likely SGD source, although similar radium activities were measured in other subtidal porewater samples collected in the Isola La Cura area. The estimated SGD flux to the study area ranged from 1 · 109 to 6 · 109 L·d− 1, the same order of magnitude as the overall riverine input to the lagoon (3 · 109 L·d− 1). A major fraction of this SGD flux is likely recirculated seawater, as evidenced by the endmember salinity. The water residence time of 2 days was estimated by both using the shortest-lived radium isotope and estimating the volume of water exchanged between the lagoon and the open sea during a tidal cycle (tidal prism approach). This SGD flux could be used to estimate the input of other chemical species (metals, nutrients, etc.) via SGD which might affect the Venice Lagoon ecosystem.  相似文献   

6.
A mass balance for the naturally-occurring radium isotopes (224Ra, 223Ra, 228Ra, and 226Ra) in Jamaica Bay, NY, was conducted by directly estimating the individual Ra contributions of wastewater discharge, diffusion from fine-grained subtidal sediments, water percolation through marshes, desorption from resuspended particles, and water exchange at the inlet. The mass balance revealed a major unknown source term accounting for 19–71% of the total Ra input, which could only be resolved by invoking a source from submarine groundwater. Shallow (< 2 m depth) groundwater from permeable sediments in Jamaica Bay was brackish and enriched in Ra relative to surface bay waters by over two orders of magnitude. To balance Ra fluxes, a submarine groundwater input of 0.8 × 109–9.0 × 109 L d− 1 was required. This flux was similar for all four isotopes, with individual estimates varying by less than a factor of 2. Our calculated groundwater flux was 6- to 70-fold higher than the fresh groundwater discharge to the bay estimated by hydrological methods, but closely matched direct flow rates measured with seepage meters. This suggests that a substantial portion of the discharge consisted of recirculated seawater. The magnitude of submarine groundwater discharge varied seasonally, in the order: summer > autumn > spring. Chemical analyses suggest that the recirculated seawater component of submarine groundwater delivers as much dissolved nitrogen to the bay as the fresh groundwater flux.  相似文献   

7.
The distributions of dissolved organic carbon (DOC), Ba, U, and a suite of naturally occurring radionuclides in the U/Th decay series (222Rn, 223,224,226,228Ra) were studied during high- and low-discharge conditions in the Loxahatchee River estuary, Florida to examine the role of submarine groundwater discharge in estuarine transport. The fresh water endmember of this still relatively pristine estuary may reflect not only river-borne constituents, but also those advected during active groundwater/surface water (hyporheic) exchange. During both discharge conditions, Ba concentrations indicated slight non-conservative mixing. Such Ba excesses could be attributed either to submarine groundwater discharge or particle desorption processes. Estuarine dissolved organic carbon concentrations were highest at salinities closest to zero. Uranium distributions were lowest in the fresh water sites and mixed mostly conservatively with an increase in salinity. Suspended particulate matter (SPM) concentrations were generally lowest (< 5 mg L− 1) close to zero salinity and increased several-fold ( 18 mg L− 1; low discharge) toward the seaward endmember, which may be attributed to dynamic resuspension of bottom sediments within Jupiter Inlet.Surface water-column 222Rn activities were most elevated (> 28 dpm L− 1) at the freshwater endmember of the estuary and appear to identify regions of the river most influenced by the discharge of fresh groundwater. Activities of four naturally occurring isotopes of Ra (223,224,226,228Ra) in this estuary and select adjacent shallow groundwater wells yield mean estuarine water-mass transit times of less than 1 day; these values are in close agreement to those calculated by tidal prism and tidal frequency. Submarine groundwater discharge rates to the Loxahatchee River estuary were calculated using a tidal prism approach, an excess 226Ra mass balance, and an electromagnetic seepage meter. Average SGD rates ranged from 1.0 to 3.8 × 105 m3 d− 1 (20–74 L m− 2 d− 1), depending on river-discharge stage. Such calculated SGD estimates, which must include both a recirculated as well as fresh water component, are in close agreement with results obtained from a first-order watershed mass balance. Average submarine groundwater discharge rates yield NH4+ and PO4− 3 flux estimates to the Loxahatchee River estuary that range from 62.7 to 1063.1 and 69.2 to 378.5 μmol m− 2 d− 1, respectively, depending on river stage. SGD-derived nutrient flux rates are compared to yearly computed riverine total N and total P load estimates.  相似文献   

8.
The absorption of anthropogenic CO2 and atmospheric deposition of acidity can both contribute to the acidification of the global ocean. Rainfall pH measurements and chemical compositions monitored on the island of Bermuda since 1980, and a long-term seawater CO2 time-series (1983–2005) in the subtropical North Atlantic Ocean near Bermuda were used to evaluate the influence of acidic deposition on the acidification of oligotrophic waters of the North Atlantic Ocean and coastal waters of the coral reef ecosystem of Bermuda. Since the early 1980's, the average annual wet deposition of acidity at Bermuda was 15 ± 14 mmol m− 2 year− 1, while surface seawater pH decreased by 0.0017 ± 0.0001 pH units each year. The gradual acidification of subtropical gyre waters was primarily due to uptake of anthropogenic CO2. We estimate that direct atmospheric acid deposition contributed 2% to the acidification of surface waters in the subtropical North Atlantic Ocean, although this value likely represents an upper limit. Acidifying deposition had negligible influence on seawater CO2 chemistry of the Bermuda coral reef, with no evident impact on hard coral calcification.  相似文献   

9.
Oxygen minimum zones in the eastern tropical Atlantic and Pacific oceans   总被引：2，自引：0，他引：2  
Within the eastern tropical oceans of the Atlantic and Pacific basin vast oxygen minimum zones (OMZ) exist in the depth range between 100 and 900 m. Minimum oxygen values are reached at 300–500 m depth which in the eastern Pacific become suboxic (dissolved oxygen content <4.5 μmol kg−1) with dissolved oxygen concentration of less than 1 μmol kg−1. The OMZ of the eastern Atlantic is not suboxic and has relatively high oxygen minimum values of about 17 μmol kg−1 in the South Atlantic and more than 40 μmol kg−1 in the North Atlantic. About 20 (40%) of the North Pacific volume is occupied by an OMZ when using 45 μmol kg−1 (or 90 μmol kg−1, respectively) as an upper bound for OMZ oxygen concentration for ocean densities lighter than σθ < 27.2 kg m−3. The relative volumes reduce to less than half for the South Pacific (7% and 13%, respectively). The abundance of OMZs are considerably smaller (1% and 7%) for the South Atlantic and only 0% and 5% for the North Atlantic. Thermal domes characterized by upward displacements of isotherms located in the northeastern Pacific and Atlantic and in the southeastern Atlantic are co-located with the centres of the OMZs. They seem not to be directly involved in the generation of the OMZs.OMZs are a consequence of a combination of weak ocean ventilation, which supplies oxygen, and respiration, which consumes oxygen. Oxygen consumption can be approximated by the apparent oxygen utilization (AOU). However, AOU scaled with an appropriate consumption rate (aOUR) gives a time, the oxygen age. Here we derive oxygen ages using climatological AOU data and an empirical estimate of aOUR. Averaging oxygen ages for main thermocline isopycnals of the Atlantic and Pacific Ocean exhibit an exponential increase with density without an obvious signature of the OMZs. Oxygen supply originates from a surface outcrop area and can also be approximated by the turn-over time, the ratio of ocean volume to ventilating flux. The turn-over time corresponds well to the average oxygen ages for the well ventilated waters. However, in the density ranges of the suboxic OMZs the turn-over time substantially increases. This indicates that reduced ventilation in the outcrop is directly related to the existence of suboxic OMZs, but they are not obviously related to enhanced consumption indicated by the oxygen ages. The turn-over time suggests that the lower thermocline of the North Atlantic would be suboxic but at present this is compensated by the import of water from the well ventilated South Atlantic. The turn-over time approach itself is independent of details of ocean transport pathways. Instead the geographical location of the OMZ is to first order determined by: (i) the patterns of upwelling, either through Ekman or equatorial divergence, (ii) the regions of general sluggish horizontal transport at the eastern boundaries, and (iii) to a lesser extent to regions with high productivity as indicated through ocean colour data.  相似文献   

10.
Multiple tracers of groundwater input (salinity, Si, 223Ra, 224Ra, and 226Ra) were used together to determine the magnitude, character (meteoric versus seawater), and nutrient contribution associated with submarine groundwater discharge across the leeward shores of the Hawai'ian Islands Maui, Moloka'i, and Hawai'i. Tracer abundances were elevated in the unconfined coastal aquifer and the nearshore zone, decreasing to low levels offshore, indicative of groundwater discharge (near-fresh, brackish, or saline) at all locations. At several sites, we detected evidence of fresh and saline SGD occurring simultaneously. Conservative estimates of SGD fluxes ranged widely, from 0.02–0.65 m3 m− 2 d− 1at the various sites. Groundwater nutrient fluxes of 0.04–40 mmol N m− 2 d− 1 and 0.01–1.6 mmol P m− 2 d− 1 represent a major source of new nutrients to coastal ecosystems along these coasts. Nutrient additions were typically greatest at locations with a substantial meteoric component in groundwater, but the recirculation of seawater through the aquifer may provide a means of transferring terrestrially-derived nutrients to the coastal zone at several sites.  相似文献   

11.
We have investigated the chemical forms, reactivities and transformation kinetics of Fe(III) species present in coastal water with ion exchange and filtration methods. To simulate coastal water system, a mixture of ferric iron and fulvic acid was added to filtered seawater and incubated for a minute to a week. At each incubation time, the seawater sample was acidified with hydrochloric acid and then applied to anion exchange resin (AER) to separate negatively charged species (such as fulvic acid, its complexes with iron and iron oxyhydroxide coated with fulvic acid) from positively charged inorganic ferric iron (Fe(III)′). By monitoring the acid-induced Fe(III)′ over an hour, it was found that iron complexed by fulvic acid dissociated rapidly to a large extent (86–92% at pH 2), whereas amorphous ferric oxyhydroxide particles associated with fulvic acid (AFO-L) dissociated very slowly with the first-order dissociation rate constants ranging from 6.1 × 10− 5 for pH 3 to 2.7 × 10− 4 s− 1 for pH 2. Therefore, a brief acidification followed by the AER treatment (acidification/AER method) was likely to be able to determine fulvic acid complexes and thus differentiate the complexes from the AFO-L particles (the dissolution of AFO-L was insignificant during the brief acidification). The acidification/AER method coupled with a simple filtration technique suggested that the iron–fulvic acid complexes exist in both the < 0.02 μm and 0.02–0.45 μm size fractions in our coastal water system. The truly dissolved iron (< 0.02 μm) was relatively long-lived with a life-time of 14 days, probably due to the complexation by strong ligands. Such an acid-labile iron may be an important source of bioavailable iron in coastal environments, as a significant relationship between the chemical lability and bioavailability of iron has been well recognised.  相似文献   

12.
Shelf break systems are highly dynamic environments. However little is known about the influence that benthic interactions and water mass mixing may have on vertical distributions of iron in these systems. Dissolved Fe (< 0.4 μm) concentrations were measured in samples from nine vertical profiles across the upper slope (150–2950 m water depth) at the Atlantic Ocean–Celtic Sea shelf break. Dissolved iron concentrations varied between less than 0.2 and 5.4 nM, and the resulting detailed section showed evidence of a range of processes influencing the Fe distributions. The near sea floor data were interpreted in terms of release and removal processes. The concentrations of dissolved Fe present in near seabed waters were consistent with release of Fe from in situ remineralisation of particulate organic matter at two upper slope stations, and possibly release from pore water upon resuspension on shelf. Lateral transport of dissolved iron was evident from elevated Fe concentrations in an intermediate nepheloid layer and its advection along isopycnals. Surface waters at the shelf break also showed evidence of vertical mixing of deeper iron-rich waters. These waters contained macronutrients that sustained primary productivity in these otherwise nutrient-depleted surface waters. The data also suggest some degree of stabilisation of relatively high concentrations of iron, presumably through ligand association or as colloids. This study supports the view that lateral export of dissolved iron to the interior of the ocean from shelf and coastal zones and may have important implications for the global budget of oceanic iron.  相似文献   

13.
In September 1993 (M26) and June/July 1996 (M36), a total of 239 surface samples (7 m depth) were collected on two transects across the open Atlantic Ocean (224 samples) and northwest European shelf edge area. We present an overview of the horizontal variability of dissolved Cd, Co, Zn, and Pb in between the northwest and northeast Atlantic Ocean in relation to salinity and the nutrients. Our data show a preferential incorporation of Cd relative to P in the particulate material of the surface ocean when related to previously published parallel measurements on suspended particulate matter from the same cruise. There is a good agreement with results recently estimated from a model by Elderfield and Rickaby (Nature 405 (2000) 305), who predict for the North Atlantic Ocean a best fit for αCd/P=[Cd/P]POM/[Cd/P]SW of 2.5, whereas the approach of our transect shows a αCd/P value of 2.6. The Co concentrations of our transects varied from <5 to 131 pmol kg−1, with the lowest values in the subtropical gyre. There were pronounced elevations in the low-salinity ranges of the northwest Atlantic and towards the European shelf. The Co data are decoupled from the Mn distribution and support the hypothesis of marginal inputs as the dominant source. Zinc varied from a minimum of <0.07 nmol kg−1 to a maximum of 1.2 and 4.8 nmol kg−1 in regions influenced by Labrador shelf or European coastal waters, respectively. In subtropical and northeast Atlantic waters, the average Zn concentration was 0.16 nmol kg−1. Zinc concentrations at nearly three quarters of the stations between 40°N and 60°N were <0.1 nmol kg−1. This suggests that biological factors control Zn concentrations in large areas of the North Atlantic surface waters. The Pb data indicated that significant differences in concentration between the northwest and northeast Atlantic surface waters presently (1996) do not exist for this metal. The transects in 1993 and 1996 exhibited Pb concentrations in the northeast Atlantic surface waters of 30 to 40 pmol kg−1, about a fifth to a quarter of the concentrations observed in 1981. This decline is supported by our particle flux measurements in deep waters of the same region.  相似文献   

14.
Aerosol (soluble and total) iron and water-column dissolved (DFe, < 0.2 μm) and total dissolvable (TDFe, unfiltered) iron concentrations were determined in the Canary Basin and along a transect towards the Strait of Gibraltar, in order to sample across the Saharan dust plume. Cumulative dust deposition fluxes estimated from direct aerosol sampling during our one-month cruise are representative of the estimated deposition fluxes based on near surface water dissolved aluminium concentrations measured on board. Iron inventories in near surface waters combined with flux estimates confirmed the relatively short residence time of DFe in waters influenced by the Saharan dust plume (6–14 months). Enhanced near surface water concentrations of DFe (5.90–6.99 nM) were observed at the Strait of Gibraltar mainly due to inputs from metal-rich rivers. In the Canary Basin and the transect towards Gibraltar, DFe concentrations (0.07–0.76 nM) were typical of concentrations observed in the surface North Atlantic Waters, with the highest concentrations associated with higher atmospheric inputs in the Canary Basin. Depth profiles showed that DFe and TDFe were influenced by atmospheric inputs in this area with an accumulation of aeolian Fe in the surface waters. The sub-surface minimum of both DFe and TDFe suggests that a simple partitioning between dissolved and particulate Fe is not obvious there and that export may occur for both phases. At depths of around 1000–1300 m, both regeneration and Meddies may explain the observed maximum. Our data suggest that, in deep waters, higher particle concentrations likely due to dust storms may increase the scavenging flux and thus decrease DFe concentrations in deep waters.  相似文献   

15.
Submarine groundwater discharge (SGD) to coastal southern Rhode Island was estimated from measurements of the naturally-occurring radioisotopes 226Ra (t1/2 = 1600 y) and 228Ra (t1/2 = 5.75 y). Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith–Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, from January 2002 to August 2003; additional porewater samples were collected in August 2005. Surface water activities ranged from 12–83 dpm 100 L− 1 (60 dpm = 1 Bq) and 21–256 dpm 100 L− 1 for 226Ra and 228Ra, respectively. Porewater 226Ra activities ranged from 16–736 dpm 100 L− 1 (2002–2003) and 95–815 dpm 100 L− 1 (2005), while porewater 228Ra activities ranged from 23–1265 dpm 100 L− 1. Combining these data with a simple box model provided average 226Ra-based submarine groundwater fluxes ranging from 11–159 L m− 2 d− 1 and average 228Ra-derived fluxes of 15–259 L m− 2 d− 1. Seasonal changes in Ra-derived SGD were apparent in all ponds as well as between ponds, with SGD values of 30–472 L m− 2 d− 1 (Winnapaug Pond), 6–20 L m− 2 d− 1 (Quonochontaug Pond), 36–273 L m− 2 d− 1 (Ninigret Pond), 29–76 L m− 2 d− 1 (Green Hill Pond), and 19–83 L m− 2 d− 1 (Pt. Judith–Potter Pond). These Ra-derived fluxes are up to two orders of magnitude higher than results predicted by a numerical model of groundwater flow, estimates of aquifer recharge for the study period, and values published in previous Ra-based SGD studies in Rhode Island. This disparity may result from differences in the type of flow (recirculated seawater versus fresh groundwater) determined using each technique, as well as variability in porewater Ra activity.  相似文献   

16.
Dissolved Fe and ligand concentrations and the Fe-binding strength of the organic ligands were measured in samples from the upper water column (150 m) of the oligotrophic waters of the Canary Basin (eastern North Atlantic Ocean). Concentrations of major nutrients, phytoplankton abundance and photosynthetic characteristics were also measured in the same samples.The concentrations of dissolved Fe and dissolved organic ligands were low with mean values of 0.31 ± 0.18 nM Fe and 1.79 ± 0.73 nEq of M Fe(n = 47), respectively. The conditional binding constant varied between 1019.8–1022.7 (n = 47). The largest variation with depth in the ligand concentrations (between 4.78 and 1.1 nEq of M Fe) was observed in the upper layer, above the Deep Chlorophyll Maximum (DCM located between 80 and 100 m), with high surface values in stations at 18 and 34.At the DCM where Fe was depleted, the ligand concentrations were still relatively high showing the same trend with depth as the amount of phytoplankton cells. Here 62% of the vertical variation in ligand concentrations can be explained by parameters describing phytoplankton cell abundance or biomass and orthosilicic acid concentration, which could reflect diatom growth. Ligand concentrations below the maximum of the DCM (n = 4) showed good linear positive relationships with the total phytoplankton biomass as well as with 2 out of 4 distinguished groups of phytoplankton (Synechococcus and pico-eukaryote I).In the maximum of the DCM and below this maximum the phytoplankton origin of the dissolved organic ligands of Fe is very probable. Data suggest a release of ligands by cell lysis and not by an active production. However, the origin in the surface layer is more difficult to explain. Although the amount of phytoplankton cells in the surface layer is reduced, it is still  25% of the cell concentration observed in the DCM. High concentrations of organic ligands could then be a remnant of past blooms or present production under nutrient depleted conditions. Input of Sahara dust can be another source of ligands.  相似文献   

17.
Measurements of submarine groundwater discharge (SGD) along the South American coast and over fractured rock aquifers are rare. The rate and distribution of SGD was measured using three types of vented benthic chambers on the floor of Flamengo Bay located at the southeast coast of Brazil. Discharge rates were found up to almost 400 cm day−1, although typically less than 100 cm d−1. Large variations in SGD rates were seen over distances of a few meters which are attributed to the geomorphologic features of the fracture rock aquifer underlying a thin blanket of coastal sediments; clustering of fractures and the topography of the rock–sediment interface might be focusing or dispersing the discharge of groundwater. SGD was modulated by the tides with the highest values occurring at times of low tide, but the interaction was non-linear and, the correlation was weak at tidal ranges less than 1 m. The effect was masked by devices that integrated the SGD, but detected on continuously recording devices.  相似文献   

18.
Natural iron fertilization processes are occurring around the Crozet Islands (46°26′S–52°18′E), thus relieving the water masses from the normally encountered High Nutrients Low Chlorophyll (HNLC) conditions of the Southern Ocean. During austral summers 2004/2005 and 2005/2006, iron and aluminium concentrations were investigated in large particles (> 53 µm) collected from just below the mixed layer at stations under the influence of island inputs, and also in adjacent HNLC waters. These large particles are anticipated to sink out of the mixed layer, and to reflect the net effects of input and cycling of these elements in the overlying mixed layer. Labile and refractory fractions were determined by a two-stage leaching technique. Data showed that water masses downstream of the islands were enriched in total iron and aluminium (0.25–2.68 nmol L− 1 and 0.34–3.28 nmol L− 1 respectively), relative to the southern HNLC control sites (0.15–0.29 nmol L− 1 for Fe and 0.12–0.29 nmol L− 1 for Al), with only a small fraction (typically < 1%) being acid leachable in both environments. Particulate iron predominantly derived from the island system represents a significant fraction of the total water column iron inventory and may complement dissolved Fe inputs that help support the high summer productivity around the Crozet islands.  相似文献   

19.
We observed the origin, behavior, and flux of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), colored dissolved organic matter (CDOM), and dissolved inorganic nitrogen (DIN) in the subterranean estuary of a volcanic island, Jeju, Korea. The sampling of surface seawater and coastal groundwater was conducted in Hwasun Bay, Jeju, in three sampling campaigns (October 2010, January 2011, and June 2011). We observed conservative mixing of these components in this subterranean environment for a salinity range from 0 to 32. The fresh groundwater was characterized by relatively high DON, DIN, and CDOM, while the marine groundwater showed relatively high DOC. The DON and DIN fluxes through submarine groundwater discharge (SGD) in the groundwater of Hwasun Bay were estimated to be 1.3 × 105 and 2.9 × 105 mol d− 1, respectively. In the seawater of Hwasun Bay, the groundwater-origin DON was almost conservative while about 91% of the groundwater-origin DIN was removed perhaps due to biological production. The DON flux from the entire Jeju was estimated to be 7.9 × 108 mol yr− 1, which is comparable to some of the world's large rivers. Thus, our study highlights that DON flux through SGD is potentially important for delivery of organic nitrogen to further offshore while DIN is readily utilized by marine plankton in near-shore waters under N-limited conditions.  相似文献   

20.
We measured significant activities of short-lived radium isotopes, 223Ra (half-life = 11 days) and 224Ra (half-life = 3.7 days), around the margins of the Hawaiian Islands to water depths of 3500 m. These measurements suggest fluid inputs from the basalt to the surrounding ocean. In general 223Ra activities were considerably greater than 224Ra in spite of the expected higher production rate of 224Ra activity in basalt. The 223Ra was not supported by dissolved 227Ac. The highest enrichments of 223Ra were measured over the Puna Ridge (2100 m depth) east of Hawaii. Here 223Ra activities reached 19 dpm/m3, similar to activities measured near sites of active submarine groundwater discharge in the South Atlantic Bight. To explain the high activities of 223Ra unaccompanied by 224Ra, we postulate that thermally-driven circulation of seawater through the Puna Ridge deposits 231Pa on basalt surfaces. With time the 231Pa produces 227Ac and 223Ra; and 223Ra desorbs into the circulating fluids. These fluids then transport 223Ra into the overlying ocean. Based on the inventory of 223Ra above the Puna Ridge, we estimate the flow of fluids through the ridge to be on the order of 20–60 cm3 cm− 2 day− 1. In less than 1000 years the incoming seawater could provide enough 231Pa to basalt surfaces to balance the inventory of 223Ra above the ridge if only 8% of the 223Ra was transported to the overlying water. These observations on the flanks of a volcanically-active ocean island have significant implications for quantifying fluid fluxes from the flanks of the mid-ocean ridge system. By mapping 223Ra inventories in the ocean above ridge flanks and measuring the activity of 223Ra in the emerging fluids, the fluid flux can be obtained.  相似文献   

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