Showing papers in "Journal of Marine Research in 1996"

A key role for iron-bound phosphorus in authigenic apatite formation in North Atlantic continental platform sediments

Abstract: A combination of pore water and solid phase analysis was used to determine whether authigenic carbonate fluorapatite (CFA) is currently forming in the sediment at two locations (OMEX I and II) on the North Atlantic continental platform Goban Spur (southwest of Ireland). Results of selective P extractions suggest that an early diagenetic redistribution of Fe-bound P to an authigenic P phase may be occurring at both stations. A steady-state diagenetic model describing the depth profiles of pore water HPO 4 2- and three solid phase forms of P (organic P, Fe-bound P and authigenic P) was developed and applied to the data of station OMEX-I. The model results indicate that CFA formation can account for the observed increase of authigenic P with depth at this station. Furthermore, the results show that an intense cycling of P between Fe-bound P and pore water HPO 4 2- at the redox interface can create conditions beneficial for CFA formation. This internal P cycle is driven by downward, bioturbational transport of mainly in-situ-formed Fe-bound P into the reduced sediment zone. Losses from the internal P cycle due to CFA formation and HPO 4 2- diffusion are compensated for by sorption of HPO 4 2- released from organic matter to Fe oxides in the oxidized surface sediment. Fe-bound P thus acts as an intermediate between organic P and CFA. CFA can account for between 25 and 70% of the total burial flux of reactive P at station OMEX-I and thus may act as an important sink for P in this low sedimentation, continental margin environment.

The dynamics of benthic nutrient pools and fluxes in tropical mangrove forests

Abstract: Variations in benthic nutrient pools and rates of dissolved nutrient exchange between the forest floor and tidal waters were examined over a 5-yr period in mixed Rhizophora forests lining Coral Creek on Hinchinbrook Island in Queensland, Australia. Seasonal and spatial changes in redox status, porewater and solid-phase nutrients, and in exchange rates were not consistent and did not correlate with temperature. Below-ground roots, on average, accounted for ∼79%, 37% and 26% of bulk sediment TOC, total N and total P pools, respectively. Porewater nutrient concentrations were dominated by Si(OH) 4 + and DON with consistently low levels of NO 2 - + NO 3 - . At most sampling periods, porewater NH 4 + and PO 4 3- concentrations were higher in creek bank sediments than in mangrove sediments indicating uptake by trees. These sediments have low adsorption capacity (K = 0.17-0.47) for NH 4 + , but a moderate capacity (K = 0.8-4.8) for PO 4 3- adsorption. Most measured benthic fluxes of dissolved nitrogen and phosphorus showed uptake by sediments, prop roots and timber lying on the forest floor. Relative (per ha) estimates indicate that low-intertidal Rhizophora forests import ∼2220 mmol N ha -1 d -1 and ∼496 mmol P ha -1 d -1 , with sediments accounting for nearly all uptake while Si is exported (∼2475 mmol ha -1 d -1 . Mid-intertidal forests import ∼1385, 93 and 4720 mmol ha -1 d -1 of N, P and Si, respectively ; sediments, prop roots and timber respectively account for 36%, 62% and 2% of the N import. Mid-intertidal sediments account for all net P uptake, but prop roots and sediments account for 60% and 40% of total Si uptake. On an absolute basis, low-intertidal forests (78 ha total area) in Coral Creek import ∼881 kgN yr -1 , 436 kgP yr -1 and export 1963 kgSi yr -1 , and the mid-intertidal forests (338 ha total area) import ∼2392 kgN yr -1 , 356 kgP yr -1 and 16300 kgSi yr -1 . The sum of these estimates equates to ∼95% of the net annual import of total dissolved N and ∼66% of the net annual import of total dissolved P into the Coral Creek tidal basin from adjacent coastal waters. By difference, ∼14337 kgSi yr -1 is imported into the system. This indicates that mangrove forests are a very efficient sink of dissolved nitrogen, phosphorus and silicon in this tidally-driven coastal ecosystem. This import may be driven by the consistently high rates of microbial and plant growth and productivity within the forests.

Biogenic Matter Diagenesis on the Sea Floor: A Comparison Between Two Continental Margin Transects

Abstract: Benthic chamber measurements of the reactants and products involved with biogenic matter diagenesis (oxygen, ammonium, nitrate, silicate, phosphate, TCOP, alkalinity) were used to define fluxes of these solutes into and out of the sediments off southern and central California. Onshore to offshore transects indicate many similarities in benthic fluxes between these regions. The pattern of benthic organic carbon oxidation as a function of water depth, combined with published sediment trap records, suggest that the supply of organic carbon from vertical rain can just meet the sedimentary carbon oxidation + burial demand for the central California region between the depths 100-3500 m. However, there is not enough organic carbon raining through the upper water column to support its oxidation and burial in the basins off southern California. Lateral transport and focusing of refractory carbon within these basins is proposed to account for the carbon buried. The organic carbon burial efficiency is greater off southern California (40-60%) compared to central California (2-20%), even though carbon rain rates are comparable. Oxygen uptake rates are not sensitive to bottom water oxygen concentrations nor to the bulk wt. % organic carbon in surficial sediments. Nitrate uptake rates are well defined by the depth of oxygen penetration into the sediments and the overlying water column nitrate concentration. Nitrate uptake accounts for about 50% of the total denitrification taking place in shelf sediments and denitrification (0. l-l .O mmolN/m*d) occurs throughout the entire study region. The ratio of carbon oxidized to opal dissolved on the sea floor is constant (0.8 t 0.2) through a wide range of depths, supporting the hypothesis that opal dissolution kinetics may be dominated by a highly reactive phase. Sea floor carbonate dissolution is negligible within the oxygen minimum zone and reaches maximal rates

Plankton production in tidal fronts : A model of Georges Bank in summer

Abstract: A two-dimensional (x,z) coupled physical-biological model of the plankton on Georges Bank during the summer was developed. The physical portion included a primitive-equation turbulence-closure model with topography-following a coordinate. The biological model was a simple N-P-Z model. Tidal forcing at the model boundary generated a well-mixed region on the top of the bank, and strong tidal fronts at the bank edges. Biological fields were homogenized on the bank, while pronounced phytoplankton patches and horizontal gradients in properties developed in the fronts. The biomasses and fluxes of biological variables in the model agreed well with field estimates from Georges Bank. The phytoplankton in the well-mixed region of the bank were found to be nutrient replete, with f ratios of about 0.3. Values up to 0.7 were found for the f ratios in the fronts, where phytoplankton patches were supported by vertical fluxes of nutrients from below the euphoric zone. While the patterns of patchiness in the fronts were stable between tidal periods, the structure of patches and fluxes changed dramatically during a tidal cycle. Enhanced vertical mixing and horizontal gradients formed during a brief period of the tide, accounting for much of the cross-frontal nutrient flux. Sampling in such a dynamic system would be very difficult, and probably miss the essential features.

A unified model for the generation and fission of internal tides in a rotating ocean

Abstract: A new two-layer model consisting of generalized Boussinesq equations is derived which contains forcing terms due to barotropic tidal flow over large-amplitude bottom topography. These equations can describe both the generation of nonlinear dispersive internal tides and their disintegration into solitary waves. Special attention is paid to the effects of Coriolis dispersion (which is due to the earth’s rotation). Numerical solutions based on observed oceanic conditions show convincingly that the earth’s rotation can be a decisive factor at mid-latitudes in that it tends to impede the disintegration of the internal tide. Oceanic observations in the Celtic Sea and in Massachusetts Bay are well reproduced by the model.

Modeling 210Pb-derived mixing activity in ocean margin sediments: Diffusive versus nonlocal mixing

Abstract: The influence of sediment mixing on activity versus depth profiles of the radionuclide *iOPb in the upper 20 cm of the sediments has been investigated along a depth transect (208 m4500 m, 17 stations) in the OMEX study area (Goban Spur, NE Atlantic Ocean). A hierarchical family of bioturbation!nonlocal exchange models was derived. Each member of the hierarchy includes all processes of the previous model, and adds a one- or two-parameter process. The significance of the additional parameters is tested using a one-tailed F-test. It was found that (1) in five cases there is a significant improvement when direct injection of part of the flux into deeper sediment layers (nonlocal exchange) is added to the diffusive mixing model. (2) In these five cases, the best model required only two additional parameters, compared to the diffusive mixing model. More elaborate models, including additional parameters did not result in a significantly better fit. (3) In four cases, the inclusion of diffusive mixing (bioturbation) to an advection/decay model does not result in a significant better fit of modeled versus measured *l”Pb activity-depth profiles. Using the simplest nonlocal exchange model, the amount of particulates that are directly injected at depth into the sediment was estimated and compared with the amount incorporated at the sediment surface. Along the OMEX transect, between 8-86% of the total flux enters the sediment by nonlocal exchange rather than by mere bioturbation/advection at the sediment surface. The importance of nonlocal exchange decreases with increasing water depth. To allow comparison with other measurements, we have also calculated the diffusive mixing coefficient using the classical bioturbation model. The sediments in the OMEX area have low bioturbation coefficients, especially at the deeper sites. Finally our models have also been used to reproduce and to explore some aberrant *‘OPb profiles reported in the literature.

Carbon export and regeneration in the coastal upwelling system of Monterey Bay, central California

Abstract: In order to quantify the role of coastal upwelling regions as source or sink areas for carbon, the relationships between particulate organic carbon (POC) production, export, remineralization, and accumulation were examined in Monterey Bay from 1989 through 1992. During a normal upwelling year (1989-90), a high positive correlation (r = 0.91) is observed between biweekly primary production and POC export at 450 m. Primary production values range from 500 mgC m -2 d -1 during the winter, to 2600 mgC m -2 d -1 in the spring and summer upwelling months. Corresponding deep-water (450 m) POC fluxes vary from a minimum of 10 mgC m -2 d -1 in December, to 120 mgC m -2 d -1 in May. In contrast, the mid-1991 through 1992 data sets obtained during the '91-92 El Nino period, show a relatively poor correlation (r = 0.23) between productivity and carbon export. Calculated ratios of POC export to POC production (defined as e-ratios) display a trend for the three-year data sets in which the e-ratio values are greatest during periods of low productivity and decrease to minimal values when surface production is high. Upwelling-induced, offshore Ekman transport of organic matter and probable seasonal changes in the planktonic community structure are the mechanisms likely to be responsible for the e-ratio trends. Based on the data sets reported from this work, a simple box model of the annual export and regeneration of particulate organic carbon is presented for the Monterey Bay region. An appreciable advective and/or recycling loss from the euphotic zone of 362.8 gC m -2 y -1 is estimated, representing primarily algal material transported offshore and/or recycled within the upper 100 m of the water column. Annual mid-water (∼100-450 m) and deep-water (>450 m) POC remineralization rates of 71.8 gC m -2 y -1 of 7.2 gC m -2 y -1 , respectively, are reported for Monterey Bay. The average POC rain rate to the underlying slope sediments is sufficient to satisfy reported benthic utilization requirements without invoking an additional input source of POC via deep lateral advection and/or the downslope movement of particulate material.

Sedimentation of phytoplankton during a diatom bloom : Rates and mechanisms

Abstract: Phytoplankton blooms are uncoupled from grazing and are normally terminated by sedimentation. There are several potential mechanisms by which phytoplankton cells may settle out of the photic zone: sinking of individual cells or chains, coagulation of cells into aggregates with high settling velocities, settling of cells attached to marine snow aggregates formed from discarded larvacean houses or pteropod feeding webs, and packaging of cells into rapidly falling zooplankton fecal pellets. We quantified the relative significance of these different mechanisms during a diatom bloom in a temperate fjord, and evaluated their potential to control phytoplankton population dynamics. Overall specific sedimentation rates of intact phytoplankton cells were low during the 11 -day study period, averaging ca. 0.1 d-l, and mass sedimentation and bloom termination did not occur. Most cells settled attached to marine snow aggregates formed from discarded larvacean houses, whereas settling of unaggregated cells was insignificant. Formation rates of phytoplankton aggregates by physical coagulation was very low, and losses by this mechanism were co.07 d-t; phytoplankton aggregates were neither recorded in the water column (by divers) nor in sediment traps. The low coagulation rates were due to a very low ‘stickiness’ of suspended particles. The dominant diatom, Thalassiosira mendiolana, that accounted for up to 75% of the phytoplankton biomass, was not sticky at all, and did not turn sticky upon nutrient depletion in culture experiments. The low particle stickiness recorded may be related to low formation rates by diatoms of transparent exopolymeric particles (TEP), that occurred in low concentrations throughout the study period. Zooplankton grazing rate did not respond to the development of the bloom and accounted for a loss term to the phytoplankton populations comparable to sinking of intact cells; fecal pellets accounted for 3&50% of settled phytoplankton and phytodetritus. While coagulation may give rise to density-dependent losses to phytoplankton populations and, hence, control blooms, neither of the other mechanisms 1. Danish Institute for Fisheries Research, Charlottenlund Castle, DK-2920 Charlottenlund, Denmark (email: tk@dfu.min.dk) 2. Marine Biological Laboratory, Strandpromenaden 5, DK-3000 Helsinger, Denmark. 3. Marine Sciences Institute, University of California, Santa Barbara, California, 93106, U.S.A. 4. Department of Oceanography, Texas A&M University, College Station, Texas, 77843, U.S.A. 5. Department of Marine Sciences, University of Connecticut, Groton, Connecticut, 06340-6097, U.S.A. 6. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02143, U.S.A. 7. Present address: School of Biological Sciences, Victoria University of Wellington, P.O. Box 600, Welling- ton, New Zealand. 8. Universidad de Cadiz, Department de Biologia Animal, Vegetal y Ecologia, Aptdo 40, E-l 1510 Puerto Real (Cadiz), Spain. 1123

Distinct year-to-year particle flux variations off Cape Blanc during 1988–1991: Relation to δ 18 O-deduced sea-surface temperatures and trade winds

Abstract: Particle fluxes measured from 1988 to 1991 adjacent to a coastal upwelling site off Cape Blanc showed significant interannual variability of fluxes and sea-surface temperatures (SST) deduced from stable oxygen isotope analysis of the planktonic foraminifera Globigerinoides ruber and, partly, of the pteropod Limacina inflata. For the duration of the study period, a decrease in the seasonality of SST's was observed, as well as a significant decrease in the average annual SST from 24.4° to 20.8°C. This cooling trend was mainly the effect of a drastic decrease in the summer to fall SST (from 27.2° to 21.8°C). In comparison, the winter-spring SST decreased only slightly from 20.3° in 1988 to 19.8°C in 1991. Concomitantly, we measured decreasing annual total, carbonate, biogenic opal and lithogenic fluxes and, in contrast, increasing marine organic carbon fluxes. During 1991, when cold SST's prevailed and the trade winds were rather high throughout, annual biogenic and lithogenic fluxes (except organic carbon) were lower by approximately a factor of two compared to the other years. Colder SST's, generally corresponding to stronger trade winds and upwelling intensity, did not result in increased biogenic opal and lithogenic matter sedimentation ; but higher marine organic carbon fluxes were recorded. Decreasing summer-fall SST from 1988 to 1991 coincided with decreased carbonate sedimentation maxima which generally occurred during the warm summer season. In the summer of 1989, when SST's were the highest of the four-year sampling period and upwelling was less intense due to weak spring-summer trades, a large sedimentation pulse of pteropod shells was observed. Our data set does not yet provide conclusive evidence that the observed year-to-year flux and SST variations represent larger-scale, periodically occurring climatic variations in the eastern Atlantic but it offers insight into the prevailing large variability in biochemical cycles and processes in the eastern Atlantic.

Water-property distributions along an eastern Pacific hydrographic section at 135W

Abstract: As part of the World Ocean Circulation Experiment, full-depth CTD/hydrographic measurements with high horizontal and vertical resolutions were made in June-August 1991 along a line extending from 34N to 33S at a nominal longitude of 135W with an additional short leg that connects it to the California coast roughly along 34N. The line spans the major part of the subtropical and intertropical circulation regime of the eastern North and South Pacific. The primary purpose of this paper is to present vertical sections of various properties from CTD and discrete water-sample measurements along this line and to give an overview of some important features as a basis for more comprehensive basin-scale studies. These features include: the frontal structures found in the surface-layer salinity field in the North Pacific; relatively high-salinity water that dominates the subpycnocline layer between the equator and 17N; troughs of the subpycnocline isopycnals for 26.8-27.5 σ θ found at 12N and 12.5S; a permanent thermostad at 9-10°C observed between 4.5N and 15N; the pycnostad of the Subantarctic Mode Water centered at 27.0-27.05 σ θ and developed south of 22S; two types of the Antarctic Intermediate Water representing the subtropical and equatorial circulation regimes; a thick tongue of high silica centered at 3000 m (45.8 σ 4 ) and extending southward across the entire section; deep (2000-3000 m) westward flows at 5-8N and 10-15S separated by an eastward flow at 1-2S; and dense, cold, oxygen-rich, nutrient-poor bottom waters, which are associated with fracture zones and believed to represent the pathways of eastward flows into the Northeast Pacific Basin of the bottom waters separated from the northward-flowing western boundary undercurrent. This work once again demonstrates the usefulness of long lines of high-quality, high-resolution hydrographic stations such as the one described herein in advancing the understanding of the large-scale ocean circulation.

The effects of seagrass patch size and energy regime on growth of a suspension-feeding bivalve

Abstract: An investigation of how the presence of seagrass and seagrass patch size in high- and low-energy environments affected growth of an infaunal bivalve (hard clam, Mercenaria mercenaria) was carried out. Two general size classes of clams were used, mean length ca. 40.0 mm (41.9 mm) and mean length ca. 20.0 mm (22.3 mm and 19.0 mm), to see if growth responses were the same for large- and small-sized clams. Seagrass density, length and species composition as well as sediment characteristics were measured at high- and low-energy sites to determine if changes in seagrass and/or sediment dynamics could explain observed growth patterns. The incidence of siphon cropping was also evaluated as a potential influence on clam growth. Large clams grew faster inside than outside vegetation at both high- and low-energy sites. Within the vegetation, growth of large clams was faster in medium-sized patches of seagrass (2-3 m across) than in small patches (1 m across) and was intermediate in large patches (> 4-5 m across). There was no apparent correlation between seagrass shoot density, blade length, or species composition and growth of clams in patches of different size. Growth of small clams (22.3 mm) within seagrass was independent of the two seagrass patch sizes tested (large vs. small), but did vary among sites. Growth of clams was not correlated with differences in shoot density, blade length or species composition among sites. Growth of small clams (19.0 mm) also varied with energy regime and with presence/absence of vegetation. Small clams grew significantly more within seagrass under both high- and low-energy conditions, but the effect was more pronounced at high-energy sites than at low. Seagrass shoot density, blade length, and species composition did not vary between high and low energy regimes, but did vary among sites from the beginning to the end of the experiment. Long-term averages of sediment stability based on grain size characteristics suggest that the baffling effect of seagrass varies greatly with energy regime. Vegetated sediments at high-energy sites contained significantly more fine material than the unvegetated sediments while there was no difference in the fine fraction between vegetated and unvegetated sediments at low-energy sites. The difference in sediment stability between seagrass cover under high- and low-energy conditions may contribute to the magnitude of the difference in the growth response of small clams to the presence of vegetation at exposed and protected sites. Other factors also contributed to the increased growth of clams in seagrass beds at the protected sites where grain size analysis suggests similar sedimentary dynamics both within and outside of the vegetation. Mean adjusted siphon weights, however, for both large and small clams were independent of seagrass cover and energy regime implying that siphon nipping differences did not influence growth patterns of clams. Based on this study, and previous investigations, the effect of seagrass cover on growth of clams appears to be the result of a complex interaction among food supply, predation disturbance, and sediment stability with the relative importance of these processes varying with size of the clam, hydrographic regime, and local site differences.

A study of the omega equation for diagnosing vertical motions at ocean fronts

Abstract: Estimation of vertical velocity is a key issue for understanding ocean physics and transport of biogeochemical tracers. We examine the accuracy of estimating vertical velocity in fronts with the omega equation. The diagnostic performance of the omega equation is evaluated by using vertical velocities obtained from simulation of frontal instabilities in a primitive equation model as a reference. We use two traditional quasigeostrophic methods in which the flow is either a geostrophic flow computed from density or a nondivergent flow derived from vorticity and also test two new formulations: a quasigeostrophic method using the total flow field and the semigeostrophic omega equation. Results show that all four formulations correctly diagnose the vertical velocity pattern. However, estimates provided by the traditional quasi-geostrophic formulations have systematic bias. In contrast, the two new techniques, which are practically equivalent, produce unbiased vertical velocity diagnostic at fronts. These results point out the importance of including higher order dynamics than quasigeostrophy to take into account the ageostrophic advection in the front. Since adequate filtering of ADCP data is not yet available to obtain a suitable total flow, the semigeostrophic omega equation is proposed as the most valuable tool to compute vertical velocities from high resolution density measurements.

Spatial variation in short-term ( 234 Th) sediment bioturbation intensity along an organic-carbon gradient

Abstract: Natural and human-induced spatial gradients provide a useful vehicle with which to better understand diverse marine processes. On the Palos Verdes margin (S. California), historical and ongoing waste-water discharge has created an along-shelf gradient in organic C and total N, as well as various trace metals and other pollutants (e.g., DDT). To better understand the impact of such pollution on bioturbation and to develop a more general understanding of the controlling factors of sediment bioturbation intensity, a series of stations representing severely, moderately and negligibly impacted sediments was studied. Vertical profiles of the naturally occurring radionuclide, 234 Th, as well as the abundance and species composition of macrofauna were measured from box cores collected at three sites during July 1992. During a March 1993 cruise, radionuclide profiles were collected at an additional eleven sites on the margin. Excess 234 Th profiles are, in general, consistent with a steady-state model that balances vertical biodiffusive mixing with radioactive decay. Biodiffusivities determined from the 234 Th profiles yield a spatial pattern in which sediments near the outfall are mixed at intensities of ∼10 cm 2 /yr, and bioturbation intensities are five times as rapid at sites 5-7 km from the outfall. Average mixing intensities are between these extremes (28 cm 2 /yr) at a nearby unimpacted site. Despite the overall consistency of this pattern the reasons behind it remain unclear. Structural aspects of the macrofauna either do not vary between the three intensively studied stations (e.g., depth distribution, size) or do so in a manner that would suggest an opposite effect on the biodiffusivity (e.g., abundance). There is also little variability in trophic groupings along the enrichment gradient. Behavioral modifications, such as: (I) sublethal pollution effects caused by elevated contaminant (e.g., organic carbon and DDT) concentrations, and (2) inhibition by a tube-building polychaete, Mediomastus sp., are postulated to suppress mixing intensities near the outfall. The results of this study suggest that, at least in shallow-water settings, the general controls of bioturbation intensity are still poorly understood.

Molecular diversity of the copepod, Nannocalanus minor: Genetic evidence of species and population structure in the North Atlantic Ocean

Abstract: The abundant calanoid copepod, Nannocalanus minor, is widespread from the Florida Straits (FS), throughout the Gulf Stream and the Sargasso Sea, to the eastern North Atlantic Ocean. Does the species represent a single, randomly-mating population across this extensive region, or does it comprise a number of genetically distinct populations or taxonomically distinct forms? What are patterns and pathways of dispersal of the copepod across the North Atlantic? These questions were addressed using population genetic analysis of DNA sequence variation of a 440 base-pair region of the mitochondrial 16S rRNA gene. This analysis separated N. minor into two genetically distinct types (distinguished by 10% sequence difference) that may represent the previously described N. m. forma major and N.m. forma minor. The two genetic types differed in size range and in geographic distribution: Type I individuals were larger and were most abundant in the western regions of the Gulf Stream; Type II individuals were smaller and became more abundant toward the eastern regions of the Gulf Stream. Significant differences in the size-frequency distributions of N. minor from different regions of the North Atlantic may result from mixtures of the two genetic types and environmental differences in food availability. Within N. minor Type I, mtDNA sequence variation defined 68 haplotypes among 155 individuals. The haplotype frequency distribution was skewed: there were 40 individuals of one haplotype, 31 individuals of a second, and 60 unique individuals. Haplotype diversity, h, was very similar across the sampled range: h = 0.886 in samples from the FS and 0.874 for samples from the Gulf Stream Meander Region (GSMR). Nucleotide diversity, p i , was significantly greater in the FS (p i = 0.00490) than in the GSMR (0.00414), largely due to a number of genetically divergent individuals. Haplotype abundances did not differ significantly either within the regions (among FS samples, P = 0.756; among GSMR samples, P = 0.336) or between the regions (P = 0.636). Molecular genetic analysis can reveal cryptic species among marine taxa, and is particularly useful for taxa characterized by morphological similarity.

Variability and sources of the southeastern Atlantic circulation

Abstract: The 1992-1993 Benguela Sources and Transport (BEST) time series provide a quantitative view of the Benguela Current transport and the eddy field crossing 3OS, as well as an estimate of the relation between its barotropic and baroclinic components. This is done by a simultaneous analysis of the BEST data derived from inverted echo sounders, pressure sensors, current meter moorings, CTD, and ADCP stations. The analysis of the time series indicates that the annual mean baroclinic transport of the Benguela Current is 13 Sv with a total transport of 16 Sv. Through the combination of instruments the total baroclinic plus barotropic transport of the upper 2600 m was obtained without making any assumption about the level of no motion. Results from this calculation corroborated the assumption that 1000 m as a level of no motion could be used as a fairly good approximation. The stationary flow of the Benguela Current is mostly confined near the African Continent while a transient flow, composed by large eddies shed from the Agulhas retroflection, dominates the western portion of the Benguela Current. In the stationary part of the Benguela Current, both barotropic and baroclinic components are equally important while in the transient part, the barotropic is more substantial. Several rings were observed during the experiment that migrated toward the west. An initial speed of 12 km/day diminished to 6 to 7 km/day at the Walvis Ridge. The water mass source of the Benguela Current includes Indian and South Atlantic subtropical thermocline water; relatively saline, low oxygen tropical Atlantic water; and the cooler, fresher subantarctic water. Changes in thermocline salinity correlate with transport: in general when the northward transport is increasing

Spin-down and the dynamics of dense pool gyres in shallow seas

Abstract: Isolated lenses or domes of dense (often cold) bottom water are found in many shelf seas and can be expected to induce significant baroclinic circulations. However, provided they have the correct thermal wind shear, many flow states can support a bottom dome structure, including: (a) an anticyclonically circulating dense dome overlain by a static surface layer or (b) cyclonic surface flow around a static dome. A one-dimensional, two-layer, quasi-geostrophic model is applied to a Gaussian vortex to argue that, on account of bottom friction in shelf seas, the bottom layer will spin down to rest over a time-scale of just a few days and that, due to layer coupling through vortex stretching/compression, cyclonic flow will concentrate in the surface layer. The association of cyclonic surface flow with static, dense domes in shallow seas is supported by reports of tank experiments and by recent observations in a cold pool system in the Irish Sea.

The solubility pump of carbon in the subtropical gyre of the North Atlantic

Abstract: The subduction of carbon is examined using abiotic models of the solubility pump in the subtropical gyre of the North Atlantic. The importance of the seasonal cycle of the mixed layer, and advection of carbon, is examined using sensitivity experiments with a Lagrangian model of the carbon system. The rate of subduction of carbon is found to be strongly influenced by the gradients in mixed-layer thickness over the gyre and, to a lesser extent, modified by the end of winter bias in the properties of subducted fluid. A seasonally-cycling geochemical model of the carbon system is then developed for the North Atlantic. The model is diagnosed to examine the seasonal exchange in carbon between the atmosphere and ocean induced by the seasonal warming and cooling. There is a net annual air-sea flux of carbon into the subtropical gyre of the model due to undersaturation ofpco, with respect to the local equilibrium with the atmosphere. The undersaturation is due to advection of carbon by the circulation. Along the path of the Gulf Stream, northward advection and cooling of the low latitude waters is so rapid that the surface waters are significantly undersaturated in carbon. Due to its long equilibration period, there is a resultant air-sea flux of carbon dioxide over the northern flank and interior of the subtropical gyre. Warm, low carbon water from the tropics is fluxed into the southern flank of the subtropical gyre in the Ekman layer, inducing an oceanic uptake of carbon there. The model experiments suggest that it is necessary to account for advection to close the carbon budget in the observed time-series measurements at Bermuda.

Diatom and silicoflagellate fluxes at the Walvis Ridge : An environment influenced by coastal upwelling in the Benguela system

Abstract: Seasonal fluctuations in the total particle, biogenic opal, diatom and silicoflagellate fluxes were observed in sediment traps deployed at 599 m and 1648 m in the Walvis Ridge area, within the Benguela upwelling system, from March 1989 to March 1990 (station WR 2: 20°02.8'S, 09°09.3'E). Fluxes were directly related to wind stress variations (wind maxima preceding flux maxima by several weeks), and inversely related to SST changes (derived from measured concentrations of C 37 alkenones; range: 14.6°-23.6°C). The biogenic particle composition at different depths reflected the complicated hydrology of the area with a combination of tropical, temperate and subantarctic water masses. Biogenic opal content varied from about 2 to 12% of the total mass flux in the upper trap and from about 4 to 17% in the lower trap. Diatoms were the main contributor to the opal fraction (mean daily flux of ca. 5.5 * 10 6 valves m -2 day -1 ), followed by silicoflagellates (ca. 2.6 * 10 5 skeletons m -2 day -1 ). Two seasonal maxima, in May and June (austral autumn) and from October to November (austral spring), were observed; silicoflagellates yielded also a third moderate maximum in August/September (austral winter). At 1648 m fluxes peaked from May to July (data were available for the period 18 Mar to 27 Aug 1989 only). Few diatoms were abundant; 19 taxa accounted for 50% of all the diatoms identified, and about 32 for the 75% level. Specific diversity of diatoms at 599 m was highest during times of lowest fluxes, in the austral winter, late spring and summer. The diatom taxa occurring at 599 m and at 1648 m were the same, with some flux enrichment with depth due to advection of particles into the lower trap by resuspension and downslope movement. The relatively high concentrations of the Antarctic-Subantarctic species Fragilariopsis kerguelensis in the upper trap solely, was probably linked to selective entrainment and transport within a ring of southerly origin (south of the Subantarctic/Subtropical Convergence Zone). The enrichment of moderately resistant and robust taxa in the sediments in conjunction with the rarity or absence of delicate taxa points to preferential concentration in the sediments of some taxa and dissolution of others. The occurrence of phytoliths in the traps and in the sediment sample can be linked to the berg winds, which are typical for the entire Benguela region during fall and winter.

Impact of the North Equatorial Current meandering on a pelagic ecosystem: A modeling approach

Abstract: A modeling study was conducted to investigate the effects of time-dependent mesoscale meandering of the North Equatorial Current on a pelagic ecosystem in the southwestern Canary Basin. The North Equatorial Current jet was represented as a quasi-geostrophic flow using a two-layer model; a standard bulk mixed layer model is included. Two cases for the biological/physical coupled model were examined: (a) a nutrient-phytoplankton-zooplankton (NPZ) model and (b) the addition of a sinking detritus pool (NPZD) in the ecosystem. The horizontal length scale of simulated eddies is 100 to 200 km. The surface eddy kinetic energy has a peak value of 110 cm 2 /s 2 and a mean value of 26 cm 2 /s 2 in the simulated North Equatorial Current. Maximum vertical velocity is of the order of 1.5 m/ day at 100 m depth, the base of the mixed layer. The additional nutrients due to eddy upwelling lead to a maximum increase of phytoplankton biomass up to 26% located at the edge of eddies. This trend is even more pronounced when introducing a detritus pool with a 1 m/day sinking velocity into the ecosystem (33%). When upwelling events are seldom present at mooring sites, it is the particulate organic carbon input by horizontal advection which feeds the carbon loss by detritus sinking. At mooring sites undergoing upwelling events, the upwelled carbon flux largely dominates losses by sedimentation and leads to a 10% enhancement of the sinking exported carbon flux. When the eddies are resolved, the mean values of the primary and exported productions in the jet zone are doubled. The results suggest that the vertical motion due to eddies and eddy-eddy interactions in a weak (10 cm/s) horizontal current such as the North Equatorial Current can be a non-negligible source of nitrogen-nutrients for oceanic plankton production in the mixed layer.

An experimental and modeling study of pH and related solutes in an irrigated anoxic coastal sediment

Abstract: Macrofaunal irrigation is an important process in nearshore sediments, facilitating greater exchange between sediments and seawater and imparting significant lateral heterogeneity to the porewater profiles of many constituents. Like many macrofaunal activities, irrigation is a transient behavior, i.e. tubes and burrows are flushed periodically, at frequencies that generally are species-specific. As a result, transient concentrations within the dwelling arise, potentially impacting gradients, fluxes and reaction rates in the vicinity of the dwelling. We investigated the impact of periodic burrow irrigation on the distribution of several diagenetically important porewater constituents. Laboratory experiments evaluated irrigation periodicity using artificially irrigated tubes embedded in nearshore organic-rich sediments, and microdistributions of oxygen and pH in laboratory experiments were measured with microelectrodes. To help interpret our results, we also constructed a simplified time and space-dependent transport-reaction model for oxygen, pH and sulfide in irrigated sediments. Laboratory results show substantial differences in the pH field of sediments surrounding an irrigated tube as a function of irrigation frequency. Higher pH values, indicative of an overlying water signature, were observed in the vicinity of the tube wall with increasing duration of irrigation. Conversely, oxygen concentrations did not vary significantly with the amount of irrigation, most likely a result of extremely high sediment oxygen demand. Model results are consistent with laboratory findings in predicting differences in the measured variables as a function of irrigation frequency. However, the nature and extent of the model-predicted differences are often at variance with the experimental data. Overall, experimental and modeling results both suggest irrigation periodicity can substantially influence porewater distributions and diagenetic processes in sediments. Future studies should examine the influence of irrigation periodicity on the types and rates of reactions, and the attendant biological features, in the environment encompassing the tube or burrow wall.

The flow field off southwest India at 8N during the southwest monsoon of August 1993

Abstract: The flow field off the southwest coast of India at 8N was investigated during RV Sonne cruise 89 in August 1993 by direct velocity observations from shipboard- and lowered-ADCP and geostrophic computations from CTD stations. The upper ocean between 75E and 76°52'E near the South Indian shelf was governed by a northward flow with a subsurface velocity maximum of 25 cm s -1 at about 100 m depth. This flow, organized as a poleward undercurrent hugging the continental slope, is typical for the southwest monsoon season. The northward transport in August 1993 was 4.7 Sv (1 Sverdrup = 10 6 m 3 s -1 ) for the upper 300 m from the shipboard ADCP. Earlier geostrophic observations showed southward surface flow above the poleward undercurrent, but in August 1993 the northward flow reached to the surface and in the geostrophy calculations, i.e. without the southward Ekman flow near the surface, there was even no clear subsurface core. The T-S characteristics show that Bay of Bengal Water (BBW) was carried with this flow, and low wind conditions seemed to be connected to the flow of BBW from the southern tip of Sri Lanka toward the southwest coast of India. Further offshore, two meridional current bands were identified in the upper 300 m of the ocean. West of the coastal undercurrent a band of southward flow existed with velocities up to 35 cm s -1 above and to the east of the Chagos-Laccadive Ridge, from 72°10'E to 75E. The associated geostrophic transport in the upper 500 m was 5.2 Sv. As the T-S relation was different from that of the northward flow, this current band was not a local recirculation of the poleward undercurrent. Further west, the flow was weak, but intensified toward the central Arabian Sea, between 66E and 69°20'E, where another southward current band was found with velocities of up to 20 cm s -1 and a total geostrophic transport for the upper 300 m of -7.2 Sv.

The effects of differential vertical diffusion of T and S in a box model of thermohaline circulation

Abstract: A diffusive box model, consistent with geostrophy, is proposed as an alternative to more usual advective box models of the ocean thermohaline circulation. When vertical diffusion coefficients for T and S are taken as identically equal (the normal assumption in all numerical ocean models to date), the diffusive box model exhibits both steady-state modes and time-dependent behaviors which are essentially indistinguishable from those of an advective model, under both fixed flux and mixed (T restoring) boundary conditions. The thermohaline “circulation” of the diffusive box model, however, is a combination of a convective branch and a vertical diffusive branch, involving zero volume flux. Modifications in behavior of the diffusive box model are investigated for a plausible range of values for the ratio d equiv Ks/KT of the vertical turbulent diffusivities of S and T. When surface fluxes of heat and freshwater are constant, the model with d ne 1 exhibits additional steady-state modes in which convection is absent from the system, as well as a periodic oscillatory mode. Compared to results with d equiv 1 under mixed surface boundary conditions, the model with d ne 1 exhibits extended ranges of multiple equilibria, a different mode transition near present-day values of freshwater forcing magnitude, and the possibility of quasi-periodic oscillatory states. The sensitivity of the present box model, coupled with that previously observed in a primitive equation model (Gargett and Holloway, 1992), raises serious questions about the ability of numerical models to predict the evolution of the ocean thermohaline circulation under changing atmospheric forcing, even if other problems with such prediction were resolved.

Fluid flow and suspended particulates as determinants of polychaete feeding behavior

Abstract: We examined the interactive effects of fluid flow, bed characteristics and suspended load on the feeding behavior of four species of marine polychaetes. Two species of obligate deposit feeders (Marenzelleria viridis and Ampharete parvidentata) and two species of palp-coiling facultative suspension feeders (spiochaetopterus oculatus and Spio setosa) were exposed to flow and sediment-bed treatments that served to decouple fluid flow and particle flux. We employed low (no particle transport), medium (transport of flocs only) and high (transport of sand) flow speeds in factorial treatments of natural sediment, winnowed bed (flocs removed), armored bed (no sand transport at high flows), and armored bed plus fines (flocs added). For each species, worms were exposed to an increasing (low, medium and high) and then decreasing (high, medium and low) flow leg for each bed treatment. We recorded visual observations of animal behavior of the four polychaete species. We found little systematic response to flow and bed differences in the two obligate deposit feeders. When fine material was present, one of the two species exhibited higher variability in time spent deposit feeding, possibly responding to small-scale depositional pockets enriched with fine particles and organic matter. For both facultative suspension feeders, there was an increase in time spent suspension feeding with increasing flow and suspended particle concentrations. Percent suspension feeding was also greater on the decreasing flow legs in treatments with fine material available for suspension. Exploratory analyses of the data reveal a direct relationship between time spent suspension feeding and the flux of suspended high quality organic matter. For both species, compositional parameters of particulate nitrogen and enzymatically available amino acid concentrations were the best correlates of suspension feeding behavior.

Buoyant discharge on the inner continental shelf: A frontal model

Abstract: A steady state, frontal model of the arrested topographic wave type (Csanady, 1978) is developed for application to buoyant coastal discharge of large-scale and weak stratification, typically found on the inner continental shelf. The across-shore momentum balance is geostrophic, while the alongshore momentum balance includes wind stress and bottom or interfacial friction. The dynamics thus has semi-geostrophic character. No mixing dynamics is present. The model has two major purposes : first, to serve as the vehicle for a process study requiring only moderate computing resources, and second, to inquire into the general consequences of extending the original single-layer model of Csanady (1978) into a two-layer, frontal model for application to buoyant coastal discharge. Analysis of the flow near the frontal bottom intersection shows that the bottom stresses on each side of the front must be equal and match as well the interfacial stress just above. Similar analysis near the surface front shows that static stability there requires the presence of only downwelling-favorable wind stress. This implies that a statically stable, steady state is not possible for upwelling-favorable winds. The model possesses an asymptotic downshelf state that is termed frictionally adjusted flow in which alongshore gradients and across-shore velocities vanish and bottom, interfacial, and wind stresses all are equal. The front then becomes trapped to the local isobaths. Numerical experiments showed that the model contains possible spatially growing instability because of the frontal boundary. Weaker baroclinic strength and diminished bottom slopes tended to increase flow stability. Stable flows were computed in their evolution from a prescribed upshelf state intended to simulate estuarine outflow of buoyant discharge and adjacent inflow of denser ambient shelf water. A turning region developed where the front moved first offshore then back nearer the coast. Experiments showed that the turning region was a joint product of the turning isobath geometry imposed near the estuary mouth and the estuarine inflow of shelf water. Comparisons of model results with recent observations of the Delaware Coastal Current showed general qualitative agreement, but highlight the lack of model mixing processes.

Characterization of the particulate protein in Pacific surface waters

Abstract: We investigated molecular characteristics of particulate proteins in Pacific surface waters using two-dimensional electrophoresis (2DE). Most proteinaceous materials estimated by dye-binding methods were characterized by the 2DE unresolved acidic materials with a broad range of molecular mass and the 2DE unresolved low molecular mass materials with a broad range of isoelectric point. The 2DE unresolved acidic and low molecular mass materials were considered to comprise peptides conjugated with acidic saccharides and degradation products (peptides) of proteins, respectively, which indicates that almost all proteins in living organisms failed to survive in detrital particulate organic matter (POM). Nevertheless, 23 discrete proteins were distinguished by the 2DE. Electrophoretic patterns of the discrete proteins indicated that they were a component of detrital POM. Three discrete proteins were subjected to N terminal amino acid sequence analysis. Two proteins out of three could not be determined because their N termini were blocked, and one protein was determined from the N terminus to the ninth amino acid residue. A homology search revealed that the N terminal amino acid sequence of the protein agreed completely with that of 70 kDa heat shock protein (HSP70) derived from photosynthetic organisms. HSP70 is a major member of the molecular chaperones that protect or repair proteins from damage under conditions of environmental stress. The occurrence of HSP70 in this study demonstrated that phytoplankton were able to induce the molecular chaperone(s). Clarification of factor(s) controlling induction of chaperones will enable us to assess the actual environmental stress on phytoplankton at the biomolecular level. The particulate organic matter (POM) in surface water is a complex mixture of living biomass and nonliving detritus. The contribution of living biomass to the bulk POM is generally 1 order of magnitude lower than that of detrital organic matter (e.g., Volkman and Tanoue 2002). The particulate combined amino acids (PCAA) are the largest identified fraction in surface POM (e.g., Wakeham et al. 1997), and dye-binding colorimetric methods demonstrated that proteinaceous materials were a major component of POM (Setchell 1981; Long and Azam 1996). Phytoplankton is a primary producer of organic matter in the sea, and most combined amino acids in phytoplankton are in the form of proteins. However, heterotrophic processing may convert cellular proteins of phytoplankton to other forms before its incorporation into detrital pool. At present the chemical nature of the detrital combined amino acids and proteinaceous materials is not well documented, and the processes by which organic matter produced by phytoplankton is transferred to detrital POM are not clear. The characterization of proteinaceous materials in POM at the molecular level is 1

Biological removal of fine-grained lithogenic particles from a large river plume

Abstract: The pelagic tunicate, Oikopleura dioica, feeds by non-selectively filtering particles in the size range of 0.2-20 μm. In the northern Gulf of Mexico, particulate matter in this size range contains large amounts of fine-grained lithogenic material because of the influence of the Mississippi River. During May 1992, O. dioica populations filtered between 2 and 44% (mean = 20%) of the upper 5 m each day. The filtered lithogenic particles either remain in the oikopleurid house or are defecated in fecal pellets which have high sinking velocities. Either way, the larvacean populations significantly alter the fates of fine-grained lithogenic particles in these waters and thereby enhance light penetration. The widespread occurrence of oikopleurids in coastal regions of temperate and subtropical oceans suggests they could have a significant influence on the fates of fine-grained lithogenic particles in discharge plumes of many of the world's large, sediment-laden rivers.

The role of mesoscale hydrography on microbial dynamics in the northeast Atlantic: Results of a spring bloom experiment

Abstract: During RV Meteor cruise No. 10 from May to June 1989 (JGOFS pilot study) bacterial and picocyanobacterial abundance, biomass, and bacterial production were estimated at two drift stations close to 47N, 20W and 58N, 20W in the northeast Atlantic. At 47N two different mesoscale hydrographic structures were sampled which divided the drift experiment into a cyclonic and an anticyclonic circulation phase. Transition from one phase to the next was clearly reflected by changes of the biological structure in the upper water column. Phytoplankton stocks maintained during the cyclonic phase were about 1.8 times higher than those of the anticyclonic phase (1552 mg C m -2 and 880 mg C m -2 , resp., integrated over the mixed layer, Deckers, 1991). Integrated stocks of bacteria showed an opposite pattern of distribution. Picocyanobacterial biomass (PCB) was 3.4 times higher during the anticyclonic phase than during the cyclonic phase (96 mg C m -2 and 28 mg C m -2 , resp.), and the respective factor for total bacterial biomass (TBB) was 3.7 (830 mg C m -2 and 225 mg C m -2 , resp.). Our analysis indicates that the combined bacterial biomass dominated within the mixed layer during the anticyclonic phase, while the cyclonic phase was clearly dominated by eucaryotic phytoplankton. Additional evidence for a shift of biology toward the microbial food web was indicated by a strong increase of bacteria during the anticyclonic phase. Thus, simultaneously and side by side, an autotrophic and a heterotrophic system were supported by the prevailing hydrographic conditions. At 58N within an anticyclonic mesoscale hydrographic structure the phytoplankton bloom was at a developing stage, characterized by low biomass (730 mg C m -2 in the mixed layer, Deckers, 1991) but relatively high primary production. In contrast, bacterial stocks were quite high, but bacterial production was low in comparison to the anticyclonic phase at 47N (90 mg C m -2 d -1 and 153 mg C m -2 d -1 , resp., integrated from 0-300 m). It was calculated that bacterial gross production averaged 42% (47N, anticyclonic phase) and 25% (58N) of primary production. These results suggest that within a specific type of hydrographic structure either a heterotrophic or an autotrophic system can be established, depending on the stage of bloom development. In conclusion : Depending on their origin and age, mesoscale hydrographic structures can be correlated with different stages of biological development. This leads to the mesoscale patchiness of biological measurements, which is a characteristic feature of the northeast Atlantic.

Seasonal transport variations of the wind-driven ocean circulation in a two-layer planetary geostrophic model with a continental slope

Abstract: Using a simple two-layer planetary geostrophic model with a continental slope, the observed seasonal variation of the total transport of the Kuroshio is explained qualitatively for the first time in a quite concise way. During the period of weak winds in summer, the total transport is much larger than the nontopographic Sverdrup transport. This is because the joint effect of baroclinicity and bottom relief (so-called JEBAR) drives the barotropic flow with releasing the available potential energy accumulated in winter. In winter, on the other hand, the transport is much smaller than the nontopographic Sverdrup transport. This is mainly due to the topographic beta-effect. In addition to the available potential energy supplied directly by winds, the barotropic flow excited by the strong winds in winter impinges on the bottom slope to induce the baroclinic flow so that the wind-driven barotropic flow is converted into the available potential energy to be released in summer. Thus, the role of JEBAR is to make the total transport of the Kuroshio relatively insensitive to seasonal changes of winds as observed. This mechanism may be called JEBAR rectification for simplicity.

A high resolution camera system (ParCa) for imaging particles in the ocean : System design and results from profiles and a three-month deployment

Abstract: For direct optical measurement of abundance, concentration and size distribution of marine particles, a high-resolution camera system (ParCa) was designed to improve on similar systems used by Honjo et al. (1984), Asper (1987) and others. Imaging a probe volume of up to 37 1, smallest particles with diameters of 50 μm can be counted. The images provide information on particle size, shape and abundance either during profiling through the water column or while moored in a certain depth over time. Depth profiles were acquired between fall 1992 and late spring 1993 on R. V. Meteor cruises M22-1 and M23-3 at 6 stations in the equatorial Atlantic Ocean and off the west African shelf. The images show variable particle and aggregate concentrations through 550 m of the water column, with highest concentrations in the upper 80 m. A distinctive change in the depth of the upper chlorophyll maximum from about 75 m in the Brazil Basin to about 50 m in the Guinea Basin was measured with the attached INFLUX current meter (Krause and Ohm, 1996) and is as well represented in the particle abundances of two selected profiles. In contrast, both profiles show a second particle abundance maximum between 100 and 250 m, which is not visible in the chlorophyll-a and backscatter signal of the INFLUX sensors. Total particle abundance maxima raise from 677 counts per liter in the central Brazil Basin to 991 counts in the Guinea Basin, corresponding to marine snow abundances of 57 and 127 counts per liter, respectively. In order to compare high-resolution data on particle concentration and flux through time, ParCa was also deployed on a sediment-trap mooring at 995 m depth in the Canary Basin between June and September 1994. First results show similar trends in sediment-trap derived fluxes of particulate matter from 2.8 to 67.2 mg m -2 d -1 and equivalent spherical volumes of particles with diameters > 0.5 mm from 0.98 to 4.13 mm 3 l -1 .

The role of turbulence in the sedimentation loss of pelagic aggregates from the mixed layer

Abstract: A simple approach is presented to model the effect that turbulence has on the sedimentation loss of pelagic particles from the mixed layer. The approach consists in determining how turbulence affects their vertical distribution by splitting the solution of the advection-diffusion equation into two functions, one for the concentration of particles in the mixed layer and the other one reflecting the shape of the profile of particle concentration. The results of the paper indicate that the sedimentation flux is seriously underestimated if a uniform distribution of particles is assumed in the mixed layer when turbulence levels are low. A correction to this underestimation is possible in many situations without resolving the vertical scale in the mixed layer. The cases in which the correction cannot be applied are delimited in the paper in terms of dimensionless numbers. The results also demonstrate the importance of resolving the vertical scale in models of particle dynamics and add further support to the hypothesis of turbulence as the mechanism responsible for daily cycles of aggregates in the sea.