Search

International audience; For the first time a very high resolution palynological study (mean resolution of 1 to 5 years) was carried out over the last 150 years in a French estuarine environment (Bay of Brest; NW France), allowing direct comparison between the evolution of landscapes, surface water, and human practices on Bay of Brest watersheds, through continental (especially pollen grains) and marine (phytoplanktonic microalgae: cysts of dinoflagellates or dinocysts) microfossils. Thanks to the small size of the watersheds and the close proximity of the depositional environment to the mainland, the Bay of Brest represents an ideal case study for palynological investigations. Palynological data were then compared to published palaeo-genetic analyses conducted on the same core and to various available instrumental data, allowing us to better characterize past environmental variability since the second half of the 19th century in Western Brittany. We provide evidence of some clues of recent eutrophication and/or pollution that affected phytoplankton communities and which appears linked with increased runoff (higher precipitations, higher percentages of riparian forest pollen, decline of salt marsh-type indicators, and higher values of the XRF Ti/Ca signal), mainly explained by the evolution of agricultural practices since 1945 superimposed on the warming climate trend. We assume that the significant relay observed between dinocyst taxa: Lingulodinium machaerophorum and Spiniferites bentorii around 1965 then followed by Spiniferites membranaceus after 1985, attests to a strong and recent eutrophication of Bay of Brest surface waters induced by high river runoff combined with abnormally elevated air temperatures, especially obvious in the data from 1990. The structure of the dinocyst community has thus been deeply altered, accompanied by an unprecedented increase of Alexandrium minutum toxic form at the same period, as confirmed by the genetic quantification. Despite this recent major anthropogenic forcing, the fossil pollen sequence also records natural climate variability. We highlight, for the first time, a possible connection between climate (AMO modes) and fossil pollen records (especially tree pollination rates) in coastal sediments using tree percentage fluctuations as an indirect proxy for past sea surface and atmospheric temperatures.

Mechanisms controlling the transfer and retention of silicon (Si) along continental margins are poorly understood, but play a major role in the functioning of coastal ecosystems and the oceanic biological pump of carbon. Deep-sea fans are well recognized as carbon sink spots, but we lack knowledge about the importance of the fans in the global Si cycle. Here, we provide a first estimate of the role played by the Congo deep-sea fan, one of the biggest in the world, in the Si cycle. Sediment cores sampled in the deep-sea fan were analyzed to build a Si mass balance. An exceptionally high accumulation rate of amorphous silica aSiO 2 (2.29 ± 0.58 mol Si m − 2 y − 1 ) was found, due to a high sedimentation rate and the presence of aluminum in the sediments. Although favored by bioirrigation, recycling fluxes remained low (0.3 mol Si m − 2 y − 1 ) and reconstructed input fluxes could only be explained by lateral inputs coming from the canyon. Preliminary calculations show that the rapid transport of aSiO 2 through the canyon and the excellent preservation efficiency in the sediments imply that 50% of aSiO 2 river inputs from the Congo River accumulate annually in the deep-sea fan. Si:C ratios in deep-sea fan sediments were very low (0.2) and only three times as high as those measured in the river itself, which suggests that material from the river and the continental shelf was delivered directly through the canyon, with very little time for Si and C cycle decoupling to take place.

International audience; The oceans' continental margins face strong and rapid change, forced by a combination of direct human activity, anthropogenic CO2-induced climate change, and natural variability. Stimulated by discussions in Goa, India at the IMBER IMBIZO III, we (1) provide an overview of the drivers of biogeochemical variation and change on margins, (2) compare temporal trends in hydrographic and biogeochemical data across different margins, (3) review ecosystem responses to these changes, (4) highlight the importance of margin time series for detecting and attributing change and (5) examine societal responses to changing margin biogeochemistry and ecosystems. We synthesize information over a wide range of margin settings in order to identify the commonalities and distinctions among continental margin ecosystems. Key drivers of biogeochemical variation include long-term climate cycles, CO2-induced warming, acidification, and deoxygenation, as well as sea level rise, eutrophication, hydrologic and water cycle alteration, changing land use, fishing, and species invasion. Ecosystem responses are complex and impact major margin services. These include primary production, fisheries production, nutrient cycling, shoreline protection, chemical buffering, and biodiversity. Despite regional differences, the societal consequences of these changes are unarguably large and mandate coherent actions to reduce, mitigate and adapt to multiple stressors on continental margins.

International audience; The mesopelagic zone is the oceanic region through which carbon and other elements must pass in order to reach deeper waters or the sea floor. However, the food web interactions that occur in the mesopelagic zone are difficult to measure and so, despite their crucial importance to global elemental cycles, are not very well known. Recent developments in technology and new approaches have advanced the study of the variability in and controls upon the distribution and diversity of organisms in the mesopelagic zone, including the roles of respiration, recycling, and repackaging of particulate and dissolved organic material. However, there are remarkably few syntheses of the ecology and biogeochemistry of the microbes and metazoa that permanently reside or habitually visit this 'twilight zone'. Without this synthesis, it is difficult to assess the impact of ongoing changes in ocean hydrography and chemistry, due to increasing atmospheric carbon dioxide levels, on the biological carbon pump. This paper reviews what is known about the distribution of microbes and metazoa in the mesopelagic zone in relation to their activity and impact on global biogeochemical cycles. Thus, gaps in our knowledge are identified and suggestions made for priority research programmes that will improve our ability to predict the effects of climate change on carbon sequestration. (C) 2010 Elsevier Ltd. All rights reserved.

International audience; In shallow coastal areas the amplitude and range of benthic silicic acid fluxes can have a significant influence on benthic–pelagic coupling and the functioning of the pelagic system. To explore the oscillation in fluxes over the diurnal cycle and in particular the influence of microphytobenthos (MPB), an experiment was carried out in a shallow subtidal site in the Bay of Brest (France). Benthic chambers were employed over a 48 h period to measure the variability in silicic acid and oxygen fluxes; MPB migration was investigated using a diving Pulse Amplitude Modulated (PAM) fluorometer and uptake rhythms of silicic acid by natural MPB populations were measured using the 32Si isotope. It was discovered that silicic acid fluxes fluctuated greatly throughout the diurnal period resulting in an oscillation in the availability of this nutrient for phytoplankton communities. The uptake of silicic acid by the MPB was quantified for the first time and highlighted a 2-fold increase in the demand from night to afternoon periods. The combined silicic acid uptake and the concentration of cells at the sediment–water interface, forming a dense biofilm of MPB, were postulated to be the main processes reducing effluxes at midday. Our work highlighted the many processes which influence silicic acid effluxes in shallow coastal areas and the possible interaction between uptake and dissolution processes. The variations in benthic fluxes over the diurnal period were comparable to observations reported at the seasonal scale. Therefore, up-scaling hourly flux observations to daily and annual estimates should be undertaken with caution. Further we suggest that the main processes influencing flux oscillations over the diurnal period should be considered when planning sampling strategies and extrapolating to larger time scales.

International audience; The deposition of fresh phytoplankton detritus (phytodetritus) following phytoplankton blooms may influence biogenic silica (BSi) dissolution in marine sediments. We studied BSi dissolution properties before, during, and after periods of phytodetritus deposition during time-series field programs in the abyssal North Atlantic (the BENGAL project), and on the West Antarctic Peninsula Shelf (the FOODBANCS project). Dissolution experiments, performed by means of flow-through reactors, showed temporal variations in the dissolution properties of BSi in the sediment column after phytodetritus deposition. This non-steady-state character of benthic silica dynamics is an important aspect of pelagic-benthic coupling. The last FOODBANCS cruise occurred after a phytodetritus deposition event, and yielded high pore-water dissolved silica (DSi) concentrations and DSi effluxes in the upper centimetres of the sediment column, suggesting a rapid turnover of recently deposited siliceous material. Higher dissolution rates were measured in the phytodetritus-rich sediments relative to surface sediments collected during previous seasons on earlier FOODBANCS cruises. During the BENGAL project, high dissolution rates were measured at depth in the sediment column only after a summer phytodetritus deposition event. In the highly detrital sediment matrix of the abyssal North Atlantic Ocean, resolution of increased dissolution rates and experimental artefacts of the flow-through reactors can be difficult because of the low abundance of BSi. Depending on the sediment matrix, bioturbation can play a crucial role in transporting fresh BSi particles to depth, where DSi concentrations are close to experimentally determined BSi solubilities. The potential impacts of such processes on BSi preservation are discussed. We suggest that future models of BSi early diagenesis should include the rapid mixing of freshly deposited particles if we want to describe further the preservation of BSi in marine sediments.

The stable isotope of nitrogen ( 15 N) and an appropriate three-compartment model were used in two 24-h lasting feeding experiments to trace the flow of N through the copepod Acartia discaudata and Calanus helgolandicus fed on 15 N-labelled Skeletonema costatum and Thalassiosira weissflogii, respectively. Details of the labelling technique and principles of the computation of N transport rates are given. At the end of a single 24-h feeding period only about one third of the total amount of N ingested by A. discaudata was incorporated into the copepod's body N; we refer to this rate as net incorporation. Most of the N ingested was lost as ammonium (48% of total N ingested), followed by losses in the form of eggs+fecal pellets (13%) and dissolved organic N (DON, 9%). The sum of net incorporation and the latter losses is defined as gross incorporation. Net incorporation by C. helgolandicus and N losses did not vary over time during a 24 h lasting time-series feeding experiment. On average, 79% of total N ingested was actually incorporated by the copepod whereas mean N losses as ammonium, eggs+fecal pellets represented only 12 and 9%, respectively. After a 24-h feeding period only 2% of N ingested was lost as DON. Inspection of individual DON pathways showed that both A. discaudata and C. helgolandicus highly contributed to total DON production via direct excretion (79 and 64%, respectively). The remaining DON appearing in the DON pool was derived from phytoplankton via direct release and/or indirect release (copepod ‘sloppy feeding’). © 2007 Elsevier B.V. All rights reserved.

Flow-through experiments have been performed to study the thermodynamics of biogenic silica (opal) dissolution in deep-sea sediments. They were applied for the first time on sediment from the Southern Ocean Van Cappellen and Qiu 1997a , Van Cappellen and Qiu 1997b We have extended the use of these experiments to other deep-sea settings, thereby covering a wide range of in situ silicic acid asymptotic concentrations (Casympt; 200 to 900 μmol/L) and biogenic opal content (BSi; 0.5 to 80%). Performing these experiments under in situ bottom temperatures allows for the comparison between experimental apparent solubilities and Casympt concentrations. Low values of BSi apparent solubilities have been measured in the deepest sections of the multicores collected in the northeast Atlantic (229 μmol/L) and in the equatorial Pacific (505 μmol/L). They are only 10 to 20% higher than the in situ Casympt concentrations. This demonstrates a clear control of pore water silicic acid concentrations by the in situ apparent solubility of the BSi, i.e., the solubility of BSi within a complex sedimentary matrix that includes important quantities of silicate minerals. In regions where the percentage detrital/percentage biogenic ratio is low, the apparent solubility of the biogenic silica is close to that of in situ biogenic silica. In the opposite case, when the percentage detrital/percentage biogenic ratio is high, reprecipitation reactions induce strong interference on the dissolution properties of the opal, both in situ and in flow-through experiments. In such a sedimentary matrix, it is important to determine the appropriate opal solubility to be used in early diagenetic models, i.e., the solubility of the biogenic silica just before deposition on the seabed. This has been achieved by performing flow-through experiments on sediment trap material from the north Atlantic site. Comparison of apparent biogenic silica solubility measured by flow-through experiments and the silicic acid concentrations measured in the cups of the sediment traps suggested that the solubility of biogenic silica that reaches the sediment-water interface is not unique and varies spatially and temporally. In fact, it is the degree of coupling between surface waters and the sediment-water interface that will control the aging of biogenic silica in the water column and hence the dissolution properties of the biogenic silica deposited at the sediment-water interface. All these results call for a strong improvement of biogenic silica early diagenetic models that should include not only a reprecipitation term that takes into account interaction with silicate minerals but also the existence of several phases of biogenic silica and thus that should operate in a non-steady-state mode to account for seasonal variations in the quality of deposited biogenic silica.

Le probleme du decouplage entre les cycles biogeochimiques du silicium et du carbone organique particulaire est ici traite a partir de donnees de flux provenant de 5 sites situes dans l'ocean austral (dans la zone du Front Polaire, au nord et au sud de celui-ci) et de 4 sites situes dans le reste de l'ocean mondial (series temporelles dans l'Atlantique et le Pacifique). Pour l'ensemble des sites etudies les flux annuels de silice produite dans la couche de surface sont les plus eleves dans l'ocean austral (environ 3 mol m -2 an -1 ) et les flux de carbone les plus eleves dans le Pacifique Equatorial (environ 25 mol m -2 an -1 ). Les rapports molaires Si:C dans la couche de surface varient de 0.04 en moyenne pour les sites du reste de l'ocean mondial, a comparer a 0.25 pour l'Ocean Austral ou le phytoplancton est domine par les diatomees. Dans l'Ocean Austral les rapports Si:C sont particulierement eleves, tant pour la colonne d'eau (rapports variant de 2.6 a 10.4 pour les flux profonds > 3000 m) que pour less sediments (rapports de 13 a 29 pour les flux d'accumulation sur le long terme). Les rapports les plus faibles sont calcules pour le site BATS (0, 55 et 0, 57, respectivement pour les flux profonds dans la colonne d'eau et dans les sediments). Cependant, quelque soit le site etudie le rapport Si:C dans la colonne d'eau s'accroit avec la profondeur: ainsi le rapport Si:C des flux de production en surface etant pris comme reference, les rapports Si:C s'accroissent d'un facteur 24-28 pour les flux profonds et d'un facteur 106-111 dans les sediments. Dans la colonne d'eau la variation du rapport Si:C en fonction de la profondeur z suit l'equation: (Si:C) z = a(Si:C) b 0 /z c , ou (Si:C) 0 est la valeur du rapport des flux dans les eaux de surface et a, b, and c sont des parametres ajustes en fonction des donnees de flux recueillis aux sites exterieurs a l'Ocean Austral. Cette equation permet de predire correctement les valeurs des rapports Si:C pour les 5 sties d'etude pour l'Ocean Austral, dans la limite des incertitudes des determinations des flux de matiere biogene. Ceci montre que le decouplage des flux de carbone et de silice dans l'Ocean Austral n'est finalement pas different de ce qui se passe dans le reste de l'ocean mondial, contrairement a ce que l'on pouvait penser. Cette etude est une etape importante pour l'utilisation ulterieure de la silice biogene comme traceur de la paleoproductivite oceanique.

Within the framework of the EU-funded BENGAL programme, the effects of seasonality on biogenic silica early diagenesis have been studied at the Porcupine Abyssal Plain (PAP), an abyssal locality located in the northeast Atlantic Ocean. Nine cruises were carried out between August 1996 and August 1998. Silicic acid (DSi) increased downward from 46.2 to 213 μM (mean of 27 profiles). Biogenic silica (BSi) decreased from ca. 2% near the sediment–water interface to

Downward particle flux was measured using sediment traps at various depths over the Porcupine Abyssal Plain (water depth ~4850 m) for prolonged periods from 1989 to 1999. A strong seasonal pattern of flux was evident reaching a maximum in mid-summer. The composition of the material changed with depth, reflecting the processes of remineralisation and dissolution as the material sank through the water column. However, there was surprisingly little seasonal variation in its composition to reflect changes in the biology of the euphotic zone. Currents at the site have a strong tidal component with speeds almost always less than 15 cm/sec. In the deeper part of the water column they tend to be northerly in direction, when averaged over periods of several months. A model of upper ocean biogeochemistry forced by meteorology was run for the decade in order to provide an estimate of flux at 3000 m depth. Agreement with measured organic carbon flux is good, both in terms of the timings of the annual peaks and in the integrated annual flux. Interannual variations in the integrated flux are of similar magnitude for both the model output and sediment trap measurements, but there is no significant relationship between these two sets of estimates. No long-term trend in flux is evident, either from the model, or from the measurements. During two spring/summer periods, the marine snow concentration in the water column was assessed by time-lapse photography and showed a strong peak at the start of the downward pulse of material at 3000 m. This emphasises the importance of large particles during periods of maximum flux and at the start of flux peaks. Time lapse photographs of the seabed show a seasonal cycle of coverage of phytodetrital material, in agreement with the model output both in terms of timing and magnitude of coverage prior to 1996. However, after a change in the structure of the benthic community in 1996 no phytodetritus was evident on the seabed. The model output shows only a single peak in flux each year, whereas the measured data usually indicated a double peak. It is concluded that the observed double peak may be a reflection of lowered sediment trap efficiency when flux is very high and is dominated by large marine snow particles. Resuspension into the trap 100 m above the seabed, when compared to the primary flux at 3000 m depth (1800 mab) was lower during periods of high primary flux probably because of a reduction in the height of resuspension when the material is fresh. At 2 mab, the picture is more complex with resuspension being enhanced during the periods of higher flux in 1997, which is consistent with this hypothesis. However there was rather little relationship to flux at 3000 m in 1998. At 3000 m depth, the Flux Stability Index (FSI), which provides a measure of the constancy of the seasonal cycle of flux, exhibited an inverse relationship with flux, such that the highest flux of organic carbon was recorded during the year with the greatest seasonal variation.

Due to the major role played by diatoms in the biological pump of CO2, and to the presence of silica-rich sediments in areas that play a major role in air–sea CO2 exchange (e.g. the Southern Ocean and the Equatorial Pacific), opal has a strong potential as a proxy for paleoproductivity reconstructions. However, because of spatial variations in the biogenic silica preservation, and in the degree of coupling between the marine Si and C biogeochemical cycles, paleoreconstructions are not straitghtforward. A better calibration of this proxy in the modern ocean is required, which needs a good understanding of the mechanisms that control the Si cycle, in close relation to the carbon cycle. This review of the Si cycle in the modern ocean starts with the mechanisms that control the uptake of silicic acid (Si(OH)4) by diatoms and the subsequent silicification processes, the regulatory mechanisms of which are uncoupled. This has strong implications for the direct measurement in the field of the kinetics of Si(OH)4 uptake and diatom growth. It also strongly influences the Si:C ratio within diatoms, clearly linked to environmental conditions. Diatoms tend to dominate new production at marine ergoclines. At depth, they also succeed to form mats, which sedimentation is at the origin of laminated sediments and marine sapropels. The concentration of Si(OH)4 with respect to other macronutrients exerts a major influence on diatom dominance and on the rain ratio between siliceous and calcareous material, which severely impacts surface waters pCO2. A compilation of biogenic fluxes collected at about 40 sites by means of sediment traps also shows a remarkable pattern of increasing BSi:Corg ratio along the path of the “conveyor belt”, accompanying the relative enrichment of waters in Si compared to N and P. This observation suggests an extension of the Si pump model described by Dugdale and Wilkerson (Dugdale, R.C., Wilkerson, F.P., 1998. Understanding the eastern equatorial Pacific as a continuous new production system regulating on silicate. Nature 391, 270–273.), giving to Si(OH)4 a major role in the control of the rain ratio, which is of major importance in the global carbon cycle. The fate of the BSi produced in surface waters is then described, in relation to Corg, in terms of both dissolution and preservation mechanisms. Difficulties in quantifying the dissolution of biogenic silica in the water column as well as the sinking rates and forms of BSi to the deep, provide evidence for a major gap in our understanding of the mechanisms controlling the competition between retention in and export from surface waters. The relative influences of environmental conditions, seasonality, food web structure or aggregation are however explored. Quantitatively, assuming steady state, the measurements of the opal rain rate by means of sediment traps matches reasonably well those obtained by adding the recycling and burial fluxes in the underlying abyssal sediments, for most of the sites where such a comparison is possible. The major exception is the Southern Ocean where sediment focusing precludes the closing of mass balances. Focusing in fact is also an important aspect of the downward revision of the importance of Southern Ocean sediments in the global biogenic silica accumulation. Qualitatively, little is known about the duration of the transfer through the deep and the quality of the material that reaches the seabed, which is suggested to represent a major gap in our understanding of the processes governing the early diagenesis of BSi in sediments. The sediment composition (special emphasis on Al availability), the sedimentation rate or bioturbation are shown to exert an important control on the competition between dissolution and preservation of BSi in sediments. It is suggested that a primary control on the kinetic and thermodynamic properties of BSi dissolution, both in coastal and abyssal sediments, is exerted by water column processes, either occuring in surface waters during the formation of the frustules, or linked to the transfer of the particles through the water column, which duration may influence the quality of the biogenic rain. This highlights the importance of studying the factors controlling the degree of coupling between pelagic and benthic processes in various regions of the world ocean, and its consequences, not only in terms of benthic biology but also for the constitution of the sediment archive. The last section, first calls for the end of the “NPZD” models, and for the introduction of processes linked to the Si cycle, into models describing the phytoplankton cycles in surface waters and the early diagenesis of BSi in sediments. It also calls for the creation of an integrated 1-D diagnostic model of the Si:C coupling, for a better understanding of the interactions between surface waters, deep waters and the upper sedimentary column. The importance of Si(OH)4 in the control of the rain ratio and the improved parametrization of the Si cycle in the 1-D diagnostic models should lead to a reasonable incorporation of the Si cycle into 3-D regional circulation models and OGCMs, with important implications for climate change studies and paleoreconstructions at regional and global scale.

The dynamics of phytoplankton blooms were studied during spring 1992 in two typical coastal ecosystems of Western Europe, which differ in the influence of river discharges on the vertical stratification of the water column. The Bay of Brest is a semi-enclosed ecosystem, which is connected with the adjacent ocean and entered by two nutrient-rich rivers. The Western English Channel is an open ocean situation and the studied area was remote from any significant riverine influence during spring. Both areas are macrotidal environments and are usually considered as well-mixed. The beginning of the annual diatom bloom is delayed until late May in the Channel, in comparison with the Bay of Brest where the bloom starts by early April. Water column stability induced by freshwater runoff, and local topography are responsible for the earlier start of the phytoplankton bloom in the Bay of Brest. In both areas, the spring period is marked by a succession of diatom blooms which is strongly dependent upon the spring–neap tidal cycle. However, blooms develop under opposite mixing regimes: they occur during neap tides in the Bay of Brest and during spring tides in the Channel. In the Channel where light is not a limiting factor, increased mixing during spring tides enables nutrient replenishment from the water–sediment interface and phytoplankton responds immediately after nutrients have been renewed in the water column. In the more turbid waters of the Bay of Brest, relaxation of vertical mixing during neap tides is required before phytoplankton is able to utilize nutrients originating from freshwater inputs or in situ regeneration later in the season.

A study was undertaken to measure the rate of biogenic opal dissolution in equatorial Pacific sediments along the equator between 103 and 140°W, and across the equator between 12°S and 9°N. Along the equator, benthic incubation chamber measurements indicate a gradient in the opal dissolution rate, with rates decreasing from ∼0.7 mmol m−2 day−1 at 103°W to 0.4 mol m−2 day−1 at 140°W. Across the equator at 140°W, the pattern of opal dissolution is symmetrical, with dissolution rates of ∼0.4 mmol m−2 day−1 from 2°S to 2°N, decreasing to ∼0.1 mmol m−2 day−1 at the ends of the transect. Benthic fluxes calculated from pore water profiles of silicic acid are in good agreement with incubation chamber measurements. Each pore water profile fits with a function that exponentially approaches a constant value with depth (Cd), and Cd co-varies with the dissolution flux. At least three previously published models can explain this relationship: one in which Cd is regulated by the solubility of the opal present in the sediments; a second in which Cd depends on the availability of easily dissolvable opal; and the sediment mixing rate and a third rate in which Cd is controlled by the development of surface coatings. If the first model is correct, the data demonstrate that opal solubility varies spatially and that solubility is positively correlated with the opal rain rate, although the rate at which pore waters become saturated varies little among the stations between 5°N and 5°S. The implication of this model is that the opal burial rate depends on dissolution kinetics and sediment accumulation rate. If the second model is correct, fits to the pore water data and knowing the sediment mixing rate indicate that at least three types of solid phase opal must be present in the equatorial Pacific region, one that is essentially unreactive, one that has a dissolution rate constant between 0.27 ± 0.09 and 0.05 ± 0.02 year−1 , and another that has a dissolution rate constant of 6 ± 4 × 10−4 year−1. The more reactive phase dominates the dissolution flux between 5°S and 5°N, whereas the less reactive phase dominates the flux at the high latitude extremes of the transect. The implication of this second model is that sedimentary opal in equatorial Pacific sediments provides a record of only the non-reactive opal supply. If the third model is correct, surface coating development and opal preservation may depend upon the kinetics of the opal surface aging process or on the concentration of the coating material within the sediments. Storage experiments suggest that this third model may be the most realistic, but the implications of this model cannot be explored until the factors regulating coating growth are identified.

The NaOH/HF digestion method, used for the determination of particulate biogenic silica (BSi) and of particulate lithogenic silica (LSi) in oceanic waters, has been applied to coastal waters. In the course of one year, 275 samples of suspended matter were collected at two stations located in the Bay of Brest and in the English Channel (Western Europe). In contrast to previous studies, significant leaching of silicic acid from lithogenic material during the NaOH attack is demonstrated, both for natural samples and for laboratory experiments with suspension of quartz, illite, and kaolinite (usual components of clay in suspended matter of those coastal waters); the interference of lithogenic material on biogenic silica determination averaged 15%. For samples collected during the period of low biological activity, plots of apparent biogenic silica concentrations vs. apparent lithogenic silica concentrations, show linear relationships for both study sites. These relationships are used to calculate corrected biogenic silica concentrations, BSi c . During the period of active diatom production, the precision for BSi c determination is better than 10%.