European Station For Time Series Research Articles

Iron and copper complexation in Macaronesian coastal waters

Dissolved iron (dFe) and copper (dCu), the concentration and the conditional stability constants of organic binding ligands (LFe, LCu, log KcondFe3+L and log KcondCu2+L) were studied in the surface coastal waters of the Macaronesia region (Cape Verde, Canary Islands, and Madeira) using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV). Two oceanic stations were also studied: the Cape Verde Ocean Observatory (CVOO) and the European Station for Time Series in the Ocean in the Canary Islands (ESTOC). Dissolved Fe varied from 0.46 to 1.32 nM and LFe concentrations were between 0.56 and 2.96 nM. More than 98% of the total dFe was complexed with conditional stability constants (log KcondFe3+L) between 20.77 and 21.90 (L2-type ligands). Dissolved Cu concentrations ranged between 0.07 and 4.03 nM and the amount of LCu varied between 0.54 and 2.59 nM, with more than 99% of dCu organically complexed. The conditional stability constant (log KcondCu2+L) showed values between 13.40 and 14.42 (L1-type).Due to biological activity and water mixing induced by the wind around the islands, dissolved metals and ligand concentrations were greater at the coastal stations than in oceanic water. Variations were observed between the eastern and western parts of Fogo, Tenerife and Gran Canaria. On the east coasts, the increase in dissolved metals and ligand concentrations were related to wind-induced water mixing. The results of this study will contribute to the knowledge about the impact of coastal areas on the Fe and Cu biogeochemical cycles.

Long-Term Changes of Particle Flux in the Canary Basin Between 1991 and 2009 and Comparison to Sediment Trap Records Off Mauritania

Eastern Boundary Upwelling Ecosystems (EBUEs) are associated with high biological productivity, high fish catch and they highly contribute to marine carbon sequestration. Whether coastal upwelling has intensified or weakened under climate change in the past decades is controversially discussed and different approaches (e.g. time-series of chlorophyll, wind, sea surface temperature, modelling experiments) have been considered. We present a record of almost two decades of particle fluxes (1991-2009) from ca. 600 to 3100 m water depth in the Canary Basin at site ESTOC (European Station for Time series in the Ocean Canary Islands; ca. 29°N, 15°30.W, ca. 3600 m water depth), located in the offshore transition zone of the northern Canary Current-EBUE. We compare these flux records with those measured at a mesotrophic sediment trap site further south off Cape Blanc (Mauretania, ca. 21°N). The deep ocean fluxes at ESTOC in ca. 3 km recorded the evolution of the coastal Cape Ghir filament (30-32°N, 10-12°W) due to lateral advection of particles, whereas the upper water column sediment traps in ca. 1 km reflected the oligotrophic conditions in the overlying waters of ESTOC. We observed an increased emphasis in spring-time fluxes since 2005, associated with a change in particle composition, while satellite chlorophyll biomass did not show this pattern. Due to its northern location in the CC-EBUEs, spring biogenic fluxes at ESTOC provide a better relationship to the forcing of the North Atlantic Oscillation than those recorded further south off Cape Blanc. Off Cape Blanc, deep fluxes showed the best overlap with the deep ESTOC fluxes during the spring season before 2005. On the long-term, both chlorophyll and particle fluxes showed an increasing trend at ESTOC which was not observed further south at the mesotrophic Cape Blanc site. This might indicate that, depending on their location along the NW African margin, coastal upwelling systems react differently to global change.

Controls on carbon export in the subtropical North Atlantic

We present a lower trophic level pelagic ecosystem model that allows the investigation of the role of hydrography, bacterial remineralization and detritus consumption on the strength of carbon export and its attenuation. We apply the model to investigate the controls on the carbon export at the Bermuda Atlantic Time-Series Study site (BATS) and the European Station for Time series in the Ocean, Canary Islands (ESTOC) in the subtropical North Atlantic. In previous field studies, export ratios at 200 m (the ratio between particulate carbon export below the euphotic zone and primary production) were found to be 300–400% smaller at ESTOC compared to BATS. Our model results show that the magnitude and temporal variability of primary productivity and modeled carbon export are modulated by the intensity and duration of vertical mixing events. BATS, with a more dynamic physical environment, has winter mixed-layer depths on average 80 m deeper than ESTOC and is characterized by pulses of enhanced productivity and export. Our model demonstrates the influence of hydrography on export attenuation through (i) more stable water column dynamics at ESTOC that increase particle remineralization time scales and weaken export strength at ESTOC compared to BATS; and (ii) higher water temperatures at BATS in the mesopelagic between 200 and 500 m that increase remineralization rates compared to ESTOC. This results in reduced differences in export ratios within the mesopelagic between both stations that is confirmed by observations. Strengthening remineralization through (i) adding zooplankton feeding on detritus and (ii) increasing bacterial remineralization rates decrease export at all depths at both stations, and increase the modeled difference in 200 m export ratios between BATS and ESTOC from 7% to 17%, a difference that is still much smaller than observed. While we could demonstrate the skill of our model in testing the mechanisms that could lead to differences in regional carbon export, our results indicate that hydrography-driven residence times, zooplankton feeding on detritus and enhanced bacterial remineralization rates alone are insufficient in driving export ratio differences between BATS and ESTOC. On the other hand, modeled export proved to be highly sensitive to prescribed particle sinking speeds. Thus, community and site dependent processes that lead to variations in particle sinking speed and remineralization, together with potential differences in vertical migration by zooplankton and horizontal transport, may be additional processes explaining the observed differences in export ratios at BATS and ESTOC.

Mesopelagic respiration near the ESTOC (European Station for Time-Series in the Ocean, 15.5°W, 29.1°N) site inferred from a tracer conservation model

Remineralization of organic matter in the mesopelagic zone (ca. 150–700m) is a key controlling factor of carbon export to the deep ocean. By using a tracer conservation model applied to climatological data of oxygen, dissolved inorganic carbon (DIC) and nitrate, we computed mesopelagic respiration near the ESTOC (European Station for Time-Series in the Ocean, Canary Islands) site, located in the Eastern boundary region of the North Atlantic subtropical gyre. The tracer conservation model included vertical Ekman advection, geostrophic horizontal transport and vertical diffusion, and the biological remineralization terms were diagnosed by assuming steady state. Three different approaches were used to compute reference velocities used for the calculation of geostrophic velocities and flux divergences: a no-motion level at 3000m, surface geostrophic velocities computed from the averaged absolute dynamic topography field, and surface velocities optimized from the temperature model. Mesopelagic respiration rates computed from the model were 2.8–8.9mol O2 m2y−1, 2.0–3.1mol C m2y−1 and 0.6–1.0mol N m2y−1, consistent with remineralization processes occurring close to Redfield stoichiometry. Model estimates were in close agreement with respiratory activity, derived from electron transport system (ETS) measurements collected in the same region at the end of the winter bloom period (3.61±0.48mol O2 m−2y−1). According to ETS estimates, 50% of the respiration in the upper 1000m took place below 150m. Model results showed that oxygen, DIC and nitrate budgets were dominated by lateral advection, pointing to horizontal transport as the main source of organic carbon fuelling the heterotrophic respiration activity in this region.

A Time-Series View of Changing Ocean Chemistry Due to Ocean Uptake of Anthropogenic CO2 and Ocean Acidification

Sustained observations provide critically needed data and understanding not only about ocean warming and water cycle reorganization (e.g., salinity changes), ocean eutrophication, and ocean deoxygenation, but also about changes in ocean chemistry. As an example of changes in the global ocean carbon cycle, consistent changes in surface seawater CO2-carbonate chemistry are documented by seven independent CO2 time series that provide sustained ocean observations collected for periods from 15 to 30 years: (1) Iceland Sea, (2) Irminger Sea, (3) Bermuda Atlantic Time-series Study (BATS), (4) European Station for Time series in the Ocean at the Canary Islands (ESTOC), (5) CArbon Retention In A Colored Ocean sites in the North Atlantic (CARIACO), (6) Hawaii Ocean Time-series (HOT), and (7) Munida in the Pacific Ocean. These ocean time-series sites exhibit very consistent changes in surface ocean chemistry that reflect the impact of uptake of anthropogenic CO2 and ocean acidification. The article discusses the long-term changes in dissolved inorganic carbon (DIC), salinity-normalized DIC, and surface seawater pCO2 (partial pressure of CO2) due to the uptake of anthropogenic CO2 and its impact on the ocean's buffering capacity. In addition, we evaluate changes in seawater chemistry that are due to ocean acidification and its impact on pH and saturation states for biogenic calcium carbonate minerals.

Oxygen dynamics in the North Atlantic subtropical gyre

Dissolved oxygen (DO) in the ocean is a tracer for most ocean biogeochemical processes including net community production and remineralization of organic matter which in turn constrains the biological carbon pump. Knowledge of oxygen dynamics in the North Atlantic Ocean is mainly derived from observations at the Bermuda Atlantic Time-series Study (BATS) site located in the western subtropical gyre which may skew our view of the biogeochemistry of the subtropical North Atlantic. This study presents and compares a 15yr record of DO observations from ESTOC (European Station for Time-Series in the Ocean, Canary Islands) in the eastern subtropical North Atlantic with the 20yr record at BATS. Our estimate for net community production of oxygen was 2.3±0.4mol O2m−2yr−1 and of oxygen consumption was −2.3±0.5mol O2m−2yr−1 at ESTOC, and 4mol O2m−2yr−1 and −4.4±1molm−2yr−1 at BATS, respectively. These values were determined by analyzing the time-series using the Discrete Wavelet Transform (DWT) method. These flux values agree with similar estimates from in-situ observational studies but are higher than those from modeling studies. The difference in net oxygen production rates supports previous observations of a lower carbon export in the eastern compared to the western subtropical Atlantic. The inter-annual analysis showed clear annual cycles at BATS whereas longer cycles of nearly 4 years were apparent at ESTOC. The DWT analysis showed trends in DO anomalies dominated by long-term perturbations at a basin scale for the consumption zones at both sites, whereas yearly cycles dominated the production zone at BATS. The long-term perturbations found are likely associated with ventilation of the main thermocline, affecting the consumption and production zones at ESTOC.

Regional differences in modelled net production and shallow remineralization in the North Atlantic subtropical gyre

Abstract. We used 5-yr concomitant data of tracer distribution from the BATS (Bermuda Time-series Study) and ESTOC (European Station for Time-Series in the Ocean, Canary Islands) sites to build a 1-D tracer model conservation including horizontal advection, and then compute net production and shallow remineralization rates for both sites. Our main goal was to verify if differences in these rates are consistent with the lower export rates of particulate organic carbon observed at ESTOC. Net production rates computed below the mixed layer to 110 m from April to December for oxygen, dissolved inorganic carbon and nitrate at BATS (1.34±0.79 mol O2 m−2, −1.73±0.52 mol C m−2 and −125±36 mmol N m−2) were slightly higher for oxygen and carbon compared to ESTOC (1.03±0.62 mol O2 m−2, −1.42±0.30 mol C m−2 and −213±56 mmol N m−2), although the differences were not statistically significant. Shallow remineralization rates between 110 and 250 m computed at ESTOC (−3.9±1.0 mol O2 m−2, 1.53±0.43 mol C m−2 and 38±155 mmol N m−2) were statistically higher for oxygen compared to BATS (−1.81±0.37 mol O2 m−2, 1.52±0.30 mol C m−2 and 147±43 mmol N m−2). The lateral advective flux divergence of tracers, which was more significant at ESTOC, was responsible for the differences in estimated oxygen remineralization rates between both stations. According to these results, the differences in net production and shallow remineralization cannot fully explain the differences in the flux of sinking organic matter observed between both stations, suggesting an additional consumption of non-sinking organic matter at ESTOC.

Interannual variability of chlorophyll and the influence of low‐frequency climate modes in the North Atlantic subtropical gyre

The impact of low‐frequency climate modes on the large‐scale variability of chlorophyll‐a (a proxy for phytoplankton biomass) concentration in the subtropics is little known. Here we examined the concurrent monthly chlorophyll‐a and hydrographical records obtained at two subtropical time series stations (Bermuda Atlantic Time series Study, BATS and European Station for Time series in the Ocean Canary Islands, ESTOC) from the beginning of the time series (1989 at BATS and 1994 at ESTOC) to 2003, in order to analyze its seasonal and interannual variability and investigate the potential correlation with large‐scale atmospheric oscillations. At BATS, Chl‐a variations are mainly caused by differences in the convective mixing and mesoscale phenomena. Variability in winter mixing is a significant factor at ESTOC as exemplified by years with anomalously deep mixed layer depths. An additional nutrient source causing Chl‐a variability at this station likely occurs due to nutrient advection driven by the baroclinic flow caused by the Trade Winds during summertime. We found that interannual variability in mean integrated total Chl‐a (TChl‐a) is significantly correlated with temperature and salinity anomalies at BATS. Chl‐a also covaried with changes in temperature although the correlation was not significant at ESTOC. We could not find any direct correlation between TChl‐a and NAO at BATS; the correlation improved between TChl‐a and NAO +1 year but was still insignificant. However, significant correlations were found for ESTOC between TChl‐a anomalies and ENSO (El Niño Southern Oscillation) +3 years and NAO +1 year time lag.

Sensitivity analysis of an ocean carbon cycle model in the North Atlantic: an investigation of parameters affecting the air-sea CO<sub>2</sub> flux, primary production and export of detritus

Abstract. The sensitivity of the biological parameters in a nutrient-phytoplankton-zooplankton-detritus (NPZD) model in the calculation of the air-sea CO2 flux, primary production and detrital export is analysed. We explore the effect on these outputs of variation in the values of the twenty parameters that control ocean ecosystem growth in a 1-D formulation of the UK Met Office HadOCC NPZD model used in GCMs. We use and compare the results from one-at-a-time and all-at-a-time perturbations performed at three sites in the EuroSITES European Ocean Observatory Network: the Central Irminger Sea (60° N 40° W), the Porcupine Abyssal Plain (49° N 16° W) and the European Station for Time series in the Ocean Canary Islands (29° N 15° W). Reasonable changes to the values of key parameters are shown to have a large effect on the calculation of the air-sea CO2 flux, primary production, and export of biological detritus to the deep ocean. Changes in the values of key parameters have a greater effect in more productive regions than in less productive areas. The most sensitive parameters are generally found to be those controlling well-established ocean ecosystem parameterisations widely used in many NPZD-type models. The air-sea CO2 flux is most influenced by variation in the parameters that control phytoplankton growth, detrital sinking and carbonate production by phytoplankton (the rain ratio). Primary production is most sensitive to the parameters that define the shape of the photosynthesis-irradiance curve. Export production is most sensitive to the parameters that control the rate of detrital sinking and the remineralisation of detritus.

Carbonate system distribution south of the Canary Islands in spring 2000

The measurement of the surface molar fraction of CO 2 (atmosphere and sea water) and water column pH T , total alkalinity, A T , nutrients and oxygen were carried out in spring 2000 at the European Station for Time Series in the Ocean at the Canary Islands (ESTOC) and in the area located south of the Canary Islands. The significant eddy field strongly affecting the pattern of the chemical and carbonate system variables is presented and discussed. A mixing model based on the thermohaline properties of the water masses was established. The model explained over 97% of the variability found in the distribution of the chemical variables. Intermediate waters to the south of the Canary Islands show a high contribution of Antarctic waters with about 5% of pure Antarctic Intermediate Water. Moreover, the surface structure affected the atmosphere-ocean carbon dioxide exchange, making the area act as a CO 2 sink taking up 9.1 mmol m -2 week -1 , corresponding to 0.03 Mt of CO 2 which were taken up by the area in a week at the end of March 2000.

The water column distribution of carbonate system variables at the ESTOC site from 1995 to 2004

Abstract. The accelerated rate of increase in atmospheric carbon dioxide and the substantial fraction of anthropogenic CO2 emissions absorbed by the oceans are affecting the anthropocenic signatures of seawater. Long-term time series are a powerful tool for investigating any change in ocean bio-geochemistry and its effects on the carbon cycle. We have evaluated the ESTOC (European Station for Time series in the Ocean at the Canary islands) observations of measured pH (total scale at 25 °C) and total alkalinity plus computed total dissolved inorganic carbon concentration (CT) from 1995 to 2004 for surface and deep waters, by following all changes in response to increasing atmospheric carbon dioxide. The observed values for the surface partial pressure of CO2 from 1995 to 2008 were also taken into consideration. The data were treated to better understand the fundamental processes controlling vertical distributions in the Eastern North Atlantic Ocean and the accumulation of anthropogenic CO2, CANT. CT at constant salinity, NCT, increased at a rate of 0.85 μmol kg−1 yr−1 in the mixed layer, linked to an fCO2 increase of 1.7±0.7 μatm yr−1 in both the atmosphere and the ocean. Consequently, the mixed layer at ESTOC site has also become more acidic, −0.0017±0.0003 units yr−1, whereas the carbonate ion concentrations and CaCO3 saturation states have also decreased over time. NCT increases at a rate of 0.53, 0.49 and 0.40 μmol kg−1 yr−1 at 300, 600, and 1000 m, respectively. The general processes controlling the vertical variations of alkalinity and the inorganic carbon distribution were computed by considering the pre-formed values, the production/decomposition of organic matter and the formation/dissolution of carbonates. At 3000 m, 30% of the inorganic carbon production is related to the dissolution of calcium carbonate, increasing to 35% at 3685 m. The total column inventory of anthropogenic CO2 for the decade was 66±3 mol m−2. A model fitting indicated that the column inventory of CANT increased from 61.7 mol m−2 in the year 1994 to 70.2 mol m−2 in 2004. The ESTOC site is presented as a reference site to follow CANT changes in the Northeast Atlantic Sub-tropical gyre.

Cross-basin differences in particulate organic carbon export and flux attenuation in the subtropical North Atlantic gyre

We compared in- situ and satellite-derived measures of the biological carbon pump efficiency at the two seemingly similar subtropical North Atlantic gyre time series sites, the Bermuda time series (BATS, Bermuda Atlantic time-series study and OFP, ocean flux program) in the western gyre and the ESTOC time series (European station for time-series in the ocean, Canary Islands) in the eastern gyre. Satellite-derived surface chlorophyll a was slightly lower at Bermuda compared to ESTOC (annual average of 0.10±0.04 vs. 0.14±0.05-mg-m −3), as was satellite-derived primary production (annual average of 380±77 vs. 440±80-mg C-m −2 d −1). However, export production normalized to primary production (export ratio) was higher at Bermuda by a factor of 2–3 when estimated using mesopelagic traps moored at 500-m depth and by a factor of 3–4 when estimated using surface-tethered drifting traps. When averaged seasonally, flux at BATS was highest in spring (March, April, May) at all depths followed by summer (June, July, August) and decreasing towards fall, but this seasonality was less visible at ESTOC. Seasonal comparison showed the fastest flux attenuation at Bermuda in winter and spring, coinciding with the highest POC flux. POC/PIC ratios derived from the moored traps were significantly higher at BATS than at ESTOC in fall and winter, but this difference was not significant in spring ( p>0.05). This study shows that while the western and eastern Atlantic subtropical gyres have similar rates of primary production, the biological carbon pump differs between the two provinces. Higher new nutrient input observed at Bermuda compared to ESTOC might explain part of the difference in export ratio but alone is insufficient. Greater winter mixed-layer depths and higher mesoscale eddy activity at Bermuda resulting in pulsed production events of labile organic matter might explain both the higher export flux and export ratios found at Bermuda.

Decadal analysis of hydrography and in situ nutrient budgets in the western and eastern North Atlantic subtropical gyre

The current debate about the mechanisms and magnitude of new nutrient input to the euphotic zone in subtropical gyres calls for studies which consider large and mesoscale perspectives by combining in situ time series and remote observations. We carried out a first of its kind comparative analysis of hydrography and sea level anomaly (SLA) at the oligotrophic time series stations BATS (Bermuda Atlantic Time Series Study) and ESTOC (European Station for Time Series, Canary Islands) using concomitant 10‐yr in situ and satellite altimetry data. The stations are located at about the same latitude in the western and eastern boundaries of the subtropical North Atlantic gyre, respectively, and provide the opportunity to study differences that may exist between both regions. Observed SLA was 0.25 m at BATS, compared with 0.12 m at ESTOC, a consequence of the higher eddy kinetic energy in the western compared with the eastern subtropical gyre. We quantified a detailed in situ nutrient budget for both time series stations; ESTOC received about 75% of the nutrients available for new production at BATS (in average 0.28 mol N m−2 yr−1 compared with 0.38 mol N m−2 yr−1, respectively), but the difference was not significant. However, significant differences in input mechanisms existed between both stations; eddy pumping constituted the main new nutrient source BATS, whereas wintertime convection was the main nutrient supply mechanism at ESTOC. In addition, the nutricline was significantly shallower at ESTOC compared with BATS, partly compensating for shallower mixed‐layer depths and SLA variability at the western station. We found considerable interannual variability in both eddy pumping and wintertime convection which may be related to NAO‐induced changes in the pattern of the subtropical gyre.

Dust deposition pulses to the eastern subtropical North Atlantic gyre: Does ocean's biogeochemistry respond?

Enhancement of primary and export production following dust deposition pulses is well established for High Nutrient Low Chlorophyll regions, but the effect of atmospheric dust on the biogeochemistry of oligotrophic gyre regions remains unclear. Here we report atmospheric dust concentrations measured on Gran Canaria, Canary Islands, concomitantly with upper water biogeochemistry at the oligotrophic time series station ESTOC (European Station for Time series in the Ocean, Canary Islands) during a 2‐year period from winter 1997 to 1999. ESTOC is located in the eastern subtropical Atlantic gyre about 100 km north of Gran Canaria and 500 km west of the Sahara, and receives aeolian dust episodically mainly in winter and summer. We found a close correlation of the magnitude of atmospheric dust concentration in winter with the magnitude of downward particle flux at ESTOC, mainly due to a relative increase in lithogenic matter and carbonate sedimentation. Higher aerosol concentration was not accompanied by higher primary or export production, however, indicating that phytoplankton production remained unaffected by atmospheric nitrogen supply on a seasonal or yearly timescale. However, by estimating bioavailable iron input and the need of the phytoplankton population, we found that the highly episodic dust pulses might exert a feast and famine effect on the phytoplankton. Despite the high lithogenic matter input, carbonate, mainly stemming from coccolithophorids, exceeded in importance the role of lithogenic matter as ballasting agent of sinking organic matter.

Seasonal and interannual variability of sea‐surface carbon dioxide species at the European Station for Time Series in the Ocean at the Canary Islands (ESTOC) between 1996 and 2000

Seasonal patterns in hydrography, partial pressure of CO2, fCO2, pHt, total alkalinity, AT, total dissolved inorganic carbon, CT, nutrients, and chlorophyll a were measured in surface waters on monthly cruises at the European Station for Time Series in the Ocean at the Canary Islands (ESTOC) located in the northeast Atlantic subtropical gyre. With over 5 years of oceanographic data starting in 1996, seasonal and interannual trends of CO2 species and air‐sea exchange of CO2 were determined. Net CO2 fluxes show this area acts as a minor source of CO2, with an average outgassing value of 179 mmol CO2 m−2 yr−1 controlled by the dominant trade winds blowing from May to August. The effect of short‐term wind variability on the CO2 flux has been addressed by increasing air‐sea fluxes by 63% for 6‐hourly sampling frequency. The processes governing the monthly variations of CT have been determined. From March to October, when CT decreases, mixing at the base of the mixed layer (11.5 ± 1.5 mmol m−3) is compensated by air‐sea exchange, and a net organic production of 25.5 ± 5.7 mmol m−3 is estimated. On an annual scale, biological drawdown accounts for the decrease in inorganic carbon from March to October, while mixing processes control the CT increase from October to the end of autumn. After removing seasonality variability, fCO2sw increases at a rate of 0.71 ± 5.1 μatm yr−1, and as a response to the atmospheric trend, inorganic carbon increases at a rate of 0.39 ± 1.6 μmol kg−1 yr−1.

Primary productivity in the northern Canary Islands region as inferred from SeaWiFS imagery

Abstract A 19-month record from September 1997 to March 1999 is presented of the productivity gradient at 3 stations that fall along a quasi-zonal 29°N transect through the Canary Islands using the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll data and the primary production method of Antoine and Morel (Global Biogeochem. Cycles 10(1) (1996) 43). These stations correspond to 3 fixed sediments traps positioned during the same period east of the island of Fuerteventura (Eastern Boundary Current, EBC) and north of the islands of Gran Canaria (European Station for Time-Series in the Ocean, Canary Islands, ESTOC) and La Palma (LP). The mean annual productivity during the 19-month period was observed to decrease steeply westwards along the quasi-zonal 29°N transect, from 237 g C m−2 yr−1 at EBC to 164 and 145 g C m−2 yr−1 at ESTOC and LP, respectively. The high productivity at EBC indicates that this station is strongly influenced by the seasonal coastal upwelling off the adjacent Moroccan coast. This is confirmed by SeaWiFS chlorophyll images showing incursions of the Cape Yubi filament over the EBC station in March, July and October 1998 and January 1999, coincident with a decrease in sea-surface temperature (SST) of up to 1.5°C as observed by satellite. In comparison it was observed that although some cooler upwelled water does reach the ESTOC and LP stations in summer 1997 and summer/autumn 1998, there was no corresponding increase in productivity, indicating that the water was nutrient depleted. It is concluded that the ESTOC and LP stations are more characteristic of oligotrophic ocean, with minimal influence of coastal upwelling in near-surface waters. Comparison of SeaWiFS chlorophyll values with in situ data at the ESTOC station show that in the SeaWiFS generally provided good-quality data (within the ±35% accuracy goal targeted for this instrument) for the Canary Island region except for period December 1997–April 1998. Comparison of model productivity estimates with in situ 14C uptake measurements shows good agreement for oligotrophic water, but model usage in upwelling regions, in particular filaments, may be problematic. This investigation suggests that the application of the Antoine and Morel model with a limited dataset of satellite images can provide a good insight into relative primary production in the Canary Islands region.

Underwater light field and its effect on a one-dimensional ecosystem model at station ESTOC, north of the Canary Islands

Light abundance is a major prerequisite for primary production in pelagic ecosystems, influencing the evolution of the marine environment. Realistic simulations of planktonic ecosystems therefore require an appropriate representation of the underwater light field. Taking a look at the different biogeochemical models discussed in literature, one finds a variety of descriptions for the distribution of light, or more specific, photosynthetically available radiation (PAR) in the water column. This paper compares the effect of different parameterizations of PAR on the primary production and phytoplankton evolution at the European Station for Time-Series in the Ocean Canary Islands (ESTOC) station (29°10′N, 15°30′W) north of Gran Canaria, Canary Islands. Observations from two cruises in 1997 are used to illustrate the winter and spring situation at the time-series site. Four alternative PAR descriptions are used in a one-dimensional coupled biological–physical model of the upper ocean driven by daily forcing fields over a 5-year period. The biological model is a simple nitrate-phytoplankton–zooplankton-detritus model. Although the different descriptions are found to have only a small effect (±3%) on the annual primary production, we observe significant changes in the vertical distribution of simulated phytoplankton. The large variation (±32%) in the near-surface chlorophyll contents will be of particularly crucial importance when using satellite ocean-color sensors for model validation and parameter estimation. For future three-dimensional biogeochemical models, a computationally efficient and accurate parameterization of the light field will be particularly relevant.

Influence of lateral particle advection and organic matter degradation on sediment accumulation and stable nitrogen isotope ratios along a productivity gradient in the Canary Islands region

Abstract We compare total and biogenic particle fluxes and stable nitrogen isotope ratios (δ15N) at three mooring sites along a productivity gradient in the Canary Islands region with surface sediment accumulation rates and sedimentary δ15N. Higher particle fluxes and sediment accumulation rates, and lower δ15N were observed in the upwelling influenced eastern boundary region (EBC) compared to the oligotrophic sites north of Gran Canaria [European Station for Time-Series in the Ocean, Canary Islands (ESTOC]] and north of La Palma (LP). The impact of organic matter degradation and lateral particle advection on sediment accumulation was quantified with respect to the multi-year flux record at the ESTOC. Remineralisation of organic matter in the water column and at the sediment surface resulted in an organic carbon preservation of about 0.8% and total nitrogen preservation of about 0.4% of the estimated export production. Higher total and carbonate fluxes and accumulation rates in the lower traps and surface sediment compared to the upper traps indicated that at least 50% of the particulate matter at the ESTOC was derived from allochthonous sources. Low δ15N values in the lower traps of the ESTOC and LP point to a source region influenced by coastal upwelling. We conclude from this study that the reconstruction of export production or nutrient regimes from sedimentary records in regions with strong productivity gradients might be biased due to the mixture of particles originating from autochthonous and allochthonous sources. This could result in an imprint of high productivity signatures on sedimentation processes in oligotrophic regions.

Seasonal and interannual variation of coccolithophore fluxes and species composition in sediment traps north of Gran Canaria (29°N 15°W)

Coccolithophore flux variations in sediment traps of the time-series station ESTOC (European Station for Time-series in the Ocean, Canary Islands), located 60nm north of Gran Canaria, were recorded from April 1993 to December 1996. The 20 cup particle traps were deployed at 1000 and 3000m below sea surface, and sampling intervals ranged from eight days to three weeks.Coccolith and coccosphere fluxes at both trap depths were characterized by a strong seasonality. At 1000m, maximum coccolith fluxes were reached during spring from March to April 1995 and from February to March 1996. Highest coccolith fluxes of 4538×106 coccoliths m−2d−1 occurred at the beginning of March 1996, whereas lowest values of 19.8×106 coccoliths m−2d−1 were recorded in June 1994. Total coccosphere fluxes were constantly low during most of the investigated time interval and showed only high values during the main sedimentation phases from January to March 1995 (up to 6.2×106 spheres m−2d−1) and in March 1996 (36.4×106 spheres m−2d−1).At 3000m water depth, the sedimentation pattern was rather similar to the pattern observed at 1000m. Maximum coccolith and coccosphere fluxes were also reached during spring time in February 1994 (8200×106 and 15.6×106m−2d−1, respectively) and in March 1996 (8114×106 and 32.3×106m−2d−1, respectively). However, total coccolith flux peaks at 3000m were about 2–4 times higher compared to the upper traps, probably due to horizontal advection from phytoplankton material from north of the study site. This effect also influenced the coccolith–CaCO3 flux estimates. Mean calculated coccolith–CaCO3 fluxes ranged between 3.9mgm−2d−1 for the upper traps (1995/1996) and 9.9mgm−2d−1 for the lower traps (1996). Coccoliths contributed for about 8–78% of the measured total CaCO3 fluxes (<1mm fraction), with a mean contribution of 33.4% in the upper, and 41.4% in the lower traps.The coccolithophore assemblages were dominated by Emiliania huxleyi, Florisphaera profunda, and Gephyrocapsa ericsonii during the entire sampling period, whereas all other species together generally contributed less than 40% to the total assemblages. Variability of species compositions at either trap depth was very low, indicating similar seasonal flux patterns of the different species.

Seasonal and interannual pigment concentration in the Canary Islands region from CZCS data and comparison with observations from the ESTOC

We have compared historical data from the Nimbus-7 Coastal Zone Color Scanner (CZCS) with recent in situ measurements at the European Station for Time-Series in the Ocean, Canary Islands (ESTOC). CZCS data show an annual winter phytoplankton maximum in January but with a large interannual variation. In sediment traps moored at ESTOC we observed winter sedimentation peaks at 1000m and 3000m depth occurring about 1 month later. We also observed sedimentation peaks in other seasons which probably result from material advecting into the region. CZCS data suggest that one possible source is the Cape Ghir upwelling filament off the north-west African coast.