Seasonal and inter-annual variability in nutrient supply in relation to mixing in the Bay of Biscay

Post-print

2008 Inter-laboratory Comparison Study of a Reference Material for Nutrients in Seawater

Autoclaved natural seawater collected in the North Pacific Ocean was used as a reference material for nutrients in seawater (RMNS) during an inter-laboratory comparison (I/C) study conducted in 2008. This study was a follow-up to previous studies conducted in 2003 and 2006. A set of six samples was distributed to each of 58 laboratories in 15 countries around the globe, and results were returned by 54 of those laboratories (15 countries). The homogeneities of samples used in the 2008 I/C study, based on analyses for three determinants, were improved compared to those of samples used in the 2003 and 2006 I/C studies. Results of these I/C studies indicate that most of the participating laboratories have an analytical technique for nutrients that is sufficient to provide data of high comparability. The differences between reported concentrations from the same laboratories in the 2006 and 2008 I/C studies for the same batch of RMNS indicate that most of the laboratories have been maintaining internal comparability for two years. Thus, with the current high level of performance in the participating laboratories, the use of a common reference material and the adaptation of an internationally accepted nutrient scale system would increase comparability among laboratories worldwide, and the use of a certified reference material would establish traceability. In the 2008 I/C study we observed a problem of non-linearity of the instruments of the participating laboratories similar to that observed among the laboratories in the 2006 I/C study. This problem of non-linearity should be investigated and discussed to improve comparability for the full range of nutrient concentrations. For silicate comparability in particular, we see relatively larger consensus standard deviations than those for nitrate and phosphate

Constraining oceanic dust deposition using surface ocean dissolved Al

We use measurements of ocean surface dissolved Al, a global Biogeochemical Elemental Cycling (BEC) ocean model, and the global Dust Entrainment and Deposition (DEAD) model to constrain dust deposition to the oceans. Our Al database contains all available measurements with best coverage in the Atlantic. Vertical profiles and seasonal data exist in limited regions. Observations show that surface dissolved Al is distributed similarly to the dust deposition predicted by DEAD and other models. There is an equatorial Atlantic Al maximum that decreases toward higher latitudes. There are high Al concentrations in the Mediterranean Sea and the Arabian Sea and low concentrations in the Pacific and the Southern Ocean. The ocean basins maintain more distinct Al profiles than Fe profiles in the upper ocean, consistent with a weaker biological influence on Al than Fe. The BEC-predicted surface dissolved Al compares relatively well with observations. The Al distribution reflects the combined effects of Al input from dust and Al removal by particle scavenging and biological uptake by diatoms. Model-observed biases suggest a southward shift of maximum dust deposition compared to current dust model predictions. DEAD appears to overestimate deposition north of 30°N in the Pacific and to underestimate deposition south of 30°N. Observed Al concentrations and the ocean model–predicted surface Al lifetime provide a semi-independent method to estimate oceanic dust deposition. This technique indicates that DEAD may overestimate dust deposition to the north equatorial Atlantic but underestimate in other Atlantic regions, the Southern Ocean, and the Arabian Sea. However, spatial variations in aerosol Al solubility may also contribute to the model-observation mismatch. Our results have implications for all dust-borne ocean nutrients including Fe and demonstrate the potential of marine geochemical data to constrain atmospheric aerosol deposition fields

Variability of alkalinity and the alkalinity-salinity relationship in the tropical and subtropical surface ocean

The variability of total alkalinity (TA) and its relationship with salinity in the tropical and subtropical surface ocean were examined using data collected in various marine environments from a ship of opportunity. In the open ocean regions of the Atlantic, Pacific, and Indian Oceans, sea surface TA variability was observed to be mainly controlled by the simple dilution or concentration (SDC) effect of precipitation and evaporation, and the measured concentrations of TA agreed well with those predicted from salinity and temperature. Non-SDC changes in alkalinity in ocean margins and inland seas were examined by comparing the salinity-normalized alkalinity with that of the open ocean end-member. Non-SDC alkalinity additions to the western North Atlantic margin, eastern North Pacific margin, and Mediterranean Sea were identified, which mainly resulted from river inputs and shelf currents. In contrast, removal of TA through formation and sedimentation of calcium carbonate was observed to be an important control in the Red Sea. The concentration of the river end-member can only be reliably derived from the y intercept of TA-S regression (TAS0) in river-dominated systems such as estuaries and river plumes. In coastal regions where other processes (evaporation, shelf currents, upwelling, calcification, etc.) are more influential, TAS0 can significantly deviate from the river water concentration and hence be an unreliable indicator of it. Negative values of TAS0 can result from non-SDC TA removal at the low salinity end (relative to the salinity of the oceanic end-member) and/or non-SDC TA addition at high salinities (as occurs in the Mediterranean Sea)

Application and assessment of a membrane-based pCO2 sensor under field and laboratory conditions

ABSTRACT: The principle, application, and assessment of the membrane-based ProOceanus CO2-Pro sensor for partial pressure of CO2 (pCO2) are presented. The performance of the sensor is evaluated extensively under field and laboratory conditions by comparing the sensor outputs with direct measurements from calibrated pCO2 measuring systems and the thermodynamic carbonate calculation of pCO2 from discrete samples. Under stable laboratory condition, the sensor agreed with a calibrated water-air equilibrator system at –3.0 ± 4.4 μatm during a 2-month intercomparison experiment. When applied in field deployments, the larger differences between measurements and the calculated pCO2 references (6.4 ± 12.3 μatm on a ship of opportunity and 8.7 ± 14.1 μatm on a mooring) are related not only to sensor error, but also to the uncertainties of the references and the comparison process, as well as changes in the working environments of the sensor. When corrected against references, the overall uncertainties of the sensor results are largely determined by those of the pCO2 references (± 2 and ± 8 μatm for direct measurements and calculated pCO2, respectively). Our study suggests accuracy of the sensor can be affected by temperature fluctuations of the detector optical cell and calibration error. These problems have been addressed in more recent models of the instrument through improving detector temperature control and through using more accurate standard gases. Another interesting result in our laboratory test is the unexpected change in alkalinity which results in significant underestimation in the pCO2 calculation as compared to the direct measurement (up to 90 μatm)

Periodic Stratification in the Rhine ROFI in the North Sea

The nature of the physical regime in the vicinity of the Rhine ROFI (Region Of Freshwater Influence) has been determined in a series of collaborative observations. Extensive surveys with shipboard CTD/rosette systems have been used to complement time series observations by an array of moorings instrumented with currentmeters, transmissometers and fluorimeters. The observations reveal a highly variable system in which the influence of the freshwater input from the Rhine extends northeastwards from the source and out to 30 km from the coast. The mean flow within this region is generally parallel to the coast (northeastwards) and with surface speeds, determined by the HF radar, of 15-20 cm/s. The residual current at sub-tidal frequencies was strongly correlated with windstress-forcing with a transfer factor of ca. 1%. Water column structure exhibits marked periodic variations particulary on semidiurnal and semi-monthly time scales, the latter highlighted by contrasting post-springs and post-neaps surveys of the ROFI region. Springs tidal stirring was reinforced by strong wind (and wave) mixing which brought about complete vertical homogeneity everywhere except at the Rhine mouth. After the following neaps, and a period of light winds, the water column was observed to have re-stratified over the whole inshore region through the relaxation of the horizontal gradients under gravity and with the influence of rotation as in the model of Ou (1983). The switching of the water column regime between stratified and mixed conditions was observed to markedly change the coupling between low frequency surface and bottom currents and is also reflected in the suspended sediment variations. Generally high levels of seston throughout the coastal boundary layer in the post-springs period were followed by a dramatic reduction especially in the region which re-stratified. An interesting exception was combined occurrence of high turbidity and low salinity in surface waters due to the immediate influence of the Rhine outflow near the source

Phytoplankton distribution and survival in the thermocline

Observations of the vertical structure of density, concentrations of chlorophyll a and nitrate, and turbulent dissipation rates were made over a period of 25 h in a well-stratified shelf region in the Western English Channel, between neap and spring tides. Maximum turbulent dissipation at the base of the thermocline occurred almost 5 h after maximum tidal currents. This turbulence aids phytoplankton growth by supplying bottom-layer nutrients into the subsurface chlorophyll maximum but reduces phytoplankton concentrations in the thermocline by mixing cells from the base of the subsurface maximum into the bottom mixed layer. The turbulent dissipation observations were used to estimate an average nitrate flux into the thermocline of 2.0 (0.8–3.2, 95% confidence interval) mmol m22d21, which is estimated to have been capable of supporting new phytoplankton growth at a rate of 160 (64–256)mg C m22 d21. Turbulent entrainment of carbon from the base of the subsurface biomass maximum into the bottom mixed layer was observed to be 290 (120–480) mg C m22 d21. This apparent excess export from the chlorophyll maximum is suggested to be a feature of the spring-neap cycle, with export dominating as the tidal turbulence increases toward spring tides and erodes the base of the thermocline. The observed rate of carbon export into the bottom mixed layer could account for as much as 25% of the gross annual primary production in stratifying shelf seas. Such turbulent losses, combined with grazing losses and low light levels, suggest that phytoplankton need to be highly adapted to environmental conditions within the thermocline in order to survive

Constraining oceanic dust deposition using surface ocean dissolved Al

We use measurements of ocean surface dissolved Al, a global Biogeochemical Elemental Cycling (BEC) ocean model, and the global Dust Entrainment and Deposition (DEAD) model to constrain dust deposition to the oceans. Our Al database contains all available measurements with best coverage in the Atlantic. Vertical profiles and seasonal data exist in limited regions. Observations show that surface dissolved Al is distributed similarly to the dust deposition predicted by DEAD and other models. There is an equatorial Atlantic Al maximum that decreases toward higher latitudes. There are high Al concentrations in the Mediterranean Sea and the Arabian Sea and low concentrations in the Pacific and the Southern Ocean. The ocean basins maintain more distinct Al profiles than Fe profiles in the upper ocean, consistent with a weaker biological influence on Al than Fe. The BEC-predicted surface dissolved Al compares relatively well with observations. The Al distribution reflects the combined effects of Al input from dust and Al removal by particle scavenging and biological uptake by diatoms. Model-observed biases suggest a southward shift of maximum dust deposition compared to current dust model predictions. DEAD appears to overestimate deposition north of 30°N in the Pacific and to underestimate deposition south of 30°N. Observed Al concentrations and the ocean model–predicted surface Al lifetime provide a semi-independent method to estimate oceanic dust deposition. This technique indicates that DEAD may overestimate dust deposition to the north equatorial Atlantic but underestimate in other Atlantic regions, the Southern Ocean, and the Arabian Sea. However, spatial variations in aerosol Al solubility may also contribute to the model-observation mismatch. Our results have implications for all dust-borne ocean nutrients including Fe and demonstrate the potential of marine geochemical data to constrain atmospheric aerosol deposition fields