Jill Nicola Schwarz

Deep carbon export from a Southern Ocean iron-fertilized diatom bloom

Fertilization of the ocean by adding iron compounds has induced diatom-dominated phytoplankton blooms accompanied by considerable carbon dioxide drawdown in the ocean surface layer. However, because the fate of bloom biomass could not be adequately resolved in these experiments, the timescales of carbon sequestration from the atmosphere are uncertain. Here we report the results of a five-week experiment carried out in the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current, during which we tracked sinking particles from the surface to the deep-sea floor. A large diatom bloom peaked in the fourth week after fertilization. This was followed by mass mortality of several diatom species that formed rapidly sinking, mucilaginous aggregates of entangled cells and chains. Taken together, multiple lines of evidence—although each with important uncertainties—lead us to conclude that at least half the bloom biomass sank far below a depth of 1,000 metres and that a substantial portion is likely to have reached the sea floor. Thus, iron-fertilized diatom blooms may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments.

Self-shading of upwelling irradiance for an instrument with sensors on a sidearm

The self-shading measurement error of the upwelling irradiance that is due to the presence of the instrument housing of an optical spectrometer with the irradiance meter located on a sidearm was calculated with a Monte Carlo code. The dependence of the effect on the instrument dimensions, the values of real optical parameters, sea-surface roughness, and Sun zenith angle were all studied to estimate maximum errors for two possible configurations of a proposed new marine spectrophotometer.

Derivation of dissolved organic carbon concentrations from SeaWiFS data

Coloured dissolved organic matter (CDOM), which represents a fraction of dissolved organic carbon (DOC), contributes strongly to the absorption of blue light in Baltic waters. This makes the Baltic an ideal test site for deriving DOC concentrations from ocean colour imagery. Two hundred and five high‐resolution (1.3 km) Sea‐viewing Wide Field of view Sensor (SeaWiFS) images from 1998 were processed using SeaDAS and combined into monthly composites of reflectance. A simple reflectance ratio algorithm, generated from in situ data, was used to derive CDOM absorption at 440 nm (a g440) and an algorithm from the literature was applied to derive DOC concentrations, with a constant CDOM spectral slope coefficient of 0.019 nm−1 to extrapolate to the required DOC‐algorithm wavelength. Monthly composites were chosen to improve the radiometric signal‐to‐noise ratio, given limitation of spatial and temporal coverage caused by clouds, ice and low solar illumination. Surface DOC concentrations ranged between 4 mg l−1 and 5 mg l−1, with elevated values in the cloudiest months (March and October). An increase in surface DOC lagged one month behind the annual cyanobacterial bloom, which peaked in July. The Gulf of Bothnia was found to have relatively high DOC concentrations in all months and comparisons with water‐leaving radiance (555 nm, associated with particulate scattering) maps showed no consistent correlation between DOC and suspended particulates.

Optical closure in marine waters from in situ inherent optical property measurements

Optical closure using radiative transfer simulations can be used to determine the consistency of in situ measurements of inherent optical properties (IOPs) and radiometry. Three scattering corrections are applied to in situ absorption and attenuation profile data for a range of coastal and oceanic waters, but are found to have only very limited impact on subsequent closure attempts for these stations. Best-fit regressions on log-transformed measured and modelled downwards irradiance, Ed, and upwards radiance, Lu, profiles have median slopes between 0.92 - 1.24, revealing a tendency to underestimate Ed and Lu with depth. This is only partly explained by non-inclusion of fluorescence emission from CDOM and chlorophyll in the simulations. There are several stations where multiple volume scattering function related data processing steps perform poorly which suggests the potential existence of unresolved features in the modelling of the angular distribution of scattered photons. General optical closure therefore remains problematic, even though there are many cases in the data set where the match between measured and modelled radiometric data is within 25% RMS%E. These results are significant for applications that rely on optical closure e.g. assimilating ocean colour data into coupled physical-ecosystem models.

The Southampton underwater multiparameter optical-fibre spectrometer system (SUMOSS)

The SUMOSS was designed to measure simultaneously a number of inherent and apparent optical properties at high spectral resolution (<5 nm). The sensors consist of a transmissometer to measure beam attenuation, plane irradiance sensors (to measure reflectance, Kd and Ku) and a scalar irradiance sensor for calculating the absorption coefficient. The SUMOSS was designed to minimize the effects of self-shading of the sensors. The radiometric stability of the instrument across the visible spectrum (350-720 nm) was assessed in a series of dark room experiments. The linearity of the response of the passive sensors was evaluated over a range of intensities and was found to be stable to within 1%. The spectral resolution was measured using red and green lasers on the passive channels (FWHM 4 nm, sampling interval 0.4 nm). Radiometric calibrations showed that there was minimal variability in the spectral response over time. The SUMOSS has been deployed in three optically different water masses (the Baltic, Mediterranean and UK) and the spectral quality and intensity of the readings is interpreted using measurements of the concentrations of suspended particulate material and chlorophyll.

Flexible fiber-optic-based underwater spectrometer for oceanographic research

The need for new instruments to measure the optical properties of natural waters, at higher spectral resolutions than existing commercial instruments, has become apparent in recent years. Such instruments will be required to support the calibration of the next generation of remote sensing platforms by providing in-situ measurements of the optical light field. In addition, high resolution spectral measurements will also support the development of optical closure models which are used to relate the observed optical properties of the water to its biological and geological content. This paper describes the design and construction of a flexible, general purpose, high resolution underwater spectrometer which can be re- configured to perform a variety of optical measurements using interchangeable, optical fiber based sensor heads. Typical results obtained during the first deployment of the instrument at sea are also presented.

Toward optical closure in coastal waters

An instrument has been developed for the simultaneous in situ measurement of inherent and apparent optical properties, enabling various radiative transfer models to be tested. The Southampton Underwater Multi-parameter Optical Spectrometer System measures up- and downwelling irradiance, beam transmission and forward, side and backward scatter. These inherent optical properties can be measured both at 670 nm using a laser diode, or spectrally using a white light. The instrument resolution ranges form 0.47nm to 6.7nm. This extended abstract describes the new instrument, and presents preliminary data from an experiment to monitor the optical properties at the mouth of the Hamble river over a spring tidal cycle. Optical data are compared with measurements of suspended particulate matter and chlorophyll concentrations.