The use of Lagrangian platforms and of Autonomous Underwater Vehicles (AUVs) in oceanography has increased rapidly over the last decade along with the development of improved biological and chemical sensors. These vehicles provide new spatial and temporal scales for observational studies of the ocean. They offer a broad range of deployment and recovery capabilities that reduce the need of large research vessels. This is especially true for ice-covered Arctic ocean where surface navigation is only possible during the summer period. Moreover, safe underwater navigation in icy waters requires the capability of detecting sea ice on the surface (ice sheets). AUVs navigating in such conditions risk collisions, RF communication shadowing, and being trapped by ice keels. In this paper, an underwater sea-ice detection apparatus is described. The source is a polarized continuous wave (CW) diode-pumped solid-state laser (DPSS) module operating at 532 nm. The detector is composed of a polarizing beam splitter, which separates light of S and P polarization states and two photodetectors, one for each polarized component. Since sea-ice is a strong depolarizer, the ratio P/S is an indicator of the presence or absence of sea-ice. The system is capable of detecting sea-ice at a distance of 12m. This apparatus is designed to be used by free drifting profiling floats (e.g., Argo floats), buoyancy driven vehicles (e.g., sea gliders) and propeller-driven robots (e.g., Hugin class AUV).
We present two algorithms for the retrieval of 1) vertical light penetration and 2) horizontal visibility in coastal waters using ocean color data. The Secchi depth, a proxy to vertical visibility or water transparency, is related to the former two properties and can be retrieved from two irradiance reflectances. The algorithm development includes the use of classical approximations in marine optics, sensitivity analyses based on radiative transfer calculations, and the use of an extensive in situ optical data set. Finally, match-ups between in situ measurements of the Secchi depth and ocean color data were used for the parameterization of the operational algorithm. Maps of Secchi depths are presented over
different coastal regions.
We study two types of contamination of Ocean Color data related to the presence of sea ice. The first type, referred to as the adjacency effect, is the contamination of the radiance from the intended target by photons scattered in atmosphere towards the sensor but originating from a bright object such as sea ice nearby the target. The second type results from the presence of sub-pixel sea ice. In the case of the adjacency effect, the contribution of the icy environment to the top-of-atmosphere signal in the visible is not fully removed by the atmospheric correction algorithm, leading to an overestimation of the water-leaving reflectance. This is due to the strong spectral increase of atmospheric scattering with decreasing wavelength. The adjacency effect being more important at short wavelengths, the chlorophyll estimates based on the blue-to-green ratio will tend to decrease as the field of view approaches the ice edge. Conversely, contamination by sub-pixel sea ice results in an underestimation of the water-leaving reflectance, especially in the blue domain, and consequently to an overestimation of the chlorophyll concentration. The magnitude of the errors depends on the type of ice contaminating the pixel. It is more important for ice with high reflectance ratios for the wavebands 765 to 865 nm. Absolute error on the water-leaving reflectance up to 0.7% was observed, which is not acceptable for Ocean Color applications intending inversion of inherent optical properties from the absolute radiance, and for validation and vicarious calibration activities.
KEYWORDS: Carbon, In vivo imaging, Quantum efficiency, Electron transport, Carbon dioxide, NOx, Absorption, Oceanography, Atmospheric sciences, Energy conversion efficiency
Laboratory experiments already showed that the most significant photoacclimation strategy deployed by
Prochiorococcus sp. would consist of changing the number of photosynthetic units per cell' and, thereby, the minimal
turnover time for in vivo electron transport from water to CO2 (t). As a result, the saturation parameter Ek, would tightly
covary with growth irradiance. Also, because the zeaxanthin cellular content in Prochiorococcus sp. is quite stable, the
ratio of zeaxanthin-to-dv-chl a would also covary with irradiance. A study in the Atlantic Ocean already showed that, to
some extent (i.e. by a factor of 3), the maximum quantum yield of carbon fixation, , is proportional to the ratio of nonphotosynthetic
pigments (mostly zeaxanthin) to all phytoplankton pigments (hereafter denoted the "non-photosynthetic
pigment index; NPP index)2. In turn, the NNP index is linearly correlated with the mean irradiance in the mixed layer.
The present study aims at documenting the above mentioned relationships in the equatorial and sub-equatorial Pacific,
where prokaryotic photoautrotrophs dominate the phytoplankton community.
Photoacclimation in prokaryotic phytoplankton species commonly found in the ocean is not as well documented as for eukaryotes. Variations in the pigment cellular content and composition were observed in Synechococcus1 and Prochlorococcus2 in response to changes in light conditions. These variations lead to changes in the magnitude and spectral shape of light absorption capabilities3•4• Partensky et al. 2 reported a significantly higher photosynthetic capacity in Prochlorococcus grown under high light. Besides these signs of the capability of prokaryotic phytoplankton to photoacclimate, it is known that they possess no flagellae and do not operate a xanthophyll cycle5. So, keeping in mind the specific character of prokaryotes, it could be thought that the number of photoacclimation strategies is smaller compared with eukaryotes. Nevertheless, prokaryotic phytoplankton dominate large parts of the ocean, especially oligotrophic systems where it grows at high rates6. In the present study, we aim at understanding why prokaryotic phytoplankton have such an ecological success in the ocean. Specifically, our objectives are ( 1) to clearly identify the photoacclimation strategies deployed by these organisms, significant in terms or carbon fixation, and (2) to quantify the timescales on which they are deployed. Finally, we discuss how these strategies are effective in the ocean. To reach our objectives, we studied the photoacclimation kinetics of Prochlorococcus sp. We chose this species firstly because it often dominates the prokaryotic phytoplankton communities of oligotrophic systems7 . Secondly, there is a need for a better documentation on this recently discovered species, which was shown to account for up to 50 % of net primary production6 in oligotrophic systems.
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