ABSTRACT. Reports of severe stratospheric ozone depletion over the Arctic have heightened concern about the potential impact of rising ultraviolet-B (UV-B) radiation on north polar aquatic ecosystems. Our optical measurements and modelling results indicate that the ozone-related UV-B influence on food web processes in the Arctic Ocean is likely to be small relative to the effects caused by variation in the concentrations of natural UV-absorbing compounds, known as chromophoric dissolved organic matter (CDOM), that enter the Arctic basin via its large river inflows. The aim of our present study was to develop and apply a simple bio-optical index that takes into account the combined effects of attenuation by atmospheric ozone and water column CDOM, and photobiological weighting for high-latitude environments such as the Arctic Ocean. To this end, we computed values for a biologically effective UV dose rate parameter ("weighted transparency" or T*) based on underwater UV measurements in highlatitude lakes and rivers that discharge into the Arctic Ocean; measured incident UV radiation at Barrow, Alaska; and published biological weighting curves for UV-induced DNA damage and UV photoinhibition of photosynthesis. The results underscore how strongly the Arctic Ocean is influenced by riverine inputs: shifts in CDOM loading (e.g., through climate change, land-use practices, or changes in ocean circulation) can cause variations in biological UV exposure of much greater magnitude than ozonerelated effects.Key words: Arctic, CDOM, DNA, global change, ozone depletion, phytoplankton, Siberia, transparency, ultraviolet radiation RÉSUMÉ. Des rapports concernant un appauvrissement sérieux de l'ozone stratosphérique au-dessus de l'Arctique ont accru les préoccupations au sujet de l'impact potentiel d'un plus fort rayonnement ultraviolet-B (UV-B) sur les écosystèmes aquatiques du pôle Nord. Nos mesures optiques et nos résultats de modélisation indiquent que l'influence de l'UV-B relié à la couche d'ozone sur les processus du réseau trophique dans l'océan Arctique est probablement faible en comparaison avec les effets dus à la variation dans les concentrations des composés absorbants naturels de l'UV, connus sous le nom de matière organique dissoute chromophore, qui entrent dans le bassin Arctique par le biais de l'eau amenée par les cours d'eau majeurs. Le but de la présente étude était de développer et d'appliquer un index bio-optique simple qui tient compte des effets combinés de l'atténuation due à l'ozone atmosphérique et à la matière organique dissoute chromophore dans la colonne d'eau, et de la pondération photobiologique pour les écosystèmes des hautes latitudes comme l'océan Arctique. À cette fin, nous avons calculé les valeurs pour un paramètre de débit de dose UV qui ait une action biologique («transparence pondérée» ou T*) fondée sur des mesures sous l'eau du rayonnement UV dans des lacs et cours d'eau de hautes latitudes qui se déversent dans l'océan Arctique; sur des mesures du rayonnement UV incident à Barrow, en Ala...
Intensive monitoring of the maximum quantum yield of carbon fixation (&,,,) and of the minimum quantum yield of chlorophyll a in vivo fluorescence ($& was followed along the path of drifting buoys in the estuary and the Gulf of St. Lawrence, Canada. Samples were collected every third hour during two 24-h periods. Over the 170 measurements, &,,X varied by a factor of 20 [from 0.004 to 0.08 mol C (mol quanta))'].As predicted by a model of primary processes in photosystem 2, a significant and positive linear relationship was observed between $,,, and q5I;,. In the estuary, the covariability of &,,,, and $t;, seems to be driven by light, because it was observed only during daytime and was characterized by a parallel increase of &,,, and $r;, with depth. Examination of the spectral values of $FO suggests a contribution of photoprotectant carotenoids. In the gulf, because a parallel, although weaker, increase of &,, and rbr;, with depth also persisted during nighttime, nutrient deficiency is probably an additional cause of covariation between (6,,, and cbr;,. In both regions, an overall diel cycle was observed for $,,X with a maximum at around noon; $I;, did not show a clear diel periodicity. Despite the vertical covariability of &,,X and 4F0, the diel cycle of &,,X was uncoupled to $FO and thus to photosystem 2 activity. These results suggest a significant diel variation in the photosynthetic quotient of natural phytoplankton. AcknowledgmentsThis research is a contribution to the programs of the Maurice Lamontagne Institute (Dept. Fisheries and Oceans, Marine Productivity Division), GIROQ (Groupc interuniversitaire de recherches oceanographiques du Quebec), and the Fachbereich Physik, AG Laserfernerkundung, Universitat Oldenburg. M. B. received postgraduate scholarships from Fisheries and Oceans and from the "Action structurante" program of the Quebec Government.We thank A. Gag&, D. Michaud, and G. Rosenberg for technical assistance and M. R. Lewis, W. Vincent, P. G. Falkowski, A. Morel, and P. J. Nealc for comments on the manuscript. We are also grateful to D. Stramski and an anonymous referee for their criticisms and suggestions on various drafts of the manuscript.
A 64-channel Laser Environmental Airborne Fluorosensor (LEA F) system for sea surface oil detection was modified for continuous depth-resolved detection of gelbstolT and chlorophyll a spectral fluorescence, and water Raman scattering. The system was successfully tested during shipborne and airborne experiments in the SI. Lawrence Estuary. System performance was studied, and the best integration time to optimize the signal to noise ratio (SIN) was determined using dilTerent statistical procedures such as the geophysical and robust statistical methods. In general, raw data indicated SIN of about 150, 10, and 4 for Raman. gelbstolT and chlorophyll a signals, respectively. Results with calibrated LEAF spectra clearly indicated that, with this system, water Raman scattering, geibstolT and chlorophyll a signals could be detected down to a maximum depth of 4·8 m. Spatial distribution of these quantities compared well with simultaneously observed ill Silll structure of oceanographic variables, such as, underwater light attenuation, salinity and chlorophyll a fluorescence. In the perspective of modelling primary production in coastal and estuarine waters in the SI. Lawrence system (Case II waters), the utilization of LEAF should provide an adequate representation of the spatial and temporal variation pattern of oceanographic variables, at scales between those from ships and satellites.
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