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Absorption properties of phytoplankton in the Lower Estuary and Gulf of St. Lawrence (Canada)

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Models of ocean colour rely on information about phytoplankton absorption, which varies according to community composition and photoacclimation. Here we show that pigment packaging, which is strongly determined by the size structure of local algal populations, represents a dominant factor in the Estuary and Gulf of St. Lawrence, accounting for ~50%–80% of the reduction in phytoplankton absorption at 440nm during the spring bloom periods and for 24%–48% before and after the blooms. This is consistent with the importance of diatoms in this environment. Comparison between three methods of estimating packaging gave average values within less than 20% of each other during the blooms. Changes in pigment composition, which also affect phytoplankton absorption, were more important outside bloom periods (particularly in the Gulf), although this influence was relatively modest (11%–13%). This was accompanied by an increase in photoprotective pigments and an absorption peak in the ultraviolet range (~330nm). Regional variations in phytoplankton absorption reflected bloom conditions, whereas detrital particulate material was highest in the upstream Saguenay region (often more than 60% of the absorption of total particulate material at 440 nm (ap(440))) and was at least 20%–30% elsewhere. This information is a first step towards the development of regional models of ocean colour.

Les propriétés d’absorption de la lumière par le phytoplancton sont essentielles pour le développement des modèles bio-optiques de couleur de l’océan. Dans ce travail, nous montrons que l’empaquetement des pigments algaux («packaging effect»), qui est principalement déterminé par la structure de taille des populations locales, explique 50–80 % de la réduction du spectre d’absorption du phytoplancton à 440nm pendant les périodes de floraisons printanières de diatomées dans l’estuaire et le golfe du Saint-Laurent. Ce facteur est de 24–48 % avant et après les floraisons, reflétant l’importance des diatomées dans cet environnement. Une comparaison entre trois méthodes d’estimation du « packaging » donne des valeurs moyennes variant de moins de 20 % entre elles. La composition pigmentaire a une influence relativement plus modeste (11–13 %) sur l’absorption phytoplanctonique et elle est importante principalement dans le golfe après la floraison printanière, liée à une augmentation des pigments photoprotecteurs et à un maximum dans la bande UV (autour de 330nm). Les variations régionales de l’absorption phytoplanctonique sont le reflet des conditions de floraisons algales, alors que pour le matériel détritique particulaire, les valeurs sont généralement entre 20 et 30 % de l'absorption particulaire totale (ap(440)), sauf dans la région du Saguenay où elles sont nettement plus élevées (souvent plus de 60 %).

Document Type: Research Article

Publication date: August 1, 2008

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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