STUDIES OF DISSOLVED CARBOHYDRATES (OR CARBOHYDRATE-LIKE SUBSTANCES) IN AN ESTUARINE ENVIRONMENT

Variations in concentrations of dissolved sugars were studied along the salinity gradient of a small estuary (Elorn, Bay of Brest, France) from February 1985 to January 1986. Total dissolved carbohydrate (TDCHO) and dissolved monosaccharides (MCHO) were measured by the methods of Burney and Sieburth and Johnson and Sieburth respectively. It must be noted that these methods cannot distinguish between carbohydrates and carbohydrate-like substances; consequently, these methods probably do not closely reflect biologically available pools of carbohydrates. In the river, TDCHO and MCHO values ranged from 230 to 970-mu-g C l-1 and from 75 to 450-mu-g C l-1 respectively. In the estuary, TDCHO and MCHO were usually lower; they varied respectively from 20 to 570-mu-g C l-1 and from 0 to 180-mu-g C l-1. In June, some TDCHO values were much higher, probably because some polysaccharide was produced at this time by phytoplankton excretion or lysis. The relationship between Cl parts-per-thousand and TDCHO was dependent on the seasons. TDCHO was conservative in autumn, winter and early spring. TDCHO and DOC concentrations were fairly well correlated during the same period. Similar results have been previously reported by various workers. MCHO and TDCHO concentrations were well correlated throughout this study except in June. From these results, it was concluded that most dissolved carbohydrates were linked to a conservative fraction of the organic matter in periods of low biological activity, whereas newly biosynthesised carbohydrates were responsible for non-conservative behaviour.