Dissolved free and combined carbohydrates during a phytoplankton bloom in the northern North Sea

Variations in dissolved free and combined carbohydrates during a phytoplankton bloom in the northern North Sea were investigated. Large amounts of carbohydrates are released into seawater during the bloom; the major portion of this release occurs towards the end of the bloom. A considerable part of the released carbohydrates is in the combined form. Free dissolved carbohydrates are formed mainly by in situ hydrolysis of dissolved combined carbohydrates. Glucose and fructose dominate the free dissolved carbohydrate fraction. Glucose is formed biologically, and fructose biologically and abiotically from glucose. Glucose comprises more than 60 % of the combined carbohydrate fraction, followed by mannose, galactose and xylose. The production and release of dissolved carbohydrates in large amounts appear to be related to the availability of nutrients. The vertical distribution of dissolved carbohydrates is controlled by boundary layers in mid-water.