In a Mediterranean seagrass ecosystem (Posidonia oceanica ) around Ischia (Gulf of Naples) an attempt was made to study the fate of Posidonia derived particulate matter in wrack beds around and within the seagrass stands, on the shore and in the water column. Changes in total soluble carbohydrate concentrations from green and brown parts within one leaf vary from 70.7 to 25.7 mg.g-1 dry weight. Minimum values of 0.2 mg.g-1 dry weight consisting mainly of saccharose are detected for brown wrack particles. All other components found in green leaf parts, e.g. fructose, glucose and myo-inositol probably leach rapidly into the water or are transported into the rhizome prior to the loss of the brown leaf region. Only in the rhizome the trisaccharide raffinose was detected in addition to the components found in the leaves. The importance of the brown leaf fraction as a substrate for microheterotrophs is indicated by bacterial densities up to 4 x 104 cells.mm-2. This is correlated with nitrogen and carbon values, showing a decreasing C/N ratio with decreasing particle size, but increasing O2 uptake with maximum values of 10 mg.g-1.h-1 for the particle size fraction of 0.1 - 1 mm. The role of the brown dead Posidonia derived leaf tractions as energy source for consumers is discussed and attempts are made to redefine the terms "debris" and „detritus"
