ISOTOPIC AND BIOCHEMICAL-COMPOSITION OF PARTICULATE ORGANIC-MATTER INA SHALLOW-WATER ESTUARY (GREAT OUSE, NORTH-SEA, ENGLAND)

The biogeochemistry of particulate organic matter was studied in the G reat Ouse estuary draining to the North Sea embayement known as the Wa sh from March 1990 to January 1991. Eleven locations were sampled mont hly on a 50 km transect across the shallow estuary from the tidal weir to the middle of the Wash. Particulate organic carbon (POC) and total carbohydrate, protein and lipid analyses were combined with the deter mination of stable carbon isotopes. DeltaC-13 often increased from -30 parts per thousand in the river to -22 parts per thousand in the tida l freshwater reach. The mixing zone between fresh and marine tidal wat ers displayed only a slight increase in deltaC-13 to -19 parts per tho usand. The change in deltaC-13 values in the freshwater tidal reach de monstrated that mixing of riverborne and marine suspended POC was not the only process affecting the carbon stable isotope composition. Comp lementary sources, interfering considerably with the two end-member so urces, may be identified as autocthonous primary production and resusp ension of sediment that may be transported upstream. The respective im portance of these sources is subject to seasonal variation. From March to August, high concentrations in carbohydrate and protein through th e whole estuary indicate that despite turbidity significant primary pr oduction occurred. The proportional importance of the uncharacterized fraction of POC, which is considered as complex organic matter, was hi gh from September to January and low from March to August. During most of the year, the biochemical compositions of particulate organic matt er in the turbidity maximum and the rest of the estuary were similar. This contradicted the principle that owing to the long residence times of particles degradation processes largely dominate the production pr ocesses within the turbidity maximum. The occurence of significant in situ production in such shallow water estuaries may partially compensa te for the degradation of suspended particulate organics, resulting in a complex relationship between the biogeochemical cycling and the fat e of nutrients.