4.2.2.5 Sedimentation 



Sediment traps, a PVC cross with 20 cylindrical glass tubes 

 (0 26 mm, length 200 mm), were positioned at a depth of 20 m. The 

 sedimented matter was divided in two parts; one was used for deter- 

 mination of dry weight and the other was transferred to a glass jar 

 (cleaned with hexane) and stored deep frozen until analysed for oil. 



4.3 Results 



4.3.1 Phytoplankton 



4.3.1.1 Phytoplankton biomass 



At both reference stations, total phytoplankton biomass remained 



-2 

 fairly constant, around 600 mg C m , throughout the period (Fig. 4.2). 



In the impacted area phytoplankton biomasses were mostly more than a 



factor of two higher in the two weeks following the spill, but gradually 



decreased and approached the level at the reference stations by the end 



of November. There is no significant difference between the two stations 



(IV and V) in the impacted area (p = 0.32, rank sum test according to 



Dixon and Massey 1969:345), but valid statistical comparisons between 



the impacted area and the reference stations cannot be made, due to the 



long time spread and few measurements at the latter stations (see Fig. 



4.3). 



4.3.1.2 Phytoplankton species composition 



At all stations monads constituted 75 to 90% of the total biomass 

 and diatoms 10 to 15% (dominating species: Coscinodiscus granii and 

 Skeletonema costatum) throughout the investigated month. The remaining 

 few percent were peridineans ( Gymnodinium sp . and Gyrodinium sp.). A 

 few species belonging to Cyanophycea and Chlorophycea were also present, 

 but their contribution never exceeded one percent. This is not an 

 abnormal phytoplankton composition for autumn in this area (Hobro, in 

 press) . 



65 



