The use of caged mussels for monitoring produced water discharges: the Norwegian Water Column Monitoring Program

Abstract

Offshore activities, especially productions platforms might be responsible to adverse biological effect on local marine organisms. In order to assess this impact a water column monitoring program was designed and performed every year in the Norwegian sector of the North Sea till 2014. From 2017 new regulation applies and the monitoring is mandatory every three years. As part of the program, chosen living organism (caged mussels and wide fish) are used to evaluate the potential impact. The main focus of the current thesis was to analyze caged mussel overall health status and physiological conditions using four biomarkers: condition index (CI), survival in air, micronucleus formation (MN), and lysosomal membrane stability in digestive gland (LMS). The assays were performed on samples of caged mussel that were deployed offshore in order to investigate the potential biological effects of produced water (PW) on marine ecosystem around production platforms Statfjord A and B. Mussels were deployed around the platforms in different position to achieve exposure to the PW plume. The thesis experiments are based on adequate number of mussels (at least 30 individual mussels needed for each biomarker study in each station) placed in nets and divided into 19 different stations. The stations were located offshore and exposed to potential PW plume in distances of 500 m (within platform safety zone) to 1000, 2000 and 10000 m far from the platforms. Two reference stations were located up north from platforms. Mussels were kept offshore for six weeks (April/May 2017). Analyses were performed at the IRIS Environment laboratory in Mekjarvik, Stavanger. Results from the survival in air test show that the mussels caged inside platform safety zones (4, 10, 11, 12, 13, and 14) were significantly less tolerant toward stress in compare with the one caged at the reference stations (18 and 19). Generally, there were statistical differences in survival time between mussels caged at stations close to the platforms compared to the one caged farther from the platforms. As for the CI, no significant decrease in the overall health status was observed between mussels caged at the platform and the one caged at the reference stations. The MN assay showed statistically higher MN frequency in mussels from two of the six stations located closer to the platforms (4 and 13) when compared with both reference stations. No significant differences were observed. The LMS assay showed no significant difference between exposed and reference individual samples. However, closer stations to the platforms (safety zone) had slightly smaller LMS time even though the Probability-values (p-values) were higher than 0.05. Overall, the performed biomarker studies show a moderate stress experienced by the mussels placed within safety zone of the platforms (based on the data from survival in air and MN test). The performed analysis on digestive glands (LMS) showed minor impact on mussels exposed to the environment in vicinity of the platform safety zone.