The Tsesis oil spill impact on the pelagic ecosystem

Abstract

On 26 October, 1977, the tanker Tsesis grounded in the Swedish archipelago, 65 km south of Stockholm (northern Baltic proper, 59°N, 18°E). The Tsesis carried 17575 t of No. 5 fuel oil. The total spill was estimated as being somewhat more than 1000 t, of which about 600–700 t were recovered, thus leaving about 300 t in the environment. The spill occurred only about 5 km east of the Askö Laboratory, a marine ecological station, which made a relatively fast sampling response possible. The effects on the pelagic ecosystem were studied for 1 month following the spill. Severe effects were recorded only in the immediate vicinity of the wreck where zooplankton biomass declined substantially during the first few days after the spill. Within 5 days the zooplankton biomass was re-established. Oil contamination of zooplankton was recorded for over 3 weeks. It is suggested that an increased phytoplankton biomass and primary production in the impacted area was due to decreased zooplankton grazing rates. Increased bacterial numbers and the oil degradation pattern indicated a rapid bacterial degradation of hydrocarbons in the water column. Sediment traps positioned in the area demonstrated the importance of sedimentation as a pathway for removal of oil from the water column. During the second week after the spill, 0.7% of sedimented material was petroleum hydrocarbons. Using sediment trap data, a total sedimentation of 30–60 t of oil was estimated in the impacted area (42 km 2). This corresponds to 10–15% of the unrecovered oil.