The role of the copepod Calanus finmarchicus in affecting the fate of marine oil spills

Abstract

Background: Oil spills in marine environments are subject to biological, physical and chemical weathering processes, including entrainment of oil as droplets in the water column. The oil droplets with diameter < 100 μm are within the size range of particles ingested by marine filter-feeders. Ingestion of oil droplets has been reported for several species of zooplankton, including the calanoid copepod Calanus finmarchicus. C. finmarchicus is ubiquitous in the North Sea, the Norwegian Sea, and the Barents Sea. Based on their high abundance, high feeding activity, and indiscriminate feeding strategy, copepods have been suggested to contribute to weathering and transport processes of oil spills. Aim: The aim of this thesis was to investigate how the abundant marine filter-feeder C. finmarchicus influence transport and weathering of oil dispersions. The work included: 1. A modelling approach using the oil spill contingency and response model OSCAR with a filter-feeder module implemented, determining the quantity of an oil spill that can be removed by ingestion by C. finmarchicus. 2. Laboratory studies determining: a. Concentration of oil in C. finmarchicus feeding in dilute oil dispersions b. Feeding activity of C. finmarchicus in dilute oil dispersions c. Accumulation of oil compounds to C. finmarchicus from dilute oil dispersions and the corresponding water soluble fraction (WSF) d. Viable and total microbial communities in clean and oil-containing feces from C. finmarchicus e. Biodegradation of dilute oil dispersions in the presence of feces from C. finmarchicus Results and discussion: The modeling approach estimated that C. finmarchicus may ingest between 1 and 40% of an oil spill. The estimates in the lower ranges (≤ 2%) were suggested to be realistic, since the high range estimates combined extreme values for several input parameters. The input parameters that had highest impact on the quantity of oil removed by C. finmarchicus were the size limit for droplets ingested, and the population density. The laboratory studies showed that at fixed density (50 ind. L-1) and oil droplet size (diameter < 40 μm), the concentrations of oil in C. finmarchicus biomass were ranging between 3 and 14 mg oil kg-1 (exposure concentration 5.5-0.3 mg L-1). Both the concentration of oil in the biomass and the feeding activity of the copepods were low at the high concentration of oil. The feeding activity were rapidly significantly reduced at low concentrations of oil (17 h, 1.4 μL L-1,) indicating that C. finmarchicus have largest impact on oil spills at an early stage and at low concentrations. Ingestion of oil droplets contributed to rapid accumulation of all oil compounds. Accumulation from the WSF reached steady state for the low lipophilic (log Kow < 5) compounds within 24 hours, while the high lipophilic compounds (log Kow > 5) did not reach steady state within the 96 hour exposure. Over time, lower concentrations of the low lipophilic compounds were observed in oil dispersion exposed C. finmarchicus compared to WSF-exposed. This indicated elimination to the water, and may cause redistribution of these compounds during oil spills. Since the concentration of the high lipophilic compounds not was affected similarly, C. finmarchicus biomass may act as a sink for high lipophilic oil compounds. The oil-containing feces from C. finmarchicus feeding in dilute oil dispersions contained significantly higher concentrations of viable oil-degrading microorganisms. The total microbial communities were similar between the clean and oil-containing feces, and the oil-degrading activity was suggested to be mediated by indigenous feces bacteria. The presence of oil-containing feces resulted in higher biodegradation of the nalkanes in a dilute oil dispersion, while the presence of clean copepod feces resulted in lower biodegradation of the n-alkanes. This supported the suggestion that the indigenous feces bacteria were mediating the oil-degrading activity. These bacteria may have preferred carbon in feces prior to the carbon in the n-alkanes. The oil and copepod feces also formed large agglomerates. These may increase the sedimentation of relatively un-weathered oil towards the seabed during oil spills, depending on their effective density. The presence of clean C. finmarchicus feces resulted in higher biodegradation of the aromatic fraction, suggested to be caused by leaking of nutrients from the copepod feces. The presence of C. finmarchicus feces can thus increase the biodegradation of the dissolved fraction of an oil spill. Conclusion: The results indicated that a substantial concentration of oil can be contained in the C. finmarchicus biomass during oil spills. Further, C. finmarchicus biomass can contribute to redistribution of the low lipophilic oil compounds and function as a sink for dissolved high lipophilic compounds. The excretion of oil in feces increased the concentration of viable oil-degrading microorganisms in the feces, mediated by the indigenous feces bacteria. Biodegradation of the n-alkanes was dependent on the quantity of feces present, while the biodegradation of the aromatic compounds was increased in the presence of copepod feces.