Aarhus University Seal / Aarhus Universitets segl

No 216: Store Hellefiskebanke, Greenland

Wegeberg, S., Rigét, F., Gustavson, K. & Mosbech, A. 2016. Store Hellefiskebanke, Grønland. Miljøvurdering af oliespild samt potentialet for oliespildsbekæmpelse. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 98 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 216.  http://dce2.au.dk/pub/SR216.pdf

 

Summary

 

The potential for combating oil spills at Store Hellefiskebanke has been evaluated. This evaluation has focused on minimization of the combined environmental impacts of both oil spill and oil spill response techniques, which include chemical dispersion of oil and in situ burning (burning of oil directly on the sea surface). Hence an assessment has been completed to evaluate the environmental pros and cons of the different oil spill response techniques, for which the concept of a strategic Net Environmental Benefit Analysis (sNEBA) was developed.

 

The sNEBA includes an analysis, which is based on oil spill scenarios and published as well as expert knowledge on the environment in the area in question. The environmental knowledge included in the sNEBA, to achieve the overall environmental optimal oil spill combating strategy, is biodiversity (on sea surface, in sea and sea bed, and seasonality), biology, ecotoxicology of oil (naturally and chemically dispersed as well as oil burning residues). The sNEBA is hence a planning tool, and thus a desktop analysis finished in advance of a potential oil spill, for environmentally assessing and preparing of oil spill combating potential and strategy.

 

The analysis is based on simulation of dispersion of dissolved oil components during an oil spill in the Store Hellefiskebanke area. Five scenarios have been selected with variables of location of oil spill (locality and surface / bottom), date, degradation rate and ice conditions. For the simulation, the size of the oil spills is fixed at 1000 t per 24 hours for 28 days. This is in accordance with modelling of oil spill scenarios performed in connection with previous actual drilling operations in Greenland. It should be noted that the historical largest offshore oil spills were up to five times larger in volume. This means that in case of accidents in the size of Macondo and Ixtoc, the water volumes with oil concentrations of toxic levels also will be up to five times larger than simulated here, a fact which also applies to the distribution of oil on the sea surface. The meteorological data provided in connection with these simulations were used for simple calculation of oil trajectories for the same five scenarios.

 

An environmental impact assessment of oil spills was performed for the organisms associated to sea surface, water column, sea bed, coast and ecosystems characteristic of these spatial compartments. Based on the simulation results, analysis of organisms’ sensitivity to potential oil exposure was performed.

 

Considered benefits by oil dispersion and ISB for seabirds on sea surface

 

To the benefit of the organisms associated to the sea surface, removal of the oil from the sea surface will prevent smothering of feathers and fur. It is assessed that the rehabilitation time of the Store Hellefiskebanke king eider population can be more than 20 years after an oil spill causing comprehensive mortality. Thick-billed murre and little auk are also present in the Store Hellefiskebanke area, especially in the spring.

 

The response techniques, chemical dispersal and ISB, are thus considered as beneficial in protection of seabirds congregated on the sea surface for feeding or moulting. However, it should be noted that burning residues from ISB also may possess a risk for smothering of seabird feathers, but by ensuring a high burning efficiency this risk can be further reduced.

 

Analysis of potential effects of dispersed oil on copepods in the water column

 

Simulation of the tidal dynamics indicates complete vertical mix of the water body across the bank, and this may explain the high primary production in the entire bank area. The production is thus not restricted to the up-welling points in the area.

 

The dispersion model simulations also show horizontal distribution of oil concentrations. From the oil spill scenarios located on Store Hellefiskebanke and close to the coast at the northern part of the bank the simulations show that the dispersed oil may stay on the bank, beach on nearby coasts or spread into Disko Bay.

 

Calculations of water volumes with toxic oil concentrations, based on the simulations of dissolved oil components dispersion (1000 t per 24 hours), show that there is risk of effects on the organisms in the water column. The vertical distribution of toxic oil concentrations covers the upper part of the water column (down to app. 10 m), which also is occupied by a high fraction of plankton (0-50 m). Hence, there is an overlap between the zones with toxic concentrations (acute lethal and sublethal) and high density of plankton. This overlap is estimated to potentially cover 0.4-3% of Store Hellefiskebanke’s surface area for lethal concentrations and 7-30% for sublethal concentrations. Besides the toxic concentrations of oil components, dispersal of oil may result in oil droplets which can be perceived as food items and taken up by copepods. This may pose a risk, especially during summer, when the copepods are feeding and lead to accumulation of oil components in these organisms. However, dispersal of oil during winter time may not pose the same risk as the copepods do not feed during this season.

 

Calanus-species and larger crustaceans (e.g., Themisto libellula) in the water column provide an important food ressource for whales and birds (little auks and thick-billed murres (in particular during spring)), but also for fish larvae (Atlantic halibut and sandeel). Sandeel is considered to be a key species in the food web of the bank, e.g. as food item for cod, birds and seals. However, it is also considered that the species is sensitive to oil spills, as sandeel will spend less time in its main habitat, sediments of sand and gravel, if contaminated by oil. The Disko West area, including the entire Store Hellefiskebanke, is furthermore feeding and mating ground for bowhead whale. The food web of Store Hellefiskebanke therefore seem to be very tightly linked and complex and with a fast turnover. This may be the reason for surprisingly low abundance of some of the ecosystem’s key species in some of the studies conducted in the area.

 

Oil spill in this area may thus have direct and indirect impacts on the ecosystem due to the toxic effects on organisms at lower trophic levels and the derived impact on the higher trophic levels due to changes in food availability. The toxic effects of oil components may hence be transmitted through the food web and cause so called cascade effects.

 

Dispersion of large oil spills cannot be recommended in the summer time, as it may provide a risk of impacting ecosystem key species such as copepods (Calanus spp.) by relatively large water volumes and areas with toxic oil concentrations. During winter month, the copepods may be less sensitive to oil exposure and hence dispersion may be a solution for combating oil spill during that season.

 

It is also expected that dispersion of smaller oil volumes may have only insignificant effects on the environment as simulations show that concentrations of dispersed oil in the water column is positively correlated with the volume of oil spill dispersed.

 

Analysis of potential effects of dispersed oil and ISB residues on seabed
organisms

 

The sea bed organisms (benthos), e.g., bivalves, corals, brittle stars, sea anemones, polychaetes etc., may not be impacted by toxic concentrations of dispersed oil which may reach a depth of app. 7 m offshore and 15 m in coastal areas, according to the simulations. However, there are areas on Store Hellefiskebanke which are shallower and in these areas impacts on benthic organisms, including fish species, cannot be excluded.

 

Oil spill from a well head at the seabed is not expected to cause stronger effects, as simulations of such oil spill show that the oil, due to oil’s buoyancy and shallow waters, will be transported to the sea surface at a fast rate.

 

The risk of indirect effects of toxic oil concentrations through food web impacts may be more imminent. If the primary and secondary production in the water column above the sea bed decline, food propagated to the benthic organisms will also decline. The production in the upper part of the water column and the relative shallow waters across Store Hellefiskebanke ensure food for the benthic organisms as it is not turned over in the water column, and filtering organisms such as bivalves dominate the benthos community until 150 m’s depth. Bivalves provide food for king eiders and walruses, for which Store Hellefiskebanke is an extremely critical habitat.

 

It is assessed that burning residues from ISB may pose a risk of more direct effect on the benthos if they sink. Mats of partly burned oil may lie on the sea bottom. Environmental effects of these residues on benthos and, in particular, demersal fish has only been sporadic elucidated.

 

Analysis of potential effects of dispersed oil on the coastal ecosystems

 

Simulations of oil concentrations show that dispersed oil may pose a risk of relatively high oil concentrations until 15 m depth at exposed coasts. The richest part of the kelp forest is in the depth of 0-20 m at wave exposed coastlines.

 

If an oil spill is not combated offshore and drifts towards a wave protected coast, and where the oil hence not will be dispersed naturally, it may beach. On such coasts, also protected from ice scouring, the richest tidal communities are found, which may be impacted by the beaching oil.

 

Protected coasts, may moreover have very limited self-cleaning potential, why there is risk of preserving oil for example buried in the beach sediment or between boulders and in crevices. Such oil may pose a source of continuous contamination to the environment. In connection with the Exxon Valdez accident in 1989, small amount of buried oil is still present on the coast, also surprisingly far away from the location of the accident.

 

Beaching oil will furthermore pose a risk for spawning areas for capelin and lumbsucker, which both are important commercial fish species abundant along the Greenland west coast up to Upernavik and Uummannaq, respectively. The capelin spawns in the shallow waters (in the tidal zone) along the coasts in spring and summer. When not spawning, the capelin stays in deeper fjord waters and on the banks. The lumbsucker spawns in shallow waters from May to June. These two species will be particularly sensitive to oil beaching in this time window from early spring and through summer.

 

Conclusion on analysis (sNEBA)

 

It is assessed that the potential for combating oil spills on Store Hellefiskebanke is limited, as the use of chemical dispersant is not, in overall, to the environments advantage according to the strategic NEBA (sNEBA). However, the use of chemical dispersion will be of least risk during months with low productivity in the water column and low abundance of pelagic organisms (e.g. copepods and fish larvae). It is also assessed that dispersion of smaller oil volumes can be considered as concentrations of dispersed oil in the water column is smaller accordingly.

 

In situ burning is assessed as a potential oil spill response technique that may be used all year round, however, with a reservation for potential impact of burning residues on birds and on benthos from sinking residues.