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After the Deepwater Horizon oil disaster in the Gulf of Mexico in 2010, substances were added to the seawater to disperse the oil and help clean it up. These substance had an unforeseen side-effect: vast amounts of marine snow were generated. This is a kind of snot produced by bacteria and plankton, probably under stress. Many things stick to it, including organic matter and oil residues. This mixture then sinks to the seabed, forming a layer of sludge. ‘When people think of an oil disaster they tend to picture pitiful seabirds with polluted wings, but seabed animals are extremely vulnerable too because they can’t easily get away,’ says Van Eenennaam, who recently graduated with a PhD on this topic in the Marine Animal Ecology chair group.
The experimental setup of different aquariums in the climate chamber. ©Justine van Eenennaam
In the lab, Van Eenennaam studied the impact of oil and marine snow on four species of seabed creatures: mud shrimps, aquatic snails, saltwater clams and Foraminifera. She made artificial marine snow that was similar in composition to the snow that formed when the oil spill was cleaned up, and added it to the aquaria. She saw that the marine snow suffocated the seabed creatures. Van Eenennaam: ‘How vulnerable they are to it depends partly on their mobility.’ Snails, for example, can crawl away. ‘In the aquarium they crawled up the glass, which of course they can’t do in the sea, but they can let themselves float in the water to a cleaner place.’ Unfortunately, though, the snails also ate the toxic marine snow.
Marine snow also hampers the breakdown of oil residues by bacteria. This is because they prefer to break down the marine snow first, as it contains more easily digestible sugars and proteins. Also, the bacteria need oxygen to break down the oil. Van Eenennaam: ‘Creatures such as mud shrimps dig up the seabed, which increased the amount of oxygen in it. If these seabed creatures are suffocated, that doesn’t happen so much.’
Eenennaam cannot say for certain to what extent the same effects occur in the Gulf of Mexico. ‘We studied Dutch species. Besides, the situation at a depth of 1500 metres is not the same as in an aquarium of 25 by 25 centimetres.’ But it has been ascertained, she says, that marine snow has negative effects. ‘This should be taken into account when selecting the best method of cleaning up oil.’