Could secondary flows make possible the cross-strait transport of passive floating organisms in the Strait of Gibraltar?

Abstract

The Gibraltar Strait suffers an unprecedented invasion of the alien alga Rugulopteryx okamurae of North Pacific origin. Seemingly, algae first settled in the south shore around year 2015, probably following commercial exchanges with French ports, but there is no certainty that algae first colonized the south shore and then spread to the north one. The opposite could well have happened. Whatever the case, it spread with amazing rapidity over the whole area. Human-mediated vectors (algae attached to ship hulls or fishing nets) can be behind the spread from the shore initially settled to the opposite one. But secondary cross-strait flows within frictional Ekman boundary layers associated to the large along-strait velocity typical of this region could also have propitiated the connection without human intervention. Historical currentmeter profiles collected in the Strait show an intermediate layer of north-going cross-strait velocity near the interface of the mean baroclinic exchange, and an overlying surface layer of southward velocity, whose lower part overlaps the interfacial zone. The first one would facilitate south-to-north transport of algal fragments (or any other neutrally buoyant material) able to settle near the interface depth, while the second one would do the opposite. Cross-strait currents at this depth are of few cm/s, which implies crossing times of several days in low-illuminated conditions. Living organisms must be able to overcome these demanding conditions of darkness and maintain good photosynthetic activity after such period for a successful colonization. Rugulopteryx okamurae can do it.