Idotea metallica in the German Bight - an indicator of a warming trend in the North Sea

The marine isopod Idotea metallica exhibits a strictly neustonic life-style. The species is associated with surface drifting objects such as macroalgae, wood, plastic bottles, or remains of fishing nets. Due to their resistance to natural decay, abiotic substrates, in particular, represent a suitable vehicle for dispersal by large scale water currents sometimes resulting in a cosmopolitan distribution. Permanent populations of I. metallica occur predominantly in subtropical waters of the Mediterranean or the east coast of North America. The species spreads from America over the entire North Atlantic area. It is carried along by the Gulf stream to Western European waters from Scandinavia down to the Iberian Peninsula. Apart from a few records from the Dutch and the Norwegian coasts the species had never been found in the North Sea proper.

However, in 1994, we encountered I. metallica for the first time off the island of Helgoland (German Bight). This was in the course of a long-term survey program (made since 1988) on the isopod fauna associated with surface drifting objects. A slight increase in population density in 1995 was followed by the species' complete disappearance from that area after the severe winter 1995/96. However, since 1998 I. metallica re-appeared in higher numbers than ever before, with a relatively stable proportion of 1-2 % of the total Idotea fauna found on drift material.

A comprehensive data set on sea surface temperatures measured daily at Helgoland since the early 1960s revealed that the last decade of the 20th century was characterised by extraordinarily mild winter temperatures. Whether this temperature anomaly is a first sign of a long-term global warming trend or just a temporary phenomenon caused by the North Atlantic Oscillation (NAO) has as yet to be decided. However, the simultaneous occurrence of the warm adapted I. metallica and mild winter temperatures appears to be more than purely accidental.

Since drift material could only be collected from spring to autumn, laboratory experiments were performed in order to reveal the species' status in winter. The question was whether I. metallica was able to survive winter periods in the North Sea under typical and slightly warmer temperature conditions. - The answer was: no.

Since 1998 I. metallica has re-appeared in higher numbers than ever before

Experimental populations reared under such winter temperatures were not able to produce offspring and finally went extinct within about six months. The lower temperature limit for reproduction was determined as 13 °C. Since the period with water temperatures below 13 °C lasts for about 8 months in the German Bight, animals cannot survive until spring to be able to establish a new population.

Consequently, summer populations found in the German Bight must derive from specimens introduced into the North Sea every year anew. Presumably, specimens of I. metallica may have always been there but in too low numbers to be easily detected. However, as the North Sea warmed up this resulted in an extended reproductive period for I. metallica allowing the population to become more prolific and thus more conspicuous. Accordingly, the recent conspicuous and regular occurrence of I. metallica is potentially a suitable and sensitive indicator for the ecological relevance of a warming trend in the North Sea.