Jellyfish are an important part of the ocean ecosystem, but when they appear in large numbers they cause problems: shutting down beaches for swimmers, clogging fishing nets, closing power plants and desalination facilities by blocking their water intakes, and gobbling fish larvae and plankton.

In the Adriatic Sea blooms of moon jellyfish have become more common. These harmless jellyfish – their sting is painless for humans – were first recorded in the region in 1834 but were mostly a rare occurrence. But in the 1950s, 60s and 70s they appeared once or twice per decade, and by the 80s and 90s they were present around 80% of the time. In the last two decades they have been present every year.

Moon jellyfish have a number of different stages in their life cycle, including a phase as a "polyp". The polyps need an overhanging substrate to attach themselves to. In nature such substrates tend to be rare, but the increase in gas platforms – the first one arrived in the region in 1968 and now there are around 140 – may have inadvertently helped moon jellyfish to thrive, by providing the polyps with an ideal home. Other factors that may also have helped moon jellyfish numbers to increase include over-fishing, pollution and climate change.

Martin Vodopivec from the Slovenian National Institute of Biology and colleagues decided to investigate the influence of offshore marine constructions on the moon jellyfish population in the Adriatic. The team used a computer model to simulate the jellyfish lifecycle and dispersal patterns over the course of five years. The model included accurate representations of the ocean currents and positions of the gas platforms in the area.

Observational data on jellyfish is scarce because it is hard to perform regular observations and difficult to cover enough area to give a proper spatial representation of jellyfish distribution. Models provide a way of giving a spatio-temporal picture, and exploring the impact of changes over time.

The model results show that offshore platforms have been responsible for a large increase in connectivity between populations in the Adriatic Sea, which helps to sustain populations that might otherwise be wiped out at times of difficulty (due to pollution, low water temperatures and predation, for example).

But some platforms had more influence over jellyfish numbers than others. In particular, platforms located close to the Western Adriatic current have the potential to influence jellyfish populations over 1000 km away.

"Our simulation shows that jellyfish can travel surprisingly far, especially in a strong current like the Western Adriatic current," said Vodopivec, whose findings are published in Environmental Research Letters (ERL) .

In this case the simulation suggests that the Italian coast, particularly the area south of the Emilia-Romagna region, has suffered most from invasions of jellyfish originating from the gas platforms.

Offshore marine constructions are increasing worldwide, particularly with the push towards renewable energy, including offshore wind farms. For example, the power capacity of European offshore wind installations has more than doubled in three years.

"We believe that this could be cause for considerable concern in some places, especially those that are downstream from big offshore wind farms," said Vodopivec.

So how can we avoid this marine construction boom leading to plagues of jellyfish? Vodopivec and his colleagues think that models like theirs could be used to foresee which areas are most likely to be affected, and to evaluate the connectivity of jellyfish populations and avoid bridging natural gaps.

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