More jellyfish and toxic fish expected as sea water temperatures rise, marine biologist warns

Until a few years ago the landmark temperature of 30°C in surface waters used to be reached at the end of summer, data compiled by The Oceanography Marine Research Group within the Department of Geosciences of the University of Malta shows.

But the same temperature is being reached progressively earlier during the summer season. Surface waters surpassed the 30°C mark in August for the first time in the summer of 2021, whilst in 2022 this benchmark was reached during the last days of July.

Moreover, sea water temperatures, especially of surface waters, are extending for a longer period in the autumn calendar season, with the first significant cooling of Maltese seas only being observed in latter half of November. And while water temperatures are rising in summer, they are also becoming cooler in winter, an indication of temperature extremities.

Data shows sea temperature dipping below the 15°C mark in March for the first time in several years, with the unseasonal cold temperatures persisting till early May this year.

Even thickness of the surface layer is also running amok. During the summer season, the water column is stratified, with a surface warm layer of water ‘sitting’ on a cooler bottom one, with the two layers being separated by the thermocline layer, such that there is very little vertical mixing in the water column. But in recent years, the thermocline has ventured deeper and deeper, resulting in a deeper penetration of normally warm surface water.

According to marine biologist Alan Deidun rising sea temperatures represent an “abrupt departure from the norm” and are expected to exert a significant long-term impact, both on native marine communities but also on socio-economic aspects linked with the sea.

For instance, one of the consequences is the proliferation of alien marine species including the dreaded invasive ones from tropical waters. Some of these invasive species like the silver-cheeked toadfish, which has already been recorded locally, are toxic. This fish is extremely poisonous if eaten because it contains tetrodoxin in its ovaries and to a lesser extent its skin, muscles and liver, which protects it from predators.

Others like the nomadic jellyfish whose stinging can cause very painful injuries to humans and was indigenous to tropical warm waters of the Indian and Pacific Oceans, has already been recorded in Maltese waters. Other species like the blue swimmer crab which is recorded in Maltese waters are known to decimate native biota. Another lurking danger is posed by the lionfish whose sharp spines contain painful venom. Although the lionfish has so far not been recorded in Maltese waters its presence has become more frequent in the Mediterranean.

Some native marine species, including the bearded fireworm (‘Busuf’ in Maltese) and the ornate wrasse (the ‘Lhudi’ in Maltese) are also favoured by the higher temperatures and their increasing numbers disrupt the stability of marine ecosystems.

Higher than normal water temperatures also facilitate the reproduction of many jellyfish species, such that blooming of some of these species is more intense and lasts for longer, as documented within the Spot the Jellyfish citizen science campaign. For example, a jellyfish bloom in Israel this summer, has been described as one of apocalyptic proportions.

Warmer sea temperatures carry less dissolved oxygen such that sessile (non-moving) species are stressed, especially in poorly-flushed waters (like harbours), whilst a number of motile species (including sea urchins) respond by relocating to deeper waters so as to escape the higher incidence of infection by pathogens (e.g. viruses, fungi) which the higher sea temperatures invariably bring along.

Crucially Posidonia Oceanica meadows which like forests on land oxygenate the sea are also debilitated by warmer temperatures. This is because their shoots are increasingly colonised by higher densities of competing epiphytes (organisms that grow on the surface of the plant), which shade the plant from the sunlight it needs.

Apart from the ecological considerations, persistently high-water temperatures in autumn are also associated with more intense storms once a cooler front trundles in, resulting in flooding and soil erosion.

How do we know?

The Oceanography Marine Research Group within the Department of Geosciences of the University of Malta has deployed, for the past few years, vertical sets of water temperature loggers, extending from the surface down to a depth of 40m. These loggers record the water temperature every hour and are left in the water for a whole year prior to them being retrieved and being replaced with a new set of loggers.

This data, combined with data emerging from satellites and from mathematical models, provides the most reliable picture of how water temperature fluctuates over the course of one year in Maltese coastal waters.