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NEWS 7 maltatoday | WEDNESDAY • 3 AUGUST 2022 More jellyfish and toxic fish expected as sea water temperatures rise, Diedun warns JAMES DEBONO DATA compiled by The Ocean- ography Marine Research Group within the Department of Geo- sciences of the University of Malta shows that till a few years ago the landmark temperature of 30 C in surface waters used to be reached at the end of sum- mer. But progressively the same temperature is being reached progressively earlier during the summer reason. For instance, 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 temper- atures, especially of surface wa- ters, are extending for a longer period in the autumn calendar season, with the first significant cooling of Maltese seas only be- ing observed in latter half of No- vember. And while water tem- peratures are rising in summer, they are also becoming cooler in Winter in 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 tem- peratures persisting till early May this year. Even thickness of the sur- face 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 sepa- rated by the thermocline layer, such that there is very little verti- cal mixing in the water column. But in recent years, the thermo- cline has ventured deeper and deeper, resulting in a deeper penetration of normally warm surface water. According to marine biologist Alan Deidun rising sea tem- peratures represent an "abrupt departure from the norm" and are expected to exert a signifi- cant long-term impact, both on native marine communities but also on socio-economic aspects linked with the sea. For instance, one of the con- sequences is the proliferation of alien marine species including the dreaded invasive ones from tropical waters. Some of these invasive like the silver-cheeked toadfish, which has already been recorded locally are toxic. This fish is extremely poisonous if eaten because it contains tet- rodoxin in its ovaries and to a lesser extent its skin, muscles and liver, which protects it from predators. Others like the no- madic 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 recorded in Maltese waters pose a danger sting- ing. 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 con- tain painful venom. Although the lionfish has so far not been recorded in Maltese waters its presence has become more fre- quent in the Mediterranean. Some native marine species, including the bearded fireworm ('busuf' in Maltese) and the or- nate wrasse (the 'lhudi' in Mal- tese) are also favoured by the higher temperatures and their increasing numbers disrupts the stability of marine ecosystems. Higher than normal water tem- peratures also facilitate the re- production of many jellyfish spe- cies, such that blooming of some of these species is more intense and lasts for longer, as docu- mented within the Spot the Jel- lyfish 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 tempera- tures invariably bring along. Crucially Posidonia oceanica meadows which like forests on land oxygenate the sea are al- so debilitated by warmer tem- peratures. This is because their shoots are increasingly colo- nised by higher densities of com- peting 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 temper- atures in autumn are also asso- ciated with more intense storms once a cooler front trundles in, resulting in flooding and soil erosion. How do we know? The Oceanography Marine Re- search Group within the Depart- ment of Geosciences of the Uni- versity of Malta has deployed, for the past few years, vertical sets of water temperature loggers, ex- tending 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 log- gers. This data, combined with data emerging from satellites and from mathematical models, provides the most reliable pic- ture of how water temperature fluctuates over the course of one year in Maltese coastal waters. Surface water temperatures normally surpass the 30 C mark towards the end of Summer. But for the first time this temperature was reached at the end of July. MaltaToday caught up with marine biologist Alan Deidun who warns that higher water temperatures will have lasting consequences in the shape of more jellyfish blooms, the arrival of more alien species from tropical zones and increased likelihood of Autumn storms Marine biologist Alan Deidun