What do we know about marine heatwaves?

“The temperature of coastal waters around the islands have been inexorably climbing each summer season over the past few years,” Deidun explained.

“For instance, the landmark temperature of 30 degrees was first reached within surface waters at the end of summer a few years ago, with the same temperature being reached progressively earlier during the summer reason.”

“For instance, surface waters surpassed the 30-degree mark during the month of August for the first time in the summer of 2021, whilst this benchmark was reached this year during the last days of July.”

“Water sea temperatures, especially of surface waters, are extending for a longer period in the autumn calendar season, with the first significant cooling of our seas only being observed in the latter half of November.”

“It’s not just warming, however. Water temperature extremities, in general, are on the rise, with the sea temperature in March dipping, for the first time in several years, below the 15-degree mark, with the unseasonal cold temperatures persisting till early May this year, signalling a potential shift, of a few weeks, of the usual seasonality.”

“Besides the absolute temperature of coastal waters, the 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’ aloft and a cooler bottom one, with the two layers being separated by the thermocline, such that there is very little vertical mixing in the water column.”

“In recent years, the thermocline has ventured deeper and deeper, resulting in deeper penetration of normally warm surface water.”

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 replaced with a new set of loggers,” he explained. 

“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.”

“Water temperature loggers are small instruments which were first procured by the Geosciences Department as part of its participation in the Tropical Signals programme run by CIESM, the International Commission for the Scientific Exploration of the Mediterranean Sea.”

“The loggers are placed in the water column in a vertical fashion, with a logger being attached to the permanent mooring shot-line every 5 metres, with the set extending from the surface down to 40m.”

Why is this important?

“The rising sea temperatures, given that they are an abrupt departure from the norm, are expected to exert a significant long-term impact, both on native marine communities but also on socio-economic aspects linked with the sea.”

Deidun also mentioned a number of species that are on the increase in Malta’s waters due to such marine heatwaves. 

“For instance, as documented by the Spot the Alien citizen science campaign run by the same research team at the University, the spread and proliferation of alien marine species, including the dreaded invasive ones, has spiralled, mainly since the same species are of largely of tropical origin (native to the Indian, Pacific and tropical Atlantic Oceans).”

“The Invasive Alien Species (IAR) exert a significant socio-economic impact, either by being toxic (e.g. silver-cheeked toadfish, which has already been recorded locally), or by being venomous/stinging (e.g. the nomadic jellyfish, already recorded in our waters) or by decimating native biota (e.g. the blue swimmer crab, already recorded in local waters and the lionfish, not recorded to date in Maltese waters).”

“Some native marine species, including the bearded fireworm (‘busuf’ in Maltese) and the ornate wrasse (the ‘lhudi’ in Maltese) are favoured by the higher temperatures and their increasing numbers further disrupt the stability of marine ecosystems.”

“Besides alien species, higher than normal water temperatures also facilitate the reproduction of many jellyfish species, such that the blooming of some of these species is more intense and lasts for longer, as documented within the Spot the Jellyfish citizen science campaign.”

“Warmer sea temperatures carry less dissolved oxygen such that sessile (non-moving) species are stressed, especially in poorly-flushed waters (e.g. those within harbours and embayments), whilst a number of motile species (including sea urchins) will relocate 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.”

“Keystone species, such as Neptune grass (Posidonia Oceanica) meadows, also bear the brunt of higher sea temperatures given that their shoots are colonised by higher densities of competing epiphytes, which shade the plant from the sunlight it needs.”

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

Share to raise awareness about this phenomenon