Marine species taking refuge in unknown waters due to higher temperatures, Bristol and Exeter Uni study finds

By Julia Riopelle, SciTech Editor

The joint study between the University of Bristol and Exeter, compiled over 540 marine species abundance surveys and how their distribution has shifted as a result of warming waters.

Over the last century, the Earth’s oceans have increased by an average of 1°C. 93% of anthropogenic – human induced greenhouse emissions are absorbed by oceans, where 30% of this is Carbon Dioxide. The result of our warming climate has drastic effects on marine life, even more rapidly than terrestrial environments, due to the simple fact that water warms quicker than air.

Climate change has already drawn global attention to the melting glaciers, rising sea levels and ocean acidification. However, the temperature increase has also forced its inhabitants to undergo behavioral changes, in order to survive these unprecedented conditions.

Martin Genner, Professor of Evolutionary Ecology at the University of Bristol’s, has observed a latitudinal shift poleward in marine species distribution ranges. This is likely due to the fact that the poleward range becomes more tolerable as the ocean warms, and the equatorward boundary is increasingly too hot.

A joint study between the University of Bristol and Exeter, compiled over 540 marine species abundance surveys, in order to analyze how their distribution has shifted across their latitudinal range boundaries in response to this temperature change. Latitudinal range boundaries of marine ectotherms are dependent on their optimal temperature, the temperature a species is observed to perform at its highest fitness.

From this one can map out a ‘thermal performance curve’, which determines at which temperature the species’ performance shifts from ‘good to poor’ and at which temperature their survival deteriorates.

Both the tropical and polar marine species are at particular risk, as their narrow habitat ranges makes them extremely sensitive to small changes

General trends have shown marine ectotherm’s optimal temperature to be closer to their warmer equatorward boundary. However, as temperature has an exponential effect on the physiological mechanisms in the body, their performance rapidly degrades past their threshold survival temperature. In too warm conditions, the organisms aerobic, circulatory and ventilatory systems begin to deteriorate.

Though some species can adapt to the change, many cannot keep up with the warming temperatures. It seems that both the tropical and polar marine species are at particular risk, as their narrow habitat ranges makes them extremely sensitive to small changes. Studies have also shown that thermal sensitivity is experienced earlier on by larger species and older individuals, which in turn decreases their performance and increases their mortality.

Data from southern Norway showed that of 1600 species recorded in the area, 565 of them have exhibited a 500-800km shift polewards from 1997-2010. Over the last 40 years plankton populations in the Greater North Sea have also moved polewards over 10° latitude, at a rate of 250km every decade.

Plankton are primary producers of all ocean ecosystems. Their photosynthetic ability harvests energy and provides the vital dissolved organic matter for higher biota other organisms in the ecosystem to survive. Phytoplankton are the fastest moving pelagic – open ocean species in their adaptivity and distribution, thus shifts in this foundation directly affects all other communities depending on them.

It is hypothesized that many species will have to resort to increased depths to take refuge from the heat

The European Environment Agency reported that since the 1980s, North-East Atlantic fish species that are listed as ‘warm-adapted’ have increased 250% (eg. red mullet, hake, grey gurnard), whereas those considered ‘cold-adapted species’ (eg. cod, haddock, whiting) have halved in abundance.

We observe this contrasting response, as the effects of climate change are not homogenous. Warmer waters could have some positives outcomes, as they may open up new habitat niches. For instance, after decades of absence, Western Australia saw reappearance of the subtropical wrasse, Choerodon rubescens, at their polewards boundary. Warmer Icelandic waters have also led to an increase in the reproductive success of monkfish, Lophius piscatorius, over the last decade.

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Though there are also alarming consequences to a warming ocean, as some species have already completely vanished from their equatorward boundary. Temperature increase has the strongest effects in coral reefs, tropical waters and polar regions. Obvious effects being melting ice glaciers decreasing the habitat of polar species and decreasing Antarctic krill, which form the base of most Antarctic food webs.

It is hypothesized that many species will have to resort to increased depths to take refuge from the heat. Although, currently many species rely on the Eutrophic zone – uppermost ocean level for photosynthetic processes and rich nutrient availability. Phytoplankton and zooplankton may be able to adapt to new depths, due to their short generation times, though the larger marine species which rely on them have much slower evolutionary rates.

The species we come to know and love today may not be able to keep up with the anthropogenic pressures we subject them to.

Featured: Unsplash / Sebastian Pena Lambarri

What do you make of the joint Bristol and Exeter University study? Does it surprise you or not at all?