The blue marble

The blue marble

Increased UV radiation:a hidden driver of marine declines?

Australians, with one of the highest prevalence of health problems in the world caused by exposure to UV radiation, are very aware of the dangers of elevated UV radiation. The prevalence of skin cancer in Australian is nearly four times the rates of citizens in Canada, the US and the UK.

However, the role of elevated UVB radiation on marine biota and its possible role as a driver of widespread declines of marine biota has not been assessed in a systematic manner. The misconception that the Montreal Protocol succeeded in reverting UVB levels to its pre-disturbance values. Following the compelling demonstration of the role of CFC gases in the erosion of the stratospheric ozone layer, and hence, elevated UVB radiation, the Montreal Protocol ruled the banning of production and emission of CFCs. The Montreal Protocol has been invoked recurrently as a success-story in environmental regulation. Indeed, CFCs emissions and production declined sharply. However, UVB levels have not recovered as yet, and are not expected to recover before 2050. Hence, incident UVB levels have remained elevated for over four decades, a fact that stats on prevalence of UVB-induced health problems in humans provide evidence of.

Evidence that marine life is vulnerable to UVB radiation abounds, and indeed the Hunter action spectrum, used to evaluate damaging UVB doses, is based on the levels causing mortality to anchovy eggs (Hunter et al. 1979). However,the believe that UVB radiation would not penetrate to significant depths in the ocean lead to the assumption that elevated UVB index was not a reason for concern for marine biota. Yet, development of submarine UVB profiling instruments showed that UVB doses sufficient to cause mortality of marine plankton penetrated below 30 m in clear ocean waters (LLabrés et al. 2010).

On the light of abundant evidence on the vulnerability of marine biota to UVB radiation, a team of Australian, Spanish and Chilean researchers led by UWA Professor Susana Agustí conducted a meta-analysis of available experiments assessing the response of marine biota to elevated UVB radiation or removal of UVB radiation. The results of this research has just been reported on a paper (LLabrés et al. 2012) published on line in Global Ecology and Biogeography.

The analysis revealed a general deterioration of organismal performance with elevated doses of UVB radiation and a general improvement when incident UVB was removed. The marine life most affected by UVB are protists (such as algae), corals, crustaceans and fish larvae and eggs, thereby affecting marine ecosystems from the bottom to the top of the food web. Mortality was the trait most sensitive to increased UVB radiation.

The analysis also provided evidence that marine organisms in the Southern Hemisphere are more resistant to elevated UVB radiation than those in the Northern Hemisphere, and that resistance of organisms in the Southern Hemisphere has increased slowly over time. We interpreted these observations as evidence that high mortality of sensitive marine organisms in the Southern Hemisphere, where UVB levels have increased the most, has already selected for the more resistant organisms.

The experiments included in this research involve organisms and species that have survived after the erosion of the ozone layer caused by CFCs. Therefore, the results suggest that an increase in UVB radiation could have a heavy impact on marine biota. A clear evidence of this impact is the finding of a reduction of mortality rates by up to 81 per cent when reducing exposure to UVB present in larvae of commercial fish such as cod, anchovies and other organisms,.

The effects of ultraviolet radiation detailed in this study mainly affect organisms growing near the ocean surface, such as eggs and larvae of invertebrates and fish, which are exposed to very high UVB levels.

Our results strongly suggest that increased UVB radiation over the past four decades may be a hidden driver of the widespread decline of marine life, from corals to fish, often attributed to other pressures, such as climate warming, overfishing and other impacts.

Most global impacts on marine biota have been documented since the 1970’s, including those attributed to overfishing, ocean warming, ocean acidification and hypoxia, concurrent with elevated UVB levels at the global scale. The evidence is to a large extent correlational. However, because a global increase in UVB radiation, greatest at higher latitudes and in the Southern Hemisphere, has occurred in parallel to all these other impacts there effects maybe linked.

It is very likely that the general decline of marine life in the past three decades reflect the compound effects of multiple stresses, including that to UVB radiation, and not warming alone. The additional mortality, particularly of early life stages of marine life, due to elevated UVB radiation may have sufficed for already stressed populations to shift to negative net population growth, further accelerating their decline.

Because current models do not anticipate the stratospheric UVB levels to recover before the middle of this century, the impacts of elevated UVB radiation will continue to operate. Understanding the role of elevated UVB radiation as an additional driver of the decline of marine life is of fundamental importance to predict and catalyse, through effective managerial actions, their recovery.

Carlos M. Duarte and Susana Agustí The UWA Oceans Institute and School of Plant Biology, The University of Western Australia

REFERENCES

Hunter, J.H., J.H. Taylor, and H.G. Moser, 1979. The effect of ultraviolet irradiation on eggs and larvae of the northern anchovy, Engraulis mordax, and the pacific mackerel, Scomber japonicus, during the embryonic stage", Photochemistry and Photobiology, 29, 325-338.

Llabrés, M., Agustí, S., Alonso-Laita, P. & Herndl, G. 2010. Synechococcus and Prochlorococcus cell death induced by UV radiation and the penetration of lethal UVR in the Mediterranean Sea. Marine Ecology–Progress Series, 399, 27–37.

Llabrés, M., S. Agustí, M. Fernández, A. Canepa, F. Maurin, F. Vidal, and C.M. Duarte. 2012. Impact of Elevated UVB Radiation on Marine Biota: a Meta-Analysis. Global Ecology and Biogeography (published on line).