Lockdown led to quieter but not cleaner oceans

Andrew Twelves

In February 2020, Italy became the first country in Europe to impose a lockdown in response to the Covid-19 pandemic.

In the following months, amidst the stress of those initial lockdowns, the sounds of birdsong freed from traffic noise were a source of comfort and optimism to many around Europe and the rest of the locked-down world. This hiatus in noise pollution was accompanied by reductions in other forms of pollution, including those which usually blight the oceans. 

However, in a new paper published in Science of the Total Environment, researchers from the Italian Institute of Marine Sciences (ISMAR) question the degree to which lockdown policies were in fact responsible for cleaner seas during the spring of 2020.

Italy’s Covid clean seas were a coincidence.

Satellites orbiting the Earth are constantly observing the world’s oceans and coastal seas.  Analysis of satellite images can detect the change in colour which occurs when microscopic algae – phytoplankton – proliferate near the ocean surface. These organisms, using chlorophyll to photosynthesise and generate energy, are consumed by zooplankton and thus form the basis of marine ecosystems.

NASA image of a phytoplankton bloom

When these phytoplankton blooms are fertilised by industrial runoff, however, they can grow so large that they crowd out other forms of life, starving them of oxygen and light. This means that the intense green patches of phytoplankton seen in satellite images can sometimes be used to locate the ecologically harmful transport of pollutants from land to sea.

The scientists at ISMAR were well placed to examine the impacts of lockdown on coastal pollution. Northern Italy is characterised by extensive industry, with the potential to release pollutants; as well as by large rivers, which can carry these pollutants onwards into the Adriatic Sea. From early in the lockdown, locals reported seeing unusually clear waters, particularly in the Venice lagoon. 

Venice Lagoon saw unusually clear waters during lockdown

It seemed that the closing of factories had shut off the supply of pollutants to the rivers and coastal waters of northern Italy. When the team at ISMAR examined a record of weekly chlorophyll estimates stretching from 2016 to 2020, they confirmed that chlorophyll concentrations during lockdown were lower than average for that time of year.

However, when the team looked at data from weather stations, a different picture started to emerge.

Phytoplankton can be supplied with nutrients from different sources and from different directions, but in general large blooms are favoured by a strong upwards supply of nutrients from deeper in the water column. The ISMAR analysis showed that the lockdown period coincided with unusually low winds across the region, leading to seas with less mixing and therefore a poorer supply of nutrients from the depths. 

Furthermore, the coastal current which connects river outflows to phytoplankton blooms appeared to be unusually weak during lockdown. In their paper, the scientists at IMS conclude that the low chlorophyll, high water quality conditions seen in the Adriatic Sea in the spring of 2020 were in fact a response to natural environmental variability, rather than a consequence of lockdown.

So did the 2020 Covid lockdowns pass the oceans by?

Not necessarily – another paper in a different journal found strong evidence that the lockdown in China, which started some months earlier, had indeed led to reduced chlorophyll concentrations in the Yellow Sea.

In this case the weakened phytoplankton blooms are attributed to a reduction in the nitrogen-rich pollutants, carried not through waterways but through the air, which are ordinarily deposited at the surface of the Yellow Sea. Furthermore, whilst both these studies have examined impacts on these smallest of marine organisms, many other researchers are interested in the ways that large marine mammals may have felt the impacts of lockdown.

Sound evidence points to benefits for mammals.

In California, scientists have developed a sophisticated system of acoustic measurements to monitor whale behaviour in the Monterey Bay National Marine Sanctuary. Baleen whale species, such as fin and blue whales, feed on swarms of krill which themselves consume phytoplankton.

Within Monterey Bay, these phytoplankton are nourished by the upwelling of nutrients from the California current. Using a network of highly sensitive underwater microphones, known as hydrophones, scientists from the Monterey Bay Aquarium Research Institute (MBARI) have shown that whales use sound signals to alert other individuals when they find rich feeding grounds, helping them adjust their migration routes from year to year to follow changes in phytoplankton and krill abundance.  

Humpback whales use sound to communicate

Just as lockdown led to quieter streets on land, it seems that there was a major reduction in harmful marine noise pollution. When scientists at MBARI examined data recorded during lockdown, they found that their acoustic instruments were picking up far less noise from marine traffic such as container vessels, temporarily eliminating the negative impact of this traffic on blue, fin, and humpback whales. 

The quieter waters would have allowed dolphins to communicate across several more kilometres than usual. In New Zealand, scientists demonstrated the extent to which the cessation of both commercial shipping and pleasure boating reduced noise pollution in the Hauraki Gulf Marine Park. For every 10% drop in marine traffic, their study finds a 2 decibel drop in man-made noise within biologically important frequencies.

The Intergovernmental Panel on Climate Change (IPCC) has confirmed that the positive impact of lockdown in slowing carbon emissions was short-lived, with levels of greenhouse gases in the atmosphere now rising once again.

As climate change accelerates, this will impact where and when phytoplankton blooms and krill swarms occur – trends which are hard to forecast. The way these changes will shape the survival chances of larger organisms such as whales is even more difficult to predict.

However, results from the lockdown suggest that we might be able to help some marine animals adjust to change by reducing the other forms of human disturbance which currently pervade the blue planet.

Andrew Twelves (he/him) recently submitted his PhD thesis at the University of Edinburgh on the topic of how phytoplankton blooms around the Antarctic coastline are responding to climate change.

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