Monthly Ocean Science Roundup

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Welcome to the September 2022 issue of the OneOcean Science Mailout.

In this edition we’re bringing you three new papers – these are focused on the role of industrialised fishing nations on floating plastic pollution in the North Pacific subtropical gyre, global seaweed productivity and compound marine heatwaves and ocean acidity extremes.

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Industrialised fishing nations largely contribute to floating plastic pollution in the North Pacific subtropical gyre.

Authors: Lebreton, L., Royer, S.-J., Peytavin, A., Strietman, W.J., Smeding-Zuurendonk, I. & Egger, M.

– WHAT THIS PAPER EXAMINES –

The subtropical oceanic gyre in the North Pacific Ocean is currently covered with tens of thousands of tonnes of floating plastic debris, dispersed over millions of square kilometres. This study analysed over 500 kg of hard debris retrieved from this accumulation zone, known as the North Pacific Garbage Patch, and determined the possible sources of origin.

– TOP HIGHLIGHTS –

A large fraction of the North Pacific Garbage Patch is composed of fishing nets and ropes while the rest is mostly composed of hard plastic objects and fragments, sometimes carrying evidence on their origin. This debris is sometimes mistaken for food by turtles and other animals and can trap wildlife.

In 2019 The Ocean Cleanup retrieved over 6,000 hard plastic debris items measuring greater than 5 cm, totalling 547 kg. The debris was later sorted, counted, weighed, and analysed for evidence of origin and age.
The most common plastic objects were unidentifiable fragments, followed by fishing and aquaculture gear such as fish boxes, oyster spacers and eel traps. Other items found included plastic floats and buoys, plastic items associated with food and drinks and containers, drums, jerry cans and baskets.
Of the 6,093 debris items found, 232 objects could be traced back to their country of origin, either by the language on the item, a local brand name or a logo. The top five identified origins were Japan (34%), China (32%), Korea (10%), USA (7%) and Taiwan (6%). The oldest identified item was a buoy dating from 1966.

Recent assessments for plastic inputs into the ocean point to coastal developing economies and rivers as major contributors to oceanic plastic pollution. The five countries that were identified by this study as the greatest contributors to the North Pacific Garbage Patch are not recognised as major contributors to land-based emissions of plastics into the ocean, indicating that most of the garbage-patch plastic has been dumped into the ocean directly by passing ships.
These five countries are major fishing nations in the North Pacific Garbage Patch and given that much of the debris found is associated with fishing, these findings show that fishing activities are mostly responsible for the accumulation of floating plastics in the North Pacific subtropical gyre.
A greater transparency from the fishing industry and strengthened cooperation between countries to regulate and monitor the generation of abandoned, lost or otherwise discarded fishing gear would help reduce emissions from the ‘other tap’ of ocean plastics.

– CONCLUSION –

Analysis of debris collected from the North Pacific Garbage Patch reveals most floating plastic can be traced back to five industrialised fishing nations (Japan, China, Korea, USA & Taiwan), highlighting the important role the fishing industry plays in the solution to this global issue.

The paper is open access and published in Nature Climate Change, available from Nature:

READ THE PAPER

Global seaweed productivity.

Authors: Pessarrodona, A., Assis, J., Filbee-Dexter, K., Burrows, M.T., Gattuso, J.-P., Duarte, C.M., Krause-Jensen, D., Moore, P.J., Smale, D.A. & Wernberg, T.

– WHAT THIS PAPER EXAMINES –

Net primary productivity is a major driver of global carbon cycling and roughly half of the planet’s net primary productivity occurs in the ocean. Satellites can be used to measure net primary productivity of phytoplankton in the open ocean and of forests on land, but satellites cannot take measurements at the depths to which coastal marine forests grow. Using data from hundreds of studies worldwide, this study provides global predictions of the productivity of seaweed habitats.

– TOP HIGHLIGHTS –

Seaweeds form the largest and most productive coastal vegetated biome, drawing an annual global CO₂ flux comparable to that of the Amazon rainforest. Seaweed productivity represents a key source of carbon to nearshore food webs but also to the open ocean, where it can support food webs and carbon burial.

Despite the immense carbon fixation capacity of seaweeds and their contribution to carbon cycling having been known for decades, global assessments of the distribution and determinants of their net primary productivity remain elusive.
This study collates the most comprehensive dataset of wild seaweed net primary productivity measurements to date and models area-based seaweed productivity rates across the coastal ocean. Data from over 400 sites in 72 geographic ecoregions, spanning seaweed habitats from pole to pole and from the high-tide mark to depths of >50 m are used.
Unlike on land where forest productivity peaks in tropical rainforests, productivity in the sea peaked around temperate regions, which are usually bathed in cool, nutrient-rich water. These productive regions are dominated by forests of large brown seaweeds. Productivity decreased toward the tropics and polar regions.
Seaweed forests exhibit exceptionally high per-area production rates, being up to 10 times higher than coastal phytoplankton in temperate and polar seas.
The observations from this research suggest that the contribution of seaweeds to coastal productivity may be greater in temperate and polar environments. This has important consequences for coastal food webs, suggesting that seaweeds are key sources of carbon and energy in these environments.
The findings also suggest that the export of that productivity to the open ocean or deep sea may be greater toward high latitudes.
Environmental conditions such as light, temperature, nutrients and wave exposure influence the rate of net primary productivity in seaweeds, and climate change may have important effects on the global carbon assimilated by seaweed habitats.
The productivity of ocean forests also means they assimilate great quantities of CO₂ from the atmosphere and could play an overlooked role in climate change mitigation if some of that carbon ends up being sequestered.
Harnessing the productivity of seaweeds also offers promising opportunities to help meet the world’s future food security and contribute toward greater sustainability.

– CONCLUSION –

Temperate seaweed forests are among the most productive ecosystems on Earth. This analysis of the global productivity of seaweed habitats reveals these ocean forests are important primary producers at a global scale and highlights the need to integrate these coastal ecosystems in the oceanic carbon cycle.

The paper is open access and published in Science Advances, available from Science:

READ THE PAPER

Compound marine heatwaves and ocean acidity extremes.

Authors: Burger, F.A., Terhaar, J. & Frölicher, T.L.

– WHAT THIS PAPER EXAMINES –

Climate change has led to an increase in frequency and intensity of ocean extreme events, such as marine heatwaves and ocean acidity extremes, and this trend is projected to continue through the 21st century. When two extremes occur together, known as compound events, the harmful impacts on marine ecosystems may become more severe. This research quantified, for the first time, the frequency and drivers of compound events in which marine heatwaves happen together with extreme acidity.

– TOP HIGHLIGHTS –

While the processes that lead to extreme events on land, such as droughts, heatwaves or floods and how they interact with each other have been well studied, marine compound events, such as marine heatwaves and ocean acidity extreme events, are only just starting to receive more attention.

Using monthly open-ocean observations from 1982 to 2019 the study characterised patterns, identified drivers and assessed future changes of compound marine heatwave and ocean acidity extreme events.
Compound marine heatwave and ocean acidity extreme events are most likely to occur in the subtropical ocean and less likely in mid-to-high latitudes and the tropical Pacific Ocean.
The likelihood of these extreme events occurring is caused by changes in ocean chemistry and circulation. In regions such as the subtropical ocean the likelihood is higher due to an increase in acidity at higher temperatures. At lower latitudes, such as the Southern Ocean, and in the Pacific Ocean, the temperature increase during a marine heatwave causes other effects, such as less mixing of relatively more acidic subsurface water with surface water, thus reducing acidity and leading to a decrease in the frequency of compound events.
With continuing CO₂ emissions and climate change, extreme events such as marine heatwaves and ocean acidity extremes will continue to increase in frequency, and so will compound marine heatwave and ocean acidity extreme events.
Model simulations project that the long-term warming and acidification trends associated with climate change will have the largest effect on the number of marine heatwave and ocean acidity extreme days per year, increasing it from 12 to 265 days per year at 2 °C global warming relative to a fixed pre-industrial baseline.
The capacity for marine organisms to adapt to these extreme events will vary. Even if organisms can acclimate and adapt to long-term ocean warming and acidification or can relocate to favourable habitats, they may still be impacted by a 60% increase in compound marine heatwave and ocean acidity extreme days under 2 °C global warming that emerges mainly from increasing variability in oceanic pH.
The biological impacts of these changes in marine heatwave and ocean acidity extreme events across different species and ecosystems are currently largely unknown21. The potential threat from rising numbers of marine heatwave and ocean acidity extreme days highlights the urgent need to better understand the organism and ecosystem responses to such ocean compound events.

– CONCLUSION –

For the first time scientists have studied the occurrence of ocean extreme events occurring concurrently. “Compound” marine heatwave-ocean acidity extreme events are predicted to increase 22-fold at 2°C of global warming and could have severe impacts on marine ecosystems.

The paper is open access and published in Nature Communications, available from Nature:

READ THE PAPER

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