Ocean Discoveries in 2023

 

Human activity and the changing climate are affecting life and life cycles in the oceans. Dr Anjani Ganase discusses some of the observations made this year.

 

Marine plastics on deep coral reefs

Scientists from the California Academy of Sciences in collaboration with scientists around the world have conducted over 1,200 surveys on coral reefs from 83 countries to quantify plastic pollution on coral reefs. This study included deeper sections of coral reefs (the mesophotic zone) that can extend to depths of 150 m in some locations and accessible only to a handful of technical divers and remotely operated vehicles. Surveyed sites varied from reefs adjacent to cities and populated areas to remote and uninhabited islands. As expected, areas closer to human populations had greater amounts of plastics. However, plastics were found even on some of the most remote sites. The coral reef with lowest density of plastic occurred around the Marshall Islands in the Pacific, while the reef around Comoros island off the coast of Mozambique in the Indian Ocean had the highest density of plastic. Surprisingly, the amount of plastic pollution increased with depth, peaking in the mesophotic zone. In this zone, the largest contributor of plastic pollution came from fishing gear (ropes, lines, and nets).

 

Plastic on a coral reef. Credit: Alex Mustard / Ocean Image Bank

The study reveals the need for responsible disposal of fishing equipment and management to implement measures to fish away from reef sites to avoid damaging the fragile structure. This is another example of the pervasiveness of human activities even on deeper habitats, very few of which have been studied or explored. The study also highlighted that deeper reefs are just as vulnerable, if not more, to surface disturbances, especially considering the limited knowledge and conservation efforts that currently exist. Unfortunately, such habitats and the impacts we exert on these continue to be “out of sight out of mind.” The limited capacity to monitor at such depths also makes the management and protection of these areas challenging.

 

Satellite image of an algae (coccolithophore) bloom in Barents Sea, Norway July 24, 2003. Image courtesy Jacques Descloitres, MODIS Rapid Response Team at NASA GSFC

Climate change is altering the colour of our oceans

Using colour satellite imagery of oceans recorded over the last two decades, scientists are able to show that the colour of the ocean’s surface is changing with climate change. Scientists are careful to differentiate between the seasonal variations observed from year to year in order to be able to gauge this long-term trend. Long-term changes related to climate are detected  in several colour wave bands within the visible light spectrum. This detection reflects the changes in the ecology of microbial communities and materials that occur on the ocean’s surface. A large portion is the plankton community that forms the base of the marine food web. Significant changes in the ocean have been observed in 56 % of the oceans. The tropical, sub-tropical and parts of the temperate oceans (between the equator and 40 degrees latitude) have shown significant changes in colour. Many areas, including around Bermuda, Indonesia and off Peru have become greener in colour. The increase in green may be attributed to the increase in the populations of phytoplankton which house chlorophyll – a green pigment similar to that in plants - that is used in the process of photosynthesis. There’s also a general increase in “backscatter” in the surface waters owing to larger amounts of particulates (other plankton, sediment flux and other matter). While it is difficult to precisely determine specific changes given the scale of the model, it is aligned to the timing of long-term shifts and changes in the plankton and microbial communities observed in other studies due to ocean warming.  Knowing the changes in colour patterns may be used to highlight areas of the ocean most vulnerable to climate change and to signal the need for protection of the biodiversity beyond national jurisdictions. 

 

Coral Reef and mangroves in Indonesia. Credit: Alex Mustard / Ocean Image Bank

The “saltiness” of the ocean is also changing

Climate change is dramatically altering the world’s water cycle. More extreme weather events are expected, and typically wet areas are likely to become wetter with more intense bouts of rainfall. Add to the mix chronic activities of deforestation and development that flush more and more freshwater out to sea. Conversely, drier areas are likely to suffer from more severe drought events, leaving estuarine and brackish habitats becoming more saline (or salty) or being completely dried out. Many adjacent marine nurseries and habitats – seagrass, mangrove and coral reefs – will be impacted by the sudden changes in salinity. Many marine creatures are restricted by the thermal and salt tolerances. Sudden changes in salinities, are likely to result in biodiversity loss and shifting habitats. On the biological level, saltier conditions can alter metabolic function, natural immunity, respiration and primary production (photosynthesis). Sudden changes in salinities can alter currents, create stratifications in the ocean (fresh water is less dense and floats over salt water) that are barriers to resources (light, nutrients etc). For example, the extensive melting of glaciers in the Arctic has rapidly reduced the salinity of the surrounding seas. Along with warmer temperature, there is a dramatic blooming of plankton. In contrast, in the north of Australia, large sections of mangrove forests have died back because of extreme drought conditions. Trends in the changed salinities along coastal areas are more difficult to predict owing to many additional influences including human activities.

 

 

Manta ray with plastic pollution. Credit: Vincent Kneefel / Ocean Image Bank

Fossil evidence of once connected oceans

During the late Miocene Epoch (6.4 – 5.8 million years ago), the Central America isthmus was in its early stages of formation, to create the barrier between the Pacific and the Atlantic Oceans. The formation of the isthmus resulted in significant alterations in the ocean circulation that caused the extinction of many marine flora and fauna in the Caribbean. However, scientists in Panama have found fossil records of whales that reveal species common to north and south Pacific that also occurred in the shallow waters along the Caribbean coast of Panama. There must have been a period of many shallow habitats and shared spaces for marine creatures before the separation became complete. Except for some coral fossil records, there are not many fossil records present in the Caribbean region that give insight into the marine flora and fauna before this barrier between the oceans.

 

References

 

Aldo Benites-Palomino, Jorge Vélez-Juarbe, Carlos De Gracia, Carlos Jaramillo. Bridging two oceans: small toothed cetaceans (Odontoceti) from the Late Miocene Chagres Formation, eastern Caribbean (Colon, Panama). Biology Letters, 2023; 19 (6) DOI: 10.1098/rsbl.2023.0124

 

Cael, B.B., Bisson, K., Boss, E. et al. Global climate-change trends detected in indicators of ocean ecology. Nature (2023). https://doi.org/10.1038/s41586-023-06321-z

 

Pinheiro, H.T., MacDonald, C., Santos, R.G. et al. Plastic pollution on the world’s coral reefs. Nature 619, 311–316 (2023). https://doi.org/10.1038/s41586-023-06113-5