The Latest Research from the Ocean

Although the ocean occupies more area than the land, land-based species are impacting the ocean in untold ways.  Dr Anjani Ganase looks at some findings from current research in ocean science.

Deep ocean impacts from hurricanes

Scientists from New Zealand discovered that some cyclones in the Pacific Ocean may leave behind a biological marker in the ocean that is stored in the sediments of the ocean. Scientists observed a large phytoplankton (micro-organisms living suspended in the water) bloom following Cyclone Oma off the coast of Vanuatu.

Cyclones form as the ocean surface heats up. The cyclone cools any area in its path by churning up the water and at times even drawing cold and nutrient rich waters up from the deep. The phytoplankton take the opportunity to feed on the nutrients and multiply. In nearshore environments, cyclones ejected large amounts of water over land masses resulting in significant run off of land-based nutrient that results in algal blooms in the surrounding water. This had never been observed offshore in the middle of the ocean. Using satellite imagery capable of detecting chlorophyll (a pigment that absorbs sunlight) scientists were able to track a sizeable phytoplankton bloom two weeks after Cyclone Oma which was a Category One storm that moved at a relatively slow pace.

So what happens to the plankton biomass after blooming? While some of the plankton will serve to feed marine life, a large portion of the plankton would sink to the bottom of the ocean. Coring the sediment at the site revealed that such a bloom event at the scale of the tropical cyclone Oma is incredibly rare. However, other storms have been shown to produce other bloom events in the South Pacific (14 observations for the area). The intensity of the bloom depends on the size of the storm and the speed of movement. With respect to climate change, as oceans heat up, we expect more severe cyclone events to churn up and drive more phytoplankton blooms in the ocean. Researchers are uncertain of the positives and negatives of this.

We are all islands bathed and nourished by the vast Ocean. This is one of the Maldives in the Indian Ocean. Credit: Fabrice Dudenhofer / Ocean Image Bank

 

Loss of oxygen on coral reefs

Researchers collaborating across several universities found that hypoxia (limited availability of oxygen) on coral reefs was more prevalent than previously thought. The scientists monitored 32 reefs across 12 locations including Japan, Panama, Hawaii and Taiwan. They put out oxygen sensors and other probes to track the variability in oxygen levels on the reefs during the day and night. Researchers found that the oxygen dropped to the lowest levels in the early mornings (after night-time respiration and absorption of oxygen by marine organisms on the reef). The highest levels of oxygen were found in the afternoon with a build-up of oxygen in the water column because of photosynthesis by coral and algae during the day. Shockingly, they found that up to 84 % of the reefs observed may have suffered weak to moderate exposure to hypoxia at some point. Considering the warming ocean conditions, where warmer waters absorb less oxygen, there is concern that the potential for hypoxia is likely to increase with climate change and will increase detrimental impacts on the health of marine life.

 

Sea Otters sick with toxoplasmosis

Between 2020 and 2022 scientists in California found four stranded Southern Sea Otters infected with a rare parasite (a variant known as Toxoplasma gondii). All the sea otters were located within 26 km of each other when rescued; the otters were trapped in the respective areas because of flooding conditions. The otters suffered from steatitis, which is severe inflammation of the fat in the body, and systemic toxoplasmosis that eventually resulted in death. While other versions of the parasite are found in sea otters, this was the first time this variant has been found in marine mammals. Toxoplasma is commonly spread through the faeces of cats, and while most infections of humans is mild it can cause miscarriages and birth defects. Researchers speculate that the otters may have been infected while they were trapped due to the heavy rains and the run off from land that carried the parasite. While the parasite only infects warm blooded animals, there is concern that the parasite may potentially get into marine food supplies and be transmitted to humans this way. More investigation on how the parasite is being spread and consequences of being in the marine environment continue to be investigated.

Rats and fishy behaviour

Using the islands of the Chagos Archipelago as their study site, scientists found that the presence of invasive rats on the islands changed the behaviour of the jewelled damselfish when compared to the island without rats. During the 1700, rat stowaways on ships were released on some of islands in the archipelago. As most of the islands serve as key nesting grounds for marine birds, the release of the rats devastated the populations of the birds. The seabirds would fly over the ocean to forage for food to feed their young in the nests.

The nutrients and waste from the daily catches of the bird colonies would run off into the nearshore marine ecosystems largely composed of coral reefs. The nutrients that entered the coral reef were particularly nutritious for ocean algae gardeners, such as the jewelled damselfish. Damselfish are called gardeners because they are known to plant and tend algae gardens on the reef. This means that they are known to be aggressively territorial about their gardens. Here in the Caribbean they are known to chase and butt divers that are unaware.

Scientists found that the damselfish on reefs surrounding rat free islands tended smaller turf gardens and were much more aggressive. On the reefs of the rat invaded island, the turf algae territories were larger and the fish guarding less aggressively. The turf gardens were bigger on the rat invaded island because the turf algae were overall less nutritious, and the damselfish needed to forage larger areas. With less nutrients, the fish expended less energy on being aggressively territorial.

All these studies highlight the co-evolution of terrestrial and marine ecosystems and the need for maintaining balance between the ocean and land; not least of which is the need to manage human impacts.

 

References

Rachel L. Gunn, Cassandra E. Benkwitt, Nicholas A. J. Graham, Ian R. Hartley, Adam C. Algar, Sally A. Keith. Terrestrial invasive species alter marine vertebrate behaviour. Nature Ecology & Evolution, 2023; DOI: 10.1038/s41559-022-01931-8

Ariel K. Pezner, Travis A. Courtney, Hannah C. Barkley, Wen-Chen Chou, Hui-Chuan Chu, Samantha M. Clements, Tyler Cyronak, Michael D. DeGrandpre, Samuel A. H. Kekuewa, David I. Kline, Yi-Bei Liang, Todd R. Martz, Satoshi Mitarai, Heather N. Page, Max S. Rintoul, Jennifer E. Smith, Keryea Soong, Yuichiro Takeshita, Martin Tresguerres, Yi Wei, Kimberly K. Yates, Andreas J. Andersson. Increasing hypoxia on global coral reefs under ocean warming. Nature Climate Change, 2023; DOI: 10.1038/s41558-023-01619-2

Melissa Ann Miller, Cara A. Newberry, Devinn M. Sinnott, Francesca Irene Batac, Katherine Greenwald, Angelina Reed, Colleen Young, Michael D. Harris, Andrea E. Packham, Karen Shapiro. Newly detected, virulent Toxoplasma gondii COUG strain causing fatal steatitis and toxoplasmosis in southern sea otters (Enhydra lutris nereis). Frontiers in Marine Science, 2023; 10 DOI: 10.3389/fmars.2023.1116899

Peter Russell, Christopher Horvat. Extreme South Pacific Phytoplankton Blooms Induced by Tropical Cyclones. Geophysical Research Letters, 2023; 50 (5) DOI: 10.1029/2022GL100821