32 days on the Indian Ocean exploring climate change

May 13, 2019
Biological oceanographer Raleigh Hood joins a 32-day ocean expedition to reveal the effects of climate change on the physics, chemistry, and biology of the waters of the southeast Indian Ocean.

Sixty years ago, marine scientists aboard ships from 14 countries combined efforts to explore the largest unknown area on earth, the Indian Ocean. The expedition generated a wealth of information and formed the basis of our modern understanding of the Indian Ocean basin.

Beginning next week, an Australian voyage will retrace part of the that historic International Indian Ocean Expedition to reveal the effects of climate change on the physics, chemistry, and biology of the waters of the southeast Indian Ocean and produce a unique snapshot into how much the oceans’ marine life has changed over time.

University of Maryland Center for Environmental Science professor and biological oceanographer Raleigh Hood will be among the 40 marine scientists and technicians from 18 institutions who will spend 32 days at sea on this second International Indian Ocean Expedition (IIOE2), led by Professor Lynnath Beckley from Murdoch University in Australia, beginning May 14.

The voyage will retrace part of the historic International Indian Ocean Expedition 60 years ago to reveal the effects of climate change on the southeast Indian Ocean.

They will leave from Freemantle Australia near Perth and work their way north all the way to Jakarta, sampling along the 110°E longitudinal meridian in the deep ocean—approximately 500-600 km offshore of the continent—passing through big waves and wind in the southern ocean to temperate and productive tropical waters and even stunning blue waters of the southern Indian Ocean subtropical gyre.  Researchers will study the entire marine food web from phytoplankton to whales, including currents and chemistry, and will make direct comparisons with data from the expedition 60 years ago.

“We are all aware of the potential negative consequences of global warming and warming of the oceans. The Indian Ocean seems to be accumulating heat faster than the other ocean basins,” said Hood. “We look at it as a canary in the coalmine in terms of how ocean ecology and chemistry might be impacted by warming and acidification.”

The Indian Ocean drives the region's climate, including extreme events such as cyclones, droughts, severe rains and waves. Research and observations supported through the Second International Indian Ocean Expedition will result in an improved understanding of the ocean's physical and biological oceanography, and related air-ocean climate interactions.

“There hasn’t been very much research along the lines of what we’re doing now to try to understand how the Indian Ocean is changing, so we are at the vanguard of efforts to do that,” said Hood. 


Involved in Indian Ocean activities for decades, biological oceanographer Raleigh Hood coauthored one of the first biogeochemical models of the Indian Ocean.

Hood has been involved in Indian Ocean activities for decades, doing a combination of modeling and observational work in the Bay of Bengal and Arabian Sea. He coauthored one of the first biogeochemical models of the Indian Ocean more than 20 years ago and founded the Sustained Indian Ocean Biogeochemistry Ecosystem and Research Program to encourage research in the region. He was one of the principal people who motivated this expedition.

“We construct models and run them in the Indian Ocean to simulate physical and biological processes and we can compare those models with observations—both in situ and satellite. If you can get your model to reproduce what you see in the ocean, you can use the model to sort out what’s going on,” he said. “Once you capture that variability, you can take the model apart and say why it is this happening.”

On the cruise, Hood will be working with Scripps Institute of Oceanography biological oceanographer Mike Landry, an expert on marine food webs, to quantify zooplankton grazing control on phytoplankton. They will take samples of water from various depths in the upper ocean and manipulate them on the on deck of the research ship using incubators to control temperature and the light, removing a few grazers at time to sort out how phytoplankton growth is impacting by zooplankton. They will also be taking isotopic measurements aimed at understanding how nutrients are propagating through the food web.

“The Indian Ocean is super complicated and not very well understood. It’s really complex in terms of physical and biological dynamics,” said Hood. “It’s is the final frontier of the final frontier.”

The voyage leaves on May 14 and returns on June 14.

Modeling the Indian Ocean Pelagic Ecosystem

Aboard RV Investigator, Raleigh Hood aims to use many of the variables collected along the 110°East meridian to develop biogeochemical models of the south-east Indian Ocean. (Micheline Jenner AM)

This voyage (IN2019_V03) on the RV Investigator will traverse and sample one of the most poorly understood regions of the world’s oceans along110°East in the south-east Indian Ocean. An important goal of this IIOE-2 voyage is to characterize the physical, chemical and biological properties of the waters and determine how they change from the temperate waters at the southern most stations to the northern most stations in the tropics. In particular, how do the species composition and biomass of microscopic planktonic organisms vary along this 110°East meridian transect and how do ocean currents, nutrient concentrations and the availability of light in the ocean drive this variability?

Characterizing these patterns is important for developing computer models that simulate the circulation and biogeochemistry of the Indian Ocean. They also serve to effectively test if existing models are correct. Once we have established that our models can simulate the present day Indian Ocean, we can use them to ascertain how much it might change in the future, as a result of the impacts of humans.

Existing oceanographic model for the Indian Ocean that shows the sea surface temperature of the ocean during February. (NOAA).

We know that increasing atmospheric carbon dioxide concentrations and global warming are having significant effects in the Indian Ocean, including higher water temperatures, more dissolved carbon dioxide and lowering of pH (ocean acidification). Our validated models will allow us to predict how these properties might change in fifty years, or even a century into the future, and how these alterations might impact the Indian Ocean food web.

Dilution Experiments

Raleigh Hood and Michael Landry (Scripps Institution of Oceanography) filtering water from the CTD in preparation for experiments conducted on the aft deck of RV Investigator. (Micheline Jenner AM)
By Michael Landry

Phytoplankton can go through one or two generations (cell divisions) per day and are often eaten almost as fast as they divide, by protozoan grazers. Because the size ranges of phytoplankton and microzooplankton broadly overlap, they cannot be mechanically separated from one another to determine these two different rates. We do, however, have a technique for this, which involves dilution of the grazing impact. This entails changing the encounter frequency of predators and prey using filtered water with the same chemistry, from the same depth. We create two conditions­–one with natural concentrations of predators and prey and the other in which the grazing has been slowed by about 2/3rds. From the differences in the measured rates of increase of phytoplankton of the two treatments, we can solve the equations for the two unknown rates. 

We run our experiments in a seawater-cooled, shipboard incubator system that has 6 light levels simulating the conditions of underwater light at the six depths where we collected our samples from the CTD. The change in phytoplankton concentration in each bottle is measured by chlorophyll and by flow cytometry analyses of dominant populations. After a lot of filtering, analyses and calculations, we will have a pretty good idea of how much of phytoplankton productivity is consumed by microzooplankton each day, how that varies with depth in the euphotic zone, and if there are significant differences among the stations or regions that we are sampling along the 110°East line.

Simulating different light levels experienced at six different depths in the ocean, these seawater-cooled incubation tanks allow the calculation of phytoplankton growth and microzooplankton grazing rates. (Micheline Jenner AM)
Each incubation box contains two bottles, one with the natural conditions of predators and prey and the other where the grazing conditions have been reduced or diluted. (Micheline Jenner AM)
Raleigh Hood removes the dilution experiment bottles that have incubated for 24 hours. (Micheline Jenner AM)