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Indian Ocean Nutrients - One Ten East Log

17 May 2019

RV Investigator

One Ten East Logs from the IIOE-2 voyage aboard RV Investigator will be posted on the WAMSI website during the month long voyage.

The wind and swell was up a little yesterday. This seemed to keep a few participants in their bunks a bit longer in the morning but by evening, most were feeling fine again. Fortunately, this was a transit day and the weather has improved as we have begun the first station on 110 East meridian. 

The yellow-nosed and black-browed Albatross were soaring beautifully in the fresh 20-30 kts and all is well on the fine RV Investigator.

 

Log from One Ten East

The RV Investigator is currently undertaking oceanographic research along the 110°E meridian off Western Australia as part of the second International Indian Ocean Expedition. The voyage is led by Professor Lynnath Beckley of Murdoch University and the research is supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility.

 

Date:  May 17, 2019

Time:  1200 AWST

Latitude: 39.5°S

Longitude: 110°E

Wind direction: NNW 

Wind speed: 10 knots

Swell direction: SSW

Depth: 4600 m

Air temperature: 9.6°C

Sea temperature: 12.6°C

Notes: Five species of albatrosses, cape petrels and a minke whale sighted today. The galley is providing wonderful food for a hungry and appreciative crew!

Indian Ocean Nutrients

By Dr Peter Thompson

The CSIRO scientists involved in the original International Indian Ocean Expedition during the 1960s described it as a desert. Huge regions of the Indian Ocean had almost no plant life. The thousands of species and billions of microscopic plants found in most oceans were largely absent. These plants, just like the ones in your garden or paddock, require nutrients to grow. In the garden, we add fertiliser with these nutrients, particularly nitrogen and phosphorus, to ensure a successful crop. These nutrients are in very short supply in the surface waters of the Indian Ocean.

To find out how much nutrient is present it is necessary to get samples of water from deep in the ocean. Special tubular containers, called Niskin bottles, located on the CTD are sent down with the ends open (can you guess why?) on a wire to depths of 5000 m where they are electronically triggered to close and collect the samples. Several hours later when the bottles are winched back to the surface the chemists can start the process of measuring the nutrients. To do this we use sophisticated on board instruments and internationally agreed high precision methods. It is a bit like a blood test to check the health of the ocean.

Dr Peter Thompson (CSIRO) and Dr Karlie McDonald (CSIRO) take water from two of the 36 Niskin bottles in the CTD rosette, to analyse nutrient levels of the south-east Indian Ocean. Photo: Micheline Jenner.

 

In the nearly 60 years since the first International Indian Ocean Expedition we have learned much about the cycling of nutrients in the ocean. A thousand meters below the surface where the water is only 5°C, nitrate is plentiful (Figure 1). However, enough light for plants to grow only penetrates to about 100 m where nitrate is particularly scarce. Processes that we did not understand in the 1960s, we now know are changing the supply of nutrients to the surface and some parts of the Indian Ocean are becoming more productive (more nutrients = more plants = more fish) while others are less productive. On this second voyage (IIOE-2) we are exploring how nutrient supply has changed over the last six decades in the Indian Ocean offshore of Western Australia.

Figure 1. Plots of temperature, light and nitrate in the upper water column. Temperature declines slowly with depth but light disappears dramatically at about 100 m depth in the open Indian Ocean. The amounts of nutrients, such as nitrate, are very low until around 200 m depth. Data courtesy of CSIRO.

 

In particular, we are looking for evidence of changes in nutrients within the deep ocean as a result of changing global ocean circulation patterns. This mix of deep and shallow circulation brings deep nutrients from the North Atlantic and some nutrients from the Pacific into the Indian Ocean (Figure 2).

 

Figure 2. The global ocean and its thermo-haline circulation bring nutrients from deep regions in the North Atlantic and the surface of the Pacific into the Indian Ocean. Image courtesy of NASA/JPL.

These processes are changing faster than ever due to global warming. In addition, we will investigate nutrient recycling near the surface of the ocean. There, an expanding area of low oxygen is impacting on the microbial processes that recycle organic matter. Understanding the impacts of these changes on the ecology of the Indian Ocean is one of the key goals of the 110° East voyage.

Dr Peter Hughes (CSIRO) in the hydro-chemistry lab aboard RV Investigator uses sophisticated and precise equipment to measure chemical composition of the water sampled by the CTD. Photo: Micheline Jenner.

 

Be sure to follow our daily Log from One Ten East at https://iioe-2.incois.gov.in or www.wamsi.org.au .