The aim was to investigate the potential for deep-water carbon sequestration as a strategy

to reduce atmospheric CO2 levels and potentially offset emissions from gas operations off the NWShelf.  The technique involves addition of macronutrients (principally nitrate) to the surface of oligotrophic (nutrient-poor) waters in the tropical ocean.

The bad news at this stage is that we have still not bent the curve on GHG emissions and global warming predictions. This is indeed bad for the Ocean, as the bending of that curve is the answer to most of the Ocean’s problems; and here I refer to dying coral, acidification, deoxygenation, deteriorating ecosystems, proliferating dead zones, changing Ocean currents, Ocean warming, and rising sea levels. To that list should be added such other anthropogenic effects as declining fish-stocks and marine pollution, led of course by the plastic plague.

The study site is on the edge of the Australian continental shelf, within the Australian EEZ. It is well removed from ecologically sensitive areas, in highly stratified waters which are suitable for long-term sequestration.

This addition of nutrients stimulates productivity in the planktonic food web, and some of the resulting organic material is deposited in the deep ocean, where its associated carbon will be isolated from the atmosphere for hundreds of years. Modelling simulations included testing the biological response of the system to nitrate addition in different seasons, at different intensities, and with or without the simultaneous addition of phosphate.

The model explored what the effects of addition of nitrogen are on bacteria, phytoplankton and zooplankton populations and total biomass. These results were assessed to determine the impact of the process on carbon export to deep ocean waters.

We also investigated potentially adverse impacts on the marine environment  such as toxic algal blooms, eutrophication and anoxia, emission of N₂O and changes in ocean acidification.

Schematic of the NW Shelf Model

The results suggest that effective long-term sequestration is possible in this location. Addition of relatively low levels of nitrate (equivalent to a nitrate concentration of 2 mmol/m3. in the mixed layer) produced approximately four-fold increase in biological productivity.

In each scenario tested the N-addition produced a significant increase in overall biomass. In all cases, there was a short-term spike in productivity of very small (pico- and Nano-) phytoplankton,

followed by a strong increase in diatoms. This is not unexpected, as diatoms are

typically, the fastest phytoplankton to take advantage of increased availability in nutrients.

Over the month following the N-addition, mesozooplankton biomass increased strongly in response to the greater abundance of food for this group.  To evaluate the changes in the plankton, we looked at the “total plankton biomass” over 3 and 12 months following the nutrient addition experiments.

There is some seasonal variation in total productivity: in absolute terms, the greatest biomass productivity over three months occurred during spring, but the proportional gain was highest in autumn. Nitrate addition in summer or spring produced increases of approximately 340% of the baseline biomass, whereas an experiment in autumn produced over 400% of the baseline scenario.

By biomass, the largest gains were in microzooplankton and nanoflagellates. However, the largest proportional gains were in diatoms, mesozooplankton, and (to a lesser extent) microphytoplankton. The gains in these latter groups are more important in terms of carbon sequestration, as they in turn produce larger organic particles.

As we have found that most of the particulate carbon sequestration comes from zooplankton, this would mean that Chlorophyll a (via satellite monitoring) may not be an accurate means of measuring total carbon sequestration. 

No significant negative impacts were detected in the modelling exercise. Small changes in pH of an

an increase of about 0.03 in the surface ocean, and a decrease of 0.0005 in the pH of the

deeper water. This could be investigated further as a means of mitigating ocean acidification.

In conclusion, the most important biological group for sequestration appears to be large zooplankton.



Adding nitrate equivalent to a concentration of 2 mmol/m3 in the mixed layer resulted in a

290-fold increase in the biomass of large zooplankton over the year following the nutrient

addition. Doubling the concentration of added nitrate increased the zooplankton to 785

times the baseline biomass. The results suggest that effective long-term ocean carbon sequestration is possible on the North West Shelf of Australia. The next step would be to validate this modelling with at sea experimentation over a 12-month period.