UWA Oceans Institute

The ocean’s microscopic unsung heroes

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Diatom species (Photo: P. Thomson).


THIS issue the UWA Oceans Institute’s Dr Paul Thomson exposes the diverse nature of microscopic phytoplankton on WA’s North West Shelf (NWS) and explains how local institutions are working to bolster the body of research in this field.

Just looking at phytoplankton through a microscope you would never believe that these minuscule troopers are responsible for producing an estimated half of the world’s oxygen. These single-celled plants float freely in the ocean and combat greenhouse gases by using chlorophyll (like terrestrial plants) in photosynthesis to draw carbon dioxide for the atmosphere.

So try and spare them a thought when you take your next breath and think about how they alleviate this greenhouse gas. As the phytoplankton multiply, mostly by dividing in half (called binary fusion), they form the pastures of the ocean that are grazed upon by zooplankton which in turn support all animals in the marine food chain, including fish, rays, sharks, and whales. Yet despite supporting this wealth of marine life, little research has been done on phytoplankton of WA’s North West Shelf (NWS) where the tourism industry relies heavily on fishing and sightings of charismatic megafauna such as manta rays, whales and whale sharks. When scientists talk about understanding the phytoplankton, they want to know what species form large blooms and what oceanographic conditions initiate these blooms.

Understanding the phytoplankton and factors influencing their distribution and abundance is important in understanding how our coastal oceans and marine life may change with our changing climate. In Western Australia for example, long term records over the past five decades show a persistent warming trend in the Leeuwin Current that flows alongside most of the state. How such a warming trend will affect our marine life and our ocean-related pastimes, such as rock lobster fishing for example, is poorly understood.

The Indian Ocean Marine Research Collaboration (IOMRC) between the University of Western Australia’s Oceans Institute, CSIRO, the WA Department of Fisheries and the Australian Institute of Marine Science (AIMS) is working to change this by making research on the NWS more accessible. In January, we sampled along the NWS on board the AIMS research vessel the RV Solander with the aim of determining the community composition of phytoplankton and the physical factors affecting their distribution and abundance. We found phytoplankton of fantastic shapes and sizes ranging from tiny, almost featureless spherical cells of around 0.002mm in diameter to large elaborate cells up to 0.1mm in length with spines, armour plates and flagella (tails).

The most abundant phytoplankton were the diatoms, a group with intricate glass walls (silica frustules) that are credited as being most important in supporting the marine food chain. Dinoflagellates, cells that can have armoured plates and be both plant and animal at the same time, were also important across the sampling area. Overall, the complex oceanography of the NWS greatly influenced the distribution and abundance of phytoplankton. For instance near the shallow waters of the Montebello Islands (40 metres), we found communities of very small phytoplankton and low biomass (measured as chlorophyll in the water) but immediately north of the Montebello Islands, the communities included large diatoms and it appeared that the higher biomass here was influenced by the Ningaloo Upwelling, which brings cold, nutrient rich deep water into the surface layers of the ocean. Further north still and west of Broome, the ocean was well stratified; meaning a warm, less salty upper layer of water overlaid a cooler and more saline bottom layer. Here we found impressively large blooms of diatoms occurring at the junction of these layers, which occurred at depths of at least 45m or deeper.

Surface waters of the NWS are notoriously low in nutrients and phytoplankton grow along this boundary, utilising nutrients mixed upwards from the deeper waters. The mechanism for mixing nutrients into the surface waters from below is interesting. The tidal range on the NWS is high (up to 10m) and can generate internal tides or waves which cause a vertical displacement of the two layers across their boundary. The end result of internal tides is the mixing nutrients from the depths into the upper sunlit waters that encourage phytoplankton growth.

Our results will join a growing body of knowledge on the phytoplankton of the NWS and factors that influence their distribution and abundance. By understanding the phytoplankton and the oceanography that affects them, we can monitor and understand change in our coastal oceans.

Acknowledgements: Our research was also supported by Australia’s Integrated Marine Observing System (IMOS) who will make our data freely available on their website (https://imos.aodn.org.au/imos123). Thanks also to the helpful and professional AIMS staff and crew of the RV Solander.


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Map of North West Shelf sampling locations also showing chlorophyll biomass of phytoplankton in micrograms per litre (Figure: P. Thomson).