More than mud

More than mud

Photo: Lara Ainley

Photo: Lara Ainley

Photo: Lara Ainley

How will disruption of soft-sediments threaten coastal biodiversity?

This is a $310,000 question that one of Climate Future’s brightest stars is uncovering for an Australian Research Council grant.
Doctor Melanie Bishop is a marine biologist who is knee deep in marine mud, for it is these soft-sediments which are the focus of her scientific enquiry. She intends to uncover how both coastal development and climate change will influence decomposition processes in these sediments - the process of organic matter decay which fuels the food web of estuaries.

A major concern for Bishop is the possibility of ecosystem collapse. This may occur if the building blocks of brown food-webs are seriously disrupted leading to anoxic, or oxygen depleted water columns.

In aquatic systems the most palatable thing to consume is algae such as seaweeds. However, a lot of plants have chemical defences to deter them from being consumed. So instead of being eaten, the plants eventually shed dead matter which rots at the bottom – thus starting the brown food-web. This detritus is then broken down by bacteria, which are preyed upon by invertebrates. The fish then eat the invertebrates, and the energy slowly makes its way up the food chain.

The part of this food-web that is most likely to be disrupted is when there is increased algal growth – a response to increased nutrient loading (eutrophication). Coastal development is a major contributor to this loading: estuaries become the dumping ground for all sorts of human discharges such as farm effluent, wastes from roads and sewage, of which large quantities of nitrogen is of most concern.

A little bit of decomposition is a good thing and is needed for fuelling food webs. However a lot of decomposition can result in ecosystem tipping points being breached. This can occur when there is an increase of rotting matter – a product of eutrophication.

Taking advantage of the larger food source, the bacteria go into 'overdrive', with their populations multiplying very quickly. As bacteria eat, they respire and this consumes oxygen, which normally results in a natural anoxic layer deep within the sediment. However, when rotting matter is increased, this layer is brought to the surface which can begin to affect the water column. Eventually this results in depletion of oxygen levels overlying the sediment. The fallout is a crash of invertebrates which in turn reverberates up through the food chain. This is when an estuarine ecosystem can begin to collapse.

Fortunately Eastern Australian estuaries are characteristically nutrient deficient. Therefore the release of nutrients from coastal development has until now, not been of much concern. The question is thus related to tipping points.

In most cases these tipping points have not yet been reached. However, changes in water temperature can also influence eutrophication and anoxia. Biological processes tend to increase as water is warmed, so the breakdown of this dead plant matter is likely to accelerate with the warming water.

So with future climatic changes, if the bacteria are breaking down plant material more rapidly the question is: are we now going to see tipping points where we didn't see them in the past?

Dr Bishop accepts that we may not have a lot of control over the warming of the coastal waters in the near future, however this is not the case with the amount of nutrients that we allow to enter these estuarine environments.  Therefore she intends to work out what level of nutrient loading is likely to cause tipping points in the future. Once this is better understood, it will be easier to advise governments and policy makers on how to keep our nutrient loading below that point, so that in the future we don't get the collapse of these productive systems that fuel coastal industries.

Written by Eden Ottignon - Climate Futures placed SCOM300 intern.

Photos: Lara Ainley

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