Doing the ‘mesocosm dance’ to investigate food webs and flows
Written by James Hitchcock
Centre for Applied Water Science, University of Canberra
Food webs provide a useful way to think about life in rivers and wetlands. Food webs describe the interactions between organisms, who is eating who, from the smallest bacteria to the largest Murray cod. They can illustrate how much energy is moving between organisms or groups, and the role of individual animals or connections in sustaining life across an ecosystem. In this way, food webs as a concept allow us to understand how changes in the environment influence plants and animals, both at the individual scale and the ecosystem scale.
A core part of ‘food web thinking’ is about hierarchy. At the bottom or base of the food web are producers, these include algae, plants, and microbes. These components, whilst often only microscopic, make up the majority of living creatures, both in weight and numbers, in rivers. It is the responses of these organisms to changes in the environment that often control outcomes for larger animals. In the middle of food webs are primary consumers which include zooplankton, insects, and small fish. At the top of the food web are animals like fish, birds, turtles, frogs, as well as aquatic mammals like the Platypus and Rakali.
Flows and flooding provide food and sustain life in river and wetland ecosystems. Inundating a wetland helps plants grow by wetting dry areas and creating habitat and places for algae to flourish. Flows bring organic matter into rivers fuelling microbes and small zooplankton growth. They allow fish to move, forage and spawn, as well as helping insects grow which, in turn, enables birds to forage and boom. Our work in monitoring environmental responses at monthly, yearly, and decadal time scales, have shown that flows are central to the life-giving food webs in the Murray-Darling Basin.
Understanding and predicting the precise nature of how these organisms and their interactions change following environmental flows still remains tricky. This is in part due to the complexity of the rivers and landscape, as well as the fact that the critters driving these food web changes are incredibly small and dynamic.
Dr James Hitchcock and Ann Bennett taking zooplankton and microbe sampling from the mesocosms. Ann Bennett (Central Queensland University) is undertaking a student work placement at the University of Canberra. Photo credit: Rhiannon Newton
To help unravel the complexities of how ecosystems respond to environmental flows the Flow-MER Food Web theme is conducting a series of experimental studies at the University of Canberra’s purpose built E2R mesocosm facility.
A ‘mesocosm’ is an experimental enclosure built to replicate the natural environment, but which also allows important factors to be controlled. For example, if we find very small numbers of zooplankton in a river it is difficult to know if that is because very few have grown there or, rather, that they have all been eaten by fish. In our experiments we can control for this by keeping the numbers of fish in experimental enclosures the same. Or conversely, we could test for this by altering the number of fish in different enclosures.
Conducting mesocosm experiments requires a particular ‘dance’ between keeping things as ‘natural’ or similar to the river environment as possible, whilst also manipulating and controlling factors to allow for testing. Our outdoor mesocosm facility consists of ten 2000 L circular enclosures. Each is fitted with an electric paddle to move water around the enclosures, which can then be programmed to simulate a slow meander to raging current. Water from a nearby waterway is carted in to allow for a natural mix of animals and water chemistry. We have fitted each mesocosm with loggers that are constantly monitoring the temperature and oxygen levels in the water, 24 hours a day. Bricks, pipes, and wood have been added that aim to balance consistency between enclosures whilst mimicking the diverse structures present in waterways that create turbulence, habitat for animals to hide, and surfaces for algae and microbes to grow. We created a water chemistry solution made from native plants to half the enclosures to mimic the inputs of resources that rivers receive during floods.
Dr James Hitchcock (University of Canberra) and Dr Darren Gilling (University of Canberra/CSIRO Land and Water) taking reading from mesocosms experiments at the University of Canberra. Photo credit: Rhiannon Newton
Our current experiments aim to unravel the trophic upsurge that occurs during environmental flows in the Murray-Darling Basin. What exactly is trophic upsurge?
Scientists often use a particularly strange set of business-speak to describe food webs. The amount of food and life growing within the river is labelled production. When an animal eats a plant or another animal it’s called a trophic relationship. When a leaf or an insect enters the river from the outside world and a fish eats them, this is a resource subsidy. When nutrients encourage microbes and algal growth, in-turn providing more food for larger animals to grow, this is a bottom-up effect. The concept of trophic upsurge captures the idea that additional organic matter and nutrients brought in from upstream and the surrounding catchment during a flood, act as a resource subsidy, causing increased production from the smallest animals in the food web. Following this is a bottom-up affect, as the energy is transferred via trophic relationships between predators and prey. This is what we call a ‘trophic upsurge’.
The outcomes of these experiments will reveal at fine detail how energy moves between organisms in food webs, and how these alter when rivers receive large inputs of resources, as happens during environmental flow events. By observing these complex changes from microbes to fish over a number of weeks, we hope to not only better understand the data we observe in rivers from our monitoring programs, but to help more accurately develop models that can be used to enhance how environmental flows are delivered in the Murray-Darling Basin to promote a diverse array of life to grow and flourish.
Food webs show how food and energy resources such as microbes, algae and reeds are connected with consumers such as waterbugs, fish and waterbirds. Water quality and stream metabolism provide the oxygen and environment for aquatic plants and animals to thrive, and both respond to flow management to provide the energy that fuels riverine food webs. Our work will investigate how Commonwealth environmental water impacts food webs and water quality in the rivers, floodplains and wetlands of the Murray-Darling Basin.
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