Golidlocks and the Goulburn River:
measuring stream metabolism to get flows ‘just right’
Author: Mike Grace
Rivers are the lifeblood of fish, plants, insects, bacteria and algae, in fact anything that lives within, and relies upon the riverine ecosystem. The health of a river ecosystem affects these organisms, and we want to know if water for the environment is making a difference to river health. In the Goulburn River we are investigating this by measuring ‘stream metabolism’ to see if water for the environment is breathing life into this important waterway.
Stream metabolism consists of two key ecological processes which generate and recycle organic matter (carbon). These processes are called gross primary production (GPP) and ecosystem respiration (ER). Healthy aquatic ecosystems need both processes to generate new organic matter (which becomes food for organisms higher up the food web), break down plant and animal remnants, and recycle nutrients to enable growth to occur.
Many factors can affect stream metabolism, including changes to environmental flows and unseasonal weather events. These factors also affect the amount of food available to sustain all the fish and bugs living in the river’s ecosystem.
Stream metabolism is a quick way to measure the ‘health’ of a river, or how much oxygen it is producing and using. In a human context, the doctor will measure your breathing, looking at whether it is at a normal rate. If the rate is too high or too slow, it’s likely something is not right. In the context of stream metabolism, we measure the following:
Photosynthesis creates new plant cells, providing food resources for the river’s inhabitants (its fish and bugs), whilst respiration provides the nutrients to enable new cells to be made. Photosynthesis creates organic carbon, releasing oxygen as a byproduct, and respiration uses that oxygen to break down the organic carbon for energy, releasing CO2 as a waste product. The processes are strongly linked! We’ll look at how these processes are measured shortly, but firstly it’s important to highlight that, just like in human health, the stream organisms (fish, invertebrates, turtles, etc) need enough food to survive and grow.
The ‘Goldilocks’ analogy can be applied here. Just as the porridge that Goldilocks ended up eating needed to be ‘just right,’ so too for rivers; we want the amount of photosynthesis and respiration to be ‘just right’ to provide these important functions. For example, if photosynthesis is too high, it is probably because there are too many algae (algal blooms) that are photosynthesizing during the daylight (this needs sunlight) but still respiring 24-hours a day. So overnight, without photosynthesis, respiration can drive the oxygen in the water to very low levels (or even zero) – this can cause fish deaths as not enough reaeration is taking place. Respiration uses organic carbon (plant and animal detritus – what plants and animals leave behind when they die), so if there’s too much of this, which can happen in “black water” events, then respiration will bring the oxygen down to zero. In both cases the stream metabolism is not functioning properly and the river is in poor health.
To measure stream metabolism, we simply measure how the oxygen in the water changes over time each day. Just like a human heartbeat, oxygen goes up during the daytime when the plants are “awake”, especially when the weather is warm and there is a lot of sunshine – when night falls, oxygen goes down. We place oxygen and temperature probes below the water surface to provide us with information for our computer model to calculate how much photosynthesis and respiration is happening in every litre of river water each day. We then relate the amount of oxygen created by photosynthesis to the amount of new ‘fish food’ – the new cells – also known as organic carbon material.
So we’ll now examine how healthy the Goulburn River is based on our key processes and how the amount of ‘fish food’ produced depends on flow.
Since 2014, we have been measuring stream metabolism at five sites in the Goulburn River between the Goulburn Weir and McCoy’s Bridge. The sites being monitored every day are at Murchison, Arcadia Downs, Shepparton Golf Club, Loch Garry and McCoy’s Bridge. One of the great benefits of running a program over several years is that it allows us to start teasing apart effects of seasons from individual weather events (e.g. storms), forming longer-term patterns of ‘wet’ and ‘dry’ years.
Along with continuous monitoring of oxygen levels, water quality parameters and nutrient samples (of nitrogen, phosphorus and organic carbon), as well as the chlorophyll-a concentration as an indicator of algae in the water, are collected every month.
As metabolism varies on a daily, as well as seasonal basis due to differing temperatures, our long-term data set records amounts of sunlight, cloud cover, nutrient and organic carbon concentrations in the water. This enables us to estimate the typical rates of photosynthesis and respiration in each season, for a range of different flows, from very low to medium, and sometimes high ‘freshes’.
Freshes are short-duration flow events that submerge the lower parts of the river channel. They are important for plants that grow low on the banks and provide opportunities for fish and other animals to move more easily along the river.
From a management perspective, our work has demonstrated that even relatively small increases in flow can have positive benefits that increase food resources in the river. These higher flows are rewetting areas of banks that now allow algae to grow – and it’s these algae growing on snags and rocks that are a key factor in creating more ‘food’ for fish and other organisms.
It is important, however, that these higher flow events are not at a constant level for long periods of time, but vary in size throughout the duration of the event. This will support other parts of a river healths, such as its geomorphology (to avoid bank slumping and notching from oversaturation of water) and vegetation – some plants cannot survive if they are fully submerged in water. Another important finding associated with this is that there is no benefit from a metabolism perspective of increasing flows during winter time.
A related and interesting project undertaken by a group of Masters students over summer 2020-21 for their final year project at the University of Melbourne aimed to provide additional information about where in the river the majority of photosynthesis is occurring. Is it the water column itself? If not, then it must be from algae (biofilms) on the stream bed and snags, plus any plants etc under the water surface. This information will link in very nicely with the Stream Metabolism insights from Flow-MER Food webs and Water Quality Theme. A couple of photos of the team in action are shown below.
Featured Photo: The Goulburn River near Shepparton. Source: Greater Shepparton City Council.
The Goulburn/Kaiela River is one of the hardest working rivers in the Murray-Darling Basin, providing water to support agriculture and rural communities, as well meeting growing downstream Murray River demands. At the same time, it is home to diverse flora and fauna. Our work is assessing how environmental and consumptive (irrigation) flows are affecting the ecosystem of the Lower Goulburn/Kaiela River, to help waterway managers to protect and enhance this vital water source into the future.
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