Measuring stream metabolism

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:

1. Photosynthesis (also known as gross primary production or GPP) is a major part of stream metabolism. It is the creation of new plant cells in water plants and algae. The process of photosynthesis also creates oxygen. To create new cells, plants and algae need nutrients (just like we need nutrients from our food) like phosphorus and nitrogen. These are typically generated within the stream by the second of stream metabolism’s key processes:
2. Respiration – this is similar to human respiration (breathing in and out) – where an organism consumes oxygen and ‘exhales’ carbon dioxide (CO2) to release the energy from organic carbon. In streams, respiration breaks down dead plant and animal cells, returning phosphorus and nitrogen to the water so that they can be used for new cell growth in photosynthesis. Although fish and invertebrates (and other animals) also respire CO2, in streams, most of the respiration is performed by bacteria in the water and surface sediments.
3. Another process involved in stream metabolism is reaeration. Reaeration is when oxygen is absorbed from the air into the water to replace oxygen consumed by respiration. This is a physical, not a biological process, and is vital in maintaining a balanced level of oxygen and healthy riverine habitats for fish and other organisms. 

The relationship between photosynthesis, respiration and Goldilocks!

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. 

Measuring stream metabolism

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 how healthy is the Goulburn?

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’.

What we have found

• Stream metabolism as measured by the rates of photosynthesis and respiration,  generally does not change dramatically between sites. The Goulburn Weir to McCoy’s Bridge section of the river has a relatively uniform metabolism for most of the time. One exception to this is the effect of summer storms in the Seven Creeks catchment which brings very poor water quality (nearly no dissolved oxygen in the water) into the river which results in a short term (up to a week or two) decline in water quality in the Goulburn. We also found that metabolism varies across seasons, with rates highest in summer and early autumn, and lowest in winter.
• Increases in flow result in lower rates of photosynthesis and respiration per litre of river water. This is due to the dilution effect of more water. When we examine the amount of organic carbon (‘fish food’) created, the greater volume of water provided by increased flows, means that there are many more litres of water. We found that the increase in flow produces more food for aquatic animals than lower flows at the same time of year. In any season (apart from winter), the higher the flow category, the more organic carbon (‘food’) is created.

So what?

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.