Isotopes aid food web study

The flow of energy in the lower Waikato River ecosystem, from simple organic matter to the largest fish, is far from a simple process. Waikato University PhD student Michael Pingram is getting to grips with this by investigating food webs, in a study which he hopes will aid the river’s restoration.

“Food webs consist of networks of food chains, and they can be quite complex,” he says. “For example species such as eels could be feeding on a variety of small fish and invertebrates, which themselves may play a role in other food chains, all of which then become linked, forming a web.”

The food supply for the river’s fish ultimately comes from humble material such as algal films on rocks or logs, leaf litter washed in from the land, or fine debris and planktonic algae washed down from further up the river. In order for scientists to understand better how the ecology of the river functions, it is necessary to identify the relative importance of these various inputs.

Michael is using a technique called stable isotope analysis to do this. Elements exist in different forms, known as isotopes, which vary in atomic weight. He is focusing on two elements which are fundamental to life, carbon and nitrogen, both of which have two stable (non-radioactive) isotopes.

“It’s fairly widely used these days for tracking the flow of carbon through food webs. Because carbon from different sources has different isotope ratios, we’re able to differentiate between carbon from algae that has grown in the river, and carbon from detritus washed in off the land. And that ratio remains pretty much the same as it works its way through the food webs. So you can theoretically figure out which of the carbon sources has eventually made its way to a top predator such as an eel.” For example, fish such as rudd, which primarily eat aquatic plants, are likely to have rather different isotope signatures to those of smelt, which eat zooplankton.

The nitrogen isotopes behave rather differently. Each time one organism consumes another and assimilates its nitrogen, the proportion of the heavier isotope increases relative to the lighter one. “So if you know the difference between an animal’s nitrogen ratio and that of the food resources, you can work out that animal’s position in the food web. And if you combine the ratios for both elements you can start to work out how much the animal depends on each carbon source.”

Results to date indicate that bacteria and algae growing locally in the river form the basis for a significant amount of the energy flowing through the food webs, with smaller contributions from terrestrial vegetation, aquatic plants, and material washed down from further upriver. Michael still has to analyse samples collected after the willows and alder along the riverbanks shed their leaves in autumn, and these could make a substantial seasonal contribution of carbon to the food web.

The results also highlight the importance of the river’s tributaries, which is in line with similar studies elsewhere. “Research overseas has found these lateral off-channel habitats, where water flow is reduced but fish still have access to the main channel, can have much more complex food webs because they provide a greater range of habitats for a greater range of species.”

Though his findings are still preliminary, it appears leaf litter and other land-derived material may play a greater role in some of these tributaries, particularly the outlets to some of the wetlands. The outlets to Lakes Waahi and Whangape, on the other hand, could provide important amounts of planktonic algae and zooplankton to the river at certain times.

Michael’s study forms part of a large river research programme led by Dr Kevin Collier. In time, Michael hopes his work will assist with restoration efforts on the lower Waikato. “We’ve got quite a unique river in many ways, given it’s a temperate river with a fairly aseasonal flood pattern. Hopefully this will provide some good information about where to restore and what might be important, particularly with regard to important food sources and habitats.”