Quality food supports stream ecosystems

By Fen Guo

The animals that keep stream ecosystems healthy rely on high quality food. My PhD studies at the Australian Rivers Institute showed that the changes humans make to streams, like cutting down trees, can affect the quality of food stream animals get to eat.

Microscopic algae are a higher quality food compared with other organic matter for the growth and reproduction of small animals and fish in aquatic ecosystems. In part, this is because of their higher essential fatty acid content.

Fig 1
Mt Barney Creek in the Logan-Albert catchment, South-East Queensland, Australia (one of my sampling streams)

Essential fatty acids are necessary in the diets of all animals, including humans. However, most aquatic animals cannot synthesize the essential fatty acids themselves. These are only abundant in algae, and animals depend on getting them from their diets.

My PhD work investigated the role of algal food quality, as assessed by EFA, in stream ecosystems.

Diatoms and cryptophytes are two types of algae that are considered high-quality food sources for aquatic invertebrates and fish because of their higher content of high quality omega-3 essential fatty acids. Green algae are considered to be of medium dietary quality and contain primarily lower quality fatty acids. In contrast, blue-green algae (cyanobacteria) are poor quality food for aquatic consumers.

My study found algal fatty acid content was sensitive to environmental change. In particular removal of riparian vegetation and addition of nutrients to streams changes algal fatty acids from high quality to medium quality fatty acids, and sometimes to very low levels of important fatty acids.

These changes in algal food quality resulted in changes in aquatic animal growth. Increases in algal fatty acid content significantly improved the growth of insect grazers.

I also found that stream insects (shredders) that are known to eat fallen leaves from the riparian zone grew much larger if the leaves had a film of high-quality algae growing on them. Although the shredders obtained most of their body carbon from the leaves they acquired and selectively retained fatty acids from high-quality algae.

Thus, even organisms that seem to eat leaves rely on algae for part of their diets.

Fig 2
Wassercluster Lunz, Austria, where I work as a post-doc now

My PhD work highlights the importance of high-quality algae in stream ecosystems. High quality algae not only benefits animal growth and energy transfer to upper trophic levels but also regulates the incorporation of low quality leaf litter into stream food webs. This study complements previous findings of algal importance in freshwater ecosystems, and improves our understanding of how algal EFA transfer in stream food webs.

Now I have completed my PhD I have been empolyed on a post-doc position in Wassercluster Lunz, Austria. In Austria I am continuing to look at the importance of high quality food in stream ecosystems.

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