The Challenge

Water level variations from managed flow releases are known to influence both rates of biofilm productivity and structural attributes of biofilms that inhabit hard surfaces within rivers. However, little attention has been directed at understanding how managed flows may alter the potential food quality of river biofilms and the subsequent implications for higher consumers (e.g. invertebrates, fish and water birds). Flow directly influences biofilm communities that inhabit hard surfaces within rivers via mobilization of carbon, nutrients (nitrogen and phosphorus), and sloughing. Hard surfaces within streams are hot spots for production, and support diverse macroinvertebrate communities; in effect, the biofilm communities are the first responders to changed flow. Biofilms are also important food sources, providing the basal resource for higher order consumers. As such, responses in biofilms can have a significant impact on community compositions at higher trophic levels. Dissolved organic matter (DOM) within freshwaters is also essential for a broad range of ecosystem functions. Concentrations and types of DOM within rivers depend on the relative contributions of allochthonous sources and the production and consumption of DOM by microbes. However gaps persist in our understanding of biofilm succession patterns and ecosystem dynamics in Australian lowland rivers, and in particular, their responses under varying hydrological scenarios.

 

Our Response

This project employed biochemical and DNA techniques to measure structural and functional changes of biofilms over an eight week period, commencing three weeks after initial inundation. Biochemical tools (analysis of changes to fatty acid profiles) were used to provide information about nutritional quality, while DNA tools were used to provide rapid, detailed analyses of structural aspects of microbial communities. We examined temporal patterns in DOM quality and productivity among three lowland streams (Wakool River, Neimur River and Yallakool Creek) using fluorescence excitation emission scans. We also assessed the production and consumption of DOM within light and dark bottle assays to quantify the relative contribution of bacteria and algae to the DOM pool and compared them to whole-stream metabolism estimates in order to calculate the contribution of surface biofilms to overall metabolic activity.

•  Science and Innovation

Applying fatty acid analyses, next generation DNA sequencing techniques and metabolic experiments simultaneously represents a novel approach to demonstrating ecological responses to hydrology in freshwater ecosystems.

Results

Results from this project can broadly be separated into three main parts:

• Importance of allochthonous DOC in the Edward – Wakool river system

Fluorescence excitation emission scans of dissolved organic matter in the Niemur River, Wakool River and Yallakool Creek water followed by PARAFAC analysis revealed the presence of three main DOC components. We demonstrated the presence of a more heterotrophic microbial community within the water column. The lack of a significant difference between the light and dark bottles and the production of protein-like DOC within all three rivers also suggests that protein-like DOC in the water column is linked to bacterial production in place of algal activity. The high abundance of the humic-like component found within these systems is indicative of the important role allochthonous DOC plays in these systems. Protection of allochthonous sources of DOC (e.g. riparian vegetation, inundation of floodplains) is an important management consideration.

• Biofilm succession

Chlorophyll- a in biofilms (an a proxy for algae) remained low (<10µg/L) for the first 5 weeks before peaking at the end of the 11 week study. We demonstrate a range of key metrics to which biofilms are responding (e.g. discharge, nutrients, turbidity). This response coincided with increases in some essential fatty acids critical for animal development. Green algae is known to be an excellent source of PUFA, and here we show that some essential fatty acids peaked in concentration after 11 weeks deployment. This was supported by C:N data, which was lowest (lower is reflective of higher food quality) after 11 weeks. We demonstrate that biofilm quality was retained for up to 11 weeks following inundation, and based on some key metrics (e.g. C:N, fatty acids profiles), peak quality was reached at the end of the 11 week study.

• Stream metabolism

Our work indicates that during steady high flows over spring and summer that streams within the Edward-Wakool River system are strongly heterotrophic. Importantly, our technique showed that gross primary production in these ecosystems is predominantly generated within the water column by phytoplankton. Surfaces (combining hard surface biofilms on snags and soft benthos) are predominantly heterotrophic and consume more oxygen than they produce via microbial processing of organic material. These data provide important baseline information of ecosystem dynamics within these systems that will be useful for comparison with metabolism responses to managed flows.

Contact Person

Paul McInerney

Additional Information

• Publish article: Dissolved organic matter and metabolic dynamics in dryland lowland rivers