By Ryan Burrows
Article Read Time: 623 words about 4 minutes.
Groundwater upwelling to rivers not only sustains surface flow but also delivers nutrients and creates favourable conditions for algae growth, importantly algae is a critical food source for aquatic animals.
What did we do and how?
Algae is a nutritious and major food source for many species in rivers, including aquatic bugs and fish. Because of this, algae can form the energetic basis of many rivers.
Researchers need to know what controls the distribution of algae in rivers so that we can then manage human activities that may negatively impact its growth.
Our research team investigated the factors driving the spatial distribution of benthic algae (i.e. growing on the river bottom) in the Fitzroy River, in the Kimberley region of tropical northern Australia. We did this in the dry season by measuring algal biomass and recording environmental conditions (e.g. river depth, water velocity, nutrients, light availability), at hundreds of locations along the river.
However, we did not just measure surface-water characteristics. Like many rivers in northern Australia, the Fitzroy River relies on groundwater upwelling during the dry season to maintain flow. Because of this, we also investigated the potential role that groundwater inputs may have for algal growth by measuring the strength of groundwater inputs. We did this using radon gas concentrations in surface water, because radon is an excellent indicator of groundwater inputs.
What did we find?
Our findings show that two major things were driving the distribution of algae along the Fitzroy River.
- First, algae biomass was lower in parts of the river with higher water velocities. This was probably because the riverbed was very sandy, and sand moves very easily at a higher water velocity. When sand moves a lot it can be very hard for algae to establish and grow.
- Second, algae biomass was greater in parts of the river with relatively greater inputs of groundwater. This was probably because the groundwater contained higher concentrations of nutrients that the algae required to grow.
All-in-all, algal growth was constrained by higher water velocity but was promoted by groundwater inputs.
Why was our research important?
There is increasing interest in exploiting the ground and-surface water resources of northern Australian rivers (including the Fitzroy River) to support expanded agricultural development. For example, the Australian Government wants to double the agricultural output of northern Australia over the next 20 years. But the Fitzroy River remains largely undeveloped and contains many aquatic species including the critically endangered Largetooth sawfish (See video below). So there may be negative ecological impacts from development.
Modification to northern Australian rivers that alters dry-season flows and water velocity will likely influence the biomass of benthic algae along sandy reaches, and this may have implications for aquatic food webs.
Further, reductions in groundwater levels via groundwater extraction will diminish the strength of groundwater–surface water interactions along the Fitzroy River. And as our study revealed that areas of groundwater inputs are potential ‘hotspots’ of benthic algal biomass, modifications to groundwater levels can directly influence river productivity.
Where can I read more?
This research was just published in Hydrobiologia. Please contact the authors for a copy of the paper.
Who funded this research?
This project was supported through funding from the Australian Government’s National Environmental Science Program (NESP Northern Australia Environmental Resources Hub Project 1.3.3), Griffith University and the University of Western Australia.