Which lakes can geo-engineering help to rehabilitate?
Many of the riverine and peat lakes in the Lower Waikato are degraded, with turbid water and algal blooms predominating.
Recent research has focused on assessing the potential use of geo-engineering products, to clear the water column and cap lake sediments, to minimise the release of the nutrient phosphorus that can fuel algal blooms.
From riverine to peat
The Waikato has a diverse range of lake types including volcanic lakes found on the plateau, peat and riverine lakes in the Lower Waikato and dune lakes in coastal areas. Many of the riverine and peat lakes in the Lower Waikato have become degraded due to drainage of the surrounding landscapes for farming and associated inputs of sediments and nutrients.
Excessive inputs of nutrients and sediment reduce light reaching the lakebed causing submerged macrophytes to be outcompeted by phytoplankton. This can cause the loss of rooted macrophytes and lake primary productivity to be phytoplankton dominated. This change in primary producer dominance has been described as a “flip” in ecological state. In this “flipped” state lakes are resilient to attempts to flip back to a submerged plant community. This resilience can be attributed in part to the legacy effects of phosphorus bound to lake sediments, that can be released to the water column fueling phytoplankton production. One pathway that releases sediment phosphorus occurs when water overlying the sediments becomes anoxic. This often occurs during summer when the upper layer of water is warmed by the sun, reducing its density causing it to remain on the surface of a lake. This means the colder, denser water deeper in the lake is isolated from atmospheric oxygen re-supply. If the oxygen contained in this layer is consumed, it becomes anoxic and phosphorus release from the sediments occurs. Geo-engineering products, that bind phosphorus, can be used to create a “capping” layer over the sediments which can capture this phosphorus before it enters the water column.
Are these products useful in all lake types?
Through the Waikato River Clean-up Trust, and with co-funding from NIWA, Waikato Regional Council and DairyNZ, the Waikato River Authority recently funded a project to explore where geo-engineering products could be useful to help rehabilitate degraded lakes. In a previous project, funded by the same organizations, it was hypothesized that a high concentration of humic materials found in a peat lake would inhibit the functionality of the geo-engineering products tested. These humic materials, we believe, were competing with phosphorus for binding sites on the products, causing them to become “saturated” and reducing their effectiveness.
To test this hypothesis using two geo-engineering products, one natural and one man-made, we collected sediment and water from seven Waikato lakes. These lakes had a gradient of low to high concentrations of humic materials. For both geo-engineering products, we found that with high concentrations of humic materials the products were less able to reduce the movement of phosphorus from the sediments into the water. Additionally, sediment density was found to affect the utility of the products. In sediments that were less dense, the products were unable to form a cohesive “capping” layer, allowing phosphorus to continue to move from the sediments to the water column.
We now have a good idea in which types of lakes these products will work, when a cohesive layer is formed. But in many shallow lakes such a layer will be disturbed by wind-driven wave action that re-suspends sediments. Next, we plan to test how geo-engineering products are affected by wind-driven sediment re-suspension and what mitigation, including the use of native plants, can be implemented to reduce the re-suspension of sediments and the geo-engineering product.
Dr Ben Woodward, Biogeochemist.
Dr Deborah Hofstra, Freshwater Ecologist.