Lake Rotorua sediment-capping trial
Many New Zealand lakes are suffering from nutrient enrichment, causing potentially toxic blooms of blue-green algae. Blocking the release of phosphorus from lakebed sediments can be the most effective way of controlling algal blooms. NIWA is testing a range of methods to manage phosphorus release from lake sediments – including sediment-capping agents.
Many New Zealand lakes suffer as a consequence of nutrient enrichment from surrounding land use, particularly pastoral farming. This is exacerbated by inputs from pumice soils in the volcanic zone of the central North Island, which are naturally high in phosphorus.
Human-derived inputs of nitrogen and phosphorus have been particularly damaging in some of the well known Te Arawa/Rotorua lakes in the central North Island.
In nutrient-enriched lakes, nitrogen and phosphorus are also released into lake water from the sediments by decomposition of organic matter. These 'internal loads' sometimes outweigh nutrient loads from surrounding land use. This is particularly a problem in the summer months, when stratification (lack of mixing) of lake waters starves the bottom waters of oxygen.
Controlling algal blooms requires reducing both nitrogen and phosphorus loads. Microbes can permanently remove internal nitrogen loads through natural denitrification processes, but don't remove phosphorus. Excess phosphorus can promote the growth of potentially toxic cyanobacteria (blue-green algae) blooms, which have plagued lakes throughout New Zealand over the past decade.
Internationally recognised techniques for managing internal loads of phosphorus already exist but these may not work as expected in New Zealand because of differences in water characteristics such as hardness and geothermal influences. Consequently we cannot simply take an overseas recipe and apply it to our lakes - the international techniques need to be tested and adjusted to work correctly under New Zealand conditions.
As a part of the Bay of Plenty Regional Council 's long-term programme to rehabilitate the Te Arawa lakes, NIWA ,in collaboarion with the University of Waikato, has tested four sediment-capping agents on Te Arawa/Rotorua lake sediments to determine and compare:
- their effectiveness in reducing internal phosphorus loads
- how long their effects may last
- what side effects they may have on the lake ecosystem and bottom-dwelling organisms.
In the future, this will enable regional councils to develop the best treatment strategy for large-scale lakes, suffering from nutrient enrichment, by providing them with information about the most appropriate product to use.
The four sediment-capping agents are:
- Alum – an aluminium sulphate solution commonly used to treat drinking water and swimming pools
- Aqual P – a locally-sourced modified zeolite clay
- Allophane – a natural volcanic ash from the central North Island
- Phoslock™ – a bentonite clay mineral modified specifically for sediment capping, sourced from Australia.
NIWA has a study funded through the Ministry of Science and Innovation which is designed to improve understanding of techniques for restoring the water quality of degraded lakes in New Zealand. The study will evaluate internationally recognised remediation techniques under New Zealand conditions and test modifications to improve them.
Lake Rotoehu trial
During the summer of 2011, NIWA is testing a range of methods to manage phosphorus release from sediments in Lake Rotoehu. Sediment-capping agents will be applied inside mesocosms (lake tubes) which enclose the full water column from the surface to the lake bed.
What is a mesocosm?
A mesocosm consists of a thin-walled flexible plastic tube, 1.6 m in diameter, sealed against the lake bed with a weighted ring and supported at the lake surface with a floatation collar. When installed, the mesocosm encloses a volume of natural lake water over a small area of natural lake sediment in order to study lake processes. A separate mesocosm will be used to study the effect of each remediation technique and be repeated 3 times. To prevent the mesocosms drifting, they will be tethered in groups to paired ropes stretched between 2 securely anchored mooring floats.
Scientists will compare several measures inside and outside the mesocosms including sediment and water chemistry, particularly changes in phosphate levels, and interactions between the sediments and the capping agents.
The trial will run each summer until 2012. The trial is authorised by the Te Arawa Lakes Trust and Bay of Plenty Regional Council under Resource Consent Number 66470.
In 2007 and 2009, NIWA provided guidance on the application of Aqual P to Lake Okaro, one of the smaller Rotorua lakes, where it successfully prevented blue-green algal blooms in the summer of 2009/2010. NIWA also tested the effectiveness of the four agents listed above in laboratory studies. These indicated that all of these materials have the capability and capacity to block the release of phosphorus from the sediments.
The tests also indicated that alum may be of limited used in a large shallow lake such as Lake Rotorua, where it can be easily disturbed by lake currents. Based on these tests, NIWA recommended that sediment capping agents should only be applied below the level where the lake periodically stratifies.
Hickey, C.W.; Gibbs, M.M. (2009). Lake sediment phosphorus release management—Decision support and risk assessment framework. Journal of Marine and Freshwater Research 43: 819–856.
Gibbs et al (2010). Sustainability assessment and comparison of efficacy of four P-inactivation agents for managing internal phosphorus loads in lakes: sediment incubations. Hydrobiologia 658: 253-275.
Gibbs, M.; Hickey, C.; Özkundakci, D. (2011). Sustainability assessment and comparison of efficacy of four P-inactivation agents for managing internal phosphorus loads in lakes: sediment incubations. Hydrobiologia 658(1): 253-275.