Nitzanim Reservoir retains water for irrigation purposes. Prevention of blooms in the reservoir is key to its continuous operation. It is required to supply its clients with water that meets both bacterial standards as well as filterability standards at all times.
Nitzanim reservoir has a surface area of 15 acres, near the city of Ashkelon, Israel.
Israeli water associations operate some ~600 reservoirs (10-190 acres) all over the country, designed to retain and manage recycled wastewater for irrigation.
Cyanobacterial outbreaks occur regularly in these reservoirs likely due to multiple reasons including a high level of nutrients (e.g. phosphates and nitrates), high temperatures and sun light intensity. Noticeably, water alkalinity is very high ranging between 500-800 mg/l CaCO3.
Over the years, Israeli irrigation ponds have been continuously treated with raw copper at a dose rate of 10-20 kg/acre (20-40 lb/acre). Applied either from crop-dusters or manually, from a boat. The effectiveness of the treatment was rather poor thus demanding frequent treatment. In many cases, the superintendents are forced to open and clean up pumps and filters, sometimes on daily basis to maintain the water flow. Eventually, as water levels decreased towards the end of the irrigation season most reservoirs are forced to arrest the water flow due to the condensed bloom clogging and damaging the pumps.
Materials and methods:
The reservoir has a surface area of 15 acres and is ~50 ft deep (~2.6 million cubic ft). It was monitored 2-3 times every week between January-June 2018.
- Chlorophyll-a (as an indicator for total phytoplankton) was measured by a handheld device (FluoroSense™, by Turner Designs, USA).
- Total particulate matter was assessed using a Clogging Potential Meter (Israel Water Works Association, Israel) with a 33 µm sieve filter. This device measures the time it takes for the sieve to clog under constant water pressure. In principle, the longer it takes for the filter to clog – the better is the water quality.
Water was sampled from the intake flow in a fixed location in the middle of the reservoir, ~6 feet above the bottom of the reservoir, 45 feet below the surface when the reservoir is full.
Sampling was conducted in triplicates. All results were averaged for each sampling point. Algal population analysis was conducted by a microscope observation using hemocytometer cell count chamber.
The treatments were conducted in accordance with the status of the algal biomass as well as the water’s filterability status, one representative example is shown below. The parameters presented were measured in the field and the company’s laboratory.
Results and Conclusions:
A mix of toxic cyanobacteria species (Anabaena sp. and Microcystis sp.) constituted over 95% of the entire phytoplankton populations. An initial treatment with Lake GuardTM Oxy followed by treatments with Lake GuardTM Blue (as specified in the Figure below) caused the total collapse of the toxic bloom, keeping it for months to-come below dangerous levels. Analysis of the phytoplankton population clearly indicated that the treatment outcome underscored “Killing the Winner” paradigm, whereby the dominant species were severely affected by the treatment, allowing non-harmful eukaryotic algae species mostly Monorapridium sp. and Pediastrum sp. (far less sensitive to the treatment) to occupy the “vacant” ecological niche (Fig. 1).
While in principle, non-toxic species should be welcomed as a natural buffer to toxic species this is not the case in wastewater reservoir where they may clog filter pumps as well. For this reason – treatment was conducted under intensifying conditions of non-harmful green-algae. Needless to say, that in freshwater bodies, unlike irrigation ponds, the thriving of green algae in the aquatic system is a positive ‘Resistance Factor’ against toxic species.
Fig. 1. Seasonal treatment with Lake Guard™ Oxy and Lake Guard™ in an irrigation reservoir, Kibbutz Nitzanim indicating the dramatic impact of the treatment on algal levels, its prolonged effect, as well as its ability to influence species-variety in favor of non-toxic ones (1kg/ha. ≈ 1lb/acre).
The impact of a seasonal treatment with Lake Guard™ on the Lowest Lethal Concentration of Copper needed:
The overall amount of copper applied in 2018, using Lake GuardTM was 1/3 that used in the year before (Fig. 2) despite the intensification of toxic blooms in a nearby water body. Considering the ~200%/yearly rise in cyanobacteria populations in various water bodies in Israel between 2014-2017 (represented in Fig. 2), the actual reduction in copper applied in 2018, using Lake GuardTM is closer to ~85%.
Fig. 2. Copper used as an algaecide in the Kibbutz Nitzanim irrigation reservoir during 2014-2018.
Fig. 2. Relative changes in comparison to time 0. (A) cell-density of phytoplanktons and (B) biomass of phytoplanktons.
A simple, cost-effective treatment with one small dose of Lake GuardTM Blue surgically removed one dominant toxic species and enabled non-harmful, beneficial phytoplankton species to occupy the ecological niche and to further outcompete the toxic cyanobacteria throughout the entire season.
The dramatic shift between the toxic species (cyanobacteria) to non-toxic ones (green-algae) is a repeating motive in Lake Guard™ treatments throughout the world. Reshaping the balance of species in the water, in fact, ‘restarts’ the disrupted ecological sphere, allowing it to keep a healthy balance between these coexisting species in the water naturally and uninterruptedly.