LG Sonic

Why are we monitoring nutrients at different depths?

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Courtesy of LG Sonic

Recently, LG Sonic introduced the MPC-Lab, an innovative monitoring buoy that can monitor algae, physico-chemical variables, and even nutrients such as nitrogen and phosphorus in near real-time and at different depths in a body of water. Combined with our other monitoring tools, this system offers a unique way to detect the spatial and temporal patterns of nutrient concentrations which play a critical role in algae-related problems in lakes.In combination with a series of physiochemical data, this information can assist in determining the most optimal solution. You can read here why the MPC-Lab is such an important tool to identify the best solution for nutrient pollution in lakes.

Eutrophication

Most lakes suffering from severe algal blooms are eutrophic or even hyper-eutrophic. The trophic state of a lake is determined by the concentration of nutrients, mainly phosphate and total phosphorus, in the water and is assessed by the concentration of algae (chlorophyll-a) and water transparency. A hyper-eutrophic or eutrophic lake has an increased concentration of nutrients in the water, leading to persistent algal blooms or, in most cases, to harmful cyanobacteria blooms. Eutrophication is a problem for 53% of lakes, ponds, reservoirs, and dams in Europe. The numbers for Asia at 48% and North America at 54% are more or less the same. The most obvious solution for algae related problems is to reduce the number of nutrients flowing into a lake or reservoir in order to reduce the effects of eutrophication.

Although preventing the inflow of nutrients into our rivers and waterbodies is key to effectively deal with eutrophication, unfortunately, the mitigation of algal blooms is rarely this simple. Many affected waterbodies have been receiving excess nutrients for many years, leading to a build-up of phosphate in the sediments of these lakes and reservoirs.

Nutrient source build-up and organic cycle

The source of nutrients in a lake or reservoir can be diverse; some common examples include agricultural run-off, domestic sewage, pollution through rivers flowing into the reservoir, cattle grazing around a reservoir, and organic materials that may end-up in a reservoir. Running into the reservoir, nutrients such as phosphate (PO4) or nitrate (NO3) may be used by algae, water plants or other organisms. When these organisms die, the organic material sinks to the bottom and builds a nutrient rich sediment layer and an oxygen-poor water column through biological decay. When the nutrient source is mainly from organic materials, these nutrients may build-up directly in the sludge. When lakes turn eutrophic, the phenomenon is often the result of years of nutrient pollution that have led to a build-up of nutrient-rich sediment in the bottom of the lake. These nutrients may be released in low or high concentrations from the sediments (internal nutrient loading) either during a specific part of the season or continuously, depending on the lake dynamics. As a result, they are recycled back in to the water and eventually foster the growth of algae and water plants.

Stratification

Many deep lakes experience stratification, a process through which the water divides into different layers with varying temperatures and density. Near the interface of these thermal zones, the phytoplankton or other substances such as nutrients may accumulate. The bottom waters of a mesotrophic or eutrophic lake usually remain anoxic, meaning lacking oxygen, during stratification, leading to an accelerated release of nutrients and other toxic substances (hydrogen sulphate, ammonia, etc.) from the sediments. When nutrient-rich sediment builds-up in a lack for a prolonged time, it may provide nutrients to the algae for years after external nutrient inflow in the lake has been reduced.

The ratio between nutrients coming from the sediment and nutrients flowing into a reservoir through various sources of pollution differs from lake to lake.

Water Plants

Most healthy waterbodies have water plants growing at the bottom or surface. The growth of plants is important in a lake, as they are able to act as nutrient sinks, improve water clarity, and provide important habitat for various aquatic organisms. Specifically, benthic or submerged plants, growing at the bottom of a body of water, benefit water quality. They consume nutrients coming from the sediment and the water layer and increase the dissolved oxygen levels at the bottom of the lake. The oxygen is in turn used by the aerobic bacteria that breakdown organic material and control the nutrient accumulation in the sediments. In a balanced ecosystem, plants and bacteria function synergistically in order to reduce nutrients in a lake or reservoir. In lakes and reservoirs with a high turbidity, due to an abundance of algae, benthic plants cannot grow and this cycle is disturbed, often leading to excessive build-up of nutrients in the sediment.

To measure is to know

LG Sonic works with several solutions to control and reduce algal blooms and their harmful effects. The MPC-Buoy has a direct impact on algal growth, by monitoring the algae and controlling them by using ultrasound. This technology is chemical free and only affects algae; therefore, it also can be used to improve the light conditions for the growth of aquatic plants and bacteria in the sediment.

To deal with the source of the problem of algae -the nutrients-  LG Sonic uses different technologies based on effective lake or reservoir aeration (provision of oxygen), direct and systematic phosphorus or nitrate removal, or a combination of all of these. All the technologies that we approve for lakes and reservoirs are non-invasive, chemical-free, and should not negatively affect any other organisms in the water.

To know which solution may be best for a lake, we monitor and assess the status of the lake.

3D mapping

As the source of nutrients and how a lake responds to nutrient enrichment is always complex and unique for each lake, before implementing any treatment, the status of the lake must be assessed. For this, we monitor vertical, spatial, and temporal patterns of a series of water quality variables.

The MPC-Lab can profile the vertical water-column of a lake. This system is based on a solar powered buoy system that works autonomously and can be used to measure many different variables, such as nitrate (N-NO3), phosphate (P-PO4), ammonium (NH4), turbidity, Chlorophyll-a, phycocyanin, dissolved oxygen, temperature, pH, and many more. These measurements can be taken at different depths in the lake. The measurements are calculated in near-real time and are outputted several times a day.

Because this system also provides a temporal overview as a vertical, the data can be used to identify the source and dynamics of the nutrients within the lake. The MPC-Lab can timely detect nutrient release from the sediment and relate that information to nutrient input from other sources.

A spatial overview is being created through remote sensing. Although the MPC-Lab can monitor relatively precisely and in real-time, it still only captures a few points in a large waterbody. A lake or reservoir is rarely a homogenous body of water. Besides the vertical stratification of a lake, variations in water quality, nutrient concentrations, and algal growth also differ horizontally in the lake.

Remote sensing image showing the chlorophyll levels in a water body.

Remote sensing is the use of satellite data, such as images from Landsat or Sentinel, to determine water quality parameters that normally can only be measured in the laboratory or through in-situ sensors. The use of remote sensing however, has an unparalleled advantage when assessing a lake’s status, because remote sensing is the only data-source that provides a spatial overview of water quality parameters over an entire water surface.

In combination with the more specific, real-time data from the MPC-Lab, remote sensing can be used to interpolate the data from the MPC-Lab to other areas of the lake. Through the determination of variables, such as chlorophyll a, turbidity and phycocyanin, and algal blooms, lake dynamics may be assessed. An additional bathymetric study may compliment this information to determine where the highest level of sediment build-up in the lake is located.

The result

All these data sources are processed in the MPC-View, a web-based software. With this data, the exact relationship between nutrient inflow and release from the sediment can be determined. Also, the peak time of year for sediment releasing critical loads of nutrients can be determined. This information can be used to plan the right timing for an intervention to effectively reduce nutrients.

MPC-View water quality software

In addition, through the measurement of nutrients in relation to other physico-chemical variables, the magnitude of nutrient availability /supply along the water column and the role of sediments in the nutrient cycling can also be determined.

The actions that we may propose will be based on nutrient reduction, aeration or ultrasound. The use of our MPC-Buoy systems in these applications is often a first step to better water quality. The MPC-Buoy systems will reduce algal growth, increasing the clarity of the water. In many reservoirs, this allows light to penetrate to the bottom of the reservoir, supporting benthic/submerged plant growth and aerobic digestion of nutrients.

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