Buoy-based water quality profiling takes a look below the surface

By on August 17, 2010

Buoy-based water profiling systems with real-time telemetry can provide streaming data on important water quality parameters all day, every day.

This information helps managers, agencies, and universities care for and understand the world’s precious freshwater resources. Most buoys, however, have sensors located primarily in the surface waters — the epilimnion — of lakes, usually with only a temperature string taking measurements deeper in the water.

Often the data from such setups can only provide insight that’s surface deep. A number of water parameters fluctuate along the water column, and the only way to gather an accurate picture of a water body’s quality is to measure at various depths.

What happens deeper in the water column and why, and what does this mean for lake science?

Temperature: Surface water temperatures are usually close to uniform over the first several meters due to wind-driven mixing. In the case of this reservoir, the temperature is nearconstant over the uppermost three meters and decreases quickly deeper in the reservoir. The temperature near the bottom of the lake remains cold and changes much more slowly than surface temperatures. Because temperature is one of the most important measurements made on a lake, a temperature string that measures at various depths along the water column is one of the most common and important tools used in water profiling. (Units: °C)

Chlorophyll: Chlorophyll is an indicator of algae. Often there is a below-surface peak in chlorophyll levels (termed a deep chlorophyll maximum) that depends on a lake’s transparency and nutrient levels. A rule of thumb is that the chlorophyll peak occurs at about the 1% light level. Because of this, the chlorophyll peak helps to estimate transparency. In the case of this reservoir, the chlorophyll peak occurs at about five meters. Chlorophyll can also regulate the pH and dissolved oxygen (DO) of a lake, and algal productivity increases pH and DO. (Units: ug/L)

pH: pH can change throughout the water column depending on the physical, chemical, and biological processes in a lake. For example, algal photosynthesis increases the pH; thus, pH can vary from day to night and from summer to winter. Since algal photosynthesis occurs more in surface waters, pH can also fluctuate near the surface. Note how, in the reservoir depicted in the graph, pH is higher near the surface, where chlorophyll is higher. pH can also become lower deeper in the water column especially when lower oxygen conditions, called anoxia, develop.

Dissolved Oxygen: Water’s DO concentration depends on water temperature and the balance between algal photosynthesis and respiration. Oxygen is less soluble at higher temperatures. Moreover, photosynthesis produces oxygen, meaning dissolved oxygen can be extremely high when algae are abundant. Note how in the example reservoir, the peak in dissolved oxygen occurs at about the same depth as the chlorophyll peak. Lower in the water column, there isn’t enough light to support photosynthesis. Here, respiration consumes oxygen, and anoxic conditions can develop. Anoxia is harmful to aquatic organisms, such as fish that will struggle to breathe in such conditions. (Units: mg/L)

Turbidity: Turbidity is a measure of the light-scattering properties of particles in the water — in other words, the water’s cloudiness. Particles can range from dust and dirt from nearby roads to algae and materials from vegetation in the land around a lake. Because variation in particulate size, shape, and distribution affect turbidity readings, its measurement is site- and depth-specific depending on local geology, soil conditions, land use, and river inputs. Therefore, it is difficult to make predictions about turbidity throughout the water column based on surface sensor measurements. (Units: NTU).

Glacial lakes are a classic and beautiful example of how turbidity affects lakes. Glacial lakes usually have extremely high turbidity because glaciers grind against rocks to release fine sediments, called glacial flour, into downstream lakes. The underlying geology and soils determine the size, shape, and distribution of particles, and this creates the many beautiful colors seen in glacial lakes

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