Dig into the different parameters that affect lake health.
The level of oxygen gas dissolved in water (DO) is vital for the survival of most aquatic organisms. It is therefore an important indicator of the water quality in the lagoon. The concentrations can range from 0 to 15mg/L depending on the water temperature, salinity and atmospheric pressure. For this reason, when comparing water quality of different samples, percent saturation is generally used instead of mg/L.
Excellent DO levels are between 90% and 100%; adequate levels between 80% and 89%; acceptable between 60% and 79%; and poor levels percent saturation of DO <60%. Water can be supersaturated for short periods of time, i.e., percent saturation of DO ≥101%.
Oxygen is dissolved into the waters of the lagoon through contact with the air, where it will remain near 100% saturation. 100% air saturation means that the water is holding as many dissolved gas molecules as it can in equilibrium with the atmosphere. DO also enters the lagoon as a byproduct of photosynthesis by submersed plants and algae. DO from this latter source are the cause of supersaturation in surface waters and can be seen as an indicator of excess growths.
The concentration of DO in the water therefore typically peaks mid-day and can be measured at its lowest concentration just before sunrise. Fish kills resulting from low DO routinely occur in the early morning hours and when sufficient data is collected, they are often predictable and sometimes avoidable.
Water temperature can be a catalyst for many internal lake processes. It is one of the primary drivers of the growth of plants and algae and is also directly correlated to the depletion of oxygen. DO has an inverse relationship with water temperature, so high temperatures can result in dangerously low concentrations of DO. Fish kills typically occur when water it at its highest seasonal temperatures and DO is at its lowest.
The trend in the temperature of surface waters typically mimics that of ambient air temperatures, but lags behind by several days or even weeks. Collecting sufficient data on the relationship between these temperatures can reveal the unique hydrologic phenomenon within the lake and provide an evaluation tool of flushing and circulation tactics being employed.
pH stands for the “power of hydrogen.” It is a numerical value of the molar concentration of hydrogen ions (H+) ³ in the water. It is a logarithmic scale ranging from 1 to 14. The lower the number in the range, the more acidic and higher the number, the more basic. A pH of 7 is considered neutral. A logarithmic scale means that each number below neutral (7) is 10 times more acidic than the previous number, and when counting up above neutral, each number is 10 times more basic than the previous number. In other words, a pH of 5 is 100x more acidic than neutral (7).
Similar to dissolved oxygen, pH will fluctuate throughout the day. Typically, pH reaches its highest basicity in mid-day and its most acidic just prior to sunrise. This is primarily driven by the photosynthesis of plants and algae which consume carbon dioxide from the water, reducing the formation of carbonic acid, and driving pH up.
Conductivity is a measure of a water’s ability to pass an electric current. The measure indicates the concentration of dissolved ions in the water – sometimes referred to as electrolytes. These ions come from dissolved salts like chloride, sodium, magnesium, sulfate, calcium, potassium, bicarbonate, bromine and other compounds – collectively referred to as salinity. The measure of salinity is therefore derived from conductivity and not a direct measurement of the concentration of salts in the water.
Salinity fluctuates significantly in response to the rainy season, freshwater flows from the rivers, streams, and even from groundwater. Salinity plays a very important role in ecosystem health and can be a driver for aquatic life. Salinity can also impact equipment and infrastructure significantly and it should be monitored to understand its impacts a lake's characteristics. Understanding how salinity correlates with algae and plant growth, and then understanding how changing trends may impact management and operations can be a useful metric.
Total Suspended Solids (TSS) is the direct measure of the amount of particulate matter drifting in the water. It includes sediment of many types, decaying material, living plankton and algae. These drifting particles scatter light in the water and can impart a discoloration (grey, brown, green, hazy, etc). Turbidity is the optical measurement of the water’s clarity and is closely related to TSS but not a direct measure for it. Turbid water is not necessarily unhealthy water, rather, turbidity can be a useful metric to identify trends or to explain or describe phenomenon in a lake. For example, high turbidity will likely reduce the amount of sunlight penetration through the water column thereby reducing submersed plant and algae growth.
Chlorophyll-a is the green pigment found in all photosynthesizing plants, algae and phytoplankton; it is the main photosynthetic molecule responsible for absorbing wavelengths of sunlight and converting them into energy. Measurement of chlorophyll-a in a waterbody is used to quantify planktonic algal biomass. It can be measured directly by lab analysis which reports results in a concentration such as micrograms per liter (µg/L) or it can be measured in-situ via a fluorometer sensor which reports in Relative Fluorescence Units (RFUs). This latter unit can be correlated with water sample concentrations if required. For this reason, sensors are often used to measure trends and relative abundance for management purposes but not for regulatory reporting or evaluation of management tactics.
Although algae are a natural part of aquatic ecosystems, too much algae can cause discoloration and other aesthetic problems such as scums and bad odors. When algal blooms occur, they can result in dangerous fluctuations in the concentration of dissolved oxygen which can lead to fish-kills