Turbidity is the amount of cloudiness in the water. Turbidity can vary from a river full of mud and silt where it would be impossible to see through the water (high turbidity), to a spring which appears to be completely clear (low turbidity).
Lake Pontchartrain typically has low turbidity, but wave action increases the amount of sand and lake-bottom material, which can dramatically increase turbidity levels in the lake. Mississippi River water has high turbidity levels due to sand, silt and clay particles carried by the current.
Too much algae in water can also affect water clarity. Water clarity is measured by the sensor as turbidity. USGS defines turbidity as “an optical characteristic of water and is a measurement of the amount of light that is scattered by material in the water when a light is shined through the water sample. The higher the intensity of scattered light, the higher the turbidity.” Turbidity is affected by any type of suspended material in water. High turbidity is often caused by suspended sediment: sand, silt and clay. However, it can also be caused by an abundance of floating algae. High turbidity can be harmful to aquatic organisms as it can block sunlight from important aquatic plants, affect the ability of fish to absorb dissolved oxygen through their gills, and affect predators’ ability to find prey in otherwise clear water systems. High turbidity can also affect humans by increasing the risk of bacterial or viral attachment to suspended particles that may be ingested in drinking water or during swimming. It also can make a lake look “dirty”. The chart below depicts turbidity levels on August 25, 2021 when turbidity appeared to rise and fall with the rise and fall of algae concentrations throughout the day. This would seem to indicate that water clarity was being impacted by an algal bloom in the water column of Lake Pontchartrain.
Water sensor measurements at the Lighthouse were also continuously made both before and after the landfall of Hurricane Ida on August 29, 2021:
- Turbidity levels were already rising early on the morning of August 29, likely due to onshore winds and waves from Ida that were already beginning to impact Lake Pontchartrain. Wind and waves will often cause an increase in water turbidity as bottom sediments are violently picked up and mixed into the water column. This pattern continued, with three separate peaks in turbidity observed (likely due to the arrival of 3 separate severe wind squalls) until the evening of August 30 when turbidity levels began to decline.
Hurricane Ida’s effects on suspended algae were also evident. Algae measurements were somewhat low during and immediately following the passage of Ida, but began to spike again on September 2, perhaps due to calmer conditions, enhanced nutrient availability after the storm, and a return of prolonged sunlight. A notable difference in causes of turbidity spikes is also evident in the chart below: spikes in turbidity during the late August storm are associated with suspended lake bottom sediment stirred up by wind and waves while the spike observed on September 2 may be more closely associated with the 7-day peak in chlorophyll (algae) concentrations on the same day.