Water Quality Parameter Relationships

In the RMB Environmental Laboratories Lakes Monitoring Program, volunteers collect three water quality parameters: Total Phosphorus, Chlorophyll-a, and Secchi disk depth. These three parameters are inter-related, and as a whole produce a comprehensive picture of lake water quality. As Total Phosphorus and Chlorophyll-a concentrations increase, Secchi disk depth decreases (Figure 1). These three parameters are also collectively used to determine Mean Trophic State Index.

Total phosphorus (TP) is a "cause" parameter, while chlorophyll-a and Secchi depth are "effect" parameters.  When TP increases, that means there is more food available for algae, so algal concentrations increase.  When algal concentrations increase, the water becomes less transparent (cloudier) and the Secchi depth decreases.  By measuring each of the three parameters, we can better pinpoint the source of water quality improvement or decline (Figure 2). 

Water Quality Improvement

If over time there is a decrease in TP, it should eventually result in a decrease in chlorophyll-a and an increase in Secchi depth.  If there is an increase in Secchi depth, but no change in the other parameters, it could indicate a decrease in erosion in the watershed.  Stabilizing streambanks and lakeshore decreases the amount of particles entering the water, making it less cloudy.

Water Quality Decline

If there is an increase in TP over time, but no change in chlorophyll-a and Secchi depth, it could indicate that excess phosphorus is entering the lake.  If this TP increase continues, chlorophyll-a will increase and Secchi depth will decrease.  If there is a decrease in Secchi depth but no change in TP and chlorophyll-a this could indicate that erosion is occurring along the shoreline of the lake or the streambank of an inlet (more on Best Management Practices).