This work presents empirical data from several databases. Empirical data ultimately form the basis for most ecological/environmental studies, and this work uses what may be the most comprehensive data set ever compiled concerning relationships between chlorophyll-a concentrations, nitrogen, phosphorus, and salinity, since the data in this study concerns about 500 lakes and coastal areas. The focus has been on how variations between systems in median summer values of chlorophyll-a concentrations (Chl) depend on variations in total nitrogen, total phosphorus, and salinity. The salinities range from 0 to 275 PSS78 in hypersaline systems; the median salinity is 12.5. The range in nutrients is from ultraoligotrophic (totalP, or TP, is less than 1 μg/L) to hypertrophic systems (TP is greater than 1000 μg/L). The chlorophyll values vary from 0.12 to 60 μg/L. Results showed that there is a minimum in the Chl/TP ratio in the salinity range between 2 and 5, followed by an increase up to a salinity range of 10–15, and then a continuous reduction in the Chl/TP ratio until a minimum value of about 0.012 is reached in hypersaline systems. We have also identified a threshold value for the salinity. In systems dominated by freshwater influences, the ratio of Chl to total nitrogen, or TN, increases steadily from about 0.008 in lakes to 0.02 when the salinity is 10. At higher salinities than 10, the Chl/TN ratio decreases in a nonlinear fashion. We have also presented several empirical models to predict chlorophyll concentrations from levels of TP, TN, and salinity. The model predicting Chl from TP and salinity is generally the best one. The next best model according to the criteria (r² value, median error, and standard deviation for the error) is the model using TN and salinity. Models not including salinity predict chlorophyll less well.