The phosphorus-chlorophyll relationship in lakes:: Potential influences of color and mixing regime

We used summer epilimnetic means from a large dataset (369 lakes from North America, Europe, Asia, and New Zealand) to examine whether color or mixing regime significantly influence the log-log relationship between chlorophyll a (Chl a) and total phosphorus (TP) in lakes. We found no significant difference in regression models for clear (color < 10 Pt units) vs. humic takes (color > 20 Pt units), even when data were screened so that both types of lakes were represented by the same range of TP. Likewise, there was no significant difference in regression models for mixed vs. thermally stratified lakes. Knowing that a particular lake is clear vs. humic, or mixed vs. stratified, therefore is not helpful in developing a predictive model of Chl a from TP. However, when we simultaneously considered water color and mixing regime, a potentially useful feature was identified. Mixed humic lakes have a somewhat lower (significant at p = 0.10) ratio of Chl a / TP than mixed clear water lakes, whereas no such difference exists between humic and clear water lakes that are thermally stratified. Likewise, when we plotted the Chl a / TP ratio vs. color, there was a negative slope for mixed takes, but no significant relationship for stratified lakes. We suggest that in stratified humic lakes, which are often sheltered deep systems with a high Osgood Ratio, phytoplankton can compensate for reduced underwater irradiance by migrating toward the water surface, whereas in mixed humic lakes, that response is not possible. Lake managers dealing with mixed humic lakes might expect a lower yield of Chl a / TP than in mixed clear water lakes. Further research is needed to test this hypothesis. In any case, caution is warranted when applying such results to lake management, because given the numerous effects of high P inputs (e.g., changes in benthic invertebrate communities, biological oxygen demand, and sediment metabolism) it is unwise to focus solely on the Chl a response to protect water quality.