Growth limitation status and its role in interpreting chlorophyll a response in large and shallow lakes: A case study in Lake Okeechobee

Understanding the response of Chlorophyll a concentration (Chl-a, an indicator of phytoplankton biomass) to environmental factors is critical for eutrophication management. Light and nutrients often act as two main limiting environmental factors in large and shallow lakes. However, the limitation status is usually not considered explicitly when building empirical relationships even though the growth limitation is the possible mechanism controlling the behaviors of these relationships. Here we chose a typical large and shallow eutrophic lake (Lake Okeechobee) to study the response of Chl-a concentration under different growth limitation conditions. Using an existed decision tree model followed by Carlson's trophic state index, monitoring data from 1994 to 2020 were classified into light-limitation, nitrogen-limitation, or phosphorus-limitation. The spatio-temporal patterns of limitation status were revealed. By subdivision of observations according to these growth limitation classes, our results demonstrated three main findings. First, algae responded differently between light limitation and nutrient limitation. Chl-a concentrations were lower with smaller variability when light was limiting than those when nutrient was limiting. In addition, the evolution of Chl-a in enduring nutrient limitation events were more dynamic. Second, limitation-specific regressions provided a more straightforward interpretation compared with those without consideration of limitation status. Chl-a ∼ nutrient relationship based on limitation classification displayed a higher R² with a positive slope. This positive slope indicates the sensitivity of Chl-a to that specific nutrient. Moreover, response of Chl-a to phosphorus was successfully detected by identifying P-limited samples. Otherwise, the Chl-a ∼ TP response would be muted since nitrogen is the main limiting nutrient in Lake Okeechobee. Third, a spatial heterogeneity of Chl-a ∼ TN relationship was revealed by Bayesian hierarchical modelling. This indicates the necessity of focusing more on hot spots where Chl-a displays a higher sensitivity to increase of nutrient. Our findings demonstrate the advantage of developing the limitation-specific and zone-specific relationships between algal biomass and environmental factors.