Relationships between nutrient, chlorophyll a and Secchi depth in lakes of the Chinese Eastern Plains ecoregion: Implications for eutrophication management.

Eutrophication and associated algal blooms are principal environmental challenges confronting lakes in China, particularly in the Eastern Plains ecoregion. The empirical relationships between nutrient and chlorophyll a (Chla) level and Secchi depth (SD) are widely used as a theoretical basis for lake eutrophication management. However, these relationships are largely influenced by hydromorphological conditions and biogeochemical processes. Thus, there is a need to establish a type-specific understanding of these interactions. In the current study, lakes in the Chinese Eastern Plains ecoregion were subdivided into four lake types according to water retention time (LRT), water depth, and water area. Regression analyses indicated that the impacts of nutrient (total nitrogen, TN; total phosphorus, TP) concentrations on summer Chla were significantly reduced in lakes with high inorganic suspended solids (ISS) (P<0.05). Meanwhile, the decrease in SD in these lakes were found to relate mainly to non-algal turbidity. In lakes characterized by both short LRT and high ISS content, the Chla exhibited limited response to nutrients. In contrast, in lakes with low ISS content and long LRT, the observed slopes of both Chla=f(TP) and SD=f(Chla) were significantly steeper (P < 0.05). The factors limiting summer algal growth and the development of type-specific nutrient criteria (TN and TP) of all four investigated lake types in the Eastern Plains ecoregion are discussed in the context of specific nutrients. Based on these results, we establish type-specific eutrophication assessment equations of TN, TP, Chla, and SD in our study lakes. Our results may provide essential information for achieving the cost-effective eutrophication management of lakes both in the Eastern Plains ecoregion and elsewhere with similar climatic and hydromorphological conditions. Moreover, we believe that the subdivision of lakes to allow type-specific eutrophication management framework may prove valuable for other ecoregions where the interpretation of empirical nutrient-Chla and SD relationships suffer from similar serious limitations.