Trophic status index discrepancies as a tool for improving lake management: insights from 160 Polish lakes

The Trophic State Index (TSI) proposed by Carlson is widely used to assess the trophic status of lakes, however, it has rarely been used to interpret environmental conditions and provide management decisions by examining discrepancies between partial indicators. Comprehensive studies were conducted on 160 Polish lakes (231 investigations including repetitions) spanning a wide range of trophic conditions during the summer season. Eutrophic conditions dominated (56 %), followed by mesotrophic (21 %), hypertrophic (16 %), and oligotrophic (7 %). Polymictic lakes generally exhibited higher trophic levels than dimictic lakes. The chlorophyll index was strongly correlated with the Secchi disk index, whereas the phosphorus index showed weak correlations with both of them. The phosphorus index mostly overestimated the trophic status of temperate lowland lakes, with an average of 16.3 ± 15.7 units and, in many cases, exceeding 30 units. Nevertheless, we demonstrate that TSI discrepancies may provide crucial insights into lake-specific environmental dynamics, allowing for tailored management strategies - either top-down (biomanipulation) or bottom-up (nutrient reduction). Top-down approaches should be prioritized in dimictic lakes with lower trophic status or lakes with potentially higher zooplankton grazing, complemented by bottom-up measures to further reduce nutrient levels. Bottom-up strategies are more crucial in effectively managing dimictic lakes with higher trophic status and polymictic lakes. This study highlights the utility of TSI in providing deeper insights into the ecological status of lakes and proposes practical guidelines for effective lake management and biomanipulation techniques.