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Phytoplankton Metric of Ecological Status Assessment for Polish Lakes and Its Performance along Nutrient Gradients

ABSTRACT: 
 This paper outlines the Phytoplankton Multimetric for Polish Lakes (PMPL) — an ecological status assessment method which can be used to implement the European Water Framework Directive (EC, 2000). The PMPL includes abundance parameters of phytoplankton: the metrics: “chlorophyll a”, “total biomass” while the taxonomic composition is partly evaluated by the metric “biomass of cyanobacteria”. All of these three single metrics as well as the final PMPL index values are the same scale and range from 0 to 5. The PMPL can be transformed to a normalized Ecological Quality Ratio (EQR) range from 0 (the worst status) to 1 (the best status). The PMPL method differentiates “stratified” and “unstratified” lake types as well as subdivisions into lakes characterized by a high (>2) or low (<2) lake volume-to-catchment area ratio (VQ). The metrics used and the PMPL index respond to eutrophication pressure expressed by total phosphorus and total nitrogen concentration, the significance of the relationships depends on the tested parameter and the abiotic type of lake. From among the single metrics, the metric “chlorophyll a” was usually found to reveal the strongest significant correlation to both total phosphorus and total nitrogen (Spearman`s coefficient varied from -0.41 to -0.75 and from -0.34 to -0.56, respectively). The weakest correlation with nutrients was noted for metric “biomass of cyanobacteria” (R=-0.13 to -0.62). The PMPL correlated best with TP and TN in unstratified lakes (R=-0.49 to -0.70). The testing of the PMPL index shows a statistically important distinction between Good/Moderate ecological status and confirmed designated boundaries of High/Good and Good/Moderate states.