Trophic State Indexes and Phytoplankton in the Trophic Status Assessment of Waters of a Small Retention Reservoir at an Early Stage of Operation

This paper will focus on determining the trophic status of the two basins of the Niwa Reservoir – Niwa I and Niwa II (Sawin commune, Lubelskie Province, SE Poland) at the sixth year of operation, by using partial TSI indexes – TSI(SD), TSI(TP), TSI(chl-a), as well as qualitative and quantitative structures of phytoplankton. As a supplementary, index TSI(TOC) was employed. We show that the phytoplankton abundance, as well as TSI(chl-a) and TSI(TP) were relatively low and typical for mesotrophic waters (with the exception of July TSI(TP) values for the Niwa II basin), although the evaluation of trophy based on other partial TSI indexes – TSI(SD), TSI(TOC) pointed to a eutrophic or even a hypertrophic water characteristic. The obtained results were discussed in detail and referenced to other, non-nutrient related trophy factors: morphometric features and geology of the basins, as well as the physicochemical properties of its water.