Soft water lakes with isoetids (SLI) are ecosystems prone to degradation due to the low buffer capacity of their waters. One of the main threats resulting from human impact is eutrophication due to agriculture, catchment urbanization and recreational use. In this paper, changes in the water chemistry and transformation of biocoenoses of one of the largest Polish SLI, Lake Jeleń, over the past 30 years are presented. The lake is located within the borders of a city, and a significant part of its catchment is under agriculture and recreation use. The physicochemical (concentration of nutrients, organic matter, electrical conductivity, oxygen saturation and water pH) and biological parameters (macrophytes and phytoplankton) were measured in summer 1991, 2004, 2013 and 2018. Since the beginning of the 1990s, a gradual increase in the trophy of the lake has been observed as indicated by increased nutrient availability, deterioration of oxygen conditions and a decrease in water transparency. The alterations of water chemistry induce biological transformations, in particular, an increase in phytoplankton abundance (4-fold increase of biomass in epilimnion) as well as a gradual reduction in the range of the phytolittoral (from 10 to 6 m), a decrease in the frequency of isoetids, Lobelia dortmanna and Isoetes lacustris, and expansion of plant species characteristic for eutrophy.

Relationship between areal hypolimnetic oxygen depletion rate and the trophic state of five lakes in northern Poland: The oxygen content in a lake is a fundamental factor in lake ecology. In stratified lakes, deep waters are isolated from the atmosphere for several months during the summer; therefore, oxygen (substantially consumed by biological and chemical processes at this time) cannot be replaced before the autumnal mixing period. Hypolimnetic oxygen depletion has been considered an indicator of lake productivity since the early twentieth century. Many recent studies have been in opposition to this view by showing that the areal hypolimnetic oxygen depletion rate (AHOD) is poorly correlated with seston biomass and/or phosphorus concentration. The objective of this study is to show relationships between the mean values of total phosphorus (TP), total nitrogen (TN), chlorophyll a, and water transparency (Secchi disk depth, SDD) during the thermal stratification formation period and the AHOD rate. Hypolimnetic oxygen conditions in five dimictic lakes in northern Poland were examined in 2009 and 2010. Two of them were studied in the previous year. Monthly oxygen profiles taken from April to August, midsummer temperature profiles, and morphological data of the lakes were used to determine the AHOD rate. Standard water quality parameters such as concentrations of chlorophyll a, TP, and TN, as well as water transparency measured at the same time were used to calculate the trophic state indices (TSI) according to the Carlson-type formulas. On the basis of the collected data it is shown that AHOD is highly correlated with the TSI value for chlorophyll a, and poorly correlated with the TSI values for water transparency and phosphorus content. The best correlation between AHOD and TSI has been found for chlorophyll a (r²=0.702; p<0.001), as well as for overall TSI, determined by averaging separate component indices (r²=0.826; p<0.000). No correlation was found between AHOD and total nitrogen concentration. The research also confirmed previous observations, which pointed to a significant role of the hypolimnion depth in increasing oxygen deficits.