Seasonal variability in phosphorus concentrations in strongly eutrophic Lake Chełmżyńskie
The results of the monthly examinations of Lake Chełmżyńskie carried out during the vegetation periods in the years 2007-2009 made it possible to define seasonal variability in the concentrations of phosphorus compounds. The examinations showed that the bottom deposits constitute a considerable source of phosphates. The release of phosphates from the deposits began as early as in mid May and with the occurrence of anoxic conditions. A particularly high concentration of phosphates in the near-bottom layer was recorded in the southeast pool of the lake (max. 0.658 mg P l-1). The central part of the lake showed a lower concentration of phosphates near the bottom, yet due to the partial thermal stratification of its waters, it was the main place for the transport of the bio-element to the euphotic zone. The intense transport of phosphates occurred mainly during the short periods of strong winds. The analysis of the meteorological data obtained from the Integrated Natural Environment Monitoring - Station Koniczynka prove that winds, whose mean daily velocity values exceeded 8.0 m s-1, brought about particularly intense and deep water mixing. A very significant increase in the concentration of phosphorus compounds in the epilimnion was recorded in the years 2007 and 2008, and it resulted in the violent development of phytoplankton. The results of the examinations conducted in the years 2007-2009 showed that Lake Chełmżyńskie was classified as a strongly eutrophic lake. The ecological condition of the lake was defined as poor according to the Water Framework Directive. Reclamation works including the inactivation of phosphates in the bottom deposits are indispensable in order to improve the ecological and trophic state of the lake.
Dariusz Borowiak, Kamil Nowiński, Jacek Barańczuk, Włodzimierz Marszelewski, Rajmund Skowron and Adam Solarczyk
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.