© by PSP Volume 17 – No 2. 2008 Fresenius Environmental Bulletin 164 PHOSPHORUS FRACTIONS IN SEDIMENT PROFILES OF THE EUTROPHIC LAKE MOGAN, TURKEY Akasya Topcu 1* and Serap Pulatsü 1 1 Ankara University, Faculty of Agriculture, Department of Aquaculture and Fisheries, 06110, Ankara, Turkey ABSTRACT This study was conducted between July 2004 and June 2005 to determine the seasonal and spatial patterns of phosphorus in the littoral sediment of Lake Mogan, and its potential release into the lake water. Total phosphorus and phosphorus fractions in the sediment and overlying water samples were taken mainly from a selected station in the lake, and additionally, some chemical and physical pa- rameters were measured. The amount of released phospho- rus was estimated using the total filterable orthophosphate concentrations related to the sediment pore as well as over- lying waters with the water content of the sediment, and the diffusion coefficient depending on the temperature. During the research period, sediment phosphorus re- lease fluctuated between 0.002-0.062 µg.m -2 d -1 reaching a minimum in November and a maximum in June. The esti- mated values of phosphorus release into the lake quantita- tively suggest that the effect of sediment over the trophic level of the lake was minimal. During the research period, total phosphorus (TP) concentrations of the sediment sam- ples varied between 286.00 µg g -1 DW and 892.25 µg g -1 DW, and the trophic level of the lake was found to be eu- trophic. The greater proportion of phosphorus fractions pre- venting phosphorus release into the lake was estimated to be organically-bound (Org≈P; 36%), and this fraction was followed by the calcium-bound (Ca≈P; 35%), iron+alumi- nium-bound (Fe+Al≈P; 16%) and carbonate-bound (CO 3 ≈P; 13%) phosphorus fractions, respectively. With regard to low-sediment phosphorus release into the lake, the iron+ aluminium-bound fraction (Fe+Al≈P) was effective because of the high iron content of the sediment (13450-44450 µg g -1 DW). During the research, another effectual compo- nent in the low-release of phosphorus was a high TFe/TP ratio (mean 45.33). High sediment clay content and macro- phyte presence at the station were the other effective fac- tors of low-sediment phosphorus release. In this study, the internal phosphorus load was esti- mated to be 43 kg y -1 for the lake`s area, and it was deter- mined that in order to protect the trophic level of the lake, a priority monitoring programme should be carried out, while phosphorus adsorption kinetics are examined. KEYWORDS: phosphorus release, sediment, phosphorus, phos- phorus fractions, iron, Lake Mogan, Turkey. INTRODUCTION In recent years, lake eutrophication has become a se- rious problem in shallow aquatic ecosystems. Therefore, the fundamental role of sediments must be considered in scien- tific examinations. Phosphorus may enter an aquatic system in particulate form, or dissolved P may become associated with particles settling out of the water column. Sedimenta- tion is a major P-sink for the epilimnia of lakes, transport- ing P to the hyoplimnion, and ultimately the sediments, the main components of limnological studies, giving informa- tion about a lake and its environment. Furthermore, they directly affect the trophic level of water-bodies by produc- ing an internal phosphorus load. In aquatic systems, rooted macrophytes are the crucial sources of nitrogen and phosphorus. Upon the macrophytes’ death, nutrients are released into the sediment; hence, they contribute to phosphorus accumulation [1-4]. To improve the water quality of eutrophic lakes, water management measures have often focused on the reduction of the ex- ternal phosphorus load. Additionally, in most studies, the internal phosphorus load exceeded the external one into the lake [5, 6]. In this context, studies concerning sediment should be taken into consideration in eutrophication re- search. TP concentration of the sediment is a poor measure of the potential phosphorus release from the sediments. Chemical phosphorus fractionation of sediments has served as a tool for predicting the phosphorus-binding capacity of sediments under different environmental conditions [7]. This process is more evident in shallow lakes than in deeper lakes. Mogan Lake was chosen in this study due to the fact that it has been heavily influenced by human actions rang- ing from agricultural and domestic to partially industrial pollution sources. Lake Mogan, a recreation area near the capital city of Turkey, was given legal environmental pro- tection status in 1990 by a decree of the Turkish Cabinet