J. Limnol., 70(1): 3-10, 2011 DOI: 10.3274/JL11-70-1-02 Water pH and sediment resuspension regulating internal phosphorus loading in a shallow lake – field experiment on diurnal variation Juha NIEMISTÖ*, Heidi HOLMROOS and Jukka HORPPILA Department of Environmental Sciences, Viikinkaari 1, P.O. Box 65, 00014 University of Helsinki, Finland *e-mail corresponding author: juha.niemisto@helsinki.fi ABSTRACT Resuspension-mediated aerobic release of P can be crucial to the phosphorus cycling of a shallow lake. The effect of resuspension on the aerobic release of phosphorus was studied in the field with experimental water columns. The study included four experiments that were conducted in May and August during the day and following night in the shallow Kirkkojärvi basin of Lake Hiidenvesi. In all the experiments, the concentrations of suspended solids (SS) and total phosphorus (TP) increased substantially, due to the 10-min resuspension treatment. However, the concentration of soluble reactive phosphorus (SRP) was strongly affected only at night in August, when high pH levels promoted SRP release by ligand-exchange reactions. During the day experiment in August, no SRP release was detected, due to high assimilation by phytoplankton. The results pointed out that coupling of resuspension and high algal blooms induced pH in the water column can liberate significant amounts of soluble P into the water column. The importance of this phenomenon for the lake studied was emphasized by the fact that the P pools susceptible to pH- dependent P release (Al-P and Fe-P) formed a large part of the total extractable P of the surface sediment. Additionally, the conditions of high pH are common from early July to late September and the water pH can stay at a high level also at night during strong phytoplankton blooms. Key words: resuspension, internal loading, aerobic phosphorus release, experimental approach 1. INTRODUCTION In the majority of lakes, the phosphorus (P) burial or release is controlled by a complex coupling of sediment composition, external load, catchment hydrology, lake morphometry, and biogeochemical reactions (Hupfer & Lewandowski 2008). Periodically, some of these factors can take over others. In shallow lakes, the water body is well mixed and the surface sediment oxidized due to active surface wave action (Søndergaard et al. 2003). In addition to oxidized conditions, frequent surface wave action also leads to increased sediment resuspension, which can markedly contribute to internal P loading of a shallow lake (Kristensen et al. 1992; Niemistö & Horppila 2007). Although, beneath the aerobic sedi- ment-water interface, anoxic conditions can govern P dynamics, resuspension-induced oxidized conditions play an important role in the P dynamics in the sedi- ment-water interface and water column. The importance of sediment-water interactions in shallow lakes is fur- ther enhanced by the high sediment surface:water col- umn ratio, which implies that the potential influence of resuspension on lake water concentrations is stronger than in deeper lakes (Søndergaard et al. 2003). In a shallow lake, the aerobic release of P via ligand- exchange reactions can be substantial, because resus- pension transports sediment P bound to Fe and Al oxides to the water layers where pH is elevated due to primary production (Hingston et al. 1967; Golterman 1976; Lijklema 1980; Koski-Vähälä & Hartikainen 2001; Søndergaard et al. 2003). In a previous study, the pH-dependent P release under oxic conditions was sig- nificant in the shallow Kirkkojärvi basin (Holmroos et al. 2009). The release was most pronounced in late summer due to coupling of sediment resuspension and high water pH induced by strong phytoplankton blooms. The seasonal variation in pH-dependent P release was also previously observed (Holmroos et al. 2009). How- ever, the diurnal variation of aerobic P release was not studied and experiments conducted on this topic in the field are scarce. The present study was conducted to illuminate the possible diurnal alternation of aerobic P release during the different phases of the growing sea- son. It was hypothesized that the P release would be most pronounced in daytime in late summer during high algal blooms when the water pH is high. 2. MATERIAL AND METHODS 2.1. Study area The study was conducted in the shallow (mean depth 1.1 m, max. depth 3.5 m) Kirkkojärvi basin of Lake Hiidenvesi (area 30.3 km 2 ), which is situated in south- ern Finland (60°24'N, 24°18'E) (Fig. 1). The restricted Kirkkojärvi basin (area 1.6 km 2 ) is the most eutrophic basin of Lake Hiidenvesi due to agricultural actions in the catchment. The summertime total phosphorus (TP) concentration varies between 80 and 120 μg L -1 and total nitrogen (TN) concentration between 1000 and 1500 μg L -1 . Due to the shallowness of the lake, sedi- ment resuspension is often high, constituting 85-95% of