Interactions between sediment resuspension and sediment quality decrease the TN : TP ratio in a shallow lake

The relative contributions of sediment resuspension and external nutrient loading to seasonal fluctuations of the total nitrogen to total phosphorus (TN : TP) ratio in the water column of the shallow Kirkkojärvi basin were studied. In May, the TN : TP mass ratio was above 30, but it decreased to below 10 in July–August. In October, the ratio increased again. Water quality was regulated by external loading in spring, but the sharp decrease in the TN : TP ratio in July–August resulted mainly from the elevated sediment resuspension rate. In May and October, the resuspension rate was below 20 g dry weight (wt) m22 d21, while in July–August it exceeded 60 g dry wt m22 d21. Sediment resuspension decreases the TN : TP ratio in the water because the ratio in the surface sediment is low. The high resuspension rate in July–August could not be explained by wind effects, water level fluctuations, or bioturbation, but was instead attributed to the seasonal change in the sediment quality. During the summer, fresh material having low critical shear stress settled on the bottom until the critical shear stress was lower than the actual shear stress, and the resuspension rate increased steeply, although no increment in wave effects occurred. For a shallow lake, a single representative TN : TP ratio is misleading, because the seasonally fluctuating resuspension rate substantially affects the ratio and may induce switches between N and P limitation. Seasonal fluctuations in the resuspension rate may have important effects on algal communities, because low N to P ratios can favor N-fixing cyanobacteria over other algal groups. In Kirkkojärvi, cyanobacteria became the dominant algal group during the period of intensive resuspension. The bottom sediments of lakes contain large pools of nutrients. For numerous reasons, these nutrients stored in the sediment may return to the water column and cause internal nutrient loading, with consequences for the water quality (Forsberg 1989; Søndergaard et al. 2003). Factors causing internal nutrient loading include low redox potential, elevated pH, and sediment resuspension caused by water currents and animal activities (Forsberg 1989). In shallow lakes, wind-induced sediment resuspension is often the main cause of internal loading (Kristensen et al. 1992; Niemistö and Horppila 2007). Most resuspension studies have focused on phosphorus dynamics, because phosphorus has been regarded as the factor that most often limits algal growth in lakes, but the importance of resuspension on nitrogen recycling has also been documented (Gálvez and Niell 1992; Reddy et al. 1996). Moreover, sediment resuspension tends to decrease the total nitrogen to total phosphorus (TN : TP) ratio in the water, because the ratio is usually lower in the surface sediment than in the water, which is again attributed to denitrification (Kaspar 1985; Hamilton and Mitchell 1988; Gálvez and Niell 1992). For instance, Schelske et al. (1995) reported that during a period of wind-induced intensive sediment resuspension, the total phosphorus concentration in the water column of a shallow lake increased twofold, but the total nitrogen concentration increased by only one fifth. Sediment resuspension may therefore induce switches between phosphorus and nitrogen limitation (Hamilton and Mitchell 1997), which may have important implications for algal communities, because low nitrogen to phosphorus ratios can favor nitrogen-fixing cyanobacteria in relation to other algal groups (Smith 1983; Levine and Schindler 1999). Resuspension may also bring sedimented phytoplankton (meroplankton) back into the water column (Hamilton and Mitchell 1988; Carrick et al. 1993). The effect of resuspension on the TN : TP ratio has been linked to the strength of the resuspension events, and it has been shown that the TN : TP ratio in the water is negatively dependent on wind-induced wave action and bottom shear stress (Hamilton and Mitchell 1988, 1997). Hence, the TN : TP ratio in the water could fluctuate according to weather conditions. Another, more sparsely studied aspect is that the effects of resuspension on water quality depend not only on the bottom shear stress but also on the sediment characteristics, which may change during the growing season. Over the course of the growing season, organic material is deposited on the sediment surface, and this newly deposited material is brought into suspension more easily than is old compacted material (Bengtsson and Hellström 1992; Weyhenmeyer 1998). If the sediment becomes increasingly resuspension-prone during the summer, it can be hypothesized that the effects of resuspension on TN : TP ratio should be more pronounced in late summer than in the beginning of the growing season. In order to study whether this hypothesis is valid, we examined the seasonal variations in the sediment quality, the rate of sediment resuspension, and the effects of resuspension on nutrient dynamics in a shallow eutrophic 1 Corresponding author ( [email protected]).