Nutrient fluxes induced by disturbance in Meiliang Bay of Lake Taihu

Abstract

A wave flume experiment was conducted to study nutrient fluxes at water-sediment interface of Meiliang Bay under different hydrodynamic conditions. The results reveal that hydrodynamics has remarkable effects on nutrient fluxes in this area. With a bottom wave stress of 0.019 N m⁻² (equivalent to disturbance caused by wind SE 5–7 m s⁻¹ at the sediment sample site of Meiliang Bay), the fluxes of TN, TDN and NH₄ ⁺-N were separately 1.92 × 10⁻³, −1.81 × 10⁻⁴ and 5.28 × 10⁻⁴ mg m⁻² s⁻¹ (positive for upward and negative for downward), but for TP, TDP and SRP, the fluxes were 5.69 × 10⁻⁴, 1.68 × 10⁻⁴ and −1.29 × 10⁻⁴ mg m⁻² s⁻¹. In order to calculate the released amount of nutrients based on these results, statistic analysis on the long-term meteorological data was conducted. The result shows that the maximum lasting time for wind SE 5–7 m s⁻¹ in this area is about 15 h in summer. Further calculation shows that 111 t TN, 32 t NH₄ ⁺-N, 34 t TP and 10 t TDP can be released into water (the sediment area was 47.45% of the whole surface area), resulting in concentration increase of 0.025, 0.007, 0.007 and 0.002 mg L⁻¹ separately. With stronger disturbance (bottom wave stress is 0.217 N m⁻² which is equivalent to disturbance caused by wind SE 10–11 m s⁻¹ at the same site), there has been significant increase of nutrient fluxes (1.16 × 10⁻², 6.76 × 10⁻³, 1.14 × 10⁻² and 2.14 × 10⁻³ mg m⁻² s⁻¹ for TN, DTN and NH₄ ⁺-N and TP). The exceptions were TDP with flux having a decrease (measured to be 9.54 × 10⁻⁵ mg m⁻² s⁻¹) and SRP with flux having a small increase (measured to be 5.42 × 10⁻⁵ mg m⁻² s⁻¹). The same statistic analysis on meteorological data reveal that the maximum lasting time for wind SE 10–11 m s⁻¹ is no more than 5 h. Based on the nutrient fluxes and the wind lasting-time, similar calculations were also made suggesting that 232 t TN, 134.9 t TDN, 228 t NH₄ ⁺-N, 42.7 t TP, 2.0 t TDP and 1.1 t SRP will be released from sediment at this hydrodynamic condition resulting in the concentration increases of 0.050, 0.029, 0.049, 0.009, 0.0004 and 0.0002 mg L⁻¹. Therefore in shallow lakes, surface disturbance can lead to significant increase of nutrient concentrations although some components in water column had negative flux with weak disturbance (e.g. TDN and SRP in this experiment). In this case, sediment looks to be a source of nutrients. These nutrients deposited in sediment can be carried or released into water with sediment resuspension or changes of environmental conditions at water-sediment interface, which can have great effects on aquatic ecosystem and is also the characteristics of shallow lakes.