Phosphorus dynamics and submerged aquatic macrophytes in hell kettles
A study was made of the phosphorus ecology of Hell Kettle ponds, a Site of Special Scientific Interest in County Durham, UK, in order to help establish the causes of the temporary "whitening" of Chara hispida in summer 1996. Chara hispida, the most abundant organism in Croft Kettle, one of the two small calcareous ponds comprising Hell Kettles, was first reported in 1777 and since then its presence has been accounted many times, perhaps being the longest continuous record for a freshwater algal species anywhere in the world. The aims were to assess the concentration and variability of aqueous N and P as well as a number of other physical and chemical variables on spatial and temporal scales and the response of Chara hispida to these changes. Studies focused on Croft Kettle and key aspects included analysis of water chemistry, P sequential fractionation of sediments, tissue N and P contents and surface phosphatase activities of Chara hispida. This involved monthly surveys of surface as well as depth profiles of water chemistry during 1999-2001 and many other visits. Data was also collected from Double Kettle and the farm-borehole (representing groundwater) for comparative means. Croft Kettle was stratified in summer (approximately May to October) with severe deoxygenation in the hypolimnion. Aqueous N and P concentrations were about 150 \ig L"' TN and 15 fig L"' TP respectively, but showed high within-year and intra-annual variability. Episodic events and autumnal turnover caused only short-term increase in aqueous P concentrations and co-precipitation with CaCO] was suspected. Depth profile studies of sediments (0-35 cm) for TN and TP content and N:P ratio suggest historical changes in N and P dynamics. Seasonal changes as well as a high range of tissue N and P contents were observed in C. hispida apical tips during this survey, possibly indicating that C. hispida is capable of rapid nutrient uptake and storage. Supportive evidence for this hypothesis arose from incubations of C. hispida under a series of aqueous P concentrations as well as the seasonal study on phosphatase activity. C. hispida apical tips collected from Double Kettle had on average higher tissue N and P contents than those collected from Croft Kettle, probably corresponding to the higher nutrient content of Double Kettle's water. Possible reasons for aquatic vegetation changes as well as the "whitening" of C. hispida during summer 1996 are discussed. Unusually warm summer along with wind- protected shores due to the dense reed vegetation may have resulted to reduced water mixing, light penetration and increased deoxygenation in the hypolimnion and thus stress on C. hispida. However, there is no indication of a long-term destabilization of the system indicating that the event of 1996 was only a "temporary instability".