Whitemere, a lake that defies some conventions about nutrients

Abstract

1. Whitemere, a kettle-hole lake in north-west England (z(m),14 m, area 22.5 ha) has extremely high maximal concentrations (around 1 mg L-1) of total phosphorus (TP) and soluble reactive phosphorus (SRP), and comparatively low maximal concentrations of dissolved inorganic nitrogen (<0.5 mg N L-1). Bioassays indicate that its phytoplankton, is nitrogen limited, and it has surface blue-green algal blooms in summer. Palaeoecological investigations have shown that high populations of cyanophytes occurred more than 6000 years ago, even before human settlement of the catchment. 2. The reasons for this combination of features are not immediately clear. This paper attempts to distinguish between two hypotheses: that the high phosphorus concentrations arise from input of phosphate-rich groundwater; and that the high phosphorus concentrations arise from concentration and recycling mechanisms within the lake. 3. The lake is entirely fed by ground water, direct rainfall and surface sheet flow. The concentrations of phosphorus in the ground water are much lower than in the lake water. The lake is stratified and has high phosphorus concentrations in the anaerobic hypolimnion in summer as a result of substantial release from the sediment. Vertical turbulence in the relatively weakly stratified water column may transfer substantial amounts to the epilimnion. There is also substantial phosphorus release from the sediments under the aerobic epilimnion. 4. There is little outflow of water and phosphorus. Most of the phosphorus is returned to the sediment under winter isothermal conditions, to be released again the following summer. The lake has probably maintained these mechanisms for millennia. Recent eutrophication is likely to have been driven by nitrogen inputs from greater intensification of agriculture in the catchment.