Birds as eutrophicating agents: a nutrient budget for a small lake in a protected area

Abstract

Sources of nitrogen and phosphorus were measured for a small pool, Brown Moss, which is a Site of Special Scientific Interest in the UK. The site is designated for its macrophyte and bird interest and comprises several pools flanked by heath and woodland. Currently the largest pool has high mean concentrations of total N and P (4 ± 2.4 mg N l⁻¹ and 389 ± 254 μg P l⁻¹), and also suffers from water table fluctuation and recreational impact. There are no point sources of nutrients but potential diffuse sources include direct rainfall, drainage from surrounding heathland and potentially from distal agricultural land, some septic tank drainage and bird excreta. Nutrients are also released from the sediments. Amongst external sources of phosphorus, birds contributed 234 mg m⁻² yr⁻¹ (73%), land drainage 63 mg m⁻² yr⁻¹ (20%), and rainfall 24 mg m⁻² yr⁻¹ (7%). Major bird species were dabbling ducks and geese. For external sources of nitrogen, the most important was land drainage (2,860 mg m⁻² yr⁻¹ and 64%), followed by rainfall (870 mg m⁻² yr⁻¹ and 19%) and birds (741 mg m⁻² yr⁻¹ and 11%) There was substantial internal loading of phosphorus (1670 mg m⁻² yr⁻¹) so that internal loading dominated the overall phosphorus budget (84%). The nitrogen budget was dominated by land drainage, though for short periods, geese made a major contribution. Restoration of the site to the status required by European legislation will be very difficult, if not impossible in the medium-term.