The Mineral Nutrient Levels in Three Norfolk Broads Differing in Trophic Status, and an Annual Mineral Content Budget for One of Them

The seasonal range and variation of phosphorus, nitrogen, silica and manganese were studied in three Norfolk Broads: Alderfen Broad, Barton Broad and Upton Broad. At all of the sites nitrate-nitrogen was maximal during the winter and usually undetectable in the summer. At Alderfen and Barton Broads a large increase of soluble reactive phosphorus occurred during the summer. At Alderfen Broad this was accompanied by an increase of ammonium-nitrogen, manganese and soluble reactive silica. A nutrient budget was calculated for Alderfen Broad, and this showed that the increase of these nutrients could not be accounted for by the input stream. It is suggested that the sediment was an important source of these nutrients during the summer. The algal crop, the range of nutrients in these broads, and the nitrogen and phosphorus loading of Alderfen Broad are compared with those in other lakes in Britain. It is suggested that these three broads are typical of the range of conditions present in the Norfolk Broads as a whole. Barton Broad is highly enriched and often contains blue-green algal blooms; Alderfen Broad is less enriched; and Upton Broad is typical of a broad as yet unaffected by increased enrichment. The implications of internal loading for the possible recovery of Alderfen Broad are discussed.