Algal and other fossil evidence for major changes in Strumpshaw Broad, Norfolk, England in the last two centuries

Abstract

Strumpshaw Broad is a medieval man-made lake, excavated about 500 years ago in valley peat deposits. Evidence from diatom fossils, and other remains (filamentous algae, Chrysophycean cysts, sponge spicules, snail shells) indicates a series of changes since the eighteenth century probably related to development of the city of Norwich and sophistications of its sewerage system. Progressive nutrient-enrichment led to replacement around 1912 of low-growing macrophytes (Chara) and sparse plankton populations by tall-growing vascular aquatic macrophytes, with increased epiphytic diatom numbers. A blanketing phase of filamentous algae (probably Vaucheria) was associated with the transition between 1795 and 1903. Around 1950, after a phase of increasing plankton populations and declining epiphyte and macrophyte numbers, the macrophyte-epiphyte complex disappeared altogether, to be replaced by dense plankton populations for a few years. These changes and the progressive decrease in volume of the lake owing to increased build up of sediment throughout the period covered by the core, led to alterations in hydrology which now prevent even plankton development. All data are expressed on absolute bases (amount sedimented per unit area per year) and the necessity for this, as opposed to relative measures (percentage representation, concentration per unit weight) in valid interpretation of palaeolimnological data is emphasized.