The changing ecosystem of a shallow, brackish lake, Hickling Broad, Norfolk, U.K. II. Long‐term trends in water chemistry and ecology and their implications for restoration of the lake

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Abstract
SUMMARY: Hickling Broad underwent major changes from a clear water, charophyte‐dominated state in the decades previous to 1970 to a turbid, phytoplankton‐dominated state by the mid 1970s. These changes were complexly linked with increasing eutrophication by black‐headed gulls and increased salinity due to agricultural changes in the catchment. At the turn of the 1970s, the lake began to change again and during the 1980s a submerged plant community, of tall, vigorously growing species (e.g. Myriophyllum spicatum, Patamogeton pectinatus) had recovered, despite a major reduction in the roosting gull population, no change in salinity, and only small reductions in phytoplankton biomass and total phosphorus concentration. Recovery of the plants may be linked to grazing of periphyton on them by an increased population of a mysid Neomysis integer which had been suppressed by toxicity from an alga, Prymnesium parvum formerly stimulated by the ingress of gull guano. A cladoceran community present in the clear‐water phase has not recovered and may be suppressed by continued high salinities. Further restoration of the lake requires displacement of the large phytoplankton biomass and this might best be contemplated by land use changes leading to lowered salinity and predicted recovery of grazing Cladocera. Models are given which summarize the likely workings of the system in the early twentieth century, the mid‐twentieth century, the 1970s and the late 1980s.