Historical changes (1905-2005) in external phosphorus loads to Loch Leven, Scotland, UK

This article reviews historical changes in
the total phosphorus (TP) inputs to Loch Leven,
Scotland, UK. Data derived from palaeolimnological
records suggest that inputs in the early 1900s were
about 6 t TP year-1 (0.45 g TP m-2 year-1). By
1985, this had risen to about 20 t TP year-1
(1.5 g TP m-2 year-1) due to increases in runoff
from agricultural land and discharges from point
sources. By the late 1970s, increased TP inputs were
causing serious degradation of lake water quality.
Most noticeably, there had been an increase in
cyanobacterial blooms. A catchment management
plan was implemented in the early 1990s. This
resulted in a 60% reduction in the annual TP input
between 1985 (20 t TP year-1/1.5 g TP m-2 year-1)
and 1995 (8 t TP year-1/0.6 g TP m-2 year-1). The
main reduction was associated with better control of
point source discharges, but attempts were also made
to reduce inputs from diffuse sources. The reduction in
external TP loading to the lake led to a marked decline
in TP retention by the lake each year.