Introduction Lak e Pamvotis is a m ode rately-sized shallow lake nestled in the Balkan mountains of north-western Greece (area o f22 km2, average d epth of 4-5 111). (Figure 1). T he lake is ringed by 1500-2000111 mountains that provide a dramatic backdrop for th e sm all bustl ing city o f l oan nina (po pul ation of ca. 100,000) , the regional gove rnm ent and the cultural centre. The lake does not have a natural surface overflow but rath er, this karstic watershed is riddled with sinkh o les which are probably the source waters for many of the sprin gs in the neighboring river valleys. A man-m ad e d itch and tunn el have co nnected the lake to th e Kalmpas River sin ce 1959. In 1997, the local auth ori ties commissioned the Centre for Water R esearch to d eve lop m an agem en t strategies to improve the aesth etics and water qua lity of th e lake. Our study was based on: • historical data ana lysis, • seasonal (one-year) and sub-dai ly (one-week) field data acquisition and analysis an d • numerical modelling of the wate r 30 quality. a 1':
Today the lake is eutrophi c with high algal bio mass throughout the year (> 10 (~1g ch la/ L) with summ er blooms in excess of 30 (~1g chla/ L. The local community wants reco mm endations to fu rther improve the water quality, particularly strategies to decrease alga l biomass. A chrono logy of anthropogenic influences on the lake ecology over the past 40 yea rs is summarised in Table 1. Period ic measurements o f fi lterable reactive phosphorus (FRP) from the 1970s to the prese nt clearly illustrate the effec ts of external P-loading on th e lake water (see Table 2) . Low FRP during the 1970s atta ined hi gh levels by the ea rly 1980s. The highest levels were m easured in the early 1990s shortly after the cessa ti on of direct sewerage and industrial e ffiu ent o ut falls into th e lake. C urr e nt p h osphorus leve ls h ave continu ed to dec rease though th ese are still substa ntially greater than the 1970s 38
WATER MARCH 2001
Dcpth (m) ~
Figure 1. Location of Lake Pamvotis given by * on map. Bathymetry of Lake Pamvotis with the location of the five seasonal stations, the two inflows, the two outlets (irrigation and outflow) , the meteorology station (x) and t he city of loannina (n).
Figure 2. Average surface , mid-depth and bottom measu rements over the five seasonal stations of (a) temperature, (b) dissolved oxygen, (c) pH, (d) chla, (e) FRP, (f) NH 4 , (g) N0 3 and (h) DIN(=NH4 +N03 ):FRP. The bold line in (b) represents t he saturated dissolved oxygen value and in (h) the dotted line is the 16:1 line.