Yallop 11 - Lake Finjasjon Flashcards
characteristics of Lake Fijasjon
Sweden 1100 ha 3m mean depth max depth 13m retention time 3 months, v popular swimming lake, historically in good condition, secchi depth 2m.
problem with lake fijasjon
raw sewage pumped into lake in first half of 20th C. eutrophic by 1940s.
5 feeder streams, likely to bring in P from fertiliser.
first sign of eutrophication was appearence of cyanobacteria - Gloeotrichia echinulatum.
first STW introduced 1949 but continues to be eutrophic.
anabena and microcystis appeared.caused skin rashes in swimmers.
historic monitoring of lake finjasjon
long term data since 1950s. in 70s started to reduce monitoring after P content seen to reduce.
what allowed P to reduce?
1963, sludge step formed to reduce P entering lake.
however, Halssleholm pop increased causing P the peak at 65 tonnes per annum.
1977 added chemical flocculation step for treating sewage + extra filtering – P dropped to 2 tonnes per annum.
what is a typical graph illustrating vollenweider model?
see graph
why doesnt lake finjasjon follow the vollenweider model?
clear relationship for deep lakes, whereas shallow lakes dont always fit, due to internal loading of P. continues to support algal growth. like barton broad. required to remove sediments to reduce P loading.
why are so many lakes polluted today?
reason 1: flow of N and P from land to water increased. industrial revolution and urbanisation has caused increased disposal of sewage directly into rivers = extreme de oxygenisation.typhoid and cholera outbreaks moved sewage dumping onto land, left to rot.
STW initiated at end of 19C, using bacteria and protozoa to break down organic matter.
today sewage contains inorganic waste too, is rich in ammonium, nitrate and phosphate and discharged into rivers. ammonium toxic to fish.
changing farming practises over time has massively increased N content of rivers and lakes.
deep lakes, high correlation btw chl a and P. shallow lakes, more complex relationship as trophic interactions v imp in controlling phytopl.
what are some simple solutions to external loading of lakes?
- more treatment at STWs,
- divert effluent away from lakes
- dose intake of lake with chemicals such as iron sulphate to precipitate phosphate.
Works well for deep lakes – but shallow lakes work differently! - understanding the major biota and the factors controlling them will help us to implement the most effective restoration procedures.
table of values for different trophic classes
see table
why was there a lack of waterfowl between 1970s and 1994?
no change in no. macrophytes.
waterfowl attracted by charophytes, which need low nutrients and clear water to thrive.
when was a major restoration program initiated for finjasjon?
1980s
‘Lake Finjasjön- possible to swim, in our time, for our children.’
used as drinking water supply for 25,000 people.
dredged sediments covering 60% of lake bed. remove 0.5m thick top layer. which contained 30% of P. suction degrading began 1987.
1990-1 P measured in sediments to see effectiveness. very little difference in P in sediments removed and those left. no obvious change.
1991 - 134 tonnes of P removed from the lake, at a cost of £5 million and cyanobacteria blooms remained.
why was it decided to try biomanipulation
1987-1993 – large inter and intra-annual variations in cyanobacterial biomass which contained hepatotoxins
no one allowed to swim and cows sick when drank water.
blooms situated in same region as intake to water treatment works, so decided to try biomanipulation.
dredging ceased in 1992
effects of trawling on fish stocksin 1994
1994 - trawling and dredging reduced 20% of all fish catch weights.
found 72% cyprinids (zooplanktivorous), and 28% piscivores. found that a 1:1 ration was most beneficial. molluscivores are also useful.
when and why was a reed bed set up
what was the effect of this?
30 ha constructed wetland
reduced N and P from sewage effluent.
to address diffuse sources, put 5m wide buffer strips at edge of feeder streams which werent fertilized.
Constructed wetland reduced P by 25% and N by 32%.
what changes in zooplankton species caused by biomanipulation?
D cucullata and D. galeata increased abundance.
Adapt to predation pressure, change body form. inducible defences.
