Chemistry of Freshwaters - Principles of Environmental Chemistry - page 145

mercury compounds in the lake are declining. 132 Kim and co-workers 133
have simulated mercury transport and speciation (Hg II ,Hg 0 ,CH 3 Hg 1 )
in both the water column and the benthic sediment using a Water
Quality Simulation Program. Model predictions generally agreed with
measured values for the water column, in which advection, sorption, and
settling were important mechanisms for mercury transport. Reduction,
methylation, and demethylation affected mercury speciation in the water
column and the benthic sediment. Natural attenuation showed no
positive impact for remediation when compared with dredging and
capping.
3.3.4 Nutrients in Water and Sediments
3.3.4.1 Phosphorus and Eutrophication. Eutrophication can be con-
sidered as the excessive primary production of algae and higher plants
through enrichment of waters by inorganic plant nutrients, usually
nitrogen and phosphorus. The latter, in the form of phosphate, is
normally the limiting nutrient because the amount of biologically
available phosphorus is small in relation to the quantity required for
algal growth. 134 Sources of nutrients can be discrete (e.g. specific sewage
outfall) or diffuse (e.g. farmland fertilizers). Eutrophic lakes, highly
productive and often turbid owing to the presence of algae, can be
contrasted with oligotrophic lakes, which exhibit low productivity and
are clear in summer. There have been many examples of unsightly algal
blooms affecting freshwater bodies throughout the world, from Lake
Erie in North America to the Norfolk Broads in East Anglia, England. 59
Public concern has increased along with the reported incidences of
toxicity of the bloom-forming organisms, in particular the cyanobacter-
ia (blue-green algae), which have been implicated in fish fatalities, for
example in Loch Leven, Scotland. 135 The environmental damage costs
of freshwater eutrophication in England and Wales have been estimated
at d 75-114.3 M per year. 136
The chemical form of phosphorus in the water column available for
uptake by biota is important. The biologically available phosphorus is
usually taken to be 'soluble reactive phosphorus (orthophosphate)', i.e.
which, upon acidification of a water sample, reacts with added molyb-
date to yield molybdophosphoric acid, which is then reduced with SnCl 2
to the intensely-coloured molybdenum blue complex and is determined
spectrophotometrically (l max ¼ 882 nm). 137 Reduction in inputs of
phosphate, for example from point sources or by creating water
meadows and buffer strips to contain diffuse runoff, has obviously been
one of the major approaches to stemming eutrophication trends and