Eutrophication
Eutrophication is defined by the European Commission (in the Urban Waste Water Treatment Directive of 1991) as: 
"the enrichment of water by nutrients especially compounds of nitrogen and phosphorus, causing an accelerated growth of algae and higher forms of plant life to produce an undesirable disturbance to the balance of organisms and the quality of the water concerned." 
Origin of the concept of eutrophication in fresh waters
The idea of eutrophication began with limnologists. Early in the 20th century, northern European limnologists distinguished oligotrophic from eutrophic lakes. The former were nutrient-poor, the latter, nutrient-rich.

Subsequently it was hypothesised that lakes evolved from oligotrophic to eutrophic as a result of human activity, in particular the input of sewage, industrial and agricultural nutrients. This was called cultural eutrophication. Eutrophic freshwaters have consistently higher biomass of (usually) phytoplankton than is found in oligotrophic lakes, and eutrophication is generally the result of addition of phosphate. Cyanobacteria, common in freshwaters, can 'fix' nitrogen. 

The evidence for eutrophication
Identifying a water body as subject to eutrophication may have legal and economic consequences, for (under the Urban Waste Water Treatment Directive) it requires urban waste water to receive more costly treatment. At present, any UK water where summer chlorophyll exceeds 10 mg/m3 is regarded as eutrophic (CSTT, 1997). However, the enhanced phytoplankton biomass might be benign, providing food for zooplankton and hence for fish. The ascription of anthropogenic eutrophication should thus take account of all of the three parts of the European Commission definition of eutrophication. This is considered here for the Firth of Clyde 
the enrichment of water by nutrients especially compounds of nitrogen and phosphorus - nutrient concentrations in Loch Striven (Tett et al., 1986) and in the Firth of Clyde in Winter (Grantham & Tett, 1993) increase as salinity decreases, implying a source in freshwater, especially the River Clyde, which is enriched by urban waste water and leaching from farmland; 
causing an accelerated growth of algae and higher forms of plant life - it is difficult to demonstrate accelerated growth, but the graph given above shows that there are larger blooms of phytoplankton in the nutrient-enriched Loch Striven than in the pristine Loch Creran; 
to produce an undesirable disturbance to the balance of organisms and the quality of the water concerned - there have been several harmful algal blooms in Loch Striven, and in other parts of the Firth of Clyde (Jones et al., 1982; Tett et al., 1986), and in addition the deep water of Loch Striven sometimes becomes partly depleted in oxygen (Tett et al., 1986), presumably as a result of the decay of organic matter that has sunk from the surface. At issue here is the unknown extent to which nutrient enrichment has increased the sedimentation as well as the production of organic matter in Striven. 
www.lifesciences.napier.ac.uk/teaching/MB/MBeu.html 
Agriculture As A Cause
Farmers are frequently at fault by adding excess fertilisers (usually nitrates or ammonium compounds, containing nitrogen, which plants use for making proteins). It is these salts that are most likely to be leached and so pass into streams, rivers, lakes etc... 
These salts are normally in short supply and hence act as "limiting factors" restricting the growth of plants in such bodies of water. In the presence of leachate, however, the algae grow in huge numbers causing an "algal bloom" . The salts are soon used up and so most of the algae die and sink to the bottom. 
Here they are decayed by saprophytic bacteria (and fungi). The bacteria therefore increase in numbers and, as the decay (digestive) processes are energy requiring, the bacteria use "dissolved oxygen" for their respiration. 
This may result in the death of all other oxygen requiring organisms, such as fish, water fleas etc... Also, even large flowering plants growing in the water may die. 
Thus the bodies of water become lifeless except for the bacteria which can survive the anaerobic conditions. The whole sequence of events caused by the extra salts leading to death of organisms is called "Eutrophication". 
Eutrophication may also be caused by sewage / slurry etc... which gets into the water supply because these too contain nitrate rich chemicals. (Washing powders containing phosphates can have the same effect as phosphorus, also a limiting factor).
www.revisioncentral.co.uk/ gcse/biology/leaching_eutrophication.html