What is eutrophication?

‘On a clear, sunny day, the Potamogetae, flourishing at a great depth amid the transparent waters, animated by numerous members of the insect and finny races present a delightful spectacle, and the long stems of the white and yellow water lilies may be traced from their floating flowers to the root. . . . . .One feels in such a place estranged for a time from the cares and vicissitudes of the world and the charms of nature penetrate, with their refining influences, the deepest recesses of the heart, denying to human language the power to give them full expression.’ W. Gardiner, 1848. The Flora of Forfarshire.