Paul, I suspect you might have a long wait for a response to the question of how the horticulture/gardening industry contributes to 'excessive richness of nutrients in a lake or other body of water, frequently due to runoff from the land, which causes a dense growth of plant life and death of animal life from lack of oxygen'. The industry would perhaps prefer to keep such information under wraps.
It might be more fruitful to reframe the question based on what you have found out so far. Why not pose the question more directly to the sectors of the industry which are likely to cause eutrophication: off the top of my head these might be nursery stock, greenhouse edibles (including hydroponic-based systems), and field vegetable production.
Chemical/oil-based systems are likely to be responsible for more pollution than organic systems (but not exclusively).
A good strand to your project would be to find out what measures are in place to avoid the pollution. What, for example, if anything, happens to the unused nutrients from a hydroponic system?
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