But the fertility seems to have helped plants, especially the naiad, far more than the planktonic bloom. Even with periodic liming & fertilization, my average visibility is probably around four feet. Three feet is unusually good.
Three to four feet is perfectly OK in my way of thinking. To be sure, it won't grow as much lettuce and the standing weight will likely be less as a result but probably not as much as you might think ... (more on that later). But as long as the reason for that clarity isn't an excessive stand of submerged veggies ... you would be in the sweet spot at that level secchi depth. There are benefits to high secchi readings, among which are better water quality, greater night time DO, enhance visibility of prey, etc. Fish grow well and live long under such conditions.
Think a bit about why the bloom was so intense after killing the naiad. The naiad had starved out the zooplankton that utilize micro-algae to point their population was very low. When the naiad died, the phytoplankton could respond more quickly than the zooplankton. So it got ahead of the zooplankton which likely later caught up with it.
Frank, a person cannot really tell how productive water is by secchi alone. There are a number of proposed reasons for this but among them are two worth noting.
1. Community structure may be such that phytoplankton consumption is very high that it is efficiently grazed. The production of phytoplankton can be very good even while the standing weight of phytoplankton is very modest. Think a pasture with cows. The height of the grass isn't really an effective indicator of how much grass was grown ... the reliable indicator is a combination of what weight of cows were carried and how much they gained.
2. Alternate primary forage is produced. So when clarity is high, the sun penetrates to greater depths and support communities of periphyton that contribute to the primary and secondary trophic production that support higher organisms.
Consider this excerpt on fish standing weights where trophic status was determined by available nitrogen.
Lastly, in the case of the22 lakes studied by Kautz (1980), his plot of sport fish biomass versus trophic state as determined by total nitrogen content did indicate a downward trend for hyper-eutrophic lakes. However ,when he grouped the lakes by trophic state he found that the standing crops of sport fishes for oligotrophic, mesotrophic/eutrophic, and hyper-eutrophic lakes were 52, 89, and 65 kg⋅ha–1, respectively, but these values were not statistically different from each other.
So what does this tell us? Sport-fish can be grown to comparable biomass density in Oligotrophic water. What a lot of people don't get is that the standing weight of prey fish is primarily composed of adults that are relatively safe from predators. The apex predators benefit only marginally when eutrophic conditions prevail ... certainly not proportionately to the increase in primary production.
Just some food for thought.