You mentioned filling with water from the river ... so ... it kind looks like maybe it doesn't have much watershed and water is pumped to maintain level? If so then there is very little flushing of the nutrients you are adding by feeding and so they accumulate. Any idea of how much weight of feed you've added through the years?
If you witnessed a sudden intense bloom ... it may be that a fish/crayfish kill has already occurred. When I look at the image, the fish population is not evident at all. I am seeing thousands of GAMs (mosquitofish), for example, in most ponds that have them near me. They have predators in those ponds and yet the every square meter within a couple of meters of shore has dozens visible near the surface. The bloom is reminiscent to me of a nearby pond that fish-killed three years ago. Only mosquito fish remain as far as I can tell and their numbers are very sparse. My son and I went to see if the water had recovered this summer but absolutely no fish were caught on small flies (size 12). Nor were there any takes or nibbles. My son discovered the GAMs ... just 2 of them in a 2.5 acre pond. Was a very sad sight for us to see as only 5 years ago it was a vibrant fishery with lots of fair sized LMB and various species of lepomis.
Anyways, you have a hyper-eutrophic pond now and this is what you can expect if you don't do something. Get with esshup. He can tell you how to kill it out and then floc the nutrients to roll back clock a bit. Be ready to perform these tasks periodically ... especially if you continue feeding.
