A few BG may start spawning after they pass 3" in length. There is a chance your rapidly growing fish from last year did already spawn!
Stopping the sand/silt from going into the pond needs to be a priority. That sediment is going to go right to your lowest spots and "steal" your depth.
Not a horse guy, but most of the arenas that I have seen have been pretty clean of animal waste. However, I don't know about your stable/paddock areas? Your erosion plan also needs to divert large-animal waste from your pond. The manure will have a very strong fertilization effect, and enough urine will begin to change your water chemistry.
If the animal waste cannot go anywhere else other than into your pond, then you need to start some beneficial plants to take up the nutrients into vegetation you like, rather than plants/algae that you don't like!
Lots of threads on the forum about beneficial and slow-spreading plants. Especially if you do riding lessons, etc. and need the pond to be attractive, rather than an eyesore.
P.S. What is going to be the top predator to control your BG? Once a large number hit spawning size, they are going to make LOTS of baby BG!
