Thomas - You're to be commended for the detail and forethought shown in your post. Based upon the spring-induced outflows, your pond probably experiences a 100% "turnover" every 30 to 40 days. Such a dynamic pond may prove a challenge for fertilization programs, lake dyes or other non-chemical means of thwarting filamentous algae. I'm certainly not experienced with WI ponds - so DO solicit other opinions. But, I think your best options may be 1) physical removal of heavy filamentous algal growths 2) followed by CUTRINE-treatments to any remaining concentrations of growth on an as-need basis. The judicious use of CUTRINE shouldn't pose a threat to your fish or other desirable aquatic plants. One other thing (but also not in my area of experience): have you investigated the possibility of stocking tilapia? Since your pond is spring-fed, this otherwise tropical algae-eater might survive during the few months of summer in which your algae problems are at their peak. But, when water-temps drop below 50F, these fish are history. KD