String Algae in Winter (Zone 6)

I just checked and the nice rich green carpet on the bottom of my pond
is string algae. The stuff beginning to cover the surface of the pond
is not duckweed, but azolla. I am hoping the azolla through shade and
filtering during the winter will help hold down the string algae growth
until the water warms up in the spring.

The golden shiner minnows are still swimming. The tadpoles are still
moving about. No sign of the frogs. All my marginal plants are sitting
on the bottom so the pots won't crack. There are some huge anacharis
"bushes" growing. I have tubs of water hyacinth in my greenhouse ready
to put in after the last frost.

Since my pond is spring fed, nutrient filtering is important all year
long. Is there anything I can do now or do I just play catchup in the
spring?
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18,000 gallon (17'x 47'x 2-4') lily pond garden in Zone 6
Cheers, Steve Henning in Reading, PA USA