Your pond is teeming with animals that are better adept to winter conditions. It all starts with everything that dies when it gets cold. The decay causes an influx in Bacteria and Protozoans which no longer compete with their warmer water cousins who are now dormant. Plus many Metazoans, Worms, and Mollusks who compete for the same food go dormant as well. In turn the Cladocerans (Daphnia), eat this soup containing these organism which boast more nutrients than the green soup of summer time (which is also eaten and competed for by many other Metazoans). Also since the Daphnia's metabolism is slower because of the cold you get larger Daphnids. which you can see better and more of (depending on species of course I am guessing colder climate Daphnia Magna or some derived species).
O as for the "MEZZIE'S", I think they are glass worms that eat organic build up just under the Ice. They are the larval stage of a gnat that can overwinter in abundant numbers and take a year to form into adults(depending upon temperature). Thanks to them I catch many a Sunfish and or Crappies just under the Ice when I ice fish during the daytime in winter.
