I was kind of thinking they might be clam shrimp since they were sort of big (2-3mm), but I think they're just Daphnia. Very abundant in the vernal pools, pretty much the only animals I could see.
The unseasonal warmth might be eerie, but it's giving me a chance to appreciate some of New England's evergreen fern's without exsanguination by the local mosquito population. This is Dryopteris marginalis, or the marginal wood fern, which bares its sori on the margins of its leaflets.
A lot like our species in California.