Why haven't I seen a Tardigrade with my eyeball?
It seems like they are the size of a spot on a ladybug from the pics.
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Our local parks department has several annual events where they ask for volunteers to help perform benthic macroinvertebrate surveys. It basically amounts to meeting up at a local park with a couple people in waders dragging special nets along the bottom, dumping scoops of material into buckets and large, shallow, white trays, and others sitting at picnic tables with spoons, magnifying glasses, and muffin tins sorting out the critters that get caught in the nets.
The cool part is that at the end, you can score the creek based on the quantity and types of larvae that you find: Caddisfly, mayfly, and stonefly larvae are very sensitive to factors like runoff from agriculture and road salt, sediment, water oxygenation, and other factors, beetles, crayfish, dragonflies, and scuds are moderately tolerant, while leeches, worms, midges, and flies will grow in anything. Thousands of these surveys happen every year, so you can compare the relative frequency and quantity of various species and determine the relative health of the stream.
I don't know how many tardigrades you'll find just scooping 4-8mm nymphs and larvae by eye, but I've brought my microscope to a couple and put random droplets of water under a cover and slide: there are an astonishing number of tiny critters swimming around at any zoom level.