Mini-Ecosystem in a Pool

[Fromage]

A couple years ago my family had a pool in our backyard. It was fairly big, and about 4.5 feet deep. Anyways, we left it untreated for quite some time. The water got dirty, turned green, froze in the winter, and thawed out again in the spring. That's when I noticed all of the things that lived in it. It was actually pretty fascinating, and I have a lot of questions.

 

The first life to appear in the pool was algae, and that doesn't surprise me much. Soon after there were snails, though. Where exactly did they come from? I don't think I've ever seen another snail in the area where I live, so how did they get in my pool? After snails came some sort of spider that glided on top of the water, and little black wormy things that would swim around. And like the snails, I've never seen these things anywhere else in my area, so how did they get there? My mother said that a bunch of stuff probably fell from the tree above the pool into the water, but I'm not too sure about that...

 

Anyways, sorry if these are dumb questions, but I've been thinking a lot about it lately.

[Paralith]

I know that with a lot of freshwater aquatic snails, they're farily amphibious and can exist both out of and under water quite well. And when they're not in the water, then they're probably in moist, muddy, cool areas, like under plants and such, where they're not as visible. The same probably goes for the worms. And again, a spider that can glide on water can probably also walk on land.