Okay, my farmer can't fish anymore b/c of all the algae, which I can't get rid of yet b/c of my lack of tech.....but what am I supposed to do about food in the mean time? I have three villagers building a dam, which will hopefully help with that...but in the meantime?

If you have enough tech points (1500), you can buy the 2nd level of Farming and harvest the coconuts. If not, watch the mushrooms that children may pick up and put the more people you can on building the Dam. It will coming soon

Do as Laurence suggests, you can get L2 farming, which will enable you to collect coconuts. In the meantime put as many villagers on building the dam as possible. The villagers are stronger than you might think; it takes quite a long while before they are severely weakened or die because of starvation. If you are playing on 2 x speed, try reducing it to normal or even half speed. Your villagers will eat less over a period. Grab as many mushrooms as possible, and pray!