How to get rid of weeds and algea on virtual villagers 2 for my iPod?

I don't know what todo I do have a master scientist but not a master farmer now I have algea in the water and there are no coconuts left so they don't have anyfood and I keep seeing my villagers go back to these weeds and I don't know how to get the weeds to go away...oh and my master scientist has 6531 Tec points so please help thankyaaa