Now the term "algae" is not very precise; it refers to any number of small green aquatic organisms, members of several clades (kingdoms), both prokaryotic and eukaryotic organisms. Under the right conditions unicellular green organisms can multiply at a remarkable rate (2, 4, 8, 16, 32, keep going for a few hours) resulting in algal blooms, vast population explosions of tiny green organisms. Blooms are not usually good things. Here's a great picture of an incredibly massive algal bloom in the shallow western basin of Lake Erie. That end of the lake is the warmest and it gets a lot of nitrogen laden runoff via a river from agricultural lands. The organism in question is Microcystis, a blue-green algae or more accurately a cyanobacterium, what my old phycology professor used to call "itty-bitty blue-green cells" in a Texas drawl. The number of cells it takes to turn a big lake green such that it can be seen from Earth orbit boggles the mind. Now here's the problem: it's toxic. Don't drink it; don't touch it. Don't let your dopey Lab lap it up. Now stop and think about how many people rely on Lake Erie for their drinking water. If what comes out of the tap looks like what's in this glass, that's a warning. Actually water purification plants do a pretty good job and you really have to concentrate a fantastic number of these tiny cells to get that much green, but you get the idea. These algal blooms are nothing new, but a warming climate will make such algal blooms larger, longer, and more frequent, something to get excited about. And to top it off, then human activities add fertilizer!