Where is all the algae going?

Hippos and wildebeest load a lot of nutrients into the Mara River… like, a lot. All of those nutrients should fertilize the river and produce a lot of algae. It’s what the textbooks say should happen, and it’s what research in other rivers show should happen. We see it happen in some portions of the river where there are only moderate levels of hippo inputs. However, it is not what happens where the hippo and wildebeest inputs are highest. Why not? I don’t know. 

This– getting results that don’t fit your expectations, that surprise you– is simultaneously the most exciting and the most frustrating part of science. It’s the part that makes science addictive, that keeps you coming back for more, that makes you think grand discovery is on your doorstep. It’s also the part that can drive you crazy and make it difficult to publish your research. If you’re getting an unexpected result, you need to 1) be absolutely sure it’s a real result and not an artifact of something you screwed up on accident, and 2) figure out why you’re getting it. “This is weird, and I don’t know why it’s happening,” will only get you so far. 

So, back to our algae… Why don’t we see more in the river? Is it not growing in the first place? Or is it growing and going somewhere? I have been asking this question a lot over the last year as I have been working on the final papers from my dissertation, and this summer I was able to run a pilot experiment to test what is becoming my primary hypothesis… hippo and wildebeest inputs provide so much food for bugs and fish that their populations increase and they then graze down the algae. This would be really cool if it turns out to be true, because it would show that animal inputs can enter the river food web through various pathways, and which pathway they follow determines how those inputs shape the river ecosystem. Or I could be wrong and it could be something else entirely… See? Exciting, and a little frustrating. 

So, to test this hypothesis, I set out a grazing exclusion experiment this summer. I basically grew algae in the river on glass discs (the white circles in the picture below). Some discs were protected from grazing (either by a cage to restrict fish or by insecticide to restrict bugs) and others were not. If there was more algae growing on the protected discs than on the not-protected discs, that would suggest that algae can grow in the river, but it doesn’t accumulate there because it gets grazed down. 

I wasn’t originally planning to run this experiment this year, as we already had a full schedule, but I was excited to be able to put this together almost completely with supplies we had stored around our camp. I built two of these that I deployed at two different sites in the river. I left them out for two weeks and hoped they didn’t get stepped on by a hippo, washed away in a flood or collected by an enterprising Maasai person with a need for some glass discs. I always consider it a major win when we put something out in the river and return later to find it still there.