Thursday, November 29, 2012

What do carcasses do to a river?

Carcasses contribute a lot of nutrients and carbon to a river system. Depending on the nutrient and carbon levels already in the river, these inputs can either cause an increase in primary production, like algae, or an increase in respiration, caused by bacterial decomposition. Although there are a relatively small number of carcasses left in the river, the river flows have also started going down over the last two weeks, making the effect of the carcasses more noticeable. We have found that most of the equipment we have in the river now has pieces of rotting meat and hair hanging off of it. Here are some tiles we place in the river to measure the growth of biofilm. Biofilm is the community of algae and bacteria that grows on the rocks and sand in the river-- this is what makes river stones slippery. You can see here a big piece of meat hanging off the tiles, and likely affecting what's growing on them.

Carcass meat hanging off my experiment
After we pull the tiles out of the river, we put them in sealed containers of water, and measure the dissolved oxygen in the water before and after exposing the tiles to sunlight and darkness. If there is a lot of algae, they will undergo photosynthesis in sunlight and produce oxygen, resulting in an increase in dissolved oxygen levels. If there is a lot of bacteria, they will just undergo respiration during both light and dark and take up oxygen, resulting in a decrease.

Measuring biofilm activity on carcasses
This is one way we measure how the input of wildebeest carcasses in the river affects the other things living in the river. We also can look directly at the carcasses to see what's eating them. Investigating a few carcasses up close, we found they had lots of a certain kind of aquatic insect called Baetidae (which is a type of mayfly) hanging out on them. By collecting the insects and measuring their stable isotope composition in the lab, we can learn if they are actually eating the carcasses as well as living on them.

Mayfly larvae on a wildebeest carcass
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