A Post from the P.I. Yvonne Vadeboncoeur

As one of the principle investigators on the Lake Tanganyika Littoral Project, I spend most of my time figuring out the logistics of getting science done while keeping seven people reasonably well-looked after in paradise.  Given that its paradise this ought to be easy, but as our fish-man George Kazumbe always says TIA. This Is Africa.  Roughly translated this means – don’t expect anything to be on time, as planned, or uncomplicated. 

So why bring all of these folks to East Africa?  Lake Tanganyika is one of the most amazing freshwater ecosystems on earth.  It is a huge ancient tropical lake that boasts not only a stunning diversity of cichlids, but also snails, crabs, shrimp and jellyfish.  When Europeans first encountered Lake Tanganyika, they speculated that it had once been connected to the ocean, which is an understandable mistake given its fauna.

The lake is what we would normally call “unproductive,” meaning that the water doesn’t have many of the nutrients that allow photosynthetic organisms (in this case algae) to grow. “Unproductive” lakes are what you would think of as “clean”--nice clear water with no green pond scum.  The clear water allows photosynthesis to occur to great depths in the lake.  When plants photosynthesize, they use sunlight to convert carbon dioxide and water into carbohydrates - which the plant uses to grow.  Even though this process of making life with light occurs wherever there is light on earth, I find it miraculous. Non-living (or inorganic) compounds are transformed into living tissue using chlorophyll and the energy from sunlight.

If you have house plants or a garden, you know that you don’t give plants carbon dioxide to make them grow faster, you give them fertilizer or manure. These fertilizers contain nitrogen and phosphorus (N & P) which are critical for plant growth. So what has all this to do with fish and Lake Tanganyika? As I said, Lake Tanganyika would be categorized as “unproductive” because nutrients like N and P are extremely scarce in the water.  However, the algae on the rocks are growing crazy fast and the fish that eat the algae are very abundant.  In fact, densities of fish in Lake Tanganyika rival those of a coral reef.  So, the lake ISN’T unproductive with respect to fish, and the algae themselves are very productive, but the fish eat the algae pretty much as fast as they can grow. We are trying to figure out how this seemingly nutrient poor lake supports all these fish and this high algal growth.

Dr. Vadeboncoeur measuring photosynthesis (photo by E Gaines)

We think that in the very shallow waters at the lake edge, the algae get fertilized by the fish pee and poop. Basically, the fish are critical to fertilizing their own food source, much as we would fertilize a farm or garden.  Take away the fish and you take away the nutrients, and the algae would not be able to grow as well. The fish “store” nitrogen and phosphorus in their bodies and leak the nutrients out is small amounts that help the algae grow.  The algae, in turn, use photosynthesis to create carbon compounds that the fish need to meet their energy demands.  It gets pretty circular if you think about it too hard, but ecologists like getting mesmerized by cycles.

Fish eating algae (photo by E Gaines)

My crew spends all day underwater measuring photosynthesis of algae growing on rocks and watching fish.  We try to detect patterns between fish abundance and algal growth. Ecologists use common things in ways that they were never intended in order to understand how organisms interact with their environment.  Limnology was once described as “the science of ropes and buckets”.  Now we would add PVC, duct tape and zip ties to that description.  I get people to work with me by telling them about the fish and the SCUBA diving,  Then I ask them to haul around all these weird contraptions to do these underwater experiments.  For the first few weeks, we were measuring photosynthesis.  I won’t go into details, but we used a bunch of sausage weights to hold our chambers against the rocks. A sausage weight is the circular tube of fabric filled with lead shot.  We use about 130 lbs of sausage weights. Len and Ryan got pretty tired of hauling them up from 5 m at the end of a long day underwater.  So they were happy when we moved on from photosynthesis to “rugosity”.  Rugosity is a measure of the lumpiness of the bottom of the lake.  Fish like to have places to hide, so the more rocky areas are preferred habitats for most of the fish we are studying.  Measuring lumpiness sounds innocuous and without the sausage weights, our boat got much lighter.  Sometimes we even went a little fast. 

researchers work with tropical fish all around (photo by E Gaines)

But how to measure lumpiness?  Well of course you take a 10 m long chain and drape it over all the rocks and compare the length of chain you use with a straight line distance between the your beginning and ending point.  Now instead of moving buckets of lead around, they are doing underwater surveying with an unwieldy chain.  Ryan decided today that he misses doing photosynthesis.

Jane Goodall and Yvonne Vadeboncoeur (photo by E Gaines)