Observing plankton - A birdwatcher's approach to plankton ecology

We need first to collect undisturbed plankton. In the Sea Core Sampler a small (50-L), almost undisturbed piece of the ocean is taken onboard the research vessel, and plankton can be observed directly through the transparent walls of sampler by a video camera equipped with a macro lens (Kiørboe 2007).

A slab of water is illuminated by a 1 mm thick laser sheet. The total volume of water viewed is about 100 µl (Central North Sea in September). Shining light this way emphasises the high concentration of particles in water at the spatial scale of a copepod. The particles are phytoplankton cells, protozoans, and detritus.

Feeding appendicularians (Oikopleura sp.)
We can use the particles to visualise the feeding current and see how the animal "breathes", pumps and relaxes, pumping water in and out, with a net inward flow. The reversal of the feeding current helps to rinse the outer filter for particles. To some extent it pumps the same water again and again, and if the slab of light is focused on the exhalent current, one can see how this is rinsed for particles. The critter therefore has to change position every now and then to get "new" food. By simple observations of the live animal in its environment we get a first understanding of how it functions.

Note that the critter is almost stationary in the water; it is hovering. One would expect that the feeding current would drive the animal through the water, but the weight of the egg-sac allows it to be tethered by gravity. From small-scale fluid dynamics we know that the efficiency of the feeding current is increased by abt 50 % in a tethered vs non-tethered suspension feeder. Carrying eggs has a fitness cost in terms of elevated susceptibility to predation, but this may be partly compensated by this higher feeding efficiency. Note also that not all particles entrained in the feeding current are captured – there is some sort of selection and remote identification of particles.