Wednesday, December 28, 2011
What's down there?
Beyond the bounds of the scientific questions I like to ask about biological communities, I frequently just want to know what organisms live where. In shallow streams and ponds I can simply reach down with a net for the answer. Deeper lakes present a slightly larger challenge. Biologists before me have risen to the challenge and designed an array of amazing devices to get to the bottom of things.
A collaborator recently needed benthic samples from 5m deep in Lake Champlain. My petite Ponar dredge was the obvious tool for the job. It's called 'petite', because the full size version must be lifted using a winch. The petite version can be lifted, deployed, and recovered with just a little elbow grease.
The device is lowered on a line into the lake and consists of two weighted jaws held apart by a sprung pin. When the sampler embeds in the lake floor and the line goes slack, the pin springs out. A tug on the retrieval line brings the jaws together and a sample of the lake floor is collected. The lake floor organisms can be seen after some washing in a sieve bucket to remove the fine sediment.
So what's down there? Well, the sample I took in November was overwhelmingly dominated by striped mollusks native to the Baltic region of Europe. Zebra mussels came to Lake Champlain via the Laurentian Great Lakes. Because of their filter feeding activity, they shift food resources from the water column to the lake floor. One result of this is increased abundance of other benthic organisms. The other organisms in my sample included the larvae of midges, caddisflies, and mayflies along with 2 snail species. That's quite a bit of diversity for a square of lake floor measuring just 9cm on each side.
An important lesson can be learned from this sample. Zebra mussels are extraordinarily abundant and can easily attach to any equipment used in an infested lake. To contain these invaders, it is essential to clean and then dry all equipment between uses in different water bodies.