The Aquatic Food Web: Who's eating whom?

Those of you who fish, probably already know a lot about the aquatic food web without realizing it.  The aquatic food web is a conceptual way to look at who is eating who and what in a lake.  The reason biologists use the term "food web" now instead of "food chain" is because it is a better way to visualize that everything is interrelated in a lake.

Let's start at the bottom and build up from there.  The first "ingredients" needed are nutrients (mainly phosphorus and nitrogen) and light.  Algae and plants need nutrients and light to grow.  Algae and plants are at the bottom of the food chain, and are important for the survival of everything else living in the lake.  They are important as food, shelter and also produce oxygen as a byproduct of photosynthesis, which fish and other aquatic organisms need to breathe underwater.

The next step includes the zooplankton and aquatic insects.  Zooplankton are tiny little animals (mainly crustaceans) that eat algae.  Some aquatic insects also eat algae, while some are predators and eat other insects or zooplankton.  This group of organisms is very numerous in a healthy ecosystem.

Now we come to the planktivorous fish, the fish that eat zooplankton and aquatic insects.  These fish are also called foragers, and include sunfish, crappies, and perch. 

Here is where the food web starts getting complicated and interrelated, because when predatory fish are young and small, they can feed on zooplankton and aquatic insects, and when they are adults, they will feed on minnows and fish.

Bass are sort of in the middle of these steps on the food web, because they eat large invertebrates such as crayfish and mayfly nymphs, but also eat minnows and small fish.

At the top of the food web are the piscivorous fish, or the predatory fish that eat other fish.  This is where most of the game fish are included.  Muskies and northern pike are the top predators, while walleyes are part of this group too.  This group can also include animals that live outside of the lake but eat fish such as eagles, ospreys, mink and fishers.

Finally, bringing the food web back around to the bottom again are the decomposers, or the bottom feeding fish and aquatic invertebrates such as crayfish, leeches and midge larvae.  These organisms recycle all the dead material back into nutrients to feed the plants and algae.

The food web must stay in balance for a lake ecosystem to be healthy.  Each part of the food web keeps the other parts in check.  If you remove one part of the web, the rest of it will fall apart.  For example, if the food web is top-heavy, meaning it has too many northern pike and walleye, they will eat up all the small fish until they run out of food and die or get growth-stunted.  Then, with no small fish around to eat the zooplankton and invertebrates, they multiply and run out of food, and so on down the line.

Next time you go fishing or are sitting out gazing at the lake, think about all the things that go into a healthy food web that contains healthy organisms.  It's quite an amazing balancing act!

Until next week, enjoy the lakes!

Moriya Rufer is the Lakes Monitoring Program Coordinator for RMB Environmental Laboratories in Detroit Lakes, 218-846-1465, lakes.rmbel@eot.com