By Drew Jack
How can you tell if a creek is healthy or not? One way is to look at the bugs living in the creek. These bugs are called Benthic Macroinvertebrates (BMIs)– Macro, because we can see them without a microscope and benthic or benthos, meaning bottom, because they live on the stream bottom. BMIs allow us to evaluate the health of a creek because they are an important link between lower level resources such as algae, leaves, woody debris, and water plants and organisms in higher level organisms such as fish, frogs, and birds.
If BMIs are missing then we can say that the creek might be polluted or degraded. Why can we say this solely based on the absence of BMIs? Not only because BMIs are the vital link between plants and animals, but also because they are also very sensitive to changes in the ecosystem. They can only live in well-balanced ecosystems with steady pH levels, dissolved oxygen, and water temperature. Human actions like fertilizer and pesticide use can cause the physical and chemical characteristics of a creek to fluctuate wildly. Therefore, these bugs allow us to identify creeks and streams that may be impaired as a result of human actions.
So, how do we use BMIs to determine the health of a creek ecosystem? First, we need to know which BMIs should be in a healthy creek. The types of BMIs vary from watershed to watershed and among locations in one stream itself. Fortunately, scientists have done much work regarding macroinvertebrate types. This work is compiled into an Index of Biological Integrity (IBI). An IBI describes which BMI types should be in the stream and the particular sensitivities of the BMI.
The next step to determine creek health is to actually sample the creek for the BMIs. Using a special net, called a kick net, that is placed flush against the stream bottom, the sampling team gently scrubs the rocks about a foot in front of the net which allows the macroinvertebrates to be swept downstream and collected in the net. This is usually performed several times in different sections of the stream to obtain a good representation of the stream. The captured BMIs are then placed in a sample jar which is taken to an expert taxonomist– a person trained to identify all of the BMIs.
The last step in a BMI survey is to compare the report from the taxonomist to the IBI for the creek. You must pay attention to dominant bugs and bugs that are scarce or missing altogether. If major differences are found between your sample and the IBI, then you can say that the stream is unhealthy or stressed.
The next time you are near local stream, take a look under a rock or two and see what kind of bugs you find, you might be surprised at what you find. If you are interested in participating in a BMI survey of a local creek, contact Juliana@thewatershedproject.org.