We've been doing these macroinvertebrate samplings - the informal name for them is critter counts, because we collect the macroinvertebrates in a kick net, then pull them off the net to identify them to order and count them - for the past three years at this site. I've also taken part in sampling a tributary of the River des Peres in University City as a member of Stream Team 1437 from 2001 through 2010.
The most interesting thing about the March 20 critter count was the high (for an urban stream) diversity of critters. The usual pattern for the River des Peres has been a low diversity in the spring sampling and a somewhat higher, though still low, diversity in the fall sampling. Our suspicion is that winter road salting adversely affects the macroinvertebrate population, as the conductivity and chloride levels go much higher once road salting begins. We've measured chloride levels well over chronic toxicity during winter chemical monitoring events on the River des Peres sites. During the spring and summer, rain flushes the salt out of the soil and stream, perhaps allowing for a repopulation of some of the macroinvertebrates from nearby locations that are less affected by runoff from salted roads. If this is what's happening, it would explain why we observe a higher diversity during fall samplings compared to spring samplings.
For the Coal Bank site of Watkins Creek, we recorded a higher diversity of critters in fall 2009 compared to spring 2009, but lower diversity of critters in fall 2010 compared to spring 2010. Critter diversity in both spring and fall 2011 continued at about the same low level as noted in fall 2010. This spring, the critter diversity took a marked jump up from fall 2011, returning to about the same level as noted in fall 2009. What might be going on here?
In addition to road salting, the Coal Bank site is also affected by occasional flooding when the nearby Mississippi River floods up into the creek. While the winter of 2009-2010 did not bring excessive snowfall and therefore not much road salting was required, the Mississippi River experienced several weeks of flooding during late spring and summer, flooding that backed up into the Coal Bank site. Some critters cannot survive in the slack water caused by flooding. It may be that any repopulation of some kinds of critters that would normally occur during spring and summer was set back by flood-related habitat destruction, leading to the lower diversity of critters we noted in fall 2010 compared to fall 2009.
Not only did we experience excessive snowfall during the winter of 2010-2011 (we measured a chloride level about 3 times higher than the acute toxicity level on January 28, 2011, during the worst of the snowy pattern), but the Mississippi River flooded for weeks again during late spring and summer of 2011. If salt toxicity and flooding do indeed lead to a lowered diversity of macroinvertebrates as I suggested above, we would expect to see that reflected in our critter counts for spring and fall of 2011, as indeed they are. Compared to the water quality rating of 21 (good according to Missouri Stream Team criteria) we obtained in fall 2009, the water quality rating in spring 2011 was 12, and in fall 2011 it was 13 (fair in both cases).
So why might we have seen a rebound in diversity this spring, to a water quality rating of 22? First, the Mississippi has stayed below flood levels during the past fall and winter, perhaps allowing some critters to repopulate into the creek from less-affected waters nearby. In addition, we had very little snow this past winter, so very little salt was applied to area roads. Our one winter measurement of chloride was just under the chronic toxicity level. Perhaps the low level of added salt allowed for better critter survival over the winter. If the Mississippi River stays out of flood this year, we might see at least as high a diversity of critters this fall, if not higher than what we measured on March 20. Call this a hypothesis; we'll see what next fall brings. It would be good news for Watkins Creek.
