You may have noticed that not all macroinvertebrates on the tally sheet are included in one of the metric calculations (for example, true flies). This is because for some macroinvertebrates, there is too much variability in pollution tolerance at the order level to definitively say whether they indicate a healthier or less healthy stream. These bugs still contribute to the overall stream health score since they are included in the total organism count, but they can’t be grouped into any one of the metrics with the level of information we have.

For more information about how the VA SOS protocol and metrics were developed, check out The History of VA SOS!