Summary of Trends in Six Years of the Longview Watershed Study
As is expected in a small, intermittent, silt-laden stream, macroinvertebrate diversity is low, though higher in the summer with the emergence of aquatic insects. Diversity does not appear to be increased by adding leaves, on the basis of comparing experimental samples with leaves and controls without leaves. Indications that leaves reduce dissolved oxygen and nitrate concentrations are not clear. Field experiments have been subjected to the vicissitudes of rains, floods, siltation, net destruction, and problematic chemical analysis. The complete set of methods and results can be seen in the archive. Two trends of note over nine years of the study include an increase in summer nitrate levels and decrease in summer SQI. It looks like the overall quality of the stream is decreasing in summer over time.
Why is there a trend toward increasing nitrate each year (with one exception in 2008) measured in May, June, or July. Bernhardt et al (2005) observed decreasing nitrate export in the stream at the Hubbard Brook site. What is the difference in Little Mouse Creek on the Longview Campus? Longview is a much more urban site with continued disturbance and continual addition of fertilizer in the watershed. The facilities crew tells me that the amount of fertilizer added has actually decreased over the years. According to Sebilo et al. (2013) there is a long time lag before fertilizer added to the soil is transferred to the stream, even decades. If the peak fertilizer added was a decade ago, that peak may just now be leaving the soil and entering the creek.
Longview campus Watershed Study showing trends in long term study by Stephen L Reinbold.
Figure 1. Comparison of summer nitrate-N concentrations between experimental sites with leaves and control sites without leaves over ten years of the field study. The increase continues in 2013.
Figure 2. Comparison of summer dissolved oxygen between experimental sites with leaves and control sites without leaves over ten years of the field study.
Figure 3. Experimental sites with leaves and control sites without leaves, comparing number of organisms in summer over ten years of the field study.
Figure 4. Experimental sites with leaves and control sites without leaves, comparing Stream Quality Index in summer over ten years of the field study.
Figure 5. Change in nitrate-N (mg/L) from time of experimental set-up to completion of experiment. The duration was 12 weeks except in 2004 (8 weeks), 2011 (4 weeks), and 2013 (six weeks). Positive values indicate decrease from spring to summer and negative values increase from spring to summer. Note that in 2012, the original control pools with nets were dry and could not be sampled, so that the nearest pools with water (no leaves added) were sampled as controls. In 2013 data were missing from control #6 in March so only control # 2 is included. There was a slight decrease in the amount of nitrate from March to May in the experimentals but increase in the control in 2013.