Long-term trends in total suspended sediments from agricultural tributaries to Lake Erie

Background/Question/Methods

Suspended sediment delivered to Lake Erie through riverine transport is a major concern. Sediment can not only carry other pollutants, such as phosphorus, but sedimentation in receiving harbors and shipping channels requires costly remediation and removal. The National Center for Water Quality Research has been monitoring total suspended sediments (TSS) and nutrients in the major US tributaries to Lake Erie for many years. The primary goal of this analysis was to examine the outcomes of efforts to reduce erosion from agricultural lands via trends in TSS from 1975 to 2012 in the Maumee and Sandusky Rivers, two agriculturally-dominated watersheds in northwestern Ohio. Water samples were collected daily using automated samplers (ISCO’s) located at USGS gaging stations on the main stem of the rivers and at two smaller tributaries to the Sandusky River, Rock and Honey Creeks. TSS was measured as the dry mass of particles remaining on a pre-weighed glass fiber filter after filtering a known volume of sample.   

Results/Conclusions

Although discharge has been increasing in the Maumee and Sandusky Rivers over the past decade, TSS flow-weighted mean concentrations (mg/L) and loads (metric tons) have been decreasing (linear regression, P<0.05) or have changed little. Both rivers exhibited positive linear relationships between ln-transformed TSS concentrations and ln-transformed discharge. When examined by year, the slopes of these linear relationships were not significantly different from 1975-2005 (ANCOVA interaction, P>0.05), yet the intercepts have consistently declined (ANCOVA, P<0.05). This indicates a general decline in mean TSS over time, or more specifically that there was a lower TSS concentration for a given discharge in more recent years. The largest declines in TSS concentrations have been in the summer and fall compared to winter and spring. When comparing the tributaries to the Sandusky River with the mainstem, we found substantial effects of watershed size on TSS export. In Rock and Honey Creeks, a higher proportion of the annual TSS load was transported during storm-events compared to the larger Sandusky River. Overall, these declines in TSS over the past 30 years suggest best management practices implemented throughout the watersheds to reduce erosion and prevent sediment loss have been successful. Higher variability in storm transport of TSS in smaller watersheds may make it more difficult to detect positive results such as these. Future management should focus on sediment losses in the winter and spring to further reduce TSS export to downstream ecosystems.