Since 1992, Raritan Headwaters Association (RHA), a non-profit environmental organization, has conducted annual biological, chemical, and habitat monitoring throughout the 1217 km2 (470 mile 2) North and South Branch Raritan River Watershed of New Jersey, USA. Temporal trends and geographic patterns were explored using RHA’s data on benthic macroinvertebrates collected from 105 monitoring sites for two long-term periods (1992-2017 and 2000-2024), and 4 shorter time-periods in between, for the watershed overall and combinations of major river (North Branch vs. South Branch) and physiographic province/region (Highlands vs. Piedmont) using Spearman Rank correlation. In addition, relationships of macroinvertebrate metrics to local habitat, chemical, and land use-land cover parameters were explored.
A total of 3,608 Spearman Rank correlation analyses of time with 15 macroinvertebrate community metrics were run using site-level data and results were tallied. Most of the sites did not exhibit significant trends in the High Gradient Macroinvertebrate Index (HGMI) during the 2 long-term periods, 1992-2024 and 2002-2024. Several trends in underlying pollution-tolerant species metrics account for these trends. There has been a trend of increasing proportion of biologically impaired sites watershed-wide since about 2010. This trend is most pronounced in the North Branch, especially in the Highlands.
This study identifies specific sites and regions where preservation of forest and/or watershed restoration should be targeted. This study shows that both local stream conditions as well as catchment-level land use are impacting the benthic macroinvertebrate community likely due to increased frequency of major storms with climate change in this headwaters and urban source water region.