Long-term trends in aquatic ecosystem bioassessment metrics are not influenced by sampling method: empirical evidence from the Niobrara River
Choosing an aquatic invertebrate sampling method for biomonitoring depends upon study goals, resources, and ecosystem conditions. In this study, we compared two methods that are widely used in stream ecology, but have not been directly compared: Hester–Dendy (HD) and Hess sampling. Hester–Dendy sampling uses artificial substrate that invertebrates colonize over a specific period of time. In contrast, Hess samplers surround a fixed area of natural substrate with a net. To compare approaches, we combined 5 years of simultaneous HD and Hess data collection (2010–2014) from the Niobrara River with a 14-year (1996–2009) historical HD data set for the same study sites. We used this full 19-year data set to assess how ecosystem health has changed in the Niobrara River over time, while also testing the influence of HD versus Hess data (2010–2014) on historical trends (1996–2009). Our results showed that HD samples are more taxonomically variable and bias bioassessment metrics because they collect more sensitive taxa versus Hess sampling. However, when combined with the 1996–2009 HD data set, both recent HD and Hess data sets recovered the same trend of declining ecosystem health in the Niobrara River. These results provide empirical evidence that even when historical HD data are combined with recent Hess data, long-term bioassessment trends remain unchanged despite more accurate perspectives of invertebrate assemblages being collected.