Metal Accumulation in Net-Spinning Caddisflies, Hydropsychidae, of the Rouge River, Michigan

Abstract

Biological indicators are useful proxies for assessing water quality. By analyzing historic biological samples collected over many years, stakeholders can evaluate the health of a watershed and how it has changed over time. Measuring metal accumulation over a decade can reveal positive, negative, and geographical trends in a watershed. During the late 1990s and early 2000s, chemical analysis of the bed sediment, surface water, soil, and shallow groundwater indicated the Rouge River Watershed had elevated levels of toxic metal. Directly sampling soil, sediment, and water may not always accurately reflect the uptake of metals by living organisms. Therefore, a direct biological indicator is needed to more definitively examine the impact of toxic metals. Toxic metal contamination is an issue because at heightened concentrations many metals inhibit growth, development, motility, and neural function in animals; some are also known carcinogens. Here, I measured the metal concentrations in aquatic larvae of net-spinning caddisflies (Hydropsychidae) collected from the Rouge River watershed in 2006, 2008, 2012, and 2015. This type of caddisfly larvae can tolerate a wide range of metals, which makes them useful as a biological indicator because they will persist even in degraded streams, and can also be collected and analyzed years later. For example, the accumulation of Pb ranged from 0.5 – 127 ppm (mg/kg). Similarly, a wide range of concentrations were detected for Cr (0.2 – 16.4 ppm), Mn (80 – 8413 ppm), As (0.4 – 20.4 ppm), and Ba (6.0 – 329 ppm). Using 182 specimens of this biological indicator, I identified spatial and temporal patterns of metal accumulation across the Rouge River watershed. Hydropsychidae are suitable monitors for metal pollution, and can be used to complement other sampling methods to evaluate the extent of contamination in a watershed. The metal contents of sediment and surface water are limited in their ability to express the biological impact of metal contamination as the exposure pathways, uptake, and elimination of metals is heterogeneous across different taxa. The tissue analysis of these larvae can more accurately express the deleterious impact of toxic metals on freshwater organisms.