Long-term comparison of invertebrate communities in a blackwater river reveals taxon-specific biomass change

Abstract

Around the world, researchers are reporting declines in insect fauna. Although uncommonly evaluated in high-profile studies of insect declines, the community context of population trends can facilitate interpretation of the causes and consequences of such losses. Here, we aimed to explore the shifts in a well-studied invertebrate community of a blackwater river and identify potential catalysts of such change. We compared the density, biomass and community structure of freshwater invertebrate assemblages separated by more than 30 years in the Ogeechee River, in the southeastern U.S.A., and found biomass declines. We also evaluated long-term trends in river discharge, water temperature and precipitation. Overall, the biomass in the 2010s was approximately 60% of the total in the 1980s. Community analyses indicated that this decline was associated with reduced densities of large-bodied, filter-feeding insects, particularly Hydropsychidae caddisflies (Trichoptera). Conversely, predators and small-bodied primary consumers increased in density, although their contributions to overall biomass were minimal and their increased density was not sufficient to compensate for biomass declines. Seasonal shifts in both invertebrate populations and environmental parameters were evident, especially when focusing on discharge and dissolved organic carbon. Through a combination of direct analysis and the use of established research on the metabolic dynamics of the study site, we determined that the overall decline of freshwater invertebrate biomass may have been driven by climate-related changes in flood dynamics: seasonal flooding that facilitates delivery of floodplain carbon to filter-feeding consumers had decreased over several decades. Water temperature also had increased and was likely to have had effects on the invertebrate assemblages. Whole-community evaluations such as this one, in contrast to single-taxon and abundance-based studies, provide critical information to elucidate the dynamics of freshwater impairment and insect loss in the Anthropocene.