Cumulative impacts of agricultural disturbance and sustained drought on stream invertebrate community recovery: Implications for restoration and management
Streams in anthropogenically dominated landscapes are increasingly imperiled by various land management practices and climate change, making them the focus of restoration efforts. Although restoration goals vary, they often include physical manipulations of in-stream habitat, and thus the restoration process can be a potential disturbance to an already stressed ecosystem. Our understanding of how these cumulative impacts negatively affect biota and the ability of communities to recover after these perturbations is lacking. To determine the ability of macroinvertebrate communities to recover from disturbances related to restoration, we compared modes of benthic macroinvertebrate recolonization at two sites of varying degradation (low vs. high) in an agriculturally impacted 3rd order stream in western Colorado. We placed traps designed to allow recolonization from one of four routes (upstream, downstream, aerial, and hyporheic) at both sites for three weeks during two consecutive drought years (2012 and 2013). Control traps (open to all routes) were used to establish overall macroinvertebrate community recovery and were also compared to natural benthic communities (surber samples). We predicted upstream traps at both sites would capture communities most similar to those of controls and surbers, but that overall recolonization would be lowest in 2013 due to lower stream discharge.
There was an overall community difference between sites during both years of the study. When compared to other trap types, upstream traps captured the most similar communities to control traps and surber samples at both sites in 2012 and 2013, but captured significantly lower abundance and biomass in 2013. Natural and experimental trap communities remained similar between years at the low impact site. In contrast to the increase in natural community diversity and biomass at the high impact site between 2012 and 2013, all recolonization traps declined in these measures between years. Communities captured by downstream, bottom, and aerial recolonization traps were highly variable, as expected. Reduced discharge in 2013 may have led to a decline in drifting invertebrates, which appear to be the main source of colonization, resulting in lower upstream and control trap abundance and biomass. Although natural community diversity and biomass increased at the high impact site, experimental traps experienced a decrease, suggesting much slower recovery in highly disturbed areas when coupled with prolonged drought. As climate change makes hydrology more unpredictable, successful stream restoration planning should consider the potential for altered recovery trajectories in disturbed communities when designing and implementing project goals.