Macroinvertebrate community responses to disturbance in a fragmented river with contrasting legacies of alteration

Flow is a critical factor determining riverine ecosystem structure and function. Widespread hydrologic alteration, however, has impacted the ecological integrity of rivers in ways that are not well understood, including responses of biological communities to increasingly frequent and severe climatic disturbances. Our study compared responses of invertebrate communities on woody debris to large flooding and extreme drought in two highly contrasting segments of an impaired low-gradient river. The upstream segment, which according to previous research has higher α-diversity and production of large-bodied and sensitive invertebrates, maintained higher flows and longitudinal connectivity throughout the four-year study. Communities in this upper segment resembled one another among sites (lower spatial turnover), but experienced greater temporal shifts in composition associated with hydrological disturbances. Conversely, invertebrate communities in the highly altered downstream segment, which is impaired by reduced flow, sedimentation, and hypoxia, were comprised of smaller-bodied and pollution-tolerant taxa with lower α-diversity. Unlike the upper segment, communities were patchily distributed among sites (higher spatial turnover), which made it more difficult to detect system-wide temporal variation in composition throughout the study. Our study underscores the benefit of including measures of connectivity and spatial heterogeneity when assessing the ecological integrity of lotic systems. Understanding system-wide response to disturbances across longer time frames can help better predict and mitigate the impacts of climate change on ecosystem integrity in degraded rivers.

Methods
These data sets contain macroinvertebrate community structure and environmental data from the Cache River in southern Illinois, USA, from Jun-Aug of 2010-2013.

Macroinvertebrates: We sampled eight sites (100-m reaches), four each upstream (UCR) and downstream (LCR) of the Post Creek Cutoff. We collected three macroinvertebrate samples at each site from large woody debris (snags) using a 250-μm mesh net, scrubbing the snag in a bucket of stream water, and preserving in ~8% formalin. We measured the length of the snag and the circumference in three places to estimate surface area. Preserved invertebrates were enumerated and identified using a dissecting microscope. Lengths were measured to the nearest mm and used to estimate biomass using published length-mass relationships. We estimated abundance and biomass m-2 of wood using the surface area of the snags collected and averaged three samples per site for each sample date at each study site. We then used site-specific wood habitat quantification data to convert estimates m-2 wood to m-2 channel bottom.

Metrics of integrity: Diversity was calculated using the diversity() function in the vegan package in R v4.2.1 using abundance data and the Shannon Diversity Index. To calculate total biomass (mg AFDM), we summed the biomass of all taxa for each site and sampling event. Body size (mg AFDM) was calculated by dividing total biomass (mg AFDM) by total abundance (number of individuals) per m2 channel bottom. EPT biomass (mg AFDM m-2 channel bottom) was calculated by summing the biomass values for all invertebrates within the orders Ephemeroptera, Plecoptera, and Trichoptera. We calculated Hilsenhoff Biotic Index (HBI) using published tolerance values. For voltinism analysis, we assigned taxa a voltinism category (semivoltine, univoltine, and bi/multivoltine) from the US EPA Freshwater Biological Traits Database based on geographic proximity when possible and calculated percent multivoltine taxa as a proportion of total abundance.

Environmental parameters: UCR discharge data were obtained from the USGS gage station at Foreman, IL. We measured LCR discharge during February-May 2013 and combined these with measurements by Illinois Department of Natural Resources personnel from May 2011 and January-May 2013. We then established a correlation between measured LCR discharge events and daily UCR discharge from USGS, performed using Spearman rank correlation in R and extrapolated to estimate LCR discharge for 2011-2013. Using these discharge measurements, we calculated two discharge-related parameters: mean monthly discharge (m3 s-1) (hereby referred to as Qmo) and coefficient of variation of mean daily discharge (QCV) for each month, calculated as the standard deviation divided by the mean. We measured dissolved oxygen (DO) every 15 minutes for 24 hours once per month using a Hydrolab mini-sonde 5 (OTT Hydromet, Loveland, CO), ~0.4 m below the water surface at one site each in the UCR and LCR and calculated three DO parameters: mean (DOmean), minimum (DOmin), and maximum (DOmax) values (mg L-1) for the 24-hour period measured each month. We recorded hourly water temperature (ºC) with HOBO® data loggers at four of the eight study sites and calculated two temperature-related parameters: mean monthly temperature (Temp) and cumulative degree days (above 0ºC) as of each sampling date starting January 1 of each year (DegDays). For chlorophyll, we collected benthic algae samples monthly from three snag habitats at each study site by delineating and scrubbing a 4-cm2 area using a plastic template and steel brush. We also collected three sestonic algae samples monthly from the water column. Benthic slurries and sestonic samples were filtered through 0.7-μm glass fiber filters and extracted in 90% buffered acetone overnight in the dark and on ice, and the chlorophyll-a concentration was determined using fluorometry following EPA method 445.0. We then calculated two chlorophyll parameters: mean benthic (BenChl) and sestonic (SesChl) chlorophyll-a concentrations (μg m-2) for each site per sampling date.