Size spectra and temporal synchrony patterns in temperate and tropical river networks

Abstract

Body size is one of the most important aspects of an organism, and is one of the major determinants of metabolism, a vital life function. The distribution of community biomass in a system are measurable variables that respond to temperature, disturbance, and species interactions. Temporal and regional trends in biomass are less understood. To investigate how regional climatic variables shape temporal and spatial distribution of biomass in stream ecosystems, we sampled benthic macroinvertebrates across a gradient of stream sizes in one tropical and one temperate river network over 4 sample periods. We conducted a size spectra analysis to quantify change occurring over time and across space. We were interested in quantifying how similar communities were between sample periods, across stream sizes, and between regions. Analyzing temporal synchrony is the method we used to quantify these similarities and differences. We expected lower variability in tropical size spectra, and asynchrony patterns over time in tropical sites due to metabolic constraints and high turnover of individuals caused by consistent high temperatures. We expected higher variability in temperate size spectra and synchronous dynamics as communities would be compositionally similar over sample periods but follow seasonal cues for growth and emergence. Tropical stream communities did not exhibit change in response to seasonal or spatial variables, and communities were asynchronous over time across stream sizes. Temperate streams responded to seasonal as well as spatial variables, and communities were asynchronous at small stream sizes and variable at large stream sizes. Our findings support the importance of local as well as regional factors in driving biomass patterns, and point to the increased importance of within-network sampling as a mode to understand how future shifts in climatic regimes will impact freshwater communities.