Materials and Methods
Quantitative samples of benthic macroinvertebrates from NEON
wadeable-stream sites from 2017-2019 were downloaded to determine
size-spectra relationships (National Ecological Observatory Network
2020). NEON is a National Science Foundation (USA) funded program which
collects standardized samples from 81 sites (24 streams) across a
variety of terrestrial and aquatic ecosystems in North America. Repeat
collections include automated instrument recordings and observational
field sampling throughout the year, and data are available as open
source data products (https://data.neonscience.org/home). The
stream sites range across a broad environmental gradient (Figure 1),
spanning from 18 to 68° N latitude (majority between 33 to 45° N), mean
annual temperatures of -4 to 25° C, and mean annual precipitation of 331
to 2530 mm (Table 1). Additionally, sites are in varying biomes and
terrestrial plant communities, and subject to different local geologies
and flow regimes.
Sites were sampled 1-4 times each year across the local growing season.
Most sites had at least two years with three samples, except for COMO
which only had one year with three samples (2018), and two samples
collected in both 2017 and 2019. Additionally, three sites only had data
available from a single collection in a single year; REDB in 2018, WALK
and WLOU both in 2019. Details of sample collection and processing
protocols are available at the NEON website. Briefly, macroinvertebrate
samples were collected from a known area using the sampling method most
suited to a site. Laboratory processing included subsampling for
taxonomic identification and size class measurement (nearest mm) and
estimating the total count per sample. Estimated total counts were
standardized to individuals per m2 by dividing by the
area sampled (Chesney 2019). Macroinvertebrate size classes (mm) were
converted to individual dry mass (M, in mg) using published
length-weight regression coefficients. Most (96%) taxa had
taxon-specific length-weight regressions and these were used for model
development and assessment. Approximately 8% of the observations were
flagged by NEON as being damaged, affecting their length measurements,
and were removed from the data. The final data set included length
measurements for >82,000 individuals and counts that
totaled >17 million individuals.
Size spectra slopes have been estimated using a variety of methods, with
binning methods being common in the published literature (White et
al. 2008; Sprules & Barth 2015; Edwards et al. 2017). However,
recent comparative studies have shown binning methods to be inaccurate,
providing biased slope estimates, and recommend using maximum likelihood
methods (Edwards et al. 2017). We fitted body size data (dry
weight estimated from length-weight regressions) of macroinvertebrates
from each collection to a bounded power law distribution with a
probability density function:
\begin{equation}
f\left(x\right)=\ \frac{\left(b+1\right)x^{b}}{x_{\max}^{b+1}-x_{\min}^{b+1}},\ \ b\ \neq 1\nonumber \\
\end{equation}\begin{equation}
f\left(x\right)=\ \frac{1}{\log x_{\max}-logx_{\min}},\ \ b=1\nonumber \\
\end{equation}Where x is body mass, b is the scaling exponent (size
spectra slope), and the distribution is bounded by the minimum
(xmin ) and maximum (xmax )
body sizes observed in a collection. Maximum likelihood methods were
used to estimate the exponent, b , using code modified from
Edwards et al. (2017). Using these methods, a more negative bexponent represents a steeper slope of the size spectra. Slopes can be
steeper due to either a relative increase in the abundance of small body
sizes, or a relative decrease in the abundance of large body sizes, or a
combination of the two. Regardless, steeper slopes represent a
relatively smaller proportion of large individuals compared to small
individuals within a community.
In addition to the size spectra slopes, we also examined how total
community biomass varied across the collections. First, we multiplied
the estimated biomass of individuals by their estimated density to
calculate mg of dry mass per m2 for each sample within
a collection (n = 8 samples per collection). The 8 samples per
collection were used as individual observations in the hierarchical
Bayesian analyses (see Statistical Analyses ).
Macroinvertebrates were generally sampled three times within the growing
season from each site. However, due to the wide geographical
distribution of sites, there was significant variation in the sample
dates. For example, samples in Alaska (latitude 65-68° N) were collected
between May and August, whereas samples in Alabama (latitude 33° N) were
collected between February and November. Because we are interested in
the effects of temperature on size spectra, we used the mean annual
temperature in degrees Celsius for each site.