BIODIVERSITY MONITORING
We collected benthic macroinvertebrates in three replicates from each of
the 33 shallow coastal habitats of Lake Biwa in winter (February 2006)
using a 475-μm-mesh Sarvar net with a 30 × 30 cm quadrat (see Karube et
al. 2010 for details). At each sampling site, we also collected basal
resources, particulate organic matter (POM) and epilithic organic matter
(EOM), in November 2005, February, May, and July 2006, and measured the
quarterly dissolved oxygen (DO) concentration (mg/L) and water
temperature (WT) of the coastal waters (see Sakai et al., 2013 for
details). The average chlorophyll-a concentration for quarterly
POM and EOM samples was used as an indicator for phytoplankton and
benthic algal biomass, respectively, and the average DO and WT were used
as indicators for coastal environments. Among these coastal sites, 29
were located near the mouth of tributary rivers, which greatly vary in
catchment size and land use patterns, whereas four sites separate from
the river mouth were considered control sites with the least fluvial
impacts (see Sakai et al. 2013 for details). In this watershed,
agricultural land use is regarded as the most critical driver of
biodiversity loss in stream macroinvertebrate communities (Ko et al.,
2021), which can also impact lake macroinvertebrate diversity viariver inflows (Okano et al., 2018).
For three sub-samples at each site, macroinvertebrates were sorted,
identified, and counted, after which their dry mass was measured for
each taxon. The average biomass per unit area (mg/m2)
was calculated from the average individual body mass (mg) multiplied by
the average numerical abundance (individuals/m2) for
three replicates of each taxon at each site. The Shannon–Wiener
diversity index (H ′) was calculated as follows:
H ’ = \(\sum_{i=1}^{S}{(p_{i}\times\text{ln\ }p_{i})}\) (1)
where S and pi denote the total number of
taxa (i.e., taxonomic richness) and the relative numerical abundance of
each taxon in each of the local communities. Some small-sized animals,
such as oligochaetes, chironomids, and midges, were difficult to quickly
identify to a fine taxonomic resolution under fresh conditions for
stable isotope analysis, and leeches (Hirudinea) were difficult to
identify without anatomical observation. Since these animals were
classified into higher taxonomic levels, which can be characterized by
functional feeding groups, H ’ is regarded as an indicator of
functional diversity when considering numerical abundance and equality.
For carbon (δ13C) and nitrogen
(δ15N) stable isotope analysis, each taxon at each
site was provided individually or in bulk in one to three replicates,
depending on individual mass (see Karube et al., 2010; Sakai et al.,
2013 for methods of isotope sample treatment).