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).