The Use of Molluscan Fauna as Model Taxon for the Ecological Classification of River Estuaries

Abstract

River estuaries are important aquatic environments characterized by large environmental gradients in their water quality, riverbed material, and microtopography in the longitudinal and transverse directions. The geography or habitats in river estuaries differ depending on the energy from the tide, waves, and river; therefore, the biota inhabiting river estuaries vary depending on the river estuary type. In view of this, for effective conservation in river estuaries, there is a need for information about potential habitats and biota based on objective data about the river estuary type. The objective of this study thus was to classify river estuaries by their molluscan fauna and physical indicators to reveal the relationship between molluscan fauna and the physical environment. The classification results using physical indicators indicated three types of river estuaries (wave energy-dominated group, tide energy-dominated group, and low tide and wave energy group). This classification result was similar to the classification of molluscan fauna. Therefore, it was suggested that molluscan fauna is extremely useful as a variable representing the river estuary environment. From the comparison between molluscan fauna and the physical environment, some rivers were not classified into the same group in the classification of molluscan fauna, despite them having similar physical environments. Some of these rivers with a molluscan fauna that diverged from expectations had undergone channel modification, which is expected to have caused a shift in this fauna group. These results suggest that this approach could be used to identify rivers that have been degraded by human activities.

Plankton and benthos similarity indices as indicators of the impact of mangrove plantation on the environmental quality of silvofishery ponds

“…The presence of Cerithidea cingulata in the ponds indicates the effect of mangrove development on the environment. The species C. cingulata is adapted to habitats with a large tidal range, abundant silt and vegetation (Itsukushima et al 2017). Cerithidea is known to be a mangrove-associated benthic species (Zvonareva et al 2015).…”

Differences in conservation candidate tidal rivers by cross‐taxon analysis in the Japanese temperate zone

“…In total, 14 environmental characteristics were used in this study. Previous studies found that these factors affect species richness and distribution in temperate estuaries, including tidal rivers (Inui et al, 2016; Itsukushima et al, 2017, 2019; Nicolas et al, 2010). In the present study, estuarine environmental characteristics refer to factors measured at or in river mouths, and basin environmental characteristics refer to factors measured within river systems.…”

“…The environmental characteristics of the Kyushu region are diverse because the island is surrounded by the Sea of Japan, the Pacific Ocean, and the East China Sea (Fujikura et al, 2010). As a result, various benthic communities adapted to an estuarine environment occur (Itsukushima et al, 2017; Itsukushima, Yoshikawa, & Morita, 2019). Although these estuarine environments have been devastated in central Japan, such as Tokyo Bay, as a result of anthropogenic impacts (Yuhara & Furota, 2014), natural environments remain in the Kyushu region (Henmi et al, 2014, 2017), which is an important conservation area for estuarine ecosystems in Japan.…”

“…The mean tidal range was calculated from the average of the difference between the monthly maximum and minimum tide levels from January to December 2012. As with the previous study (Itsukushima et al, 2017) tidal data were obtained from the Japan Coastguard website (https://www1.kaiho.mlit.go.jp/ KANKYO/TIDE/tide_pred/index.htm). Thirty-four observation points were checked, and the mean tidal length of each river was calculated from the data for the nearest observation point from the river mouth.…”

“…These topographic characteristics result in the limited to narrow extent of Japanese estuaries, from downstream to the river mouth, except in large semi‐closed bays such as Ariake Sound and Tokyo Bay. As various estuarine habitats are found in these narrow stretches of rivers and are used by a variety of species (Itsukushima, Morita, & Shimatani, 2017; Koyama et al, 2019), river management that combines biodiversity conservation and coastal protection is required. In the present study, the section of rivers affected by tides is defined as the tidal river and this includes the tidal freshwater zone (Hoitink & Jay, 2016).…”