Present status of submerged macrophyte growth in Lake Biwa: Recent recovery following a summer decline in the water level

Abstract

Whole‐lake areal distribution of submerged macrophyte growth in Lake Biwa was surveyed using aerial photographs in 1994 and 2000, when unusual declines in the water level of approximately 1 m occurred in the dry summers. The estimated total areas of the submerged macrophyte communities were 1441 ha for 1994 and 2825 ha for 2000. In the shallow and eutrophic southern basin, the bottom area covered by vegetation gradually increased year by year after 1994, and the ratio of this area to the total surface area of the basin in 2000 amounted to 52%. The dominant species were two natives, Hydrilla verticillata and Myriophyllum spicatum, and one exotic, Egeria densa. Changes in physical conditions and water quality associated with the decline in the water level, as well as the reactions of submerged plants to aquatic environments and ecosystem dynamics, are discussed.

A Satellite-Based Assessment of the Distribution and Biomass of Submerged Aquatic Vegetation in the Optically Shallow Basin of Lake Biwa

“…Conversely, due to the time difference of ±5 days in the observed water transparency data and the Landsat-8 image acquisition time, a good agreement between the observed and satellite-derived water transparency was not expected (the obtained R 2 is 0.77). In the south basin of Lake Biwa, the amplified SAV growth is considered as a key driving factor for improved water clarity [22,61]. In this study, the mapped SAV coverage area does not rise as expected, with increased water clarity for the same year.…”

“…However, with the increase in turbidity the detection reduces to a maximum canopy depth of 0.46 m to 0.5 m. The result further suggests that increased turbidity significantly affects SAV detection. With increased water level in Lake Biwa, it is not possible to detect all the SAV (i.e., canopy depth increases) [22]. In the south basin, based on the in situ survey and previous studies [61], we found that SAV stand height varies (i.e., <0.2 m to >3 m) with location and water depth.…”

“…In general, the changes in SAV percent cover, stand height, and biomass are monitored using the traditional site-specific diver survey collecting the sample from a grid size of 50 cm × 50 cm, which cannot accurately account for the spatial variation in the basin. In Lake Biwa, the basin-wise assessment of the spatial extent of SAV cover and biomass is limited, primarily because the in situ survey of large lakes using traditional methods is time-consuming and cost-prohibitive [22,57,59,62].…”

“…Consequently, submerged macrophytes recovered in the lake with increased transparency and improved water quality [57]. The coverage of submerged macrophytes in the southern basin increased dramatically from 6 km 2 in 1994 to 50 km 2 in 2014, covering more than 90% of the lake bottom [22,59]. The prolific growth of invasive alien species such as Egeria Densa and Elodea Nuttallii has reduced the dispersal and biomass of the native SAV species, Vallisneria Asiatica var.…”

“…The SAV samples were harvested along with their roots (i.e., pulled out completely) from the 0.25 m 2 of the quadrat area by the scientific diver (professional diver working for LBERI). Because the survey was conducted with LBERI, we followed their method of SAV sample collection in order to maintain the consistency of the data using the same method used for the past years by LBERI to monitor the SAV biomass [22,62]. The position of each quadrat was recorded using a global positioning system (GPS) followed by the camera photographs of the sampled site.…”