09:00 〜 09:15
[AHW24-01] 古代湖・琵琶湖のベントス多様性に及ぼす温暖化影響
キーワード：底生大型無脊椎動物、地球温暖化、貧酸素化、循環不全、生物多様性、気候影響
1. Introduction
The increasing number of researches have reported warming trends in the world’s lakes over the last half century but we have poorly understood climate impacts on lake biodiversity. Simple thermal models used to project population extinction of cool water species or community shifts from the cool to warm water species under climate change scenarios. However, such a thermal model is too simple to assess the climate impacts on benthic communities in deep lakes because they are more vulnerable to climate-induced hypoxia in the benthic habitats rather than to warming itself. In addition, there is limited evidence to show that benthic biodiversity has been decreasing in deep lakes with the warming trend.
A deep ancient Lake Biwa with high biodiversity and endemism can provide a rare opportunity to assess climate impacts on benthic communities in deep waters through the long-term monitoring since 1965. The long-term data revealed that species richness and biodiversity index for benthic macroinvertebrates at the offshore monitoring station drastically decreased in the late 1980’s when climate regime shift occurred (Strategic Research Project S-9, 2016). However, these two indices recovered thereafter though the warming trend continued. Such a recovery can be explained by colonization of warm water and hypoxia-tolerant species, which are dwelling in warm and eutrophic coastal waters. The biodiversity index showed significant but weak negative correlation with dissolved oxygen (DO) concentration of profundal waters, while there was no significant correlation between the species richness and the profundal DO. Such unclear correlations may also be partly due to difficulty in in-situ assessment of benthic microhabitats in the early period of long-term monitoring when sophisticated measuring instrument is unavailable.
Here we conducted synoptic monitoring to relate benthic macroinvertebrate community compositions to the lake bottom environments in the offshore after a serious warming event in 2019 when winter vertical mixing was incomplete. We took two different approaches to assess oxic/anoxic condition in the boundary layer of lake bottom. While assuming that deeper habitats are more susceptible to oxygen depletion due to the incomplete mixing, we examine how the lake bottom environments can affect benthic biodiversity across the depth gradient of the whole basin.
2. Materials & Methods
We conducted synoptic monitoring for lake bottom environments and benthic macroinvertebrates at 15 offshore sites, whose depths ranged from 3.6 to 91.2 m, in the south and north basins during 4-15 October, 2021. One site was selected from an artificial depression with a depth of 11.6 m, which was created by dredging in the south basin with an average depth of ca. 4 m. For each site, we measured water temperature (WT) and DO concentrations on the lake bottom using a CTD profiler and oxidation-reduction potential (ORP) on the lake sediment surface using a HR type core sampler and portable ORP censor. We also collected benthic macroinvertebrates using a 15×15 or 20×20 grab sampler. We sorted, identified and counted the macroinvertebrate samples to calculate species richness and Shannon–Wiener diversity index (H′).
3. Results & Discussion
Bottom DO concentrations varied from 8.3 mg-O2/L at the shallowest site to 0.1 mg-O2/L at the artificial depression site. The bottom DO was tightly correlated with the lake depth, except for the artificial depression site with the lowest DO in spite of its shallowness. Sedimentary ORP was lower at shallower sites around the south basin with cultural eutrophication, suggesting higher organic loading. Species richness varied from 3 to 10 and H′ from 0.06 to 1.63. The deepest and artificial depression sites under hypoxic condition were overwhelmingly dominated by oligochaete but their taxonomic compositions were quite different between them: the former was characterized by cool water species and the latter by warm water species. Species richness and H′ showed significantly negative correlation with the bottom DO but not with the bottom WT and sedimentary ORP.
Our synoptic monitoring demonstrated that benthic diversity could be strongly affected by the bottom DO. A limno-physical model projects that the hypoxic area of lake bottom will expand to the shallower part of lake basin due to weakening of vertical mixing under climate change scenarios. According to this projection, there is concern that many benthic macroinvertebrate taxa, especially cool water and hypoxia-sensitive endemic species, may experience population extinction in the near future.
The increasing number of researches have reported warming trends in the world’s lakes over the last half century but we have poorly understood climate impacts on lake biodiversity. Simple thermal models used to project population extinction of cool water species or community shifts from the cool to warm water species under climate change scenarios. However, such a thermal model is too simple to assess the climate impacts on benthic communities in deep lakes because they are more vulnerable to climate-induced hypoxia in the benthic habitats rather than to warming itself. In addition, there is limited evidence to show that benthic biodiversity has been decreasing in deep lakes with the warming trend.
A deep ancient Lake Biwa with high biodiversity and endemism can provide a rare opportunity to assess climate impacts on benthic communities in deep waters through the long-term monitoring since 1965. The long-term data revealed that species richness and biodiversity index for benthic macroinvertebrates at the offshore monitoring station drastically decreased in the late 1980’s when climate regime shift occurred (Strategic Research Project S-9, 2016). However, these two indices recovered thereafter though the warming trend continued. Such a recovery can be explained by colonization of warm water and hypoxia-tolerant species, which are dwelling in warm and eutrophic coastal waters. The biodiversity index showed significant but weak negative correlation with dissolved oxygen (DO) concentration of profundal waters, while there was no significant correlation between the species richness and the profundal DO. Such unclear correlations may also be partly due to difficulty in in-situ assessment of benthic microhabitats in the early period of long-term monitoring when sophisticated measuring instrument is unavailable.
Here we conducted synoptic monitoring to relate benthic macroinvertebrate community compositions to the lake bottom environments in the offshore after a serious warming event in 2019 when winter vertical mixing was incomplete. We took two different approaches to assess oxic/anoxic condition in the boundary layer of lake bottom. While assuming that deeper habitats are more susceptible to oxygen depletion due to the incomplete mixing, we examine how the lake bottom environments can affect benthic biodiversity across the depth gradient of the whole basin.
2. Materials & Methods
We conducted synoptic monitoring for lake bottom environments and benthic macroinvertebrates at 15 offshore sites, whose depths ranged from 3.6 to 91.2 m, in the south and north basins during 4-15 October, 2021. One site was selected from an artificial depression with a depth of 11.6 m, which was created by dredging in the south basin with an average depth of ca. 4 m. For each site, we measured water temperature (WT) and DO concentrations on the lake bottom using a CTD profiler and oxidation-reduction potential (ORP) on the lake sediment surface using a HR type core sampler and portable ORP censor. We also collected benthic macroinvertebrates using a 15×15 or 20×20 grab sampler. We sorted, identified and counted the macroinvertebrate samples to calculate species richness and Shannon–Wiener diversity index (H′).
3. Results & Discussion
Bottom DO concentrations varied from 8.3 mg-O2/L at the shallowest site to 0.1 mg-O2/L at the artificial depression site. The bottom DO was tightly correlated with the lake depth, except for the artificial depression site with the lowest DO in spite of its shallowness. Sedimentary ORP was lower at shallower sites around the south basin with cultural eutrophication, suggesting higher organic loading. Species richness varied from 3 to 10 and H′ from 0.06 to 1.63. The deepest and artificial depression sites under hypoxic condition were overwhelmingly dominated by oligochaete but their taxonomic compositions were quite different between them: the former was characterized by cool water species and the latter by warm water species. Species richness and H′ showed significantly negative correlation with the bottom DO but not with the bottom WT and sedimentary ORP.
Our synoptic monitoring demonstrated that benthic diversity could be strongly affected by the bottom DO. A limno-physical model projects that the hypoxic area of lake bottom will expand to the shallower part of lake basin due to weakening of vertical mixing under climate change scenarios. According to this projection, there is concern that many benthic macroinvertebrate taxa, especially cool water and hypoxia-sensitive endemic species, may experience population extinction in the near future.