2:30 PM - 2:45 PM
[AHW18-14] The fate of benthic biodiversity in a deep ancient Lake Biwa under changing climate
Keywords:Benthic macroinvertebrate, Global warming, Hypoxia, Biodiversity loss, Simulation model, Incomplete mixing
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 warming scenarios. However, such a thermal model is too simple to assess the climate impacts on benthic biodiversity in deep lakes because they are more vulnerable to climate-induced hypoxia in the benthic habitats rather than to warming itself. In some subtropical lakes, benthic faunas have recently become extinct due to hypoxia caused by incomplete vertical mixing in lasting warm years. Such a mass extinction in the subtropical lakes may mirror the fate of benthic biodiversity in temperate lakes. However, we still have limited evidence to show that benthic faunas in the temperate region are facing the biodiversity loss under changing climate.
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. In 2007 when vertical mixing was incomplete in abnormally warm winter, profundal hypoxia caused the mass mortality for an endemic benthic fish and some other macroinvertebrates. It was fortunate that the benthic habitat quickly recovered from the hypoxia after the complete mixing due to strong cold weather in the following year. However, warm winter and subsequent profundal hypoxia occurred again in 2019. We expect that frequency and intensity of recurring hypoxia will increase under changing climate, putting the benthic faunas at the risk of accelerating mass extinction.
Here we construct a simulation model to assess the long-term climate impacts on benthic biodiversity in Lake Biwa under scenarios of climate changes and social adaptation, in combination with synoptic monitoring to incorporate community responses to spatial environmental variations into the 3D physical-biogeochemical lake model.
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 of Lake Biwa during October 4-15, 2021. As environmental variables at each site, we measured water temperature (WT), electric conductivity, dissolved oxygen (DO) and chlorophyll a concentrations on the lake bottom using a CTD profiler and oxidation-reduction potential 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 density (number of species per unit area) and Shannon–Wiener diversity index (H′). After statistically testing community responses to spatial environmental variations, we integrated a regression model with significant environmental variables into a 3D physical-biogeochemical lake model which can project environments on the bottom of north basin on a 1000-m mesh. Using the integrated model, we visualize the spatial pattern of benthic biodiversity along the depth gradient of the north basin up to the next century under a combination of IPCC climate and social adaptation scenarios.
3. Results & Discussion
Bottom DO concentrations varied from 0.1 to 8.3 mg-O2/L through the whole basin. 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. Species density (/100cm2) varied from 0.75 to 4.44 and H′ from 0.06 to 1.63. Some species were sensitive to the variations in DO as well as in WT. As a whole community, the species density and H′ showed significantly negative correlation with the bottom DO. The integrated model projected that the lake bottom area with lower biodiversity would expand more quickly over the whole basin toward the next century, because of serious oxygen depression under the highest (RCP8.5) than the lowest (RCP2.6) CO2 emission scenarios. However, the model also suggests that there are some scopes to mitigate the biodiversity loss if nutrient loadings from the catchment can be substantially reduced, considering the fact that many endemic species persisted in oligotrophic state during the last interglacial period when climates were much warmer than present.
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 warming scenarios. However, such a thermal model is too simple to assess the climate impacts on benthic biodiversity in deep lakes because they are more vulnerable to climate-induced hypoxia in the benthic habitats rather than to warming itself. In some subtropical lakes, benthic faunas have recently become extinct due to hypoxia caused by incomplete vertical mixing in lasting warm years. Such a mass extinction in the subtropical lakes may mirror the fate of benthic biodiversity in temperate lakes. However, we still have limited evidence to show that benthic faunas in the temperate region are facing the biodiversity loss under changing climate.
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. In 2007 when vertical mixing was incomplete in abnormally warm winter, profundal hypoxia caused the mass mortality for an endemic benthic fish and some other macroinvertebrates. It was fortunate that the benthic habitat quickly recovered from the hypoxia after the complete mixing due to strong cold weather in the following year. However, warm winter and subsequent profundal hypoxia occurred again in 2019. We expect that frequency and intensity of recurring hypoxia will increase under changing climate, putting the benthic faunas at the risk of accelerating mass extinction.
Here we construct a simulation model to assess the long-term climate impacts on benthic biodiversity in Lake Biwa under scenarios of climate changes and social adaptation, in combination with synoptic monitoring to incorporate community responses to spatial environmental variations into the 3D physical-biogeochemical lake model.
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 of Lake Biwa during October 4-15, 2021. As environmental variables at each site, we measured water temperature (WT), electric conductivity, dissolved oxygen (DO) and chlorophyll a concentrations on the lake bottom using a CTD profiler and oxidation-reduction potential 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 density (number of species per unit area) and Shannon–Wiener diversity index (H′). After statistically testing community responses to spatial environmental variations, we integrated a regression model with significant environmental variables into a 3D physical-biogeochemical lake model which can project environments on the bottom of north basin on a 1000-m mesh. Using the integrated model, we visualize the spatial pattern of benthic biodiversity along the depth gradient of the north basin up to the next century under a combination of IPCC climate and social adaptation scenarios.
3. Results & Discussion
Bottom DO concentrations varied from 0.1 to 8.3 mg-O2/L through the whole basin. 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. Species density (/100cm2) varied from 0.75 to 4.44 and H′ from 0.06 to 1.63. Some species were sensitive to the variations in DO as well as in WT. As a whole community, the species density and H′ showed significantly negative correlation with the bottom DO. The integrated model projected that the lake bottom area with lower biodiversity would expand more quickly over the whole basin toward the next century, because of serious oxygen depression under the highest (RCP8.5) than the lowest (RCP2.6) CO2 emission scenarios. However, the model also suggests that there are some scopes to mitigate the biodiversity loss if nutrient loadings from the catchment can be substantially reduced, considering the fact that many endemic species persisted in oligotrophic state during the last interglacial period when climates were much warmer than present.