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[AHW24-P18] Different responses between copepod and cladoceran zooplankton to their growth and production by weather-mediated disturbance in Lake Biwa, Japan
Keywords:Zooplankton, population dynamics, copepod, cladoceran, typhoon disturbance, freshwater lake
Two zooplankton taxa of copepod Eodiaptomus japonicus and cladoceran Daphnia spp., including D. galeata and D. pulicaria, were always dominant in Lake Biwa, the largest lake in Japan, contributing >80% of total zooplankton biomass over 4 decades. A water column of the lake was severely disturbed by a couple of typhoon in 2018, whereas relatively calm in 2019 because of no typhoon visit. In these different conditions between the consecutive two years, we investigated population dynamics of the two dominant zooplankton. In this study, we determined in situ growth rates and calculated production for these two zooplankton in the contrasted two years, discuss about how weather-mediated disturbance influences the two different zooplankton taxa.
Zooplankton samples were collected with a vertical plankton net haul (diameter, 30 cm; mesh size, 100 μm) from the bottom to the surface at a pelagic site (35º18′32.6′′N, 136º8′38.9′E, 70 m deep) in north basin of Lake Biwa from Apr. 2018 to Dec. 2019, to determine zooplankton growth, biomass and production. Water temperature throughout the water column was measured using a CTD profiler. Average chlorophyl a (chl. a) concentrations in the 0-20 m water column were measured as a proxy of phytoplankton biomass. Methodology of calculating zooplankton production was referred to the previous studies (Liu et al. 2021a, b). Biomass (Bi) was calculated from Di × Wi, where Di and Wi are population density and body dry-weight in taxon i, respectively; production of taxon i was calculated from Bi × Gi × 0.447, where Gi was population growth rate, and a factor of 0.447 was used to convert body dry-weight into carbon weight. G1 for E. japonicus was calculated from G1 = (0.1074T − 0.8587)f− 0.09078T + 0.7556, where T was average water temperature for copepod developmental period, f was copepod food index, that is the ratio of female prosome length to the potential one; G2 for Daphnia spp. was calculated from G2 = eg – 1, where g was a growth coefficient calculated from equation obtained with bottle incubation experiments: g = 0.006T + 0.003TP – 0.004D – 0.037, where T and TP was the average water temperature and total phosphorous concentration in the 0-20 m water column.
Average T above 20 m showed similar seasonal variation among the two years, but TP concentrations were almost 2-fold higher in 2018 comparted to that in 2019, being 13.4 and 7.7 μg L-1, respectively, on annual average. Chl. a concentration showed clear bimodal seasonal variation; first peak occurred in June and second in Oct. to Nov. in both years, but higher in 2018, being 5.5 and 4.3 μg L-1 in 2018 and 2019, respectively, on annual average. Therefore, weather-mediated disturbance might enhance phytoplankton growth in the lake. Whereas, f-index showed different seasonal and year-to-year variations from those of chl. aconcentrations; it was high until June, but declined after that, then increased again after Oct. The f-indexes were rather higher in 2019 than those in 2018 until Sep., suggesting better food conditions for the copepod in 2019.
In 2018, biomass of E. japonicus declined from Apr. to May, increased in Aug., and then decreased in Sep., while no spring high biomass and one month delay of summer peak were found in 2019. Seasonal variation of biomass in Daphnia spp. was completely different from that of E. japonicus; Spring increases were found in May but declined after that in both years. Growth rates also showed different seasonal pattern between these two taxa. Higher growth rates of Daphnia spp. were obtained in 2018 compared to that in 2019, whereas opposite result was found in E. japonicus until Sep. because different food conditions for the two taxa between 2018 and 2019. Consequently, annual productions in Daphnia spp. were higher in 2018 than 2019, being 12.3 and 7.6 gC m-2 yr-1, respectively, whereas those in E. japonicus were similar, 4.2-5.8 gC m-2 yr-1 between the two years. Therefore, enhancement of phytoplankton growth by typhoon-mediated disturbance may enhance production of Daphnia spp. but not that of E. japonicus. These different responses found in the two taxa might be related to their feeding habits; Daphnia spp. are herbivores while E. japonicus is omnivore, mainly depending on microzooplankton instead of phytoplankton. Comparing the present results with the previous decadal variation, lower growth rates were found in E. japonicus, but not in Daphnia spp. even under similar chl. a concentrations. This implies severe food conditions for E. japonicus in recent years compared with those before 2010 even if phytoplankton production was not so depressed.
Reference:
Liu et al. (2021a) JpGU meeting 2021, AHW22-P05
Liu et al. (2021b) Limnol. Oceanogr., 66:3783-3795
Zooplankton samples were collected with a vertical plankton net haul (diameter, 30 cm; mesh size, 100 μm) from the bottom to the surface at a pelagic site (35º18′32.6′′N, 136º8′38.9′E, 70 m deep) in north basin of Lake Biwa from Apr. 2018 to Dec. 2019, to determine zooplankton growth, biomass and production. Water temperature throughout the water column was measured using a CTD profiler. Average chlorophyl a (chl. a) concentrations in the 0-20 m water column were measured as a proxy of phytoplankton biomass. Methodology of calculating zooplankton production was referred to the previous studies (Liu et al. 2021a, b). Biomass (Bi) was calculated from Di × Wi, where Di and Wi are population density and body dry-weight in taxon i, respectively; production of taxon i was calculated from Bi × Gi × 0.447, where Gi was population growth rate, and a factor of 0.447 was used to convert body dry-weight into carbon weight. G1 for E. japonicus was calculated from G1 = (0.1074T − 0.8587)f− 0.09078T + 0.7556, where T was average water temperature for copepod developmental period, f was copepod food index, that is the ratio of female prosome length to the potential one; G2 for Daphnia spp. was calculated from G2 = eg – 1, where g was a growth coefficient calculated from equation obtained with bottle incubation experiments: g = 0.006T + 0.003TP – 0.004D – 0.037, where T and TP was the average water temperature and total phosphorous concentration in the 0-20 m water column.
Average T above 20 m showed similar seasonal variation among the two years, but TP concentrations were almost 2-fold higher in 2018 comparted to that in 2019, being 13.4 and 7.7 μg L-1, respectively, on annual average. Chl. a concentration showed clear bimodal seasonal variation; first peak occurred in June and second in Oct. to Nov. in both years, but higher in 2018, being 5.5 and 4.3 μg L-1 in 2018 and 2019, respectively, on annual average. Therefore, weather-mediated disturbance might enhance phytoplankton growth in the lake. Whereas, f-index showed different seasonal and year-to-year variations from those of chl. aconcentrations; it was high until June, but declined after that, then increased again after Oct. The f-indexes were rather higher in 2019 than those in 2018 until Sep., suggesting better food conditions for the copepod in 2019.
In 2018, biomass of E. japonicus declined from Apr. to May, increased in Aug., and then decreased in Sep., while no spring high biomass and one month delay of summer peak were found in 2019. Seasonal variation of biomass in Daphnia spp. was completely different from that of E. japonicus; Spring increases were found in May but declined after that in both years. Growth rates also showed different seasonal pattern between these two taxa. Higher growth rates of Daphnia spp. were obtained in 2018 compared to that in 2019, whereas opposite result was found in E. japonicus until Sep. because different food conditions for the two taxa between 2018 and 2019. Consequently, annual productions in Daphnia spp. were higher in 2018 than 2019, being 12.3 and 7.6 gC m-2 yr-1, respectively, whereas those in E. japonicus were similar, 4.2-5.8 gC m-2 yr-1 between the two years. Therefore, enhancement of phytoplankton growth by typhoon-mediated disturbance may enhance production of Daphnia spp. but not that of E. japonicus. These different responses found in the two taxa might be related to their feeding habits; Daphnia spp. are herbivores while E. japonicus is omnivore, mainly depending on microzooplankton instead of phytoplankton. Comparing the present results with the previous decadal variation, lower growth rates were found in E. japonicus, but not in Daphnia spp. even under similar chl. a concentrations. This implies severe food conditions for E. japonicus in recent years compared with those before 2010 even if phytoplankton production was not so depressed.
Reference:
Liu et al. (2021a) JpGU meeting 2021, AHW22-P05
Liu et al. (2021b) Limnol. Oceanogr., 66:3783-3795