5:15 PM - 6:30 PM
[AHW22-P05] Long-term trends in zooplankton production and ecological transfer efficiency over four decades in Lake Biwa, Japan
Keywords:zooplankton, long-term production dynamics, ecological transfer efficiency, lake
Lake Biwa is the largest and the oldest lake in Japan. Cladoceran Daphnia spp., and copepod Eodiaptomus japonicus and Cyclopoida spp. were always dominant zooplankton in this lake over the past half century, contributing >90% of total zooplankton biomass. In this study, we tried to make prediction equations for estimating population growth rates of the three dominant zooplankton species from several abiotic and biotic variables, i.e. temperature, food conditions and individual density, for calculating the zooplankton production. Finally, we calculate ecological transfer efficiency between phytoplankton and dominant zooplankton productions, and discuss about the long-term trends over the four decades (1971-2010) in a large temperate lake.
Zooplankton assemblages were collected at pelagic stations (60-70 m deep) from June 2018 to July 2020, and used for incubation experiments to determine the somatic growth rates (g) in different temperature, food and density conditions. Each 10−15 L lake water with zooplankton assemblages was collected using a 20-L Schindler-Patalas zooplankton trap from 0−20 m with 5 m interval, combined in a 130-L plastic tank, and concentrated with a 100- or 200-μm-mesh to 1 L. The lake water with concentrated zooplankton was poured to each of six 3-L or four 5-L transparent plastic bottles, filled with the filtered lake water with a 100-μm mesh. Zooplankton densities (D) were arranged at 1-5 fold higher than those in situ to make broader range of density. Remained zooplankton assemblages were used to determine the initial zooplankton body size.
In 2018 and 2019, zooplankton assemblages were incubated in an incubator for 4 days at average in situ water temperature (T) and light intensity in 0−20 m of water column and photoperiod when the zooplankton was collected. In 2020, the incubation experiments were made with almost similar procedure but included additional higher temperatures. Total phosphorus (TP) and chlorophyll a (Chl. a) concentrations were measured as a proxy of food condition (F). After incubation, zooplankton number were counted, and the body sizes were measured. The g was calculated using equation: g = (lnW4−lnW0)/4, where W0 and W4 were the mean body weights in zooplankton before and after incubation, respectively. Body weights were estimated from body size using the length-weight equations. To estimate the g from multiple environmental parameters, the g was fitted with a multiple linear regression model, g = aT + bF + cD + d using RStudio software; each of TP and Chl. a as F was fitted in the model 1 and model 2, respectively. The g was transformed to population growth rate (G) using equation: G = eg−1. Zooplankton production (P) was calculated using equation: P = B × G × 0.447. The ecological transfer efficiency (E) was calculated from total zooplankton production divided by primary production of phytoplankton (Pp). Parameters of T, F, D, B and Pp referred to previous studies.
The g in Daphnia spp., E. japonicus and Cyclopoida spp. showed positive correlation with T and F but negative with D, largely varied 0.01−0.19, 0.01−0.08, 0.01−0.13 day-1, respectively, due to seasonal variation. R2 of the fitted multiple regression models for the g was 0.6−0.7 in Daphnia spp., whereas it was only 0.1−0.2 and 0.3 in E. japonicus and Cyclopoida spp. respectively, indicated that multiple regression model estimated in this study can well describe somatic growth in Daphnia spp. but not in copepods. In Daphnia spp., the model 1 is more suitable to estimate g in Daphnia spp. due to the smaller AIC. Therefore, we estimated g of Daphnia spp. in Lake Biwa using model 1 and projected the P from 1971 to 2010. Over the four decades, annual P of Daphnia spp. varied 0.5−4.8 gC m-2 y-1, 2.8 gC m-2 y-1 on average. It showed similar trend as the biomass, decreased from 1970s until 1993 and exhibited an increasing trend after 1993. Productions of E. japonicus in Lake Biwa were calculated using another method of length-based food model in our previous study. Since sum of Daphnia spp. and E. japonicus biomasses accounts for >80% of total zooplankton biomass in this lake, secondary production can be calculated from these two taxa. The calculated production varied 5.7−56.4 gC m-2 y-1,on average 20.9 gC m-2 y-1, whereas the E showed a relative constant trend over four decades, averaging in 9.2%. It also showed 2-fold high value, ca. 20% in late 2000s might be due to the low primary production. Pp in Lake Biwa were measured by different methods and locations. Variance of E might be partially regulated by estimating accuracy of Pp.
Zooplankton assemblages were collected at pelagic stations (60-70 m deep) from June 2018 to July 2020, and used for incubation experiments to determine the somatic growth rates (g) in different temperature, food and density conditions. Each 10−15 L lake water with zooplankton assemblages was collected using a 20-L Schindler-Patalas zooplankton trap from 0−20 m with 5 m interval, combined in a 130-L plastic tank, and concentrated with a 100- or 200-μm-mesh to 1 L. The lake water with concentrated zooplankton was poured to each of six 3-L or four 5-L transparent plastic bottles, filled with the filtered lake water with a 100-μm mesh. Zooplankton densities (D) were arranged at 1-5 fold higher than those in situ to make broader range of density. Remained zooplankton assemblages were used to determine the initial zooplankton body size.
In 2018 and 2019, zooplankton assemblages were incubated in an incubator for 4 days at average in situ water temperature (T) and light intensity in 0−20 m of water column and photoperiod when the zooplankton was collected. In 2020, the incubation experiments were made with almost similar procedure but included additional higher temperatures. Total phosphorus (TP) and chlorophyll a (Chl. a) concentrations were measured as a proxy of food condition (F). After incubation, zooplankton number were counted, and the body sizes were measured. The g was calculated using equation: g = (lnW4−lnW0)/4, where W0 and W4 were the mean body weights in zooplankton before and after incubation, respectively. Body weights were estimated from body size using the length-weight equations. To estimate the g from multiple environmental parameters, the g was fitted with a multiple linear regression model, g = aT + bF + cD + d using RStudio software; each of TP and Chl. a as F was fitted in the model 1 and model 2, respectively. The g was transformed to population growth rate (G) using equation: G = eg−1. Zooplankton production (P) was calculated using equation: P = B × G × 0.447. The ecological transfer efficiency (E) was calculated from total zooplankton production divided by primary production of phytoplankton (Pp). Parameters of T, F, D, B and Pp referred to previous studies.
The g in Daphnia spp., E. japonicus and Cyclopoida spp. showed positive correlation with T and F but negative with D, largely varied 0.01−0.19, 0.01−0.08, 0.01−0.13 day-1, respectively, due to seasonal variation. R2 of the fitted multiple regression models for the g was 0.6−0.7 in Daphnia spp., whereas it was only 0.1−0.2 and 0.3 in E. japonicus and Cyclopoida spp. respectively, indicated that multiple regression model estimated in this study can well describe somatic growth in Daphnia spp. but not in copepods. In Daphnia spp., the model 1 is more suitable to estimate g in Daphnia spp. due to the smaller AIC. Therefore, we estimated g of Daphnia spp. in Lake Biwa using model 1 and projected the P from 1971 to 2010. Over the four decades, annual P of Daphnia spp. varied 0.5−4.8 gC m-2 y-1, 2.8 gC m-2 y-1 on average. It showed similar trend as the biomass, decreased from 1970s until 1993 and exhibited an increasing trend after 1993. Productions of E. japonicus in Lake Biwa were calculated using another method of length-based food model in our previous study. Since sum of Daphnia spp. and E. japonicus biomasses accounts for >80% of total zooplankton biomass in this lake, secondary production can be calculated from these two taxa. The calculated production varied 5.7−56.4 gC m-2 y-1,on average 20.9 gC m-2 y-1, whereas the E showed a relative constant trend over four decades, averaging in 9.2%. It also showed 2-fold high value, ca. 20% in late 2000s might be due to the low primary production. Pp in Lake Biwa were measured by different methods and locations. Variance of E might be partially regulated by estimating accuracy of Pp.