17:15 〜 18:30
[AHW22-P06] High resolution monitoring for subsidiary nutrient loadings and phytoplankton production in north basin of Lake Biwa
キーワード：琵琶湖、補助的栄養塩供給、高解像度モニタリング、係留系システム、内部波、植物プランクトン一次生産
In Lake Biwa, nutrient loading from non-point source is considered to be an one of the reasons why total phosphorus (TP) concentration did not decline to the level before eutrophication period. Our previous studies suggested that horizontal transportation of nutrients from littoral slope may enhance phytoplankton growth at a pelagic site in north basin of Lake Biwa around rice planting season. It implies that drainages from rice paddies may enhance phytoplankton production and prevent to make further reduction of TP concentration in the lake; that is, particulate phosphorus (PP) from the rice paddies may be deposited to littoral slope, resuspended from the bottom, transported to offshore through internal wave driven by wind, and finally enhance phytoplankton growth. We still do not have enough evidence to confirm this hypothesis, because such wind-driven events occur sporadically. In this study, we simultaneously monitored nutrient concentrations, phytoplankton production, and water movements with high resolution, i.e. minutes to daily, to detect such sporadic events.
A mooring system for monitoring phytoplankton production and lake water movement was situated nearby a buoy of Japan Water Agency (35o18.69'N, 136o08.68'E) from 28 April to 27 July 2020. Water temperatures were measured at 10 min interval at 1 and 5 m, and at 2-m interval from 10 to 30 m with a thermistor chain. In situ phytoplankton production was measured using light intensity and chlorophyll a (chl. a) concentration monitored by sensors attached at 5 m, 10 m, and 15 m and monthly obtained photosynthesis - light intensity curve. Water movements was measured at 10 min interval with an acoustic doppler current profiler (ADCP). Lake waters at 5 and 20 m were collected daily at noon for measuring nutrient concentrations, NO3-N, NH4-N, and PO4-P, with a water pump from 28 April to 17 June 2020. Apart from them, we weekly investigated vertical profiles of water temperature, chl. a and nutrient concentrations.
Water temperature gradually increased above 20 m from early May, being >15oC at surface, and then distinct thermocline developed in 10-20 m in late May. Chl. a concentration increased above 20 m to exceed 5 µg/L, from early May to late June. Daily nutrient monitoring showed that PO4-P concentrations sporadically increased over 0.05 µmol/L in early May at both 5 and 20 m, but were always low, around 0.02 µmol/L, thereafter. A 0.015 µmol/L of PO4-P has been shown to be a critical concentration for phytoplankton growth. NH4-N concentrations varied 0.03 – 4.57 µmol/L at 5 and 20 m from late April to late June. NO3-N concentrations at 5 m gradually decreased from 6.12 to 2.14 µmol/L by mid-May, further declining to < 1.00 µmol/L thereafter, while those at 20 m were always 4.52–10.49 µmol/L throughout the study period. This decline of NO3-N at 5 m in May might be caused by phytoplankton growth, implying subsidiary supply of PO4-P.
Phytoplankton productions were relatively high, 0.5–1.0 gC/m2/d, during May, and gradually decreased in June. Almost 80% of variation of the daily phytoplankton productions could be explained by fluctuation of light intensity. Residuals from ones predicted from a regression equation between the production and light intensity were positive during May but almost zero after June, meaning that some factors other than light, e.g. nutrient supply, might enhance the production during May, when sestonic C:P ratios were quite low, < 100, which indicates that phytoplankton may be relaxed from P limitation. Additional microscopic observations showed that large phytoplankton dominated in May, implying that there was large-scale pulsed nutrient supply.
These high resolution monitoring of nutrients and production suggested that subsidiary nutrient supply enhanced phytoplankton production in May. We can also show water movement data with an ADCP and a relationship between the production and water movement at the conference presentation.
A mooring system for monitoring phytoplankton production and lake water movement was situated nearby a buoy of Japan Water Agency (35o18.69'N, 136o08.68'E) from 28 April to 27 July 2020. Water temperatures were measured at 10 min interval at 1 and 5 m, and at 2-m interval from 10 to 30 m with a thermistor chain. In situ phytoplankton production was measured using light intensity and chlorophyll a (chl. a) concentration monitored by sensors attached at 5 m, 10 m, and 15 m and monthly obtained photosynthesis - light intensity curve. Water movements was measured at 10 min interval with an acoustic doppler current profiler (ADCP). Lake waters at 5 and 20 m were collected daily at noon for measuring nutrient concentrations, NO3-N, NH4-N, and PO4-P, with a water pump from 28 April to 17 June 2020. Apart from them, we weekly investigated vertical profiles of water temperature, chl. a and nutrient concentrations.
Water temperature gradually increased above 20 m from early May, being >15oC at surface, and then distinct thermocline developed in 10-20 m in late May. Chl. a concentration increased above 20 m to exceed 5 µg/L, from early May to late June. Daily nutrient monitoring showed that PO4-P concentrations sporadically increased over 0.05 µmol/L in early May at both 5 and 20 m, but were always low, around 0.02 µmol/L, thereafter. A 0.015 µmol/L of PO4-P has been shown to be a critical concentration for phytoplankton growth. NH4-N concentrations varied 0.03 – 4.57 µmol/L at 5 and 20 m from late April to late June. NO3-N concentrations at 5 m gradually decreased from 6.12 to 2.14 µmol/L by mid-May, further declining to < 1.00 µmol/L thereafter, while those at 20 m were always 4.52–10.49 µmol/L throughout the study period. This decline of NO3-N at 5 m in May might be caused by phytoplankton growth, implying subsidiary supply of PO4-P.
Phytoplankton productions were relatively high, 0.5–1.0 gC/m2/d, during May, and gradually decreased in June. Almost 80% of variation of the daily phytoplankton productions could be explained by fluctuation of light intensity. Residuals from ones predicted from a regression equation between the production and light intensity were positive during May but almost zero after June, meaning that some factors other than light, e.g. nutrient supply, might enhance the production during May, when sestonic C:P ratios were quite low, < 100, which indicates that phytoplankton may be relaxed from P limitation. Additional microscopic observations showed that large phytoplankton dominated in May, implying that there was large-scale pulsed nutrient supply.
These high resolution monitoring of nutrients and production suggested that subsidiary nutrient supply enhanced phytoplankton production in May. We can also show water movement data with an ADCP and a relationship between the production and water movement at the conference presentation.