[AHW22-P05] Spatio-temporal dynamics of orthophosphate with implications for limitation of phytoplankton growth in north basin of Lake Biwa, Japan
Keywords:Orthophosphate dynamics, Phosphorus limitation, Phytoplankton growth rate, Ion chromatography, Biogeochemical processes, Freshwater ecosystem
Field investigations were conducted monthly from April to December 2018 at station K4 (35º19'9.6''N, 136º11'2.7''E; 50 m deep). Phosphate enrichment experiments were also made at each sampling occasion to clarify P limitation for phytoplankton growth. Lake water including phytoplankton assemblage collected from epilimnion (0−20 m) was enriched with 1 μM of PO4-P (+P) and incubated together with untreated one (control) for 2 days under ambient light and temperature conditions. To evaluate the effect of potentially nutrient-rich bottom water, the filtered bottom water was mixed with untreated epilimnion water including phytoplankton at 1:1 (+B) was incubated at the same conditions as the +P treatment. Phytoplankton growth rate (g) was calculated with equation of g=ln(Ct/C0)/2, where C0 and Ct are chlorophyll a (chl. a) concentrations at start and end of the experiment, respectively.
Water temperature increased in the epilimnion from May, and strict thermocline was developed from June to September. Chl. a concentrations increased from June to July in 10−20 m, and from October to November in 0−20 m. PO4-P concentrations were always low, <20 nM, in the epilimnion from April to June. The concentrations below 30 m increased up to 140 nM in May, but decreased after that until July. In August and September, the concentrations increased up to 30−40 nM even in the epilimnion probably due to disturbance by typhoon. The concentrations below 30 m increased with depth, and the highest values were frequently observed near the bottom, up to 220 nM. On average throughout the water column, PO4-P concentrations were 24−48 nM from April to June, then slightly increased up to 73 nM until September, being constant, 50−63 nM until December.
Total P, total dissolved P, dissolved organic P and particulate P exhibited similar spatio-temporal distributions, but different pattern from those of PO4-P. NH4-N increased in 10−20 m from May to July, whereas the NO3-N increased in the hypolimnion as season progressed. Sestonic C:N:P ratios suggested a terrible P deficiency in July and October when phytoplankton biomass increased.
Phosphate enrichment experiments showed that phytoplankton in the epilimnion was mostly limited with P for its growth. Phytoplankton g in control varied −0.05-0.08 d-1, being always lower than those in +P (−0.04-0.44 d-1) and/or +B (−0.04-0.49 d-1) treatments except for November and December. In November, no growth was found even in +P treatment. This might be caused by nitrogen limitation because of <6 µM of NO3-N in the epilimnion. The g of control was similar as that of +P and +B (ca. 0.1 d-1) in December, indicating that P deficiency for phytoplankton growth was relaxed until December by vertical convection.
We firstly determined completely vertical distributions of PO4-P in north basin of Lake Biwa from spring to winter, and detected 20−40 nM of PO4-P in the epilimnion. Phosphate enrichment experiments showed that 20 nM of PO4-P might be critical concentration for phytoplankton growth in the lake. No phytoplankton growth even in +P treatment in November implied that nitrogen limitation for phytoplankton growth will be possible in Autumn. Further study for clarify this will be needed.