Spatio-temporal variations of orthophosphate in a large mesotrophic lake and implications for phytoplankton productivity

Information on orthophosphate (PO₄-P) distributions in large oligo- and mesotrophic lakes is limited because PO₄-P concentrations are low. Using a new procedure based on ion chromatography, we determined spatio-temporal distributions of PO₄-P concentrations and biogeochemical parameters in the north basin of mesotrophic Lake Biwa in 2018–2019 to clarify the effects of horizontal transportation of nutrients on phytoplankton growth. Above 20 m depth, chlorophyll <i>a</i> peaked in June–July and October–November for 2018 and 2019 while PO₄-P concentrations usually remained &lt; 0.03 μmol L⁻¹. Distinct subsurface PO₄-P maxima of &gt; 0.07 μmol L⁻¹ were detected at 5−10 m depths in May 2019 along a transect connecting littoral and pelagic sites. At &gt; 40 m depths, PO₄-P concentrations increased with depth and were frequently elevated near the bottom, displaying inconsistent seasonality. In both years, PO₄-P increases throughout the water column in May were followed by phytoplankton blooms; PO₄-P increased before the autumnal phytoplankton bloom in 2018 but not in 2019. The spring bloom may be related to horizontal transportation of PO₄-P from the littoral slope by wind-driven internal waves, whereas the autumnal bloom may be fed by nutrients transported from the hypolimnion by vertical convection. Phosphate enrichment experiments showed that phytoplankton growth was restricted at PO₄-P concentration &lt; 0.014 μmol L⁻¹, reached to the potential maximum growth at &gt; 0.05 μmol L⁻¹. The horizontal transportation of PO₄-P by internal waves in spring may play an important role in supplying sufficient nutrients to the euphotic zone in the pelagic region and enhance phytoplankton production.