Spatiotemporal variations of orthophosphate in a large mesotrophic lake and implications for phytoplankton productivity

Information on orthophosphate (PO4-P) distributions in large oligotrophic and mesotrophic lakes is limited because PO4-P concentrations are low. Using a new procedure based on ion chromatography, we determined spatiotemporal 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 a peaked in June–July and October–November for 2018 and 2019 while PO4-P concentrations usually remained −1. Distinct subsurface PO4-P maxima of >0.07 μmol L−1 were detected at 5−10 m depths in May 2019 along a transect connecting littoral and pelagic sites. At >40 m depths, PO4-P concentrations increased with depth and were frequently elevated near the bottom, displaying inconsistent seasonality. In both years, increases in PO4-P 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 −1 and reached potential maximum growth at >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, resulting in enhanced phytoplankton production.