Vertical Mixing, Light Penetration and Phosphorus Cycling Regulate Seasonal Algae Blooms in an Ice‐Covered Dimictic Lake

Many temperate lakes accumulate sediment derived orthophosphate (PO4) in their hypolimnion during late‐summer deep‐water hypoxia. In dimictic lakes, fall turnover will mix the PO4 through the water column. However, the fate and transport of this primary production limiting nutrient, during winter, is unknown. Does it remain available for the spring bloom, and why does it not trigger a fall bloom in many dimictic lakes? We conducted field observations and supplemented these with three‐dimensional physical biogeochemical numerical simulations to gain a deeper understanding of PO4 transport and cycling within a small dimictic lake from 2011 to 2020. Our focus was particularly on the often‐ice‐covered winter season. We found, the sediment derived PO4 to be only a small portion (∼1%) of the total PO4 load, with most of the load from mineralization (49% ice free, 29% ice covered) and tributary inflows (22%). The accumulated hypolimnetic PO4 increased the water column concentration during fall turnover, but a fall bloom was not initiated, because the associated mixing transported phytoplankton beneath the photic zone. This PO4 remained available in the water column during winter and was combined with under‐ice mineralized PO4 to initiate the spring bloom, in a thin stable layer beneath the ice, as solar radiation increased seasonally during spring.