Regular build-up of the spring phytoplankton maximum before ice-break in a boreal lake

Abstract

The development of phytoplankton biomass and composition in a eutrophic boreal lake was studied during the evolution of under‐ice convection in spring. The results from 8 yr showed that, within a few weeks before ice‐break, phytoplankton biomass regularly increased by up to two or three orders of magnitude, reaching or exceeding the biomass in summer. Accordingly, this may be the most significant single annual phytoplankton episode in the lake. The development of phytoplankton was closely coupled with that of convection created by solar radiation at water temperatures < 4°C. In addition to vertical convection which keeps phytoplankton suspended, there was also horizontal convection which transported water with likely higher abundance of phytoplankton from the shallow lake margins. The effect of mechanical mixing, which was meant to prevent anoxia in the deep water layers of the study basin, was overridden by natural convection. Stochastic variations in weather played a key role controlling light penetration into the lake and hence the abundance and composition of under‐ice phytoplankton. After snow melt the proportion of motile algae was at first higher, sometimes with a maximum nearest to the ice, but later diatoms flourished throughout the convection layer. Only in the years of the shortest convection period did the proportion of motile algae remain high until the end of ice cover. More detailed information about the interaction between littoral and pelagial water masses under different mixing regimes is needed for a more profound understanding of the development of phytoplankton under‐ice.

Ecology under lake ice

“…; Jewson et al . ; Salmi & Salonen ). As with many aspects of climate change, the extremes and the timing of shifts, in addition to average changes, are important (reviewed in Adrian et al .…”

“…Observed and anticipated shifts in precipitation, wind and solar radiation patterns associated with climate change are heterogeneous across and within regions, and can greatly alter the under-ice environment by changing the amount of incident light that penetrates the ice. Surface snow accumulation of as little as 10 cm can reduce light penetration to levels insufficient for photosynthesis and convective mixing that influences algal suspension as well as nutrient concentrations in the photic zone (Granin et al 2000;Mackay et al 2006;Jewson et al 2009;Salmi & Salonen 2016). As with many aspects of climate change, the extremes and the timing of shifts, in addition to average changes, are important (reviewed in Adrian et al 2012).…”

Using a decadal diatom sediment trap record to unravel seasonal processes important for the formation of the sedimentary diatom signal

“…3). During these warm late-winter periods, stratified conditions in the surface water under the ice, increased light availability due to reduced snowpack and thinning ice, and excess nutrient concentrations likely favored the growth of phytoplankton (Salmi and Salonen 2016), including C. glomerata, which is known to favor conditions of shallow stratification and high light availability (Hutchinson 1967;Saros et al 2012). These conditions are usually observed during early shallow spring stratification after over-turn (Wiltse et al 2016), but also occur in under-ice surface water layers which are rarely sampled.…”

“…While field and laboratory studies provide critical insights into how specific seasonal processes are functioning over experimental timescales (up to a few years; e.g. Salmi and Salonen 2016), lake ecosystems are controlled by complex interactions among lake properties, catchment characteristics, climate conditions, and human activity which play out over longer timescales. Therefore, long time series are needed to understand how information from these various processes is integrated in sediment archives.…”

Seasonality of chrysophyte cyst and diatom assemblages in varved Lake Nautajärvi – implications for palaeolimnological studies

“…During autumnal overturn phytoplankton biomass increases as nutrients 'trapped' in the hypolimnion are brought to the epilimnion resulting in an autumnal bloom (Rautio et al, 2000). Small numbers of phytoplankton cells survive during winter season in the water column, attached to the bottom of the ice, or as resting cells in the bottom sediment (Salmi and Salonen, 2016).…”