Lake Water Body in the Ice Season - Freezing of Lakes and the Evolution of their Ice Cover - page 241

7.5.3 Nutrients

Circulation and mixing processes under ice, such as shear-driven resuspension at the
water-sediment interface and pore-water convection, affect directly the nutrient release
from the sediments. For open water conditions, several bulk estimates exist for nutrient
supply by shear-driven resuspension (Kristensen et al. 1992; Hamilton and Mitchell 1996)
and by convective mixing (Golosov and Ignatieva 1999; Kirillin et al. 2009) as a function
of mixing intensity. These dependencies generally stay valid in ice-covered lakes, though
reveal speci

c aspects. Anoxic conditions produce chemical and ecological consequences,
such as release of phosphorus from the bottom sediments (Lepp ranta et al. 2012).

First, the decoupling of wind, waves, currents and the sediment surface leads to an
unhindered particle settling and incorporation into the sediment structure. Thus, the quiet
winter conditions considerably reduce the entrainment of particulate and dissolved
nutrients by resuspension, which in turn should favour the consolidation of sediment.
H

ä

kanson and Bryhn (2008) reported that the longer the particles have stayed at the
bottom, the larger the potential gluing effect will be and the faster the settling velocity is if
the particles are resuspended. Thus, the shorter the period of ice coverage the less is the
degree of microbial gluing of surface sediment. Second, less consolidated sediment is
more prone to wind/wave-induced bottom shear stresses. As a consequence, the

å

' start

conditions for the phytoplankton succession after ice thaw change considerably. It needs

a lower critical bottom shear stress for resuspension, which is related to the entrainment of
nutrients (algal recruitment), algal cells, and overwintering cyanobacterial cysts. Hence,
climate-driven shortening of ice-cover duration would not only increase resuspension and
related nutrient loading (Niemist

'

and Horppila 2007), but would also shorten the calm
period for settling of particles and their integration into sediment as well as for its
consolidation.
Another source of nutrients, speci

ö

c for the ice-covered period only, is the ice sheet,
which contains nutrients originating from the water body, sediments or atmospheric
fallout. The capturing of impurities by the ice depends on the mode of ice growth, and
their release to the water body occurs in the melting season (Lepp

ranta et al. 2003a).
There is almost no research done on the nutrients in lake ice and their release into the
water, the only publications concern Antarctic lakes (Fritsen and Priscu 1998).

ä