No abstract

Miniaturized hyperspectral imaging sensors are becoming available to small unmanned airborne vehicle (UAV) platforms. Imaging concepts based on frame format offer an attractive alternative to conventional hyperspectral pushbroom scanners because they enable enhanced processing and interpretation potential by allowing for acquisition of the 3-D geometry of the object and multiple object views together with the hyperspectral reflectance signatures. The objective of this investigation was to study the performance of novel visible and near-infrared (VNIR) and shortwave infrared (SWIR) hyperspectral frame cameras based on a tunable Fabry-Pérot interferometer (FPI) in measuring a 3-D digital surface model and the surface moisture of a peat production area. UAV image blocks were captured with ground sample distances (GSDs) of 15, 9.5, and 2.5 cm with the SWIR, VNIR, and consumer RGB cameras, respectively. Georeferencing showed consistent behavior, with accuracy levels better than GSD for the FPI cameras. The best accuracy in moisture estimation was obtained when using the reflectance difference of the SWIR band at 1246 nm and of the VNIR band at 859 nm, which gave a root mean square error (rmse) of 5.21 pp (pp is the mass fraction in percentage points) and a normalized rmse of 7.61%. The results are encouraging, indicating that UAV-based remote sensing could significantly improve the efficiency and environmental safety aspects of peat production.

A small range (, 1uC) of under-ice water temperature is shown to result in remarkably different circulation regimes under spring ice in a deep, oligotrophic boreal lake. With the water column at , 4uC, melting of snow led to deepening vertical convection before ice break and a final depth of convection inversely correlated with earlier deep-water temperature. We attribute that to the nonlinear dependence of water density on temperature, albeit further affected by stochastic weather factors. In four of nine study years, convection led to complete under-ice overturn of the lake, indicating that this may not be uncommon in similar lakes with steep topography. River inflow and more intensive warming of water in the littoral zone also created a horizontal density differential, convection that involved flow down the sloping bottom and a lateral intrusion of this sinking water at a depth between the vertical convection and the quiescent deep-water layers. The vertical and horizontal convection together produced a profile of temperature slightly increasing from the surface to the bottom of the convection layer. The contribution of horizontal convection to under-ice mixing was interannually variable, and in one of the study years it eventually dominated under-ice mixing. A thermal bar circulation regime developed occasionally and only in the open water between ice and shoreline. We identified five different under-ice mixing regimes that form an interannually variable continuum of behavior during the ice melting period. The dependence on a narrow temperature range likely makes the circulation regime sensitive to a warming climate.

Distribution and development of phytoplankton were studied in the deep and large Lake Päijänne from mid-winter until the disappearance of ice. Diatoms were an important part of the phytoplankton assemblage and, with cryptophytes and chrysophytes, made up 50-80% of the phytoplankton biomass. In mid-winter, chlorophyll a and phytoplankton biomass were uniformly distributed over the whole water column down to a depth of 90 m. Thus, most of the phytoplankton was in virtual darkness and there was negligible growth. Only motile cryptophytes were concentrated in the layers below the ice and were rare in deep water. After the disappearance of snow, convection developed, but at first cryptophytes were able to resist mixing. When convection turned from penetrative to predominantly horizontal, all phytoplankton were generally uniformly distributed in the water column. In spite of the full under-ice overturn with low average availability of light, the phytoplankton biomass doubled in April. The growth of cryptophytes was higher than that of diatoms, suggesting that motile species gained an advantage by being able to maintain themselves in the upper, illuminated layers. The results show that knowledge of the basic physical framework is essential for interpretation of under-ice phytoplankton results.

In deep ice-covered lakes with temperatures below 4 °C the heat flux from the bottom sediment results in a horizontal density gradient and a consequent flow along the bottom slope. Measurements in Lake Pääjärvi, Finland, show a stable temperature field where a heat gain through the bottom and a heat loss through the ice nearly balance each other. The circulation is thermal with low velocities (less than 1.5 cm s–1). We used the 3D hydrodynamic Princeton Ocean Model as a tool to simulate the water circulation and the temperature distribution under the ice. The model forcing was based on field temperature measurements. The model simulations suggest that in midwinter the velocity field of the upper water layers is anticyclonic while that of deep layers is cyclonic. Comparison with current measurements at one site showed good agreement between the modelled and observed results. On the basis of the modelled results it is possible to better understand the distributions of some micro-organisms and the accumulation of oxygen depleted waters in the deepest part of the lake

The quality of lake ice is of uppermost importance for ice safety and under-ice ecology, but its temporal and spatial variability is largely unknown. Here we conducted a coordinated lake ice quality sampling campaign across the Northern Hemisphere during one of the warmest winters since 1880 and show that lake ice during 2020/2021 commonly consisted of unstable white ice, at times contributing up to 100% to the total ice thickness. We observed that white ice increased over the winter season, becoming thickest and constituting the largest proportion of the ice layer towards the end of the ice cover season when fatal winter drownings occur most often and light limits the growth and reproduction of primary producers. We attribute the dominance of white ice before ice-off to air temperatures varying around the freezing point, a condition which occurs more frequently during warmer winters. Thus, under continued global warming, the prevalence of white ice is likely to substantially increase during the critical period before ice-off, for which we adjusted commonly used equations for human ice safety and light transmittance through ice.

Climate-induced changes in water temperature can thus have a considerable influence on the structure and functioning of lake ecosystems worldwide. A detailed understanding of long-term change in lake water temperature, and its associated drivers, is therefore important for climate change impact studies, and for anticipating the repercussions of climate change on lake ecosystems.Previous studies, notably those involving detailed satellite images, have suggested that lake surface water temperatures are increasing globally (O'Reilly et al., 2015;Schneider & Hook, 2010;, with deep lakes situated at high-latitude typically experiencing the greatest change (Woolway & Maberly, 2020;Woolway & Merchant, 2017). The rapid warming of high-latitude lakes under climatic change partially reflects the substantial increase in air temperature in polar regions (Noori, Bateni, et al., 2022;Post et al., 2018;Stuecker et al., 2018). However, some high-latitude lakes, as well as many others situated at lower latitudes, also experience summer surface temperature trends that are sometimes greater than local changes in air temperature (O'Reilly et al., 2015;Schneider et al., 2009). This suggests an additional source of warming for lakes, such as an increase in incoming solar radiation (Schmid & Köster, 2016) or changes in water transparency which can influence the depth at which solar radiation is absorbed within a lake (Persson & Jones, 2008;Read & Rose, 2013;Rose et al., 2016). In some cases, an earlier break-up of winter ice cover (Sharma et al., 2021) and/ or an earlier onset of thermal stratification (Woolway et al., 2021) can lead to rapid lake surface warming due to a lengthening of the summer stratified season (Austin & Colman, 2007;Woolway & Merchant, 2017). In

We studied vertical distribution of oxygen under the ice of 5 medium-sized, morphologically variable lakes that cooled well below 4 °C before freezing. In the upper part of the water column, dissolved oxygen and dissolved inorganic carbon concentrations generally remained vertically almost uniform, but in the deepest water, concentrations changed rapidly near the bottom. The coincidence of the changes with an increase in deep water temperature shows that they were due to advection of water made heavier by the heat flux from the sediment. Consequently, water with low concentrations of dissolved oxygen and high concentrations of dissolved inorganic carbon accumulated in the deepest part of the lake (i.e., outcome of sediment respiration on a large area was focused to a limited volume of the lake). This conclusion was supported by the results of an experiment in which water samples incubated at different depths showed no vertical differences in oxygen consumption. Our results show that temperature-dependent hydrodynamics affect under-ice oxygen conditions in medium-sized temperate lake basins. Interannual variation in water temperature and differences in morphology between lake basins probably cause significant variations in the accumulation of water in the deepest layers during winter.