The influence of winter lake surface cover conditions on under-ice light regimes and primary productivity in small, hydrologically disconnected lentic systems

Abstract

As the Earth undergoes continued climatic change, shifts in the cryosphere are occurring at increasingly rapid rates. As a significant proportion of global freshwater is located in the mid- to high-latitudes of the northern hemisphere and is often seasonally ice covered, it is vital to understand how surface cover quality influences biological activity in these systems. Ice and surface cover conditions have been noted to be very effective at limiting light availability, and therefore influencing the photosynthetically active radiation (PAR) available for primary production. To quantify the relationship between winter surface cover and under-ice hydroecological variables, two complimentary controlled experiments were conducted over two winter periods at the University of Calgary Aquatic Experimental Facility. Above-ground polyethylene mesocosms were utilized to remove the influence of littoral and benthic activity influences on measured biological endpoints. Two paired experimental ponds, that are hydrologically disconnected and located immediately adjacent to the mesocosm enclosures were utilized to further develop the relationships, while including a slightly more complex food web structure. Surface cover manipulations were done by either adding snow (snow-on-ice), or slushing snow (white ice), and resulted in distinct differences in under-ice light regimes, dissolved oxygen (DO) levels, but not primary production measured as chlorophyll-α values. However, in the pond systems, surface cover had minimal impact on the DO levels, with both the control and treatment systems trending towards hypoxic conditions quickly after ice-on conditions, as measured using high temporal resolution probes embedded in the ponds. Chlorophyll-α levels in the pond systems, however, was significantly different with the snow-on-ice (control) pond having lower values than the pond where snow was mechanically removed, over two adjacent winter observation periods. In the second observation period, dissolved organic carbon (DOC) values were also manipulated in the mesocosm systems, with the elevated DOC systems exhibiting a decrease in DO values when compared to control systems. The utilization of controlled experimental systems and high-resolution data loggers allowed this study to offer unique insights into the relationship between ice and surface cover composition, under-ice light regimes, and corresponding biological activity under-ice. As shifts in annual winter climatic and associated meteorological conditions are predicted to continue to occur, generally towards increased precipitation and warmer mean air temperatures, the relationships derived in the study will be valuable in understanding and potentially predicting the implications of climate variability and change on seasonally ice-covered systems