The permanent ice cover of Lake Bonney, Antarctica: The influence of thickness and sediment distribution on photosynthetically available radiation and chlorophyll-a distribution in the underlying water column

Abstract

The thick permanent ice cover on the lakes of the McMurdo Dry Valleys, Antarctica, inhibits spatial lake sampling due to logistical constraints of penetrating the ice cover. To date most sampling of these lakes has been made at only a few sites with the assumption that there is a spatial homogeneity of the physical and biogeochemical properties of the ice cover and the water column at any given depth. To test this underlying assumption, an autonomous underwater vehicle (AUV) was deployed in Lake Bonney, Taylor Valley. Measurements were obtained over the course of 2 years in a 100 × 100 m horizontal sampling grid (at a 0.2 m vertical resolution). Additionally, the AUV measured the ice thickness (in water equivalent) and collected images looking up through the ice, which were used to quantify sediment distribution on the surface and within the ice. Satellite imagery was used to map sediment distribution on the surface of the ice. We present results of the spatial investigation of the sediment distribution on the ice cover and its effects on biological processes, with particular emphasis on photosynthetically active radiation (PAR). The surface sediment is a secondary controller of the ice cover thickness, which in turn controls the depth-integrated PAR in the water column. Our data revealed that depth-integrated PAR was negatively correlated with depth-integrated chlorophyll-a (r = 0.88, p < 0.001, n = 83), which appears to be related to short-term photoadaptation of phytoplanktonic communities to spatial and temporal variation in PAR within the water column. IntroductionLake sampling of Antarctic perennially ice-covered lakes is usually limited to only a few sites per year because of the challenge of getting access to the water column. Sampling is usually performed at the deepest part of the lake, which is assumed to represent the entire lake at any given depth. Such an assumption might be valid in temperate lakes due to the seasonal overturning of the water column. However, because of the strong stratification and lack of mixing in the Antarctic lakes [Spigel and Priscu, 1998], biogeochemical horizontal homogeneity of the water column might not exist. The low kinetic energy of the Antarctic lake systems (diffusion dominates the spatial transport of constituents) may produce an ecosystem and ecosystem limits that vary significantly in three dimensions, variations which will not be detected with lake data sets collected at one site at various depths. Spigel and Priscu [1998] have shown spatial and temporal stability of the water column based on 3 years of conductivity and temperature profiles in perennially ice-covered Lake Bonney (Figure 1). However, variations of underwater photosynthetically active radiation (UW PAR) and chlorophyll-a under the ice have not been previously documented in high spatial and temporal resolution.In this paper, we present the results of an investigation by the Environmentally Non-Disturbing Under-ice Robotic ANtarctiC Explorer (ENDURANCE), deployed during the

Growth dynamics of a laminated microbial mat in response to variable irradiance in an Antarctic lake

“…400% variability, both between years and within years on a spatial basis. Neither between nor within years was this variability in ice clarity linked to ice thickness, and in previous studies, the importance of the albedo and sediment burden of the upper part of the ice layer on transmission, rather than thickness per se, has been emphasised (McKay, Andersen & Wharton, 1994;Obryk et al, 2014). This is the first time that the depth of propagation of the patchiness of lake ice light transmission has been measured in Lake Hoare, and it is clear that it extends sufficiently deep to create several-fold spatial variability to at least 20 m. A similar finding has been recorded in nearby Lake Joyce, though to lesser depth (Hawes et al, 2011).…”

“…While incident irradiance varies by only 10% between years, the transmission of light through the ice cover shows a 400% variability, both between years and within years on a spatial basis. Neither between nor within years was this variability in ice clarity linked to ice thickness, and in previous studies, the importance of the albedo and sediment burden of the upper part of the ice layer on transmission, rather than thickness per se , has been emphasised (McKay, Andersen & Wharton, 1994; Obryk et al ., ).…”

“…() assembled data to show interannual variation in the transmission of the ice cover of Lake Hoare, one of the MDV lakes, from 0.1 to 2.5% of surface incident. More recently, an order of magnitude spatial variability of light penetration has been documented in Lake Joyce (Hawes et al ., ) and the west lobe of Lake Bonney (Obryk, Doran & Priscu, ), and these authors note that this heterogeneity propagates to depths of at least 10 m. Such variability, in a severely light‐limited environment (Priscu et al ., ; Moorhead, Schmeling & Hawes, ; Hawes et al ., ), can be expected to lead to substantial changes in community photosynthesis and potential for growth over time. For example, Doran et al .…”

“…The optical properties of the ice change over the course of the austral summer (Howard-Williams et al, 1998), and Quesada et al (2008) assembled data to show interannual variation in the transmission of the ice cover of Lake Hoare, one of the MDV lakes, from 0.1 to 2.5% of surface incident. More recently, an order of magnitude spatial variability of light penetration has been documented in Lake Joyce (Hawes et al, 2011) and the west lobe of Lake Bonney (Obryk, Doran & Priscu, 2014), and these authors note that this heterogeneity propagates to depths of at least 10 m. Such variability, in a severely light-limited environment (Priscu et al, 1999;Moorhead, Schmeling & Hawes, 2005;Hawes et al, 2001), can be expected to lead to substantial changes in community photosynthesis and potential for growth over time. For example, Doran et al (2002) related the gradual reduction in planktonic photosynthesis over more than a decade in Lake Bonney to a corresponding increase in ice thickness and reduction in irradiance.…”

Prediction of Ice‐Free Conditions for a Perennially Ice‐Covered Antarctic Lake

“…This sediment migrates to approximately 2-m depth within the ice cover as a result of melting caused by absorption of solar radiation (Jepsen et al, 2010). Consequently, ice covers have a distinct heterogeneous sediment layer at 2-m depth (Dugan et al, 2016;McKay et al, 1994;Obryk et al, 2014;Priscu et al, 1998).…”