Legacies of recent environmental change in the benthic communities of Lake Joyce, a perennially ice-covered Antarctic lake

Abstract

Many Antarctic lakes provide habitat for extensive microbial mats that respond on various timescales to environmental change. Lake Joyce contains calcifying microbialites and provides a natural laboratory to constrain how environmental changes influence microbialite development. In Lake Joyce, depth-specific distributions of calcitic microbialites, organic carbon, photosynthetic pigments and photosynthetic potential cannot be explained by current growth conditions, but are a legacy of a 7-m lake level rise between 1973 and 2009. In the well-illuminated margins of the lake, photosynthetically active benthic communities colonised surfaces submerged for just a few years. However, observed increases in accumulated organic material with depth from 5 to 20 m (2-40 mg ash-free dry weight cm(-2)) and the presence of decimetre-scale calcite microbialites at 20-22 m depth, apparently related to in situ photosynthetic growth, are inconsistent with the current distributions of irradiance, photosynthetic pigments and mat photosynthetic potential (as revealed by pulse-amplitude-modulated fluorometry). The microbialites appeared photosynthetically active in 1986 and 1997, but were outside the depth zone where significant phototrophic growth was possible and were weakly photosynthetically competent in 2009. These complex microbial structures have persisted after growth has ceased, demonstrating how fluctuating environmental conditions and the hysteresis between environmental change, biological response and microbialite development can be important factors to consider when interpreting modern, and by inference ancient, microbially mediated structures.

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

“…() 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 .…”

“…While the best known of the primary production pathways in the MDV lakes is the phytoplankton (Priscu et al ., ; Laybourn‐Parry & Pearce, ), these have a limited ability to record change. In contrast, the microbial mats are less well known, but are conspicuous contributors to diversity and productivity in the euphotic zones of these lakes, are perennial and contain laminations that record annual net accumulation (Hawes & Schwarz, ; Hawes et al ., ; ; ; Quesada et al ., ). Furthermore, there is evidence that irradiance directly limits the rate of photosynthesis in the mat communities (Vopel & Hawes, ; Hawes, Giles & Doran, ), and hence their potential for growth.…”

Key microbial drivers in Antarctic aquatic environments

“…In Lake Joyce, the lake level has risen 7 m between 1973 and 2009 due to increased meltwater influx, at least partly due to an increase in solar radiation and a gradual summer warming trend in the McMurdo Dry Valleys (Bomblies et al ., ). New microbial mats are forming at the margins of Lake Joyce at a rate of millimeters per year, while established mats and microbialites, which formed in shallow water but are now located below 13–16 m, are no longer receiving sufficient irradiance for growth (Hawes et al ., ). Thus, the larger microbial structures in deep water of this lake now represent legacies of previously favorable environmental conditions for phototrophic growth.…”

“…Less commonly reported are Deinococcus‐Thermus , Planctomycetes , Verrucomicrobiales , and others (Brambilla et al ., ; Antibus et al ., ; Peeters et al ., ; Varin et al ., ). Diatoms may be absent (Anderson et al ., ), present at low levels alongside dominant Cyanobacteria (Sutherland, ; Hawes et al ., ), or in some cases present as the dominant phototrophs (Fernández‐Valiente et al ., ). Other Eucarya such as green algae, fungi, and tardigrades have been noted in a wide range of mats (Verleyen et al ., ).…”

Microbial Mat Communities along an Oxygen Gradient in a Perennially Ice-Covered Antarctic Lake

“…Instead, absorption at 755 nm was recorded, and Bchl a was estimated using the absorption coefficient given by Scheer (44). HPLC analysis was carried out as described by Hawes et al (45), and Bchl c, Bchl d, and Bchl e were identified using diode array spectra (400 to 800 nm) and extinction maxima assembled by Scheer (44). The absorption of each of these pigments at 665 nm was recorded, and the percent contributions of Chl a, Bchl c, Bchl d, and Bchl e to the measured 665-nm absorption were calculated, assuming absorption to be additive.…”