Abstract:
Lake Louise, a temperate lake in south-central Georgia, USA, is 6.5 meters deep at its deepest point and approximately 6 hectares in area. Despite being relatively shallow, the lake stratifies each year, typically stabilizing from mid-March until mid-November, with a thermocline located at a depth of 2.0 – 2.5 meters. During the summer, high tannin concentrations in the water also cause a sharp decrease in light levels with less than 0.1% of incident light reaching a depth of 2.5 m. However, spectrophotometric pigment analysis indicates that a distinct deep chlorophyll maximum (DCM) develops at an approximate depth of 3.0 m, well below the photic zone, and contains higher apparent chlorophyll b concentrations than the surface waters. Photosynthetic activity at this depth suggests organisms which are capable of compensating for extremely low-light. Preliminary data at 3.0 m indicate an abundance of filamentous, photosynthetic organisms that are visually consistent with cyanobacteria. Chlorophyll spectra from sampling events suggested bacteriochlorophyll d as the primary photosynthetic pigment in the DCM, while initial imaging of filters and sequencing of 16s RNA PCR amplicons further indicated the presence of eubacteria in the phyla Chlorobi and Chloroflexi. A Chloronema-type trichome has been identified through light micrographs that may be indicative of a novel morphospecies.
Keywords: Chlorophyll; Phytoplankton; Chloronema; Thermal stratification;