Sustained viability of aphotic phytoplankton in Lake Tahoe (California-Nevada)

Biomass, chlorophyll a, and photosynthetic potential of phytoplankton were measured year-round throughout a 450-m water column of ultraoligotrophic Lake Tahoe. Phytoplankton biomass was uniform from the surface to the maximum depth while the lake mixed completely in March, but was lower in aphotic waters during thermal stratification. Aphotic chlorophyll a concentrations decreaked progressively in fall and reached a miniI~UI~ of 17% of mean euphotic concentrations in January, before the mixing depth exceedcd the euphotic depth. Photosynthetic rates measured in an incubator under constant light conditions were significantly correlated with pigment concentrations in both euphotic and aphotic phytoplankton and demonstrated viability of the aphotic phytoplankton throughout the year. High surface : volume ratios of the small species dominating the phytoplankton assemblage suggest low sinking velocities. Extremely slow decomposition is postulated to explain the vertical pigment concentration gradients in the aphotic zone. Re-entry of viable aphotic phytoplankton into the euphotic zone during deep mixing of the lake can diminish the dilution of euphotic phytoplankton, thereby increasing the magnitude of the subsequent vernal bloom. In deep lakes and oceans phytoplankton are lost as a result of sinking into aphotic layers. Algal cells decay by bacteria1 attack and autolysis, thereby releasing nutrients which can be reintroduced to euphotic waters by vertical mixing or upwelling. These processes have been studied in Lake Tahoe ( Paerl et al. 1975a,h). In spite of its high transparency (Secchi depths vary between 20 and 40 m), twothirds of Lake Tahoe’s water volume is aphotic because of its great mean depth (313 m). In eutrophic lakes the aphotic degradation of phytoplankton occurs under anaerobic and frequently toxic conditions ( e.g. development of H2S). In Lake Tahoe, however, even the deepest layers remain fully oxygenated. Environmental conditions in deep euphotic and aphotic waters are similar except for the lack of light and enhanced (but still very low ) nutrient levels. Phytoplankters on the average have to sink from deep in the cu--’ Present address : Technische Universitaet Berlin, Institut fuer Oekologie-Limnologie, EngleralIce 19-21, D-l Berlin 33, West Germany. a Present address: Department of Scientific and Industrial Research, Ecology Division, Vreshwatcr Section, P.O. Box 415, Taupo, New Zealand. photic zone through a water column of about 200 m before reaching the lake bottom. Because of the extreme clarity of the lake, layers of water at different optical depths are farther apart than in a less transparent environment. In high altitude and high latitude lakes phytoplankton can endure extended periods of extremely low light intensities or complete darkness under thick ice and snow cover (Rodhe 1955; Tilzer 1972; KaIff and Welch 1974). In Lake Tahoe, which never freezes, previous work has revealed the existence of high phytoplankton biomass in deep aphotic layers (Kiefer et al. 1972). We here provide evidence for the viability of these populations throughout the year by measuring their photosynthetic potential under constant light and temperature. We then evaluate their importance in influencing the vernal bloom. R. C. Richards helped with sampling, S. Paulson with sample analyses, R. L. Leonard, R. C. Richards, and B. Jost reviewed the text. P. Towns typed the final manuscript. The work was supported by NSF-RANN grants GI-22 and AEN 7422675, LIMNOLOGY AND OCEANOGRAPIIY 84 JANUARY 1977, V. 22( 1) Viability of nphotic phytoplankton 85 Materials and methods Between March 1974 and March J-975, phytoplankton samples were collected at monthly intervals from 13 depths throughout a midlake water column with opaque, nonmetallic Van Dorn water bottles. Chlorophyll a concentrations were determined in 3.5-4.0 liters of water filtered through Whatman GF/C filters followed by extraction in 90% acetone. Absorbance was read in 4-cm glass cells in a Beckman DB-G spectrophotometer. The trichromatic equations of Strickland and Parsons (1968) were used to calculate the amount of pigment. Pheopigments were analyzed after acidification. Since fairly constant C : ATP ratios have been found (HolmHansen and Booth 1966)) concentrations of ATP (adenosine triphosphate) were used as an approximate measure of live cellular carbon assuming a C : ATP ratio of 250 ( IIolm-Hansen and Booth 1966). Using the luciferin-luciferase ATP assay following extraction in 0.02 M tris buffer as modified by Holm-IIansen and Booth (1966) and as described by Holm-IIansen and Paerl ( 1972), we performed ATP analyses on 1-Z ter subsamples filtered through precombusted Whatman GF/C filters. Water for both analyses was strained through a 76-pm plankton net befort filtration, The photosynthetic potential of algal populations was examined by labeling subsamples with 0.5 ml of 14C-bicarbonate (specific activity 8.24 i microbial activity in Lake Tahoe is low. In two incubator experiments, euphotic phytoplankton was held in complete darkness in replicate 4-liter plastic containers at 7-8” C for 22 days. Samples were taken every 3-5 days and chlorophyll a and photosynthetic potential measured.