THERMAL STRATIFICATION, NUTRIENT DYNAMICS, AND PHYTOPLANKTON PRODUCTIVITY DURING THE ONSET OF SPRING PHYTOPLANKTON GROWTH IN LAKE-BAIKAL, RUSSIA

Lake Baikal, Russian Siberia, was sampled in July 1990 during the peri od of spring mixing and initiation of thermal stratification. Vertical profiles of temperature, dissolved nutrients (nitrate and soluble rea ctive phosphorus), phytoplankton biomass, and primary productivity wer e determined in an eleven-station transect encompassing the entire 636 km length of the lake. Pronounced horizontal variability in hydrodyna mic conditions was observed, with the southern region of the lake bein g strongly thermally stratified while the middle and north basins were largely isothermal through July. The extent of depletion of surface w ater nutrients, and the magnitude of phytoplankton biomass and product ivity, were found to be strongly correlated with the degree of thermal stratification. Horizontal differences likely reflected the contribut ion of two important factors: variation in the timing of ice-out in di fferent parts of the lake (driving large-scare patterns of thermal str atification and other limnological properties) and localized effects o f river inflows that may contribute to the preliminary stabilization o f the water column in the face of intense turbulent spring mixing (dri ving meso-scale patterns). Examination of the relationships between su rface water inorganic N and P depletion suggested that during the spri ng and early summer, phytoplankton growth in unstratified portions of the lake was largely unconstrained by nutrient supplies. As summer pro gressed, the importance of co-limitation by both N and P became more a pparent. Uptake and regeneration rates, measured directly using the st able isotope N-15, revealed that phytoplankton in stratified portions of the lake relied primarily on NH4 as their N source. Rates of NH4 re generation were in approximate equilibrium with uptake; both processes were dominated by organisms <2 mu m. This pattern is similar to that observed for oligotrophic marine systems. Our study underscores the im portance of hydrodynamic conditions in influencing patterns of biologi cal productivity and nutrient dynamics that occur in Lake Baikal durin g its brief growing season.