High Variability of Phytoplankton Photosynthesis in Response to Environmental Forcing in Oligotrophic Lake Malawi/Nyasa Stephanie J. Guildford 1,* , Harvey A. Bootsma 2 , William D. Taylor 1 , and Robert E. Hecky 1 1 Department of Biology University of Waterloo Waterloo, Ontario N2L 3G1 2 Great Lakes WATER Institute University of Milwaukee 600 E. Greenfield Ave. Milwaukee, Wisconsin, 53204 ABSTRACT. In southern Lake Malawi, seasonal pelagic chlorophyll means were 1.0 ± 0.3 μg L –1 in the deep mixing season (DMS) (May–August), 0.8 ± 0.3 μg L –1 in the dry stratified season (DSS) (Sep- tember to November) and 0.7 ± 0.3 μg L –1 in the wet stratified season (WSS) (December to April). Despite the low variability in chlorophyll, there was a wide range in chlorophyll specific photosynthetic activity. The photosynthetic parameters, P b m (the light saturated rate) and α b (the light limited slope), varied significantly among seasons and were highly positively correlated, with lowest values in the DSS and highest values in WSS. During deep mixing, P b m did not covary with α b ; and the light saturation index, E k (=P b m /α b ), varied in response to changes in α b rather than in P b m . Phytoplankton appeared to be nutrient deficient at all times but less deficient during deep vertical mixing in the DMS. Average daily rates of integrated phytoplankton primary productivity were lowest in the DSS (337 mg C m –2 d –1 ) and highest in the WSS (629 mg C m –2 d –1 ) despite nearly identical mean chlorophyll concentrations. Along a near shore transect off the Linthipe River, chlorophyll concentrations were higher and more variable (1.4 ± 1.3 μg L -1 ), phytoplankton were not strongly nutrient deficient and chlorophyll specific photosyn- thetic activity was as high or higher than at the offshore station. Estimates of phytoplankton productivity in this tropical great lake must account for spatial and temporal variability in photosynthetic parameters imposed by seasonal changes in mixing dynamics. INDEX WORDS: Lake Malawi, phytoplankton, photosynthesis, nutrient limitation, light limitation. J. Great Lakes Res. 33:170–185 Internat. Assoc. Great Lakes Res., 2007 * Corresponding author. E-mail: sguildfo@sciborg.uwaterloo.ca 170 cooling and wind while water below this depth is permanently isolated from the atmosphere (Bootsma and Hecky 2003) and therefore anoxic. Denitrification at the oxycline is thought to cause the extremely low nitrate (NO 3 – ) and low total ni- trogen (TN) concentrations here (Hecky et al. 1996). In this regard, Lake Malawi differs from temperate great lakes and the oceans which have oxic deep waters that can effectively recycle N as NO 3 – to the euphotic zone with minimal losses to denitrification. Denitrification in LakeMalawi re- sults in some of the lowest mean TN concentrations and TN:T phosphorus (P) ratios of any great lake (Guildford and Hecky 2000). There are two major threats to this large lake— increased nutrient loading and climate change. Lake Malawi has always been known as a low chloro- INTRODUCTION Tropical Lake Malawi, at the southernmost end of the African Rift Valley (Fig. 1), is one of the world’s largest (29,500 km 2 ), deepest (Z max 700 m), and oldest (5–10 million y) lakes (Bootsma and Hecky 2003). Its ancient age, isolation from other water bodies, and tropical climate are thought to ex- plain why this lake is the most species-rich world- wide. For example, an estimated 500–1,000 fish species inhabit Lake Malawi (Fryer and Iles 1972, Snoeks 2000) compared to approximately 142 in the North American Great lakes (Scott and Cross- man 1973). Another consequence of its depth and tropical climate is meromixis. Above 220 m, waters of Lake Malawi are mixed annually by seasonal