Phytoplankton and bacterioplankton dynamics in a polymictic tropical lake

Sammanfattning: Phytoplankton and bacterioplankton dynamics in lakes are regulated by a multitude of factors such as light, temperature, water column mixing, nutrients, organic substrates, grazing, and food web structure. Tropical lakes are especially interesting to study because of the relative constancy of several of these factors. This thesis deals with interactions between phytoplankton, bacterioplankton and environmental factors in tropical, polymictic Lake Xolotlán, Nicaragua.Phytoplankton biomass per unit area showed only small temporal and spatial variations, with an average of 65 g C m-2. The photic zone:mixing depth ratio was relatively constant at 0.19, which is a critical lower limit for algal survival. Frequent mixing of the water column caused the algae to spend much time in darkness, thereby loosing by respiration what was gained by photosynthesis during short periods in the photic zone, and this caused high algal mortality.Areal primary production was limited by the biomass of actively photosynthesizing algal cells in the photic zone and varied within the range 6.0-7.5 g C m-2 d-l. The depth of the photic zone was mainly regulated by light extinction by active algae, but also by dead algae. The rates of light saturated and light limited photosynthesis were close to theoretical maximum values.Bacterial production (1 g C m-2 d-1) was comparable to known empirical models, but the bacterial biomass (5 g C m-2) was much greater than predicted. This resulted in a very low production per biomass. Bacterial biomass and production were significantly correlated to algal degradation products suggesting that bacteria were regulated by algal mortality. Allochtonous organic carbon was an insignificant source of substrate for the bacteria. This study shows that bacteria were sinks for organic carbon and regenerators of nutrients. By rapid decomposition of dead algae they made nutrients available for new algal growth, thus maintaining a high biomass specific primary production. Both algal and bacterial mortality by grazing was very low. The system was thus characterized by a tight coupling between algae and bacteria with little transfer of carbon and energy to higher trophic levels.