Photosynthetic algal production, accumulation and release of phytoplankton storage carbohydrates and bacterial production in a gradient in daily nutrient supply

Abstract

In a mesocosm experiment providing a gradient of semi-continuous addition of mineral nutrient, production rates and mortality of phytoplankton were estimated. Heterotrophic bacterial biomass and production rates and their responses to the mineral nutrients additions were also estimated. The purpose of the experiment was to establish responses of the major biological factors as a function of nutrient amendments. Initial primary production was ∼0.47 µg C L−1 day−1. In the most fertilized mesocosm, phytoplankton biomass increased at a specific rate of 0.4 day−1 during the first week of the experiment, and on day 9 primary production reached a peak at 1027 µg C L−1 day−1. The responses in the other fertilized mesocosms were intermediate, and in an unfertilized control the variables measured stayed almost constant throughout the experiment. The termination of the blooms in the fertilized mesocosms was a consequence of nitrogen limitation, and nitrogen limitation subsequently induced storage of intracellular organic material in the phytoplankton. In the mesocosm receiving the highest daily dose of nutrients, strong post-bloom nutrient limitation resulted in high phytoplankton mortality, and release of organic material from the algae supported the gradient’s highest heterotrophic bacterial production.