Nutrient limitation of phytoplankton biomass or growth rate: an experimental approach using marine enclosures

Abstract

The paradoxical limitation pattern of phytoplankton, i.e. the increase in carbon biomass in response to increased nutrient loading without an increased community growth rate, was examined using marine enclosures with and without addition of nutrients. The phytoplankton carbon biomass, determined from particulate carbon, chlorophyll a, and incorporation of 14C into chlorophyll a, increased in enclosures added both nitrate and phosphate. Phytoplankton growth rates derived from primary production (uptake of 14C into particulate carbon) and from rates of incorporation of 14C into chlorophyll a and protein remained, however, unaltered after nutrient additions, indicating that increased cell volume induced by phytoplankton succession could be responsible for the increased biomass without changing the relative growth rate. Though, no major changes were detected in the phytoplankton species composition in any enclosures, the population mean cell volume increased in enclosures enriched with both nitrogen and phosphate when compared to the control enclosure. The increased mean cell volume could explain the enhanced carbon biomass and the unchanged specific growth rates.