The precision of chlorophyll a as an indicator of phytoplankton biomass and phytoplankton production

Sammanfattning

Chlorophyll a is widely used as a proxy for phytoplankton biomass and phytoplankton production, including in the Marine Strategy Framework Directive (MSFD). The estimation of phytoplankton biomass and production is used to assess the nutritional status of water bodies and is therefore important for gaining knowledge about the eutrophication status. Eutrophication is one of the biggest threats to the Baltic Sea and is monitored by HELCOM and MSFD by measuring chlorophyll a. Since chlorophyll a is widely used to estimate phytoplankton biomass and production, the aim of the study was to examine with what precision chlorophyll a can indicate phytoplankton biomass and production, and if the precision varies with sea basin. The aim was answered by analyzing data from three different basins in the Gulf of Bothnia. Results indicated a moderate precision of chlorophyll a indicating phytoplankton biomass and a low to moderate precision of chlorophyll a indicating phytoplankton production. It was concluded that the reason for the observed precision could be due to factors such as phytoplankton community composition and light availability, affecting the chlorophyll a content in phytoplankton. The precision seemed to vary slightly between the three different basins. The variation between different basins could be explained by differences in salinity, affecting the community composition of phytoplankton. The precision that chlorophyll a indicated phytoplankton production was generally lower compared to biomass which was explained by varying influence of additional factors such as temperature, nutrient availability, light and predation.