Chlorophyll in North Sea coastal and offshore waters does not reflect long term trends of phytoplankton biomass

• We analyse 20 years of phytoplankton monitoring data in Dutch sea waters.
• We model the changes in Carbon:chlorophyll ratios of the phytoplankton community.
• C:Chl ratios show an increasing trend during the study period.
• C:Chl ratios are affected by underwater PAR irradiance and nutrient concentrations
• These changes complicate the use of chlorophyll as phytoplankton biomass proxy.

This study analyses long-term and seasonal changes of phytoplankton community Carbon: Chlorophyll a ratio (θ) during the period 1991 to 2010 in North Sea waters and its relationship to environmental drivers. Based on the data from the Dutch water monitoring programme covering 20 years, major trends in phytoplankton abundance, community structure and chlorophyll were identified. Overall C:Chla increased during the study period, particularly in coastal areas. This increase was related to an increase in average underwater photosynthetically active radiation (PAR) and a decrease in nutrient concentrations. A mismatch was detected between chlorophyll a and biomass carbon trends caused by both a decrease in chlorophyll a values and an increase in biomass. In coastal waters, physiological adaptation to higher light and lower nutrient levels may have enhanced the θ, increasing from yearly averages of 12 g C ∗ g Chl a− 1 in 1990 to 69 g C ∗ g Chl a− 1 in 2010. Offshore, the increased stratification coincided with a shift towards dinoflagellate dominance. This dominance of dinoflagellates co-occurred with an increased θ yearly averages from 62 g C ∗ g Chl a− 1 in 1991 to 119 g C ∗ g Chl a− 1 in 2010. Because of these changes detected in C:Chl a ratio of multispecies phytoplankton communities, we question the validity of chlorophyll a as a proxy for phytoplankton biomass and argue its possible misrepresentation of phytoplankton dynamics.