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

• Published in: Journal of sea research 2014-08, Vol.91, p.35-44
• Main Authors: Alvarez-Fernandez, Santiago, Riegman, Roel
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Aquatic Ecology and Water Quality Management; Aquatische Ecologie en Waterkwaliteitsbeheer; Biological and medical sciences; Biomass; Carbon; Carbon:chlorophyll ratio; Chlorophyll; Chlorophylls; Dinoflagellates; Dominance; Ecosystemen; Fundamental and applied biological sciences. Psychology; IMARES Ecosystemen; Leerstoelgroep Aquatische ecologie en waterkwaliteitsbeheer; North Sea; Nutrients; Phytoplankton; Phytoplankton biomass; Phytoplankton dynamics; Sea water ecosystems; Synecology; Trends
• ISSN: 1385-1101; 1873-1414
• DOI: 10.1016/j.seares.2014.04.005

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 20years, 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 12gC∗gChla−1 in 1990 to 69gC∗gChla−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 62gC∗gChla−1 in 1991 to 119gC∗gChla−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. •We analyse 20years 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.