Effects of phytoplankton species distribution on particulate organic carbon dynamics along a coastal gradient
2025
Uth, Catharina | Wiethase, Joris | Stoffers, Tjardo | Asmala, Eero | Lewandowska, Aleksandra | Geologian tutkimuskeskus | Geological Survey of Finland
Phytoplankton communities affect carbon dynamics worldwide, strongly influencingthe quality and quantity of organic carbon in coastal ecosystems. Yet, we still know littleabout the impacts of changing phytoplankton community composition on the poten-tial carbon pathways in estuaries and coasts. Here, we sampled 25 sites along a coastalsalinity and nutrient gradient, collecting water for water chemistry and phytoplanktonfor community composition analyses. For each site, we determined phytoplanktontaxonomic diversity and used Bayesian joint species distribution models consideringspecies interactions, taxonomic relatedness and traits to identify key environmentalfactors driving phytoplankton community composition. Subsequently, we used struc-tural equation modelling to establish direct and indirect links between the identifiedkey environmental factors, taxonomic diversity (richness and evenness) and particu-late organic carbon (POC). We found that the phytoplankton distribution along theestuarine gradient was mainly driven by changes in salinity. Increasing salinity (rangingbetween 0.8–6.4) benefited motile species and reduced the phytoplankton richness,which resulted in a decrease in POC concentration. This indirect effect of salinity onPOC was stronger than a direct one, highlighting the mediating role of phytoplanktonrichness. This emphasizes the importance of diversity regulating coastal biogeochemi-cal processes and suggests that future changes in salinity might shift coastal carbondynamics due to changes in phytoplankton community composition
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