Abstract
It is well-known that nutrients shape phytoplankton communities in marine systems, but in coastal waters allochthonous dissolved organic matter (ADOM) may also be of central importance. We studied how humic substances (proxy of ADOM) and other variables influenced the nutritional strategies, size structure and pigment content of the phytoplankton community along a south–north gradient in the Baltic Sea. During the summer, the proportion of mixotrophs increased gradually from the phosphorus-rich south to the ADOM-rich north, probably due to ADOM-fueled microbes. The opposite trend was observed for autotrophs. The chlorophyll a (Chl a): carbon (C) ratio increased while the levels of photoprotective pigments decreased from south to north, indicating adaptation to the darker humic-rich water in the north. Picocyanobacteria dominated in phosphorus-rich areas while nanoplankton increased in ADOM-rich areas. During the winter–spring the phytoplankton biomass and concentrations of photoprotective pigments were low, and no trends with respect to autotrophs and mixotrophs were observed. Microplankton was the dominant size group in the entire study area. We conclude that changes in the size structure of the phytoplankton community, the Chl a:C ratio and the concentrations of photoprotective pigments are indicative of changes in ADOM, a factor of particular importance in a changing climate.
Highlights
Phytoplankton communities are shaped by bottom-up and top-down factors (Chisholm, 1992; Kiørboe, 1993; Mousing et al, 2014), and their community properties are in turn important for aquatic food web functioning (Havens, 1998; Dahlgren et al, 2010)
Our study indicates that allochthonous dissolved organic matter (ADOM), in addition to the more well-known factors, governs the large-scale patterns of phytoplankton nutritional strategy, size structure and pigment content in the Baltic Sea
We found that in the summer ADOM was one of the major factors governing the phytoplankton community, while Tot N played a major role in the winter–spring
Summary
Phytoplankton communities are shaped by bottom-up and top-down factors (Chisholm, 1992; Kiørboe, 1993; Mousing et al, 2014), and their community properties are in turn important for aquatic food web functioning (Havens, 1998; Dahlgren et al, 2010). Due to the interaction with the heterotrophic microbial food web, the size distribution and function of the phytoplankton community is influenced by factors like nutrient availability and temperature. In coastal areas and semi-enclosed seas like the Baltic Sea, the influence of allochthonous dissolved organic matter (ADOM) can be strong, fueling the heterotrophic microbial food web (Andersson et al, 2015; Figueroa et al, 2016). Under such conditions mixotrophic phytoplankton may be enhanced, since they feed, e.g. on heterotrophic bacteria (Andersson et al, 1989). How an increase in ADOM will affect phytoplankton structure in the Baltic Sea is poorly understood
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