Response of Coastal Phytoplankton to High Inflows of Terrestrial Matter

PACZKOWSKA, JOANNA; ROWE, OWEN; LEFÉBURE, ROBERT; ANDERSSON, AGNETA; BRUGEL, SONIA; BRUTEMARK, ANDREAS

FRONTIERS IN MARINE SCIENCE

Frontiers Media SA

Año: 2020 vol. 7

2296-7745

Climate change scenarios project that precipitation will increase in northern Europe,causing amplified inflows of terrestrial matter (tM) and inorganic nutrients to coastalareas. How this will affect the plankton community is poorly understood. A mesocosmexperiment was carried out to investigate the influence of two levels of tM inputs onthe composition, size-structure and productivity of a natural coastal phytoplanktoncommunity from the northern Baltic Sea. The tM addition caused browning of thewater and decreased underwater light levels, while the concentrations of dissolvedorganic carbon (DOC) and inorganic nutrients increased. Microphytoplankton werepromoted by tM addition, while in the controls picophytoplankton dominated thephytoplankton community. Inorganic nutrient availability was instrumental in definingthe phytoplankton community composition and size-structure. As a response to tMaddition, the phytoplankton increased their chlorophyll a content. This physiologicaladaptation helped to maintain high primary production rates at the low tM enrichment,but at the high tM load the primary production decreased as did the biomass ofmesozooplankton. The ciliate biomass was high when the mesozooplankton biomasswas low, indicating that a trophic cascade occurred in the system. Structural equationmodeling showed that tM borne DOC promoted ciliates, while primary and bacterialproduction were disfavored. Thus, DOC originating from soils had an indirect negativeeffect on the mesozooplankton by reducing their food availability. Although, a positivecorrelation between heterotrophic bacteria and phytoplankton suggested couplingbetween phytoplankton produced carbon and heterotrophs growth. The results fromour study indicate that river-borne DOC and inorganic nutrients have a large impacton the phytoplankton community, driving the system to the dominance of largediatoms. However, since river-borne humic substances cause browning of the water,phytoplankton increase their light harvesting pigments. At moderate inflow this helpsto support the primary production, but at high inflows of terrestrial material the primaryproduction will decrease. As high river inflows have been projected to be a consequenceof climate change, we foresee that primary production will decrease in coastal areas inthe future, and the impacts of such changes on the food web could be significant