Denitrification and Anammox Shift Nutrient Stoichiometry and the Phytoplankton Community Structure in the Benguela Upwelling System

Abstract

AbstractA shift in nutrient stoichiometry and phytoplankton community structure due to denitrification and anammox is investigated using coupled physical‐biogeochemical model experiments in the northern Benguela. Denitrification depletes more dissolved inorganic nitrogen (DIN) than anammox, increasing phosphate accumulation and this creates a shift in nutrient stoichiometry. This decreases the nutrient stoichiometry to below the Redfield ratio with an effect on phytoplankton composition in the system. Flagellates dominate the community structure in the northern Benguela as compared to diatoms, picophytoplankton and large phytoplankton. Denitrification limits the growth of picophytoplankton resulting in a flagellate‐dominated community structure and diatom succession at depth. The phytoplankton community structure shift and succession correspond with a negative correlation between the phytoplankton groups, denitrification, anammox and DIN. The nutrient stoichiometry and a decline in phytoplankton concentrations from denitrification decrease Dissolved inorganic carbon (DIC) uptake more than gross primary production. Annual mean diatoms decline from denitrification is 0.012 Gg N , reducing DIC uptake by 2.793 Gg C and resulting in an estimated gross primary production of 0.080 Gg N in the northern Benguela. Anammox drives an estimated flagellates decline of 0.008 Gg N , a diatoms increase of 0.873 Gg N and DIC uptake of up to 46.08 Gg C . The direct links between denitrification, anammox, DIC uptake and primary production as demonstrated in this study present considerable implications for the biological pump in the northern Benguela.