AME prepress abstract - doi: 10.3354/ame01202
Effects of inorganic nutrients, glucose and solar radiation on bacterial growth and exploitation of dissolved organic carbon and nitrogen in the northern Baltic Sea
Risto Lignell*, Laura Hoikkala, Titta Lahtinen
ABSTRACT: Factors controlling bacterial growth and degradation of dissolved organic carbon (DOC) and nitrogen (DON) in the productive surface layer were investigated during the main summer stages of phytoplankton growth in the Gulf of Finland. The effects of different combinations of ammonium, phosphate, glucose, flagellates, and sunlight pre-exposure treatments were followed for 2-3 weeks in natural bacterial (<0.7 µm pre-filtered) samples. Bacteria degraded the indigenous labile DOC and DON pools within 1 week. The labile shares of total DOC and DON were <1-5% and 13-21%, respectively, and their depletion showed no important treatment effects. Nevertheless, photochemical transformations of DOC and DON (sunlight pre-treatment over a day) resulted in increased bacterial production. The phytoplankton system was N-limited in early summer, but showed a shift towards combined P and N deficiency during the late summer bloom of filamentous, N2-fixing cyanobacteria. Ambient labile DOC:DON ratios were low, increasing from <1-3 (mol/mol) in early summer to 3-7 in late summer, and addition of glucose led to a significant bacterial production response. Thus, it appears that bacteria were consistently limited by the low availability of labile DOC, while phytoplankton exhausted the available free mineral nutrient pools, i.e. creating a situation combining C-limited bacterial growth with mineral nutrient-limited phytoplankton growth.