Effects of inorganic nutrients, glucose and solar radiation on bacterial growth and exploitation of dissolved organic carbon and nitrogen in the northern Baltic Sea

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 ammo- nium, phosphate, glucose, flagellates, and sunlight pre-exposure treatments were followed for 2 to 3 wk in natural bacterial (<0.7 µm pre-filtered) samples. Bacteria degraded the indigenous labile DOC and DON pools within 1 wk. The labile shares of total DOC and DON were <1 to 5 and 13 to 21%, respectively, and their depletion showed no important treatment effects. Nevertheless, photo- chemical transformations of DOC and DON (sunlight pre-treatment over 1 d) 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 -1 ) 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, thereby creating a situation combining C-limited bacterial growth with mineral nutrient-limited phytoplankton growth.