Nitrogen bound to humic matter of terrestrial origin a nitrogen pool for coastal phytoplankton?

The effects of additions of river isolated hurnic compounds on biomass formation and primary production of a natural phytoplankton community were compared with the effects of additions of nitrate or phosphate in a batch culture experiment lasting for 10 d during late summer. Mesozooplankton (> 150 pm) were excluded in the experiment, but micrograzers such as heterotrophic nanoflagellates, ciliates and copepod nauplii were present. The humic substances were isolated from river water using a macroporous resin (XAD-8). Trace metals, iron and vitamins were added in order to avoid growth limitation caused by these compounds. Phytoplankton biomass and primary production became significantly higher in treatments where nitrate or humic substances were added than in control flasks. Bacterial numbers became significantly higher in the treatments where humic substances were added compared with controls. Ammonium concentrations became elevated in the treatments where nitrate or hurnic substances were added. It seems likely that humic substances were utilized by bacteria both as a carbon and a nitrogen source and that nitrogen was incorporated in bacteria and later regenerated as inorganic nitrogen due to the activity of bacterial grazers and other grazers of higher trophic levels. We suggest that this is the mechanism by which nitrogen in hurnic substances becomes available for coastal phytoplankton communities.