Influences of riverine humic substances on bacteria, protozoa, phytoplankton, and copepods in a coastal plankton community

Abstract

The effects of additions of humic substances (derived from river water) on bacteria, protozoa, phytoplankton, and copepods in a coastal plankton community from the Skagerrak (west coast of Sweden) were studied in a n enclosure experiment. Bacterial numbers and bacterial production were significantly higher in the humic treatment compared with non-hurnic treatment. Tintinnids and oligotrich chates also attained higher densities in the humic treatment than in non-humic treatment, probably reflecting higher food availability due to the increased bacterial production. Phytoplankton biomass and primary production were slightly higher in the humic treatment compared with the nonhumic treatment. The growth of dinoflagellates and small flagellates was enhanced by humic addition. The number of copepods and nauplii became significantly lower in the humic treatments. Thus, the humic addition affected the 'microbial loop' positively, but the increase in nutrient regeneration substantially increased neither phytoplankton growth nor copepod production.