BACTERIAL-ACTIVITY AND ITS TROPHIC ROLE IN A EUTROPHIC POND

The trophic role of bacteria in the microbial loop was examined in a e utrophic fish pond receiving organic fertilizer. Two types of experime nts were carried out: the first in situ aimed at studying the trophic pathways after the addition of a nitrogen-rich organic fertilizer; the second in vitro aimed at defining the role of bacteria in this partic ular aquatic environment. In situ experiments showed that addition of a nitrogen-rich fertilizer decreased the C/N ratio of the water and th en enhanced mineralization of organic matter by bacteria. A strong phy toplankton outbreak occurred as a result, which then caused the develo pment of a larger-sized filter-feeding zooplankton population with a p hytoplankton grazing tendency. On the other hand, according to the lit erature, a fertilizer having a high C/N should favour bacterial organo trophic activity and growth, which ought to cause the growth of a smal l-sized bacterioplankton grazer. This present study discusses the vari ations of mineralization rates and of biosynthesis rates, two tightly connected processes that occurred in fish ponds: these two processes o f the trophic pathway govern prey quality for fishes. Small-sized bact eria (under GFF filters) were not much influenced by the fertilizer; c onversely, large-sized bacteria (over GFF filters) exhibited an increa se in activity due to primarily fertilizer input and later due to the lysis of algal cells.