Distribution of Oligotrophic and Eutrophic Bacteria in Fish Culturing Inland Bays.

Abstract

The distribution of oligotrophic and eutrophic bacteria in fish culturing inland bays was in-vestigated by the most probable number (MPN) method using both poor (ca. 0.3mgC/l) and rich (ca. 300 mgC/l) culture media. Bacterial growth in the rich media (eutrophic and facultatively oligotrophic bacteria) was detected by turbidity and that in the poor media (obligately and facul-tatively oligotrophic bacteria) by epifluorescence microscopic observation after DAPI (4', 6-di-amidino-2-phenylindole) staining. The total counts of heterotrophic viable bacteria in bottom sediments, seawater and particle fractions (>5μm) were ca 107 cells/g, 103-105 cells/ml, and 102-103 cells/ml, respectively. In seawater samples, oligotrophic bacteria, especially obligate oligotro-phs, were often predominant. While, facultatively oligotrophic bacteria were predominant in the fractionated water samples (>5μm). As particulate organic matters provided nutrient rich microspheres for microorganisms in seawater, non-random distribution of eutrophic and facul-tatively oligotrophic bacteria were observed. Different from the bottom sediments in the open sea, obligate oligotrophs were never detected in the bottom sediments of the fish culturing areas. The occurrence of eutrophic bacteria in bottom sediments of fish farms seems to be mainly due to the heavy load of sedimented organic matter which is derived from left-over feed and fish excreta.