Some aspects of the decomposition process in the mangrove swamp at Gazi Bay, Kenya
The number and activity of several physiological groups of heterotrophic bacteria in Gazi Bay were estimated between September 1992 and February 1993. The study was done in three plots made of pure stands of Aviccenia marina, Ceriops tagal and Rhizophora micronata trees. Average numbers of aerobic heterotrophic bacteria in sediments ranged between 7.44 x107 x 108 cfu/g dwt of sediment. Average numbers of anaerobic heterotrophic bacteria in sediments ranged between 2.00 X106 and 1.86 x 108 cfu/g. There was no significant difference between the three plots with respect to the numbers of aerobic bacteria in the sediment. The average number or aerobic heterotrophic bacteria in water ranged between 8.6 x 104 and 8.90 x 106 cfu/ml of water. Water in the Rhizophora plot had a significantly higher population of aerobic heterotrophic bacteria than water in the other plots. Aerobic heterotrophic bacteria populations on leaves ranged from 2.00 x 102 of leaf area. A. Marina leaves had the highest number of heterotrophic bacteria while C. tagal leaves had the least. Numbers of nitrate reducing bacteria in the sediment ranged from 2.37 x 102 - 9.21 x 103 bacteria cells/g dwt. The highest numbers of nitrate reducing bacteria were observed in the Rhizophora plot. The fastest cellulolytic activity was observed in the Aviccenia plot where 92% of filter paper dry weight was lost in 28 days. The slowest cellulolytic activity was in the Ceriops plot where only 11% of filter paper dry weight was lost in 28 days. Rates of acetylene reduction ranged between 0 and 51.474 n moles of C2H/4h/g dwt of sediment. The Ceriops plot showed the lowest rates of nitrogen fixation while the Rhizopthora plot had the highest. Aviccenia marina leaves in their native plot decomposed faster than either Rhizophora mucronata or Ceriops tagal leaves in their native plots. Organic mater in the mangrove swamp sediments ranged from 2.25 - 37.1%, while water content ranged from 17.48 - 76.36%. The levels of combined NO3- and NO2- in sediment pore water ranged between 0.85 and 113.92g/1. Sediment in the Rhizophora plot had the highest levels of nutrients compared to the other plots. Characterization of bacteria revealed that, the majority of the heterotrophic bacteria strains in the mangrove swamp were Gram positive and spherical in shape. NB: c fu = colony forming units. g dwt = gram dry weight. G wwt = gram wet weight.