SIGNIFICANCE OF BACTERIA IN THE FLUX OF ORGANIC-MATTER IN THE TIDAL CREEKS OF THE MANGROVE ECOSYSTEM OF THE INDUS RIVER DELTA, PAKISTAN

We studied bacterial biomass and production in 3 tidal creeks (Isaro, Gharo and Phitti Creeks) in the mangrove forests in the Indus River de lta, Pakistan, to assess the significance of bacteria-mediated carbon fluxes in the creek ecosystem. Bacterial biomass, bacterial carbon pro duction (BCP) and primary productivity (PP) were measured periodically for over a year during 1991-92. BCP was high, generally 50 to 300 mu g C l(-1) d(-1). Despite such high BCP, bacterial abundance remained b etween 1 x 10(6) ml(-1) and 4 x 10(6) ml(-1) (20 to 80 mu g C l(-1)) i ndicating tight coupling between bacterial production and removal. Spe cific growth rates generally ranged from 1 to 7 d(-1) but the rate rea ched 24 d(-1) during a phytoplankton bloom, apparently a red tide, and this was an unprecedented growth rate for a natural assemblage. The a bundance of attached bacteria exhibited a large variation, ranging fro m 4 to 92 % (mean 35 +/- 21 %, n = 41) in Isaro Creek and from 14 to 8 4% (mean 37 +/- 28%, n = 10) in Gharo Creek. Bacterial production due to attached bacteria was 73 to 96 % of the total. Thus, a major fracti on of BCP may have been directly available to metazoan grazers. BCP wa s generally much higher than net PP; the yearly integrated average BCP /PP for all sites was 2.0. Thus, the growth of bacteria, attached and free, probably represented the major pathway of the production of high quality (low C:N) biomass potentially available to the grazers. Avera ge yearly integrated bacterial carbon demand (BCD), estimated conserva tively by assuming a 30 % growth efficiency for all sites, was 6.9 tim es net PP. Thus, the creek water columns were strongly and persistentl y net heterotrophic. Data integrated over the entire study period show that even if all phytoplankton production was utilized by bacteria it would satisfy only 7 to 20% of the BCD; the remaining 80 to 93% of BC D would be met by reduced carbon from other sources. Phytoplankton pro duction was light Limited due to high turbidity and, apparently, the m ajority of BCP could be supported by the input of mangrove detritus. E stimates of utilizable dissolved organic carbon (UDOC) in selected sam ples were 97 to 656 mu g C l(-1), indicating that in order to sustain the measured BCD (643 +/- 671 mu g C l(-1) d(-1)) the UDOC pool would turnover in <1 d to a few days. Limited data suggest that bacterial pr oduction was carbon rather than N or P limited, consistent with sustai ned high levels of inorganic N and P in the surface water. Since mangr ove detritus provides most of the energy for bacterial production, whi ch in turn is a significant source of high quality food for grazers, p articularly via ingestion of attached bacteria, we predict that the on going destruction of mangrove forests in the Indus delta and elsewhere could have a major impact on mangrove ecosystem structure and functio ning and the production of economically important fish and shrimp in m angrove creeks.