Significance of bacteria in the flux of organic matter in the tidal creeks of the mangrove ecosystem of the Indus River delta, Pakistan

Abstract

We studied bacterial blomass and production in 3 t ~ d a l creeks (Isaro, Gharo and Ph~tt l Creeks) in the mangrove forests in the Indus R~ver delta, Pakistan, to assess the signif~cance of bacteria-mediated carbon fluxes in the creek ecosystem. Bacterial biomass, bacterial carbon production (BCP) and primary productivity (PP) were measured periodically for over a year during 1991-92. BCP was high, generally 50 to 300 pg C 1-' d-l. Despite such high BCP, bacterial abundance remained between 1 X 106 ml-' and 4 X 106 ml-' (20 to 80 p g C I-') indicating tight coupling between bacter~al production and removal. Specific growth rates generally ranged from 1 to 7 d.' but the rate reached 24 d.' during a phytoplankton bloom, apparently a red tide, and this was an unprecedented growth rate for a natural assemblage The abundance of attached bacteria exhlblted a large variation, ranglng from 4 to 9 2 % (mean 35 2 21 ,,, n = 41) in Isaro Creek and from 14 to 84 L. (mean 37 -t 28%,, n = 10) in Gharo Creek Bacterial production due to attached bacteria was 73 to 96'Y of the total Thus, a major fract~on of BCP may have been directly ava~lab lc to m.etazoan grazers. BCP was generally much higher than net PP, the yearly integrated average BCP/13P 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. Average yearly integrated bacterial carbon demand (BCD), estimated conservatively by assuming a 30-0 growth efficiency for all sites, was 6.9 times net PP. Thus, the creek water columns were strongly and persistently 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':0 of the BC:D; the remaining 80 to 93% of BCD would be met by reduced carbon from other sources. Phytoplankton production was l ~ g h t limited due to high turbidity and, apparently, the majority of BCP could be supported by the input of mangrove detritus. Estimates of utilizable dissolved organic carbon (UDOC) in selected samples were 97 to 656 pg C I-', indicating that In order to sustain the measured BCD (643 + 671 pg C I ' d') the UDOC pool would turnover in < l d to a few days. Limited data suggest that bacterial production was carbon rather than N or P limited. consistent w ~ t h sustained high levels of Inorganic N and P in the surface water. Since mangrove de t r~ tus provides most of the energy for bacterial production, which in turn is a significant source of high quality lood for grazers, particularly via ingestion of attached bacteria, w e predict that the ongoing destruction of mangrove forests in the Indus delta and elsewhere could have a major impact on mangrove ecosystem structure and functioning and the production of economically important flsh and shrimp in mangrove creeks.