The enigma of aerobic vs. anaerobic decomposition in marine sediments was addressed by means of a thin-layer incubation technique. Two different 14C-labeled plant materials, aged diatoms (Skeletonema costatum) and fresh barley hay, were each mixed into intertidal sediment and spread in a 1.5-mm layer on the bottom of oxic and anoxic chambers. After a 27-d incubation, conditions in all chambers were switched from aerobic to anaerobic and vice versa for 11 d. Rates of 14C0, evolution in diatom chambers showed that aerobic carbon mineralization was -10 times faster than anaerobic both before and after the switch. Low rates of [14C]DOC release suggested that the limiting step of an,aerobic decay was the initial hydrolytic and fermentative enzymatic attack on the predecomposed diatoms. Initial carbon mineralization of barley hay was not affected by the presence or absence of oxygen. Leaching of DOC from the fresh barley hay supplied anaerobic respirers with labile substrates. When leaching ceased and after the aerobic-anaerobic switch, the rate of anaerobic mineralization was reduced. Mineralizat:.on of leachable and easily hydrolyzable compounds from fresh plant detritus is equally fast under aerobic ant! anaerobic conditions. When structural components dominate the particulate remains, anaerobic processes are hampered by inefficient and slow bacterial hydrolysis of structurally complex macromolecules.

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Abstract.Extensive observations were made during the late Southwest Monsoon of 2004 over the Indian and Omani shelves, and along a transect that extended from the southern coast of Oman to the central west coast of India, tracking the southern leg of the US JGOFS expedition (1994)(1995) in the west. The data are used, in conjunction with satellite-derived data, to investigate long-term trends in chlorophyll and sea surface temperature, indicators of upwelling intensity, and to understand factors that control primary production (PP) in the Arabian Sea, focussing on the role of iron. Our results do not support an intensification of upwelling in the western Arabian Sea, reported to have been caused by the decline in the winter/spring Eurasian snow cover since 1997. We also noticed, for the first time, an unexpected development of high-nutrient, low-chlorophyll condition off the southern Omani coast. This feature, coupled with other characteristics of the system, such as a narrow shelf and relatively low iron concentrations in surface waters, suggest a close similarity between the Omani upwelling system and the Peruvian and California upwelling systems, where PP is limited by iron. Iron limitation of PP may complicate simple relationship between upwelling and PP assumed by previous workers, and contribute to the anomalous offshore occurrence of the most severe oxygen (O 2 ) depletion in the region. Over the much wider Indian shelf, which experiences large-scale bottom water O 2 -depletion in summer, adequate iron supply from reCorrespondence to: S. W. A. Naqvi (naqvi@nio.org) ducing bottom-waters and sediments seems to support moderately high PP; however, such production is restricted to the thin, oxygenated surface layer, probably because of the unsuitability of the O 2 -depleted environment for the growth of oxygenic photosynthesizers.

We studied bacterial blomass and production in 3 t~d a l creeks (Isaro, Gharo and Ph~ttl 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~lablc 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 detr~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.

We conducted a sequential tracer leaching study on a 24.4 by 42.7 m field plot to investigate the temporal behavior of chemical movement to a 1.2-m deep field drain during irrigation and subsequent rainfall events over a 14-d period. The herbicides atrazine [6-chloroN-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], and alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] along with the conservative tracer Br were applied to a 1-m wide strip, offset 1.5 m laterally from a subsurface drain pipe, immediately before an 11.3-h long, 4.2-mm h −1 irrigation. Three additional conservative tracers, pentafluorobenzoate (PF), o-trifluoromethylbenzoate (TF), and difluorobenzoate (DF) were applied to the strip during the irrigation at 2-h intervals. Breakthrough of Br and the two herbicides occurred within the first 2-h of irrigation, indicating that a fraction of the solute transport was along preferential flow paths. Retardation and attenuation of the herbicides indicated that there was interaction between the chemicals and the soil lining the preferential pathways. The conservative tracers applied during the later stages of irrigation arrived at the subsurface drain much faster than tracers applied earlier. The final tracer, applied 6 h after the start of irrigation (DF), took only 15 min and 1 mm of irrigation water to travel to the subsurface drain. Model simulations using a twodimensional, convective, and dispersive numerical model without an explicit preferential flow component failed to reproduce Br tracer concentrations in the drain effluent, confirming the importance of preferential flow. This study showed that preferential flow in this soil is not a uniform process during a leaching event. ABSTRACTalso found Cl concentrations in tile water to peak within 1 h of the onset of irrigation, which they attributed toWe conducted a sequential tracer leaching study on a 24.4 by

SummaryThis study evaluated the growth performance and body composition of Oreochromis niloticus fingerlings (average initial weight 16.53 ± 0.44 g) fed 9 experimental diets (A, B, C, D, E, F, G, H and I) containing three different levels of protein (26, 31 and 36 g 100 g )1 ) at three different gross energy (GE) levels (16, 19 and 22 MJ kg) for a period of 64 days. Significant differences were observed in the feed consumption, body weight gain, specific growth rate (SGR), condition factor (k), feed conversion ratio (FCR), protein efficiency ratio (PER), net protein retention (NPR) and apparent net energy retention (ANER) values of fish when the energy level of diet was increased at different protein levels. The maximum weight gain, SGR and k were observed on diet F containing 36% protein and an energy level of 19 MJ kg )1 of dry feed with a protein to energy (P ⁄ E) ratio of 18.96 (g protein MJ )1 GE). A further increase in the energy content of the diet (22 MJ kg )1 ) at the same protein level (Diet I) did not produce any improvement in the growth performance. Lowering the energy level at the same protein level significantly affected the growth performance. Fish fed diet B containing 31% protein and a lower energy level of 16 MJ kg )1 with the same P ⁄ E ratio of 18.61 as diet F showed significantly lower weight gain and growth performance than diet F. Diets E and H containing 31% crude protein at all three energy levels produced similar results as diet B. The poorest FCR was observed when the diet contained both lower levels of protein and energy. Fish fed diet G, containing 26% protein at high energy level (22 MJ kg ). The body moisture content at all protein levels decreased (P < 0.05) with the increasing level of dietary energy whereas the body fat content increased (P < 0.05). Similar trends were observed in the body ash and energy content. Increasing the dietary energy content at lower protein levels did not show any difference (P > 0.05) in body protein content. Our results indicated the optimum P ⁄ E ratio for O. niloticus as 18.96 g protein per mega joule of gross energy at 36% dietary protein level and a dietary gross energy value of 19 MJ kg )1.

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