Aquatic sediments represent a key component for understanding CH4 dynamics and emission to the atmosphere. Once produced in the sediments, CH4 is released either by diffusion at the sediment–water interface or by bubbling out to the atmosphere when total gas pressure in the sediment exceeds local ambient pressure due to high CH4 production. Although bubbling is one of the dominant CH4 emission pathways in lakes, direct measurements of this flux are hampered by its high spatiotemporal variability and methodological limitations. Here, we develop a conceptual approach to quantify CH4 production in lake sediments and particularly its release as bubbles based on simple measurements of bubble gas content and depth. Its main assumptions were empirically tested using > 200 long-term bubble trap deployments collected from 4 temperate lakes. We then applied the developed methodology to a suite of 408 Canadian lakes to produce the first standardized large-scale assessment of lakes CH4 ebullitive flux during summer. Our results show that lake sediments produced CH4 at a median rate of 3.3 mmol m−2 d−1 (ranged from 0.2 to 11.8 mmol m−2 d−1), releasing 33% via ebullition to the atmosphere. These rates are remarkably similar in magnitude to other regional estimates in the literature. Moreover, our approach revealed that catchment slope was an important determinant of both the lake-wide ebullitive fluxes and the fraction of sediment CH4 production released as bubbles.

Rivers are globally significant sources of atmospheric carbon dioxide (CO2). However, the processes governing supersaturation of CO2 in large tropical fluvial networks are poorly understood. In particular, strikingly little is known about the role of land use in shaping CO2 variability in South Asian river basins, which are undergoing rapid urbanization. Here, we show that the wide variability in the partial pressure of CO2 (pCO2: 246.3-21271.2 μatm) in an agriculture-dominated river basin (Krishna River basin, India) is primarily shaped by the extent of urbanization. Specifically, a strong positive correlation between pCO2 and built-up area (%) was observed when the built-up area exceeded 2%. Furthermore, machine learning analysis showed that pCO2 variability was predicted by built-up area (%), Strahler order, and altitude, together explaining ∼77% of the spatial variability. Additionally, a strong negative correlation between excess CO2 and oxygen relative to atmospheric equilibrium indicates that in-stream metabolism, fueled by organic matter inputs from urbanized areas, is the primary cause of CO2 supersaturation, establishing a mechanistic link between pCO2 and the built-up area. Given that pCO2 increases with urbanization, limiting urban inputs is crucial for reducing fluvial CO2 emissions from South Asian river basins.

Aquaculture plays a crucial role in meeting the increasing demand for protein-rich food. However, aquaculture production also comes with a large carbon footprint, partly due to the substantial emission of greenhouse gases, particularly methane (CH₄), during the aquaculture production. Yet, our understanding on the magnitude, pathways, and drivers of CH4 emission from aquaculture ponds is limited, particularly in the Asian continent where more than 90% of global aquaculture production occurs. In this study, we quantified CH4 concentrations, air–water fluxes, production in anoxic sediments, and oxidation in the water column across multiple tropical aquaculture ponds where one of the most commonly cultivated shrimp species, Litopenaeus vannamei, is farmed. Field measurements showed that the diffusive CH4 emissions, with a mean value of 3.40 ± 1.76 mg m−2 day−1, varied greatly—ranging from 0.68 to 7.12 mg m−2 day−1—and were regulated by a suit of environmental variables. Salinity and total phosphorus (TP) concentration were the key determinants of diffusive CH4 flux: CH4 emission decreased with increasing salinity, while it increased with TP. Accordingly, our results suggest that shifting from freshwater to saline water aquaculture can decrease CH₄ emissions, thereby reducing the carbon footprint of aquaculture production. However, an increase in phosphorus concentration can offset this salinity-driven emission reduction. Therefore, management practices should prioritize reducing phosphorus loads to effectively mitigate CH₄ emissions and enhance the sustainability of aquaculture.

The ubiquitous oversaturation of methane (CH4) in fluvial environments is hypothesized to be sustained by both proximate and distal factors, which reflect their respective roles in modulating CH4metabolism through water chemistry as well as hydrological, morphological, and landscape features. Yet, efforts to disentangle their complex interplay in regulating riverine CH4variability remain limited. Herein, we used machine learning (ML) approach to examine drivers of CH4variability in five South Asian river basins where CH4concentration varied widely (0.01–455.75 μmol L–1). Among proximate variables, dissolved oxygen (DO) emerged as the strongest predictor of CH4concentration, explaining 61–75% of CH4variability explained by different ML models, followed by total phosphorus (TP) and dissolved organic carbon. Conversely, the extent of built-up area (%) was the key predictor among the distal variables. When combining proximate and distal variables in ML analysis, proximate factors emerged as the dominant drivers, whereas distal factors had only a marginal impact suggesting that local biogeochemical conditions outweigh broader landscape features in determining fluvial CH4variability. Our ML analysis reveals that while remote-sensing-derived distal variables can assist in predicting CH4concentrations, they offer limited mechanistic insights. Therefore, integrating proximate factors with landscape variables is important in deriving a comprehensive and mechanistic understanding of CH4dynamics in river networks.

Oxygenated surface layers of aquatic systems are ubiquitously oversaturated with methane (CH4). A growing number of studies suggest that CH4 oversaturation in surface waters can be sustained, at least partly, by methanogenesis occurring under oxic conditions. Although we are gaining a better understanding of the extent and drivers of oxic CH4 production (OMP) in oceanic and lake environments, the existence and variability of OMP in rivers and streams remain unknown. Here, we present experimental evidence for the occurrence and a large variability of OMP rates in a tropical river network. The positive correlation between chlorophyll a concentration and OMP rates and reduction of OMP during the experimental inhibition of photosynthesis establishes a clear link between OMP and photosynthesis. At the same time, a general decrease of the OMP rates with increasing total phosphorus (TP) concentration and the correlation between stable carbon isotopic (δ13C-CH4) values of the OMP-derived CH4 and TP suggest the likely involvement of P-availability as well in regulating the OMP rates. While our estimation suggested a minor contribution of the OMP in the CH4 cycling of the studied tropical system, we show that the OMP in the fluvial environment may be highly sensitive to the current and future changes in algal and nutrient dynamics.

Aerobic methane oxidation (MOX) significantly reduces methane (CH4) emissions from inland water bodies and is, therefore, an important determinant of global CH4 budget. Yet, the magnitude and controls of MOX rates in rivers – a quantitatively significant natural source of atmospheric CH4 – are poorly constrained. Here, we conducted a series of incubation experiments to understand the magnitude and environmental controls of MOX rates in tropical fluvial systems. We observed a large variability in MOX rate (0.03 - 3.45 μmol l-1d-1) shaped by a suit of environmental variables. Accordingly, we developed an empirical model for MOX that incorporate key environmental drivers, including temperature, CH4, total phosphorus, and dissolved oxygen (O2) concentrations, based on the results of our incubation experiments. We show that temperature dependency of MOX (activation energy: 0.66 ± 0.18 eV) is lower than that of sediment methanogenesis (0.71 ± 0.21 eV) in the studied tropical fluvial network. Furthermore, we observed a non-linear relationship between O2 concentration and MOX, with the highest MOX rate occuring ∼135 μmol O2l-1, above or below this “optimal O2” concentration, MOX rate shows a gradual decline. Together, our results suggest that the relatively lower temperature response of MOX compared to methanogenesis along with the projected decrease of O2 concentration due to organic pollution may cause elevated CH4 emission from tropical southeast Asian rivers. Since estimation of CH4 oxidation is often neglected in routine CH4 monitoring programs, the model developed here may help to integrate MOX rate into process-based models for fluvial CH4 budget.

Rivers are globally significant natural sources of atmospheric methane (CH4). However, the effect of land use changes on riverine CH4 dynamics, particularly in tropical zones, remain ambiguous, yet important to predict and anticipate the present and future contribution of rivers to the global CH4 budget. The present study examines the magnitude and drivers of riverine CH4 concentration and emission in the tropical Krishna River (KR) basin, India. The large spatial variability of CH4 concentration (0.03 to 185.34 μmol L −1) and emissions (0.04 mmol m−2 d−1 to 1666.24 mmol m−2 d−1) in the KR basin was linked to the site-specific features of the catchments through which rivers are draining. Several fold higher CH4 concentration and emission was observed for the urban river sites (64.63 ± 53.17 µmol L−1 and 294.15 ± 371.52 mmol m2 d−1, respectively) than the agricultural (1.05 ± 2.22 µmol L−1 and 3.45 ± 9.72 mmol m2 d−1, respectively) and forested (0.49 ± 0.23 µmol L−1 and 1.26 ± 0.73 mmol m2 d−1, respectively) sites. The concentrations of dissolved oxygen, total phosphorus, and Chlorophyll-a were significant hydrochemical variables strongly coupled with the dissolved CH4 concentrations. On the other hand, percentage of built-up area emerged as the most important landscape-level driver indicating that urbanization has an overriding effect on riverine CH4 concentration in the agriculture dominated KR basin. Our study supports the growing notion that tropical urban rivers are hotspot of CH4 emission. Furthermore, we show that the pattern of increasing in riverine CH4 concentration with built-up area (%) is a general feature of Asian river basins. As the urban land cover and population following an exponential increase, Asian rivers might contribute substantially to the regional and global CH4 budget.

Methanogenesis is traditionally considered as a strictly anaerobic process. Recent evidence suggests instead that the ubiquitous methane (CH4) oversaturation found in freshwater lakes is sustained, at least partially, by methanogenesis in oxic conditions. Although this paradigm shift is rapidly gaining acceptance, the magnitude and regulation of oxic CH4production (OMP) have remained ambiguous. Based on the summer CH4mass balance in the surface mixed layer (SML) of five small temperate lakes (surface area, SA, of 0.008-0.44 km2), we show that OMP (range of 0.01 ± 0.01 to 0.52 ± 0.04 μmol L-1day-1) is linked to the concentrations of chlorophyll-a, total phosphorus, and dissolved organic carbon. The stable carbon isotopic mass balance of CH4(δ13C-CH4) indicates direct photoautotrophic release as the most likely source of oxic CH4. Furthermore, we show that the oxic CH4contribution to the SML CH4saturation and emission is an inverse function of the ratio of the sediment area to the SML volume in lakes as small as 0.06 km2. Given that global lake CH4emissions are dominated by small lakes (SA of <1 km2), the large contribution of oxic CH4production (up to 76%) observed in this study suggests that OMP can contribute significantly to global CH4emissions.

The aerobic oxidation of methane (CH4) by methanotrophic bacteria (MOB) is the major sink of this highly potent greenhouse gas in freshwater environments. Yet, CH4 oxidation is one of the largest uncertain components in predicting the current and future CH4 emissions from these systems. While stable carbon isotopic mass balance is a powerful approach to estimate the extent of CH4 oxidation in situ, its applicability is constrained by the need of a reliable isotopic fractionation factor (αox), which depicts the slower reaction of the heavier stable isotope (13C) during CH4 oxidation. Here we explored the natural variability and the controls of αox across the water column of six temperate lakes using experimental incubation of unamended water samples at different temperatures. We found a large variability of αox (1.004–1.038) with a systematic increase from the surface to the deep layers of lake water columns. Moreover, αox was strongly positively coupled to the abundance of MOB in the γ-proteobacteria class (γ-MOB), which in turn correlated to the concentrations of oxygen and CH4, and to the rates of CH4 oxidation. To enable the applicability in future isotopic mass balance studies, we further developed a general model to predict αox using routinely measured limnological variables. By applying this model to δ13C-CH4 profiles obtained from the study lakes, we show that using a constant αox value in isotopic mass balances can largely misrepresent and undermine patterns of the extent of CH4 oxidation in lakes. Our αox model thus contributes towards more reliable estimations of stable carbon isotope-based quantification of CH4 oxidation and may help to elucidate large scale patterns and drivers of the oxidation-driven mitigation of CH4 emission from lakes.

Previous stable isotope and biomarker evidence has indicated that methanotrophy is an important pathway in the microbial loop of freshwater ecosystems, despite the low cell abundance of methane-oxidizing bacteria (MOB) and the low methane concentrations relative to the more abundant dissolved organic carbon (DOC). However, quantitative estimations of the relative contribution of methanotrophy to the microbial carbon metabolism of lakes are scarce, and the mechanism allowing methanotrophy to be of comparable importance to DOC-consuming heterotrophy remained elusive. Using incubation experiments, microscopy, and multiple water column profiles in six temperate lakes, we show that MOB play a much larger role than their abundances alone suggest because of their larger cell size and higher specific activity. MOB activity is tightly constrained by the local methane:oxygen ratio, with DOC-rich lakes with large hypolimnetic volume fraction showing a higher carbon consumption through methanotrophy than heterotrophy at the whole water column level. Our findings suggest that methanotrophy could be a critical microbial carbon consumption pathway in many temperate lakes, challenging the prevailing view of a DOC-centric microbial metabolism in these ecosystems.

Stable isotopic analysis is a popular method to understand the mechanisms sustaining methane (CH4) emissions in various aquatic environments. Yet, the general lack of concurrent measurements of isotopes and fluxes impedes our ability to establish a connection between the variation in the rates of CH4 emission and isotopic signature. Here, we examine the magnitude of CH4 ebullition (bubbling) and stable carbon isotopic signature (δ13C-CH4) of bubble CH4 in four northern temperate lakes and evaluate the in-lake processes shaping their variability. The ebullitive CH4 flux and bubble δ13C-CH4 varied from 0.01 to 37.0 mmol m−2 d−1 and between −71.0‰ and −50.9‰, respectively. The high emission lakes in general and high fluxing shallow zones within each lake consistently showed enriched δ13C-CH4 signature. Subsequently, in addition to the temperature dependence (1.4 ± 0.1 eV), the rates of ebullition strongly correlated with the variability of δ13C-CH4 across our study lakes. Our results suggest that higher ebullitive emissions are sustained by acetoclastic methanogenesis, likely fueled by fresh organic matter inputs. Further, the annual whole-lake estimate of bubble isotopic flux alone showed depleted δ13C-CH4 values (−64.6 ± 0.6‰ to −60.1 ± 3.2‰), yet the signature of the total CH4 emission (ebullition + diffusion) was relatively enriched (−60.7‰ to −52.6‰) due to high methanotrophic activity in the water column. We show that δ13C-CH4 signature of bubbles can be linked to the magnitude of ebullition itself, yet we suggest there is a need to account for different emission pathways and their isotopic signature to allocate CH4 source signature to northern lakes.

Lake methane (CH4) emissions are largely controlled by aerobic methane-oxidizing bacteria (MOB) which mostly belong to the classes Alpha- and Gammaproteobacteria (Alpha- and Gamma-MOB). Despite the known metabolic and ecological differences between the two MOB groups, their main environmental drivers and their relative contribution to CH4 oxidation rates across lakes remain unknown. Here, we quantified the two MOB groups through CARD-FISH along the water column of six temperate lakes and during incubations in which we measured ambient CH4 oxidation rates. We found a clear niche separation of Alpha- and Gamma-MOB across lake water columns, which is mostly driven by oxygen concentration. Gamma-MOB appears to dominate methanotrophy throughout the water column, but Alpha-MOB may also be an important player particularly in well-oxygenated bottom waters. The inclusion of Gamma-MOB cell abundance improved environmental models of CH4 oxidation rate, explaining part of the variation that could not be explained by environmental factors alone. Altogether, our results show that MOB composition is linked to CH4 oxidation rates in lakes and that information on the MOB community can help predict CH4 oxidation rates and thus emissions from lakes.

Understanding the drivers of aerobic methane (CH 4 ) oxidation (MOX) is paramount in assessing the current and potential future CH 4 emissions from freshwater aquatic systems. Regulation of MOX kinetics is a complex function of CH 4 and oxygen (O 2 ) concentrations. While MOX activity is usually proportional to the concentration of CH 4 itself, the effects of O 2 have been more conflicting, with maximum MOX rates often restricted to low O 2 concentrations. Despite the complexity involved, MOX kinetics are often modelled as monotonic positive functions of both CH 4 and O 2 concentrations. We conducted a series of incubation experiments using natural and unamended water samples obtained from multiple depths in northern temperate lakes that vary widely and independently in their CH 4 and O 2 concentrations. Our results showed the expected positive effect of CH 4 concentration and temperature but also demonstrated the strong inhibitory effects of O 2 at high concentration. We then developed a general model describing the kinetics of MOX, simultaneously integrating the effects of CH 4 concentration, temperature as well as the non-linear effect O 2 on MOX activity. The model revealed an overall temperature dependency (activation energy = 0.49 ± 0.06 eV) much lower than reported for methanogenesis and an optimal O 2 level around 15 μmol O 2 L −1 where maximum MOX activity occurs, regardless of CH 4 concentration and temperature. We further show that ignoring the inhibitory effect of O 2 can lead to significant bias in calculating the expected MOX rates in different portions of the water column.

Inland waters have a significant influence on atmospheric methane (CH4) levels. However, processes determining the strength of CH4 emissions from these systems are not well defined. Aerobic oxidation is a major sink of CH4 in freshwater environments and thus an important determinant of aquatic CH4 emissions, yet strikingly little is known about its drivers. Here we assessed the extent of water column CH4 oxidation at the whole ecosystem scale using stable carbon isotopic (δ13C) mass balance of CH4 in 14 northern lakes spanning wide range of dissolved organic carbon (DOC) concentrations. We show that the extent of oxidation can vary from near zero to near complete, and for concentrations of 1.9–11 mg/L, DOC is a key modulator of CH4 oxidation during the summer stratification period. Increasing DOC concentrations enhances oxidation in the upper layers by reducing light inhibition on methanotrophic activity, while reducing oxygen available for oxidation in the deeper layers. The effect of this light inhibition was also observable over the diurnal cycle. We developed simple predictive empirical models (r2 > 0.82) to estimate the extent of oxidation in the different layers of lakes for the summer period. Applying our surface layer model to a larger data set suggests that about 30% of CH4 transported to or generated within the epilimnion of Québec lakes is oxidized during summer. Our results imply that DOC concentration, through its effect on the light regime of lakes, has the potential to affect strongly the magnitude and patterns of summer CH4 emissions.

Single-cell PCR and gene sequencing were conducted to evaluate species and genotypic compositions of Cryptomonas in the eutrophic Lake Hira, Japan. We determined the sequences of nuclear internal transcribed spacer 2 region from single Cryptomonas cells with a high success rate (83.3–97.9 %), excluding one case (56.3 %). A total of 325 sequences were obtained over eight sampling days from May 28, 2012, to October 3, 2012, and phylogenetic analysis indicated that all sequences were divided into six groups. Four groups were clustered together with known sequences of C. curvata, C. marssonii, C. pyrenoidifera or C. tetrapyrenoidosa, although the sequences of the other two groups did not show high similarity to known Cryptomonas species. Cryptomonascurvata dominated during the study period (45–98 %), and unidentified Cryptomonas species (group 2) became dominant at high water temperatures. The genotypic composition of C. curvata also varied temporarily, suggesting that the genotypic composition of Cryptomonas was susceptible to environmental changes. These results indicated that single-cell PCR can be used to analyze the species composition and ecology of Cryptomonas.

The dynamics of fluorescent dissolved organic matter (FDOM) in the large monomictic freshwater Lake Biwa (surface area 675 km2, maximum depth 104 m) was studied from December 2010 to December 2011. The protein-like FDOM (FDOMT) and dissolved organic carbon (DOC) showed epilimnetic accumulation (FDOMT from 4.42 ± 0.22 quinine sulfate units [QSU] to 6.30 ± 0.04 QSU; DOC from 80.8 ± 2.7 μmol L-1 to 102.7 ± 3.5 μmol L-1) between nutrient-replete winter mixing to nutrient-depleted stratified periods. This accumulation is attributed to the reduced heterotrophic activity following severe P-limitation. The positive correlation between accumulated DOC and FDOMT in the epilimnion and their uniform reduction in the hypolimnion (~ 9%) suggest FDOMT as a proxy for semi-labile DOM. The humic-like FDOM (FDOMM) generally increased with depth, a pattern similar to nutrients and total carbon dioxide (TCO2), but adverse to dissolved oxygen. The significant positive correlations of FDOMM with apparent oxygen utilization (r = 0.86, p < 0.001), TCO2 (r = 0.91, p < 0.001), nitrate (r = 0.83, p < 0.001), and phosphate (r = 0.76, p < 0.001) in the deeper layers suggest that FDOMM is formed during hypolimnetic mineralization. We estimated that ~ 8% of the organic carbon degraded in the hypolimnion is transferred into humic substances. The minor contribution of DOC (6.4%) to hypolimnetic mineralization suggests that production of humic substances is mainly fueled by the mineralization of sinking biogenic particles. The production and consumption of FDOM in freshwater lakes may influence the quality and bioavailability of carbon exported from these systems. © 2013, by the Association for the Sciences of Limnology and Oceanography, Inc.

Changes in the phytoplankton biomass (chlorophyll a), production rate, and species composition were studied over two seasons using the time series measurements in the northern limb of the Cochin estuary in relation to the prevailing hydrological conditions. The present study showed the significant seasonal variation in water temperature (F = 69.4, P < 0.01), salinity (F = 341.93, P < 0.01), dissolved inorganic phosphorous (F = 17.71, P < 0.01), and silica (F = 898.1, P < 0.01) compared to nitrogen (F = 1.646, P > 0.05). The uneven input of ammonia (3.4-224.8 μM) from upstream (Periyar River) leads to the inconsistency in the N/P ratio (range 6.8-262). A distinct seasonality was observed in Si/N (F = 382.9, P < 0.01) and Si/P (F = 290.3, P < 0.01) ratios compared to the N/P ratio (F = 1.646, P > 0.05). The substantial increase in chlorophyll a (average, 34.8 ± 10 mg m -3) and primary production (average, 1,304 ± 694 mg C m -3 day-1) indicated the mesotrophic condition of the study area during the premonsoon (PRM) and it was attributed to the large increase in the population of nanoplankton (size < 20 μ ) such as Skeletonema costatum, Thalassiosira subtilis, Nitzschia closterium, and Navicula directa. In contrast, during the post monsoon (PM), low chlorophyll a concentration (average, 9.3 ± 9.2 mg m-3) and primary production (average, 124 ± 219 mg C m-3 day-1) showed heterotrophic condition. It can be stated that favorable environmental conditions (optimum nutrients and light intensity) prevailing during the PRM have enhanced the abundance of the nanoplankton community in the estuary, whereas during the PM, the light limitation due to high turbidity can reduce the nanoplankton growth and abundance, even though high nutrient level exists. © 2009 Springer Science+Business Media B.V.

Carbon biogeochemistry of a tropical ecosystem (The Cochin Estuary, India) undergoing increased human intervention was studied during February (premonsoon), April (early monsoon) and September (monsoon) 2005. The Cochin estuary sustains high levels of pCO2 (up to 6000 μatm) and CO 2 effluxes (up to 274 mmolC m-2 d-1) especially during monsoon. A first-order estimate of the carbon mass balance shows that net production of dissolved inorganic carbon is an order of magnitude higher than the net loss of dissolved and particulate organic carbon from the estuary. This imbalance is attributed to the organic inputs to the estuary through anthropogenic supplies. The bacteria-mediated mineralization of organic matter is mainly responsible for the build-up of pCO2 and increased CO 2 emission to the atmosphere indicating heterotrophy. The linear correlation between excess CO2 and apparent oxygen utilization indicates respiration as the chief mechanism for CO2 supersaturation. An increase in the net negative ecosystem production (-ve NEP) between premonsoon (-136 mmolC m-2 d-1 or -376 MgC d-1) and monsoon (-541 mmolC m-2 d-1 or -1500 MgC d -1) is supported by a corresponding increase in O2 influxes from 17 mmol O2 m-2 d-1 (126 MgC d-1) to -128 mmol O2 m-2 d-1 (-946 MgC d-1) and CO2 emissions from 65 mmolC m-2 d-1 (180 MgC d-1) to 267 mmolC m-2 d -1 (740 MgC d-1). There is a significant north-south gradient in metabolic rates and CO2 fluxes attributable to the varying flow patterns and anthropogenic inputs into the estuary. The study reveals that the Cochin estuary, a previously autotrophic (CO2 sink) system, has been transformed to a heterotrophic (CO2 source) system following rapid urbanization and industrialization. Moreover, the export fluxes from the Cochin estuary appear to be quite important in sustaining net heterotrophy in the southeastern Arabian Sea. © 2009 Springer Science+Business Media, LLC.

Bacterial productivity (BP) and respiration (BR) were examined in relation to primary productivity (PP) for the first time in a shallow tropical ecosystem (Cochin Estuary), India. The degree of dependence of BP (6.3-199.7 μg C L-1 d-1) and BR (6.6-430.4 μg C L-1 d-1) on PP (2.1-608.0 μg C L-1 d-1) was found to be extremely weak. The BP/PP (0.05-8.5) and PP/BR (0.02-7.9) ratios widely varied in the estuary depending on the season and location. There was a seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter input through rivers which enhanced the bacterial heterotrophic activity and very high pCO2 (106-6001 μatm) levels. The heterotrophic zones were characterized by low PP but high bacterial production and respiration leading to oxygen undersaturation and exceptionally high pCO2. We propose that the CO2 supersaturation caused by increased bacterial respiration (in excess of PP) was a result of bacterial degradation of allochthonous organic matter. This indicates that sources other than planktonic compartment need to be explored to understand the C-cycling in this estuary. These results are of particular relevance to tropical ecosystems in general, where the bulk of world's river discharges occur. © 2008 Elsevier Ltd. All rights reserved.

"Biolog" plates were used to study the changes in the metabolic capabilities of bacterioplankton over a complete tidal cycle in a tropical ecosystem (Cochin Estuary) along southwest coast of India. The pattern of utilization of carbon sources showed a definite shift in the community metabolism along a salinity gradient. Multivariate statistical analysis revealed two communities, namely allochthonous bacterioplankton sensitive to salinity and autochthonous bacterioplankton, which are tolerant to wide salinity fluctuations. Regression analysis showed salinity as the most important parameter influencing the physiological profile of bacterioplankton, irrespective of tide. Apart from salinity, limno-tolerant retrievable counts and halo-tolerant retrievable counts also accounted for the metabolic variation of bacterioplankton during low and high tides, respectively. The shift in the substrate utilization from carbohydrates to amino acids appears to be due to the physiological adaptation or nitrogen limitation of bacterial community with increasing salinity. © 2008 Elsevier Ltd. All rights reserved.