Dissolved primary production and heterotrophic prokaryote reassimilation in a large oligotrophic tropical lake (Lake Kivu, Eastern Africa)

Conference (2012, July 11)

Variability of methane in the epilimnion of Lake Kivu

Poster (2011, July 11)

Carbon fluxes and cycling in African Rivers

Conference (2010, October 04)

Carbon biogeochemistry of the Betsiboka Estuary (north-western Madagascar)

in Organic Geochemistry (2008), 39

Madagascar’s largest estuary (Betsiboka) was sampled along the salinity gradient during the dry season to document the distribution and sources of particulate and dissolved organic carbon (POC, DOC) as ... [more ▼]

Madagascar’s largest estuary (Betsiboka) was sampled along the salinity gradient during the dry season to document the distribution and sources of particulate and dissolved organic carbon (POC, DOC) as well as dissolved inorganic carbon (DIC). The Betsiboka was characterized by a relatively high suspended matter load, and in line with this, low DOC/POC ratios ( 0.4–2.5). The partial pressure of CO2 (pCO2) was generally above atmospheric equilibrium (270–1530 ppm), but relatively low in comparison to other tropical and subtropical estuaries, resulting in low average CO2 emission to the atmosphere (9.1 ± 14.2 mmol m 2 d 1). Despite the fact that C4 vegetation is reported to cover >80% of the catchment area, stable isotope data on DOC and POC suggest that C4 derived material comprises only 30% of both pools in the freshwater zone, increasing to 60–70% and 50–60%, respectively, in the oligohaline zone due to additional lateral inputs. Sediments from intertidal mangroves in the estuary showed low organic carbon concentrations (<1%) and d13C values (average 19.8‰) consistent with important inputs of riverine imported C4 material. This contribution was reflected in d13C signatures of bacterial phospholipid derived fatty acids (i + a15:0), suggesting the potential importance of terrestrial organic matter sources for mineralization and secondary production in coastal ecosystems. [less ▲]

Mangrove production and carbon sinks: a revision of global budget estimates

in Global Biogeochemical Cycles (2008), 22(GB2013),

Mangrove forests are highly productive but globally threatened coastal ecosystems, whose role in the carbon budget of the coastal zone has long been debated. Here we provide a comprehensive synthesis of ... [more ▼]

Mangrove forests are highly productive but globally threatened coastal ecosystems, whose role in the carbon budget of the coastal zone has long been debated. Here we provide a comprehensive synthesis of the available data on carbon fluxes in mangrove ecosystems. A reassessment of global mangrove primary production from the literature results in a conservative estimate of 218 ± 72 Tg C a 1. When using the best available estimates of various carbon sinks (organic carbon export, sediment burial, and mineralization), it appears that >50% of the carbon fixed by mangrove vegetation is unaccounted for. This unaccounted carbon sink is conservatively estimated at 112 ± 85 Tg C a 1, equivalent in magnitude to 30–40% of the global riverine organic carbon input to the coastal zone. Our analysis suggests that mineralization is severely underestimated, and that the majority of carbon export from mangroves to adjacent waters occurs as dissolved inorganic carbon (DIC). CO2 efflux from sediments and creek waters and tidal export of DIC appear to be the major sinks. These processes are quantitatively comparable in magnitude to the unaccounted carbon sink in current budgets, but are not yet adequately constrained with the limited published data available so far. [less ▲]

Importance of intertidal sediment processes and porewater exchange on the water column biogeochemistry in a pristine mangrove creek (Ras Dege, Tanzania)

in Biogeosciences (2007), 4

We sampled a tidal creek (Ras Dege, Tanzania) during a 24-h cycle to document the variations in a suite of creek water column characteristics and to determine the relative influence of tidal and ... [more ▼]

We sampled a tidal creek (Ras Dege, Tanzania) during a 24-h cycle to document the variations in a suite of creek water column characteristics and to determine the relative influence of tidal and biological driving forces. Since the creek has no upstream freshwater inputs, highest salinity was observed at low tide, due to evaporation effects and porewater seepage. Total suspended matter (TSM) and particulate organic carbon (POC) showed distinct maxima at periods of highest water flow, indicating that erosion of surface sediments and/or resuspension of bottom sediments were an important source of particulate material. Dissolved organic carbon (DOC), in contrast, varied in phase with water height and was highest at low tide. Stable isotope data of POC and DOC displayed large variations in both pools, and similarly followed the variations in water height. Although the variation of 13CDOC (−23.8 to −13.8‰) was higher than that of 13CPOC (−26.2 to −20.5‰), due to the different endmember pool sizes, the 13C signatures of both pools differed only slightly at low tide, but up to 9‰ at high tide. Thus, at low tide both DOC and POC originated from mangrove production. At high tide, however, the DOC pool had signatures consistent with a high contribution of seagrass-derived material, whereas the POC pool was dominated by marine phytoplankton. Daily variations in CH4, and partial pressure of CO2 (pCO2) were similarly governed by tidal influence and were up to 7- and 10-fold higher at low tide, which stresses the importance of exchange of porewater and diffusive fluxes to the water column. When assuming that the high dissolved inorganic carbon (DIC) levels in the upper parts of the creek (i.e. at low tide) are due to inputs from mineralization, 13C data on DIC indicate that the organic matter source for mineralization had a signature of −22.4‰. Hence, imported POC and DOC from the marine environment contributes strongly to overall mineralization within the mangrove system. Our data demonstrate how biogeochemical processes in the intertidal zone appear to be prominent drivers of element concentrations and isotope signatures in the water column, and how pathways of dissolved and particulate matter transport are fundamentally different. [less ▲]

Dynamics of organic and inorganic carbon across contiguous mangrove and seagrass systems (Gazi bay, Kenya)

in Journal of Geophysical Research. Biogeosciences (2007), 112(G02018),

We report on the water column biogeochemistry in adjacent mangrove and seagrass systems in Gazi Bay (Kenya), with a focus on assessing the sources and cycling of organic and inorganic carbon. Mangrove and ... [more ▼]

We report on the water column biogeochemistry in adjacent mangrove and seagrass systems in Gazi Bay (Kenya), with a focus on assessing the sources and cycling of organic and inorganic carbon. Mangrove and seagrass-derived material was found to be the dominant organic carbon sources in the water column, and could be distinguished on the basis of their d13C signatures and particulate organic carbon:total suspended matter (POC/TSM) ratios. Spatially, a distinct boundary existed whereby the dominance of mangrove-derived material decreased sharply close to the interface between the mangrove forest and the dense seagrass beds. The latter is consistent with the reported export of mangrove-derived material, which is efficiently trapped in the adjacent seagrass beds. There were significant net inputs of POC and dissolved organic carbon (DOC) along the Kidogoweni salinity gradient, for which the d13CPOC signatures were consistent with those of mangroves. DOC was the dominant form of organic carbon in both mangrove and seagrass beds, with DOC/POC ratios typically between 3 and 15. Dynamics of dissolved inorganic carbon in the creeks were strongly influenced by diagenetic C degradation in the intertidal mangrove areas, resulting in significant CO2 emission from the water column to the atmosphere. Although highest partial pressure of CO2 (pCO2) values and areal CO2 flux rates were observed in the mangrove creeks, and the water column above the seagrass beds was in some locations a net sink of CO2, most of the ecosystems’ emission of CO2 to the atmosphere occurred in the seagrass beds adjacent to the mangrove forest. The presence of dense seagrass beds thus had a strong effect on the aquatic biogeochemistry, and resulted in trapping and further mineralization of mangrove-derived POC, intense O2 production and CO2 uptake. The adjacent seagrass beds provide a large area with conditions favorable to exchange of CO2 with the atmosphere, thereby limiting export of mangrove-derived organic and inorganic carbon toward the coastal ocean. [less ▲]

Carbon and nitrogen stable isotope ratios of subtidal benthic invertebrates in an estuarine mangrove ecosystem (Andhra Pradesh, India)

in Estuarine Coastal and Shelf Science (2002), 54(5), 901-913

In order to assess the relative trophic importance of mangrove litterfall and aquatic primary production in the mangrove creeks of the Coringa Wildlife Sanctuary (Andhra Pradesh, India) and the adjacent ... [more ▼]

In order to assess the relative trophic importance of mangrove litterfall and aquatic primary production in the mangrove creeks of the Coringa Wildlife Sanctuary (Andhra Pradesh, India) and the adjacent semi-enclosed Kakinada Bay, carbon and nitrogen stable isotope ratios were determined in a variety of benthic invertebrate species collected at 22 sites during the pre-monsoon period (May-June) of 1997 and 1999. delta(13)C values showed little interspecific variation at any given location, but there was a distinct spatial gradient in consumer delta(13)C values of about 7parts per thousand, with more depleted values in the mangrove creeks ( - 23.6 +/- 0.6parts per thousand), and gradually increasing in the mangrove outlets ( - 21.5 +/- 0.9parts per thousand), a relatively restricted zone in the south-eastern part of Kakinada Bay adjacent to the mangroves ( - 18.8 +/- 0.8parts per thousand), and the central and northern part of the Bay ( - 16.7 +/- 1.4parts per thousand) which opens into the Bay of Bengal. This gradient is much larger than that observed during a previous study in suspended organic matter (maximum about 2.7parts per thousand) and during this study in sediment organic matter (about 1.5-2.5parts per thousand). The observed carbon stable isotope ratios thus suggest a marked selectivity of the benthic invertebrate community for pelagic and benthic microalgal food sources and indicate that mangrove-derived and other terrestrial carbon is not a significant food source for benthic invertebrate communities in this ecosystem during the pre-monsoon period. Furthermore, delta(13)C values of sediment organic matter (SOM) suggest that terrestrial carbon is not a major contributor to the SOM-pool in this ecosystem. Evidence for seaward migration of Penaeid prawns was provided by some individuals caught in the North Bay which displayed low delta(13)C values of characteristic of fauna found in the mangrove creeks or outlets. Nitrogen stable isotope ratios were found to be a useful indicator of trophic level, even though there remained some overlap between delta(15)N values of presumed low and higher trophic levels. Benthic invertebrates showed a delta(15)N gradient of about 3.2parts per thousand between the mangrove creeks and the Central and North Bay whereas sediment delta(15)N values showed a smaller spatial gradient of about 1.6parts per thousand. This gradient which is hypothesized to reflect differences in inorganic nitrogen sources and availability. (C) 2001 Elsevier Science Ltd. All rights reserved. [less ▲]