Enhanced biodegradation of diesel fuel through the addition of particulate organic carbon and inorganic nutrients in coastal marine waters

Diesel fuel pollution in coastal waters, resulting from recreational boating and commercial shipping operations, is common and can adversely affect marine biota. The purpose of this study was to examine the effect of additions of particulate organic carbon (POC) in the form of naturally-occurring marsh grass (Spartina alterniflora), inorganic nutrients (nitrogen and phosphorus), inert particles, and dissolved organic carbon (DOC) on diesel fuel biodegradation and to attempt to formulate an effective bioremedial treatment for small diesel fuel spills in marine waters. Various combinations of treatments were added to water samples from a coastal marina to stimulate diesel fuel biodegradation. Diesel fuel was added in concentrations approximating those found in a spill and biodegradation of straight chain aliphatic constituents was estimated by measuring mineralization of ¹⁴C hexadecane added to diesel fuel. All treatments that included POC showed stimulation of biodegradation. However, the addition of inert particles (glass fiber filters and nylon screening) caused no stimulation of biodegradation. The addition of nitrogen and phosphorus alone did not result in stimulation of biodegradation, but nitrogen and Spartina (although not phosphorus and Spartina) did result in stimulation above that of Spartina alone. Maximum biodegradation rates were obtained by the addition of the Spartina POC, ammonium, and phosphate. The addition of mannitol, a labile DOC source with POC and phosphate resulted in a decrease in diesel fuel biodegradation as compared to POC and phosphate alone. The seasonal pattern of diesel fuel biodegradation showed a maximum in the summer and a minimum in the winter. Therefore, of the treatments tested, the most effective for bioremediation of diesel fuel in marine waters is the addition of POC, nitrogen, and phosphorus.     

Stimulation of Diesel Fuel Biodegradation by Indigenous Nitrogen Fixing Bacterial Consortia

Abstract Successful stimulation of N₂ fixation and petroleum hydrocarbon degradation in indigenous microbial consortia may decrease exogenous N requirements and reduce environmental impacts of bioremediation following petroleum pollution. This study explored the biodegradation of petroleum pollution by indigenous N₂ fixing marine microbial consortia. Particulate organic carbon (POC) in the form of ground, sterile corn-slash (post-harvest leaves and stems) was added to diesel fuel amended coastal water samples to stimulate biodegradation of petroleum hydrocarbons by native microorganisms capable of supplying a portion of their own N. It was hypothesized that addition of POC to petroleum amended water samples from N-limited coastal waters would promote the growth of N₂ fixing consortia and enhance biodegradation of petroleum. Manipulative experiments were conducted using samples from coastal waters (marinas and less polluted control site) to determine the effects of POC amendment on biodegradation of petroleum pollution by native microbial consortia. Structure and function of the microbial consortia were determined by measurement of N₂ fixation (acetylene reduction), hydrocarbon biodegradation (¹⁴C hexadecane mineralization), bacterial biomass (AODC), number of hydrocarbon degrading bacteria (MPN), and bacterial productivity (³H-thymidine incorporation). Throughout this study there was a consistent enhancement of petroleum hydrocarbon degradation in response to the addition of POC. Stimulation of diesel fuel biodegradation following the addition of POC was likely attributable to increases in bacterial N₂ fixation, diesel fuel bioavailability, bacterial biomass, and metabolic activity. Toxicity of the bulk phase water did not appear to be a factor affecting biodegradation of diesel fuel following POC addition. These results indicate that the addition of POC to diesel-fuel-polluted systems stimulated indigenous N₂ fixing microbial consortia to degrade petroleum hydrocarbons. Received: 29 December 1998; Accepted: 6 April 1999     

Carbon flux between an estuary and the ocean: a case for outwelling

The classical outwelling hypothesis states that small coastal embayments (e.g. estuaries, wetlands) export their excess production to inshore marine waters. In line with this notion, the present study tested whether the Swartkops estuary acts as source or sink for carbon. To this end, concentrations of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and particulate organic carbon (POC) were determined hourly during the first monthly spring and neap tides over one year in the tidal waters entering and leaving the estuary. Each sampling session spanned a full tidal cycle, yielding a total of 936 concentration estimates. Carbon fluxes were calculated by integrating concentrations with water flow rates derived from a hydrodynamic model calibrated for each sampling datum. Over the year, exports to marine waters markedly exceeded imports to the estuary for all carbon species: on the basis of total spring tidal drainage area, 1083 g m^–2 of DIC, 103 g m^–2 of DOC, and 123 g m^–2 of POC left the estuary annually. Total carbon export from the estuary to the ocean amounted to 4755 tonnes, of which 83% was in the inorganic form (DIC). Thus, the bulk of carbon moving in the water column is inorganic - yet, DIC seems to be measured only rarely in most flux studies of this nature. Salt marshes cover extensive areas in this estuary and produce some carbon, particularly DOC, but productivity of the local Spartina species is low (P:B=1.1). Consequently, the bulk of carbon exported from the estuary appears to originate from the highly productive macroinvertebrate and the phytoplankton component and not from the salt marsh plants.     

An experimental investigation of phytoplankton nutrient limitation in two contrasting low arctic lakes

We investigated whether phytoplankton communities in two lakes in SW Greenland were phosphorus or nitrogen limited. The study lakes have contrasting water chemistry (mean conductivities differ ten fold) and are located near Kangerlussuaq, SW Greenland (~67°N, 51°W). A microcosm nutrient enrichment experiment was performed in June 2003 to determine whether nitrate or phosphate addition stimulated phytoplankton growth. Samples were analysed for species composition, biomass, and alkaline phosphatase activity (APA). Initially, both lakes had extremely low total phosphorus but high total nitrogen concentrations and high APA, suggesting that the phytoplankton were phosphorus limited prior to the start of the experiment. The phytoplankton composition and biomass (mainly Ochromonas spp.) responded to phosphate but not to nitrate addition. In both lakes, chlorophyll a increased significantly when phosphate was added. Furthermore, APA was significantly lower in the two lakes when phosphate was added compared to the control and the nitrogen addition treatment. The dominance of mixotrophic phytoplankton and high DOC values suggest that these lakes may be regulated by microbial loop processes.     

Bioremediation of diesel-contaminated soils: Evaluation of potential in situ techniques by study of bacterial degradation

The development of a simple laboratory methodology allows theimplementation of in situbioremediation of polluted soils with diesel fuel. In thisinvestigation microbiological and chemical analyses and a suitable bioreactor design, were veryuseful for suggesting the best ways to improve biodegradation extents in a diesel-enrichedsoil. Biostimulation with inorganic nitrogen and phosphorus produced the best resultsin a simple bioreactor, with biodegradation extents higher than 90% after 45 days. Also,the addition of activated sludge from a domestic wastewater plant increased the degradationrate to a great extent. In both cases, microbiological studies showed the presence ofAcinetobacter sp. degrading most of thehydrocarbons. Simultaneously, a diesel fuel release(approximately 400,000 l) was studied. Samples taken in polluted soil and water revealed thatbacteria from the genus Acinetobacterwere predominant. In plate studies, Acinetobacter coloniesproduced a whitish substance with the characteristics of a biosurfactant. Remarkably, thepresence of this product was evident at the field site, both in the riverbanks and in the physicalrecovery plant. The study of the similarities between laboratory results and the diesel spillsite strongly suggested that natural conditions at the field site allowed the implementationof in situ bioremediation after physical removal of LNAPL (light nonaqueous-phase liquids).     

海水中藻菌共培养体系对碳氮磷的吸收转化

海洋环境中,细菌和微藻之间的物质交换是生源要素在自然界中迁移转化的重要方式。为进一步了解生源要素的生物地球化学循环,在实验室模拟条件下,研究了共培养体系中营养盐和有机物在细菌和微藻之间的转换。通过纯培养中肋骨条藻(Skeletonema costatum)、东海原甲藻(Prorocentrum donghaiense)、天然海水中的细菌以及藻菌混合培养,分析了营养盐和有机物随藻菌生物量的变化情况,并计算了溶解有机碳(DOC)和溶解有机氮(DON)的浓度比值[(DOC/DON)a]。结果发现,在共培养体系中,细菌对中肋骨条藻的生长有抑制作用,对东海原甲藻影响不明显;中肋骨条藻有利于细菌生长,东海原甲藻抑制细菌生长,这种不同可能与微藻的粒径有关。海洋细菌在2种藻的指数生长均期均会促进微藻吸收氨氮(NH_4-N),但在生长末期NH_4-N以释放为主。硝氮(NO_3-N)的浓度与藻的生长呈负相关,但在衰亡期NO_3-N略有增加,表明NO_3-N再生所需时间较长。细菌对硝氮的吸收量较少,但对其再生有贡献。细菌和中肋骨条藻对磷酸盐(PO_4-P)的吸收存在竞争,但与东海原甲藻的竞争关系不明显。不同培养体系中DOC浓度变化不同,在藻菌共培养体系中增加较快,纯藻培养体系中增加缓慢,在纯菌培养体系中缓慢减少。通过对DOC与DON浓度比值的分析,发现用判断颗粒有机碳(POC)来源的方法可以分析DOC的来源。     

High Organic Carbon Export Precludes Eutrophication Responses in Experimental Rocky Shore Communities

We studied the effect of nutrient inputs on the carbon (C) budget of rocky shore communities using a set of eight large experimental mesocosms. The mesocosms received a range of inorganic nitrogen (N) and phosphorus (P) additions, at an N:P ratio of 16. These additions were designed to elevate the background concentration, relative to that in eutrophic Oslofjord (Norway) waters, by 1, 2, 4, 8, 16, 32 μmol dissolved inorganic nitrogen (DIN)l⁻¹ (and the corresponding P increase). Two unamended mesocosms were used as controls. The nutrients were added continuously for 27 months before gross primary production (GPP), respiration (R), net community production (NCP), and dissolved organic carbon (DOC) production were assessed for the dominant algal species (Fucus serratus) and for the whole experimental ecosystem. Inputs and outputs of DOC and particulate organic carbon (POC) from the mesocosms were also quantified. The F. serratus communities were generally autotrophic (average P/R ratio = 1.33 ± 0.12), with the GPP independent of the nutrient inputs to the mesocosms, and maintained a high net DOC production during both day (0.026 ± 0.008 g C m⁻² h⁻¹) and night (0.015 ± 0.004 g C m⁻² h⁻¹). All the experimental rocky shore ecosystems were autotrophic (P/R ratio = 2.04 ± 0.28), and neither macroalgal biomass nor production varied significantly with increasing nutrient inputs. Most of the excess production from these autotrophic ecosystems was exported from the systems as DOC, which accounted for 69% and 58% of the NCP of the dominant community and the experimental ecosystem, respectively, the rest being lost as POC. High DOC release and subsequent export from the highly energetic environments occupied by rocky shore communities may prevent the development of eutrophication symptoms and render these communities resistant to eutrophication. Received 10 October 2001; accepted 18 July 2002.     

Effects of phosphorus and nitrogen enrichment on the yield of some strains of Stigeoclonium Kütz. (Chlorophyceae)

SUMMARY.

• 1 The combined effect of nitrate and phosphate concentrations on the yields of five selected strains of Stigeoclonium is demonstrated in a synthetic medium as well as in natural waters.
• 2 Increase of yield after addition of one nutrient depends on the concentration of the other. A graphic growth model based on the experiments in synthetic medium is presented, which allows prediction of yields at different combinations of nitrate and phosphorus concentrations.
• 3 In general, yields in natural waters before and after addition of nitrate and/or phosphate agree with the model. In some cases, other limiting factors appear to be involved.
• 4 The possible effects of nitrogen and phosphorus limitation on the phosphorus and nitrogen metabolism of the alga are discussed.
• 5 Neither an exponential model (Baule-Mitscherlich) nor a hyperbolic model adequately describe the interaction demonstrated between nitrogen and phosphorus limitation.

     

UAF radiorespirometric protocol for assessing hydrocarbon mineralization potential in environmental samples

Following the EXXOn Valdez oil spill, a radiorespirometric protocol was developed at the University of Alaska Fairbanks (UAF) to assess the potential for microorganisms in coastal waters and sediments to degrade hydrocarbons. The use of bioremediation to assist in oil spill cleanup operations required microbial bioassays to establish that addition of nitrogen and phosphorus would enhance biodegradation. A technique assessing 1-¹⁴C-n-hexadecane mineralization in seawater or nutrient rich sediment suspensions was used for both of these measurements. Hydrocarbon-degradation potentials were determined by measuring mineralization associated with sediment microorganisms in sediment suspended in sterilized seawater and/or marine Bushnell-Haas broth. Production of ¹⁴CO₂ and CO₂ was easily detectable during the first 48 hours with added hexadecane levels ranging from 10 to 500 mg/l of suspension and dependent on the biomass of hydrocarbon degraders, the hydrocarbon-oxidation potential of the biomass and nutrient availability. In addition to assessment of the hydrocarbon-degrading potential of environmental samples, the radiorespirometric procedure, and concomitant measurement of microbial biomass, has utility as an indicator of hydrocarbon contamination of soils, aqueous sediments and water, and can also be used to evaluate the effectiveness of bioremediation treatments.     

Intracellular Metabolic Changes of <Emphasis Type="Italic">Rhodococcus</Emphasis> sp. LH During the Biodegradation of Diesel Oil

In recent years, some marine microbes have been used to degrade diesel oil. However, the exact mechanisms underlying the biodegradation are still poorly understood. In this study, a hypothermophilous marine strain, which can degrade diesel oil in cold seawater was isolated from Antarctic floe-ice and identified and named as Rhodococcus sp. LH. To clarify the biodegradation mechanisms, a gas chromatography-mass spectrometry (GC-MS)-based metabolomics strategy was performed to determine the diesel biodegradation process-associated intracellular biochemical changes in Rhodococcus sp. LH cells. With the aid of partial least squares-discriminant analysis (PLS-DA), 17 differential metabolites with variable importance in the projection (VIP) value greater than 1 were identified. Results indicated that the biodegradation of diesel oil by Rhodococcus sp. LH was affected by many different factors. Rhodococcus sp. LH could degrade diesel oil through terminal or sub-terminal oxidation reactions, and might also possess the ability to degrade aromatic hydrocarbons. In addition, some surfactants, especially fatty acids, which were secreted by Rhodococcus into medium could also assist the strain in dispersing and absorbing diesel oil. Lack of nitrogen in the seawater would lead to nitrogen starvation, thereby restraining the amino acid circulation in Rhodococcus sp. LH. Moreover, nitrogen starvation could also promote the conversation of relative excess carbon source to storage materials, such as 1-monolinoleoylglycerol. These results would provide a comprehensive understanding about the complex mechanisms of diesel oil biodegradation by Rhodococcus sp. LH at the systematic level.     

Organic matter release by dominant hermatypic corals of the Northern Red Sea

Particulate organic matter (POM) and dissolved organic carbon (DOC) release by six dominant hermatypic coral genera (Acropora, Fungia, Goniastrea, Millepora, Pocillopora and Stylophora) were measured under undisturbed conditions by laboratory incubations during four seasonal expeditions to the Northern Red Sea. In addition, the influence of environmental factors (water temperature, light availability and ambient inorganic nutrient concentrations) was evaluated. Particulate organic carbon (POC) and particulate nitrogen (PN) release were always detectable and genus-specific, with Stylophora releasing most POM (6.5 mg POC and 0.5 mg PN m⁻² coral surface area h⁻¹) during all seasons. The fire coral Millepora released significantly less POM (0.3 mg POC and 0.04 mg PN m⁻² coral surface area h⁻¹) than all investigated anthozoan genera. The average POC:PN ratio of POM released by all coral genera was 12 ± 1, indicating high carbon/low nitrogen content of coral-derived organic matter. POM release showed little seasonal variation, but average values of POC and PN release rates correlated with water temperature, light availability and ambient nitrate concentrations. DOC net release and elevated DOC:POC ratios were detectable for Acropora, Goniastrea and Millepora, revealing maximum values for Acropora (30.7 mg DOC m⁻² coral surface area h⁻¹), whilst predominant DOC uptake was observed for Pocillopora, Fungia and Stylophora. Depth-mediated light availability influenced DOC fluxes of Acropora and Fungia, while fluctuations in water temperature and ambient inorganic nutrient concentrations showed no correlation. These comprehensive data provide an important basis for the understanding of coral reef organic matter dynamics and relevant environmental factors.     

Nitrogen addition does not reduce belowground competition in a salt marsh clonal plant community in Mississippi (USA)

Previous studies have suggested that belowground competition for nutrients influences plant zonation in salt marshes. In this study, I tested the hypothesis that competition for nitrogen structured a clonal plant community in a nitrogen-limited salt marsh in coastal Mississippi, USA. In contrast to most previous field studies that have investigated mechanisms of competition, I examined clonal growth responses of established genets of a nitrogen-demanding low-intertidal species (Spartina alterniflora) to nitrogen addition and the removal of a nitrogen-conserving high-intertidal species (Juncus roemerianus). Nitrogen addition stimulated clonal invasion of the Juncus zone by Spartina but did not reduce the significant competitive effects of Juncus on Spartina. Simulated Juncus shade did not reduce invasion of the Juncus zone by Spartina, indicating that belowground competition reduced clonal invasion. In the last year of the study, the border shifted unexpectedly towards the Spartina zone, resulting in competitive displacement of Spartina by Juncus. Nitrogen addition did not prevent or slow this displacement, further contradicting the nitrogen competition hypothesis. Although growth rates were much more strongly limited by nitrogen in Spartina than in Juncus, nitrogen addition did not cause the displacement of Juncus by Spartina after three growing seasons. I conclude that zonation of Spartina and Juncus is maintained by preemption of space and greater tolerance of low nitrogen supplies by Juncus in the high marsh. These results contrast sharply with findings of reduced belowground competition with nutrient addition in previous studies and highlight the important role of nutrient-mediated competition for space between clonal plants.     

Consequences of nitrogen and phosphorus limitation for the performance of two planthoppers with divergent life-history strategies

Phytophagous insects have a much higher nitrogen and phosphorus content than their host plants, an elemental mismatch that places inherent constraints on meeting nutritional requirements. Although nitrogen limitation is well documented in insect herbivores, phosphorus limitation is poorly studied. Using factorial experiments in the laboratory and field, in which levels of soil nitrogen and phosphorus were manipulated, we studied the relative consequences of macronutrient limitation for two herbivores, namely the phloem-feeding planthoppers Prokelisia dolus and P. marginata. These planthoppers inhabit the salt marshes of North America where large stands of their Spartina host plant are found. Notably, these congeners differ in their dispersal abilities; P. marginata is dispersive whereas P. dolus is sedentary. Both nitrogen and phosphorus subsidies enhanced the nitrogen and phosphorus content of Spartina. When P. dolus and P. marginata were raised on plants with an enriched nitrogen signature, they exhibited greater survival, grew to a larger size, developed more rapidly, and achieved higher densities than on nitrogen-deficient plants. However, P. marginata experienced greater fitness penalties than P. dolus on nitrogen-deficient plants. Phosphorus limitation and associated fitness penalties were not as severe as nitrogen limitation for P. marginata, and were not detected in P. dolus. The tempered response of P. dolus to N- and P-deficient Spartina is probably due to its greater investment in feeding musculature and hence ability to compensate for nutrient deficiencies with increased ingestion. To cope with deteriorating plant quality, P. dolus employs compensatory feeding, whereas P. marginata disperses to higher quality Spartina. When its option of dispersal is eliminated and P. marginata is confined on nutrient-deficient plants, its performance is drastically reduced compared with P. dolus. This research highlights the importance of interfacing herbivore life-history strategies with ecological stoichiometry in order to interpret the consequences of macronutrient limitation on herbivore performance and population dynamics.     

Biostimulation of n-Alkane Degradation in Diesel Fuel-Spiked Soils

Nutrient enhancement of bioremediation with nitrogen, namely biostimulation, increases process performance. Selection of a proper nitrogen source is critical for bioremediation applications. In this study, the effects of different nitrogen sources on biodegradation of C₁₀–C₂₅ n-alkane compounds in diesel fuel-spiked soil were revealed, and the most appropriate nitrogen source for biodegradation of semi- and non-volatile n-alkanes was investigated. Bioremediation of diesel fuel contaminated soil was monitored in lab-scale reactors for 15 days. Ammonium sulfate, potassium nitrate and urea were used as nitrogen sources. Carbon dioxide and oxygen levels in the reactors were recorded to monitor microbiological activity. Contaminant removal process was investigated by pH, heterotrophic plate count, total petroleum hydrocarbons (TPH) and C₁₀–C₂₅ n-alkane analyses. First-order kinetic constants were calculated via respirometric and contaminant concentration data. According to total C₁₀–C₂₅ n-alkane removal levels and degradation rate constants, ammonium sulfate addition resulted in the most efficient contaminant removal followed by potassium nitrate and urea. Simultaneous degradation of individual n-alkanes was observed for all of the nitrogen sources. Urea addition changed the distribution of individual n-alkane concentrations relative to the pre-experimental concentrations. Nitrogen source type had no differential effect on degradation rates of semi- (C₁₀–C₁₆) and non-volatile (C₁₇–C₂₅) fractions.     

Response of irrigated dry-seeded rice to nitrogen and phosphorus in a semi-arid environment

Summary Under semi-arid conditions, three field experiments were conducted at Gezira Research Station to determine response of irrigated dry-seeded rice (Oryza sativa L. var IR 2053-206-1-3-6) to addition of nitrogen and phosphorus fertilizers. The experimental treatments included the factorial combinations of seven levels of nitrogen applied as urea and four levels of phosphorus applied as super phosphate. Plant growth and grain yield were significantly and progressively increased with the rise in the levels of added nitrogen and phosphorus. However, response to added phosphorus was restricted by the applied level of nitrogen. The responses of grain yield to nitrogen and phosphorus levels are given by quadratic regression equations. Without addition of nitrogen or phosphorus grain yield averaged 1.52 t/ha compared to 6.07 t/ha with addition of the optimal levels (160 kg N plus 35 kg P/ha). The high potential for rice production in semi-arid environment is evidently restricted by addition of relatively high rates of nitrogen and phosphorus.     

Anaerobic degradation of No. 2 diesel fuel in the wetland sediments of Barataria-Terrebonne estuary under various electron acceptor conditions

The biodegradation of No. 2 diesel fuel under anaerobic conditions was investigated using sediments collected from wetlands of Barataria-Terrebonne estuary in Louisiana. The results indicated enhanced biodegradation of diesel fuel under sulfate-reducing, nitrate-reducing, methanogenic, and mixed electron acceptor conditions. However, the rate of diesel degradation was the highest under mixed electron acceptor conditions followed in order by sulfate-reducing, methanogenic, and nitrate-reducing conditions. Under mixed electron acceptor condition, 99% removal of diesel fuel was achieved within 510 days, while under sulfate-reducing condition 62% degradation of diesel fuel was observed for the same period. Diesel fuel was also degraded to a smaller extent in the culture condition where electron acceptors were not supplemented (natural attenuation condition). This study showed evidence for enhanced diesel fuel metabolism in a mixed microbial population system similar to any contaminated field site, where a heterogeneous microbial population exists.     

Scavenging of dissolved organic matter (DOM) by amorphous iron hydroxide particles Fe(OH)3(s)

Scavenging of dissolved organic matter (DOM) by particulate metal oxides like Fe(OH)_3(s) is one of three processes that can influence the concentration and composition of DOM in aquatic systems. The other two possible processes include photodegradation and biodegradation. In combination, these processes alter the concentration and composition of DOM systematically with increasing time, measured as hydrologic residence time (HRT). The objective of this research was to determine the change in Fe(OH)_3(s)-scavengable dissolved organic carbon (DOC) with increasing HRT (0–80 yr). In addition, DOC from allochthonous and autochthonous sources were included in this study. The susceptibility of DOC from surface waters to scavenging by Fe(OH)_3(s) was found to decrease as a function of HRT, from approximately 90% to 79%. The lowest HRT system was operationally considered equivalent to allochthonous DOC, while autochthonous DOC was scavenged similarly to DOC from the 80 yr HRT system. These results indicate that scavenging of bulk DOC may be limited by metal loading in aquatic systems, and that the bulk of Fe(OH)_3(s)-reactive DOC is from allochthonous sources. In addition, all surface waters treated with Fe(OH)_3(s) contained approximately 1 mg l^–1 of DOC that was resistant to scavenging (SD = 0.50, n = 5), which suggests that a refractory fraction of DOC persists in surface waters.     

Temporal variability of particulate organic carbon,nitrogen and phosphorus in the Northern Adriatic Sea

The particulate organic carbon (POC), nitrogen (PN) and phosphorus (PP) vertical distribution along the water column and temporal variability in coastal and offshore waters of the Northern Adriatic Sea were related to the hydrodynamic conditions and biological processes. Fresh water inputs from the Po and Adige rivers enhance primary production, resulting in high POC, PN and PP concentrations at the surface. In offshore waters, POC and PN concentrations were about 3–4 times less than in the coastal waters, while PP were up to 10 times lower, highlighting a marked phosphorus depletion. In the bottom layer, the POC content decreases due to the strong density gradients which separate bottom waters with prevailing degradation processes. Short term 48 h-variability of POC, PN and PP in the coastal waters was determined to a great extent by variations in the spreading of river plumes at the surface and by nepheloid layers and resuspension processes in the bottom waters. The particulate matter in the Adriatic offshore waters is extremely depleted as regards particulate phosphorus and is characterised by C_org:P and N:P ratios higher than the Redfield ratio.     

A worldwide view of organic carbon export from catchments

Growing interest in the effects of global change on the metabolism, stoichiometry and cycling of carbon in aquatic ecosystems has motivated research on the export of organic carbon (OCE) from catchments. In this article, quantitative and functional features of the annual export rates of total, particulate and dissolved organic carbon (TOC, POC and DOC) were reviewed, and the stoichiometry of export (OC:N, OC:P and N:P) from 550 catchments worldwide was reported. TOC export ranged 2.1–92,474?kg?C?km^?2?year^?1, POC export ranged 0.4–73,979?kg?C?km^?2?year^?1 and DOC export ranged 1.2–56,946?kg?C?km^?2?year^?1. Exports of TOC and DOC were strongly linked, but POC export was unrelated to DOC. The DOC fraction comprised on average 73?±?21% of TOC export. The export rates of organic carbon were poorly related to those of total nitrogen and total phosphorus. Discrete and continuous environmental variables failed to predict TOC export, but DOC export was influenced by discharge and catchment area worldwide. Models of OCE in different catchment types were controlled by different environmental variables; hydrological variables were generally better predictors of OCE than anthropogenic and soil variables. Elemental ratios of carbon export in most catchments were above the Redfield ratio, suggesting that phosphorus may become the limiting nutrient for downstream plant growth. These ratios were marginally related to environmental data. More detailed hydrological data, consideration of in-stream processes and the use of quasi-empirical dynamical models are advocated to improve our knowledge of OCE rates and those of other nutrients.     

Fluxes of dissolved and nonfossil particulate organic carbon from an Oceania small river (Lanyang Hsi) in Taiwan

Concentrations of dissolved and particulate organic carbon (DOC & POC) in river waters were measuredduring 1993–1994 in the Lanyang Hsi watershed, which representsa typical small Oceania river. The DOC concentrations varied in the range of 0.5–4 mg/l during non-typhoon period, but rose to as highas 8 mg/l during Typhoon Tim in July, 1994. Based on the log-linearrelationship between the DOC load and the discharge rate, weestimated the DOC export to be 3.4 ± 0.6 ktC/yr,and the DOC yield to be 4.1 ± 0.7 gC/m²/yr,which is considerably higher than a former estimate (ca.0.1 gC/m²/yr) for the Oceania. On the other hand, the DOC yield is less than the concurrent POC yield (21.7 ± 4.7gC/m²/yr) by a factor of five, but most of theexported POC is fossil carbon. Under the assumption that the suspended sediments contain a mean fossil POC content of0.5%, the nonfossil POC yield was calculated to be 4.6± 3.0 gC/m²/yr, comparable to theDOC yield. Since DOC and nonfossil POC are directly related to theecosystem, their combined fluxes give a biogenic organic carbonyield of 8.7 ± 3.1 gC/m²/yr.