DOC) And Nitrogen Research Articles

Fate of terrigenous dissolved organic matter (DOM) in estuaries: Aggregation and bioavailability

Abstract When dissolved organic matter (DOM) from terrestrial and freshwater sources is mixed with estuarine waters at the land-sea interface, the change in salinity has been suggested to cause fast aggregation and an increase in the bioavailability of dissolved organic carbon (DOC) and nitrogen (DON). These processes were investigated in different Danish freshwaters. Aggregation of DOC in short-term (hours) mixing events at increasing salinity was low. In one stream with forest and wetland runoff and in a humic lake, the decrease of DOC over a 0 to 25 ppt salinity gradient was 2 to 5%. Optical analyses by absorption and fluorescence, revealed changes in the composition of the humic components due to salt. The bioavailability of terrestrial DOC was also investigated and found not to change moving from limnic to estuarine conditions. Although the yield of freshwater bacteria cells was about twice the yield of estuarine bacteria, the utilization of DOC was identical and not influenced by the bacterial commu...

Resource control on the production of dissolved organic carbon and nitrogen in a deciduous forest floor

The forest floor in temperate forests has become recognized for its importance in the retention of elevated inputs of dissolved inorganic nitrogen (DIN) and as a source of dissolved organic matter (DOM). A laboratory leaching experiment was conducted over the period of 98d to examine the origin of dissolved organic carbon (DOC) and nitrogen (DON) in a deciduous forest floor, and the effect of resource availability and microbial activity on the production mechanisms involved. The experiment was composed of different types of treatments: exclusion of specific forest floor layers (no Oi, no Oe) and addition of carbon sources (glucose, cellulose, leaf, wood) and NH4NO3 (nitrogen). The cumulative amount of CO2 evolution was positively related to the availability of C sources at each treatment: glucose>leaf=wood=cellulose>control=no Oe=nitrogen>no Oi. DOC release was related to the amount of C sources but showed no clear correlation with CO2 evolution. An increase in C availability generally led to a reduction in the release of DON as well as DIN. In contrast, the amendment of NH4NO3 reduced the cumulative DOC release but enhanced the release of both DON and DIN. Fresh leaf litter was a more important DOC source than labile substrates (glucose and cellulose) as well as more stable substrates (forest floor materials and wood). Among forest floor layers, more humified horizons (Oe and Oa) were the primary source of DIN and made a similar contribution to DOM release as the Oi layer. The changes in DOM composition detected by a humification index of the leachates, in combination with a shift in the final microbial biomass C, suggested that DOM released from the soluble pools of added litter or the Oi layer contained a substantial amount of microbially processed organic matter. Our study demonstrated the importance of C availability in regulating microbial activity and immobilization of dissolved N in an N-enriched forest floor. However, the discrepancy between substrate lability and DOC production, in combination with a rapid microbial processing of DOC released from labile C pools, illustrated the complicated nature of microbial production and consumption of DOC in the forest floor.

Impacts of nutrients and grazing mortality on the abundance of Aureococcus anophagefferens during a New York brown tide bloom

Although nutrients and grazing both contribute to the formation of harmful algal blooms, research on these events has rarely considered both factors simultaneously. To ascertain the impact of nutrients and grazing on brown tides of Aureococcus anophagefferens, nutrient bioassays were conducted in parallel with dilution‐style microzooplankton grazing experiments during an intense bloom that occurred throughout Great South Bay (GSB), New York, in fall of 1999. During the study, Aureococcus represented between 25 and 85% of phytoplankton biomass and attained peak cell densities > 6 × 105 cells ml−1. Concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in GSB were high (mean = 430 µM and 32 µM, respectively) during the bloom, while dissolved inorganic nitrogen (DIN) levels were low (mean = 2.5 µM). Although the experimental additions of nitrogen (nitrate or urea) typically enhanced the growth rates of the non‐brown tide phytoplankton community, such additions often had no impact on, or decreased, growth rates of Aureococcus relative to unamended control treatments. These observations suggest that growth of non‐brown tide phytoplankton depended on ambient N supply rates, while Aureococcus experienced nutrient replete growth. Dilution experiments indicated that microzooplankton grazing rates on A. anophagefferens were significantly lower than those on other algal populations. This reduced grazing pressure contributed toward higher net growth rates for Aureococcus relative to non‐brown tide phytoplankton. In sum, these results demonstrate that both top‐down (low grazing mortality rates) and bottom‐up (a high DOC/DON, low DIN nutrient regime) factors can contribute to the proliferation of brown tide blooms in New York waters.

Different effects of peat degradation on dissolved organic carbon and nitrogen

The dynamics of dissolved organic matter (DOM) are closely related to organic matter processes such as decomposition, humification, and stabilization of organic matter in soils. Nevertheless, it is unknown whether dissolved organic carbon (DOC) and nitrogen (DON) have an equal response to these processes. We analyzed DOC and DON concentrations of topsoil, groundwater, and surface water of six differently used sites characterized by differences in peat degradation. Carbon ( 13C, 14C) and nitrogen ( 15N) isotopes of DOM were related to a humification index in order to detect differences between DOC and DON at different stages of humification. The results show that intact peatlands had higher DOC concentrations than degraded peatlands. Clearly, DOC concentration depends on soil organic carbon content and therefore on the degree of peat degradation. However, intact and degraded peatlands showed similar DON and inorganic N concentrations suggesting that DON release is dependent on soil inorganic N rather than the organic N pool. Moreover, a high degree of peat degradation resulted in lower DOC/DON ratios than the C/N ratio of the solid phase indicating a preferential release of DON from soil organic matter. Further, δ 13C ratios and the radiocarbon age of DOM increase with peat degradation and humification indicating a high C turnover, an increased microbial modification and age of DOC. On the other hand δ 15N ratios decrease, probably as a result of N fertilization. The promoting effect of inorganic N on DON release and a high humification of DOM at sites treated with inorganic N fertilizers suggest that N fertilization promotes a release of amino acids depleted in 15N and subsequent condensation with carbohydrates to humic substances.

Organic matter distributions in the Eastern North Atlantic–Azores Front region

Temperate, transitional and subtropical waters of the remote Azores Front region east of Azores (24–40°N, 22–32°W) were sampled during three cruises conducted under increasing stratification conditions (April 1999, May 1997 and August 1998). Despite the temporal increase of surface temperature (by 5 °C) and stratification (by 2.1 min −2), as well as the thermocline shoaling (by ∼15 m), dissolved organic carbon (DOC) and nitrogen (DON) in the surface layer were not significantly different for the early spring, late spring and summer periods, with average concentrations of 69±2 μM-C and 5.2±0.4 μM-N, respectively. The surface excess of semi-labile DOC, compared with the baseline DOC concentration in the deep ocean (47±2 μM-C), represents 33% of the bulk DOC concentration and as much as 85% of the TOC (=POC+DOC) excess. When compared with the winter baseline (56±2 μM-C), the seasonal surface DOC excess is 20% of the bulk DOC concentration and 87% of the seasonal TOC excess. These results confirm the major role played by DOC in the carbon cycle of surface waters of the Azores Front region. The total amount of bioreactive DOC transported from the temperate to the subtropical North Atlantic by the Ekman flux between March and December represents only ∼15% of the average annual primary production, and ∼15% and ∼30% of the measured sinking POC flux+vertical DOC eddy diffusion during early spring and summer, respectively. Vertical eddy diffusion is 35% and 2% of the spring and summer sinking POC flux, respectively. On the other hand, DOC only contributes 13% to the local oxidation of organic matter in subsurface waters (between the pycnocline and 500 m) of the study region.

Fluxes and concentrations of dissolved organic carbon and nitrogen - a synthesis for temperate forests

Dissolved organic carbon (DOC) and nitrogen (DON) represent an important part of the C and N cycles in forest ecosystems. Little is known about the controls on fluxes and concentrations of these compounds in soils under field conditions. Here we compiled published data on concentrations and fluxes of DOC and DON from 42 case studies in forest ecosystems of the temperate zone in order to evaluate controls on a larger temporal and spatial scale. The focus was on annual fluxes and concentrations in throughfall, forest floor leachates and soil solutions. In all compartments considered, concentrations and fluxes differed widely between the sites. Highest concentrations of DOC and DON were generally observed in forest floor leachates and in A horizons. Highest fluxes occurred in forest floor leachates. The fluxes of DOC and DON in forest floor leachates increased with increasing annual precipitation and were also positively related to DOC and DON fluxes with throughfall. Variation in throughfall fluxes could explain 46% and 65% of the variation in DOC and DON fluxes from the forest floor, respectively. No general difference in DOC and DON concentrations and fluxes in forest floor leachates was found when comparing coniferous and hardwood sites. Concentrations of DOC in forest floor leachates were positively correlated to the pH of the forest floor. Furthermore, there was no relationship between organic C and N stocks, soil C/N, litterfall or mineral N inputs and concentrations and fluxes of DOC and DON in forest floor leachates. Including all compartments, fluxes of DOC and DON were highly correlated. Ratios of DOC to DON calculated from fluxes from the forest floor were independent of the amount of annual precipitation, pointing to a similar response of DOC and DON to precipitation conditions. A decrease in the ratio of DOC to DON with soil depth as observed on a plot-scale, was not confirmed by data analysis on a large scale. The controls observed on annual fluxes and concentrations of DON and DOC at regional scale differed from those reported for smaller time and space scales.

A stormflow/baseflow comparison of dissolved organic matter concentrations and bioavailability in an Appalachian stream

Patterns of dissolved organic carbon (DOC) and nitrogen (DON) delivery were compared between times of stormflow and baseflow in Paine Run, an Appalachian stream draining a 12.4 km2 forested catchment in the Shenandoah National Park (SNP), Virginia. The potential in-stream ecological impact of altered concentrations and/or chemical composition of DOM during storms also was examined, using standardized bacterial bioassays. DOC and DON concentrations in Paine Run were consistently low during baseflow and did not show a seasonal pattern. During storms however, mean DOC and DON concentrations approximately doubled, with maximum concentrations occurring on the rising limb of storm hydrographs. The rapid response of DOM concentration to changes in flow suggests a near-stream or in-stream source of DOM during storms. Stormflow (4% of the time, 36% of the annual discharge) contributed >50% of DOC, DON and NO3− flux in Paine Run during 1997. In laboratory bacterial bioassays, growth rate constants were higher on Paine Run stormflow water than on baseflow water, but the fraction of total DOM which was bioavailable was not significantly different. The fraction of the total stream DOC pool taken up by water column bacteria was estimated to increase from 0.03 ± 0.02% h−1 during baseflow, to 0.15 ± 0.04% h−1 during storms. This uptake rate would have a minimal effect on bulk DOM concentrations in Paine Run, but storms may still have considerable impact on the bacterial stream communities by mobilizing them into the water column and by supplying a pulse of DOM.

Different effect of drying on the fluxes of dissolved organic carbon and nitrogen from a Norway spruce forest floor

The forest floor represents the major source of dissolved organic carbon (DOC) and nitrogen (DON) in forest soils. The release mechanisms of DOC and DON from forest floors and their environmental controls as well as the dynamics of concentrations and fluxes are still poorly understood. We investigated the effect of drying and rewetting on the release of DOC and DON from a Norway spruce forest floor. Undisturbed soil columns of 17 cm diameter and 15-20 cm height were taken with 7 replicates from the forest floor of a mature Norway spruce (Picea abies [L.] Karst.) site and established at 10 degreesC in the laboratory. Columns were exposed to different periods of drying (3, 5, 10, 20 days). Each drying period was followed by a rewetting for 5 days at an irrigation rate of 10 mm d(-1) with a natural throughfall solution. The percolates from the forest floor were collected daily and analyzed for DOC, total N, NH4, NO3, pH, electrical conductivity and major ions. Drying for 10 and 20 days decreased the water content of the Oi horizon from 280% dry weight to about 30%. The water content of the Oe and the Oa horizon only changed from about 300% to 200%. The fluxes of DOC from the forest floor were moderately effected by drying and rewetting with an increase after 3 and 5 days of drying, but a decrease after 10 and 20 days. On the contrary, the drying for 10 and 20 days resulted in a drastic increase of the DON fluxes and a subsequent decrease of the DOC/DON ratios in the forest floor percolates from about 50 to 3.3. These results suggest that the mechanisms for DOC release in forest floors differ from those for DON and that drying and rewetting cause temporal variations in the DOC/DON ratios in forest floor percolates.

Use of 13C and 15N Mass Spectrometry to Study the Decomposition of Calamagrostis epigeios in Soil Column Experiments with and Without Ash Additions

The dynamics of C and N in terrestrial ecosystems are not completely understood and the use of stable isotopes may be useful to gain further insight in the pathways of CO2 emissions and leaching of dissolved organic carbon (DOC) and nitrogen (DON) during decomposition of litter. Objectives were (i) to study the decomposition dynamics of Calamagrostis epigeios, a common grass species in forests, using 13C-depleted and 15N-enriched plants and (ii) to quantify the effect wood ash addition on the decomposition and leaching of DOC and DON. Decomposition was studied for 128 days under aerobic conditions at 8°C and moisture close to field capacity in a spodic dystric Cambisol with mor-moder layer. Variants included control plots and additions of (i) Calamagrostis litter and (ii) Calamagrostis litter plus 4 kg ash m−2. 1. Decomposition of Calamagrostis resulted in a CO2 production of 76.2 g CO2–C m−2 (10% of added C) after 128 days and cumulative DOC production was 14.0 g C m−2 out of which 0.9g C m−2 was Calamagrostis-derived (0.1% of added C). The specific CO2 formation and specific DOC production from Calamagrostis were 6 times higher (CO2) and 4 times smaller (DOC) than those from the organic layer. The amount of Calamagrostis-derived total N (NH4 +, NO3 −, DON) leached was 0.7g N m−2 (4.8% of added N). Cumulative DON production was 0.8g N m−2 which was slightly higher than for the control. During soil passage, much of the DOC and DON was removed due to sorption or decomposition. DOC and DON releases from the mineral soil (17cm depth) were 6.3g C m−2 and 0.5g N m−2. Addition of ash resulted in a complete fixing of CO2 for 40 days due to carbonatisation. Afterwards, the CO2 production rates were similar to the variant without ash addition. Production of DOC (98.6g C m−2) and DON (2.5g N m−2) was marked, mainly owing to humus decay. However, Calamagrostis-derived DOC and Culamagrostis-derived total N were only 3.9g C m−2 (0.5 YO of added C) and 0.5g N m−2 (3.4% of added N). The specific DOC production rate from the organic layer was 6 times higher than that from Cularnagrostis. The results suggest that with increasing humification from fresh plant residues to more decomposed material (OF and OH layers) the production ratio of DOC/CO2-C increases. Addition of alkaline substances to the forest floor can lead to a manifold increase in DOC production.

Long-Term Nitrogen Additions and Nitrogen Saturation in Two Temperate Forests

This article reports responses of two different forest ecosystems to 9 years (1988–96) of chronic nitrogen (N) additions at the Harvard Forest, Petersham, Massachusetts. Ammonium nitrate (NH4NO3) was applied to a pine plantation and a native deciduous broad-leaved (hardwood) forest in six equal monthly doses (May–September) at four rates: control (no fertilizer addition), low N (5 g N m-2 y-1), high N (15 g N m-2 y-1), and low N + sulfur (5 g N m-2 y-1 plus 7.4 g S m-2 y-1). Measurements were made of net N mineralization, net nitrification, N retention, wood production, foliar N content and litter production, soil C and N content, and concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in soil water. In the pine stand, nitrate losses were measured after the first year of additions (1989) in the high N plot and increased again in 1995 and 1996. The hardwood stand showed no significant increases in nitrate leaching until 1995 (high N only), with further increases in 1996. Overall N retention efficiency (percentage of added N retained) over the 9-year period was 97–100% in the control and low N plots of both stands, 96% in the hardwood high N plot, and 85% in the pine high N plot. Storage in aboveground biomass, fine roots, and soil extractable pools accounted for only 16–32% of the added N retained in the amended plots, suggesting that the one major unmeasured pool, soil organic matter, contains the remaining 68–84%. Short-term redistribution of 15N tracer at natural abundance levels showed similar division between plant and soil pools. Direct measurements of changes in total soil C and N pools were inconclusive due to high variation in both stands. Woody biomass production increased in the hardwood high N plot but was significantly reduced in the pine high N plot, relative to controls. A drought-induced increase in foliar litterfall in the pine stand in 1995 is one possible factor leading to a measured increase in N mineralization, nitrification, and nitrate loss in the pine high N plot in 1996.

Microbial Metabolism and Nutrient Cycling in the Mississippi and Atchafalaya River Plumes

Spatial distributions of chlorophyll, bacterial abundances and production, community respiration, and dissolved C, N, P and Si were measured in the Mississippi River (MRP) and Atchafalya River (ARP) plumes during July 1993. Dark bottle incubations were used to estimate net flux rates of inorganic nutrients, community respiration, and changes in chlorophyll concentrations in unfiltered water samples. Concentrations of total dissolved N (TDN) and soluble reactive P (SRP) in the Mississippi River were 55μM and 3μM higher, respectively, compared with those in the Atchafalaya River. Concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in the Atchafalaya River, however, were 35 and 11μM higher, respectively than in the Mississippi River. Elevated chlorophyll concentrations, bacterial abundances and production, and community respiration rates were observed at intermediate (5–25) salinities of both plumes. Property-salinity plots indicated net sinks of dissolved N, P and Si at intermediate salinities consistent with photosynthetic utilization of these substances within the plumes. The distribution of dissolved P, N and chlorophyll suggested phytoplankton-mediated transformation of riverine-NO−3 to DON at intermediate salinities of the MRP, and a similar transformation of riverine SRP to dissolved organic P (DOP) at intermediate salinities of the ARP. Net regeneration of dissolved Si and NH+4 was observed in regions of elevated chlorophyll concentrations and net removal rates in both plumes. Nitrification rates in the MRP were c. 10-fold higher than in the ARP. Estimates of C fixation by nitrifying bacteria equalled or exceeded heterotrophic bacterial C production in the low salinity region of the MRP, but were negligible compared to heterotrophic bacterial production in the ARP. Dissolved inorganic N:P, Si:P and DOC:DON:DOP ratios suggested the potential for P limitation in both plume systems during the period investigated.

Bioavailability and sources of DOC and DON in macrophyte stands of a tropical coastal lake

Dissolved organic carbon (DOC) and nitrogen (DON) derived from aquatic and terrestrial vascular plants provide a major energy and nutrient source for freshwater and coastal marine biota. The bioavailability of this material may to a large extent depend on plant species. In this study, we have compared the bioavailability of DOC and DON sampled in two distinct stands of Typha domingensisand Eleocharis mutatain a coastal tropical lake and in the adjacent ocean in the state of Rio de Janeiro, Brazil. Bioavailability of organic matter was assessed by regrowth bioassays using natural bacterial inocula. Nutrients were added to achieve carbon or nitrogen limitation. At all sampling sites, DON comprised over 95% of the total bioavailable nitrogen, suggesting its dominant role as a nitrogen source. The bioavailability of lacustrine DON (22% in the Typhastand and 34% in the Eleocharisstand) exceeded the bioavailability of DOC (8 and 10%, respectively) and exhibited a larger difference between the stands. 3H-leucine incorporation studies showed that lake bacterioplankton had a well balanced supply of C, N and P. Therefore, an accumulation of labile DON due to an excess nitrogen supply is not probable. We propose that a substantial part of the lake DON was newly formed within the macrophyte stands, while DOC was predominantly of terrestrial origin and more diagenetically changed.

Export of dissolved organic carbon and nitrogen from Gleysol dominated catchments – the significance of water flow paths

In this study, we estimated whether changes in hydrological pathwaysduring storms could explain the large temporal variations of dissolvedorganic carbon (DOC) and nitrogen (DON) in the runoff of threecatchments: a forest and a grassland sub-catchment of 1600m2 delineated by trenches, and a headwater catchment of 0.7km2. The average annual DOC export from the sub-catchments was 185 kg DOCha−1 y−1 for the forest, 108 kg DOCha−1 y−1 for the grassland and 84 kgDOC ha−1 y−1 for the headwatercatchment. DON was the major form of the dissolved N in soil and streamwater. DON export from all catchments was approximately 6 kg Nha−1 y−1, which corresponded to 60% ofthe total N export and to 50% of the ambient wet N deposition. DOC andDON concentrations in weekly samples of stream water were positivelycorrelated with discharge. During individual storms, concentrations andproperties of DOC and DON changed drastically. In all catchments, DOCconcentrations increased by 6 to 7 mg DOC l−1 comparedto base flow, with the largest relative increment in the headwatercatchment (+350%). Concentrations of DON, hydrolysable amino acids, andphenolics showed comparable increases, whereas the proportion ofcarbohydrates in DOC decreased at peak flow. Prediction of DOC and DONconcentrations by an end-member mixing analysis (EMMA) on the base ofinorganic water chemistry showed that changes in water flow pathslargely explained these temporal variability. According to the EMMA, thecontribution of throughfall to the runoff peaked in the initial phase ofthe storm, while water from the subsoil dominated during base flow only.EMMA indicated that the contribution of the DOC and DON-rich topsoil washighest in the later stages of the storm, which explained the highestDOC and DON concentrations as the hydrograph receded. Discrepanciesbetween observed and predicted concentrations were largest for thereactive DOC compounds such as carbohydrates and phenolics. Theyoccurred at base flow and in the initial phase of storms. This suggeststhat other mechanisms such as in-stream processes or a time-variantrelease of DOC also played an important role.

Influence of dolomitic lime on DOC and DON leaching in a forest soil

The influence of liming on leaching and distribution of dissolved organic carbon (DOC) and nitrogen (DON) in mineral soil was investigated in a leaching experiment with soil columns. Soil samples from separate horizons (O, A and B horizons) were collected from control and limed plots in a field liming experiment in a spruce forest in southern Sweden. The field liming (0.88 kg m−2) had been carried out 8 years before sampling. To minimize the variation among replicates, soil profiles were reconstructed in the laboratory so that the dry weight was the same for each individual soil horizon regardless of treatment. Two soil column types were used with either the O+A horizons or the O+A+B horizons. One Norway spruce seedling (Picea abies (L.) Karst) was planted in each soil column. Average pH in the leachate water was greater in the limed treatment than in the control treatment (5.0 versus 4.0 for O+A columns and 4.3 versus 3.8 for O+A+B columns). After reaching an approximate steady state, the leaching of DOC was 3–4 times greater from the limed O+A and O+A+B columns than from the corresponding control columns but the leaching of DON increased (3.5 times) only in the limed O+A columns. There was a significant correlation between DOC and DON in the leachates from all columns except for the control O+A+B columns, which indicated a decoupling of DOC and DON retention in the B horizon in the control treatment. This might be explained by a selective adsorption of nitrogen poor hydrophobic compounds (C/N ratio: 32–77) while there was a lower retention of nitrogen rich hydrophilic compounds (C/N ratio: 14–20). Proportionally more hydrophobic compounds were leached from the limed soil compared to the unlimed soil. These hydrophobic compounds also became more enriched in nitrogen after liming so in the limed treatment nitrogen might be adsorbed at nearly the same proportion as carbon, which might explain the fact that there was no decoupling of leached DOC and DON from the B horizon after liming.

Dissolved and particulate organic carbon and nitrogen in the Northwestern Mediterranean

The distribution of dissolved organic carbon (DOC) and nitrogen (DON) and particulate organic carbon (POC) and nitrogen (PON) was studied on a transect perpendicular to the Catalan coast in the NW Mediterranean in June 1995. The transect covered a hydrographically diverse zone, including coastal waters and two frontal structures (the Catalan and the Balear fronts). The cruise was conducted during the stratified period, characterized by inorganic nutrient depletion in the photic zone and a well established deep chlorophyll a maximum. DOC concentrations were measured using a high-temperature catalytic oxidation method, and DON was determined directly, with an update of the Kjeldahl method, after removal of inorganic nitrogen. The ranges of DOC and DON concentrations were 44–95 μM-C and 2.8–6.2 μM-N. The particulate organic matter ranged between 0.9 and 14.9 μM-C and from 0.1 to 1.7 μM-N. The DOC : DON molar ratio averaged 15.5±0.4, and the mean POC : PON ratio was 8.6±0.6. The distribution of dissolved organic matter (DOM) was inverse to that of the salinity. The highest concentrations of DOM were found in coastal waters and in the stations affected by the Catalan front, located at the continental shelf break. It was estimated that recalcitrant DOM constituted 67% of the DOM pool in the upper 50 m. The data suggest that accumulation of DOC due to the decoupling of production and consumption may occur in the NW Mediterranean during stratification and that the organic matter exported from the photic layer is dominated by C-rich material.

Dissolved organic matter in shelf waters off the Rı́a de Vigo (NW Iberian upwelling system)

Net in situ production and export of dissolved organic carbon (DOC) and nitrogen (DON) have been studied in shelf waters off the Rı́a de Vigo (NW Spain), as part of a comprehensive hydrographic survey carried out from September 1994 to September 1995 with a fortnight periodicity. DOC and DON correlated well ( r=+0.78), the slope of the regression line being 12.0±0.7 mol-C mol-N −1, about twice the Redfieldian slope of particulate organic matter, 6.5±0.2 mol-C mol-N −1 ( r=+0.95). Labile DOC and DON accumulated in the upper 50 m during the upwelling season (March–September), mainly after prolonged periods of wind relaxation, when horizontal flows were reduced. This labile material represented 50% and 35% of the total (dissolved+particulate) organic carbon and nitrogen susceptible of microbial utilisation, which assert the key contribution of dissolved organic matter (DOM) to the export of new primary production in the NW Iberian upwelling system. This surface excess in shelf waters appeared to be formed into the highly productive Rı́a de Vigo (a large coastal indentation) at net rates of ∼4.4 μM-C d −1 and ∼1.3 μM-C d −1 in the inner and outer segments of the embayment respectively, and subsequently exported to the shelf. Once in the shelf, simple dilution with the inert DOM pool of recently upwelled Eastern North Atlantic Central Water (ENACW) occurred. Eventually, the DOM excess produced during the upwelling season is exported to the adjacent open ocean waters by the coastal circulation. Conversely, during the unproductive downwelling season (October–February), the lowest DOC and DON levels were recorded and export was prevented by the characteristic downwelling front associated to the seasonal poleward slope current.

Dissolved organic matter and nutrients in the Lena River, Siberian Arctic: Characteristics and distribution

Dissolved organic carbon (DOC) and nitrogen (DON), amino acids, carbohydrates and inorganic nutrients were measured on samples taken in July 1994 at 18 stations between Yakutsk and the Lena delta, East Siberia. There were no obvious gradients or features along the river, except in the tributaries, the Aldan and Vilyuy rivers, where significantly higher concentrations of several parameters were measured. Concentrations of DOC varied between 300 and 1000 μM C, with most values varying between 500 and 700 μM C (mean 570 μM C). DON concentrations ranged between 9 and 28 μM N (mean 13 μM N). The C/N ratios of bulk dissolved organic matter (DOM) varied from 30 to 58, with 75% of the values being between 45 and 55 (mean 48). Total dissolved amino acids (TDAA) ranged between 1.6 and 5.4 μM, averaged about 3.5 μM, mostly in the combined form, and represented about 28% of the DON. Free amino acids were only about 2% of TDAA. Glycine, aspartic acid and glutamic acid predominated, accounting for about 41% of TDAA. Total dissolved carbohydrates ranged from 190 to 470 μg glucose equivalents l−1 and averaged 299 μg l−1, forming only 1.2 to 2.5% of the DOC pool. The following ranges of inorganic nutrients were measured: nitrate, 0.01 to 1.4 μM N (mean 0.6 μM N); nitrite, 0.03 to 0.1 μM N (mean 0.07 μM N); ammonium, 0.01 to 0.3 μM N (mean 0.13 μM N); phosphate, 0.2 to 1 μM P (mean 0.5 μM P); silicate, 59 to 87 μM Si (mean 66 μM Si). Carbon isotope data of the suspended organic material suggest that the low inorganic nitrogen values are not due to algal uptake, but rather an inherent characteristic of the river and the catchment area. This, together with positive correlations between silicate, DOC and DON and high C/N values, suggests that the composition of DOM in the Lena River is mainly determined by the input of soil-derived, recalcitrant material and not by autochthonous sources.

Dissolved organic matter and its utilization by bacteria during spring in the Southern Ocean

Concentrations of dissolved organic carbon (DOC) and nitrogen (DON) were measured during early austral Spring 1992 at a number of stations along the 6°W meridian between 47° and 60°S. This included the Polar Front in the north, the zone of melting sea-ice in the south, and waters of the Antarctic Circumpolar Current in between. Concentrations of DOC were low in deep water (34–38 μM) with generally similar or slightly higher values in the surface mixed layer (38–55 μM). DOC:DON ratios are wider in surface water than in deep water, i.e. surface accumulations contain relatively C-rich dissolved organic matter. The highly variable distribution of the surface DOC was not related to hydrographic or biotic features (fronts, plankton development) indicating the lability and transient occurrence of this material. Growth rates of bacteria were determined in subsamples from 51 0.8-μm-filtered batches of seawater incubated in the dark at in-situ temperature. Thymidine and leucine uptake and bacterial biomass change as well as changes in dissolved organic carbon in the batches, and oxygen consumption in parallel incubations correlated linearly over 2 weeks of incubation which allowed extrapolation to in-situ conditions. Bacterial growth in these experiments depended strongly on the amount of initial DOC. Growth in water from greater depth (1000 m) containing 38 μM DOC was minimal, as were DOC-decrease and oxygen consumption. Higher rates were observed in surface water slightly enriched with DOC, and highest rates in surface water amended with DOC-rich melted sea ice. Bacterial growth efficiencies (biomass C-increase vs DOC consumed) were about 30%. The experiments showed that at least 40–60% of the DOC in excess of deep water concentrations was available to bacteria.

Dissolved organic matter in Arctic multi-year sea ice during winter: major components and relationship to ice characteristics

Ice cores were collected between 10.03.93 and 15.03.93 along a 200 m profile on a large ice floe in Fram Strait. The ice was typical of Arctic multi-year ice, having a mean thickness along the profile of 2.56 ±0.53 m. It consisted mostly of columnar ice (83%) grown through congelation of seawater at the ice bottom, and the salinity profiles were characterized by a linear increase from 0 psu at the top to values ranging between 3 and 5 psu at depth. Distributions of dissolved organic carbon (DOC) and nitrogen (DON) and major nutrients were compared with ice texture, salinity and chlorophyll a. DOC, DON, dissolved inorganic nitrogen (DIN), NH4 + and NO2 − were present in concentrations in excess of that predicted by dilution curves derived from Arctic surface water values. Only NO3 − was depleted, although not exhausted. High DOC and DON values in conjunction with high NH4 + levels indicated that a significant proportion of the dissolved organic matter (DOM) was a result of decomposition/grazing of ice algae and/or detritus. The combination of high NH4 + and NO2 − points to regeneration of nitrogen compounds. There was no significant correlation between DOC and Chl a in contrast to DON, which had a positively significant correlation with both salinity and Chl a, and the distribution of DOM in the cores might best be described as a combination of both physical and biological processes. There was no correlation between DOC and DON suggesting an uncoupling of DOC and DON dynamics in multi year ice.

Comparative analyses of DOC and DON in natural waters

Abstract Extensive comparisons of the measured dissolved organic carbon (DOC) and nitrogen (DON) contents of four different natural waters were carried out prior to the Seattle DOC/DON Workshop in July 1991 to assess the current variability in these measurements among aquatic scientists. This study included 34 DOC analyses with over 20 different instruments and 13 DON analyses of the same samples using nine different instruments. In spite of good precision for individual instruments (5.9% average sample mean deviation), independently measured DOC concentrations of the same water ranged over an order of magnitude and varied on average approximately ±40% of the mean value. Variability among similar instruments operated in different labs ( ± 15–42% of the mean) was almost as large. This observation plus evidence for constant offsets among DOC analyses made in different labs suggest that unaccounted for blanks contribute more toward the observed variability than other instrument characteristics such as oxidation method, catalyst or oven temperature. The corresponding relative analytical precisions for total dissolved nitrogen and DON were 6.8% and 27%, respectively. Independent measurements of DON varied on average ±30% of the mean value and could not be related to instrument characteristics within this highly variable data set. During the Workshop, manufacturers of four different DOC analyzers measured the DOC content of untreated and spiked lake and seawater samples using common blank and calibration procedures. Excellent agreement was obtained, supporting indications from the pre-workshop exercise that operator-dependent factors, such as blank determination and treatment, are more important sources of measurement variability than instrument design.