Particulate Organic Matter Concentrations Research Articles

Influence of river discharge on zooplankton diet in the Godavari estuary (Bay of Bengal, Indian Ocean)

In estuaries, detrital (i.e., non-living) organic matter (OM) contributes significantly to the particulate organic matter (POM) pool and we hypothesize that it may be a major source of estuarine zooplankton diet. To test this hypothesis, the isotopic composition of carbon (d13C) and nitrogen (d15N) of phytoplankton, zooplankton, and POM was assessed in the Godavari estuary (Bay of Bengal, Indian Ocean) during wet (November) and dry periods (January). As a result of higher riverine discharge, POM concentrations and values of the C/Chl-a ratio during the wet period were higher than those measured during the dry one. Relatively lower δ13CPOM values were observed during wet than dry period and contrasting to that was found for δ15NPOM. Detritus from fresh water algae and C3 plants contributed significantly to the POM pool during the wet and dry period, respectively. Based on isotopic mixing model, detrital OM and phytoplankton mostly characterized the POM pools during the wet and dry periods, respectively. Accordingly, our results suggest also that the zooplankton diet was mostly supported by detrital OM during the wet period and by both phytoplankton and detrital OM during the dry one. The zooplankton trophic level (TL, 2.7) during the wet period was relatively higher than that (1.9) during the dry one, suggesting a relative higher preference for detritus than phytoplankton during the wet period. The results of this study allowed us confirming that detrital OM can significantly support zooplankton production in the Godavari estuary.

Effects of Climate Variability on Snowmelt and Implications for Organic Matter in a High‐Elevation Lake

AbstractFew coupled lake‐watershed studies examine long‐term effects of climate on the ecosystem function of lakes in a hydrological context. We use 32 years of hydrological and biogeochemical data from a high‐elevation site in the Sierra Nevada of California to characterize variation in snowmelt in relation to climate variability and explore the impact on factors affecting phytoplankton biomass. The magnitude of accumulated winter snow, quantified through basinwide estimates of snow water equivalent (SWE), was the most important climate factor controlling variation in the timing and rate of spring snowmelt. Variations in SWE and snowmelt led to significant differences in lake flushing rate, water temperature, and nitrate concentrations across years. On average in dry years, snowmelt started 25 days earlier and proceeded 7 mm/day slower, and the lake began the ice‐free season with nitrate concentrations ~2 μM higher and water temperatures 9°C warmer than in wet years. Flushing rates in wet years were 2.5 times larger than those in dry years. Consequently, particulate organic matter concentrations, a proxy for phytoplankton biomass, were 5–6 μM higher in dry years. There was a temporal trend of increase in particulate organic matter across dry years that corresponded to lake warming independent of variation in SWE. These results suggest that phytoplankton biomass is increasing as a result of both interannual variability in precipitation and long‐term warming trends. Our study underscores the need to account for local‐scale catchment variability that may affect the accumulation of winter snowpack when predicting climate responses in lakes.

Hydrography of shellfish harvesting areas in the western Cantabrian coast (Rías Altas, NW Iberian Peninsula)

Intertidal shellfish banks in estuarine areas are influenced by a wide variety of environmental conditions, including the physical and chemical characteristic of the water that floods the grounds. In this study we carry out a cross-comparison of the hydrographic characteristics and nutrient fertilization patterns of the waters that laps the shellfish grounds of a group of five drowned valleys collectively known as “Rías Altas”, located in the western Cantabrian coast (NW Iberian Peninsula). This region is affected by coastal upwelling from June to August resulting in a water exchange between the embayments and the shelf varying between 31.5 and 220.5 m3s−1. The fresh water discharge followed a seasonal cycle too, with maxima in winter and minima in summer, and long-term average flow rates ranging from 5.9 m3s−1 to 22.2 m3 s−1. Significant differences were observed among the five embayments with regard to temperature and salinity and also with inorganic nutrients, chlorophyll a and particulate organic matter concentrations in the continental waters.

Shift towards larger diatoms in a natural phytoplankton assemblage under combined high-CO2 and warming conditions.

An indoor mesocosm experiment was carried out to investigate the combined effects of ocean acidification and warming on the species composition and biogeochemical element cycling during a winter/spring bloom with a natural phytoplankton assemblage from the Kiel fjord, Germany. The experimental setup consisted of a “Control” (ambient temperature of ~4.8 °C and ~535 ± 25 μatm pCO2), a “High-CO2” treatment (ambient temperature and initially 1020 ± 45 μatm pCO2) and a “Greenhouse” treatment (~8.5 °C and initially 990 ± 60 μatm pCO2). Nutrient replete conditions prevailed at the beginning of the experiment and light was provided at in situ levels upon reaching pCO2 target levels. A diatom-dominated bloom developed in all treatments with Skeletonema costatum as the dominant species but with an increased abundance and biomass contribution of larger diatom species in the Greenhouse treatment. Conditions in the Greenhouse treatment accelerated bloom development with faster utilization of inorganic nutrients and an earlier peak in phytoplankton biomass compared to the Control and High CO2 but no difference in maximum concentration of particulate organic matter (POM) between treatments. Loss of POM in the Greenhouse treatment, however, was twice as high as in the Control and High CO2 treatment at the end of the experiment, most likely due to an increased proportion of larger diatom species in that treatment. We hypothesize that the combination of warming and acidification can induce shifts in diatom species composition with potential feedbacks on biogeochemical element cycling.

Response of microalgae to large-seaweed cultivation as revealed by particulate organic matter from an integrated aquaculture off Nan'ao Island, South China

Large seaweed cultivation has proven an effective means to inhibit harmful microalgae at experimental scales and battle eutrophication in Chinese coastal waters, but essentially there is a lack of field-scale studies to explore the underlying mechanism. Here we present a 1.5-year-long time series of particulate organic matter (POM) and settling particulate matter (SPM) concentrations from an integrated aquaculture of Gracilaria lemaneiformis off the coast of Nan'ao Island, South China from April 2014 to August 2015. The microscopic examination and geochemical characteristics show that the POM mainly consisted of microalgae. The mean POM concentration increased 99.8%, 71.2%, 45.8% and 111.9% at the four sampling sites during the non-cultivation period, while decreased 25.5%, 17.3%, 12.2% and 20.3%, respectively, during the seaweed cultivation period. These results suggest that the large scale seaweed cultivation can remove excess nutrients and inhibit microalgal growth, thereby contributing to the improvement of coastal marine aquaculture environment.

Distribution and Feeding of Herbivorous Zooplankton in the Laptev Sea

Sampling was conducted along the quasi meridional transect at 130° E from the Lena River estuary to northern deep-sea regions of the Laptev Sea in September 2015. The latitudinal zonality and the impact of river runoff are manifested in the temperature and salinity distribution, concentration of particulate organic matter, and the structure of plankton communities. The differences in the chl a concentration and primary production along the transect are insignificant. The feeding rate of mesozooplankton herbivores was assessed by a fluorescence technique. The total consumption of phytoplankton biomass and primary production are estimated based on the feeding rate, abundance of zooplankton species, and their diel migrations. The daily grazing impact of zooplankton on phytoplankton biomass increases from 2% on the inner shelf to 3% on the mid-shelf, 5% on the outer shelf, and 10% in the deep-sea part of the basin. The consumption of primary production also increases: 1, 4.5, 5.7, and 13.9%, respectively. In the fall, the consumption of phytoplankton does not compensate the energy demands for respiration. The latitudinal zonality of the Laptev Sea appears not only in the hydrophysical water parameters and the structure of plankton communities, but also in their functional characteristics.

Spatiotemporal changes in the concentration and composition of suspended particulate matter in front of Hansbreen, a tidewater glacier in Svalbard

Summary Tidewater glaciers supply large amounts of suspended particulate matter (SPM) and freshwater to fjords and affect oceanographic, sedimentological and biological processes. Our understanding of these processes, is usually limited to the short summer season. Here, we present the results of a one-year-long monitoring of the spatial variability in SPM characteristics in a context of oceanographic and meteorological conditions of a glacial bay next to Hansbreen, a tidewater glacier in Hornsund (southern Spitsbergen). The observed range of SPM concentrations was similar to ranges measured in other sub-polar glaciated fjords, especially in Svalbard. The major source of SPM is the meltwater discharge from the glacier. The maximum water column-averaged SPM concentrations did not correlate with peaks in freshwater discharge and were observed at the beginning of the autumn season, when the fjord water transitioned from stratified to fully mixed. The observed spatiotemporal variations in the total SPM, particulate organic matter (POM) and particulate inorganic matter (PIM) are likely controlled by a combination of factors including freshwater supply, water stratification and circulation, bathymetry, the presence of sea ice, biological productivity and sediment resuspension. During the ablation season, the SPM maximum concentrations were located within the upper water layer, whereas during the winter and spring, the greatest amounts of SPM were concentrated in deeper part. Thus, typical remote sensing-based studies that focus on SPM distributions may not reflect the real SPM levels. POM and PIM concentrations were correlated with each other, during most of the time suggesting that they may have a common source.

Corn residue inputs influence earthworm population dynamics in a no-till corn-soybean rotation

Temporal dynamics of earthworm populations in temperate agroecosystems are related to climatic and edaphic conditions, agricultural management and crop residue inputs. The amount of crop residue present and its suitability as a food resource for earthworms changes as the material decomposes. A readily-decomposable crop residue (low C:N ratio, low lignin content) may be a transient food resource for earthworms, whereas residues that decompose more slowly could be a persistent food resource to sustain the earthworm populations. Chemical composition of the crop residue and the particulate organic matter (POM) content in soil, a measure of the partially-decomposed residues, are indicators of the food resources for earthworms. The objective of this study was to determine how the quantity and chemistry of crop residues, and the soil POM content, were related to earthworm population dynamics during a two-year field experiment in no-till corn-soybean rotations. The high residue treatment provided an additional 3–5 Mg ha−1 y−1 in corn residue, compared to the low residue treatment. As hypothesized, earthworm abundance and biomass were strongly affected by the quantity of crop residues left in the agroecosystem after harvest. Greater corn residue inputs in the high residue treatment supported an earthworm community that had similar species composition and age structure, but was nearly twice as large as the earthworm community in the low residue treatment. Soybean residue appeared to be a transient food resource for earthworm populations in the field. Earthworm abundance and biomass were related to the amount of surface residue present, but were not correlated to the chemical composition of crop residue and the soil POM content during this two-year study. Under field conditions, earthworm populations respond to the quantity of residues present as a food resource rather than the chemical composition of the residue.

Effects of long-term application of various organic amendments on soil particulate organic matter storage and chemical stabilisation of vertisol soil

ABSTRACTCurrent understanding of the effects of long-term application of various organic amendments on soil particulate organic matter (POM) storage and chemical stabilisation remains limited. Therefore, we collected soil samples from the soil profile (0–100 cm) under six treatments in a 31-year long-term fertilisation experiment: no fertiliser (CK), mineral fertilisers (NPK), mineral fertilisers plus 3.8 or 7.5 t ha−1 year−1 (fresh base) the amount of wheat straw (1/2SNPK and SNPK) and mineral fertilisers plus swine or cattle manure (PMNPK and CMNPK). Long-term incorporation of wheat straw and livestock manure amendments significantly (p < 0.05) increased crop yield and sustainable yield index, and POM storage compared with CK and NPK treatments. The mole ratios of H/C in the POM under organic amendment treatments significantly (p < 0.05) decreased by 13.8% and 37.1%, respectively, compared with the NPK treatment. Similarly, solid state NMR spectroscopy showed that the O–alkyl carbon content of POM was greatly decreased, whereas aromatic carbon contents and alkyl to O–alkyl carbon ratios were substantially increased under PMNPK and CMNPK treatments. In conclusion, we recommend long-term livestock manure application as a preferred strategy for enhancing POM quantity and quality (chemical stability), and crop yield of vertisol soil in northern China.

Corn Residue Baling and Grazing Impacts on Soil Carbon Stocks and Other Properties on a Haplustoll

Core Ideas Corn residue baling reduced SOC stocks by 2.16 Mg ha–1 yr–1 compared with no removal after 3 yr. Baling increased erosion potential and reduced soil microbial biomass. Cattle grazing of corn residues did not generally affect soil properties. Baling and grazing effects on soil properties did not vary with irrigation and tillage system. Baling affected soil properties more than grazing regardless of irrigation and tillage. Baling and grazing of corn (Zea mays L.) residues are common practices in irrigated systems to meet the increasing demand for forage. Our understanding of how such practices affect soil properties under different tillage and irrigation levels is, however, still limited. This study assessed the impacts of corn residue baling and grazing on soil organic C (SOC) stocks, particulate organic matter (POM) concentration, soil microbial communities, sorptivity, and wind and water erosion potential under continuous corn managed with two irrigation (full and limited) and two tillage (no‐till and strip till) levels after 3 yr on a silt loam in the central Great Plains. On average, residue removal was 66% for baling and 24% for grazing. Baling reduced SOC stocks by 2.16 Mg ha–1 yr–1 for the 0‐ to 20‐cm depth compared with no residue removal, but residue grazing, irrigation, and tillage had no effects. Full irrigation decreased mean weight diameter (MWD) of wet aggregates by 19% compared with limited irrigation, attributed to Na accumulation. No‐till had lower wind‐erodible fraction and greater microbial biomass than strip till. Regardless of irrigation and tillage, baling increased wind‐erodible fraction by 43% and decreased MWD by 56%, POM concentration by 41%, sorptivity by 57%, and microbial biomass in the upper 5‐cm depth compared with grazing and no residue removal. Grazing increased POM concentration and actinomycete biomass compared with no residue removal. Overall, changes in soil properties due to baling were larger and more rapid than in most previous studies, while grazing had fewer effects on soil properties.

Influence of Manure Type and Bedding Material on Carbon Content of Particulate Organic Matter in Feedlot Amendments Using 13C NMR-DPMAS

ABSTRACTFeedlots in southern Alberta apply composted (CM) or stockpiled (SM) manure with straw (ST) or wood-chip (WD) bedding to cropland, but few studies have examined the effect of manure type and bedding material on carbon composition of these feedlot amendments prior to land application using solid-state 13C NMR-DPMAS (nuclear magnetic resonance-direct polarization, magic angle spinning). The particulate organic matter fraction was extracted from four amendment treatments (CM-ST, CM-WD, SM-ST, SM-WD) to avoid possible paramagnetic interference from considerable mineral soil in the manure from the unpaved feedlot. The hypothesis was that O-alkyl C of POM should be lower for more decomposed manure types (CM than SM) and bedding materials (ST than WD), and that alkyl C, aromatic C, carboxyl C, aromaticity, and alkyl:O-alkyl (A:O-A) ratio should be greater for the more decomposed amendments. The C composition of all feedlot amendments was dominated by aromatic C (8%–14%) and O-alkyl (7%–14%) C and had considerable less contribution from carboxyl (2%–4%) and alkyl C (1%–3%). The manure type hypothesis was supported for O-alkyl C (but not for the other three C groups), aromaticity, but not the A:O-A ratio. The bedding hypothesis was supported for O-alkyl C, aromaticity, and A:O-A ratio, but not for alkyl C, aromatic C, and carboxyl C. A decrease in O-alkyl C, increase in aromaticity, and increase in A:O-A ratio (bedding only) with more decomposed manure types or bedding materials suggested that these 13C NMR parameters may have potential to evaluate the maturity and stability of composted feedlot manures.

Biogeochemical characteristics and fluxes of suspended particulate organic matter in response to low‐tide rainfall

AbstractWe evaluated the effects of low‐tide rainfall on salt marsh particulate organic carbon (POC) cycling. Suspended sediment samples from intertidal creeks were collected during rain and no‐rain conditions. Bulk (carbon, nitrogen) and molecular (e.g., lignin) compositions of organic constituents in particulates were analyzed. Temporal patterns of suspended sediment concentration mimic fluctuations in rainfall intensity, but all POC constituents do not necessarily follow these patterns. For example, particulate organic matter concentrations in water column covaried with rain intensity yet organic matter (OM) contents in the particles showed trivial correlations with rain. In general, rainfall effects raised the suspended sediment and organic constituent concentrations in the water column by up to three orders of magnitude. Multivariate statistical analysis revealed that the rainfall‐entrained particulates were compositionally distinct from calm weather suspended particulates, from salt marsh surface sediment, and from creek bank surface sediment. Rainfall‐entrained particulates are depleted in bulk organic carbon and phytoplankton‐derived OM but enriched in lignin phenols. These observations indicate that rainfall‐runoff processes preferentially mobilize POC derived from Spartina alterniflora from marsh platforms and effectively transport these materials to intertidal creeks following low‐tide rainfall events. Moreover, average net transport rates by rainfall are 0.020 ± 0.002 gPOC m−2 mmRain−1, and this POC turnover rate amounts to ∼ 19% of the annual POC export from estuary to ocean. Therefore, estuarine outwelling estimates typically made under calm weather conditions represent minimum levels of POC export. Hence, low‐tide rainfall can be expected to substantially enhance the delivery of marsh‐derived POC to the coastal ocean.

Particle Size Characteristics of Fluvial Suspended Sediment in Proglacial Streams, King George Island, South Shetland Island

In this study, the characterization of particle size distribution of suspended sediment that is transported by streams (Ornithologist Creek, Ecology Glacier Creeks, Petrified Forest Creek, Czech Creek, Vanishing Creek, Italian Creek) in the area of the Arctowski Polish Antarctic Station is presented. During the first period of the summer season, the aforementioned streams are supplied by the melting snow fields, while later on, by thawing permafrost. The water samples were collected from the streams at monthly intervals during the Antarctic summer season (January - March) of 2016. The particle size distribution was measured in the laboratory with a LISST-25X laser diffraction particle size analyser. According to Sequoia Scientific Inc., LISST-25X can measure particle sizes (Sauter Mean Diameter) between 2.50 and 500 μm. The results of particle size measurements were analysed in relation to flow velocity (0.18–0.89 m/s), the cross-sectional parameters of the streams, suspended sediment concentration (0.06–167.22 mg/dm3) and the content of particulate organic matter (9.8–84.85%). Overall, the mean particle size ranged from 28.8 to 136 μm. The grain size of well-sorted sediments ranged from 0.076 to 0.57, with the skewness and kurtosis values varying from -0.1 to 0.4, and from 0.67 to 1.3, respectively. Based on the particle size characteristics of suspended sediment, the streams were divided into two groups. For most of the streams, the sediment was very well sorted, while fine sand and very fine sand were dominant fractions displaying symmetric and platykurtic distributions, respectively. Only in two streams, the suspended sediment consisted of silt-size grains, well or moderately well sorted, with coarse-skewness and mostly mesokurtic distribution. The C-M chart suggested that the transportation processes of suspended sediment included the suspended mode only. The grain-size distribution of suspended sediment was mainly influenced by the stream runoff, surface sediment type and biological processes.

Climate warming impacts on soil organic carbon fractions and aggregate stability in a Tibetan alpine meadow

Alpine meadows in the Tibetan Plateau contain a large amount of soil organic carbon (SOC), which is highly vulnerable to climate change and has thus been a research priority for scientists in recent decades. However, how climate warming influences the composition and stability of SOC remains unclear. In this study, a warming experiment (2010–2015) using open-top chambers was established (with unwarmed control [CK], winter warming [WW], and year-round warming [YW]) to investigate the effects of warming on the contents of SOC and its fractions, chemical composition of SOC, and water stability of aggregates in an alpine meadow located at the Damxung Grassland Station in the northern Tibetan Plateau. Experimental warming had no apparent effect (p > 0.05) on SOC content (17.8 ± 1.96, 17.8 ± 1.13 and 17.8 ± 1.09 g kg−1 under CK, WW and YW, respectively) in the 0–20 cm soil layer. However, warming significantly (p < 0.05) increased soil water-soluble organic carbon (C) content by 46.2% and 69.2%under WW and YW, respectively, and affected SOC chemical composition with decreasing phenol C (by 2.6% under WW, p > 0.05; and 8.4% under YW, p < 0.05) and increasing carboxyl C (by 11.6% under WW, p < 0.05; and 5.0% under YW, p > 0.05). Warming decreased (p > 0.05) the proportions of macroaggregates (2–0.25 mm) and free microaggregates (0.25–0.053 mm), whereas warming significantly increased the proportion of non-aggregated silt- and clay-sized fractions (<0.053 mm) by 41.0% and 55.7% under WW and YW, respectively. The variation in the aggregate size distribution resulted in the decline of the mean weight diameter and geometric mean diameter of water-stable aggregates by 5.1% (p > 0.05) and 8.5% (p < 0.05) under WW and by 8.5% and 6.6% (both p < 0.05) under YW, respectively. Of importance, the organic C content in free microaggregates, which provides greater physical protection to stabilize SOC, decreased by 10.9% (p > 0.05) under WW and by 22.4% (p < 0.05) under YW. The organic C contents of particulate organic matter and silt- and clay-sized fractions inside free microaggregates also significantly decreased under YW by 49.4% and 16.9%, respectively. However, the organic C content in non-aggregated silt- and clay-sized fractions significantly increased under WW and YW by 52.9% and 46.8%, respectively. The results suggest that short-term climate warming did not affect total SOC stocks, whereas it had a positive effect on WSOC, carboxyl C and non-aggregated silt- and clay-associated C and exhibited a negative effect on phenol C, free microaggregates-associated C and water stability of aggregates. These variables may act as sensitive indicators of climate warming in the Tibetan alpine meadows, which will in turn affect grassland ecosystem C fluxes in response to further climate change. The findings of the present study help improve our understanding of the responses of terrestrial ecosystem C cycling to future climate change.

Simultaneous organic matter removal and nitrification of an inert self-supporting immersed media to upgrade aerated lagoons.

A pilot study was performed to evaluate the potential of an inert self-supported immersed fixed film media to upgrade aerated lagoons. Simultaneous organic matter removal and nitrification was assessed under different loading rates and temperatures (near 0 °C) using 12 laboratory-scale reactors operated in parallel. Test results showed that both the temperature and the load have an influence on organic matter effluent concentrations. Effluent quality seemed related to the observed biofilm thickness. Thicker biofilm is believed to have contributed to biomass detachment and increased particulate organic matter concentrations in the effluent. Simultaneous organic removal and nitrification was obtained at loads above 5 g CBOD5/m2·d. The highest nitrification rate at 0.4 °C was obtained for the smallest load, which showed a nitrification limitation close to freezing point.

Relationships between electron transport system (ETS) activity and particulate organic matter features in three areas of the Ross Sea (Antarctica)

Electron transport system activity (ETSa) and particulate organic matter (POM) concentrations and composition were measured in three areas of the continental shelf of the Ross Sea during summer 2014, in the framework of the Ross Sea Mesoscale Experiment (ROME) project. We aimed at testing whether in the epipelagic layer (0–200m) ETS showed different activity depending on the geographical position and on the different hydrological structures that characterised each area, as eddy and fronts, and whether the ETSa of the microplanktonic fraction depended on POM quantitative and qualitative features. ETSa showed differences between the three areas, but within each of them the different hydrological conditions did not influence significantly the respiration activity. ETSa displayed significant correlations with POM, especially in the offshore areas characterised by residual ice influence and by a mesoscale eddy structure. In these zones ETSa was enhanced by good trophic value of POM, i.e. showing dominance of proteins and PON over structural carbohydrates and POC, respectively. The role of the phytoplanktonic fraction in ETSa was higher in the eddy-influenced area, that showed significantly higher chlorophyll-a concentrations. On the other hand, in the area placed coastward, the relationships between ETSa and POM changed. High ETSa were found in the subsurface layer and down to 100m depth and were related to more refractory POM, whose utilization would require higher energy. Different ETSa-POM relationships were consistent with the anomalous phytoplanktonic bloom detected in the coastward area, characterised by Phaeocystis. Thus, the anomalies of the primary producers are reflected by changes in POM respiration and potential C utilization.

Seasonal sources of carbon to the Brazilian upwelling system

Environmental heterogeneity on coastal areas is an outcome of several hydrodynamic forces. Particularly, wind-driven upwelling is usually associated with alternating periods of water mixing and stratification. In addition, the effects of near shore oceanographic conditions may vary with coastline topography and anthropogenic impacts. Herein we evaluated the seasonal differences in the hydrodynamics of the Cabo Frio upwelling system (Brazil) in function of its local bay. Surface and deep water from 5 contrasting coastal areas were sampled 13 times during the spring/summer and in the fall/winter periods to use two-way Analysis of Variance comparing the measured variables in function of season and depth. Northeastern wind was predominant during both seasons but it was more intense during the spring/summer period when water temperature was colder and concentration of nutrients peaked. Southwestern wind was more common during the fall/winter period and was associated with cold fronts that decreased water salinity inner the bay. Consequently, the concentrations of nutrients, chlorophylls, prokaryotic secondary production (PSP) as well as fluorescent and non-fluorescent particles were significantly higher during the upwelling season while the concentration of particulate organic matter (POM) was highest during the non-upwelling season. Respectively, mean nitrate concentration varied from 2.2 to 0.9 μM, ammonium from 2.7 to 1.0 μM, chlorophyll a from 2.4 to 1.4 mg m−3, PSP from 1.8 to 1.0 μgC.L−1 h−1 and POM from 2.6 to 6.4 mg L−1. Contrasting surface and deep waters, mean nitrate concentration ranged from 0.9 to 2.2 μM, POM from 4.2 to 5.2 mg L−1 and PSP from 1.9 to 0.8 μgC.L−1 h−1. Three scenarios were identified: water stratification, upwelling and water homogenization. The first two scenarios were more common outside the bay during the upwelling season. When upwelling was intense, deep water temperature in the bay dropped to less than 20 °C resulting in the stratification of water column in shallow sites. Water homogenization was common in the whole system through the non-upwelling season and was more pronounced during cold fronts. Our results evidenced a strong seasonal variation of carbon origin which was mainly associated with phytoplankton during the upwelling season and to POM during the non-upwelling season. The seasonal effect of upwelling and cold fronts on the hydrodynamics of the Cabo Frio upwelling system varied at a small spatial scale in function of site depth.

The effects of different environmental factors on the biochemical composition of particulate organic matter in Gwangyang Bay, South Korea

Abstract. The biochemical composition of particulate organic matter (POM) produced through phytoplankton photosynthesis is important in determining food quality for planktonic consumers as well as the physiological conditions of phytoplankton. Major environmental factors controlling the biochemical composition were seasonally investigated in Gwangyang Bay, South Korea, which has only natural conditions (e.g., no artificial dams). Water samples for the biochemical compositions were obtained from three different light depths (100, 30, and 1 %) mainly at three sites in Gwangyang Bay from April 2012 to April 2013. Different biochemical classes (carbohydrates, CHO; proteins, PRT; and lipids, LIP) were extracted, and then the concentrations were determined by the optical density measured with a spectrophotometer. The highest and lowest PRT compositions among the three biochemical classes were found in April 2012 (58.0 %) and August 2012 (21.2 %), whereas the highest and lowest LIP compositions were found in August 2012 (49.0 %) and April 2012 (24.8 %), respectively. The CHO composition was recorded as high in January 2013 and remained above 25 % during the study period. The calorific contents of the food material (FM) ranged from 1.0 to 6.1 Kcal m−3 (annual average ± SD = 2.8 ± 1.1 Kcal m−3). Based on a Pearson's correlation coefficient analysis, a major governing factor in the biochemical composition of POM was dissolved inorganic nitrogen loading from the river input in Gwangyang Bay. In conclusion, a relatively larger amount of FM and the higher calorific contents of POM found in this study compared to other regions reflected good nutritive conditions for sustaining productive shellfish and fish populations in Gwangyang Bay. Continuous observations are needed to monitor the marine ecosystem response to potential environmental perturbations in Gwangyang Bay.

Nitrogen dynamics in a nutrient-poor soil under mixed-species plantations of eucalypts and acacias

Introducing nitrogen-fixing species (NFS) in eucalypt plantations is a useful practice to compensate nitrogen loss at harvest, reduce fertilizer inputs, improve soil fertility and sustain forest productivity in low input systems. Nitrogen (N) and carbon (C) were evaluated in the active part of soil organic matter (SOM) i.e., the particulate organic matter (POM), which was obtained after soil (0–0.05 m) physical fractionation at the end of a first 7-year rotation (R1Y7) and at year 2 of a second rotation (R2Y2) in pure acacia (100A), pure eucalypt (100E) and mixed-species (50A50E) stands in an experimental plantation established on an Arenosol in the Congolese coastal plains. N concentration (in g kg−1 of soil) was higher in coarse POM (cPOM, 4000–250 μm) in 100A and 50A50E compared to 100E at 2YR2, while no difference was found in fine POM (fPOM, 250–50 μm) and in the organo-mineral fraction (OMF, < 50 μm). N content in cPOM was more than 3 times higher at R2Y2 than at R1Y7. A slight increase was also observed in fPOM, while no difference was observed in OMF between R1Y7 and R2Y2. Lower C:N ratios in the two POM fractions in 100A and, to a lesser extent, in 50A50E compared to 100E suggests an improved soil N status after acacia trees have been introduced in eucalypt plantations. The lack of difference in N content of coarse POM between 100A and 50A50E at R1Y7, despite higher amount of N in the forest floor and N returning to the soil (harvest residues and litterfall) in 100A than in 50A50E, suggests a faster cycling of nitrogen under acacia than under eucalypt.

Accumulation of organic components and its association with macroaggregation in a sandy loam soil following conservation tillage

This paper was primarily devoted to reveal the composition of soil organic carbon (C) at a molecular level and to compare the differences existing among tillage practices after 8 years of contrasting managements, aiming to identify the effects of conservation tillage on the accumulation process of soil organic C and its association with macroaggregation. A sandy loam soil following 8-year conservation tillage was collected to identify the dynamics of organic components and their associations with macroaggregation. The content of particulate organic matter within macroaggregates was determined using aggregate fractionation techniques. Conservation tillage improved the accumulation efficiency of different C functional groups in large and small macroaggregates, and resulted in a higher soil organic C level. The content of C functional groups was significantly and positively correlated with the C amount of subfractions in macroaggregates. The accumulation of organic components, particularly the methoxyl/N-alkyl C in the 0–5 cm layer and the O-alkyl C in the 5–10 cm layer, in turn, accelerated soil macroaggregation. Overall, organic component accumulation and soil macroaggregation were interacted on each other. Our results suggested that the enrichment of methoxyl/N-alkyl C and O-alkyl C in the test soil following conservation tillage could effectively promote organic C accumulation and macroaggregation.