Quality Of Particulate Organic Matter Research Articles

Particulate organic matter quality influences nitrate retention and denitrification in stream sediments: evidence from a carbon burial experiment

Organic carbon supply is linked to nitrogen transformation in ecosystems. However, the role of organic carbon quality in nitrogen processing is not as well understood. We determined how the quality of particulate organic carbon (POC) influenced nitrogen transformation in stream sediments by burying identical quantities of varying quality POC (northern red oak (Quercus rubra) leaves, red maple (Acer rubrum) leaves, red maple wood) in stream mesocosms and measuring the effects on nitrogen retention and denitrification compared to a control of combusted sand. We also determined how POC quality affected the quantity and quality of dissolved organic carbon (DOC) and dissolved oxygen concentration in groundwater. Nitrate and total dissolved nitrogen (TDN) retention were assessed by comparing solute concentrations and fluxes along groundwater flow paths in the mesocosms. Denitrification was measured by in situ changes in N2 concentrations (using MIMS) and by acetylene block incubations. POC quality was measured by C:N and lignin:N ratios and DOC quality was assessed by fluorescence excitation emission matrix spectroscopy. POC quality had strong effects on nitrogen processing. Leaf treatments had much higher nitrate retention, TDN retention and denitrification rates than the wood and control treatments and red maple leaf burial resulted in higher nitrate and TDN retention rates than burial of red oak leaves. Leaf, but not wood, burial drove pore water to severe hypoxia and leaf treatments had higher DOC production and different DOC chemical composition than the wood and control treatments. We think that POC quality affected nitrogen processing in the sediments by influencing the quantity and quality of DOC and redox conditions. Our results suggest that the type of organic carbon inputs can affect the rates of nitrogen transformation in stream ecosystems.

Benthic fluxes of dissolved organic nitrogen in the lower St. Lawrence estuary and implications for selective organic matter degradation

Abstract. The distribution of dissolved organic nitrogen (DON) and carbon (DOC) in sediment porewaters was determined at nine locations along the St. Lawrence estuary and in the gulf of St. Lawrence. In a previous manuscript (Alkhatib et al., 2012a), we have shown that this study area is characterized by gradients in the sedimentary particulate organic matter (POM) reactivity, bottom water oxygen concentrations, and benthic respiration rates. Based on the porewater profiles, we estimated the benthic diffusive fluxes of DON and DOC in the same area. Our results show that DON fluxed out of the sediments at significant rates (110 to 430 μmol m−2 d−1). DON fluxes were positively correlated with sedimentary POM reactivity and varied inversely with sediment oxygen exposure time (OET), suggesting direct links between POM quality, aerobic remineralization and the release of DON to the water column. DON fluxes were on the order of 30 to 64% of the total benthic inorganic fixed N loss due to denitrification, and often exceeded the diffusive nitrate fluxes into the sediments. Hence they represented a large fraction of the total benthic N exchange, a result that is particularly important in light of the fact that DON fluxes are usually not accounted for in estuarine and coastal zone nutrient budgets. In contrast to DON, DOC fluxes out of the sediments did not show any significant spatial variation along the Laurentian Channel (LC) between the estuary and the gulf (2100 ± 100 μmol m−2 d−1). The molar C / N ratio of dissolved organic matter (DOM) in porewater and the overlying bottom water varied significantly along the transect, with lowest C / N in the lower estuary (5–6) and highest C / N (> 10) in the gulf. Large differences between the C / N ratios of porewater DOM and POM are mainly attributed to a combination of selective POM hydrolysis and elemental fractionation during subsequent DOM mineralization, but selective adsorption of DOM to mineral phases could not be excluded as a potential C / N fractionating process. The extent of this C- versus N- element partitioning seems to be linked to POM reactivity and redox conditions in the sediment porewaters. Our results thus highlight the variable effects selective organic matter (OM) preservation can have on bulk sedimentary C / N ratios, decoupling the primary source C / N signatures from those in sedimentary paleoenvironmental archives. Our study further underscores that the role of estuarine sediments as efficient sinks of bioavailable nitrogen is strongly influenced by the release of DON during early diagenetic reactions, and that DON fluxes from continental margin sediments represent an important internal source of N to the ocean.

Fluorescence Tracking of Dissolved and Particulate Organic Matter Quality in a River-Dominated Estuary

Excitation-emission matrix (EEM) fluorescence was combined with parallel factor analysis (PARAFAC) to model base-extracted particulate (POM) and dissolved (DOM) organic matter quality in the Neuse River Estuary (NRE), North Carolina, before and after passage of Hurricane Irene in August 2011. Principle components analysis was used to determine that four of the PARAFAC components (C1-C3 and C6) were terrestrial sources to the NRE. One component (C4), prevalent in DOM of nutrient-impacted streams and estuaries and produced in phytoplankton cultures, was enriched in the POM and in surface sediment pore water DOM. One component (C5) was related to recent autochthonous production. Photoexposure of unfiltered Neuse River water caused an increase in slope ratio values (S(R)) which corresponded to an increase in the ratio C2:C3 for DOM, and the production of C4 fluorescence in both POM and DOM. Changes to the relative abundance of C4 in POM and DOM indicated that advection of pore water DOM from surface sediments into overlying waters could increase the autochthonous quality of DOM in shallow microtidal estuaries. Modeling POM and DOM simultaneously with PARAFAC is an informative technique that is applicable to assessments of estuarine water quality.

Longitudinal and seasonal changes in the origin and quality of transported particulate organic matter along a gravel-bed river

Particulate organic matter (POM) plays an important role in nutrient dynamics in river ecosystems, but little is known about changes in the origin and quality of POM in relation to physical and seasonal changes along rivers. Using stable isotope and stoichiometric analyses, we investigated the changes in origin and quality of POM of three different size fractions (fine [FPOM], 1.2–100 μm; medium [MPOM], 100–250 μm; and coarse [CPOM], 250–1,000 μm) at 14 sites along a gravel-bed river over four seasons. FPOM and MPOM accounted for 90% of all POM at all study sites. At each site, the δ13C level was lower for FPOM (range: −29.0 to −21.1‰) than for MPOM (−26.9 to −17.2‰) and CPOM (−27.5 to −16.3‰). The C:N ratio was lower for FPOM (6.9–15.6) than for MPOM (6.3–17.4) and CPOM (5.7–27.1). The contribution of periphyton to POM of all size fractions had a tendency to increase downstream, though the trend was less clear and varied seasonally for MPOM and CPOM between sites in middle and downstream reaches. Contrastively, the C:N ratio in all size fractions of POM consistently decreased downstream. The downstream decrease in the C:N ratio of POM can be partly explained by the increase in the contribution of periphyton, which seems to be associated with increased discharge and enhanced periphyton dislodgement, especially in winter. In addition, an increase in bacterial biomass associated with the greater nutrient availability in pool areas is another possible reason for the decrease in the C:N ratio of POM downstream.

Distribution of soil organic matter between fractions and aggregate size classes in grazed semiarid steppe soil profiles

Grazed steppe ecosystems are discussed as one of the big global carbon sinks that may have the potential to sequester large amounts of atmospheric CO2 and mitigate the effects of global change if grazing is abandoned or management improved. But until today, little is known about sequestration potentials and stabilisation mechanisms in complete soil profiles of semiarid grasslands and how these systems react to grazing cessation. We applied a combined aggregate size, density and particle size fractionation procedure to sandy steppe soils under different grazing intensities (continuously grazed = Cg, winter grazing = Wg, ungrazed since 1999 = Ug99, ungrazed since 1979 = Ug79). Higher inputs of organic matter in ungrazed treatments led to higher amounts of OC in coarse aggregate size classes (ASC) and especially in particulate organic matter (POM) fractions across all depth. These processes started in the topsoil and took more than 5 years to reach deeper soil horizons (>10 cm). After 25 years of grazing cessation, subsoils showed clearly higher POM amounts. We found no grazing-induced changes of soil organic matter (SOM) quantity in fine ASC and particle size fractions. Current C-loading of fine particle size fractions was similar between differently grazed plots and decreased with depth, pointing towards free sequestration capacities in deeper horizons. Despite these free capacities, we found no increase in current C-loading on fine mineral soil fractions after 25 years of grazing exclusion. Silt and clay fractions appeared to be saturated. We suppose empirical estimations to overestimate sequestration potentials of particle size fractions or climatic conditions to delay the decomposition and incorporation of OM into these particle size fractions. POM quality was analysed using solid-state 13C NMR spectroscopy to clarify if grazing cessation changed chemical composition of POM in different ASC and soil depths via changing litter quality or changing decomposition dynamics. We found comparable POM compositions between different grazing intensities. POM is decomposed hierarchically from coarse to fine particles in all soil depths and grazing cessation has not affected the OM decomposition processes. The surplus of OM due to grazing cessation was predominately sequestered in readily decomposable POM fractions across all affected horizons. We question the long-term stabilisation of OM in these steppe soils during the first 25 years after grazing cessation and request more studies in the field of long-term OM stabilisation processes and assessment of carbon sequestration capacities to consider deeper soil horizons.

Chemical characterization and dynamics of particulate combined amino acids in Pacific surface waters

Particulate organic matter (POM) in surface water plays an essential role in maintaining marine ecosystem and is a complex mixture of living POM (organisms) and non-living POM (detritus). Transformation of organic constituents of organisms to detritus may be the most active part in the decomposition processes of organic matter in marine biogeochemistry. Distribution and chemical fractionation of particulate combined amino acids (PCAAs) in surface waters from Southern Ocean to tropical waters along a transect of 170°W were examined to clarify the transfer processes of organic constituents of organisms to detritus. The observational facts may be summarized as follows. 1) Degradation Index (DI) based on principal component analysis of amino acid composition of bulk PCAAs agreed with the traditional degradation indicators of bulk POM, i.e., PCAAs-carbon (C)/particulate organic carbon (POC), PCAAs-nitrogen (N)/particulate nitrogen (PN) and C/N. 2) PCAAs were distinguished into 3 fractions, i.e., Urea-soluble, Sodium Dodecyl Sulfate (SDS)-soluble and SDS/Urea-insoluble fractions, by means of solubility against reagents frequently used for protein extraction from biological samples. Proportions of PCAAs of the 3 fractions to bulk PCAAs were almost constant, although concentrations of the PCAAs of 3 fractions as well as bulk in seawater varied two orders of magnitude over the transect. 3) There was no significant difference among DI values of the 3 fractions, although molecular mass distributions of the Urea-soluble and SDS-soluble fractions determined by SDS-polyacrylamide gel electrophoresis were different. The results indicated that no one fraction was particularly susceptible to degradation and that there is a common mechanism for transferring and maintaining the similar chemical quality of bulk POM over the transect.

Temperature and seston quantity and quality effects on field reproduction of farmed oysters,Crassostrea gigas, in Bourgneuf Bay, France

The proliferation of the voluntarily-introduced cupped oyster, Crassostrea gigas, has attained the propor- tions of species invasion in many intertidal habitats in Europe, presumably resulting from successful reproduction of farmed individuals. It is thus imperative to better understand the reproductive characteristics of farmed oysters, since they are directly under human control. We quantified the dry tissue mass (DTM), gametosomatic index (GSI), and reproductive cycle of farmed oysters at two sites in Bourgneuf Bay, France, in relation to environmental parameters using continuously-recording probes in 2005 and 2006. The GSI was developed for this study, based on the actual area occupied by gametes, rather than the area of the gonad previously used for quantitative histological estimation of reproductive effort. The two sites, intermediate - (IT) and high-turbidity (HT), differed markedly in the amount and quality of particulate suspended matter, and also in fine-scale temperature variations. Oysters at both sites presented two spawning periods in both 2005 and 2006; Bourgneuf Bay is thus near the northernmost European limit for a 2- spawning cycle in Crassostrea gigas. Gonad maturation was initiated when spring water temperature reached 8-10 ◦ C, and gamete atresia occurred when water temperatures transiently dipped to 15-18 ◦ C. Spawns, which occurred above 18 ◦ C, were timed by fine-scale water temperature variations. Particulate organic matter quality peaks, coinciding with gonad maturation, were related to DTM variations before spawning periods, for the IT oysters in both years, and for the HT oysters in 2006. The reproductive effort (GSI) of oysters was similar at both sites; however, the fates of the gametes differed according to site. At the first spawning, the IT oyster gamete emissions were +1 month delayed, as were peak water temperatures greater than 18 ◦ C, and more pronounced, compared to the HT site. Although the second spawning showed high proportions of atretic oocytes at both sites in both years, the IT oysters evacuated twice as many gametes as the HT oysters in 2005. The IT conditions therefore appear more suited to Crassostrea gigas gamete evacuation than the HT conditions.

Seasonal and spatial distribution of particulate organic matter in the Bay of Bengal

In this paper, we have assessed the temporal, spatial and depth related variation of suspended particulate organic matter (POM) in the Bay of Bengal. For this purpose, suspended particulate matter (SPM) samples were collected from eight depths (2 to 1000 m) at 9 locations in the Bay of Bengal during July/August 2001 (Southwest monsoon, SWM), September/October 2002 (Fall Inter-monsoon, FIM), and March/April 2003 (Spring Inter-monsoon, SPIM). The SPM samples were analyzed for chlorophyll a (Chl a), particulate organic carbon (POC) and total particulate nitrogen (TPN). The surface concentrations of POC varied from 4.3 to 11.1 µM C, 3.1 to 10.9 µM C, and 4.3 to 9.0 µM C during SWM, FIM, and SPIM, respectively. The levels of SPM, and the concentrations of Chl a, and POC were relatively higher in the offshore compared to the near shore stations, especially during SWM. The POC concentration and its contribution to the SPM were higher in the surface waters, and both decreased with increasing water column depth at most of the stations. The observed decrease with depth of POC indicates heterotrophic uptake and/or dilution by inorganic material, poor in POC. This was also reflected in the C/N ratio which generally increased with water column depth. The relatively low C/N and POC/Chl a ratios, and high Chl a and % POC during the SWM and FIM indicate the presence of relatively fresh POM in the Bay during these two seasons. The observed seasonal differences in the quality of POM appears to be governed by river run-off and the physical forces such as eddies which pump nutrients into the surface waters thereby enhancing biological production.

Crayfish effects on fine particulate organic matter quality and quantity

Coarse particulate organic matter (CPOM), especially leaves, plays a key role in food webs of many streams and can be converted to fine particulate organic matter (FPOM) by specific macroinvertebrate functional groups (i.e., shredders). In this study, we examined crayfish [Orconectes obscurus (Hagen)] effects on FPOM generation from two leaf types, red maple and white oak, that differ in recalcitrance and decomposition rates. Further, we examined potential quality differences between FPOM generated by crayfish via fragmentation and defecation. Crayfish were fed stream-conditioned maple or oak leaves in hanging 1-mm mesh-bottom baskets in aquaria. After 12 h, C:N ratios and dry/ash-free dry weights of remaining CPOM, FPOM fragments that fell through the mesh, and crayfish feces (collected using finger cots that encased the crayfish abdomens) were determined. Loss of CPOM attributable to crayfish feeding was higher for maple than oak; fragment FPOM and crayfish feces generation were also higher for maple. Maple CPOM percent organic matter was lower than oak CPOM but feces and fragment FPOM percent organic matter did not differ among leaf species suggesting that crayfish actions homogenize the properties of particulate organic matter. Both C:N ratios and bacterial abundance were also altered by crayfish processing and digestion underscoring potential crayfish effects on FPOM bioavailability. Overall, crayfish altered the ontogeny of the detritus, which may, in turn, affect stream FPOM dynamics.

The Contribution of Crab Burrow Excavation to Carbon Availability in Surficial Salt-marsh Sediments

Geomorphology, vegetation and tidal fluxes are usually identified as the factors introducing variation in the flushing of particulate organic matter (POM) from tidal marshes to adjacent waters. Such variables may, however, be insufficient to explain export characteristics in marshes inhabited by ecosystem engineers that can alter the quantity and quality of POM on the marsh surface that is subject to tidal flushing. In this study we evaluated the balance between transfer of buried sedimentary organic carbon (C) to the marsh surface due to crab excavation (measured from the mounds of sediment excavated from burrows) and outputs of C from the surface due to sediment deposition within crab burrows (estimated from sediment deposited within PVC burrow mimics), in a Southwestern Atlantic salt marsh supporting dense (approximately 70 ind m−2) populations of the crab Chasmagnathus granulatus. C excavation by crabs was much greater than deposition of C within crab burrow mimics. Per area unit estimates of the balance between these two processes indicated that crabs excavated 5.98 g m−2 d−1 and 4.80 mg m−2 d−1 of total and readily (10 d) labile C, respectively. However, sediments excavated by crabs showed a significantly lower content of both total and readily-labile C than sediment collected in burrow mimics. This indicates that ecosystem engineering by burrowing crabs causes a net decrease in the concentration of C in the superficial sediment layers and, thus, an overall decrease in the amount of C that can be washed out of the marsh by tidal action. Incorporating the in situ activities of ecosystem engineers in models of marsh export should enhance understanding of the function of marshes in estuarine ecosystems.

The mollusc fauna along a depth transect in the Faroe Shetland Channel: Is there a relationship with internal waves?

From boxcore and dredge samples taken along a depth transect across the Faroe Shetland Channel (FSC) in 1997 and 1999, molluscs were sorted and identified. On the basis of their numerical abundance, the stations could be clustered into three groups, reflecting the depth extension of the main water masses in the FSC. Between 400 and 500 m depth, both the number of taxa per sample and the number of specimens, were highest. Bivalves were mainly caught by the boxcorer and the dredge was best suited to collect the epifaunal gastropods. According to the literature, at mid-slope depths, resuspension is increased by the action of internal waves, which leads to the development of intermediate nepheloid layers. This increased concentration of suspended matter may serve as a food source of low quality. Likewise the low amounts of suspended matter with a relatively high quality of particulate organic matter, such as from ∼400 and ∼700 m may act similarly and explain the relative high abundance of filter feeders at these depths. However, in the various multivariate analyses we could not distinguish the fauna from these depths from that collected from other sites. We observed, however, a pattern in the depth distribution of feeding guilds across the slope of the FSC. Deposit feeders demonstrated a trend of decreasing numbers with depth. Filter-feeding taxa were found to have peaks at depths of approximately 350 m, between 500 and 600 m and between 700 and 800 m. This observation supports the idea that the fauna at the deep sites reflects the locally high cross-slope currents and the subsequent increased amounts or availability of food with a better quality, which seems to be related to resuspension events.

Origin, composition and nutritional quality of particulate matter at deep-sea hydrothermal vents on Axial Volcano, NE Pacific

Deep-sea hydrothermal vent communities are supported by local microbial chemolitho- autotrophic production. While nutritional symbiotic associations between microbial primary producers and their metazoan hosts are well characterised, food sources used by the diverse and abundant non symbiont-containing vent species remain poorly known. Vent suspension- and deposit-feeders are usually considered as primary consumers directly relying on free-living microbial primary produc- tion, but other sources of particulate organic matter (POM) may also be part of their diet. We investi- gated the origin, composition and nutritional quality of POM at Axial Volcano (NE Pacific) vents, using microscopic observations, stable isotopic and biochemical analyses. A positive correlation between the stable carbon isotopic composition of POM and that of vent fluid dissolved inorganic car- bon (DIC) indicates that the bulk of vent particulate organic carbon is derived from the local chemolithoautotrophic fixation of vent DIC. Low estimates of bacterial and total microbial carbon might reflect a rapid turnover of free-living bacterial biomass in the vent ecosystem. The low micro- bial fraction, together with the presence of abundant debris, point to the existence of a large detrital fraction in the POM pool. This implies that the previously largely overlooked detrital compartment may be a significant part of the diet of consumers, and that organic matter recycling may be a major process in these ecosystems.

The influence of dissolved nutrients and particulate organic matter quality on microbial respiration and biomass in a forest stream

Summary1. Although dissolved nutrients and the quality of particulate organic matter (POM) influence microbial processes in aquatic systems, these factors have rarely been considered simultaneously. We manipulated dissolved nutrient concentrations and POM type in three contiguous reaches (reference, nitrogen, nitrogen + phosphorus) of a low nutrient, third‐order stream at Hubbard Brook Experimental Forest (U.S.A). In each reach we placed species of leaves (mean C : N of 68 and C : P of 2284) and wood (mean C : N of 721 and C : P of 60 654) that differed in elemental composition. We measured the respiration and biomass of microbes associated with this POM before and after nutrient addition.2. Before nutrient addition, microbial respiration rates and biomass were higher for leaves than for wood. Respiration rates of microbes associated with wood showed a larger response to increased dissolved nutrient concentrations than respiration rates of microbes associated with leaves, suggesting that the response of microbes to increased dissolved nutrients was influenced by the quality of their substrate.3. Overall, dissolved nutrients had strong positive effects on microbial respiration and fungal, but not bacterial, biomass, indicating that microbial respiration and fungi were nutrient limited. The concentration of nitrate in the enriched reaches was within the range of natural variation in forest streams, suggesting that natural variation in nitrate among forest streams influences carbon mineralisation and fungal biomass.

Chemical properties and soil organic matter assessment under fallow systems in the forest margins benchmark

Fallow management plays a key role in supplying nutrients to the growing crop, suppressing weeds, reducing pests and diseases, and improving the sustainability of cropping systems in the humid forest zone of West and Central Africa. We evaluated soil nutrient concentration, soil acidity and soil organic matter status under natural Chromolaena fallow, fallow without Chromolaena (where Chromolaena was removed by hand), and planted Pueraria fallow in three different sites with different soil physico-chemical properties in southern Cameroon. Human population pressure is high and fallow length is short in the northern site, while an intermediate position and the contrary were observed in the central and southern sites, respectively.The highest nutrient concentrations (Ca2+ and Mg2+), and effective cation exchange capacity (ECEC) were found in the sandy clayey soils in the northern site. The lowest Al saturation and soil acidity were noticed in the sandy soils in the central site, while in the clayey soils in the southern site the lowest nutrient concentration, ECEC, and the highest soil acidity and Al saturation were found. Pueraria decreased soil acidity and increased nutrient concentration relative to Chromolaena in the northern site. The beneficial effects of Pueraria on soil acidity and nutrient concentration became less pronounced in the central site and were absent in the southern site. In the northern and central sites, the highest maize yield was observed under Pueraria, showing the responsiveness of maize to N.In the soils of the southern site with the smaller weight (<1.50mg per g of soil) of particulate organic matter (POM) (53–4000μm) and greater chemical constraints (high soil acidity and Al saturation), higher in N content in medium (250–2000μm) and fine (53–250μm) POM was found under Pueraria compared to natural fallow. N content in medium POM under Pueraria was greater by 47% compared to the natural fallow. The N content in fine POM under Pueraria was greater by 59% relative to the natural fallow. In the southern site, chemical constraints induced very low maize yields under all fallow types. It appears that POM quality, i.e. N content, was more important for maize than POM quantity, i.e. weight, while groundnut and cassava were more sensitive to POM quantity.Pueraria have a beneficial ecological and environmental sustainability in the soil with low acidity, while Chromolaena seems to be more sustainable than Pueraria in the acidic soils. However, crop yields found in this study were low. Maize yield was lower than 2000kgha−1, groundnut grain dry matter yield was lower than 600kgha−1 and cassava was lower than 400 tuber dry matter kgha−1. Therefore, to attain, highest productivity levels it is necessary to combine fallow management (primarily planted such as Pueraria which can increase N content even where there are chemical constraints or soil degradation) with the use of fertilisers (organic and mineral).

Relationships between microbial extracellular enzymatic activity and suspended and sinking particulate organic matter: seasonal transformations in the North Water

Abstract Despite the importance of hydrolytic activities by bacterial extracellular enzymes (EE) in the temperate ocean, little is known about the role of extracellular enzymatic activity (EEA) in determining the fate of particulate organic matter (POM) in polar seas. To explore the issue further, we measured various chemical and bacterial parameters in the near-0°C waters of the North Water during the months of May and July of 1998. Seawater (SW) samples were collected by Niskin bottle at the depth of the chlorophyll fluorescence maximum (8–90 m), while samples of sinking particles and aggregates were collected in short-term (0.5–1.2 d), unpoisoned, floating sediment traps deployed at depths typically below the mixed layer (50–136 m). Samples were analyzed for POC, PON, and abundance of total and actively respiring bacteria. They were also incubated with fluorescently tagged substrate analogs to measure potential maximal rates of three classes of EE (leucine-aminopeptidase, chitobiase, and β -glucosidase) at –1°C. The percentage of actively respiring bacteria was always higher in sediment trap samples than in SW (medians of 38% and 24% versus 10% and 12% in May and July, respectively). Cell-specific rates of EEA were also higher in the trap samples and, for both sample types, similar to published rates from temperate waters. Rates of EEA when scaled to the abundance of actively respiring bacteria, however, did not differ between sample types, suggesting that the elevated EEA associated with sinking material is due to the greater abundance of metabolically active cells supported by such material and not due to enhanced enzyme expression in general, as suggested by previous studies. In this study, leucine-aminopeptidase activity was always much higher than the other classes of EEA, becoming even more dominant later in the season; it always correlated positively with the abundance of both total and actively respiring bacteria. Enzyme ratios indicating protease dominance corresponded with the seasonal increases in C/N ratios of both suspended and sinking POM. Overall the results suggest bacterial responses to seasonal changes in POM quality through differential expression of EE and an important role for proteases in influencing the nitrogen content of organic matter even at near-0° temperatures in this polynya.

Soil organic matter assessment in natural regrowth, Pueraria phaseoloides and Mucuna pruriens fallow

Biological and chemical components of soil fertility were quantified under three different fallow types and related to soil quality of an Ultisol in southern Cameroon at the end of a 9-month fallow. Soil organic matter (SOM), soil exchangeable Ca2+, Mg2+ and K+ and available P concentrations, effective cation exchange capacity (ECEC) and, soil acidity in the 0–10 and 10–20cm layers were evaluated under: natural regrowth mainly composed of Chromolaena odorata and the legume cover crops velvet bean (Mucuna pruriens var. utilis) and kudzu (Pueraria phaseoloides). SOM quality was assessed by C mineralisation during a 4-week incubation at 28°C in the laboratory. In addition, particulate organic matter (POM), the most active part of SOM, was fractionated by wet sieving into coarse (4000–2000μm), medium (2000–250μm) and fine (250–53μm) particle size classes and analysed for C and N contents. Under Mucuna, Ca2+, K+ and P concentrations, ECEC and soil pH were higher and C mineralisation was lower than under natural regrowth and Pueraria in 0–10cm depth. Soil under natural regrowth had a higher C mineralisation in 0–10cm indicating more labile SOM than in Pueraria and Mucuna fallow. There was no difference in weight of total POM, for any of the fractions between the three fallow types. However, both leguminous fallow species increased POM quality through a higher N content. Compared to natural regrowth, Pueraria increased N content in coarse POM by 36% in the 0–10cm layer and by 19% (coarse POM) and 35% (medium POM) in the 10–20cm layer. Mucuna increased N content in the 0–10cm layer by 12% (coarse POM), and by 19% (fine POM), compared to natural regrowth. According to the differences in nutrient concentrations, soil acidity and the biological stability of SOM, the three fallow types ranked: Mucuna≥Pueraria>natural regrowth. However, in terms of POM quality the ranking was: Pueraria>Mucuna>natural regrowth.

Seasonality of phytoplankton production in the Columbia River: A natural or anthropogenic pattern?

This study evaluated seasonal variability in the quantity and quality of particulate organic matter exported by the Columbia River into its estuary. Samples of suspended particulate material (SPM), collected monthly over a 1 year period (November 1995–October 1996) from a freshwater site in the Columbia River, near the head of its estuary, were analyzed for total aluminum, particulate organic carbon and nitrogen (POC and PN, respectively) and algal pigments (chlorophyll and carotenoids). High Al content, averaging 7.9% by weight throughout the year, indicated that detrital minerals accounted for the majority of SPM mass at all times. Organic matter (approximately 2 × POC) varied on a seasonal basis both in terms of its mass contribution to SPM (5–26% by weight) and its chemical composition. In winter, organic matter originated mainly from allochthonous sources, most likely from erosion of soils. Diatoms, the major phytoplankton group present as inferred from pigment analysis, “bloomed” in April–June 1996, resulting in increased autochthonous contribution to POC during spring and summer as well as increased organic matter content of SPM introduced to the estuary. Comparison with results for the same site in the mainstem Columbia obtained between 1990 and 1999 as part of a land-margin ecosystem research project shows that enhanced algal production in springtime is not a feature unique to our data set, at least in the Columbia River’s present highly regulated state of hydrographic operation. The possibility is discussed that recent human influence, principally dam construction, has significantly altered the historical SPM concentration and the chemical composition of its associated organic matter.

Relations between river flow, primary production and fatty acid composition of particulate organic matter in San Francisco and Chesapeake Bays: a multivariate approach

Estuarine environments influence particulate organic matter (POM) composition in the coastal zone as regions of high primary production and through their roles in the modification and exchange of organic materials. In this study, fatty acids (FA) associated with surface water particulate organic matter (POM) were collected seasonally and used to identify spatial and temporal variations in organic matter sources within two important estuaries in the USA–San Francisco Bay and Chesapeake Bay. Factor analysis was used to identify the dominant source signatures and the environmental processes important in controlling FA distributions within each estuary. In the San Francisco Bay dataset, Factor 1 explained 29% of the variance and had positive loadings for odd–numbered branched FA (C13, C15, C17 and C19), 16:1ω7 and long-chain FA (LCFA). Factor 1 was also negatively correlated with salinity and temperature,` suggesting that concentrations of these FA were controlled by seasonal fluctuations in freshwater inflow and the delivery of allochthonous materials. In contrast, loadings on Factor 2 (16% variance) in SFB were most positive for 16:0 and 18:0 and most negative for 20:5ω3, 14:0, 16:1ω7 and C16 polyunsaturated FA. These compounds are generally attributed to plankton sources. Factor 2 was inversely correlated with chlorophyll concentrations suggesting it was useful in resolving between FA derived from the spring (diatom) vs. non-spring plankton community. In Chesapeake Bay, Factor 1 explained 24% of the variance and was characterized by high positive loadings for 12:0 and C22 polyunsaturated FA and negative loadings for 16:1ω7 and n- and branched C13, C15 and C17 acids. Factor 1 was significantly correlated with salinity, suggesting it was useful for identifying regional differences in FA composition. Factor 2 (18% of the variance) was weighted positively for 18:1ω9 and negatively for 14:0 and 16:1ω7. This factor was inversely related to samples collected under bloom conditions (chl a>10 μg l−1) and chl a concentrations (r2=0.543; P <0.0001), indicating its usefulness for identifying temporal and regional differences in phytoplankton production and species composition. POM quality was, on average, higher in Chesapeake Bay vs. San Francisco Bay. Concentrations of total FA and polyunsaturated FA were higher in Chesapeake Bay, suggesting that POM is generally more labile and potentially more useful to heterotrophic organisms than in San Francisco Bay. Moreover, POM quality increased along the Chesapeake Bay salinity gradient. Overall, this study demonstrates that biomarker methods can provide insights useful in unraveling the complex factors that control the quality of POM in estuaries, particularly when coupled with environmental data (salinity, temperature and phytoplankton distributions) and multivariate statistical methods.

Enzymatic activities associated with decomposition of particulate organic matter in two shallow ponds

Particulate organic matter (POM) is an important source of energy in aquatic systems, where decomposition rates have traditionally been studied using the litter-bag technique. However, this method has limitations for estimating in situ decomposition rates, especially for fine particles. In this study the litter-bag technique is combined with measurements of microbial enzyme activities to estimate POM mass loss rates in situ. Litter-bags containing benthic POM in two size ranges, coarse (>1 mm) and fine (0.063–0.5 mm), were placed in two contrasting ponds (Toro and Oro) located in the Doñana area in southern Spain. Litter-bags were collected over a period of a year and analysed for mass loss and the activities of five extracellular enzymes (β-glucosidase, β-xylosidase, β- N-acetylglucosamidase, phenol oxidase, alkaline phosphatase). In situ sediment POM samples were also collected, sorted in the same size ranges, and assayed for the same enzyme activities. Using separate regression models relating mass loss to average cumulative enzyme activity, the mean turnover times for in situ coarse particulate organic matter (CPOM) and fine particulate organic matter (FPOM) were 149 and 2778 days in Toro pond and 382 and 4347 days in Oro pond, respectively. This compared with turnover times of 578 and 2109 days for Toro and 778 and 5296 days in Oro pond for POM confined in bags. Integrating enzyme measurements and litter-bag techniques provides in situ organic matter turnover rates and may allow the assessment of global controlling factors (e.g. flooding regime, temperature and POM quality) in the decomposition process in temporally and spatially heterogeneous environments.

Leaf pack breakdown and macroinvertebrate colonization: bioassessment tools for a high-altitude regulated system?

Leaf pack processing and invertebrate colonization for bioassessment of metals pollution in a stream also impacted by a dam were examined. Differences in leaf breakdown rates at sites upstream and downstream of a metals point source were not detected. However, multivariate and logistic regression techniques indicated there were differences in aquatic macroinvertebrate assemblages that colonized leaf packs above and below the metal inflow. Development of a logistic regression model allowed for prediction of leaf pack location from indicator taxa in the leaf pack invertebrate community. Collector-gatherers, rather than shredders, were the most important predictors of whether leaf packs were located above or below the metal inflow, and were perhaps related to algae growing on leaf packs and/or fine particulate organic matter quality. Biotic structure, in this study, provided more information for evaluating pollution impacts than did measurement of leaf breakdown rates.