1,720,979 research outputs found
Bacterial nitrification activity directly associated with isolated benthic marine animals
No full textPotential nitrification rates (PNR) directly associated with isolated marine macrobenthic invertebrates were measured for a range of benthic epifaunal and infaunal species (bivalves, gastropods, polychaetes and crustaceans) collected from the Sacca di Goro, Po River delta, Italy. In the case of the filter-feeding bivalves,
Tapes philippinarum and Mytilus galloprovicialis the PNR associated with the shell surfaces and dissected animal tissues (gills, siphons and residual tissue) were determined separately, in order to assess the distribution of the nitrifier populations. Significant PNR was found associated with all the tested macrofaunal species with activities ranging between 12 and 2,250 nmol ind.)1 day)1 and specific activities between 150 and
18,400 nmol g)1 dry weight day)1. However, no simple relationships were observed between PNR and the animals taxonomic or functional group, or with animal comportment (infaunal or epifaunal) or size class, indicating that more complex interactions may regulate the degree of colonisation of the animals by nitrifier populations. Incubations of shells alone and dissected tissues of the bivalves T. philippinarum and M. galloprovicialis demonstrated that approximately 50% of the total PNR activity was associated with the shell surfaces and 50% with the internal animal tissues, with the highest specific activities of 950 and 1,970 nmol g)1 dry weight day)1 determined for the gills of T. philippinarum and M. galloprovicialis, respectively. Thus, specific relationships may exist between the nitrifiers and their animal hosts. Overall, our data indicate that the macrofaunal stimulation of nitrification and/or coupled nitrification–denitrification observed in previous studies may not be solely due to the animals burrow walls serving as sites for nitrification, but also to the fact that the internal and external surfaces of the animals themselves are also colonised by nitrifying bacteria. Tentative calculations based on reported animal densities in the Sacca di Goro and the determined PNRs indicate that animal-associated nitrifier populations could contribute significantly to overall nitrification rates in situ, although further experiments are required to determine to what extent the potential rates measured in this study are realised under in situ conditions
Denitrification and benthic metabolism in lowland pit lakes: The role of trophic conditions
No full textOver recent decades, a great number of pit lakes have been formed, as a result of sand and gravel quarrying in river floodplains that are often also heavily exploited for agriculture. These lakes can act as nutrient filters and regulate the nitrogen pollution resulting from agricultural fertiliser use. In this paper we report the main outcomes of a study of the major nitrogen pathways in five pit lakes of differing trophic status, located along a lowland stretch of the Po river (Northern Italy). Benthic nitrogen fluxes and denitrification rates were determined in the hypolimnion and denitrification and reactive nitrogen assimilation by microphytobenthos in the littoral zone. We tested the hypothesis that lake depth and trophic status can impair denitrification and/or reactive nitrogen assimilation, compromising the function of the lakes as nutrient filters. In the studied lakes, denitrification and reactive nitrogen assimilation by primary producer communities accounted for substantial nitrogen removal rates, which were among the highest reported in the literature. Benthic nitrogen fluxes and denitrification varied between and within lakes, with depth. The littoral zone and surface waters also supported primary production, favouring nitrogen assimilation and temporal retention in the primary producer biomass. In all lakes, denitrification rates decreased from littoral to hypolimnetic sites. Denitrification rates and net nitrogen assimilation also diminished from oligotrophic to eutrophic conditions. To some extent, in eutrophic lakes there was a transfer of primary production from the benthos to the water column and the benthic system became heterotrophic, reducing the capacity for net nitrogen removal. Overall these results highlight that floodplain pit lakes can provide ecosystem services formerly supplied by natural wetlands. An important factor for management is the development of extensive littoral and shallow water zones, which are critical for maximising the nitrogen removal
Impact of clam and mussel (Tapes philippinarum and Mytilus galloprovincialis) farming on benthic metabolism and nitrogen cycling, with emphasis on nitrate reduction pathways.
No full textThe influences of suspended mussel and infaunal clam cultivation on benthic metabolism and nutrient cycling were compared in Goro lagoon, Italy. Both aquaculture types stimulated benthic metabolism, with sediment oxygen demand (SOD), CO2 and ammonium effluxes of up to 14, 16 and 1.2 mmol m–2 h–1. However, whilst mussel farming preferentially stimulated anaerobic metabolism and sediment reduction, clam farming did not. The mussel ropes were also large oxygen sinks and ammonium sources, with oxygen consumption and ammonium production rates of 1.4 to 1.5 and 0.18 to 0.43 mmol kg–1 h–1. Consequently, the overall impacts of mussel farming on oxygen and nutrient dynamics were much greater than those of clam farming. There were also differences in nitrate-reduction processes and the nitrate sources that fuelled them. In winter, at high water column nitrate concentrations, highest nitrate reduction rates (~320 μmol m–2 h–1) occurred at the mussel farm. Nitrate reduction was driven predominantly by water column nitrate and ~30% of nitrate reduced was recycled to ammonium via dissimilatory nitrate reduction to ammonium (DNRA). At the control and clam farm sites, nitrate reduction rates were lower (~180 μmol m–2 h–1), nitrification supplied ~30% of nitrate and denitrification was dominant. In summer under low nitrate conditions, nitrate reduction was highest (~130 μmol m–2 h–1) at the mussel farm site, but this activity was completely dependent upon water column nitrate and 95% of nitrate was reduced via DNRA. In contrast, at the clam farm station, DNRA was unimportant and nitrification was the major nitrate source for denitrification. Consequently, whilst nitrate reduction processes eliminated fixed N from the clam farm sediments via coupled nitrificationdenitrification, the dominance of DNRA at the mussel farm site resulted in a net N input to the sediment compartment. These large differences in the impacts of clam and mussel farming can be explained by the fact that infaunal clams stimulate transfer of both organic matter and oxygen to the sediment, whereas suspended mussels enhance only organic matter inputs
Denitrification in a intertidal seagrass meadow, a comparison of 15N isotope and acetylene block technique: dissimilatory nitrate reduction to ammonia as a source of N2O?
No full textTests of the isotope-pairing technique over a range (15-200 μM) of 15N-nitrate additions to the water column of intact or defoliated cores of the seagrass Zostera noltii demonstrated that whilst overall rates of denitrification were highly dependent on the nitrate concentration in the water column, the calculated in situ rates, D14, DW and DN, were not significantly affected. Rates of coupled nitrification-denitrification determined following additions of 15N-ammonium to the sediment porewater were within the range of DN values determined by isotope pairing. Thus, this technique appears to accurately measure denitrification rates in these seagrass-colonised sediments, probably due to the effective limitation of coupled nitrification-denitrification to the surficial sediments, which ensures the homogenous mixing within the denitrification zone of the added 15N- nitrate tracer and the 14N-nitrate generated by nitrification in the sediment. Denitrification rates of nitrate diffusing from the water column were compared using the isotopepairing and acetylene-block techniques. Whilst rates were similar during dark incubations, during light incubations, rates of N2O accumulation during acetylene blocking were consistently twofold greater than denitrification rates measured by isotope pairing. We propose that this excess N2O could have been generated during dissimilatory nitrate reduction to ammonium by fermentative and or sulphate-reducing bacteria
Iron, sulphur and phosphorus cycling in the rhizosphere sediments od a eutrophic Ruppia cirrhosa meadow (valle Smarlacca, Italy)
No full textDenitrification, nitrogen fixation, community primary productivity and inorganic-N and oxygen fluxes in an inter-tidal Zostera noltii meadow
No full textDenitrification in an-intertidal meadow, a comparison of 15N-isotope and acetylene block techniques: dissimilatory nitrate reduction to ammonia as a source of N2O?
No full textImplications for oxygen, nutrient fluxes and denitrification rates during the early stage of sediment colonisation by the polychaete Nereis spp
No full textBiogeochemical indicators as tools for assessing sediment quality/vulnerability in transitional aquatic ecosystems
No full textWe review the major variables that can be used to monitor sediment characteristics and benthic processes in transitional water bodies with respect to ecological significance, cost/time demands, method consensus, and uncertainty of the current techniques. 2. Most of the state variables, namely organic matter, total nitrogen and phosphorus contents, are easily determined at low monetary/time cost using standardized techniques. However, they are not sufficiently informative to be used as individual-specific indicators. 3. The speciation analysis of nitrogen, phosphorus, iron and reduced sulphur provides much more precise information on the sedimentary buffering capacity. However, analytical techniques are cost/ time expensive and often present some critical step, which biases their generalized application. 4. A good understanding of sedimentary processes can also be achieved by measuring benthic fluxes and process rates, but analytical techniques are too expensive to be applied in conventional monitoring programmes. 5. A tentative integrated index, based on a few low-cost simple measures of sediment characteristics (granulometry, organic matter, carbonates, reactive iron and acid volatile sulphides) is proposed, which, in parallel with the water retention time, could provide a rapid assessment of sediment vulnerability status
Denitrification, nitrogen fixation, community primary productivity and inorganic-N and oxygen fluxes in an intertidal Zostera noltii meadow
No full textRates of denitrification, N-fixation, gross community primary productivity, inorganic-N and oxygen fluxes were determined in February, May and October 1997 in an intertidal Zostera noltii meadow of the Bassin d’Arcachon, French Atlantic coast. Rates of gross community primary productivity were high, 0.09 to 0.40 g C m–2 h–1; high P:R ratios of 1.64 to 2.82 define the system as highly autotrophic and indicate significant losses of carbon via export and/or burial of biomass. Fluxes of DIN, nitrate and ammonium were large (–0.8 to –2.4, –0.1 to –2.2 and –0.1 to –0.7 mmol N m–2 h–1, respectively) and always directed towards the plants/sediment during both light and dark incubations. The contributions of nitrate, nitrite and ammonium to total DIN fluxes reflected their relative abundance in the water column, indicating that there was no assimilatory selection of inorganic-N sources by the plants. The DIN fluxes were dominated by the N-assimilation activity of the plants even during dark incubations, as removal of the plant shoots prior to incubations essentially abolished nitrate fluxes and reversed ammonium fluxes, resulting in substantial effluxes. Thus, inorganic-N fluxes were controlled principally by the Z. noltii and epiphyte biomasses and their primary productivity, rather than the water column concentrations of DIN. Surprisingly, the plant community showed a high dark assimilation activity for inorganic-N, and differences in light and dark fluxes of DIN, nitrate and ammonium were never significant. Data indicate that, whilst DIN fluxes could supply the N-demand of primary production in spring, the plants became increasingly dependent upon sediment N-pools, N-fixation and internal N-reserves through summer into autumn. Denitrification rates determined by the 15N-isotope pairing technique were extremely low, ranging between 2 and 6 μmol N m–2 h–1. Rates of denitrification of nitrate diffusing from the overlying water were consistently below 2 μmol N m–2 h–1 during both light and dark incubations and represented only 0.1 to 0.7% and 0.2 to 1.3% of the total light and dark nitrate fluxes, respectively. Similarly, rates of denitrification coupled to nitrification were consistently low, probably due to the competition between nitrifying bacteria and the Z. noltii roots for ammonium. N-fixation rates varied between 4 and 17 μmol N m–2 h–1 and were substantially greater than N-losses via denitrification in all seasons, with net N2 inputs ranging between 2.5 and 14.6 μmol N m–2 h–1 and 0.5 and 3.8 μmol N m–2 h–1, during light and dark incubations. Overall, our data demonstrate that the Z. noltii meadows represent a highly conservative environment for nitrogen, where the N cycle is dominated by the primary productivity of the plant community and the associated assimilatory demand for fixed-N to support this productivity. Conversely, N-losses via denitrification are extremely low and are more than balanced by N-inputs from N-fixation. Thus, in this macro-tidal lagoon, export of nitrogen as plant biomass and/or N-burial in the sediments are probably the major loss mechanisms for anthropogenic N-inputs
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