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1.
Seasonal changes in aboveground plant biomass, cover, and frequency were monitored in Sweet Hall Marsh, a tidal freshwater marsh located on the Pamunkey River, Virginia, during the 1974 growing season.Peltandra virginica accumulated the most biomass, 423.40 g per m2, followed byLeersia oryzoides at 67.75 g per m2. Annual net community production was estimated to be 775.74 g per m2 by using a multiple-harvest technique. Comparisons with other studies revealed that production was somewhat low for tidal freshwater marshes but mostly higher than production in Virginia brackish and saline wetlands. Measurements revealed an annual succession of plant species from spring to fall. The pattern observed was early dominance byPeltandra followed by a rise in importance ofPolygonum spp.,Impatients capensis andLeersia.  相似文献   

2.
This paper documents the role of salt marsh algal mats in the productivity of a southern California tidal wetland. The productivity of the mats, which are composed of filamentous bluegreen and green algae and diatoms, varies both temporally and spatially in relation to tidal inundation and overstory vegetation. The estimates of net primary productivity (NPP) were highest under the canopy ofJaumea carnosa (Less.) Gray (341 g C m?2 yr?1) at low elevation. Elsewhere, NPP appeared to be limited by low light (276 g C m?2 yr?1 underSpartina foliosa Trin.) and desiccation (185 g C m?2 yr?1 underBatis martima L. and 253 g C m?2 yr?1 underMonanthochloe littoralis Engelm). Algal NPP was from 0.8 to 1.4 times that of the vascular plant overstory NPP. It is hypothesized that the arid environment of southern California and resulting hypersaline soils reduce vascular plant cover, which leads to high algal productivity.  相似文献   

3.
Aboveground live standing crop of giant cutgrass (Zizaniopsis miliacea) populations in similar freshwater tidal and impounded nontidal marshes were almost identical (peaking at 1,039 g per m2 in each). The mortality, however, was greater in the tidal marsh resulting in significantly (95% level) greater annual production of aboveground cutgrass in the tidal (1,530±103 g per m2 per yr) than the impounded (1,172±88 g per m2 per yr) marsh, a 31% difference which we consider to be a measure of tidal subsidy. Belowground production also was found to average higher in the tidal marsh, but estimates were not as satisfactory as the aboveground results due to sampling difficulties. Combined annual above and belowground net production comes to an estimated 2,048 ±101 g per m2 per yr for the tidal and 1,481±219 for the impounded cutgrass marsh. The potential of freshwater tidal marshes for tertiary treatment of wastes is briefly discussed.  相似文献   

4.
We studied interactions between animal disturbance (geese, carp, and muskrat) and elevation in a field experiment in tidal freshwater marshes of the Patuxent River, Maryland, United States. Vegetation changes were recorded in fenced and unfenced plots in high and low marsh community types for 2 yr using measurements of areal cover and within-plot frequency (which were averaged to create a dominance index), Leaf Area Index (LAI), and aboveground biomass. We related light environment to differences in vegetation using below-canopy measurements of Photosynthetically Active Radiation (PAR). In the low marsh, total cover of all species, cover of annual species, biomass, and LAI were significantly higher in plots fenced to exclude animals (exclosures) than in unfenced plots (fenced/unfenced total cover=76/40%, annual cover=45/10%, biomass=936/352 g m?2, LAI=3.3/1.4). PAR was significantly lower in fenced than unfenced plots (fenced/unfenced=115/442 μmol s-1 m?2). Despite the strong effect of fencing on biomass, species richness per plot (i.e., the number of species per plot, or species density) was not affected significantly by fencing in the low marsh. Most of the observed differences in cover, biomass, LAI, and PAR were due to variation in the abundance of the herbaceous annual speciesBidens laevis (dominance index fenced/unfenced=45/10%) andZizania aquatica (30/12%). In the high marsh community, fencing had only minor effects on plant community composition and did not significantly affect species richness, cover, biomass, PAR, or LAI. Our results show that animals can dramatically affect low marsh vegetation, primarily via physical disturbance or herbivory of shallowly rooted seedlings of annual species.  相似文献   

5.
Responses ofSpartina alterniflora marsh to combinations of feral horse grazing, clipping, simulated trampling, and a late winter burn were studied on Cumberland Island National Seashore, Georgia. Replicated 200-m2 plots were established and sampled bimonthly from July 1983 to November 1984. Clipping and trampling each reduced peak aboveground biomass by 20% in 1983 and 50% (clipping) and 55% (trampling) in 1984. A March burn reduced peak aboveground biomass by 35% in 1984. Trampling and burning earch reduced net aboveground primary production (NAPP) by 35%, but clipping did not reduce NAPP. Standing stocks of live rhizomes were correlated with aboveground biomass and were reduced with experimental treatments. Abundance of the periwinkle snail (Littorina irrorata) was also reduced. Horse grazing had a substantial impact on standing stocks and NAPP ofSpartina, but grazing was not uniform throughout the marsh. Moderately grazed plots had NAPP reduced by 25% compared to ungrazed plots. Heavily grazed plots had extremely low NAPP, and abovegroundSpartina never exceeded 40 g m?2 dry mass compared to 360 g m?2 within exclosures.  相似文献   

6.
Net primary production was measured in three characteristic salt marshes of the Ebre delta: anArthrocnemum macrostachyum salt marsh,A. macrostachyum-Sarcocornia fruticosa mixed salt marsh andS. fruticosa salt marsh. Above-ground and belowground biomass were harvested every 3 mo for 1 yr. Surface litter was also collected from each plot. Aboveground biomass was estimated from an indirect non-destructive method, based on the relationship between standing biomass and height of the vegetation. Decomposition of aboveground and belowground components was studied by the disappearance of plant material from litter bags in theS. fruticosa plot. Net primary production (aboveground and belowground) was calculated using the Smalley method. Standing biomass, litter, and primary production increased as soil salinity decreased. The annual average total aboveground plus belowground biomass was 872 g m−2 in theA. macrostachyum marsh, 1,198 g m−2 in theA. macrostachyum-S. fruticosa mixed marsh, and 3,766 g m−2 in theS. fruticosa biomass (aboveground plus belowground) was 226, 445, and 1,094 g m−2, respectively. Total aboveground plus below-ground net primary production was 240, 1,172, and 1,531 g m−2 yr−1. There was an exponential loss of weight during decomposition. Woody stems and roots, the most recalcitrant material, had 70% and 83% of the original material remaining after one year. Only 20–22% of leafy stem weight remained after one year. When results from the Mediterranean are compared to other salt marshes dominated by shrubbyChenopodiaceae in Mediterranean-type climates, a number of similarities emerge. There are similar zonation patterns, with elevation and maximum aboveground biomass and primary production occurring in the middle marsh. This is probably because of stress produced by waterlogging in the low marsh and by hypersalinity in the upper marsh.  相似文献   

7.
Changes in biomass, growth form and shoot net primary production in an eelgrass, Zostera marina L., bed were monitored along transects at three tidal heights in Netarts Bay, Oregon, from May 1979 through June 1981. During the growing season, April through October, the mean plastochrone interval was 16.5 d along the low intertidal transect and 11.6 d along the high intertidal transect. The mean export interval was 13.3 d along the low intertidal transect and 11.6 d along the high intertidal transect. The life span of a leaf averaged 48 d along the low intertidal transect and 36 d along the high intertidal transect. Shoot density was positively correlated with mean leaf area index (LAI) until the LAI reached 3.8 to 5.5, above which LAI was negatively correlated with density. The maximum Zostera biomass ranged from 143 (high intertidal transect) to 463 (low intertidal transect) g dry wt m?2. Maximum values of shoot net production ranged from 4.7 (high intertidal transect) to 13.6 (low intertidal transect) g dry wt m?2d?1. Zostera shoot net production was related to light and to the physical damage to the shoots associated with a rapid accumulation of Enteromorpha biomass in the bay. In addition, patterns of biomass accumulation were related to the duration of water coverage, as determined by both tidal height and local impoundments of water. At all transects, biomass sloughed was equal to at least 50% of the shoot net primary production in that area during that time period; sloughed leaves accounted for 25 to 97% of these losses. An estimate of the total annual net primary production of aboveground Zostera in the bed was 17,500 kg, dry wt (SE=3,080 kg dry wt), which was equivalent to a mean annual rate of 383 g C m?2 (SE=67 g C m?2)  相似文献   

8.
The monthly variations of below- and aboveground biomass of Spartina alterniflora were documented for a south Louisiana salt marsh from March 2004 to March 2005, and in March 2006 and 2007. The annual production rate above- and belowground was 1821 and 11,676 g m?2, respectively (Smalley method), and the annual production rate per biomass belowground was 10.7 g dry weight?1, which are highs along the latitudinal distributions of the plant’s range. The average root + rhizome/shoot ratio (R&R/S) was 2.6:1, which is lower than the R&R/S ratios of 4 to 5.1 reported for Spartina sp. marshes in the northeastern US. The belowground biomass increased from July to September and fluctuated between October and November, after which it declined until February when the growing season began. The belowground biomass was dominated by rhizomes, which declined precipitously in spring and then rose to a seasonal high in the month before declining again as the late summer rise in inflorescence began. Over half of the root biomass in a 30-cm soil profile was in the upper 10 cm, and in the 10- to 20-cm profile for rhizomes. The maximum March biomass above- and belowground was four to five times that of the minimum biomass over the four sampling years. The net standing stock (NSS) of N and P in live biomass aboveground compared to that in the belowground biomass was about 1.7 times higher and equal, respectively, but the NSS of N and P for the live + dead biomass was about six times higher belowground. The average nitrogen/phosphorous molar ratios of 16:1 aboveground is in agreement with the often tested N limitation of biomass accumulation aboveground, whereas the 37:1 belowground ratio suggests that there is an influence of P on R&R foraging for P belowground. Some implications for management and restoration are, in part, that salt marshes should be evaluated and examined using information on the plant’s physiology and production both below- and aboveground.  相似文献   

9.
Fluxes of methane (CH4) and carbon dioxide (CO2) to the atmosphere at 52 sites within a salt marsh were measured by a dark static chamber technique from mid July to mid September. Mean CH4 fluxes ranged from 0.2 mg m?2 d?1 to 11.0 mg m?2 d?1, with an overall average of 1.6 mg m?2 d?1. Flux of CH4 was inversely correlated (r2=0.23, p = 0.001) with salinity of the upper porewater at the site, suggesting the dominant role of SO4 2? in inhibiting methanogenesis in salt-marsh sediments. The combination of salinity and water table position was able to explain only 29% of the variance in CH4 emission. Mean soil flux of CO2 ranged from 0.3 g m?2 d?1 to 3.7 g m?2 d?1, with an overall average of 2.5 g m?2 d?1; it was correlated with aboveground biomass (positive, r2=0.38, p = 0.001) and position of the water table (negative, r2 = 0.55, p = 0.001). The combination of biomass and water table position accounted for 63% of the variance in CO2 flux. There were high variations in gas flux within the six plant communities. The sequences were CH4: upland edge > panne > pool > middle marsh > low marsh > high marsh, and CO2: middle marsh > low marsh > upland edge > high marsh > panne > pool. Compared to other salt-marsh systems, this Bay of Fundy marsh emits small amounts of CH4 and CO2.  相似文献   

10.
Aquatic primary productivity, mangrove ecology, and fish community dynamics were investigated in the Teacapán-Agua Brava lagoon-estuarine system, the most extensive mangrove ecosystem on the Pacific coast of Mexico with three species of mangroves distributed heterogeneously (Laguncularia racemosa, Rhizophora mangle, andAvicennia germinans). Tree density was 3,203 trees ha?1 and basal area was 14.0 m2 ha?1. Litterfall was 1,417 g m?2 yr?1, characteristic of a productive riverine forest. The degradation constant forLaguncularia racemosa leaves varied from 1.71 to 4.7 yr?1 and mean annual net aquatic productivity was 0.41 g C m?3 d?1. There were high concentrations of humic substances (up to 150 mg l?1) early in the wet season. Seasonal variations of the above parameters seemed closely related to the ecology of fish populations. There were 75 fish species distributed in two principal assemblages associated with wet and dry seasons. Diversity and biomass analysis indicated 18 dominant species. Total biomass of the community in this coastal system was estimated at 10 g wet wt m?2. The highest biomass occurred in the wet season. The most common fish species wereMugil curema, Achirus mazatlanus, Galeichthys caerulescens, Arius liropus, Diapterus peruvianus, Lile stolifera, Centropomus robalito, andEucinostomus sp., all of which have fishery importance. Primary productivity and fish community ecology are controlled by habitat characteristics, river discharge, and climatic seasonality.  相似文献   

11.
Plants alter biomass allocation to optimize resource capture. Plant strategy for resource capture may have important implications in intertidal marshes, where soil nitrogen (N) levels and atmospheric carbon dioxide (CO2) are changing. We conducted a factorial manipulation of atmospheric CO2 (ambient and ambient?+?340?ppm) and soil N (ambient and ambient?+?25?g?m?2?year?1) in an intertidal marsh composed of common North Atlantic C3 and C4 species. Estimation of C3 stem turnover was used to adjust aboveground C3 productivity, and fine root productivity was partitioned into C3?CC4 functional groups by isotopic analysis. The results suggest that the plants follow resource capture theory. The C3 species increased aboveground productivity under the added N and elevated CO2 treatment (P?<?0.0001), but did not under either added N or elevated CO2 alone. C3 fine root production decreased with added N (P?<?0.0001), but fine roots increased under elevated CO2 (P?=?0.0481). The C4 species increased growth under high N availability both above- and belowground, but that stimulation was diminished under elevated CO2. The results suggest that the marsh vegetation allocates biomass according to resource capture at the individual plant level rather than for optimal ecosystem viability in regards to biomass influence over the processes that maintain soil surface elevation in equilibrium with sea level.  相似文献   

12.
Coastal marshes are known as organic matter producers. The goal of this work is to study tiller demography, standing biomass, and net aerial primary productivity (NAPP) in a Spartina densiflora coastal wetland, using a method applied to permanent sample plots located at two sites differing in topographic location, a regularly flooded zone [relative low marsh (LM)] and an irregularly flooded one [relative high marsh (HM)]. Measurements were made every 2 months during the 2005–2007 period. The annual NAPP was estimated to be 2,599?±?705 gDW m?2?year?1 for the HM and 2,181?±?605 gDW m?2?year?1 and 602?±?154 gDW m?2?year?1 for the first and second period of the LM populations, respectively, showing a seasonal pattern reaching maximum values in summer. The reduced NAPP values of the LM sites in the second year was associated with an extremely high precipitation period related to the 2007–2008 El Niño event.  相似文献   

13.
Much uncertainty exists in the phosphorus (P) cycle in the marshes of the intertidal zone. This study explored the P cycling in the two Suaeda salsa marshes [middle S. salsa marsh (MSM) and low S. salsa marsh (LSM)] of the Yellow River estuary during April 2008 to November 2009. Results showed seasonal fluctuations and vertical distributions of P in different S. salsa marsh soils, and variations in P content in different parts of plants due to water and salinity status. The N/P ratios of the different S. salsa were 9.87 ± 1.23 and 15.73 ± 1.77, respectively, indicating that plant growth in MSM was limited by N, while that in LSM was limited by both N and P. The S. salsa litter in MSM released P to the environment throughout the year, while that in LSM immobilized P from the environment at all times. The P absorption coefficients of S. salsa in MSM and LSM were very low (0.0010 and 0.0001, respectively), while the biological cycle coefficients were high (0.739 and 0.812, respectively). The P turnovers among compartments of MSM and LSM showed that the uptake amounts of roots were 0.4275 and 0.0469 g m?2 year?1 and the values of aboveground parts were 1.1702 and 0.1833 g m?2 year?1, the re-translocation quantities from aboveground parts to roots were 0.8544 and 0.1452 g m?2 year?1, the translocation amounts from roots to soil were 0.0137 and 0.0012 g m?2 year?1, the translocation quantities from aboveground living bodies to litter were 0.3157 and 0.0381 g m?2 year?1, and the annual return quantities from litter to soil were less than 0.0626 and ?0.0728 g m?2 year?1 (minus represented immobilization), respectively. P was an important limiting factor in S. salsa marshes, especially in LSM. S. salsa was seemingly well adapted to the low-nutrient condition and the vulnerable habitat, and the nutrient enrichment due to the import of N and P from the Yellow River estuary would be a potential threat to the S. salsa marshes.  相似文献   

14.
Net annual productivity of tall and medium form cordgrass,Spartina alterniflora, was estimated by a new clip sampling method in a sloping foreshore salt marsh at Wallops Island, Virginia. This method measured live standing crops only, to avoid problems of measuring dead biomass inherent in other methods. Losses from live standing crops by shoot mortality and by leaf shedding were estimated from these measurements and added separately to production of live tillers and of live culms. This allowed quantification of various components of production.Spartina tillering in different zones of the marsh produced 62 to 211 g dry weight per m2 per yr. Tiller mortality removed 37 to 106 g per m2 per yr from live standing crops. Culms produced 348 to 1,132 g per m2 before flowering and die-back. Culm mortality removed 28 to 246 g per m2 before flowering. Leaf shedding removed an additional 83 g per m2 in tall formSpartina. Altogether, net annual productivity These estimates are much higher than previous estimates of productivity and standing crops inSpartina marshes nearby.  相似文献   

15.
We report the first data on belowground tissue mass and nitrogen (N) concentration forSpartina foliosa in southern California, assessing one natural and two constructed marshes on San Diego Bay. Biomass at the natural marsh was low compared to that of otherSpartina spp., but higher than values reported forS. foliosa in northern California. In sandy constructed marshes planted 5 and 10 years before this study,S. foliosa had lower belowground tissue N, lower N crop (%N×biomass), and shallower roots than in the adjacent natural marsh. We took advantage of a 2-yr, large-scale fertilization project being performed in the older constructed marsh and examined biomass and N storage after N additions. Although there was a trend toward N accumulation with fertilization, N crop remained at approximately 50% of natural marsh levels, unlike the large aboveground responses to N addition in our previous studies. Lower belowground reserves help to explain poor aerial growth in the created marshes and suggest the need for finer sediments (with greater potential for holding and supplying nutrients) to sustain (S. foliosa. While fine sediments are beginning to accumulate on the surface of the created marshes, vertical accretion is more likely to shift the plant community toward other species than to enhanceS. foliosa growth. We suggest salvaging and importing fine, organic marsh sediments or providing organic amendments to establish proper substrate conditions. Overexcavating and allowing fine sediments to accumulate remains an option, although the time scale is unpredictable due to the stochasticity of accretion events.  相似文献   

16.
N2 fixation associated with the epiphytic community on standing dead Spartina alterniflora shoots was examined in both a natural and transplanted salt marsh in North Carolina. Acetylene reduction (AR) assays were conducted over a 24-mo period to estimate N2 fixation rates on standing dead stems and leaves. In the natural salt marsh, mean AR rates ranged from 0.5 nmol C2H4 cm?2 h?1 to 14 nmol C2H4 cm?2 h?1, while in the transplanted marsh mean AR rates ranged from 1 nmol C2H4 cm?2 h?1 to 33 nmol C2H4 cm?2 h?1. Diel AR activity of epiphytic communities in both marshes varied seasonally. Midday incubations yielded higher AR rates than nighttime incubations in the spring, while midday incubations in late summer and fall generally yielded AR rates equal to or lower than nighttime incubations. Desiccation during low tides occasionally repressed AR activity, although AR rates quickly rebounded with wetting. AR activity was localized in the epiphytic community, rather than in the underlying Spartina stem material. Based on the measured AR rates and the density of standing dead stems, the annual input of new N to the natural salt marsh via epiphytic N2 fixation is estimated to be 2.6 g N m?2 yr?1. The estimate of annual input of new N to the transplanted marsh is 3.8 g N m?2 yr?1. These estimates should be added to previous estimates of N2 fixation in marsh sediments to estimate the total contribution of new nitrogen to salt marsh nitrogen budgets.  相似文献   

17.
The distribution of fiddler crab (Uca spinicarpa andU. longisignalis) burrows in coastal marsh habitats is described, and the influence of substratum characteristics on the distributions of each is examined. Burrow densities were higher in vegetated areas than in sparsely vegetated and nonvegetated areas. Elevation and landward distance from shoreline (water's edge) also co-varied with burrow densities along transects. Burrow aperture diameters and burrow numbers m?2 exhibited seasonal variations for both species ofUca. The depth of burrows increased with increased distance from the water's edge (approximate mean sea level) along transects into the marsh. A significant correlation between burrow aperture diameter and carapace width of the inhabitant enabled subsequent estimates of size and biomass in populations. Mean burrow densities forU. longisignalis ranged from a high of 182 m?2 in September 1992 to a low of 26 m?2 in May–June 1992. Mean burrow densities forU. spinicarpa ranged from a high of 46 m?2 in March April 1993 to a low of 29 m?2 in May–June 1992. Estimates of mean biomass ranged from 17.6 g AFDW m?2 to 13.2 g AFDW m?2 for aU. longisignalis-dominated site and from 6.5 g AFDW m?2 to 4.9 g AFDW m?2 for aU. spinicarpa dominated site. Field observations and laboratory soil-choice tests demonstrated that, in mixed populations,U. spinicarpa preferred to burrow in substrates of higher percent clay than didU. longisignalis.  相似文献   

18.
Net annual primary production of a sedge Carex lyngbyei dominated tidal marsh in the Fraser River estuary, British Columbia, Canada was 634 g ash-free dry weight (AFDW) per m2 per yr (687 g dry weight per m2 per yr). Mean maximum shoot elongation during the short (May to August) growing season was 1.88 cm per day from overwintering shoots. The maximum aboveground standing crop of 690 g AFDW per m2 represented only 25% of the total below-ground biomass, which appears to be controlling most of the critical life history processes of the sedge marsh. An estimate of 14 percent of the aboveground standing crop was lost through leaching of dissolved organic carbon from the growting plant. Aboveground tissue losses, which were negligible during the growing season, occurred primarily via translocation in autumn and tidal export during the winter. In situ measurements showed that of the original maximum standing crop, approximately 38%, 37%, and 25% were lost by downward translocation, tidal export, and sediment burial, respectively. Based on changes in above and belowground nutrient pools, rapid spring (May to late June) uptake rates of 109 mg N per m2 per day and 23.0 mg P per m2 per day by shoots were followed by downward translocation rates of 44.8 mg N per m2 per day and 12.2 mg P per m2 per day during late June to the end of August. Aboveground leaching rates were estimated as 23.9 mg N per m2 per day and 7.8 mg P m2 per day and belowground uptake rates as 100 mg N per m2 per day and 26 mg P per m2 per day; root uptake occurred primarily after late June. Nutrient levels in decomposing litter more than doubled over the winter period showing a pattern of nutrient enrichment characteristic of marsh ecosystems. *** DIRECT SUPPORT *** A01BY023 00004  相似文献   

19.
The first advisory to limit consumption of Florida Bay fish due to mercury was issued in 1995. Studies done by others in the late 1990s found elevated water column concentrations of both total Hg (THg) and methylmercury (MeHg) in creeks discharging from the Everglades, which had its own recognized mercury problem. To investigate the significance of allochthonous MeHg discharging from the upstream freshwater Everglades, we collected surface water and sediment along two transects from 2000 to 2002. Concentrations of THg and MeHg, ranging from 0.36 ng THg/L to 5.98 ng THg/L and from <0.02 ng MeHg/L to 1.79 ng MeHg/L, were elevated in the mangrove transition zone when compared both to upstream canals and the open waters of Florida Bay. Sediment concentrations ranged from 5.8 ng THg/g to 145.6 ng THg/g and from 0.05 ng MeHg/g to 5.4 ng MeHg/g, with MeHg as a percentage of THg occasionally elevated in the open bay. Methylation assays indicated that sediments from Florida Bay have the potential to methylate Hg. Assessment of mass loading suggests that canals delivering stormwater from the northern Everglades are not as large a source as direct atmospheric deposition and in situ methylation, especially within the mangrove transition zone.  相似文献   

20.
The role of the major biogeochemical processes in Hg cycling at the sediment–water interface was investigated in the Grado Lagoon (Northern Adriatic Sea). This wetland system has been extensively contaminated from the Idrija Hg Mine (Slovenia) through the Isonzo River suspended load carried by tidal fluxes. Three approaches were used to study the sediment–water exchange of total Hg (THg), methylmercury (MeHg), reactive Hg (RHg) and dissolved gaseous Hg (DGHg): (1) estimation of diffusive fluxes from porewater and overlying water concentrations, (2) measurements of benthic fluxes using a deployed light benthic chamber in situ and (3) measurements of benthic fluxes during oxic–anoxic transition with a laboratory incubation experiment. The THg solid phase, ranging between 9.5 and 14.4 μg g−1, showed slight variability with depth and time. Conversely, MeHg contents were highest (up to 21.9 ng g−1) at the surface; they tended to decrease to nearly zero concentration with depth, thus suggesting that MeHg production and accumulation occur predominantly just below the sediment–water interface. Porewater MeHg concentrations (0.9–7.9 ng L−1, 0.15–15% of THg) varied seasonally; higher contents were observed in the warmer period. The MeHg diffusive fluxes (up to 17 ng m−2 day−1) were similar to those in the nearby Gulf of Trieste [Covelli, S., Horvat, M., Faganeli, J., Brambati, A., 1999. Porewater distribution and benthic flux of mercury and methylmercury in the Gulf of Trieste (Northern Adriatic Sea). Estuar. Coast. Shelf Sci. 48, 415–428], although the lagoon sediments contained four-fold higher THg concentrations. Conversely, the THg diffusive fluxes in the lagoon (up to 110 ng m−2 day−1) were one- to two-fold higher than those previously estimated for the Gulf of Trieste. The diurnal MeHg benthic fluxes were highest in summer at both sites (41,000 and 33,000 ng m−2 day−1 at the fishfarm and in the open lagoon, respectively), thus indicating the influence of temperature on microbial processes. The diurnal variations of dissolved THg and especially MeHg were positively correlated with O2 and inversely with DIC, suggesting an important influence of benthic photosynthetic activities on lagoon benthic Hg cycling, possibly through the production of organic matter promptly available for methylation. The results from the dark chamber incubated in the laboratory showed that the regeneration of dissolved THg was slightly affected by the oxic–anoxic transition. Conversely, the benthic flux of MeHg was up to 15-fold higher in sediments overlain by O2 depleted waters. In the anoxic phase, the MeHg fluxes proceeded in parallel with Fe fluxes and the methylated form reached approximately 100% of dissolved THg. The MeHg is mostly released into overlying water (mean recycling efficiency of 89%) until the occurrence of sulphide inhibition, due to scavenging of the available Hg substrate for methylation. The results suggest that sediments in the Grado Lagoon, especially during anoxic events, should be considered as a primary source of MeHg for the water column.  相似文献   

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