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1.
In this paper we assemble and analyze quantitative annual input-export budgets for total nitrogen (TN) and total phosphorus (TP) for Chesapeake Bay and three of its tributary estuaries (Potomac, Patuxent, and Choptank rivers). The budgets include estimates of TN and TP sources (point, diffuse, and atmospheric), internal losses (burial in sediments, fisheries yields, and denitrification), storages in the water column and sediments, internal cycling rates (zooplankton excretion and net sediment-water flux), and net downstream exchange. Annual terrestrial and atmospheric inputs (average of 1985 and 1986 data) of TN and TP ranged from 4.3 g TN m?2 yr?1 to 29.3 g TN m?2 yr?1 and 0.32 g TP m?2 yr?1 to 2.42 g TP m?2 yr?1, respectively. These rates of TN and TP input represent 6-fold to 8-fold and 13-fold to 24-fold increases in loads to these systems since the precolonial period. A recent 11-yr record for the Susquehanna River indicates that annual loads of TN and TP have varied by about 2-fold and 4-fold, respectively. TN inputs increased and TP inputs decreased during the 11-yr period. The relative importance of nutrient sources varied among these estuaries: point sources of nutrients delivered about half the annual TN and TP load to the Patuxent and nearly 60% of TP inputs to the Choptank; diffuse sources contributed 60–70% of the TN and TP inputs to the mainstream Chesapeake and Potomac River. The direct deposition of atmospheric wet-fall to the surface waters of these estuaries represented 12% or less of annual TN and TP loads except in the Choptank River (37% of TN and 20% of TP). We found direct, although damped, relationships between annual rates of nutrient input, water-column and sediment nutrient stocks, and nutrient losses via burial in sediments and denitrification. Our budgets indicate that the annual mass balance of TN and TP is maintained by a net landward exchange of TP and, with one exception (Choptank River), a net seaward transport of TN. The budgets for all systems revealed that inorganic nutrients entering these estuaries from terrestrial and atmospheric sources are rapidly converted to particulate and organic forms. Discrepancies between our budgets and others in the literature were resolved by the inclusion of sediments derived from shoreline erosion. The greatest potential for errors in our budgets can be attributed to the absence of or uncertainties in estimates of atmospheric dry-fall, contributions of nutrients via groundwater, and the sedimentation rates used to calculate nutrient burial rates.  相似文献   

2.
We determined the monthly and annual riverine freshwater, nitrogen (N) and phosphorus (P) loading into the North Sea from Belgium, The Netherlands, and Germany for the years 1977–2000. An average of 133 km3 yr−1 of the 309 km3 yr−1 precipitation into the watershed is carried by the rivers into the sea. Total freshwater discharge fluctuates with a strong 6–7 yr periodicity, is strongly correlated with precipitation, and exhibits a slight long-term decrease. The temporal changes of regional patterns of precipitation lead to changing ratios of annual discharge of the western rivers compared to the eastern rivers, varying between 2.2 and 3.5. The long-term oscillations in discharge were more pronounced as discharge increased. The annual means of total and dissolved inorganic N and P loads were estimated to be 722 and 582 kt N yr−1 and 48 and 26 kt P yr−1, respectively. The monthly N loads were much more strongly correlated with discharge, compared to the monthly P loads. Total N and P as well as dissolved inorganic N also demonstrated a 6–7 yr periodicity. The annual N loads decreased by about 17 kt N yr−1 from 1977 to 2000. The total phosphorus and phosphate loads decreased from about 80 and 50 kt P yr−1 in the 1980s to 25 and 12 kt P yr−1, respectively, in the 1990s. The western rivers contributed the major part of the nutrient loads. The long-term oscillations in their nutrient loads were much more pronounced, compared to the eastern rivers. The area-specific loading rates estimated for all rivers are comparable to earlier estimates using shorter data records, smaller sample sizes, and a less complete watershed monitoring program. The monthly and annual average N:P ratios and their variability increased considerably for individual rivers during the study interval. These results confirm that the water quality of European continental rivers is strongly influenced by intense land use. They demonstrate the necessity for using long time series monitoring results to assess change and evaluate the effects of climate change on the North Sea coastal ecosystems, using ecosystem models on decadal time scales.  相似文献   

3.
In a large (8 ha) salt marsh restoration site, we tested the effects of excavating tidal creeks patterned after reference systems. Our purposes were to enhance understanding of tidal creek networks and to test the need to excavate creeks during salt marsh restoration. We compared geomorphic changes in areas with and without creek networks (n = 3; each area 1.3 ha) and monitored creek cross-sectional areas, creek lengths, vertical accretion, and marsh surface elevations for 5 yr that included multiple sedimentation events. We hypothesized that cells with creeks would develop different marsh surface and creek network characteristics (i.e., surface elevation change, sedimentation rate, creek cross-sectional area, length, and drainage density). Marsh surface vertical accretion averaged 1.3 cm yr−1 with large storm inputs, providing the opportunity to assess the response of the drainage network to extreme sedimentation rates. The constructed creeks initially filled due to high accretion rates but stabilized at cross-sectional areas matching, or on a trajectory toward, equilibrium values predicted by regional regression equations. Sedimentation on the marsh surface was greatest in low elevation areas and was not directly influenced by creeks. Time required for cross-sectional area stabilization ranged from 0 to > 5 yr, depending on creek order. First-order constructed creeks lengthened rapidly (mean rate of 1.3 m yr−1) in areas of low elevation and low vegetation cover. New (volunteer) creeks formed rapidly in cells without creeks in areas with low elevation, low vegetation cover, and high elevation gradient (mean rate of 6.2 m yr−1). After 5 yr, volunteer creeks were, at most, one-fourth the area of constructed creeks and had not yet reached the upper marsh plain. In just 4 yr, the site’s drainage density expanded from 0.018 to reference levels of 0.022 m m−2. Pools also formed on the marsh plain due to sediment resuspension associated with wind-driven waves. We conclude that excavated creeks jump-started the development of drainage density and creek and channel dimensions, and that the tidal prism became similar to those of the reference site in 4–5 yr.  相似文献   

4.
Spatial gradients of silver concentrations in the surface waters of San Francisco Bay reveal substantial anthropogenic perturbations of the biogeochemical cycle of the element throughout the estuarine system. The most pronounced perturbations are in the south bay, where dissolved (<0.45 μm) silver concentrations are as high as 250 pM. This is more than one order-of-magnitude above baseline concentrations in the northern reach of the estuary (6 pM) and approximately two orders-of-magnitude above natural concentrations in adjacent coastal waters (3 pM). The excess silver is primarily attributed to wastewater discharges of industrial silver to the estuary on the order of 20 kg d?1. The contamination is most evident in the south bay, where wastewater discharges of silver are on the order of 10 kg d?1 and natural freshwater discharges are relatively insignificant. The limited amount of freshwater flushing in the south bay was exacerbated by persistent drought conditions during the study period. This extended the hydraulic residence time in the south bay (≥160 d), and revealed the apparent seasonal benthic fluxes of silver from anthropogenically contaminated sediments. These were conservatively estimated to average ≈16 nmol m?2 d?1 in the south bay, which is sufficient to replace all of the dissolved silver in the south bay within 22 d. Benthic fluxes of silver throughout the estuary were estimated to average ≈11 nmol m?2 d?1, with an annual input of approximately 540 kg yr?1 of silver to the system. This dwarfs the annual fluvial input of silver during the study period (12 kg yr?1) and is equivalent to approximately 10% of the annual anthropogenic input of silver to the estuary (3,700–7,200 kg yr?1). It is further speculated that benthic fluxes of silver may be greater than or equal to waste water fluxes of silver during periods of intense diagenic remobilization. However, all inputs of dissolved silver to the estuary are efficiently sorbed by suspended particulates, as evidenced by the relatively constant conditional distribution coefficient for silver throughout the estuary (Kd≈105).  相似文献   

5.
The spatio-temporal variations in stable isotope signatures (??13C and ??15N) and C/N ratios of particulate organic matter (POM), and physicochemical parameters in a creek water column were examined in an estuarine mangrove ecosystem of Xuan Thuy National Park, Vietnam. The objective was to examine the factors influencing creek water properties, and the sources and exchange of POM in this important mangrove ecosystem. The diel and seasonal variations in water temperature, flow velocity, pH, dissolved oxygen, and salinity demonstrated that tidal level, season, and biological factors affected the creek water properties. Mangroves had relatively low ??15N and very low ??13C values, with respective average values of 1.5?±?0.9?? and ?28.1?±?1.4??. The low mangrove leaf ??15N indicated minor anthropogenic nitrogen loading to the mangrove forests. A significant positive correlation between POM?C??13C and salinity along the axis of Ba Lat Estuary, Red River, indicated that marine phytoplankton (??13C value, ?21.4?±?0.5??) was the predominant source of POM at the estuary mouth. Based on the co-variation of ??13C and C/N ratios, marine phytoplankton and mangrove detritus were predominant in POM of major creeks and small creeks, respectively. During the diurnal tidal cycle, the dynamics of POM were affected by sources of organic matter, tidal energy, and seasonal factors. The contribution of mangrove detritus to POM reached a maximum at the low tide and was enhanced during the rainy season, whereas marine phytoplankton contribution was highest at high tide.  相似文献   

6.
This paper summarizes evidence that most of the considerable nitrogen loading (～8, 470 mmol total N m?2 yr?1) to Boston Harbor (Massachusetts, USA) is expelled to shallow shelf waters of Massachusetts Bay, where it strongly influences ecological dynamics. Examination of nitrogen concentrations in the harbor, compared with loading, indicated that removal processes are active in the harbor. Comparison to other estuarine systems showed that the harbor’s nitrogen concentrations are consistent with its loading, if they are corrected for tidal flushing effects on the water residence time. Furthermore, extensive measurements of sediment denitrification confirmed that rates of N2 gas loss are high in an absolute sense (～600–800 mmol N m?2 yr?1) but nonetheless remove only a small portion (<10%) of the annual land-derived nitrogen loading. Burial in sediments apparently removes only about 2% of the N input, implying export to offshore environments as the major removal process (～88–90% of N input). Western Massachusetts Bay receiving waters were examined for a signature of export from the harbor. Data consistently show a gradient of decreasing nitrogen concentrations from the harbor to about 10–20 km into the bay. In many cases, plots of nitrogen concentrations versus salinity show nearly conservative mixing character, which implies virtual export. Seasonally, the data suggest most of the export from the harbor in winter is as dissolved inorganic forms (NH4 +, NO3 ?, NO2 ?). In summer, export is dominated by the outflow of organic nitrogen forms. Chlorophyll export is evident as well, suggesting that the nutritional coupling of the harbor and bay in summer involves organic fertilization of the bay’s surface water. Finally, high-resolution studies over different stages of the tidal cycle help refine understanding of the advection of chlorophyll and stimulation of in situ chlorophyll growth at the seaward edge of the tidal excursion into the bay.  相似文献   

7.
In Fayetteville Green Lake, past sedimentation rates can be accurately and precisely estimated by separating annual couplets or varves in dried sediment samples. Two measures were used, which serve as upper and lower limits on estimated sedimentation rate. They agree within 5 % with average annual sedimentation rate in couplets for recent years. Between 3 and 5 replicate samples are needed to reduce the half-width of 95 % confidence intervals on individual couplet sedimentation rates to 30 g m?2 yr?1 about 5 % of average recent rates. In the late 1800s sedimentation rate averaged 392 g m?2 yr?1 and ranged between 324 and 466 g m?2 yr?1, while in the 1970s the rate averaged 581 g m?2 yr?1 by the same measure, and ranged between 384 and 646 g m?2 yr?1. Sedimentation rate averaged for 13 years does not vary over short distances in the profundal zone, but lateral variation in sedimentation rate can be detected for individual years over the same distance. Not all this variation was associated with the non-uniform distribution of dark sublaminae and thin turbidites which cannot be separated from the annual layers. This indicates that although precise estimates of sedimentation rates can be made at different points in the lake, estimates will have to be made at numerous points before annual sedimentation rates for the lake as a whole can be accurately assessed.  相似文献   

8.
Calculations by others of the preindustrial deposition of inorganic nitrogen from the atmosphere in the area of Narragansett Bay compared with recent measurements suggest that this flux has increased almost 15 times over natural background. On the basis of modern studies of the export of nitrogen and phosphorus from temperate forests, the prehistoric watershed also probably contributed very little reactive N or P to the bay. New information from undisturbed old-growth forests suggests that most of the N that was exported from the watershed was probably associated with refractory dissolved organic matter and thus contributed little to the fertility of the bay. The largest source of reactive dissolved inorganic nitrogen (DIN) and phosphorus (DIP) for Narragansett Bay under prehistoric conditions was the coastal ocean water entrained in the bay in estuarine circulation. The total input of DIN to this estuary has increased about five-fold and the input of total DIP has approximately doubled as a result of human activities. Recent ecosystem-level experiments using large (13 m3, 5 m deep) mesocosms designed as living models of Narragansett Bay showed that the primary production of phytoplankton in the bay is limited by the supply of DIN and that annual phytoplankton production is strongly correlated with the rate of input of DIN. The relationship between DIN input and annual phytoplankton production in the mesocosms is consistent with observations published by others working in 10 different natural marine systems, and a functional regression of the field and experimental data provides a tool to calculate the rate of prehistoric phytoplankton production that would have been associated with the prehistoric DIN input estimates. The result of this calculation suggests that phytoplankton production in the bay has approximately doubled (from about 130 g C m?2 yr?1 to 290 g C m?2 yr?1 for a baywide average) since the time of European contact. It also seems likely that seagrasses and macroalgae once made a much larger contribution to total system production than they do today.  相似文献   

9.
The upper Klamath River experiences a cyanobacterial algal bloom and poor water quality during the summer. Diel chemical and isotopic techniques have been employed in order to investigate the rates of biogeochemical processes.Four diel measurements of field parameters (temperature, pH, dissolved oxygen concentrations, and alkalinity) and stable isotope compositions (dissolved oxygen–δ18O and dissolved inorganic carbon–δ13C) have been performed between June 2007 and August 2008. Significant diel variations of pH, dissolved oxygen (DO) concentration, and DO–δ18O were observed, due to varying rates of primary productivity vs. respiration vs. gas exchange with air. Diel cycles are generally similar to those previously observed in river systems, although there are also differences compared to previous studies. In large part, these different diel signatures are the result of the low turbulence of the upper Klamath River. Observed changes in the diel signatures vs. sampling date reflect the evolution of the status of the algal bloom over the course of the summer.Results indicate the potential utility of applying diel chemical and stable isotope techniques to investigate the rates of biogeochemical cycles in slow-moving rivers, lakes, and reservoirs, but also illustrate the increased complexity of stable isotope dynamics in these low-turbulence systems compared to well-mixed aquatic systems.  相似文献   

10.
We continuously measured dissolved silicate concentrations and fluxes discharged from various Rhode River subwatersheds for a period of 14 yr from 1984 to 1998 and for 15 mo in 1971–1972. We also measured dissolved silicate concentrations along a transect from the head of the tide in Rhode River estuary to Chesapeake Bay. The average concentration of dissolved silicate discharged from the Rhode River watershed was 10.8 mg Si l?1. There were consistent and significant differences in silicate concentrations discharged over time and space among subwatersheds. Mean annual silicate flux from the watershed was 26.6 kg Si ha?1 and 93% of this occurred during the winter and spring seasons. There were large interannual variations in silicate flux, due primarily to differences in precipitation and water discharge, rather than silicate concentration. Land use had little or no effect on silicate flux from various subwatersheds. Silicate concentrations discharged from a subset of subwatersheds in 1995–1996 were 25% to 35% lower than in a period with similar precipitation in 1971–1972. Mean annual concentrations of silicate discharged from nine subwatersheds have been declining about 1.5% yr?1 or by 0.21–0.26 mg Si l?1 yr?1 over the last 25 yr. Despite high average silicate fluxes from the watershed, at times the Rhode River estuary developed low dissolved silicate concentrations, which could have been limiting to the growth of diatoms. Examples were in the spring after a winter with low watershed discharge (as low as 0.019 mg Si l?1 in 1995) and after protracted drought (as low as 0.041 mg Si l?1 in 1993).  相似文献   

11.
Tropical peatlands of SE-Asia represent a significant terrestrial carbon reservoir of an estimated 65 Gt C. In this paper we present a comprehensive data synthesis of radiocarbon dated peat profiles and 31 basal dates of ombrogenous peat domes from the lowlands of Peninsular Malaysia, Sumatra and Borneo and integrate our peatland data with records of past sea-level and climate change in the region. Based on their developmental features three peat dome regions were distinguished: inland Central Kalimantan (Borneo), Kutai basin (Borneo) and coastal areas across the entire region. With the onset of the Holocene the first peat domes developed in Central Kalimantan as a response to rapid post-glacial sea-level rise over the Sunda Shelf and intensification of the Asian monsoon. Peat accumulation rates in Central Kalimantan strongly declined after 8500 cal BP in close relation to the lowering rate of the sea-level rise and possibly influenced by the regional impact of the 8.2 ka event. Peat growth in Central Kalimantan apparently ceased during the Late Holocene in association with amplified El Niño activity as exemplified by several truncated peat profiles. Peat domes from the Kutai basin are all younger than ～8300 cal BP. Peat formation and rates of peat accumulation were driven by accretion rates of the Mahakam River and seemingly independent of climate. Most coastal peat domes, the largest expanse of SE-Asian peatlands, initiated between 7000 and 4000 cal BP as a consequence of a Holocene maximum in regional rainfall and the stabilisation and subsequent regression of the sea-level. These boundary conditions induced the highest rates of peat accumulation of coastal peat domes. The Late Holocene sea-level regression led to extensive new land availability that allowed for continued coastal peat dome formation until the present. The time weighted mean Holocene peat accumulation rate is 0.54 mm yr?1 for Central Kalimantan, 1.89 mm yr?1 for Kutai and 1.77 mm yr?1 for coastal domes of Sumatra and Borneo. The mean Holocene carbon sequestration rates amount to 31.3 g C m?2 yr?1 for Central Kalimantan and 77.0 g C m?2 yr?1 for coastal sites, which makes coastal peat domes of south-east Asia the spatially most efficient terrestrial ecosystem in terms of long term carbon sequestration.  相似文献   

12.
In an attempt to characterize localized rates of sediment accretion, 10 sediment cores were collected from the lower reach of the Passaic River, a major tributary of Newark Bay, New Jersey. Sediments were assayed for 210Pb activity at predetermined depths and the rate of sediment accretion (cm yr?1) was estimated from the least squares regression of the log of unsupported activity versus depth. Sediment accretion rates, derived from 210Pb measurements (RPb) were used to predict the depth interval within the core containing sediments deposited around 1954; subsequent 137Cs analyses were focused on this depth interval. Sediment accretion rates derived from 137Cs measurements (RCs) were extrapolated from the depth of the 1954 horizon. Lead-210 derived sediment accretion rates in cores collected from a sediment bench extending along the inside bend on the southern shore of a meander in the river, ranged from 4.1 cm yr?1 to 10.2 cm yr?1 and averaged 6.8 cm yr?1. The RCs estimates for cores from this area ranged from 3.8 cm yr?1 to 8.9 cm yr?1 and averaged 6.6 cm yr?1. The RCs for cores collected in a more hydrologically dynamic reach of the river upstream of the sediment bench, were only 0.41 cm yr?1 and 0.66 cm yr?1. The results of this investigation indicate that this reach of the lower Passaic River is an area of high sediment accumulation, retaining much of the sediment load deposited from upstream and downstream sources. The rates of sediment accretion in the lower Passaic River are among the highest reported anywhere in the Newark Bay estuary.  相似文献   

13.
Planimetry studies of coastal geology maps prepared by the Maine Geological Survey show that there is more than an order of magnitude more tidal marsh area in the state of Maine than documented in previously published estimates. The highly convoluted coast of Maine, which is approximately 5,970 km long, contains almost 79 km2 of salt marsh, far more than any other New England state, New York, or the Bay of Fundy region. Reasonable estimates for the per-unit primary productivity of salt marshes lead to projections of total marsh productivity on the order of 1010 g dry weight yr?1 for the Maine coast and 1011 g dry weight yr?1 for the Gulf of Maine as a whole. Distribution of tidal marsh area is strongly controlled by coastal geomorphology, which varies considerably along the coast of Maine. The salt marsh area is concentrated in the southwestern coastal region of arcuate bays, where marshes have developed behind sandy beaches. A series of long islands and bedrock peninsulas in the south-central portion of the coast also provides sheltered areas where large marshes occur. Northeast of Penobscot Bay salt marshes become more numerous and smaller in average areal extent. A lack of protection from waves, along with limited sources of glacio-fluvial and glacio-marine sediments, restricts the occurrence of salt marshes in that region to the frignes of coves and tidal rivers.  相似文献   

14.
Scales of nutrient-limited phytoplankton productivity in Chesapeake Bay   总被引：1，自引：0，他引：1  
The scales on which phytoplankton biomass vary in response to variable nutrient inputs depend on the nutrient status of the plankton community and on the capacity of consumers to respond to increases in phytoplankton productivity. Overenrichment and associated declines in water quality occur when phytoplankton growth rate becomes nutrient-saturated, the production and consumption of phytoplankton biomass become uncoupled in time and space, and phytoplankton biomass becomes high and varies on scales longer than phytoplankton generation times. In Chesapeake Bay, phytoplankton growth rates appear to be limited by dissolved inorganic phosphorus (DIP) during spring when biomass reaches its annual maximum and by dissolved inorganic nitrogen (DIN) during summer when phytoplankton growth rates are highest. However, despite high inputs of DIN and dissolved silicate (DSi) relative to DIP (molar ratios of N∶P and Si∶P>100), seasonal accumulations of phytoplankton biomass within the salt-intruded-reach of the bay appear to be limited by riverine DIN supply while the magnitude of the spring diatom bloom is governed by DSi supply. Seasonal imbalances between biomass production and consumption lead to massive accumulations of phytoplankton biomass (often>1,000 mg Chl-a m?2) during spring, to spring-summer oxygen depletion (summer bottom water <20% saturation), and to exceptionally high levels of annual phytoplankton production (>400 g m?2 yr?1). Nitrogen-dependent seasonal accumulations of phytoplankton biomass and annual production occur as a consequence of differences in the rates and pathways of nitrogen and phosphorus cycling within the bay and underscore the importance of controlling nitrogen inputs to the mesohaline and lower reaches of the bay.  相似文献   

15.
A geophysical and geochemical study was carried out in the Maneadero aquifer, Baja California, Mexico, with the aim of identifying potential recharge locations for reclaimed water (RW). This coastal aquifer shows a significant decline in water quality, both as a result of salinization and the pollution by nitrates. Total dissolved solids (TDS) in an extreme case increased from 4 g l?1 in 2000 to 27 g l?1 in 2011. Nitrate as N–NO3, reaches 46 mg l?1. Based on their geochemistry and location, four water-quality zones are identified: (a) fresh water with TDS ≈ 1 g l?1 in the upper creeks, (b) mixture between seawater and freshwater in the coast-proximal sections, (c) water significantly enriched in nitrate below and adjacent to the town of Maneadero, and (d) brackish water with no signs of current interaction with freshwater. The 3D geophysics identifies the influence of modern recharge areas and also buried flow-paths down to at least 30 m depth. The locations best suitable for aquifer recharge are those with equal or higher TDS concentrations (>2.5 g l?1) than RW, which are located at the brackish water zone and/or at the coastal limits of the mixing zones.  相似文献   

16.
Estuaries are important subcomponents of the coastal ocean, but knowledge about the temporal and spatial variability of their carbonate chemistry, as well as their contribution to coastal and global carbon fluxes, are limited. In the present study, we measured the temporal and spatial variability of biogeochemical parameters in a saltmarsh estuary in Southern California, the San Dieguito Lagoon (SDL). We also estimated the flux of dissolved inorganic carbon (DIC) and total organic carbon (TOC) to the adjacent coastal ocean over diel and seasonal timescales. The combined net flux of DIC and TOC (FDIC?+?TOC) to the ocean during outgoing tides ranged from ??1.8±0.5?×?103 to 9.5±0.7?×?103?mol C h?1 during baseline conditions. Based on these fluxes, a rough estimate of the net annual export of DIC and TOC totaled 10±4?×?106?mol C year?1. Following a major rain event (36 mm rain in 3 days), FDIC?+?TOC increased and reached values as high as 29.0 ±?0.7?×?103?mol C h?1. Assuming a hypothetical scenario of three similar storm events in a year, our annual net flux estimate more than doubled to 25 ±?4?×?106?mol C year?1. These findings highlight the importance of assessing coastal carbon fluxes on different timescales and incorporating event scale variations in these assessments. Furthermore, for most of the observations elevated levels of total alkalinity (TA) and pH were observed at the estuary mouth relative to the coastal ocean. This suggests that SDL partly buffers against acidification of adjacent coastal surface waters, although the spatial extent of this buffering is likely small.  相似文献   

17.
Rates of CO2 emission from bare salt-marsh sediments in areas of short and tall formSpartina alterniflora were measured monthly for 1 yr. Maximum emission rates, as high as 325 ml CO2m?2h?1, were observed during summer months, while minimum rates, 10.2 ml CO2 m?2h?1, were observed during the winter. An exponential function of inverse soil temperature explained most of the seasonal variability, but other factors are involved in regulating CO2 emissions as demonstrated by rates that were higher in spring than in late summer at equivalent temperatures. Annual CO2 emissions from bare sediments were 27.3 and 18.6 mol C m?2 yr?1 in communities of short and tallS. alterniflora, respectively. It was estimated that losses of dissolved inorganic carbon from the turnover of pore water, up to 14.6 mol C m?2 yr?1 at the creek bank (tall,S. alterniflora) site, and diffusion of CO2 from the root system ofS. alterniflora through the culms, 12.3 to 16.2 mol C m?2 yr?1, could also be important pathways of carbon loss from marsh sediments. If the internal flux of CO2 from the root system through the culm is refixed within the leaves, then the observed rate of 9.8 μI CO2 min?1 cm?2 of culm cross sectional area appears to make a small but significant contribution to total photosynthesis.  相似文献   

18.
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19.
In an attempt to more fully understand the dissolved inorganic nitrogen dynamics of the Neuse River estuary, 15NH4 + and 15NO3 ? uptake rates were measured and daily depth-integrated rates calculated for seven stations distributed along the salinity gradient. Measurements were made at 2–3-wk intervals from March 1985 to February 1989. Significant dark NH4 + uptake occurred and varied both spatially and seasonally, accounting for as much as 95% of light uptake with the median being 33%. Apparent NH4 + uptake ranged from 0.001 μmol N 1?1 h?1 to 4.2 μmol N 1?1 h?1, with highest rates occurring during late summer-fall in the oligohaline estuary. Apparent NH4 + uptake was significantly related to NH4 + concentration (p<0.01); however, the regression explained <3% of the variation. Daily-integrated NH4 + uptake ranged from 0.1 mmol N m?2 d?1 to 133 mmol N m?2 d?1 and followed the trend of apparent uptake. Annual NH4 + uptake of the estuary was significantly lower in 1988 than for any other year. Dark uptake of NO3 ? was only 14% of maximum light uptake. Apparent NO3 ? uptake rates ranged from 0.001 μmol N 1?1 h?1 to 1.84 μmol N 1?1 h?1 with highest rates occurring in the oligohaline estuary. Apparent NO3 ? uptake was significantly related to NO3 ? concentration (p<0.01); however, the regression explained <5% of the variation. In general, NO3 ? uptake was only 20% of total dissolved inorganic nitrogen (DIN) uptake. Daily-integrated NO3 ? uptake ranged from 0.1 mmol N m?2 d?1 to 53 mmol N m?2 d?1 and followed similar patterns of apparent uptake. Annual NH4 + uptake was 11.39 mol N m?2 yr?1, 10.28 mol N m?2 Yr?1, 10.93 mol N m?2 yr?1, and 7.38 mol N m?2 yr?1, and 1.84 mol N m?2 yr?1, with the 4-yr mean being 10.0. Annual NO3 ? uptake was 3.12 mol N m?2 yr?1, 3.40 mol N m?2 yr?1, 1.96 mol N m?2 yr?1, and 1.84 mol N m?2 yr?1, with the 4-yr mean being 2.6. The total annual DIN uptake was more than twice published estimates of phytoplankton DIN demand, indicating that there is an important heterotrophic component of DIN uptake occurring in the water column. The extrapolation of nitrogen demand from primary productivity results in serious underestimates of estuarine nitrogen demand for the Neuse River estuary and may be true for other estuaries as well.  相似文献   

20.
One of the most critical problems facing many deltaic wetlands is a high rate of relative sea-level rise due to a combination of eustatic sea-level rise and local subsidence. Within the Rhône delta, the main source of mineral input to soil formation is from the river, due to the low tidal range and the presence of a continuous sea wall. We carried out field and modeling studies to assess the present environmental status and future conditions of the more stressed sites, i.e.,Salicornia-type marshes with a shallow, hypersaline groundwater. The impacts of management practices are considered by comparing impounded areas with riverine areas connected to the Rhône River. Analysis of vegetation transects showed differences between mean soil elevation ofArthrocnemum fruticosum (+31.2 cm),Arthrocnemum glaucum (+26.5 cm), bare soil (+16.2 cm), and permanently flooded soil (?12.4 cm). Aboveground and belowground production showed that root:shoot ratio forA. fruticosum andA. glaucum was 2.9 and 1.1, respectively, indicating more stressful environmental conditions forA. glaucum with a higher soil salinity and lack of soil drainage. The annual leaf litter production rate of the two species is 30 times higher than annual stem litter production, but with a higher long-term decomposition rate associated with leaves. We developed a wetland elevation model designed to predict the effect of increasing rates of sea-level rise on wetland elevation andSalicornia production. The model takes into account feedback mechanisms between soil elevation and river mineral input, and primary production. In marshes still connected to the river, mineral input decreased quickly when elevation was over 21 cm. Under current sea-level rise conditions, the annual amount of riverine mineral input needed to maintain the elevation of the study marshes is between 3,000 and 5,000 g m?2 yr?1. Simulations showed that under the Intergovernmental Panel on Climate Change best estimate sea-level rise scenario, a mineral input of 6,040 g m?2 yr?1 is needed to maintain marsh elevation. The medium term response capacity of the Rhône deltaic plain with rising sea level depends mainly on the possibility of supplying sediment from the river to the delta, even though the Rhône Delta front is wave dominated. Within coastal impounded marshes, isolated from the river, the sediment supply is very low (10 to 50 g m?2 yr?1), and an increase of sea-level rise would increase the flooding duration and dramatically reduce vegetation biomass. New wetland management options involving river input are discussed for a long-term sustainability of low coastal Mediterranean wetlands.  相似文献   

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