Productivity,detritus flux,and nutrient cycling in a Carex lyngbyei tidal marsh

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     

Comparison of giant cutgrass productivity in tidal and impounded marshes with special reference to tidal subsidy and waste assimilation

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.     

Nutrient dynamics in the Pomeranian Bay (southern Baltic): Impact of the Oder River outflow

The Pomeranian Bay is a coastal region fed by the Oder River, one of the seven largest Baltic rivers, whose waters flow through a large and complex estuarine system before entering the bay. Nutrients (NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, Ntot, PO₄ ³⁻, P_tot, DSi), chlorophylla concentrations, oxygen content, salinity, and temperature were measured in the Pomeranian Bay in nine seasonally distributed cruises during 1993–1997. Strong spatial and temporal patterns were observed and they were governed by: the seasonally variable riverine water-nutrient discharges, the seasonally variable uptake of nutrients and their cycling in the river estuary and the Bay, the character of water exchange between the Pomeranian Bay and the Szczecin Lagoon, and the water flow patterns in the Bay that are dominated by wind-driven circulation. Easterly winds resulted in water and nutrient transport along the German coastline, while westerly winds confined the nutrient rich riverine waters to the Polish coast and transported them eastward beyond the study area. Two water masses, coastal and open, characterized by different chemical and physical parameters and chla content were found in the Bay independently of the season. The role of the Oder estuary in nutrient transformation, as well as the role of temperature in transformation processes is stressed in the paper. The DIN:DIP:DSi ratio indicated that phosphorus most probably played a limiting role in phytoplankton production in the Bay in spring, while nitrogen did the same in summer. During the spring bloom, predominated by diatoms, the DSi:DIN ratio dropped to 0.1 in the coastal waters and to 0.6 in the open bay waters, pointing to silicon limitation of diatom growth, similar to what is being observed in other Baltic regions.     

Annual phytoplankton metabolism in Narragansett Bay calculated from survey field measurements and microcosm observations

Field surveys of phytoplankton metabolism, based on oxygen changes, were made in Narragansett Bay from 1971–73. Annual daytime net production varied from 218 g C per m² per yr in the East Passage to 429 g C per m² per yr in the Providence River. The area based average for the bay was 269 g C per m² per yr. The area based average night respiration was 159 g C per m² per yr resulting in an annual net carbon available for export or to the benthos of 110 g C per m² per yr. A set of microcosms, operated so as to simulate the Bay, had an annual net production of 276 g C per m² per yr and a night respiration of 163 g C per m² per yr resulting in an annual net carbon available for export or to the benthos of 113 g C per m² per yr. *** DIRECT SUPPORT *** A01BY015 00002     

Nitrous Oxide Emissions from Intertidal Zone of the Yellow River Estuary in Autumn and Winter During 2011–2012

Much uncertainty exists in spatial and temporal variations of nitrous oxide (N₂O) emissions from coastal marshes in temperate regions. To investigate the spatial and temporal variations of N₂O fluxes and determine the environmental factors influencing N₂O fluxes across the coastal marsh dominated by Suaeda salsa in the Yellow River estuary, China, in situ measurements were conducted in high marsh (HM), middle marsh (MM), low marsh (LM), and mudflat (MF) in autumn and winter during 2011–2012. Results showed that mean N₂O fluxes and cumulative N₂O emission indicated intertidal zone of the examined marshes as N₂O sources over all sampling seasons with range of 0.0051 to 0.0152 mg N₂O m^?2 h^?1 and 7.58 to 22.02 mg N₂O m^?2, respectively. During all times of day and the seasons measured, N₂O fluxes from the intertidal zone ranged from ?0.0004 to 0.0644 mg N₂O m^?2 h^?1. The freeze/thaw cycles in sediments during early winter (frequent short-term cycle) and midwinter (long-term cycle) were one of main factors affecting the temporal variations of N₂O emission. The spatial variations of N₂O fluxes in autumn were mainly dependent on tidal fluctuation and plant composition. The ammonia-nitrogen (NH₄ ⁺–N) in sediments of MF significantly affected N₂O emissions (p < 0.05), and the high concentrations of Fe in sediments might affect the spatial variation of N₂O fluxes. This study highlighted the large spatial variation of N₂O fluxes across the coastal marsh (coefficient of variation (CV) = 127.86 %) and the temporal variation of N₂O fluxes during 2011–2012 (CV = 137.29 %). Presently, the exogenous C and N loadings of the Yellow River estuary are increasing due to human activities; thus, the potential effects of exogenous C and N loadings on N₂O emissions during early winter should be paid more attention as the N₂O inventory is assessed precisely.     

Net aboveground primary production and soil properties of floating and attached freshwater tidal marshes in the Río de la Plata estuary,Argentina

In the lower delta of the Paraná River, at the head of the Río de la Plata estuary (Argentina), we compared net aboveground primary production (NAPP) and soil properties of the dominant macrophyteScirpus giganteus (Kunth) in a floating and an attached marsh community. Both marshes are tidally influenced but in different ways. The floating marsh site is relatively isolated from tidal influences because its ability to float makes it resistant to overland flow and to sediment inputs from the estuary. The attached marsh lacks the capacity to float and receives sediment supplies from the estuary through overland flow. These hydrologic differences are reflected in lower mineral content in sediments of the floating marsh. Using a leaf tagging technique, estimated NAPP was 1,109 ± 206 g m⁻² yr⁻¹ for the floating marsh and 1,866 ±258 g m⁻² yr⁻¹ for the attached marsh. We attribute the lower NAPP of the floating marsh to isolation from sediment input from overland flow.     

Nutrient transport in a riverine-influenced,tidal freshwater bayou in Louisiana

Transport of ammonium (NH₄ ⁺), nitrate + nitrite (NO₃ ^?), total Kjeldahl nitrogen (TKN), soluble reactive phosphate (SRP), and total suspended solids (TSS) was measured in a freshwater tidal bayou located in a marsh system near the mouth of the Atchafalaya River in Louisiana. Sampling was conducted six times over one year and was timed to assess effects of seasonal variation in river flow and mean sea level of the Gulf of Mexico on material fluxes. Net fluxes of all materials were large and ebb directed in all seasons except fall, when net transport was 2 to 3 orders-of-magnitude smaller than in any other season. These results demonstrate that riverine forcing was the primary influence on materials transport in all seasons except fall when tidal forcing was most important. The range of net fluxes (g s^?1) for each nutrient was as follows (a negative sign indicates a net export toward the Gulf): NO₃ ^?, ?0.006 to ?6.69; TKN, 0.09 to ?10.41; NH₄ ⁺, ?0.02 to ?1.36; SRP, ?0.001 to ?0.53; TSS, ?2 to ?81. Analysis of nutrient concentrations indicated the marsh/aquatic system removed NO₃ ^?, SRP, and TSS from the water column from late spring through early fall and released NH₄ ⁺ and TKN in summer. The results of this study show that net materials export per unit cross section channel area increased as riverine influence increased.     

Sediment-water oxygen and nutrient fluxes in a river-dominated estuary

Sediment oxygen uptake and net sediment-water fluxes of dissolved inorganic and organic nitrogen and phosphorus were measured at two sites in Fourleague Bay, Louisiana, from August 1981, through May 1982. This estuary is an extension of Atchafalaya Bay which receives high discharge and nutrient loading from the Atchafalaya River. Sediment O₂ uptake averaged 49 mg m^?2 h^?1. On the average, ammonium (NH₄ ⁺) was released from the sediments (mean flux =+129 μmol m^?2 h^?1), and NO₃ ^? was taken up (mean flux =?19 μmol m^?2h^?1). However, very different NO₃ ^? fluxes were observed at the two sites, with sediment uptake at the upper, river-influenced, high NO₃ ^? site (mean flux =?112 μmol m^?2 h^?1) and release at the lower, marine-influenced low NO₃ ^? site (mean flux =+79 μmol m^?2 h^?1). PO₄ ^3? fluxes were low and often negative (mean flux =?8 μmol m^?2 h^?1), while dissolved organic phosphorus fluxes were high and positive (mean flux =+124 μmol m^?2 h^?1). Dissolved organic nitrogen fluxes varied greatly, ranging from a mean of +305 μmol m^?2 h^?1 at the lower bay, to ?710 μmol m^?2 h^?1 at the upper bay. Total dissolved nitrogen and phosphorus fluxes indicated the sediments were a nitrogen (mean flux =+543 μmol m^?2 h^?1) and phosphorus source (mean flux =+30 μmol m^?2 h^?1) at the lower bay, and a nitrogen sink (mean flux =?553 μmol m^?2 h^?1) and phosphorus source (mean flux =+17 μmol m^?2 h^?1) in the upper bay. Mean annual O∶N ration of the positive inorganic sediment fluxes were 27∶1 at the upper bay and 18∶1 at the lower bay. Based on these data we hypothesize that nitrification and denitrification are important sediment processes in the upper bay. We further hypothesize that Atchafalaya River discharge affects sediment-water fluxes through seasonally high nutrient loading which leads to net nutrient uptake by sediments in the upper bay and release in the lower bay, where there is less river influnces.     

Patterns in seasonal abundance,growth and biomass of the atlantic silverside,Menidia menidia,in a New England estuary

Patterns in seasonal abundance (no. per m² surface area), growth and biomass (g per m² surface area) of an annual fish, the Atlantic silverside, Menidia menidia (L.) were investigated in a marsh and more seaward bay region of Essex Bay, Massachusetts from August 1976 to May 1978 using a quantitative beach seining technique. Silverside abundance varied greatly by season and year class during the study period. Abundance was high in 1976 but winter mortality (99%) left an adult density of only .01 per m² surface area in the marsh during spring 1977. Resultant 1977 year class density in the marsh was 1.88 per m² by late fall 1977 but winter mortality again produced an adult density of .01 per m² in spring 1978. Abundance was generally higher in the marsh than in the bay region especially during spring and late fall when catches in the bay were negligible. Based on catch rate comparisons, the summer and fall juvenile abundance of the 1976 year class was much higher than the juvenile abundance of the 1977 year class. Coincidentally, mean lengths and condition of the abundant 1976 year class in the late fall were significantly lower than those of the 1977 year class, suggesting density dependent population regulation. In both years, juveniles grew rapidly and reached full adult size by November when an offshore movement to deeper waters outside Essex Bay occurred. Biomass peaked in the marsh region in late fall 1977 at 7.8 g per m² wet weight. Winter mortality was size selective, favoring larger individuals. The annual life history design of M. menidia including an offshore winter movement and high winter mortality suggests that silversides represent an important pathway of energy flow from marsh to offshore trophic systems.     

The effect of chronic oil pollution on salt-marsh nitrogen fixation (acetylene reduction)

Annual acetylene reduction rates associated with interidal communities in a chronically oil polluted Virginia salt marsh were compared to rates measured in an undisturbed marsh. Chronic oil treatment resulted in visible damage to the higher plants of theSpartina alterniflora zones; however, vegetation-associated acetylene, reduction was not different from the untreated control. Sediment rates generally were affected little by oil application, except during the summer when rates in the median tidal elevation zones were considerably higher than those of the control. Acetylene reduction occurred in all transects, each of which extended from upper mudflat to theSpartina patens zone. Intertidal sediment acetylene reduction was patchy, both spatially and seasonally. Estimated rates were greatest near the surface; free-living bacterial N₂ fixation activity averaged 2.23 mg N per m² per d (range=undetectable to 365 mg N per m² per d) in the untreated and 3.17 mg N per m² per d (range=undetectable to 564 mg N per m² per d) in the oil-treated marsh during the year. Vegetation-associated N₂ fixation activity yielded highest overall mean rates (156 mg N per m² per d). The seasonal pattern of sediment and vegetation-associated fixation may be controlled by temperature and availability of oxidizable substrates.     

Productivity of cordgrass,Spartina alterniflora,estimated from live standing crops,mortality, and leaf shedding in a Virginia salt marsh

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.     

Fortnightly Tidal Modulations Affect Net Community Production in a Mesotidal Estuary

Optical in situ chemical sensors enable sampling intervals and durations that rival acoustic techniques used for measuring currents. Coupling these high-frequency biogeochemical and physical measurements in estuaries to address ecosystem-scale questions, however, is still comparatively novel. This study investigated how tides affect ecosystem metabolism in a mesotidal estuary in central California (Elkhorn Slough). Dissolved oxygen measurements were used to estimate the terms in a control volume budget for a tidal creek/marsh complex at tidal timescales over several weeks. Respiration rates were 1.6 to 7.3 g O₂ m^?2 day^?1; net community production approached 20 g O₂ m^?2 day^?1. We found that aquatic NCP integrated throughout the creek complex varied significantly over the spring-neap cycle. The intertidal contribution to aquatic metabolism was net heterotrophic during spring tides and generally in balance during neap tides because spring-tide marsh inundation was limited to nighttime, and therefore the marsh could not contribute any primary production to the water column. At the estuary scale, the fortnightly export of oxygen from the main channel to the intertidal was largely balanced by an advective flux up-estuary.     

Nitrous oxide emission from Gulf Coast wetlands

Nitrous oxide evolution may contribute to partial destruction of the ozone layer in the stratosphere. A two year study of the release of N₂O from adjoining salt, brackish, and fresh marsh sediment indicates that the annual emission was 31, 48, and 55 mg N m^?2 respectively. Emission from open water area was less than the corresponding emission from the marsh sediment. In vitro experiments indicate that the N₂O emission was increased when the sediment was drained for extended periods of time. The addition of NO₃^? significantly increased the rate of N₂O evolution, indicating that a large potential for denitrification exists in the anoxic sediment. Appreciable losses of N₂O would only be expected when the marshes receive an extraneous source of nitrate such as sewage and/or wastewater.The contribution of the Gulf Coast wetlands to the atmospheric N₂O balance is estimated to be 3.3 × 10⁹ g N₂O. The maximum average daily emission was equivalent to 1.5 g N₂O-N ha^?1, which is less than the measured emission from uncultivated soils (Mosieret al., 1981) but greater than the estimates from noncropped land (CAST, 1976).     

Comparison of production computation methods in a Southeastern North CarolinaSpartina alterniflora salt marsh

Tagging studies ofSpartina alterniflora Loisel showed no significant differences in stem longevity of short, medium, and tall height forms. Mean stem longevity was 7.9 months, and the experimental turnover rate was 1.5 crops per yr. Five methods to measure productivity (peak standing crop, Milner and Hughes, Smalley, Wiegert and Evans, and Lomnicki, et al.) yielded annual net aerial primary production (NAPP) estimates ranging from 214 to 1,038 g dry wt per m² per yr in a stand of shortSpartina. Turnover rates were computed for each of the methods by dividing the respective production value by the peak standing crop (242 g dry wt per m² per yr). Each computed turnover rate was compared with the experimental value of 1.5 crops per yr to ultimately determine that the methods of peak standing crop, Milner and Hughes, and Smalley were underestimates and that the Wiegert and Evans method was an overestimate of NAPP in tidal marsh systems. Based on its calculated turnover rate of 1.9 crops per yr, a modified Lomnicki, et al. method provided the best NAPP estimate (454 g dry wt per m² per yr).     

Vegetation and Soil Dynamics of a Louisiana Estuary Receiving Pulsed Mississippi River Water Following Hurricane Katrina

We monitored wetland biomass, decomposition, hydrology, and soil porewater chemistry at the Breton Sound estuary, which receives Mississippi River water from the Caernarvon river diversion structure. The estuary was in the direct path of hurricane Katrina in 2005, which caused a dramatic loss of wetlands in the upper basin. From March 2006 to October 2007, we made duplicate measurements at three distance classes from the diversion structure along the estuarine gradient as well as at a reference area, designated Near (N_1&2), Mid (M_1&2), Far (F_1&2), and Ref (R_1&2). Above- and belowground live biomass, porewater nutrients (NO_x, NH₄, and PO₄), salinity, sulfide, and soil Eh were measured every 2 months. Water level was monitored with gauges. Above- and belowground decomposition was measured using the litterbag (both) and cotton strip (belowground only) methods. Analysis of porewater parameters showed that stress factors affecting biomass production (porewater salinity, sulfide, flooding, and redox potential) were generally low to moderate, while measurable porewater nutrient concentrations occurred at all sites. Aboveground end of season live (EOSL) standing crop in October ranged from 423 g/m² at site M₂ to 1,515 at site F₁, and was significantly greater at site N₁ than at sites N₂, M₁, or M₂. Aboveground EOSL biomass during this study was significantly lower than previously measured in 1999, 2000, and 2001. Peak belowground biomass ranged from 8,315 g/m² at site R₂ to 17,890 g/m² at site N₁, which is among the highest reported in the literature, and there were significant increases throughout the study, suggesting recovery from hurricane Katrina. The decomposition bag data did not indicate any significant differences; however, the cotton strip decomposition rate was significantly lower at the lowest depth. Wetland surface vertical accretion ranged from 0.49 cm/year at N₂ to 1.24 cm/year at N₁, with site N₁ significantly greater than N₂, M₁, F₂, and R₁, and site N₂ significantly less than all other sites except site R₁. These findings show that marsh productivity and stability is related to a number of factors and no one factor can explain the impacts of the hurricanes.     

Spatial Distribution of Nitrous Oxide (N<Subscript>2</Subscript>O) in the Reloncaví Estuary–Sound and Adjacent Sea (41°–43° S), Chilean Patagonia

Fjords and estuaries exchange large amounts of solutes, gases, and particulates between fluvial and marine systems. These exchanges and their relative distributions of compounds/particles are partially controlled by stratification and water circulation. The spatial and vertical distributions of N₂O, an important greenhouse gas, along with other oceanographic variables, are analyzed from the Reloncaví estuary (RE) (~41° 30′ S) to the gulf of Corcovado in the interior sea of Chiloé (43° 45′ S) during the austral winter. Freshwater runoff into the estuary regulated salinity and stratification of the water column, clearly demarking the surface (<5 m depth) and subsurface layer (>5 m depth) and also separating estuarine and marine influenced areas. N₂O levels varied between 8.3 and 21 nM (corresponding to 80 and 170 % saturation, respectively), being significantly lower (11.8 ± 1.70) at the surface than in subsurface waters in the Reloncaví estuary (14.5 ± 1.73). Low salinity and NO₃ ^?, NO₂ ^?, and PO₄ ^3? levels, as well as high Si(OH)₄ values were associated with low surface N₂O levels. Remarkably, an accumulation of N₂O was observed in the subsurface waters of the Reloncaví sound, associated with a relatively high consumption of O₂. The sound is exposed to increasing anthropogenic impacts from aquaculture and urban discharge, occurring simultaneously with an internal recirculation, which leads to potential signals of early eutrophication. In contrast, within the interior sea of Chiloé (ISC), most of water column was quasi homohaline and occupied by modified subantarctic water (MSAAW), which was relatively rich in N₂O (12.6 ± 2.36 nM) and NO₃ ^? (18.3 ± 1.63 μM). The relationship between salinity, nutrients, and N₂O revealed that water from the open ocean, entering into ISC (the Gulf of Corcovado) through the Guafo mouth, was the main source of N₂O (up to 21 nM), as it gradually mixed with estuarine water. In addition, significant relationships between N₂O excess vs. AOU and N₂O excess vs. NO₃ ^? suggest that part of N₂O is also produced by nitrification. Our results show that the estuarine and marine waters can act as light source or sink of N₂O to the atmosphere (air–sea N₂O fluxes ranged from ?1.57 to 5.75 μmol m^?2 day^?1), respectively; influxes seem to be associated to brackish water depleted in N₂O that also caused a strong stratification, creating a barrier to gas exchange.     

Accumulation of plant nutrients and heavy metals through sedimentation processes and accretion in a Louisiana salt marsh

The accumulation of selected plant nutrients and heavy metals in a rapidly accreting Louisiana salt marsh was examined. Sedimentation processes were shown to be supplying large amounts of plant nutrients to the marsh. Accumulation of heavy metals was low and appeared to be associated with the natural heavy metal content of incoming sediment rather than from a pollution source. A large portion of organic carbon from primary production remained in the marsh, contributing to the aggradation process of vertical marsh accretion. Nitrogen accumulated in the marsh at rates as great as 21 g per m² per yr.     

Inter- and supratidal sedimentology of a fjord-head estuary, south-western British Columbia

Detailed examination of inter- and supratidal delta and floodplain sediments exposed in eroding bank sections at 52 locations along the Squamish River estuary provides the basis for recognizing seven distinct facies within the 5500 m-long estuary. Estuary sedimentation is initially driven by the development of sand bar complexes along the seaward edge of the intertidal delta. Sedimentation continues within interdistributary bay environments as intertidal sandflats and then tidal marshes develop, Aggradation of the delta within interdistributary bay environments results in a gradual transition from delta to alluvial plain. Of the seven facies identified, only the intertidal sands and tidal marsh deposits provide evidence of their tidal origin. Examination of deposits throughout the riverine estuary reveals a number of gradual yet distinct changes of sediment size, structure, and sequence architecture. These trends record the changing nature of tidal and riverine control on sedimentation along the tidal gradient. Generally, with increasing distance up-estuary, sediment grain-size increases, the thickness of fine-grained overbank deposits decreases, and bedding changes from fine parallel bedding to higher energy bedforms. In addition, fining-upward successions become capped by coarser sands, facies contacts change from gradational to abrupt and occasionally erosional, and facies successions become increasingly complex and less predictable. Squamish River estuary has been divided into four zones based on sedimentological and stratigraphical evidence, each zone reflecting changes in the relative influence of tidal and riverine control on sedimentation. Each zone contains distinctly different facies sequences, although zone boundaries generally are gradational.     

A chemical survey of the Mississippi estuary

A “snap shot” survey of the Mississippi estuary was made during a period of low river discharge, when the estuarine mixing zone was within the deltaic channels. Concentrations of H⁺, Ca²⁺, inorganic phosphorus and inorganic carbon suggest that the waters of the river and the low salinity (<5‰) portion of the estuary are near saturation with respect to calcite and sedimentary calcium phosphate. An input of oxidized nitrogen species and N₂O was observed in the estuary between 0 and 4‰ salinity. The concentrations of dissolved NH₄ ⁺ and O₂, over most of the estuary, appeared to be influenced by decomposition of terrestrial organic matter in bottom sediments. The estuarine bottom also appears to be a source of CH₄ which has been suggested to originate from petroleum shipping and refining operations. Estuarine mixing with offshore Gulf waters was the dominant influence on distributions of dissolved species over most of the estuary (i.e., from salinities >5‰). The phytoplankton abundance (measured as chlorophylla) increased as the depth of the mixed layer decreased in a manner consistent with that expected for a light-limited ecosystem. Fluxes of NO₃ ^?+NO₂ ^? and soluble inorganic phosphorus to the Gulf of Mexico were estimated to be 3.4±0.2×10³ g N s^?1 and 1.9±0.2 g P s^?1 respectively, at the time of this study.     

Benthic nutrient fluxes in the intertidal flat within the Changjiang （Yangtze River） Estuary

In an annual cycle from March 2005 to February 2006, benthic nutrient fluxes were measured monthly in the Dongtan intertidal flat within the Changjiang （Yangtze River） Estuary. Except for NH4^＋, there always showed high fluxes from overlying water into sediment for other four nutrients. Sediments in the high and middle marshes, covered with halophyte and consisting of macrofauna, demonstrated more capabilities of assimilating nutrients from overlying water than the low marsh. Sampling seasons and nutrient concentrations in the overlying water could both exert significant effects on these fluxes. Additionally, according to the model provided by previous study, denitrification rates, that utilizing NO3- transported from overlying water （Dw） in Dongtan sediments, were estimated to be from -16 to 193 μmol·h^-1·m^-2 with an average value of 63 μmol·h^-1·m^-2 （n=18）. These estimated values are still underestimates of the in-situ rates owing to the lack of consideration of DN, i.e., denitrification supported by the local NO3^- production via nitrification.