Modeling the hydrochemistry of the Choptank River basin using GWLF and Arc/Info: 2. Model Validation and Application

We used the hydrochemical model GWLF to estimate terrestrial diffuse fluxes from ungauged areas of a coastal plain catchment, the Choptank River basin. The gauged area of the basin is 17% of the land surface, and we divided the remaining ungauged area into 21 subbasins. Three comparative approaches were used: (1) application of area yield coefficients based on 11 years of observations from the gauged area to extrapolate over ungauged subbasins without modeling, (2) application of GWLF to estimate export from all subbasins using model parameters calibrated in the gauged subbasin, and (3) application of GWLF with parameter adjustments based on the local characteristics in each subbasin. Comparison of the predicted export from 6 selected subbasins with observed export data showed that application of GWLF with local adjustments reduced model errors of N export from 43% to 27%. With only one adjustment for sediment P, application of GWLF alone reduced errors of P export from 92%to 40–45%, with or without local adjustments for flow and sediment retention. The data supported the hypothesis that significant spatial variations in N and P yields introduce large errors when extrapolating from gauged to ungauged subbasins, and estimated TN and TP yield coefficients varied over 1–21 kg N and 0.1–0.5 kg P ha^–1 y^–1 in ungauged areas due to varying human population densities, soil drainage characteristics, and amounts of agriculture. The most accurate estimates of terrestrial diffuse sources were combined with point source discharges and wet atmospheric inputs to estimate annual average inputs of 2.5 × 10⁶ kg N and 5.8 × 10⁴ kg P y^–1 to the Choptank estuary during 1980–1996. These results illustrate the problems of spatial extrapolation from gauged to ungauged areas and emphasize the need for application of local characteristics for accurate assessment of watershed export.     

Distribution and production ofChironomus salinarius (Diptera: Chironomidae) in a shallow coastal lagoon in the Bay of Cádiz

The abundance, generation time and production ofChironomus salinarius larvae in a lagoon fish-pond system in the Bay of Cádiz were studied by taking monthly samples at 3 sites during 1991 and 1992. Numerical abundance and biomass of larvae showed considerable spatial, seasonal and interannual variation (ANCOVAs,P<0.001). The maximum mean annual density was 7048 larvae m^–2, and corresponded to a biomass of 3.08 g dry weight (DW) m^–2. It was recorded at the site with the lowest rate of water renewal. Seasonal patterns were similar at all sites, with main annual peaks of abundance and biomass in autumn-early winter. Chironomid density was positively related to the biomass of benthic macroalgae (P<0.001). The population studied was multivoltine with a probable average of five generations per year, with overlapping cohorts and a predominance of third- and fourth-instar larvae. Estimates of annual production ranged between 72.2 g DW m^–2 yr^–1 at the site with the lowest rate of water renewal in 1991 and 0.1 g DW m^–2 yr^–1 at the site with the highest rate of water renewal in 1992. Mean annual production and the production/biomass ratio for the system was estimated to be 16.8 g DW m^–2 yr^–1 and 12.7, respectively. Possible factors leading to the observed density fluctuations are discussed, as well as possible sources of error in production estimates.     

An attempt to predict long-term effects of atmospheric nitrogen deposition on the yield of Norway spruce stands (Picea abies (L.) Karst.) in southwestern Sweden

Long-term field experiments in Norway spruce stands on fertile sites (site indices 27–35 m) in southwestern Sweden were analysed with respect to volume increment. Three treatments were included (0=No fertilization, N = Fertilization with N, NP = Fertilization with N and P).Volume growth was monitored for 18 years in 10 blocks. No significant differences in annual volume increment between the treatments were detected. Volume increments in the N treatment were 97%, 99% and 107% as high as those in the 0 treatment for the periods 1–5, 6–10 and 11–15 years after the first fertilization. Corresponding values for the NP treatment were 104%, 108% and 110%, indicating that P has a small positive effect.The amount of N-fertilization would correspond to an annual N deposition of 20 kg ha^-1 during the next 30 years in southwestern Sweden. For this period, the results imply that this N deposition would not affect the growth of Norway spruce stands on fertile sites.     

Changing attitudes toward chydorid anomopods since 1769

From September 1990 through December 1991 nitrous oxide flux measurements were made at 9 intertidal mud flat sites in the Scheldt Estuary. Nitrous oxide release rates were highly variable both between sites and over time at any one site. Annual nitrous oxide fluxes vary from about 10 mmol N m^–2 at the tidal fresh-water end-member site to almost zero at the most saline stations. Along the estuarine gradient, annual nitrous oxide fluxes are significantly correlated with sedimentary organic carbon and nitrogen concentrations, ammonium fluxes and annual nitrogen turn-over rates, that are estimated using mass-balance considerations. Nitrous oxide fluxes seem to respond linearly to an increasing nitrogen load, with one out of each 17 000 atoms nitrogen entering estuaries being emitted as nitrous oxide.     

Denitrification losses from a natural grassland in the Basque Country under organic and inorganic fertilization

Denitrification losses from a poorly drained clayey loamy soil under natural pasture were measured over a two-year period using the acetylene inhibition technique. Plots received two different applications of fertilizer as calcium ammonium nitrate or cow slurry (a total of 145–290 kg N ha^–1 in 1991 and 120–240 kg in 1992). In the first year, N losses in the mineral treatments were about 4 times greater than losses in the slurry treatments. In the second year losses in the slurry treatments increased in such a way that losses in the higher slurry application became similar to those for the two mineral treatments. Soil nitrate was the factor producing differences between treatments. In this way, N mineralization in periods between fertilizations coinciding with high soil water contents was responsible in the second year for the increase in N losses in the slurry treatments. Denitrification rates greater than 0.1 kg N ha^–1 day^–1 occurred at soil water contents > 33 % (air filled porosity < 26 %) and soil nitrate contents > 1 mg N kg^–1 dry soil. Spring and autumn were the seasons of highest risk of denitrification because of N fertilizations coinciding with periods of soil saturation with water. Winter losses were low, but this is a period when there is a risk of denitrification in wetter seasons, particularly for a slurry application management.     

Chemical composition of interstitial water and diffusive fluxes within the diatomaceous sediment in Lake Myvatn, Iceland

The sub-arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere, despite an ice-cover of 190 days per year. This is due to relatively high solar radiation, nutrient rich inflow waters, N₂ fixation and internal nutrient loading. In order to define direction and magnitude of diffusive fluxes, soil water samplers were used to collect interstitial water from 25–150 cm depth, from within the diatomaceous sediment at the bottom of Lake Myvatn. Water depth at the sampling site was 225 cm. The pH of the interstitial water ranged from 7.16 to 7.30, while the pH of the lake water was 9.80–10.00. The concentrations of most solutes were similar 16 cm above the bottom of the lake at the sampling site and at the lake outlet. The concentrations of NO₃, S, F, O₂, Al, Cr, Mo, V, U, Sn and Sb were higher in the lake water than in the interstitial water. They will therefore diffuse from the lake water into the interstitial water. The concentrations of orthophosphates, PO₄, and total dissolved P were highest at 25 cm depth, but Co and NH₄ concentrations were highest at 50 to 100 cm depth. Thus they diffuse both up towards the lake bottom and down deeper into the sediments. The concentrations of Na, K, Ca, Mg, Sr, Mn, Li and alkalinity were greater within the sediments than in the lake water and increased continuously with depth. The Si concentration of the interstitial water was higher than in the lake water, it was highest at 25 cm depth and decreased slightly down into the sediments. The concentration gradient was greatest for bicarbonate, HCO₃ ^–, 1.5×10^–7 mol cm^–3 cm^–1, and then in declining order for the solutes with the highest gradient; NH₄, Si, Na, Ca, Mg, -S (diffusion into the sediments), K, PO₄, Cl, Fe and Mn. The estimated annual diffusive flux of PO₄ for Lake Myvatn was 0.1 g P m^–2 yr^–1, about 10% of the total PO₄ input to Lake Myvatn. The H₄SiO₄° flux was 1.3 g Si m^–2 yr^–1, <1% of both the input and the annual net Si fixation by diatoms within the lake and the diffusive flux of dissolved inorganic carbon was 1% of the annual net C fixation by diatoms. Annual diffusive flux of NH₄ ⁺ was 1.9 g N m^–2 yr^–1 similar to the input of fixed N to the lake and 24% of the net N fixation within Lake Myvatn. Thus it is important for the nitrogen budget of Lake Myvatn and the primary production in the lake since fixed nitrogen is the rate determining nutrient for primary production.     

Sediment nutrient accumulation and nutrient availability in two tidal freshwater marshes along the Mattaponi River, Virginia, USA

Sediment deposition is the main mechanism of nutrient delivery to tidal freshwater marshes (TFMs). We quantified sediment nutrient accumulation in TFMs upstream and downstream of a proposed water withdrawal project on the Mattaponi River, Virginia. Our goal was to assess nutrient availability by comparing relative rates of carbon (C), nitrogen (N), and phosphorus (P) accumulated in sediments with the C, N, and P stoichiometries of surface soils and above ground plant tissues. Surface soil nutrient contents (0.60–0.92% N and 0.09–0.13% P) were low but within reported ranges for TFMs in the eastern US. In both marshes, soil nutrient pools and C, N, and P stoichiometries were closely associated with sedimentation patterns. Differences between marshes were more striking than spatial variations within marshes: both C, N, and P accumulation during summer, and annual P accumulation rates (0.16 and 0.04 g P m^–2 year^–1, respectively) in sediments were significantly higher at the downstream than at the upstream marsh. Nitrogen:P ratios <14 in above ground biomass, surface soils, and sediments suggest that N limits primary production in these marshes, but experimental additions of N and/or P did not significantly increase above ground productivity in either marsh. Lower soil N:P ratios are consistent with higher rates of sediment P accumulation at the downstream site, perhaps due to its greater proximity to the estuarine turbidity maximum.     

Soil Solution Chemistry and Element Fluxes in Three European Heathlands and Their Responses to Warming and Drought

Soil water chemistry and element budgets were studied at three northwestern European Calluna vulgaris heathland sites in Denmark (DK), The Netherlands (NL), and Wales (UK). Responses to experimental nighttime warming and early summer drought were followed during a two-year period. Soil solution chemistry measured below the organic soil layer and below the rooting zone and water fluxes estimated with hydrological models were combined to calculate element budgets. Remarkably high N leaching was observed at the NL heath with 18 and 6.4 kg N ha^–1 year^–1 of NO₃–N and NH₄–N leached from the control plots, respectively, indicating that this site is nitrogen saturated. Increased soil temperature of +0.5°C in the heated plots almost doubled the concentrations and losses of NO₃–N and DON at this site. Temperature also increased mobilization of N in the O horizon at the UK and DK heaths in the first year, but, because of high retention of N in the vegetation or mineral soil, there were no significant effects of warming on seepage water NO₃–N and NH₄–N. Retention of P was high at all three sites. In several cases, drought increased concentrations of elements momentarily, but element fluxes decreased because of a lower flux of water. Seepage water DOC and DON was highly significantly correlated at the UK site where losses of N were low, whereas losses of C and N were uncoupled at the NL site where atmospheric N input was greatest. Based on N budgets, calculations of the net change in the C sink or source strength in response to warming suggest no change or an increase in the C sink strength during these early years.     

Rates and pathways of nitrous oxide production in a shortgrass steppe

Most of the small external inputs of N to the Shortgrass steppe appear to be conserved. One pathway of loss is the emission of nitrous oxide, which we estimate to account for 2.5–9.0% of annual wet deposition inputs of N. These estimates were determined from an N₂O emission model based on field data which describe the temporal variability of N₂O produced from nitrification and denitrification from two slope positions. Soil water and temperature models were used to translate records of air temperature and precipitation between 1950 and 1984 into variables appropriate to drive the gas flux model, and annual N₂O fluxes were estimated for that period. The mean annual fluxes were 80 g N ha^–1 for a midslope location and 160 g N ha^–1 for a swale. Fluxes were higher in wet years than in dry, ranging from 73 to 100 g N ha^–1y^–1at the midslope, but the variability was not high. N₂O fluxes were also estimated from cattle urine patches and these fluxes while high within a urine patch, did not contribute significantly to a regional budget. Laboratory experiments using C₂H₂ to inhibit nitrifiers suggested that 60–80% of N₂O was produced as a result of nitrification, with denitrification being less important, in contrast to our earlier findings to the contrary. Intrasite and intraseasonal variations in N₂O flux were coupled to variations in mineral N dynamics, with high rates of N₂O flux occurring with high rates of inorganic N turnover. We computed a mean flux of 104 g N ha^–1 y^–1 from the shortgrass landscape, and a flux of 2.6 × 109 g N y^– from all shortgrass steppe (25 × 106 ha).     

Retention of nitrogen and phosphorus in a Danish lowland river system: implications for the export from the watershed

In a Danish lowland river system intensive measurements were made, in four 80 m reaches, of the nitrogen (N) and phosphorus (P) stored in the stream sediment. The results were used for calculation of the total retention in the river system during two summers (June to August). In addition, the mobilization of nutrients from the stream bottom in autumn 1987 was compared with the export from the watershed.During the study period (June 1987 to September 1988) the amounts of N and P stored in stream reaches were determined fortnightly using a core-sample technique. In reaches dominated by submersed macrophytes, 25–40 g N m^–2 and 20–30 g P m^–2 were stored during two summers, against only 10–15g N and P m^–2 for sandy and gravely reaches. In riparian zones with emergent macrophytes the retention was even higher than in the submersed macrophytes. Gross retention exceeded net retention by a factor of two to three.Net retention of P in the river system during the summer of 1987 was equal to the summer export from the watershed. On an annual basis, retention in the summer constituted 20% of the P export. In contrast, retention in the summer of 1988 amounted to 60% of the total P export during the same period (38% reduction) and 22% in comparison with the annual export. The corresponding figures for N were lower, showing reductions of 16% and 12% of the export of total N in the two summer periods, and about 1% of the annual exports.In September 1987 6.4% of the total N export and 65% of the total P export from the watershed consisted of resuspended material. In 1987 the N and P retained during the summer was almost completely resuspended during storm events during September to November.     

The 1994 experimental opening of the Bonnet Carre Spillway to divert Mississippi River water into Lake Pontchartrain, Louisiana

A diversion of Mississippi River water into Lake Pontchartrain, Louisiana, USA by way of the Bonnet Carre Spillway has been proposed as a restoration technique to help offset regional wetland loss. An experimental diversion of Mississippi River water into Lake Pontchartrain was carried out in April 1994 to monitor the fate of nutrients and sediments in the spillway and Lake Pontchartrain. Approximately 6.4×10⁸ m³ of Mississippi River water was diverted into Lake Pontchartrain over 42 days. As water passed through the Bonnet Carre Spillway, there were reductions in total suspended sediment concentrations of 82–83%, nitrite+nitrate (NO_x) of 28–42%, in total nitrogen (TN) of 26–30%, and in total phosphorus (TP) of 50–59%. 3.9±1.1 cm of accretion was measured in the spillway. Nutrient concentrations at the freshwater plume edge in Lake Pontchartrain compared to the Mississippi River were lower for NO_x (44–81%), TN (37–57%), and TP (40–70%), and generally higher for organic nitrogen (−7–57%). The Si:N ratio generally increased and the N:P ratio decreased from the river to the plume edge. Nutrient stoichiometric ratios indicate water at the plume edge was not silicate limited, suggesting conditions favoring diatomic phytoplankton.     

Modelling of seasonal variation in nitrogen retention and in-lake concentration: A four-year mass balance study in 16 shallow Danish lakes

The mass balance for total nitrogen (N) was studied over a four-year period in 16 shallow mainly eutrophic 1st order Danish lakes. Water was sampled in the main inlet of each lake 18–26 times annually, and from the outlets and the lake 19 times annually. Water was also sampled from minor inlets, although less frequently. N input and output were calculated using daily data on discharge (Q), the latter being obtained either from the Q/H relationship based on automatic recordings of water level (H) for the main in- and outlet, or by means of Q/Q relationships for the minor inlets. Annual mean N retention in the lakes ranged from 47 to 234 mg N m^–2 d^–1, and was particularly high in lakes with high N loading. Annual percentage retention (N _ret –y%) ranged from 11 to 72%. Non-linear regression analysis revealed that hydraulic retention time and mean depth accounted for 75% of the variation in annual mean N _ret –y% and, in combination with inlet N concentration, accounted for 84% of the variation in the in-lake N concentration. N _ret % varied according to season, being higher in the second and third quarter than in the first and fourth quarter (median 18–19%). A simple model was developed for predicting monthly nitrogen retention (N _ret –m) on the basis of external N loading, the lake water pool of nitrogen N _pool , hydraulic loading and lake water temperature. Calibration of only two parameters on data from the randomly selected 8 out of 16 lakes rendered the model capable of accurately simulating seasonal dynamics of the in-lake N concentration and N _ret –m in all 16 lakes. We conclude that with regard to shallow, eutrophic lakes with a relatively low hydraulic retention time, it is now possible to determine not only annual mean nitrogen retention, but also the seasonal variation in N _ret–m . Prediction of seasonal variation in N loading of downstream N-limited coastal areas is thereby rendered much more reliable.     

Variability in the water chemistry of shallow ponds in southeast England, with special reference to the seasonality of nutrients and implications for modelling trophic status

The variability in water chemistry of samples taken on a monthly basis (October 1990–December 1991) from 31 shallow, artificial ponds in southeast England was examined. The survey revealed great within-year variation in the concentrations of nutrients. Total phosphorus (TP) concentrations displayed no overall marked seasonal pattern, although many sites experienced summer peaks with a simultaneous increase in soluble reactive phosphorus (SRP) concentrations, indicating that sediment P release occurred. SRP and nitrate (NO₃–N) concentrations displayed a marked seasonal pattern similar to that observed in deep, stratifying lakes, with 55% and 94% of the sites surveyed, respectively, experiencing a decline in concentrations in the spring, maintaining low levels throughout summer and the highest levels occurring in winter. Dissolved silica (SiO₂–(Si) also displayed a marked seasonal distribution with a spring decline in concentrations associated with diatom growth, followed by an increase in the summer.The study demonstrated that intra-annual variability in nutrient concentrations is high and tends to be greatest in the most enriched waters. Thus, a high sampling frequency is required to provide representative annual mean data. Furthermore, annual means rather than winter–spring means provide more appropriate estimates of TP and SRP in these waters, owing to the importance of internal cycling of nutrients in summer. The findings are especially relevant to sampling strategy design and the averaging of seasonal water chemistry data for use in predictive models of lake trophic status.     

Ecosystem respiration and organic carbon processing in a large,tidally influenced river: the Hudson River

We estimated whole-ecosystem rates of respiration over a 40-km stretch of the tidally influenced freshwater Hudson River every 2 to 3 weeks from May through November. We measured in situ concentrations of oxygen over depth at dusk and dawn at 10 stations spaced over this interval. The use of multiple stations allowed for the consideration of the influence of tidal advection of water masses. Respiration was estimated from the decrease in oxygen overnight with a correction for diffusive exchange of oxygen with the atmosphere. We estimated this flux of oxygen to or from the atmosphere using the measured oxygen gradient and a transfer velocity model which is a function of wind velocity.Integration of the data for the period of May through November yields an estimate of whole-ecosystem respiration of 591 g C m^–2 (S.E. = 66). That the standard error of this estimate is relatively low (11% of the estimate) indicates that the use of multiple stations adequately deals with error introduced through the advection of water between stations. The logarithm of average daily respiration rate was correlated with average daily temperature (p = 0.007;r ² = 0.62). We used this temperature-respiration relationship to derive an estimate of the annual respiration rate of 755 g C m^–2 yr^–1 (S.E. = 72). This estimate is moderately sensitive to the estimated flux of oxygen between the atmosphere and water; using the lower and upper 95% confidence limits of our model relating the transfer velocity of oxygen to wind speed gives a range of annual respiration estimates from 665 g C m^–2 yr^–1 to 984 g C m^–2 yr^–1.The river is strongly heterotrophic, with most respiration driven by allochthonous inputs of organic matter from terrestrial ecosystems. The majority of the allochthonous inputs to the river (over 60%) are apparently metabolized within the river. Any change in allochthonous inputs due to changes in land use or climate patterns would be expected to alter the oxygen dynamics and energy flow within this tidally influenced river.     

Response to inoculation and N fertilization for increased yield and biological nitrogen fixation of common bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is able to fix 20–60 kg N ha^–1 under tropical environments in Brazil, but these amounts are inadequate to meet the N requirement for economically attractive seed yields. When the plant is supplemented with N fertilizer, N₂ fixation by Rhizobium can be suppressed even at low rates of N. Using the ¹⁵N enriched method, two field experiments were conducted to compare the effect of foliar and soil applications of N-urea on N₂ fixation traits and seed yield. All treatments received a similar fertilization including 10 kg N ha^–1 at sowing. Increasing rates of N (10, 30 and 50 kg N ha^–1) were applied for both methods. Foliar application significantly enhanced nodulation, N₂ fixation (acetylene reduction activity) and yield at low N level (10 kg N ha^–1). Foliar nitrogen was less suppressive to nodulation, even at higher N levels, than soil N treatments. In the site where established Rhizobium was in low numbers, inoculation contributed substantially to increased N₂ fixation traits and yield. Both foliar and soil methods inhibited nodulation at high N rates and did not significantly increase bean yield, when comparing low (10 kg N ha^–1) and high (50 kg N ha^–1) rates applied after emergence. In both experiments, up to 30 kg N ha^–1 of biologically fixed N₂ were obtained when low rates of N were applied onto the leaves.     

Forest floor biomass,litter fall and nutrient return in Central Himalayan oak forests

The seasonal dynamics of forest floor biomass, pattern of litter fall and nutrient return in Central Himalayan oak forests are described. Fresh and partially decomposed litter layers occur throughout the whole year in addition to herbaceous vegetation. The highest leaf litter value is found in April and May and the minimum in September. Partially and largely decomposed litter tended to increase from January to May with a slight decline in June. The wood litter peaked in March and April. The relative contribution of partially decomposed litter to the forest floor remains greatest the year round. The maximum herbaceous vegetation development was found in September with a total annual net production of 104.3 g m^-2yr^-1. The total calculated input of litter was 480.8 g m^-2yr^-1. About 68% of the forest floor was replaced each year with a subsequent turnover time of 1.47 yr. The total annual input of litter ranged from 664 (Quercus floribunda site) –952 g m^-2 (Q. lanuginosa site), of which tree, shrub and herbaceous litter accounted for respectively 72.0–86.3%, 6.4 – 19.4% and 5.2 – 8.6%. The annual nutrient return through litter fall amounted to (kg ha^-1) 178.0 – 291.0 N, 10.0 – 26.9 P, 176.8 – 301.6 Ca, 43.9 – 64.1 K and 3.98 – 6.45 Na. The tree litter showed an annual replacement of 66.0 – 70.0%, for different nutrients the range was 64 and 84%.     

Nutrient additions by waterfowl to lakes and reservoirs: predicting their effects on productivity and water quality

Lakes and reservoirs provide water for human needs and habitat for aquatic birds. Managers of such waters may ask whether nutrients added by waterfowl degrade water quality. For lakes and reservoirs where primary productivity is limited by phosphorus (P), we developed a procedure that integrates annual P loads from waterfowl and other external sources, applies a nutrient load-response model, and determines whether waterfowl that used the lake or reservoir degraded water quality. Annual P loading by waterfowl can be derived from a figure in this report, using the days per year that each kind spent on any lake or reservoir. In our example, over 6500 Canada geese (Branta canadensis) and 4200 ducks (mostly mallards, Anas platyrhynchos) added 4462 kg of carbon (C), 280 kg of nitrogen (N), and 88 kg of P y^–1 to Wintergreen Lake in southwestern Michigan, mostly during their migration. These amounts were 69% of all C, 27% of all N, and 70% of all P that entered the lake from external sources. Loads from all external sources totaled 840 mg P m^–2 y^–1. Application of a nutrient load-response model to this concentration, the hydraulic load (0.25 m y^–1), and the water residence time (9.7 y) of Wintergreen Lake yielded an average annual concentration of total P in the lake of 818 mg m^–3 that classified the lake as hypertrophic. This trophic classification agreed with independent measures of primary productivity, chlorophyll-a, total P, total N, and Secchi disk transparency made in Wintergreen Lake. Our procedure showed that waterfowl caused low water quality in Wintergreen Lake.Contribution 824 of the National Fisheries Research Center-Great Lakes, 1451 Green Road, Ann Arbor, Michigan 48105, U.S.A. and 722 of the Kellogg Biological Station, Michigan State University.     

Hydrological and nutrient budgets of freshwater and estuarine wetlands of Taylor Slough in Southern Everglades, Florida (U.S.A.)

Hydrological restoration of the Southern Everglades will result in increased freshwater flow to the freshwater and estuarine wetlands bordering Florida Bay. We evaluated the contribution of surface freshwater runoff versus atmospheric deposition and ground water on the water and nutrient budgets of these wetlands. These estimates were used to assess the importance of hydrologic inputs and losses relative to sediment burial, denitrification, and nitrogen fixation. We calculated seasonal inputs and outputs of water, total phosphorus (TP) and total nitrogen (TN) from surface water, precipitation, and evapotranspiration in the Taylor Slough/C-111 basin wetlands for 1.5 years. Atmospheric deposition was the dominant source of water and TP for these oligotrophic, phosphorus-limited wetlands. Surface water was the major TN source of during the wet season, but on an annual basis was equal to the atmospheric TN deposition. We calculated a net annual import of 31.4 mg m^–2 yr^–1 P and 694 mg m^–2 yr^–1N into the wetland from hydrologic sources. Hydrologic import of P was within range of estimates of sediment P burial (33–70 mg m^–2 yr^–1 P), while sediment burial of N (1890–4027 mg m^–2 yr^–1 N) greatly exceeded estimated hydrologic N import. High nitrogen fixation rates or an underestimation of groundwater N flux may explain the discrepancy between estimates of hydrologic N import and sediment N burial rates.     

Water conservation and protein metabolism in northern elephant seal pups during the postweaning fast

Urine production and N output were monitored in northern elephant seal (Mirounga angustirostris) pups progressing through 10 weeks of a natural postweaning fast. Urine output declind by 84% (to 69±12 ml·day^–1) at 10 weeks (P<0.05). Glomerular filtration rate at 10 weeks was 51% of the 67±3 ml serum·min^–1 observed during week 1 (P<0.05). Urine N excretion fell by 69% to 1.2±0.17 g·day^–1, while urinary concentration increased (P<0.05). Serum urea declined from an initial 11 mmol·1^–1 to 5–7 mmol·1^–1 by 5 weeks. The fall in urinary N loss (and thus amino acid oxidation) was concomitant with depressed metabolic rate. Therefore, protein contributed little toward meeting energy demands (i.e., <4% of average metabolic rate) throughout fasting. These data indicate that fasting pups improve water conservation and minimize protein catabolism during prolonged natural fasts without an exogenous source of water.Abbreviations AA amino acid(s) - AMR average metabolic rate - ANOVA one-way analysis of variance - BMR basal metabolic rate - BUN blood urea nitrogen - EP end product - EWL evaporative water loss - [Gr]_s serum creatinine concentration - GFR glomerular filtration rate - LBM lean body mass - LML Long Marine Laboratory - MR metabolic rate - NEFA non-esterified fatty acids - RMR resting metabolic rate - TCA tricarboxylic acid - U:C ulinary urea: creatinine concentration ratio     

An assessment of the annual mass balance of carbon,nitrogen, and phosphorus in Narragansett Bay

Narragansett Bay is a relatively well-mixed, high salinity coastal embayment and estuary complex in southern New England (USA). Much of the shoreline is urban and the watershed is densely developed. We have combined our data on C, N, and P inputs to this system, on C, N, and P accumulation in the sediments, and on denitrification with extensive work by others to develop approximate annual mass balances for these elements. The results show that primary production within the bay is the major source of organic carbon (4 times greater than other sources), that land drainage and upstream sewage and fertilizer are the major sources of N, and that landward flowing bottom water from offshore may be a major source of dissolved inorganic phosphorus. Most of the nutrients entering the bay arrive in dissolved inorganic form, though DON is a significant component of the N carried by the rivers. About 40% of the DIN in the rivers is in the form of ammonia. Sedimentation rates are low in most of Narragansett Bay, and it appears that less than 20% of the total annual input of each of these elements is retained within the system. A very small amount of C, N, and P is removed in fisheries landings, denitrification in the sediments removes perhaps 10–25% of the N input, and most of the carbon fixed in the system is respired within it. Stoichiometric calculations suggest that some 10–20% of the organic matter formed in the bay is exported to offshore and that Narragansett Bay is an autotrophic system. Most of the N and P that enters the bay is, however, exported to offshore waters in dissolved inorganic form. This assessment of the overall biogeochemical behavior of C, N, and P in the bay is consistent with more rigorously constrained mass balances obtained using large living models or mesocosms of the bay at the Marine Ecosystem Research Laboratory (MERL).