Assessment of present and future nitrogen loads, water quality, and seagrass (Thalassia testudinum) depth distribution in Lemon Bay, Florida

Nitrogen loads into Lemon Bay, Florida were modeled to have increased ca. 59% between pre-development (i.e., 1850) estimates (5.3 kg TN ha⁻¹ yr⁻¹. and estimates for the year 1995 (8.4 kg TN ha⁻¹ yr⁻¹). By the year 2010, nitrogen loads are predicted to increase an additional 45% or 58%, depending upon progress being made toward replacing older septic tank systems with centralized sewerage (nitrogen loads of 12.2 and 13.3 kg TN ha⁻¹ yr⁻¹, respectively). Using 1995 estimates, nonpoint sources (stormwater runoff) are throught to be responsible for ca. 76% of the annual nitrogen load, followed by septic tank systems (14%), rainfall (10%), and an insignificant load from baseflow. Based on an empirically-derived nitrogen load:chlorophylla relationship developed for a portion of nearby Tampa Bay, a 45% increase in nitrogen loads into Lemon Bay could result in a 29% increase in annual average chlorophylla concentrations. Using the estimate of a 29% increase in future chlorophylla concentrations, an empirically-derived optical model for Lemon Bay suggests that light attenuation coefficients in the bay would increase ca. 9%, and the average depth limit ofThalassia testudinum in Lemon Bay would decrease by ca. 24%.     

Sources of nitrogen to estuaries in the United States

The purpose of this study was to quantify the nitrogen (N) inputs to 34 estuaries on the Atlantic and Gulf Coasts of the United States. Total nitrogen (TN) inputs ranged from 1 kg N ha⁻¹ yr⁻¹ for Upper Laguna Madre, Texas, to 49 kg N ha⁻¹ yr⁻¹ for Massachusetts Bay, Massachusetts. TN inputs to 11 of the 34 estuaries were dominated by urban N sources (point sources and septic systems) and nonpoint source N runoff (5% of total); point sources accounted for 36–86% of the TN inputs to these 11 urban-dominated estuaries. TN inputs to 20 of the 34 estuaries were dominated by agricultural N sources; N fertilization was the dominant source (46% of the total), followed by manure (32% of the total) and N fixation by crops (16% of the total). Atmospheric deposition (runoff from watershed plus direct deposition to the surface of the estuary) was the dominant N source for three estuaries (Barnegat Bay, New Jersey: 64%; St. Catherines-Sapelo, Georgia: 72%; and Barataria Bay, Louisiana: 53%). Six estuaries had atmospheric contributions ≥30% of the TN inputs (Casco Bay, Maine: 43%; Buzzards Bay, Massachusetts: 30%; Great Bay, New Jersey: 40%; Chesapeake Bay: 30%; Terrebonne-Timbalier Bay, Louisiana: 59%; and Upper Laguna Madre: 41%). Results from our study suggest that reductions in N loadings to estuaries should be accomplished by implementing watershed specific programs that target the dominant N sources.     

Water quality analysis of a freshwater diversion at Caernarvon, Louisiana

Since 1991, Mississippi River water has been diverted at Caernarvon, Louisiana, into Breton Sound estuary. Breton Sound estuary encompasses 1100 km² of fresh and brackish, rapidly subsiding wetlands. Nitrite + nitrate, total Kjeldahl nitrogen, ammonium, total phosphorus, total suspended sediments, and salinity concentrations were monitored at seven locations in Breton Sound from 1988 to 1994. Statistical analysis of the data indicated decreased total Kjeldahl nitrogen with associated decrease in total nitrogen, and decreased salinity concentrations in the estuary due to the diversion. Spring and summer water quality transects indicated rapid reduction of nitrite + nitrate and total suspended sediment concentration as diverted Mississippi River water entered the estuary, suggesting near complete assimilation of these constituents by the ecosystem. Loading rates of nitrite + nitrate (5.6–13.4 g m⁻² yr⁻¹), total nitrogen (8.9–23.4 g m⁻² yr⁻¹), and total phosphorus (0.9–2.0 g m⁻² yr⁻¹) were calculated along with removal efficiencies for these constituents (nitrite + nitrate 88–97%; total nitrogen 32–57%; total phosphorus 0–46%). The low impact of the diversion on water quality in the Breton Sound estuary, along with assimilation of TSS over a very short distance, suggests that more water may be introduced into the estuary without detrimental affects. This would be necessary if freshwater diversions are to be used to distribute nitrients and sediments into the lower reaches of the estuary, in an effort to compensate for relative sea-level rise, and reverse the current trend of rapid loss of wetlands in coastal Louisiana.     

Comparison of soil erosion and deposition rates using radiocesium, RUSLE, and buried soils in dolines in East Tennessee

 Three dolines (sinkholes), each representing different land uses (crop, grass, and forest) in a karst area in East Tennesse, were selected to determine soil erosional and depositional rates. Three methods were used to estimate the rates: fallout radiocesium (¹³⁷Cs) redistribution, buried surface soil horizons (Ab horizon), and the revised universal soil loss equation (RUSLE). When ¹³⁷Cs redistribution was examined, the average soil erosion rates were calculated to be 27 t ha^–1 yr^–1 at the cropland, 3 t ha^–1 yr^–1 at the grassland, and 2 t ha^–1 yr^–1 at the forest. By comparison, cropland erosion rate of 2.6 t ha^–1 yr^–1, a grassland rate of 0.6 t ha^–1 yr^–1, and a forest rate of 0.2 t ha^–1 yr^–1 were estimated by RUSLE. The ¹³⁷Cs method expressed higher rates than RUSLE because RUSLE tends to overestimate low erosion rates and does not account for deposition. The buried surface horizons method resulted in deposition rates that were 8 t ha^–1 yr^–1 (during 480 yr) at the cropland, 12 t ha^–1 yr^–1 (during 980 yr) at the grassland, and 4 t ha^–1 yr^–1 (during 101 yr) at the forest site. By examining ¹³⁷Cs redistribution, soil deposition rates were found to be 23 t ha^–1 yr^–1 at the cropland, 20 t ha^–1 yr^–1 at the grassland, and 16 t ha^–1 yr^–1 at the forest site. The variability in deposition rates was accounted for by temporal differences;¹³⁷Cs expressed deposition during the last 38 yr, whereas Ab horizons represented deposition during hundreds of years. In most cases, land use affected both erosion and deposition rates – the highest rates of soil redistribution usually representing the cropland and the lowest, the forest. When this was not true, differences in the rates were attributed to differences in the size, shape, and closure of the dolines. Received: 10 October 1995 · Accepted: 13 October 1995     

Patterns of mangrove forest structure and soil nutrient dynamics along the Shark River estuary, Florida

The basal area and productivity of managrove wetlands are described in relation to selected soil properties to understand the general pattern of optimum forest stature at the mouth of estuaries in the Everglades, such as the Shark River Slough, Florida (U.S.). The basal area of mangroves decreases from 40.4 m² ha⁻¹ and 39.7 m² ha⁻¹ at two stations 1.8 km and 4.1 km from the estuary mouth to 20.7 m² ha⁻¹ and 19.6 m² ha⁻¹ at two sites 9.9 km and 18.2 km from the mouth, respectively. The gradient in basal area at these four sites is mostly the result of approximately 34 yr of growth since Hurricane Donna. Wood productivity is higher in the lower estuary (10.7 Mg ha⁻¹ yr⁻¹ and 12.0 Mg ha⁻¹ yr⁻¹) than in the upper estuary (3.2 Mg ha⁻¹ yr⁻¹ and 4.2 Mg ha⁻¹ yr⁻¹). Porewater salinity among these four mangrove sites during seasonal sampling in 1994 and 1995 ranged from 1.6 g kg⁻¹ to 33.5 g kg⁻¹, while sulfide was generally<0.15 mM at all sites. These soil values indicate that abiotic stress cannot explain the decrease in forest structure along this estuarine gradient. Concentrations of nitrogen (N) and phosphorus (P) are more closely related to patterns of forest development, with higher soil fertility at the mouth of the estuary as indicated by higher concentrations of extractable ammonium, total soil P, and available P, along with higher ammonium production rates. The more fertile sites of the lower estuary are dominated by Laguncularia racemosa, whereas the less fertile sites in the intermediate and upper estuary are dominated by Rhizophora mangle. Relative N mineralization per unit of total N is higher in the lower estuary and is related positively to concentrations of available P, indicating the importance of turnover rates and nutrient interactions to soil fertility. Concentrations of Ca-bound P per volume soil in the lower estuary is 40-fold higher than in the upper estuary, and along with an increase in residual P in the upper estuary, indicate a shift from mineral to organic P along the estuarine gradient. Mineral inputs to the mouth of Shark River estuary from the Gulf of Mexico (rather than upland inputs) apparently control the patterns of mangrove structure and productivity.     

Direct evidence of imbalanced seagrass (Posidonia oceanica) shoot population dynamics in the Spanish Mediterranean

Direct census of shoots tagged in permanent plots was used to assess the present (2000–2002)Posidonia oceanica population dynamics in 25 meadows along the Spanish Mediterranean Coast. Shoot density ranged from 154±8 to 1,551±454 shoots m⁻², absolute shoot mortality from 5±0 to 249±53 shoots m⁻² yr⁻¹, and absolute shoot recruitment from <5 ±1 to 62±42 shoots m⁻²yr⁻¹. Specific shoot mortality and recruitment rates, which are mathematically and statistically (p>0.05) independent of shoot density, varied from 0.015±0.006 to 0.282±0.138 yr⁻¹ and 0.018±0.005 to 0.302±0.093 yr⁻¹, respectively. Absolute shoot mortality rate was scaled to shoot density (Pearson correlation, r=0.78, p<0.0001), and variability in specific shoot recruitment rate was partially due to differences in the percentage of growing apexes, which produce most of the recruits within the population (Pearson correlation, r=0.50, p<0.001), demonstrating the existence of structural constraints on shoot demography. Shoot half-life was estimated to range from 2.5 to 60.4 yr and meadow turnover times between 6.7 yr and more than a century, provided current estimates of shoot mortality, recruitment rates, and density remain uniform. There were differences in shoot mortality and recruitment at the regional scale, with the meadows developing along the coast of the Spanish mainland experiencing the highest shoot mortality (Tukey test, p<0.05) and tending to exhibit the highest shoot recruitment. The low shoot recruitment did not balance shoot mortality in most (60%) of the meadows, showing a prevalence of declining populations among the 25 meadows studied (Wilcoxon ranked sign test, p<0.0005). This study demonstrates the power of direct census of seagrass shoots in permanent plots to evaluate the present status of seagrass meadows, to detect on-going population decline, and to provide some insight onto the possible factors involved. The incorporation of direct census of seagrass meadows to monitoring programs will help provide the early-warning signals necessary to support management decisions to conserve seagrass meadows.     

Trace Metals (Cd,Cu, Hg,and Pb) Accumulation Recorded in the Intertidal Mudflat Sediments of Three Coastal Lagoons in the Gulf of California,Mexico

²¹⁰Pb geochronologies of Cd, Cu, Hg, and Pb fluxes were obtained from the intertidal mudflat sediments of the coastal lagoons Chiricahueto, Estero de Urías, and Ohuira in the Mexican Pacific. The Cu and Hg sediment concentrations at the three lagoons fell within the ranges of 6–76 μg g⁻¹ and 0.1 to 592 ng g⁻¹, respectively; Chiricahueto and Estero de Urías sediments had comparable Cd and Pb concentrations within the ranges of 0.2–2.1 μg g⁻¹ and 10–67 μg g⁻¹, respectively; whereas in Ohuira lagoon, Cd concentrations were lower (0.1–0.5 μg g⁻¹) and Pb concentrations were higher (115–180 μg g⁻¹) than in the other lagoons. The metal fluxes (μg cm⁻² y⁻¹) for the three lagoons fell within the ranges of 0.02–0.15 for Cd, 0.7–6.0 for Cu, 0.001–0.045 for Hg, and 0.7–20 for Pb. The Hg pollution in Estero de Urías was attributed to the exhausts of the thermoelectric plant of Mazatlan and the metal enrichment in Chiricahueto and Ohuira was related to the agrochemical wastes from the croplands surrounding these lagoons.     

A comparative study of nutrient transfer via surface runoff from two small agricultural catchments in north China

A field study was conducted to determine the effect of landscape spatial pattern and micro-topography on nutrient transfer via runoff from two catchments into Yuqiao Reservoir in north China. The surface runoff discharge was measured during rainfall events and water samples were analyzed in 2004 and 2005. The mean annual total nitrogen (TN) and total phosphorus (TP) exports per unit area from Caogezhuang catchment (C catchment) were 1.048 and 0.561 kg ha⁻¹ year⁻¹, respectively, while the TN and TP exports from Taohuasi catchment (T catchment) were 0.158 and 0.027 kg ha⁻¹ year⁻¹. In both catchments, village and vineyard shared the highest nutrient export ability due to the accumulated animal waste and heavy application of fertilizer and manure. In T catchment, the distance of village and vineyard was about 1,500 m away from the receiving water and in between were woodland and cropland. In the hydrological pathway, there were sink landscape structures of small stone dams, roadside swale, vegetated filter strip and dry ponds, which could detain water and nutrients. In C catchment, the distance between the village and the receiving water was about 200 m, and the hydrological pathway was compacted roads and ditches with no sink structures. It is suggested that the distance between the pollution source area and the receiving water and the micro-topographical features were the main factors to control the great difference in nutrient export rates.     

Variability of continental riverine freshwater and nutrient inputs into the North Sea for the years 1977–2000 and its consequences for the assessment of eutrophication

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.     

The chemistry of suspended particulate material in a highly contaminated embayment of Port Jackson (Australia) under quiescent,high-wind and heavy-rainfall conditions

This study investigated physico-chemical characteristics of the water column and chemistry of suspended particulate material (SPM) under quiescent, high-wind and high-wind/heavy-rainfall conditions in Homebush Bay, a highly contaminated embayment of Port Jackson (Australia) to distinguish source and possible adverse effects to benthic and pelagic animals. Mean concentrations in surficial sediment were <1, 14, 181, 141, 37, 290 and 685 μg g⁻¹ for Cd, Co, Cr, Cu, Ni, Pb and Zn, respectively. Sediment chemistry indicated these metals had multiple sources, i.e. the estuary, stormwater and industry. Mean total suspended solids (TSS) were 7, 17 and 20 mg L⁻¹ during quiescent, high-rainfall and heavy rainfall/high wind conditions, respectively, whereas SPM Cd, Co, Cr, Cu, Ni, Pb and Zn concentrations varied between 13–25, 166–259, 127–198, 38–82, 236–305 and 605–865 μg g⁻¹, respectively under these conditions. TSS and total water metal concentrations were lowest during quiescent conditions. High TSS and metal loads in surface water characterised high-rainfall events. Wind-induced resuspension contributed the greatest mass of SPM and metals to the water column. Benthic animals may be adversely affected by Pb and Zn in sediment. Total water Cu and Zn concentrations may pose a risk to filter-feeding animals in the water column due to resuspension of contaminated sediment.     

Mapping of total nitrogen,available phosphorous and potassium in Amik Plain,Turkey

Soil nitrogen, phosphorous, and potassium concentrations accurately revealed spatial distribution maps and site-specific management-prone areas through inverse distance weighting (IDW) method in the Amik Plain, Turkey. Spatial mapping of soil nitrogen, phosphorous, and potassium is a very severe need to develop an economically and environmentally sound soil management plans. The objectives of this study were (a) to map spatial variability of total N, available P, and exchangeable-K content of Amik Plain’s soils and (b) to locate problematic areas requiring site specific management strategies for the nutrient elements. Spatial analyses of Kjeldhal-N, Olsen-P, and exchangeable-K concentrations of the soils were performed by the IDW method. Mean N content for surface soils (0–20 cm) was 1.38 g kg⁻¹, available P was 28.19 kg ha⁻¹ and exchangeable-K was 690 kg ha⁻¹ with the differences between maximum and minimum being 7.63 g N kg⁻¹, 242 kg P ha⁻¹, and 2,082 kg K ha⁻¹. For the surface soil, site-specific management-prone areas of Kjeldahl-N, Olsen-P, and exchangeable-K for “low and high + very high” classes were found to be 20.1–17.8%, 24.7–10.0%, and 4.1–39.6%, respectively. Consequently, lands with excessive nutrient elements require preventive-leaching practices, whereas nutrient-poor areas need fertilizer applications in favor of increasing plant production.     

Erosion rates evaluated by the <Superscript>137</Superscript>Cs technique in the high altitude area of the Qinghai–Tibet plateau of China

Studying spatial and temporal variation of soil loss is of great importance because of global environmental concerns. Understanding the spatial distribution of soil erosion and deposition in the high-cold steppe is important for designing soil and water conservation measures. Measured ¹³⁷Cs losses (Bq m⁻²) from long-term high altitude (4,000 m above sea level) watershed plots on the Qinghai–Tibet plateau and derived soil erosion estimates (Mg ha⁻¹ year⁻¹) were significantly correlated to directly measured soil losses from the same plots, over the same period (1963–2005). The local reference inventory was estimated to be 2,468 Bq m⁻². The result of analyzing ¹³⁷Cs distribution and its intensity in the soil profiles in this area shows similarities to ¹³⁷Cs distribution in other areas. ¹³⁷Cs is basically distributed in the topsoil layer of 0–0.3 m. Soil erosions vary greatly in the entire sampled area, ranging from 5.5 to 23 Mg ha⁻¹ year⁻¹, with an average of 16.5 Mg ha⁻¹ year⁻¹ which is a moderate rate of erosion.     

Trace metals and radionuclides reveal sediment sources and accumulation rates in Jordan Cove, Connecticut

Many small estuaries are influenced by flow restrictions resulting from transportation rights-of-way and other causes. The biogeochemical functioning and history of such systems can be evaluated through study of their sediments. Ten long and six short cores were collected from the length of Jordan Cove, Connecticut, a Long Island Sound subestuary, and analyzed for stratigraphy, radionuclides (¹⁴C, ²¹⁰Pb, ²²⁶Ra, ¹³⁷Cs, and ⁶⁰Co), and metals (Ag, Cd, Cu, Pb, Zn, Fe, and Al). For at least 3,800 yr, rising sea level has gradually inundated Jordan Cove, filling it with mud similar to that currently being deposited there. Long-term sediment accumulation in the cove averaged close to 0.1 cm yr⁻¹ over the last three millennia. Recent sediment accumulation rates decrease inland from 0.84 cm yr⁻¹ to 0.40 cm yr⁻¹, and are slightly faster than relative sea-level rise at this site (0.3 cm yr⁻¹). Similarity of depth distributions of trace metals was used to confirm relative sediment accumulation rates. ⁶⁰Co and Ag are derived from sources outside the cove and its watershed, presumably the Millstone nuclear power plant and regional contaminated sediments, respectively. The combined data suggest that Long Island Sound is an important source of sediment to the cove; a minor part of total sediment is supplied from the local watershed. Trace metal levels are strongly correlated with Fe but not with either organic matter or Al. Sediment quality has declined in the cove over the past 60 yr, but only slightly. Cu, Pb, and Zn data correlate strongly with Fe but not with either organic matter or aluminum. Ratios of Ag to Fe and to trace metals suggest that Ag in the cove is derived almost entirely from Long Island Sound. This result supports the notion that Fenormalized Ag can serve as a better tracer of some kinds of contamination than more common and abundant metals, like Cu, Pb, and Zn. *** DIRECT SUPPORT *** A01BY085 00008     

Comparison of soil organic carbon content,hydraulic conductivity,and particle size fractions between a grassland and a nearby black pine plantation of 40 years in two surface depths

In highlands of semiarid Turkey, ecosystems have been significantly transformed through human actions, and today changes are taking place very rapidly, causing harmful consequences such as soil degradation. This paper examines two neighboring land use types in Indagi Mountain Pass, Cankiri, Turkey, to determine effects of the conversion of Blackpine (Pinus nigra Arn. subsp. pallasiana) plantation from grassland 40 years ago on soil organic carbon (SOC) and soil erodibility (USLE-K). For this purpose, a total of 302 disturbed and undisturbed soil samples were taken at irregular intervals from two sites and from two soil depths of 0–10 cm (D₁) and 10–20 cm (D₂). In terms of SOC, conversion did not make any statistical difference between grassland and plantation; however, there were statistically significant differences with soil depth within each land use, and SOC contents significantly decreased with the soil depth (P < 0.05) and mostly accumulated in D₁. SOC values were 2.4 and 1.8% for grassland and 2.8 and 1.6% for plantation, respectively, at D₁ and D₂. USLE-K values also statistically differed significantly with the land use, and in contrast to the statistics of SOC, there was no change in USLE-K with the soil depth. Since USLE-K was estimated using SOC, hydraulic conductivity (HC) and soil textural composition––sand (S), silt (Si), and clay (C) contents of soils––as well as SOC did not change with the land use, we ascribed the changes of USLE-K with the land uses to the differences in the HC as strongly affected by the interactions between SOC and contents of S, Si, and C. On an average, the soil of the grassland (USLE-K = 0.161 t ha h ha⁻¹ MJ⁻¹ mm⁻¹) was more erodible than those of the plantation (USLE-K = 0.126 t ha h ha⁻¹ MJ⁻¹ mm⁻¹). Additionally, topographic factors, such as aspect and slope, were statistically effective on spatial distribution of the USLE-K and SOC.     

Vertical accretion rates and heavy metal chronologies in wetland sediments of the Tijuana Estuary

This study represents the first report on sediment accretion rates using¹³⁷Cs dating for a southern California salt marsh. Vertical accretion rates ranged from 0.7 to 1.2 cm yr⁻¹, which is at the high end of sediment accretion values for coastal wetlands. This has lead to increases in elevation within the estuary from 18 to 35 cm over the last 35 years. Depth profiles of metal concentrations were converted to time-based profiles using vertical accretion rates. Chronologies for most cores indicate a consistent peak in sediment lead (Pb) concentrations in the early to mid 1980s, corresponding to the historic decline in Pb use, which was completed in the U.S. by the early 1980s, but not begun in Mexico until 1991. Sediment Pb levels ranged from about 6–56 μg g⁻¹. Other metals did not show any consistent trends in sediment chronology, except for a single core from a mid-marsh site (east-mid 2), which showed a 2–3-fold increase in levels of Cu, Ni, and Zn during the past two decades. Sediment levels of copper (Cu), nickel (Ni), and zinc (Zn) ranged from 6–34 μg g⁻¹, 11–27 μg g⁻¹, and 42–122 μg g⁻¹, respectively. Despite rapid industrial development of the watershed, a comparison of the sediment metal concentrations in the Tijuana Estuary to other anthropogenically-impacted estuaries in the United States and Europe, shows that metal levels in sediments of the north arm of the estuary are relatively low.     

Changes in soil quality indicators under long-term sewage irrigation in a sub-tropical environment

Though irrigation with sewage water has potential benefits of meeting the water requirements, the sewage irrigation may mess up to harm the soil health. To assess the potential impacts of long-term sewage irrigation on soil health and to identify sensitive soil indicators, soil samples were collected from crop fields that have been irrigated with sewage water for more than 20 years. An adjacent rain-fed Leucaena leucocephala plantation system was used as a reference to compare the impact of sewage irrigation on soil qualities. Soils were analyzed for different physical, chemical, biological and biochemical parameters. Results have shown that use of sewage for irrigation improved the clay content to 18–22.7%, organic carbon to 0.51–0.86% and fertility status of soils. Build up in total N was up to 2,713 kg ha⁻¹, available N (397 kg ha⁻¹), available P (128 kg ha⁻¹), available K (524 kg ha⁻¹) and available S (65.5 kg ha⁻¹) in the surface (0.15 m) soil. Long-term sewage irrigation has also resulted a significant build-up of DTPA extractable Zn (314%), Cu (102%), Fe (715%), Mn (197.2), Cd (203%), Ni (1358%) and Pb (15.2%) when compared with the adjacent rain-fed reference soil. Soils irrigated with sewage exhibited a significant decrease in microbial biomass carbon (−78.2%), soil respiration (−82.3%), phosphatase activity (−59.12%) and dehydrogenase activity (−59.4%). An attempt was also made to identify the sensitive soil indicators under sewage irrigation, where microbial biomass carbon was singled out as the most sensitive indicator.     

Depth colonization of eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis>) and macroalgae as determined by water transparency in Danish coastal waters

We present a comparative analysis of lower depth limits for growth of eelgrass, large brown algae and other macroalgae measured by SCUBA-diving along 162 transects in 27 Danish fjords and coastal waters, coupled to 1,400 data series of water chemistry (especially nitrogen) and Secchi depth transparency collected between March and October. Danish coastal waters are heavily eutrophied and characterized by high particle concentrations, turbid water and lack of macrophyte growth in deep water. Median values are 3.6 m for Secchi depth and median lower-depth limits are 4.0 m for eelgrass, 5.3 m for brown algae and 5.0 m for other macroalgae. Depth limits for growth of eelgrass and macroalgae increase linearly with transparency in the coastal waters. The relationships are highly significant (p<10⁻⁶) and transparency accounts for about 60% of the variability of depth limits. Eelgrass extends approximately to half the maximum depth of macroalgae, presumably because of greater respiratory costs to maintain the below-ground rhizomes and roots of eelgrass, which often constitutes half the plant weight. As a reflection of the importance of total nitrogen (TN) in controlling phytoplankton biomass and thus Secchi depth in coastal marine waters, we found that TN could explain 48–73% of the variation in depth limits of eelgrass and macroalgae, according to a multiplicative model (Y=aX^b). As with Secchi depth, the relationship to eelgrass showed a lower slope, reflecting the higher respiratory costs of eelgrass. The models show great sensitivity and a profound quantitative response with proportional effects on Secchi depth and depth limits when total-N concentrations are reduced.     

Soil erosion determinations by using 137Cs technique in the agricultural regions of Gediz Basin, Western Turkey

Gediz Basin is one of the regions where intense agricultural activities take place in Western Turkey. Erosion and soil degradation have long been causing serious problems to cultivated fields in the basin. This work describes the application of two different ¹³⁷Cs models for estimating soil erosion rates in cultivated sites of the region. Soil samples were collected from five distinct cultivated regions subject to soil erosion. The variations of ¹³⁷Cs concentrations with depth in soil profiles were investigated. Soil loss rates were calculated from ¹³⁷Cs inventories of the samples using both proportional model (PM) and simplified mass balance model (SMBM). When PM was used, erosion and deposition rates varied from −15 to −28 t ha⁻¹ year⁻¹ and from +5 to +41 t ha⁻¹ year⁻¹, respectively; they varied from −16 to −33 t ha⁻¹ year⁻¹ and from +5 to +55 t ha⁻¹ year⁻¹ with SMBM. A good agreement was observed between the results of two models up to 30 t ha⁻¹ year⁻¹ soil loss and gain in the study area. Ulukent, a small representative agricultural field, was selected to compare the present data of ¹³⁷Cs techniques with the results obtained by universal soil loss equation (USLE) applied in the area before.     

An organic carbon budget for the Mississippi River turbidity plume and plume contributions to air-sea CO2 fluxes and bottom water hypoxia

We investigated seasonal variability in organic carbon (OC) budgets using a physical-biological model for the Mississippi River turbidity plume. Plume volume was calculated from mixed layer depth and area in each of four salinity subregions based on an extensive set of cruise data and satellite-derived suspended sediment distributions. These physical measurements were coupled with an existing food web model to determine seasonally dependent budgets for labile (reactive on time scales of days to weeks) OC in each salinity subregion. Autochthonous gross primary production (GPP) equaled 1.3×10¹² g C yr⁻¹ and dominated labile OC inputs (88% of the budget) because riverine OC was assumed mostly refractory (nonreactive). For perspective, riverine OC inputs amounted to 3.9×10¹² g C yr⁻¹, such that physical inputs were 3 times greater than biological inputs to the plume. Annually, microbial respiration (R) accounted for 65% of labile OC losses and net metabolism (GPP—R) for the entire plume was, autotrophic, equaling 5.1×10¹¹ g C yr⁻¹. Smaller losses of labile OC occurred via sedimentation (20%), advection (10%), and export to higher trophic levels (5%). In our present model, annual losses of labile OC are 10% higher than inputs, indicating future improvements are required. Application of our model to estimate air-sea carbon dioxide (CO₂) fluxes indicated the plume was a net sink of 2.0×10⁹ mol CO₂ yr⁻¹, of which 90% of the total drawdown was from biotic factors. In all seasons, low salinity waters were a source of CO₂ (pCO₂=560–890 μatm), and intermediate to high salinity waters were a sink of CO₂ (pCO₂=200–370 μatm). Our model was also used to calculate O₂ demand for the development, of regional hypoxia, and our spring and early summer budgets indicated that sedimentation of autochthonous OC from the immediate plume contributed 23% of the O₂ demand necessary for establishment of hypoxia in the region.     

Sources, input pathways, and distributions of Fe, Cu, and Zn in a Chesapeake Bay tidal freshwater marsh

Tidal freshwater marshes exist at the interface between watersheds and estuaries, and thus may serve as critical buffers protecting estuaries from anthropogenic metal pollution. Bi-weekly samples of newly deposited marsh sediments were collected and analyzed for Cu, Zn, and Fe concentrations over 21 months from July 1995 to March 1997 in five distinct habitats at the head of Bush River, Maryland. Bi-weekly anthropogenic metal enrichments ranged from 0.9–4.7. Anthropogenic excess metal loadings averaged over 1996 ranged from 6–306 and 25–1302 μg cm⁻² year⁻¹ between sites for Cu and Zn, respectively. Based on Fe-normalized trace metal signatures, Susquehanna River sediment does not significantly contribute to upper Bush River. Organic matter was found to dilute total metal concentrations, whereas past studies suggested organics enhance labile metal content. Analysis of metal input pathways shows that marsh metals are primarily imported from nearby subtidal accumulations of historic watershed material by tidal flushing. Received: 29 April 1999 / Accepted: 7 December 1999