Nitrogen Retention, Removal, and Saturation in Lotic Ecosystems

Increased nitrogen (N) loading to lotic ecosystems may cause fundamental changes in the ability of streams and rivers to retain or remove N due to the potential for N saturation. Lotic ecosystems will saturate with sustained increases in the N load, but it is unclear at what point saturation will occur. Rates of N transformation in lotic ecosystems will vary depending on the total N load and whether it is an acute or chronic N load. Nitrogen saturation may not occur with only pulsed or short-term increases in N. Overall, saturation of microbial uptake will occur prior to saturation of denitrification of N and denitrification will become saturated prior to nitrification, exacerbating increases in nitrate concentrations and in N export downstream. The rate of N export to downstream ecosystems will increase proportionally to the N load once saturation occurs. Long term data sets showed that smaller lotic ecosystems have a greater capacity to remove in-stream N loads, relative to larger systems. Thus, denitrification is likely to become less important as a N loss mechanism as the stream size increases. There is a great need for long-term studies of N additions in lotic ecosystems and clear distinctions need to be made between ecosystem responses to short-term or periodic increases in N loading and alterations in ecosystem functions due to chronic N loading.     

Nutrient-enhanced productivity in the northern Gulf of Mexico: past,present and future

Nutrient over-enrichment in many areas around the world is having pervasive ecological effects on coastal ecosystems. These effects include reduced dissolved oxygen in aquatic systems and subsequent impacts on living resources. The largest zone of oxygen-depleted coastal waters in the United States, and the entire western Atlantic Ocean, is found in the northern Gulf of Mexico on the Louisiana/Texas continental shelf influenced by the freshwater discharge and nutrient load of the Mississippi River system. The mid-summer bottom areal extent of hypoxic waters (<2 mg l^–1 O₂) in 1985–1992 averaged 8000 to 9000 km² but increased to up to 16000 to 20700 km² in 1993–2001. The Mississippi River system is the dominant source of fresh water and nutrients to the northern Gulf of Mexico. Mississippi River nutrient concentrations and loading to the adjacent continental shelf have changed in the last half of the 20th century. The average annual nitrate concentration doubled, and the mean silicate concentration was reduced by 50%. There is no doubt that the average concentration and flux of nitrogen (per unit volume discharge) increased from the 1950s to 1980s, especially in the spring. There is considerable evidence that nutrient-enhanced primary production in the northern Gulf of Mexico is causally related to the oxygen depletion in the lower water column. Evidence from long-term data sets and the sedimentary record demonstrate that historic increases in riverine dissolved inorganic nitrogen concentration and loads over the last 50 years are highly correlated with indicators of increased productivity in the overlying water column, i.e. eutrophication of the continental shelf waters, and subsequent worsening of oxygen stress in the bottom waters. Evidence associates increased coastal ocean productivity and worsening oxygen depletion with changes in landscape use and nutrient management that resulted in nutrient enrichment of receiving waters. A steady-state model, calibrated to different observed summer conditions, was used to assess the response of the system to reductions in nutrient inputs. A reduction in surface layer chlorophyll and an increase in lower layer dissolved oxygen resulted from a reduction of either nitrogen or phosphorus loading, with the response being greater for nitrogen reductions.     

The Shatt al-Arab River: A nutrient salt and organic matter source to the Arabian Gulf

The distribution of dissolved reactive phosphate, nitrate and nitrite in the waters as well as total organic carbon, total phosphorus and Kjeldahl nitrogen in the sediments of the Shatt al-Arab Estuary and the NW Arabian Gulf were studied from November 1979 to April 1980. The Shatt al-Arab waters contain 0.18 to 0.70 µg-at P-PO _inf4 ^sup3– l^–1, 26.12 to 52.39 µg-at N-N0 _inf3 ^sup– l^–1 and 0.53 to 0.70 µg-at N-NO _inf2 ^sup– l^–1, indicating that this river should be considered a source of nutrients to the Arabian Gulf. It is concluded that most of the nitrate is supplied in dissolved form, while an appreciable amount of phosphate is absorbed to fine suspended particles and released at higher salinities. Total organic carbon in surficial sediments was found to vary between 0.14% and 0.96%. These rather low values are attributed to dilution by dust fallout, which is a major cource of sediments in this area.     

Plasticity in Resource Allocation and Nitrogen-use Efficiency in Riparian Vegetation: Implications for Nitrogen Retention

In this work, we summarize our current understanding of the function of riparian zones and describe an investigation of changes in the production per unit nitrogen (N) taken up, or nitrogen-use efficiency (NUE) and resource allocation of a riparian shrub in response to changes in N availability. Empirical work included measuring leaf %N and root-to-shoot ratios (R:S) of individual riparian shrubs (Baccharis salicifolia, or seepwillow) growing at a range of N availabilities in the field and growing in fertilized and unfertilized plots in a field fertilization experiment. In both observational and experimental work, N availability was related positively to %N of plant tissues and negatively to R:S. We used a simulation model to investigate feedbacks between seepwillow responses to and effects on N availability. In the model, plasticity in resource allocation and NUE in response to changes in N led to lower productivity at low N supply and higher productivity and lower retention at high N supply than was observed in plants constrained to a constant %N and R:S. Furthermore, uptake became relatively more important as a retention mechanism when plants responded to high N supply. These feedbacks could have significant effects on N retention by riparian zones in watersheds receiving large fertilizer inputs of N or on ecosystems exposed to high rates of atmospheric N deposition.     

Nitrogen solutes in an Adirondack forested watershed: Importance of dissolved organic nitrogen

Nitrogen (N) dynamics were evaluated from 1 June 1995 through 31 May 1996 within the Arbutus Lake watershed in the Adirondack Mountains of New York State, USA. At the Arbutus Lake outlet dissolved organic nitrogen (DON), NO₃ ^- and NH₄ ⁺ contributed 61%, 33%, and 6% respectively, to the total dissolved nitrogen (TDN) flux (259 mol ha^-1 yr^-1). At the lake inlet DON, NO₃ ^-, and NH₄ ^- constituted 36%, 61%, and 3% respectively, of TDN flux (349 mol ha^-1 yr^-1). Differences between the factors that control DON, NO₃ ⁺, and NH₄ ⁺ stream water concentrations were evaluated using two methods for estimating annual N flux at the lake inlet. Using biweekly sampling NO₃ ^- and NH₄ ⁺ flux was 10 and 4 mol ha^-1 yr^-1 respectively, less than flux estimates using biweekly plus storm and snowmelt sampling. DON flux was 18 mol ha^-1 yr^-1 greater using only biweekly sampling. These differences are probably not of ecological significance relative to the total flux of N from the watershed (349 mol ha^-1 yr^-1). Dissolved organic N concentrations were positively related to discharge during both the dormant (R² = 0.31; P < 0.01) and growing season (R² = 0.09; P < 0.01). There was no significant relationship between NO₃ ^- concentration and discharge during the dormant season, but a significant negative relationship was found during the growing season (R² = 0.29; P < 0.01). Biotic controls in the growing season appeared to have had a larger impact on stream water NO₃ ^- concentrations than on DON concentrations. Arbutus Lake had a major impact on stream water N concentrations of the four landscape positions sampled, suggesting the need to quantify within lake processes to interpret N solute losses and patterns in watershed-lake systems.     

Radium in the Suwannee River and estuary

A two-year study of radium in the Suwannee River has shown that groundwater discharge, via springs, is a very important source of radium both to the river and to offshore Gulf of Mexico waters. Dissolved radium is maintained within relatively narrow limits in the river by uptake into suspended particles. In the estuary, dissolved radium versus salinity profiles show distinctive nonconservative behavior with radium in significant excess of its linear mixing value at mid-salinities. Unlike the situation in many other estuaries, however, desorption of radium from particles cannot account for most of the observed excess. Thus, the anomalously high radium characteristic of much of the west Florida shelf apparently does not have a riverine source. Direct effusion of high-radium groundwater into these coastal waters is thought to be the major supplier of radium, and perhaps other elements as well.     

白洋淀水陆交错带土壤对磷氮截留容量的初步研究

１　白洋淀水陆交错带的景观特征和土壤白洋淀地处冀中凹陷,是处于发育后期的草型富营养化湖泊。在整个湖区３６６ｋｍ２的范围内,以芦苇植被为景观特征的水陆交错带约占３６％,分布在围堤内湖边的洼地上和大小淀泊之间。其精细景观类型可划分成苇园和苇地二种类型;其中苇园约占１／３［１］,主要分布在淀泊之间和近围,特征是沟壕纵横,管理精细,景观斑块类型较小,匀一化程度高;苇地约占２／３,分布在苇园外围地势较高处,连大片分布,斑块嵌接,微景观结构较复杂,多样性程度较高［１］。水陆交错带处于该湖泊和陆地系统间,是重…     

Sources of Nitrogen to the Riparian Zone of a Desert Stream: Implications for Riparian Vegetation and Nitrogen Retention

Riparian zones effectively remove nitrogen (N) from water flowing through riparian soils, particularly in agricultural watersheds. The mechanism of N removal is still unclear, especially the role of vegetation. Uptake and denitrification are the two most commonly studied mechanisms. Retention of groundwater N by plant uptake is often inferred from measurements of N in net incremental biomass. However, this assumes other sources of N are not contributing to the N demand of plants. The purpose of this work was to investigate the relative importance of three sources of available N to riparian trees in a desert stream—input in stream water during floods, input during baseflow, and mineralization of N from soil organic matter. Two approaches were used; a mass balance approach in which the mass of available N from each source was estimated, and a correlational approach in which indexes of each source were compared to leaf N for individual willow trees. Total N from all sources was 396 kg ha⁻¹ y⁻¹, with 172 kg ha⁻¹ y⁻¹ from mineralization, 214 kg ha⁻¹ y⁻¹ from the stream during baseflow, and 9.6 kg ha⁻¹ y⁻¹ from floods. Leaf N was significantly related to N mineralization rates and flood inputs; it was not related to baseflow inputs. We conclude that mineralization is a major source of available N for willow trees, subsidized by input of N from floods. Baseflow inputs are most likely removed by rapid denitrification at the stream–riparian edge, while higher rates of flood supply exceed the capacity of this “filter.” Received 18 January 2001; accepted 15 June 2001.     

Effect of seasonal hypoxia on the benthic foraminiferal community of the Louisiana inner continental shelf: The 20th century record

A species census in sediment core samples reveals significant changes in the composition of the Louisiana-shelf benthic foraminiferal community in the past century; these changes can be explained by an increase in the severity of seasonal hypoxia in bottom waters. Agglutinated and porcelaneous orders living in water depths less than 60 m suffered a noticeable decline during this time. In particular, the genus Quinqueloculina was severely affected by the progression of hypoxia, and nearly disappeared from parts of the study area. In contrast, several hyaline taxa, especially Nonionella basiloba, Buliminella morgani, and Epistominella vitrea, tolerated the progressive oxygen depletion well. Results of cluster and principal components analyses of the foraminiferal assemblage data match the observed species distribution trends and indicate that seasonal hypoxia on the Louisiana shelf, related to eutrophication and water stratification, worsened in the past century, even near the outer edge of the present-day zone of spring and summer oxygen depletion. The temporal trends in the foraminiferal record correspond to that of fertilizer use in the U.S. and nitrogen loading in the Mississippi River, suggesting that the anthropogenic factor has been particularly strong in the development of coastal hypoxia since the early 1940s.     

温带阔叶林中氮的保留机制：

 在过去的半个世纪中,森林生态系统物质能量流动模型的发展使得生态学者对矿物元素的来源、去处、转换速率及其干扰破坏的影响有了进一步的认识。其中“春坝”假设提出：在早春来临之际,林冠树木仍处于冬眠之时,林下地被植物先于林冠树木发芽开花,吸收土壤养分于植物体内,使得养分免于因雨水或雪水而造成的淋失而得以保留于森林生态系统中。本文对“春坝”假设的研究作一小结。现有的研究结果显示,早春地被植物对土壤氮的吸收量因不同生态系统而异;而土壤微生物对氮的吸收量远高于过去人们所认识的水平。早春地被物与土壤微生物都能在季节的变化中吸收土壤养分、共同起着“春坝”作用。两者所起作用大小因气候及生态系统的不同而有所改变。      

Microzooplankton grazing and the control of phytoplankton biomass in the Suwannee River estuary,USA

Microzooplankton grazing can have significant impacts on the distribution and abundance of phytoplankton, thereby influencing the frequency and duration of algae blooms. Observations of high ciliate abundances in the Suwannee River estuary, Florida, suggest a significant potential for top-down pressure on the phytoplankton community by microzooplankton. We examined the composition of the microzooplankton and determined grazing mortality losses for phytoplankton within the Suwannee River estuary from 2001 to 2002. Our results indicated grazing mortality rates of 1.4 d⁻¹, equivalent to a loss of up to 76% of phytoplankton standing crop and up to 83% of total daily primary production. The microzooplankton community was primarily composed of ciliates, dinoflagellates, and copepod nauplii. The densities of ciliates in the estuary were comparable to densities reported in highly eutrophic ecosystems (9,400–72,800 ciliates l⁻¹). Grazing pressure on small phytoplankton may be further enhanced because ciliates and small dinoflagellates have growth rates similar to those of phytoplankton, and therefore can keep up with surges in abundance. Handling editor: Judit Padisak     

Soluble Organic Nitrogen Pools in Forest soils of Subtropical Australia

Soil soluble organic N (SON) plays an important role in N biogeochemical cycling. In this study, 22 surface forest soils (0–10 cm) were collected from southeast Queensland, Australia, to investigate the size of SON pools extracted by water and salt solutions. Approximately 5–45 mg SON kg⁻¹, 2–42 mg SON kg⁻¹ and 1–24 SON mg kg⁻¹ were extracted by 2 M KCl, 0.5 M K₂SO₄ and water, on average, corresponding to about 21.1, 13.5 and 7.0 kg SON ha⁻¹ at the 0–10 cm forest soils, respectively. These SON pools, on average, accounted for 39% (KCl extracts), 42% (K₂SO₄ extracts) and 43% (water extracts) of total soluble N (TSN), and 2.3% (KCl extracts), 1.3% (K₂SO₄ extracts) and 0.7% (water extracts) of soil total N, respectively. Large variation in SON pools observed across the sites in the present study may be attributed to a combination of factors including soil types, tree species, management practices and environmental conditions. Significant relationships were observed among the SON pools extracted by water, KCl and K₂SO₄ and microbial biomass N (MBN). In general, KCl and K₂SO₄ extracted more SON than water from the forest soils, while KCl extracted more SON than K₂SO₄. The SON and soluble organic C (SOC) in KCl, K₂SO₄ and water extracts were all positively related to soil organic C, total N and clay contents. This indicates that clay and soil organic matter play a key role in the retention of SON in soil.     

Extensive reproductive disruption, ovarian masculinization and aromatase suppression in Atlantic croaker in the northern Gulf of Mexico hypoxic zone

The long-term impacts on marine ecosystems of the recent dramatic worldwide increase in the incidence of coastal hypoxia are unknown. Here, we show widespread reproductive disruption in Atlantic croakers collected from hypoxic sites approximately 120 km apart in the extensive northern Gulf of Mexico continental shelf hypoxic zone. Gonadal growth and gamete production were impaired in croakers from hypoxic sites compared with fish from reference normoxic sites east of the Mississippi River Delta. Male germ cells were detected in approximately 19 per cent of croaker ovaries collected in the hypoxic region, but were absent in ovaries from normoxic sites. In addition, the sex ratio was skewed towards males at the hypoxic sites. The masculinization and other reproductive disruptions were associated with declines in neuroendocrine function, as well as ovarian and brain expression of aromatase (the enzyme that converts androgens to oestrogens). A similar incidence of ovarian masculinization and decline in ovarian aromatase expression were observed in croaker after chronic laboratory hypoxia exposure, indicating that ovarian masculinization is a specific hypoxia response and is due to decreased aromatase activity. The results suggest severe reproductive impairment can occur over large coastal regions in marine fish populations exposed to seasonal hypoxia, with potential long-term impacts on population abundance.     

Using a Gulf of Mexico Atlantis model to evaluate ecological indicators for sensitivity to fishing mortality and robustness to observation error

This work provides a formal evaluation of 25 ecological indicators highlighted by the Southeast Fisheries Science Center’s IEA program as useful for tracking ecosystem components in the Gulf of Mexico. Using an Atlantis ecosystem model as an operating model, we select indicators that are quantifiable using simulation outputs and evaluate their sensitivity to changes in fishing mortality. Indicator behavior was examined using a multivariate ordination. The ordination is used to tell how well each indicator describes variation in ecosystem structure (termed ‘importance’) under different levels of fishing mortality and to reveal redundancies in the information conveyed by indicators. We determine importance using sample data from the operating model, with and without observation error added. Indicators whose importance is diminished least by error are considered robust to observational error. We then quantify the interannual noise of each indicator, where annual variability relates to the required sampling frequency in a management application. Red snapper biomass, King mackerel biomass and Reef fish catch ranked in the top 5 most important without error scenarios, and King mackerel biomass and Species richness were in the top 5 most important even after error was added. Red snapper biomass was consistently found to be the most important and most robust among fishing mortality scenarios tested, and all 4 of these indicators were found to have low levels of interannual noise suggesting that they need to be sampled infrequently. Our results provide insight into the usefulness of these indicators for fisheries managers interested in the impacts of fishing on the ecosystem.     

The Long-term Effects of Disturbance on Organic and Inorganic Nitrogen Export in the White Mountains, New Hampshire

Traditional biogeochemical theories suggest that ecosystem nitrogen retention is controlled by biotic N limitation, that stream N losses should increase with successional age, and that increasing N deposition will accelerate this process. These theories ignore the role of dissolved organic nitrogen (DON) as a mechanism of N loss. We examined patterns of organic and inorganic N export from sets of old-growth and historically (80–110 years ago) logged and burned watersheds in the northeastern US, a region of moderate, elevated N deposition. Stream nitrate concentrations were strongly seasonal, and mean (± SD) nitrate export from old-growth watersheds (1.4 ± 0.6 kg N ha⁻¹ y⁻¹) was four times greater than from disturbed watersheds (0.3 ± 0.3 kg N ha⁻¹ y⁻¹), suggesting that biotic control over nitrate loss can persist for a century. DON loss averaged 0.7 (± 0.2) kg N ha⁻¹ y⁻¹ and accounted for 28–87% of total dissolved N (TDN) export. DON concentrations did not vary seasonally or with successional status, but correlated with dissolved organic carbon (DOC), which varied inversely with hardwood forest cover. The patterns of DON loss did not follow expected differences in biotic N demand but instead were consistent with expected differences in DOC production and sorption. Despite decades of moderate N deposition, TDN export was low, and even old-growth forests retained at least 65% of N inputs. The reasons for this high N retention are unclear: if due to a large capacity for N storage or biological removal, N saturation may require several decades to occur; if due to interannual climate variability, large losses of nitrate may occur much sooner. Received 27 April 1999; accepted 30 May 2000.     

Seasonal and spatial variation in the reproductive biology of the dwarf seahorse Hippocampus zosterae

Factors associated with the reproductive ecology of the dwarf seahorse Hippocampus zosterae were investigated. Fish from a Tampa Bay (FL, USA) seagrass ecosystem were collected, photographed and returned to the wild, with photos analysed to determine patterns of body size, density, sex ratio and reproductive state across site and season to understand the population dynamics of H. zosterae over time. Animal density did not vary significantly with site and season, indicating there is little evidence of seasonal migration in this species. Densities reported in this study were higher than the mean density for all seahorse species Hippocampus spp. There was no sexual dimorphism in body length and both sexes reached sexual maturity at the same size. The ratio of gravid to non-gravid males was found to shift by season but not by site, with breeding detected year-round in this population compared with populations further north in their range. Peak breeding (70% gravid males) was observed in the late summer–autumn (August–October) in the site furthest from shore. The largest fish for both sexes were recorded during the summer and autumn months in the mid-shore, deepest site. Sex ratio shifted by site with even sex ratios near the shore but significantly female-biased sex ratios detected at sites near open water. Lastly, an increase in marking dates with decreased time intervals between collections did not yield a higher recapture rate, compared with sampling in 2010. However, the Tampa Bay population of dwarf seahorses demonstrated stable densities across 3 years with year-round breeding, indicating that it is a robust population worthy of long-term monitoring for conservation efforts.     

植物氮素营养与病害发生关系研究进展

氮素营养不仅影响植物的正常生长发育,还会影响植物的感病性或抗病性。该文主要综述了氮素营养及其代谢对植物病害发展的影响、诱导病原菌侵染的寄主氮营养信号和氮素营养对植物与病原菌互作相关基因表达的影响,尤其是氮素受限(饥饿)对病原菌基因的诱导表达、植物衰老基因和抗病基因的关系、植物衰老过程中防御基因的表达等国际研究的热点领域所取得的成果和进展,并讨论了有待深入研究的问题。     

Patterns in the Chemical Fractionation of Organic Nitrogen in Rocky Mountain Streams

The intraannual dynamics of particulate organic nitrogen (PON) and two fractions of dissolved organic nitrogen (DON) were investigated in two Rocky Mountain streams draining watersheds with low rates of N deposition. Organic nitrogen accounted for over 60% of the total annual nitrogen export and consisted mostly of DON. Nitrate peaked during winter months and declined considerably during the growing season (less than 10 µg/L) suggesting the importance of biotic uptake. Concentrations of PON, total DON, and two DON fractions (humic and non-humic) peaked during spring runoff and were positively related to discharge, indicating hydrologic influence. Total DON and its two fractions showed significant inverse relationships to nitrate, indicating that DON and nitrate followed different intraannual patterns. Despite its seasonal fluctuations in concentration, PON showed a consistent carbon–nitrogen (C:N) ratio suggesting that it was relatively uniform in composition. Fractionation studies indicated that DON was primarily of non-humic origin, whereas dissolved organic carbon (DOC) was mainly derived from humic sources. The two DON fractions differed from each other in seasonal patterns of concentration and C:N ratio. The proportion of humic DON increased during snowmelt, and there were diverging seasonal patterns in the C:N ratio of the two fractions implying variations in bioavailability. Although organic nitrogen is commonly treated as a single pool in ecological studies, our results indicated that DON consists of fractions that undergo large intraannual changes in proportions and chemical composition. Treatment of DON as a single pool may be misleading from the viewpoint of understanding ecosystem processes directly related to changes in its sources and biological reactivity.     

Fish larvae assemblages in the Gulf of California

The distributional diversity and assemblages of fish larvae in the Gulf of California indicated two main seasonal stages and two transitional periods: in winter, the tropical water mass is confined to the south‐east portion of the mouth of the Gulf and larval fish assemblages are dominated by subtropical and temperate‐subarctic species; in summer; tropical water invades the Gulf and assemblages are dominated by tropical species. Both seasonal stages are separated by transitional periods coinciding with strong latitudinal temperature gradients. During the autumn and spring transitional periods, the Gulf of California splits into three regions: a northern region where temperate and subarctic species spawn from autumn to spring, a southern region dominated by tropical and subtropical species year round and a central region where tropical and temperate assemblages merge. Seasonal changes in the location of the regions, as well as the borders between them, show expansion and contraction of the northern and southern faunas related to the general oceanic circulation patterns during the year.