Rapid Nitrate Loss and Denitrification in a Temperate River Floodplain

Nitrogen (N) pollution is a problem in many large temperate zone rivers, and N retention in river channels is often small in these systems. To determine the potential for floodplains to act as N sinks during overbank flooding, we combined monitoring, denitrification assays, and experimental nitrate (NO₃⁻ -N) additions to determine how the amount and form of N changed during flooding and the processes responsible for these changes in the Wisconsin River floodplain (USA). Spring flooding increased N concentrations in the floodplain to levels equal to the river. As discharge declined and connectivity between the river and floodplain was disrupted, total dissolved N decreased over 75% from 1.41 mg l⁻¹, equivalent to source water in the Wisconsin River on 14 April 2001, to 0.34 mg l⁻¹ on 22 April 2001. Simultaneously NO₃⁻ -N was attenuated almost 100% from 1.09 to <0.002 mg l⁻¹. Unamended sediment denitrification rates were moderate (0–483 μg m⁻² h⁻¹) and seasonally variable, and activity was limited by the availability of NO ₃⁻ -N on all dates. Two experimental NO₃⁻ -N pulse additions to floodplain water bodies confirmed rapid NO₃⁻ -N depletion. Over 80% of the observed NO ₃⁻ -N decline was caused by hydrologic export for addition #1 but only 22% in addition #2. During the second addition, a significant fraction (>60%) of NO₃⁻ -N mass loss was not attributable to hydrologic losses or conversion to other forms of N, suggesting that denitrification was likely responsible for most of the NO₃⁻ -N disappearance. Floodplain capacity to decrease the dominant fraction of river borne N within days of inundation demonstrates that the Wisconsin River floodplain was an active N sink, that denitrification often drives N losses, and that enhancing connections between rivers and their floodplains may enhance overall retention and reduce N exports from large basins.     

Spatial and Temporal Variability in Sediment Denitrification Within an Agriculturally Influenced Reservoir

Reservoirs are intrinsically linked to the rivers that feed them, creating a river–reservoir continuum in which water and sediment inputs are a function of the surrounding watershed land use. We examined the spatial and temporal variability of sediment denitrification rates by sampling longitudinally along an agriculturally influenced river–reservoir continuum monthly for 13 months. Sediment denitrification rates ranged from 0 to 63 μg N₂O g ash free dry mass of sediments (AFDM)⁻¹ h⁻¹ or 0–2.7 μg N₂O g dry mass of sediments (DM)⁻¹ h⁻¹ at reservoir sites, vs. 0–12 μg N₂O gAFDM⁻¹ h⁻¹ or 0–0.27 μg N₂O gDM⁻¹ h⁻¹ at riverine sites. Temporally, highest denitrification activity traveled through the reservoir from upper reservoir sites to the dam, following the load of high nitrate (NO₃⁻-N) water associated with spring runoff. Annual mean sediment denitrification rates at different reservoir sites were consistently higher than at riverine sites, yet significant relationships among theses sites differed when denitrification rates were expressed per gDM vs. per gAFDM. There was a significant positive relationship between sediment denitrification rates and NO₃⁻-N concentration up to a threshold of 0.88 mg NO₃⁻ -N l⁻¹, above which it appeared NO₃⁻-N was no longer limiting. Denitrification assays were amended seasonally with NO₃⁻-N and an organic carbon source (glucose) to determine nutrient limitation of sediment denitrification. While organic carbon never limited sediment denitrification, all sites were significantly limited by NO₃⁻-N during fall and winter when ambient NO ₃⁻-N was low.     

Stimulation of leaf litter decomposition and associated fungi and invertebrates by moderate eutrophication: implications for stream assessment

1. We investigated the effect of moderate eutrophication on leaf litter decomposition and associated invertebrates in five reference and five eutrophied streams in central Portugal. Fungal parameters and litter N and P dynamics were followed in one pair of streams. Benthic invertebrate parameters that are considered useful in bioassessment were estimated in all streams. Finally, we evaluated the utility of decomposition as a tool to assess stream ecosystem functional integrity. 2. Decomposition of alder and oak leaves in coarse mesh bags was on average 2.3–2.7× faster in eutrophied than in reference streams. This was attributed to stimulation of fungal activity (fungal biomass accrual and sporulation of aquatic hyphomycetes) by dissolved nutrients. These effects were more pronounced for oak litter (lower quality substrate) than alder. N content of leaf litter did not differ between stream types, while P accrual was higher in the eutrophied than in the reference stream. Total invertebrate abundances and richness associated with oak litter, but not with alder, were higher in eutrophied streams. 3. We found only positive correlations between stream nutrients (DIN and SRP) and leaf litter decomposition rates in both fine and coarse mesh bags, associated sporulation rates of aquatic hyphomycetes and, in some cases, total invertebrate abundances and richness. 4. Some metrics based on benthic invertebrate community data (e.g. % shredders, % shredder taxa) were significantly lower in eutrophied than in reference streams, whereas the IBMWP index that is specifically designed for the Iberian peninsula classified all 10 streams in the highest possible class as having ‘very good’ ecological conditions. 5. Leaf litter decomposition was sufficiently sensitive to respond to low levels of eutrophication and could be a useful functional measure to complement assessment programmes based on structural parameters.     

Effects of Nutrient Enrichment on Decomposition and Fungal Colonization of Sweet Chestnut Leaves in an Iberian Stream (Central Portugal)

This study assessed the effect of nutrient enrichment on rates of decomposition, ergosterol concentrations (as a measure of fungal biomass), and rates of fungal sporulation of sweet chestnut (Castanea sativa Miller) leaves in a 3rd order stream (Central Portugal), with medium to high background values of nutrients. Coarse and fine mesh leaf bags were attached to nutrient diffusing substrata containing NaNO₃, KH₂PO₄, both nutrients, or no additions. Leaf breakdown rates were similar in the four treatments and in the two mesh sizes (k=−0.0155 to −0.0219 day⁻¹). Phosphorus content of P or N + P enriched leaves was higher than in the other treatments after 28 days, but there were no differences in N concentrations. Ergosterol concentrations associated with decomposing leaves were similar among treatments. The peak sporulation rates of aquatic hyphomycetes were stimulated by the addition of N + P and N but not by P alone. Results from the experiment provide evidence that leaf breakdown in the study stream, as a model for streams with naturally medium to high level of nutrients, was not nutrient-limited, and that fungal reproductive activity was limited by dissolved N but not by dissolved P in stream water.     

Synergistic effects of water temperature and dissolved nutrients on litter decomposition and associated fungi

In woodland streams, the decomposition of allochthonous organic matter constitutes a fundamental ecosystem process, where aquatic hyphomycetes play a pivotal role. It is therefore greatly affected by water temperature and nutrient concentrations. The individual effects of these factors on the decomposition of litter have been studied previously. However, in the climate warming scenario predicted for this century, water temperature and nutrient concentrations are expected to increase simultaneously, and their combined effects on litter decomposition and associated biological activity remains unevaluated. In this study, we addressed the individual and combined effects of water temperature (three levels) and nutrient concentrations (two levels) on the decomposition of alder leaves and associated aquatic hyphomycetes in microcosms. Decomposition rates across treatments varied between 0.0041 day^?1 at 5 °C and low nutrient level and 0.0100 day^?1 at 15 °C and high nutrient level. The stimulation of biological variables at high nutrients and temperatures indicates that nutrient enrichment of streams might have a higher stimulatory effect on fungal performance and decomposition rates under a warming scenario than at present. The stimulation of fungal biomass and sporulation with increasing temperature at both nutrient levels shows that increases in water temperature might enhance fungal growth and reproduction in both oligotrophic and eutrophic streams. The stimulation of fungal respiration and litter decomposition with increasing temperature at high nutrients indicates that stimulation of carbon mineralization will probably occur at eutrophied streams, while oligotrophic conditions seem to be ‘protected’ from warming. All biological variables were stimulated when both factors increased, as a result of synergistic interactions between factors. Increased water temperature and nutrient level also affected the structure of aquatic hyphomycete assemblages. It is plausible that if water quality of presently eutrophied streams is improved, the potential stimulatory effects of future increases in water temperature on aquatic biota and processes might be mitigated.     

Bioaugmented sulfur-oxidizing denitrification system with <Emphasis Type="Italic">Alcaligenes defragrans</Emphasis> B21 for high nitrate containing wastewater treatment

The performance of enriched sludge augmented with the B21 strain of Alcaligenes defragrans was compared with that of enriched sludge, as well as with pure Alcaligenes defragrans B21, in the context of a sulfur-oxidizing denitrification (SOD) process. In synthetic wastewater treatment containing 100–1,000 mg NO₃⁻-N/L, the single strain-seeded system exhibited superior performance, featuring higher efficiency and a shorter startup period, provided nitrate loading rate was less than 0.2 kg NO₃⁻-N/m³ per day. At nitrate loading rate of more than 0.5 kg NO₃⁻-N/m³ per day, the bioaugmented sludge system showed higher resistance to shock loading than two other systems. However, no advantage of the bioaugmented system over the enriched sludge system without B21 strain was observed in overall efficiency of denitrification. Both the bioaugmented sludge and enriched sludge systems obtained stable denitrification performance of more than 80% at nitrate loading rate of up to 2 kg NO₃⁻-N/m³ per day.     

Interactions between environmental variables on growth rate and carrageenan content of Solieria chordalis (Solieriaceae,Rhodophyceae) in culture

Using vegetative propagules (ramuli) of the iota carrageenan producing red seaweed Solieria chordalis, a maximum growth rate of 6.8% d⁻¹ was achieved when cultured at 20 °C and 100 μmol photon m⁻² s⁻¹ in seawater supplemented with 20 μM NO₃-N or 10 μM NO₃-N plus 10 μM NH₄-N. Ramuli grew less well when nitrogen was supplied solely as NH ₄ ⁺ . Maximum carrageenan content was observed at the same temperature, irradiance and salinity as growth, but at lower nitrogen concentrations. These findings are discussed in relation to results obtained from studies on other iota-carrageenan producing carrageenophytes.     

Aerobic cometabolic degradation of trichloroethene by methane and ammonia oxidizing microorganisms naturally associated with <Emphasis Type="Italic">Carex comosa</Emphasis> roots

The degradation potential of trichloroethene by the aerobic methane- and ammonia-oxidizing microorganisms naturally associated with wetland plant (Carex comosa) roots was examined in this study. In bench-scale microcosm experiments with washed (soil free) Carex comosa roots, the activity of root-associated methane- and ammonia-oxidizing microorganisms, which were naturally present on the root surface and/or embedded within the roots, was investigated. Significant methane and ammonia oxidation were observed reproducibly in batch reactors with washed roots incubated in growth media, where methane oxidation developed faster (2 weeks) compared to ammonia oxidation (4 weeks) in live microcosms. After enrichment, the methane oxidizers demonstrated their ability to degrade 150 μg l⁻¹ TCE effectively at 1.9 mg l⁻¹ of aqueous CH₄. In contrast, ammonia oxidizers showed a rapid and complete inhibition of ammonia oxidation with 150 μg l⁻¹ TCE at 20 mg l⁻¹ of NH₄ ⁺-N, which may be attributed to greater sensitivity of ammonia oxidizers to TCE or its degradation product. No such inhibitory effect of TCE degradation was detected on methane oxidation at the above experimental conditions. The results presented here suggest that microorganisms associated with wetland plant roots can assist in the natural attenuation of TCE in contaminated aquatic environments.     

Factors controlling soil water and stream water aluminum concentrations after a clearcut in a forested watershed with calcium-poor soils

The 24 ha Dry Creek watershed in the Catskill Mountains of southeastern New York State USA was clearcut during the winter of 1996–1997. The interactions among acidity, nitrate (NO₃⁻), aluminum (Al), and calcium (Ca²⁺) in streamwater, soil water, and groundwater were evaluated to determine how they affected the speciation, solubility, and concentrations of Al after the harvest. Watershed soils were characterized by low base saturation, high exchangeable Al concentrations, and low exchangeable base cation concentrations prior to the harvest. Mean streamwater NO₃⁻ concentration was about 20 μmol l⁻¹ for the 3 years before the harvest, increased sharply after the harvest, and peaked at 1,309 μmol l⁻¹ about 5 months after the harvest. Nitrate and inorganic monomeric aluminum (Al_im) export increased by 4−fold during the first year after the harvest. Al_im mobilization is of concern because it is toxic to some fish species and can inhibit the uptake of Ca²⁺ by tree roots. Organic complexation appeared to control Al solubility in the O horizon while ion exchange and possibly equilibrium with imogolite appeared to control Al solubility in the B horizon. Al_im and NO₃⁻ concentrations were strongly correlated in B-horizon soil water after the clearcut (r ² = 0.96), especially at NO₃⁻ concentrations greater than 100 μmol l⁻¹. Groundwater entering the stream from perennial springs contained high concentrations of base cations and low concentrations of NO₃⁻ which mixed with acidic, high Al_im soil water and decreased the concentration of Al_im in streamwater after the harvest. Five years after the harvest soil water NO₃⁻ concentrations had dropped below preharvest levels as the demand for nitrogen by regenerating vegetation increased, but groundwater NO₃⁻ concentrations remained elevated because groundwater has a longer residence time. As a result streamwater NO₃⁻ concentrations had not fallen below preharvest levels, even during the growing season, 5 years after the harvest because of the contribution of groundwater to the stream. Streamwater NO₃⁻ and Al_im concentrations increased more than reported in previous forest harvesting studies and the recovery was slower likely because the watershed has experienced several decades of acid deposition that has depleted initially base-poor soils of exchangeable base cations and caused long-term acidification of the soil.     

Organic Matter Dynamics in a Tropical Gallery Forest in a Grassland Landscape

The high biodiversity of tropical forest streams depends on the strong input of organic matter, yet the leaf litter decomposition dynamics in these streams are not well understood. We assessed how seasonal litterfall affects leaf litter breakdown, density and biomass of aquatic invertebrates, and the microbial biomass and sporulation of aquatic hyphomycetes in a South American grassland ‘vereda’ landscape. Although litter production in the riparian area was low, leaf litter breakdown was high compared with other South American systems, with maximum values coinciding with the rainy season. Fungal biomass in decomposing leaves was high, but spore densities in water and sporulation rates were very low. Invertebrates were not abundant in litter bags, suggesting they play a minor role in leaf litter decomposition. Chironomids accounted for ~70 percent of all invertebrates; only 10 percent of non‐Chironomidae invertebrates were shredders. Therefore, fungi appear to be the drivers of leaf litter decomposition. Our results show that despite low productivity and relatively fast litter decomposition, organic matter accumulated in the stream and riparian area. This pattern was attributed to the wet/dry cycles in which leaves falling in the flat riparian zone remain undecomposed (during the dry period) and are massively transported to the riverbed (rainy season).     

Nitrogen retention in the hyporheic zone of a glacial river in interior Alaska

We examined the hydrologic controls on nitrogen biogeochemistry in the hyporheic zone of the Tanana River, a glacially-fed river, in interior Alaska. We measured hyporheic solute concentrations, gas partial pressures, water table height, and flow rates along subsurface flowpaths on two islands for three summers. Denitrification was quantified using an in situ ¹⁵NO₃⁻ push–pull technique. Hyporheic water level responded rapidly to change in river stage, with the sites flooding periodically in mid−July to early−August. Nitrate concentration was nearly 3-fold greater in river (ca. 100 μg NO₃⁻–N l⁻¹) than hyporheic water (ca. 38 μg NO₃⁻–N l⁻¹), but approximately 60–80% of river nitrate was removed during the first 50 m of hyporheic flowpath. Denitrification during high river stage ranged from 1.9 to 29.4 mg N kg sediment⁻¹ day⁻¹. Hotspots of methane partial pressure, averaging 50,000 ppmv, occurred in densely vegetated sites in conjunction with mean oxygen concentration below 0.5 mgO₂ l⁻¹. Hyporheic flow was an important mechanism of nitrogen supply to microbes and plant roots, transporting on average 0.41 gNO₃⁻–N m⁻² day⁻¹, 0.22 g NH₄⁺–N m⁻² day⁻¹, and 3.6 g DON m⁻² day⁻¹ through surface sediment (top 2 m). Our results suggest that denitrification can be a major sink for river nitrate in boreal forest floodplain soils, particularly at the river-sediment interface. The stability of the river hydrograph and the resulting duration of soil saturation are key factors regulating the redox environment and anaerobic metabolism in the hyporheic zone.     

Nitrate loss from a restored floodplain in the Lower Cosumnes River, California

Floodplain restoration has been advocated as a means to restore several ecological services associated with floodplains: water quality improvement, fish rearing habitat, wildlife habitat, flood control, and groundwater recharge. A history of agricultural encroachment on the lower Cosumnes River has resulted in extensive channelization and levee construction. In fall 1998, an experimental floodplain was established by breaching a levee in order to restore the connection between the main channel and its historic floodplain. In this study, we examined how effective this newly restored floodplain was at processing nitrate (NO ₃ ⁻ ) before reentering the main channel downstream. Two methods were used to examine nitrate loss. In December 2001, we determined denitrification potentials of the floodplain soils before seasonal flooding inundated the floodplain. Next, we conducted a series of field soil column (mesocosm) experiments from March to June 2002 to study NO ₃ ⁻ -N loss from the overlying water in both sandy and clayey soils and at three levels of NO ₃ ⁻ -N (ambient, +1 mg N l⁻¹, +5 mg N l⁻¹). In addition, we examined NO ₃ ⁻ -N loss from mesocosms with water only to determine if loss was related primarily to soil or water column processes. Denitrification potentials were highly variable ranging from 1.6 to 769 ng N₂O–N cm⁻³ h⁻¹. In addition, the denitrification potential was highly correlated with the amount of bioavailable carbon indicating that carbon was a limiting factor for denitrification. Nitrate-N loss rates from mesocosms ranged from 2.9 to 21.0 μg N l⁻¹ h⁻¹ over all treatments and all 3 months examined. Significant loss of NO ₃ ⁻ -N (60–93%) from the water only treatment only occurred in June when warmer temperatures and solar radiation most likely increased NO ₃ ⁻ -N uptake by phytoplankton. When scaled up, potential NO ₃ ⁻ -N loss from the restored floodplain represented 0.6–4.4% of the annual N load from the Lower Cosumnes River during a typical wet year and ~24% during a dry year. During dry water years, the effectiveness of the floodplain for reducing nitrate is limited by the amount of N supplied to the floodplain. Results from this study suggest that restored floodplains can be an effective NO ₃ ⁻ sink.     

Regulation of inorganic carbon acquisition by nitrogen and phosphorus levels in the Nannochloropsis sp.

Nannochloropsis sp. was grown to the exponential phase and transferred to the high CO₂ (2,800 μl l⁻¹) and irradiance (100 μmol photons m⁻² s⁻¹) condition with different levels of nitrate and phosphate for 72 h, then the photosynthetic activity and inorganic carbon acquisition of the alga were measured. The apparent photosynthetic efficiency (α) of Nannochloropsis sp. decreased with increasing NO₃ ⁻ concentration from 150 to 3,000 μM, and the high nitrate-grown cells showed the lowest levels of light-saturated photosynthetic rate (P _m), while the low nitrate-grown cells showed the highest levels of dark respiration rate (R _d). The maximal light-saturated photosynthetic rate and the minimal dark respiration rate were seen under the middle nitrate condition. When the nitrate concentration ranged from 150 to 3,000 μM, the affinity for inorganic carbons of Nannochloropsis sp. increased sharply with the increasing NO₃ ⁻ concentration to 300 μM and then decreased significantly. The middle phosphate-grown cells exhibited the highest light-saturated photosynthetic rate and apparent photosynthetic efficiency, however, the affinity for inorganic carbons of Nannochloropsis sp. was the maximum under the low phosphate condition. It was shown that the appropriate nitrogen and phosphorus levels were of vital importance to the photosynthesis of cells.     

Nitrate removal from drinking water using a membrane-fixed biofilm reactor

Biological treatment of drinking water is a cost-effective alternative to conventional physico/chemical processes. A new concept was tested to overcome the main disadvantage of biological denitrification, the intensive post-treatment process to remove microorganisms and remnant carbon source. The biological reaction zone and carbon supply were separated from the raw water stream by a nitrate-permeable membrane. Denitrification takes place in a biofilm, which is immobilized at the membrane. In a series of bench-scale runs, different types of membranes and reactor configurations were investigated. The best denitrification rates achieved were 1230 mg NO₃ ⁻-N m⁻² day⁻¹. In one run, raw water containing 100 mg NO₃ ⁻ l⁻¹ was completely freed from nitrate. The membrane and the attached biofilm also represent a barrier against the passage of the C source and nutrients into the raw water. At concentrations of 20 mg l⁻¹ ethanol and 15 mg l⁻¹ phosphate in the bioreactor no diffusion through the membrane into the treated water was observed. Without any post-treatment, the effluent met nearly all the relevant criteria for drinking water; only the colony count was slightly increased. Received: 18 December 1996 / Received last revision: 14 April 1997 / Accepted: 19 April 1997     

The saturation of N cycling in Central Plains streams: <Superscript>15</Superscript>N experiments across a broad gradient of nitrate concentrations

We conducted ¹⁵NO₃⁻ stable isotope tracer releases in nine streams with varied intensities and types of human impacts in the upstream watershed to measure nitrate (NO₃⁻) cycling dynamics. Mean ambient NO₃⁻ concentrations of the streams ranged from 0.9 to 21,000 μg l⁻¹ NO₃⁻–N. Major N-transforming processes, including uptake, nitrification, and denitrification, all increased approximately two to three orders of magnitude along the same gradient. Despite increases in transformation rates, the efficiency with which stream biota utilized available NO₃⁻-decreased along the gradient of increasing NO₃⁻. Observed functional relationships of biological N transformations (uptake and nitrification) with NO₃⁻ concentration did not support a 1st order model and did not show signs of Michaelis–Menten type saturation. The empirical relationship was best described by a Efficiency Loss model, in which log-transformed rates (uptake and nitrification) increase with log-transformed nitrate concentration with a slope less than one. Denitrification increased linearly across the gradient of NO₃⁻ concentrations, but only accounted for ∼1% of total NO₃⁻ uptake. On average, 20% of stream water NO₃⁻ was lost to denitrification per km, but the percentage removed in most streams was <5% km⁻¹. Although the rate of cycling was greater in streams with larger NO₃⁻ concentrations, the relative proportion of NO₃⁻ retained per unit length of stream decreased as NO₃⁻ concentration increased. Due to the rapid rate of NO₃⁻ turnover, these streams have a great potential for short-term retention of N from the landscape, but the ability to remove N through denitrification is highly variable.     

Aquatic Hyphomycete Communities Associated with Decomposing Alder Leaf Litter in Reference Headwater Streams of the Basque Country (northern Spain)

The community of aquatic hyphomycetes associated with decomposing alder leaf litter was studied during autumn-winter in nine headwater reference streams of the Basque Country (northern Spain). In order to study the spatial variability in composition and community structure, three streams from each of three different river basins were compared. The colonization dynamics and community changes throughout the decomposition process were also followed in three of the rivers (one per basin). The taxonomic richness and community structure of these fungi varied among rivers, including similar streams of a given watershed. However, neither species diversity nor total abundance was statistically related to environmental variables. Only the conidial production of two of the species, Flagellospora curvula and Lunulospora curvula appeared to be enhanced by nitrate availability in the water. The taxonomic richness and the reproductive activity (sporulation rate) were positively related to the leaf litter decomposition rate. The changes in conidial production along the process were similar for all the streams and helped explain leaf litter quality dynamics.     

Effect of nitrogen forms on growth,cell composition and N<Subscript>2</Subscript> fixation of <Emphasis Type="Italic">Cylindrospermopsis raciborskii</Emphasis> in phosphorus-limited chemostat cultures

The aim of this research was to test whether NH₄ ⁺ and NO₃ ⁻ affect the growth, P demand, cell composition and N₂ fixation of Cylindrospermopsis raciborskii under P limitation. Experiments were carried out in P-limited (200 μg l⁻¹ PO₄-P) chemostat cultures of C. raciborskii using an inflowing medium containing either 4,000 μg l⁻¹ NH₄-N, 4,000 μg l⁻¹ NO₃-N or no combined N. The results showed the cellular N:P and C:P ratios of C. raciborskii decreased towards the Redfield ratio with increasing dilution rate (D) due to the alleviation of P limitation. The cellular C:N and carotenoids:chlorophyll-a ratios also decreased with D, predominantly as a result of an increase in the chlorophyll-a and N content. The NH₄ ⁺ and NO₃ ⁻ supply reduced the P maintenance cell quota of C. raciborskii. Consequently, the biomass yield of the N₂-grown culture was significantly lower. The maximum specific growth rate of N₂-grown culture was also the lowest observed. It is suggested that these differences in growth parameters were caused by the P and energy requirement for heterocyte formation, nitrogenase synthesis and N₂ fixation. N₂ fixation was partially inhibited by NO₃ ⁻ and completely inhibited by NH₄ ⁺. It was probably repressed through the high N content of cells at high dissolved N concentrations. These results indicate that C. raciborskii is able to grow faster and maintain a higher biomass under P limitation where a sufficient supply of NH₄ ⁺ or NO₃ ⁻ is maintained. Information gained about the species-specific nutrient and pigment stoichiometry of C. raciborskii could help to access the degree of nutrient limitation in water bodies. Handling editor: Luigi Naselli-Flores     

Effect of nitrate concentration and UVR on photosynthesis,respiration, nitrate reductase activity,and phenolic compounds in <Emphasis Type="Italic">Ulva rigida</Emphasis> (Chlorophyta)

Seaweeds growing in the intertidal zone are exposed to fluctuating nitrate and ultraviolet radiation (UVR) levels. While it has been shown that elevated UVR levels and the decrease of nitrate concentration can reduce photosynthetic levels in seaweeds, less is known about the combined effect of nitrate levels and UVR on metabolism and photoprotection mechanisms of intertidal species. Consequently, the objective of this study was to evaluate the effect of nitrate concentration and UVR treatments on photosynthesis, respiration, nitrate reductase activity and phenolic compound levels of Ulva rigida (Chlorophyta). There was a two- to threefold increase in maximal gross photosynthesis (GP_max) and respiration rates, as nitrate increased from 0 to 50 μM NO₃⁻. Similarly, nitrate reductase activity increased linearly from low values in algae incubated at 0 μM NO₃ to high values in tissue incubated at 50 μM NO₃⁻. Phenolic compounds in the tissue of U. rigida increased approximately 60% under 50 μM NO₃⁻ relative to those incubated at 0 μM NO₃⁻. Algae exposed to UVR (8 h) showed a significant decrease in the effective quantum yield and respiration, however, no effect was observed in the phenolic compounds levels. Full recovery of effective quantum yield was observed after U. rigida was transferred for 48 h to low PAR. Nitrate reductase also decreased after an 8-h UVR exposure, but no differences were observed among the nitrate treatments. This study shows that high nitrate levels reduced the negative effect of UVR on the effective quantum yield and increased the recovery of key metabolic enzymes. It is possible that the increase of phenolic compounds in the thallus of U. rigida under high nitrate levels provide a photoprotective mechanism when exposed to high UV levels during low tides.     

The Status and Characteristics of Eutrophication in the Yangtze River (Changjiang) Estuary and the Adjacent East China Sea, China

Eutrophication has become increasingly serious and noxious algal blooms have been of more frequent occurrence in the Yangtze River Estuary and in the adjacent East China Sea. In 2003 and 2004, four cruises were undertaken in three zones in the estuary and in the adjacent sea to investigate nitrate (NO₃–N), ammonium (NH₄–N), nitrite (NO₂–N), soluble reactive phosphorus (SRP), dissolved reactive silica (DRSi), dissolved oxygen (DO), phytoplankton chlorophyll a (Chl a) and suspended particulate matter (SPM). The highest concentrations of DIN (NO₃–N+NH₄–N+NO₂–N), SRP and DRSi were 131.6, 1.2 and 155.6 μM, respectively. The maximum Chl a concentration was 19.5 mg m⁻³ in spring. An analysis of historical and recent data revealed that in the last 40 years, nitrate and SRP concentrations increased from 11 to 97 μM and from 0.4 to 0.95 μM, respectively. From 1963 to 2004, N:P ratios also increased from 30–40 up to 150. In parallel with the N and P enrichment, a significant increase of Chl a was detected, Chl a maximum being 20 mg m⁻³, nearly four times higher than in the 1980s. In 2004, the mean DO concentration in bottom waters was 4.35 mg l⁻¹, much lower than in the 1980s. In comparison with other estuaries, the Yangtze River Estuary was characterized by high DIN and DRSi concentrations, with low SRP concentrations. Despite the higher nutrient concentrations, Chl a concentrations were lower in the inner estuary (Zones 1 and 2) than in the adjacent sea (Zone 3). Based on nutrient availability, SPM and hydrodynamics, we assumed that in Zones 1 and 2 phytoplankton growth was suppressed by high turbidity, large tidal amplitude and short residence time. Furthermore, in Zone 3 water stratification was also an important factor that resulted in a greater phytoplankton biomass and lower DO concentrations. Due to hydrodynamics and turbidity, the open sea was unexpectedly more sensitive to nutrient enrichment and related eutrophication processes.     

Aerobic and Anoxic Growth and Nitrate Removal Capacity of a Marine Denitrifying Bacterium Isolated from a Recirculation Aquaculture System

Bacterial biofilters used in marine recirculation aquaculture systems need improvements to enhance nitrogen removal efficiency. Relatively little is known about biofilter autochthonous population structure and function. The present study was aimed at isolating and characterizing an autochthonous denitrifying bacterium from a marine biofilter installed at a recirculation aquaculture system. Colonization of four different media in a marine fish farm was followed by isolation of various denitrifying strains and molecular classification of the most promising one, strain T2, as a novel member of the Pseudomonas fluorescens cluster. This strain exhibits high metabolic versatility regarding N and C source utilization and environmental conditions for growth. It removed nitrate through aerobic assimilatory metabolism at a specific rate of 116.2 mg NO₃-N g dw⁻¹ h⁻¹. Dissimilatory NO₃-N removal was observed under oxic conditions at a limited rate, where transient NO₂-N formed represented 22% (0.17 mg L⁻¹) of the maximum transient NO₂-N observed under anoxic conditions. Dissimilatory NO₃-N removal under anoxic conditions occurred at a specific rate of 53.5 mg NO₃-N g dw⁻¹ h⁻¹. The isolated denitrifying strain was able to colonize different materials, such as granular activated carbon (GAC), Filtralite and Bioflow plastic rings, which allow the development of a prototype bioreactor for strain characterization under dynamic conditions and mimicking fish-farm operating conditions.