Nutrient Budget in the Seasonal Wetland of the Okavango Delta, Botswana

The nutrient (P and N species) and chloride budgets were investigated in a representative floodplain in the seasonal wetlands of the Okavango Delta, Botswana. A variety of sources of nutrients in the surface water were considered, namely ion species coming with the floodwater, those generated from dry floodplain soils and those from water-soluble dust deposition (both local and long-range sources). Concentrations of total-nitrogen and chloride in surface water were below 1 mg l⁻¹. Total-phosphorus concentrations were 0.05 mg l⁻¹, reflecting the oligotrophic character of the system. Dust deposition rates were highest for chloride at 2.44 g m⁻² year⁻¹ followed by 0.79 g m⁻² year⁻¹ for total-N, 0.40 g m⁻² year⁻¹ for ammonia and only 0.02 g m⁻² year⁻¹ for total-P, respectively. Chloride was derived primarily from long-range transport, while N and P species were of more local origin. Dissolution rates for these ions combined were calculated to be 3.9 g m⁻² for the flooded area in the 1999 season and thus all dry deposits must be re-dissolved. The accumulation of dust deposits on dry surfaces and their subsequent dissolution causes 2–5 times higher concentrations of nitrogen, phosphorus and chloride with the onset of the flood, thus boosting the nutrient stock in the crucial phase of the onset of flooding. Chloride dissolved from dry soil surfaces and dust contributed approximately 40% to the overall floodplain budget. Although contributions from the soil surface and dust to the nitrogen and phosphorus pools of the floodplain are less prominent (with 10% of total), they nonetheless represent a significant source of nutrients in the entire system. Extrapolation to annually flooded swamps (10,000 km²) indicates a maximum contribution of 40% for total-nitrogen and 60% for total-phosphorus from dust deposition on wet or dry surfaces to the nutrient pool of the water body.     

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     

Seasonal variation in nutrient limitation of microbial biofilms colonizing organic and inorganic substrata in streams

Humans have increased the availability of nutrients including nitrogen and phosphorus worldwide; therefore, understanding how microbes process nutrients is critical for environmental conservation. We examined nutrient limitation of biofilms colonizing inorganic (fritted glass) and organic (cellulose sponge) substrata in spring, summer, and autumn in three streams in Michigan, USA. Biofilms were enriched with nitrate (NO₃ ⁻), phosphate (PO₄ ³⁻), ammonium (NH₄ ⁺), NO₃ ⁻ + PO₄ ³⁻, NH₄ ⁺ + PO₄ ³⁻, or none (control). We quantified biofilm structure and function as chlorophyll a (i.e., primary producer biomass) and community respiration on all substrata. In one stream, we characterized bacterial and fungal communities on cellulose in autumn using clone library sequencing and denaturing gradient gel electrophoresis to determine if community structure was linked to nutrient limitation status. Despite oligotrophic conditions, primary producer biomass was infrequently nutrient limited. In contrast, respiration on organic substrata was frequently limited by N + P combinations. We found no difference between biofilm response to NH₄ ⁺ versus NO₃ ⁻ enrichment, although the response to both N-species was positively related to water column PO₄ ³⁻ concentrations and temperature. Molecular analysis for fungal community composition suggested no relationship to nutrient limitation, but the dominant members of the bacterial community on cellulose were different on NO₃ ⁻, PO₄³, and NO₃ ⁻ + PO₄ ³⁻ treatments relative to control, NH₄ ⁺, and NH₄ ⁺ + PO₄ ³⁻ treatments, which matched patterns for biofilm respiration rates from each treatment. Our results show discrete patterns of nutrient limitation dependent upon substratum type and season, and imply changes in bacterial community structure and function may be linked following nutrient enrichment in streams.     

Rapid solubilization of insoluble phosphate by a novel environmental stress-tolerant Burkholderia vietnamiensis M6 isolated from ginseng rhizospheric soil

We isolated and characterized novel insoluble phosphate (P)-solubilizing bacteria tolerant to environmental factors like high salt, low and high pHs, and low temperature. A bacterium M6 was isolated from a ginseng rhizospheric soil and confirmed to belong to Burkholderia vietnamiensis by BIOLOG system and 16S rRNA gene analysis. The optimal cultural conditions for the solubilization of P were 2.5% (w/v) glucose, 0.015% (w/v) urea, and 0.4% (w/v) MgCl₂·6H₂O along with initial pH 7.0 at 35°C. High-performance liquid chromatography analysis showed that B. vietnamiensis M6 produced gluconic and 2-ketogluconic acids. During the culture, the pH was reduced with increase in gluconic acid concentration and was inversely correlated with P solubilization. Insoluble P solubilization in the optimal medium was about 902 mg l⁻¹, which was approximately 1.6-fold higher than the yield in NBRIP medium (580 mg l⁻¹). B. vietnamiensis M6 showed resistance against different environmental stresses like 10–45°C, 1–5% (w/v) salt, and 2–11 pH range. The maximal concentration of soluble P produced by B. vietnamiensis M6 from Ca₃(PO₄)₂, CaHPO₄, and hydroxyapatite was 1,039, 2,132, and 1,754 mg l⁻¹, respectively. However, the strain M6 produced soluble P with 20 mg l⁻¹ from FePO₄ after 2 days and 100 mg l⁻¹ from AlPO₄ after 6 days, respectively. Our results indicate that B. vietnamiensis M6 could be a potential candidate for the development of biofertilizer applicable to environmentally stressed soil.     

Biofiltering efficiency, uptake and assimilation rates of Ulva clathrata (Roth) J. Agardh (Clorophyceae) cultivated in shrimp aquaculture waste water

The growth, biofiltering efficiency and uptake rates of Ulva clathrata were studied in a series of outdoor tanks, receiving waste water directly from a shrimp (Litopenaeus vannamei) aquaculture pond, under constant aeration and two different water regimes: (1) continuous flow, with 1 volume exchange a day (VE day^-1) and (2) static regime, with 1 VE after 4 days. Water temperature, salinity, pH, dissolved inorganic nitrogen (DIN), phosphate (PO₄), chlorophyll-a (chl-a), total suspended solids (TSS), macroalgal biomass (fresh weight) and tissue nutrient assimilation were monitored over 12 days. Ulva clathrata was highly efficient in removing the main inorganic nutrients from effluent water, stripping 70–82% of the total ammonium nitrogen (TAN) and 50% PO₄ within 15 h. Reductions in control tanks were much lower (Tukey HSD, P < 0.05). After 3 days, the mean uptake rates by the seaweed biomass under continuous flow were 3.09 mg DIN g DW day⁻¹ (383 mg DIN m⁻² day⁻¹) and 0.13 mg PO₄ g DW day⁻¹ (99 mg PO₄ m⁻² day⁻¹), being significantly higher than in the static regime (Tukey HSD, P < 0.05). The chl-a decreased in seaweed tanks, suggesting that U. clathrata inhibited phytoplankton growth. Correlations between the cumulative values of DIN removed from the water and total nitrogen assimilated into the seaweed biomass (r = 0.7 and 0.8, P < 0.05), suggest that nutrient removal by U. clathrata dominated over other processes such as phytoplankton and bacterial assimilation, ammonia volatilization and nutrient precipitation.     

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.     

Denitrification and nitrous oxide emissions from riparian forests soils exposed to prolonged nitrogen runoff

Compared to upland forests, riparian forest soils have greater potential to remove nitrate (NO₃) from agricultural runoff through denitrification. It is unclear, however, whether prolonged exposure of riparian soils to nitrogen (N) loading will affect the rate of denitrification and its end products. This research assesses the rate of denitrification and nitrous oxide (N₂O) emissions from riparian forest soils exposed to prolonged nutrient runoff from plant nurseries and compares these to similar forest soils not exposed to nutrient runoff. Nursery runoff also contains high levels of phosphate (PO₄). Since there are conflicting reports on the impact of PO₄ on the activity of denitrifying microbes, the impact of PO₄ on such activity was also investigated. Bulk and intact soil cores were collected from N-exposed and non-exposed forests to determine denitrification and N₂O emission rates, whereas denitrification potential was determined using soil slurries. Compared to the non-amended treatment, denitrification rate increased 2.7- and 3.4-fold when soil cores collected from both N-exposed and non-exposed sites were amended with 30 and 60 μg NO₃-N g⁻¹ soil, respectively. Net N₂O emissions were 1.5 and 1.7 times higher from the N-exposed sites compared to the non-exposed sites at 30 and 60 μg NO₃-N g⁻¹ soil amendment rates, respectively. Similarly, denitrification potential increased 17 times in response to addition of 15 μg NO₃-N g⁻¹ in soil slurries. The addition of PO₄ (5 μg PO₄-P g⁻¹) to soil slurries and intact cores did not affect denitrification rates. These observations suggest that prolonged N loading did not affect the denitrification potential of the riparian forest soils; however, it did result in higher N₂O emissions compared to emission rates from non-exposed forest soils.     

Dissolved Nitrogen, Phosphorus, and Sulfur forms in the Ecosystem Fluxes of a Montane Forest in Ecuador

The N, P, and S cycles in pristine forests are assumed to differ from those of anthropogenically impacted areas, but there are only a few studies to support this. Our objective was therefore to assess the controls of N, P, and S release, immobilization, and transport in a remote tropical montane forest. The study forest is located on steep slopes of the northern Andes in Ecuador. We determined the concentrations of NO₃-N, NH₄-N, dissolved organic N (DON), PO₄-P, dissolved organic P (DOP), SO₄-S, dissolved organic S (DOS), and dissolved organic C (DOC) in rainfall, throughfall, stemflow, lateral flow (in the organic layer), litter leachate, mineral soil solution, and stream water of three 8–13 ha catchments (1900–2200 m a.s.l.). The organic forms of N, P, and S contributed, on average, 55, 66, and 63% to the total N, P, and S concentrations in all ecosystem fluxes, respectively. The organic layer was the largest source of all N, P, and S species except for inorganic P and S. Most PO₄ was released in the canopy by leaching and most SO₄ in the mineral soil by weathering. The mineral soil was a sink for all studied compounds except for SO₄. Consequently, concentrations of dissolved inorganic and organic N and P were as low in stream water (TDN: 0.34–0.39 mg N l⁻¹, P not detectable) as in rainfall (TDN: 0.39–0.48 mg N l⁻¹, P not detectable), whereas total S concentrations were elevated (stream water: 0.04–0.15, rainfall: 0.01–0.07 mg S l⁻¹). Dissolved N, P, and S forms were positively correlated with pH at the scale of soil peda except inorganic S. Soil drying and rewetting promoted the release of dissolved inorganic N. High discharge levels following heavy rainstorms were associated with increased DOC, DON, NO₃-N and partly also NH₄-N concentrations in stream water. Nitrate-N concentrations in the stream water were positively correlated with stream discharge during the wetter period of the year. Our results demonstrate that the sources and sinks of N, P, and S were element-specific. More than half of the cycling N, P, and S was organic. Soil pH and moisture were important controls of N, P, and S solubility at the scale of individual soil peda whereas the flow regime influenced the export with stream water.     

Litter Decomposition and Nutrient Dynamics in a Phosphorus Enriched Everglades Marsh

A field study was conducted in a nutrient-impacted marsh in Water Conservation Area 2A (WCA-2A) of the Everglades in southern Florida, USA, to evaluate early stages of plant litter (detritus) decomposition along a well-documented trophic gradient, and to determine the relative importance of environmental factors and substrate composition in governing decomposition rate. Vertically stratified decomposition chambers containing native plant litter (cattail and sawgrass leaves) were placed in the soil and water column along a 10-km transect coinciding with a gradient of soil phosphorus (P) enrichment. Decomposition rate varied significantly along the vertical water–soil profile, with rates typically higher in the water column and litter layer than below the soil surface, presumably in response to vertical gradients of such environmental factors as O₂ and nutrient availability. An overall decrease in decomposition rate occurred along the soil P gradient (from high- to low-impact). First-order rate constant (k) values for decomposition ranged from 1.0 to 9.2 × 10⁻³ day⁻¹ (mean = 2.8 ×10⁻³ day⁻¹) for cattails, and from 6.7 × 10⁻⁴ to 3.0 ×  10⁻³ day⁻¹ (mean = 1.7 ×  10⁻³ day⁻¹) for sawgrass. Substantial N and P immobilization occurred within the litter layer, being most pronounced at nutrient-impacted sites. Nutrient content of the decomposing plant tissue was more strongly correlated to decomposition rate than was the nutrient content of the surrounding soil and water. Our experimental results suggest that, although decomposition rate was significantly affected by initial substrate composition, the external supply or availability of nutrients probably played a greater role in controlling decomposition rate. It was also evident that nutrient availability for litter decomposition was not accurately reflected by ambient nutrient concentration, e.g., water and soil porewater nutrient concentration.     

Optimization of polyhydroxybutyrate production by <Emphasis Type="Italic">Bacillus</Emphasis> sp. CFR 256 with corn steep liquor as a nitrogen source

Polyhydroxyalkanotes (PHAs), the eco-friendly biopolymers produced by many bacteria, are gaining importance in curtailing the environmental pollution by replacing the non-biodegradable plastics derived from petroleum. The present study was carried out to economize the polyhydroxybutyrate (PHB) production by optimizing the fermentation medium using corn steep liquor (CSL), a by-product of starch processing industry, as a cheap nitrogen source, by Bacillus sp. CFR 256. Response surface methodology (RSM) was used to optimize the fermentation medium using the variables such as corn steep liquor (5–25 g l⁻¹), Na₂HPO₄ 2H₂O (2.2–6.2 g l⁻¹), KH₂PO₄ (0.5–2.5 g l⁻¹), sucrose (5–55 g l⁻¹) and inoculum concentration (1–25 ml l⁻¹). Central composite rotatable design (CCRD) experiments were carried out to study the complex interactions of the variables. The optimum conditions for maximum PHB production were (g l⁻¹): CSL-25, Na₂HPO₄ 2H₂O-2.2, KH₂PO₄ − 0.5, sucrose − 55 and inoculum − 10 (ml l⁻¹). After 72 h of fermentation, the amount of PHA produced was 8.20 g l⁻¹ (51.20% of dry cell biomass). It is the first report on optimization of fermentation medium using CSL as a nitrogen source, for PHB production by Bacillus sp.     

TOC, TON, TOS and TOP in Rainfall, Throughfall, Litter Percolate and Soil Solution of a Montane Rainforest Succession at Mt. Kilimanjaro, Tanzania

Organic nutrients have proven to contribute significantly to nutrient cycling in temperate forest ecosystems. Still, little is known about their relevance in the tropics. In the present study, organic C, N, S and P were analysed in rainfall, throughfall, litter percolate and soil solution of a montane rainforest at Mt. Kilimanjaro, Tanzania. The aim was to determine the amounts of organic nutrients in different water pathways and to assess the influence of forest disturbance on organic nutrients by comparing mature forests, secondary forests and shrub vegetation in clearings. Concentrations of all studied elements increased from rainfall to throughfall and litter percolate and then exhibited a rapid decrease in the mineral soil. Concentrations of organic P were above the detection limit only in the litter percolate. Organic N (ON) as a fraction of total N increased from 50% in rainfall (0.19 mg l⁻¹) to 66% (0.45 mg l⁻¹) in throughfall followed by a decline to 39% in the litter percolate (0.77 mg l⁻¹) of the mature forest. Similarly, proportions of organic S and P amounted to 43 and 34%, respectively, in the litter percolate in mature forest. For ON, this proportion further decreased to less than 10% in the soil solution. The latter was probably attributable to a high sorption capacity of the studied Andisols, which led to overall low organic element concentrations in the soil solution (OC: 1.2 mg l⁻¹, ON: 0.05 mg l⁻¹ at 1 m soil depth) as compared to other temperate and tropical forest ecosystems. Organic element concentrations were higher in litter percolate and soil solution under the clearings, but there were no differences in the relative contribution of these elements. Organic nutrient forms at Mt. Kilimanjaro appeared to be much less susceptible to leaching than their inorganic forms.     

Effects of temperature, and availability of nitrogen and phosphorus on the abundance of Anabaena and Microcystis in Lake Biwa, Japan: an experimental approach

Under optimal nutrient conditions, both Microcystis sp. and Anabaena sp. isolated from Lake Biwa grew optimally at 28–32°C but differed in maximal growth rates, phosphate uptake kinetics, maximal phosphorus quotas, and growth responses to nitrogen and phosphorus limitation. The maximal growth rates of Microcystis and Anabaena were 1.6 and 1.25 divisions day⁻¹, respectively. With phosphate and nitrate in the growth-limiting range, the growth of Microcystis was optimal at an N : P ratio of 100 : 1 (by weight) and declined at lower (nitrogen limitation) and higher (phosphorus limitation) ratios. In contrast, Anabaena growth rates did not change at N : P ratios from 1000 : 1 to 10 : 1. Starting with cells containing the maximal phosphorus quota, Microcystis growth in minus-phosphorus medium ceased in 7–9 days, compared with 12–13 days for Anabaena. The phosphate turnover time in cultures starved to their minimum cell quotas was 7.9 min for Microcystis and 0.6 min for Anabaena. Microcystis had a higher K _s (0.12 μg P l⁻¹ 10⁻⁶ cells) and lower V _max (9.63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), than Anabaena (K _s 0.02 μg P l⁻¹ h⁻¹ 10⁻⁶ cells; V _max 46.25 63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), suggesting that Microcystis would not be able to grow well in phosphorus-limited waters. We conclude that in spite of the higher growth rate under ideal conditions, Microcystis does not usually bloom in the North Basin because of low availability of phosphorus and nitrogen. Although Anabaena has an efficient phosphorus-uptake system, its main strategy for growth in low-phosphorus environments may depend on storage of phosphorus during periods of abundant phosphorus supply, which are rare in the North Basin. Received: July 31, 2000 / Accepted: October 18, 2000     

Successional and physical controls on the retention of nitrogen in an undisturbed boreal forest ecosystem

Floristic succession in the boreal forest can have a dramatic influence on ecosystem nutrient cycling. We predicted that a decrease in plant and microbial demand for nitrogen (N) during the transition from mid- to late-succession forests would induce an increase in the leaching of dissolved inorganic nitrogen (DIN), relative to dissolved organic nitrogen (DON). To test this, we examined the chemistry of the soil solution collected from within and below the main rooting zones of mid- and late-succession forests, located along the Tanana River in interior Alaska. We also used a combination of hydrological and chemical analyses to investigate a key assumption of our methodology: that patterns of soil water movement did not change during this transition. Between stands, there was no difference in the proportion of DIN below the rooting zone. 84–98% of DIN at both depths consisted of nitrate, which was significantly higher in the deeper mineral soil than at the soil surface (0.46±0.12 mg NO⁻ ₃ –N l⁻¹ vs 0.17±0.12 mg NO⁻ ₃ –N l⁻¹, respectively), and 79–92% of the total dissolved N consisted of DON. Contrary to our original assumption that nutrients were primarily leached downward, out of the rooting zone, we found much evidence to suggest that the glacially-fed Tanana River (>200 m from these stands) was contributing to the influx of water and nutrients into the soil active layer of both stands. Soil water potentials were positively correlated with river discharge; and ionic and isotopic (δ¹⁸O of H₂O) values of the soil solution closely matched those of river water. Thus, our ability to elucidate biological control over ecosystem N retention was confounded by riverine nutrient inputs. Climatic warming is likely to extend the season of glacial melt and increase riverine nutrient inputs to forests along glacially-fed rivers.     

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.     

Response of Periphytic Algae to Gradients in Nitrogen and Phosphorus in Streamside Mesocosms

In this study we manipulated both nitrogen and phosphorus concentrations in stream mesocosms to develop quantitative relationships between periphytic algal growth rates and peak biomass with inorganic N and P concentrations. Stream water from Harts Run, a 2nd order stream in a pristine catchment, was constantly added to 36 stream-side stream mesocosms in low volumes and then recirculated to reduce nutrient concentrations. Clay tiles were colonized with periphyton in the mesocosms. Nutrients were added to create P and N concentrations ranging from less than Harts Run concentrations to 128 μg SRP l⁻¹ and 1024 μg NO₃-N l⁻¹. Algae and water were sampled every 3 days during colonization until periphyton communities reached peak biomass and then sloughed. Nutrient depletion was substantial in the mesocosms. Algae accumulated in all streams, even streams in which no nutrients were added. Nutrient limitation of algal growth and peak biomass accrual was observed in both low P and low N conditions. The Monod model best explained relationships between P and N concentrations and algal growth and peak biomass. Algal growth was 90% of maximum rates or higher in nutrient concentrations 16 μg SRP l⁻¹ and 86 μg DIN l⁻¹. These saturating concentrations for growth rates were 3–5 times lower than concentrations needed to produce maximum biomass. Modified Monod models using both DIN and SRP were developed to explain algal growth rates and peak biomass, which respectively explained 44 and 70% of the variance in algal response.     

Effects of elevated CO2 and phosphorus addition on productivity and community composition of intact monoliths from calcareous grassland

We investigated the effects of elevated CO₂ (600 μl l⁻¹ vs 350 μl l⁻¹) and phosphorus supply (1 g P m⁻² year⁻¹ vs unfertilized) on intact monoliths from species-rich calcareous grassland in a greenhouse. Aboveground community dry mass remained almost unaffected by elevated CO₂ in the first year (+6%, n.s.), but was significantly stimulated by CO₂ enrichment in year two (+26%, P<0.01). Among functional groups, only graminoids contributed significantly to this increase. The effect of phosphorus alone on community biomass was small in both years and marginally significant only when analyzed with MANOVA (+6% in year one, +9% in year two, 0.1 ≥P > 0.05). Belowground biomass and stubble after two seasons were not different in elevated CO₂ and when P was added. The small initial increase in aboveground community biomass under elevated CO₂ is explained by the fact that some species, in particular Carex flacca, responded very positively right from the beginning, while others, especially the dominant Bromus erectus, responded negatively to CO₂ enrichment. Shifts in community composition towards more responsive species explain the much larger CO₂ response in the second year. These shifts, i.e., a decline in xerophytic elements (B. erectus) and an increase in mesophytic grasses and legumes occurred independently of treatments in all monoliths but were accelerated significantly by elevated CO₂. The difference in average biomass production at elevated compared to ambient CO₂ was higher when P was supplied (at the community level the CO₂ response was enhanced from 20% to 33% when P was added, in graminoids from 17% to 27%, in legumes from 4% to 60%, and in C. flacca from 120% to 298% by year two). Based on observations in this and similar studies, we suggest that interactions between CO₂ concentration, species presence, and nutrient availability will govern community responses to elevated CO₂. Received: 12 July 1997 / Accepted: 28 March 1998     

Development and application of a nutrient-diffusing bioassay for large rivers

1. Laboratory and field experiments were performed to develop and then apply a nutrient-diffusing substratum (NDS) design suitable for use in large, fast-flowing rivers. 2. Initial laboratory experiments quantified diffusion of PO₄ and NO₃ from new and previously used clay pots, which were soaked in deionized distilled water. Mean release rates initially exceeded 2.4 and 725 μmol l^–1 day^–1 P and 0.22 and 18 μmol l^–1 day^–1 N from new and used pots, respectively, but declined rapidly with increasing time spent in deionized distilled water and were below detectable levels after about 18 and 29 days, respectively. 3. A phosphorus (P) dose–response experiment in a P-limited reach of the Athabasca River, Alberta, Canada showed that epilithic biomass and macroinvertebrate density on NDS increased with increasing concentrations of KH₂PO₄ up to about 0.5 m . Beyond this threshold, biomasses and densities were unaffected by initial KH₂PO₄ concentration. Coefficients of variation of epilithic biomass estimates declined with increasing KH₂PO₄ whereas invertebrate density appeared to be unaffected by KH₂PO₄ levels. 4. Release rates of both P and N from NDS filled with 0.5 m KH₂PO₄ or 0.5 m NaNO₃ declined at a log-negative rate from about 5000 μmol N-NO₃ l^–1 day^–1 and 3500 μmol P-PO₄ l^–1 day^–1 on day 2, to 200 μmol l^–1 day^–1 for both N and P on day 32. 5. After development, we used the diffusing substrata to identify spatial patterns in nutrient limitation at seven sites along a 120 km reach in the Athabasca River, that receives two known point-source nutrient inputs. NDS consisting of N, P, N + P and unenriched controls were attached to the river bottom for 22–23 days and then retrieved and sampled for epilithic chlorophyll a. Physicochemical parameters and epilithic biomasses on upper stone surfaces were also quantified when NDS were deployed and retrieved from each site. 6. Sites located immediately downstream of the two point source inputs had higher water column concentrations of PO₄ and epilithic biomasses than the site immediately upstream; epilithic biomass was positively related to PO₄ in the late autumn (r²⁼ 0.58) but not in early autumn. Sites located immediately below nutrient inputs were not nutrient-limited, whereas upstream reference sites were P-limited.     

Spatial variability in nutrient concentration and biofilm nutrient limitation in an urban watershed

Nutrient enrichment threatens river ecosystem health in urban watersheds, but the influence of urbanization on spatial variation in nutrient concentrations and nutrient limitation of biofilm activity are infrequently measured simultaneously. In summer 2009, we used synoptic sampling to measure spatial patterns of nitrate (NO₃ ⁻), ammonium (NH₄ ⁺), and soluble reactive phosphorus (SRP) concentration, flux, and instantaneous yield throughout the Bronx River watershed within New York City and adjacent suburbs. We also quantified biofilm response to addition of NO₃ ⁻, phosphate (PO₄ ³⁻), and NO₃ ⁻ + PO₄ ³⁻ on organic and inorganic surfaces in the river mainstem and tributaries. Longitudinal variation in NO₃ ⁻ was low and related to impervious surface cover across sub-watersheds, but spatial variation in NH₄ ⁺ and SRP was higher and unrelated to sub-watershed land-use. Biofilm respiration on organic surfaces was frequently limited by PO₄ ³⁻ or NO₃ ⁻ + PO₄ ³⁻, while primary production on organic and inorganic surfaces was nutrient-limited at just one site. Infrequent NO₃ ⁻ limitation and low spatial variability of NO₃ ⁻ throughout the watershed suggested saturation of biological N demand. For P, both higher biological demand and point-sources contributed to greater spatial variability. Finally, a comparison of our data to synoptic studies of forested, temperate watersheds showed lower spatial variation of N and P in urban watersheds. Reduced spatial variation in nutrients as a result of biological saturation may represent an overlooked effect of urbanization on watershed ecology, and may influence urban stream biota and downstream environments.     

Impact of a small cormorant (Phalacrocorax carbo sinensis) roost on nutrients and phytoplankton assemblages in the littoral regions of a submontane reservoir

Cormorants (median numbers of 54.5 ind.) excreted ca. 102.7 kg yr⁻¹ of nitrogen and ca. 80.5 kg yr⁻¹ of phosphorus in the area of the Dobczyce Reservoir (ca. 980 ha). Concentrations of N-tot and P-tot were 3 times higher in soil from the roosting area of the cormorants than outside this area. Nutrient concentrations in the littoral sediment of the reservoir were also higher in the area frequented by these birds. Differences in concentrations of pH, NO₃⁻, NO₂⁻, NH₄⁺, PO₄⁻, and P-tot in water between the area associated with the cormorants and the reference site were not found. Differences in the planktonic algal communities from around the roost and the reference site were not found, with the exception of the chlorophytes which were more abundant in the area occupied by the cormorants. Water movement and mixing in the reservoir can influence the effect of the nutrient load on the water chemistry and planktonic algae.     

Hillslope Nutrient Dynamics Following Upland Riparian Vegetation Disturbance

We investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic carbon (DOC), K⁺, Na⁺, Ca²⁺, Mg²⁺, NO₃ ⁻-N, NH₄ ⁺-N, PO₄ ³⁻-P, and SO₄ ²⁻ were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations of NO₃ ⁻-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO₃ ⁻-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO₃ ⁻-N increased four fold, and streamwater NO₃ ⁻-N doubled. Significant changes also occurred in soilwater pH, DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope. Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient uptake by canopy trees is a key control on NO₃ ⁻-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes significant nutrient loss to streams. Received 30 January 2001; accepted 25 July 2002.