Ammonium Uptake Rates Research Articles

Phytoplankton uptake of ammonium, nitrate and urea in the Neuse River Estuary, NC, USA

Uptake rates of ammonium, nitrate and urea were measured during the spring, summer and autumn (2001) in the eutrophic, nitrogen (N) limited Neuse River Estuary (NRE), North Carolina, USA. Ammonium was the dominant form of N taken up during the study, contributing approximately half of the total measured N uptake throughout the estuary. Nitrate uptake declined significantly with distance downstream comprising 33% of the total uptake in the upper estuary but only 11 and 16% in the middle and lower estuary, respectively. Urea uptake contributed least to the total pool in the upper estuary (16%), but increased in importance in the middle and lower estuary, comprising 45 and 37% of the total N taken up, respectively. The importance of regenerated N for fuelling phytoplankton productivity in the mesohaline sections of the NRE is demonstrated. The contribution of urea to the regenerated N pool suggests that internal regeneration of dissolved organic N may support a large proportion of the phytoplankton primary production and biomass accumulation in the middle and lower NRE. These results suggest that N-budgets based on dissolved inorganic N uptake rates alone will seriously under estimate phytoplankton N uptake.

DIVERSITY OF INTERTIDAL MACROALGAE INCREASES WITH NITROGEN LOADING BY INVERTEBRATES

Many ecological phenomena are characterized by context dependency, and the relationship between diversity and productivity is no exception. We examined the relationship between macroalgal diversity and nutrient availability by evaluating the effects of reduced nutrients and their subsequent replacement via local-scale nutrient loading in tide pools. Macroalgae in Oregon coast high-intertidal pools have evolved in a nitrate-rich upwelling ecosystem, but instead of settling on low-intertidal reefs (where algae are often immersed in nutrient-rich nearshore waters) these individuals have colonized high-zone pools, where they are isolated from the ocean for extended periods of time and are subjected to extended periods of nitrate depletion. In some pools, this nutrient stress was ameliorated by a positive interaction: the excretion of ammonium by invertebrates. We conducted experimental manipulations to quantify invertebrate-mediated ammonium loading and macroalgal ammonium uptake in high-intertidal pools. Variation in tide pool volume and invertebrate biomass created a gradient of local-scale nutrient inputs, allowing us to address the relationship between nitrogen loading and algal diversity. Slow-growing species tolerant of low nitrogen availability were joined by fast-growing species with higher nitrogen requirements in pools with higher ammonium loading rates. A fourfold increase in the ammonium loading rate was associated with a doubling in the number of macroalgal species, and macroalgal assemblages in more species-rich pools were characterized by higher rates of biomass-specific ammonium uptake. These patterns contrast with productivity–diversity relationships in terrestrial systems, where local-scale nutrient enrichments generally result in reduced producer diversity due to displacement of subordinate species by aggressive competitors. Our data suggest that the effect of enrichment on diversity is context-dependent. Each ecosystem has a critical level of nutrient availability, determined by the level of nutrients typically available in that system. Below this critical level, local-scale nutrient additions increase diversity, but above it, diversity declines with enrichment.

Mass‐transfer limitation of nutrient uptake by a wave‐dominated reef flat community

Maximum rates of phosphate, nitrate, and ammonium uptake by a reef flat community were estimated from rates of frictional dissipation that were calculated from the attenuation of waves propagating over the Kaneohe Bay Barrier Reef flat. Time‐averaged flow speeds across the entire study area ranged from 0.08 to 0.22 m s−1. Observed decreases in wave energy fluxes and near‐bottom orbital velocities gave a mean estimate for the bottom friction coefficient (cf) of 0.22 ± 0.03. Friction coefficients were used to estimate nutrient mass‐transfer coefficients of 5–9 m d−1 for phosphate and 9–16 m d−1 for ammonium and nitrate. Wave heights offshore of Kaneohe Bay were also used to estimate an annual phosphate mass‐transfer coefficient of 8 ± 2 m d21. This value is similar to a published value of 9 ± 2 m d−1 that was determined from measured changes in phosphate concentrations across the reef flat. We conclude that rates of phosphate uptake by the Kaneohe Bay Barrier Reef flat community are occurring near the limits of mass transfer.

Biological nitrogen and organic matter removal from tannery wastewater in pilot plant operations in Ethiopia.

The objective of this study was to set-up a pilot plant and to evaluate its effectiveness for biological nitrogen and organic matter removal from tannery wastewater in Ethiopia. A pilot wastewater treatment plant consisting of a predenitrification-nitrification process was constructed and operated for 6 months. This was fed with a raw tannery wastewater obtained from the Modjo Tannery located 70 km south of the capital, Addis Ababa. Up to 98% total nitrogen and chemical oxygen demand, and 95% ammonium nitrogen removal efficiencies were achieved in the system. The average effluent ammonium nitrogen ranged from 8.4 mg l(-1) to 86.0 mg l(-1), whereas the average effluent for nitrate nitrogen ranged from 2.9 mg l(-1) to 4.4 mg l(-1). The average values of denitrification and nitrification rates determined by nitrate and ammonium uptake rates (NUR and AUR) were 8.0 mg NO3-N [g volatile suspended solids (VSS)](-1) h(-1) and 5.4 mg NH4-N (g VSS)(-1) h(-1), respectively, demonstrating that the treatment processes of the pilot plant were effective. Further studies of the effect of chromium III on AUR showed 50% inhibition at a concentration of 85 mg l(-1), indicating that this metal was not causing process inhibition during performance operations. Thus, the predenitrification-nitrification process was found to be efficient for simultaneous removal of nitrogen and organic substrates from tannery wastewaters.

Regional differences in cerebral blood flow and cerebral ammonia metabolism in patients with cirrhosis.

Clinical and histopathological findings hint at regional differences in the brain's sensitivity to metabolic changes in cirrhosis. The aim of the present study was to examine regional differences in cerebral ammonia metabolism in patients with cirrhosis and grade 0-to-I hepatic encephalopathy (HE). (13)N-ammonia, (15)O-water positron emission tomography (PET) and magnetic resonance imaging (MRI) were performed. Quantitative values of cerebral blood flow (CBF) and the initial cerebral ammonia uptake rate (K1) were derived for several regions of interest from images of the desired parameters after interactive coregistration with the patients' MRI-studies. CBF (mL/mL/min), K1 (mL/mL/min), and the ammonia extraction fraction (K1/CBF) showed marked regional variance with the highest levels in the thalamus, the lenticular nucleus, and the cerebellum. In conclusion, the regional differences in cerebral ammonia uptake correspond to the distribution of histopathological changes in the brain of patients with cirrhosis as well as clinical features of HE, characterized by signs of basal ganglia and cerebellar dysfunction with corresponding signs of functional impairment, especially of the frontal cortex and cingulate gyrus.

CONTRASTING EFFECTS OF METHIONINE SULFOXIMINE ON UPTAKE AND ASSIMILATION OF AMMONIUM IN ULVA INTESTINALIS (CHLOROPHYCEAE)1

Ammonium is assimilated in algae by the glutamine synthetase (GS)–glutamine:2‐oxoglutarate aminotransferase pathway. In addition to the assimilation of external ammonium taken up across the cell membrane, an alga may have to reassimilate ammonium derived from endogenous sources (i.e. nitrate reduction, photorespiration, and amino acid degradation). Methionine sulfoximine (MSX), an irreversible inhibitor of GS, completely inhibited GS activity in Ulva intestinalis L. after 12 h. However, assimilation of externally derived ammonium was completely inhibited after only 1–2 h in the presence of MSX and was followed by production of endogenous ammonium. However, endogenous ammonium production in U. intestinalis represented only a mean of 4% of total assimilation attributable to GS. The internally controlled rate of ammonium uptake (Vi) was almost completely inhibited in the presence of MSX, suggesting that Vi is a measure of the maximum rate of ammonium assimilation. After complete inhibition of ammonium assimilation in the presence of MSX, the initial or surge (Vs) rate of ammonium uptake in the presence of 400 μM ammonium chloride decreased by only 17%. However, the amount that the rate of ammonium uptake decreased by was very similar to the uninhibited rate of ammonium assimilation. In addition, the decrease in the rate of ammonium uptake in darkness (in the absence of MSX) in the presence of 400 μM ammonium chloride matched the decrease in the rate of ammonium assimilation. However, in the presence of 10 μM ammonium chloride, MSX completely inhibited ammonium assimilation but had no effect on the rate of uptake.

Microbial ammonium cycling in the Mississippi River plume during the drought spring of 2000

Microbial potential uptake and regeneration rates of ammonium (NH 4 + ) were studied along a salinity gradient (salinities 0.2-34.4) in the Mississippi River plume during an extreme drought in spring 2000. Chlorophyll concentrations up to 30 μg L -1 were highest in the low- and mid-salinity regions (salinities 8.5-28.2) and comparable to records of other years but extended over smaller areas than during periods of normal river flow. Bacterial biomass (5.1-28.3 μg CL -1 ) was at the low end of the range observed in normal flow years, decreased with distance from the river mouth and did not peak with chlorophyll. Heterotrophic mncflagellate abundance (1.4-4.0 μg CL -1 ) did not reflect phytoplankton and bacterial spatial distribution but peaked at 9.2 μg C L -1 at salinity 8.5. Microbial NH 4 + regeneration rates were estimated by 15 NH 4 + isotope dilution experiments for the whole microbial community, under light and dark conditions, and for the <2 μm bacterium-dominated size fraction. Microbial. NH 4 + regeneration rates (0.018-0.124 μmol N L -1 h -1 ) were low relative to previous reports and peaked at salinity 28. Total NH 4 + regeneration rates were higher than those in the <2 μm size fraction at only four stations, suggesting that bacterial mineralization was a significant component of NH 4 + recycling in some parts of the river plume. Higher AH 4 + regeneration in whole-water samples versus <2 μm fractions provided evidence for microbial grazing in regions where chlorophyll and regeneration rates peaked and at two full-salinity stations.

Rate of Ammonium Uptake by Cranberry (Vaccinium macrocarpon Ait.) Vines in the Field is Affected by Temperature

Nitrogen fertilizer application is a universal practice among cranberry growers. Cranberries only use ammonium nitrogen sources. This study was undertaken to discover how quickly cranberries in the field would take up fertilizer-derived ammonium nitrogen. Ammonium sulfate labeled with 15N was applied in field locations in Oregon, Massachusetts, New Jersey, and Wisconsin. Samples of current season growth were collected daily for 7 days beginning 24 hours after fertilizer application. In all cases 15N was detectable in the plants from treated plots by 24 hours following application. Additional nitrogen was taken up for the next 3 to 5 days depending on the location. With the exception of Oregon, the maximum concentration of 15N was found by day 7. Oregon was the coolest of the sites in this research. To determine a temperature response curve for N uptake in cranberry, cranberry roots were exposed to various temperatures in aeroponics chambers while vines were at ambient greenhouse temperatures. The optimum temperature for N uptake by cranberry vines was 18 to 24 °C. This research suggests that ammonium fertilizers applied by growers and irrigated into the soil (solubilized) are taken up by the plant within 1 day following application. Soil and root temperature is involved in the rate of N uptake.

Distribution and dynamics of nitrogen and microbial plankton in southern Lake Michigan during spring transition 1999–2000

Ammonium and amino acid fluxes were examined as indicators of N and microbial food web dynamics in southern Lake Michigan during spring. Either 15NH4+ or a mixture of 15N‐labelled amino acids (both at 4 μM N final concentration) was added to Lake Michigan water. Net fluxes were measured over 24 h under natural light and dark conditions using deck‐top incubators and compared to microbial food web characteristics. Isotope dilution experiments showed similar light and dark NH4+ regeneration rates at lake (6 versus 5 nM N h−1) and river‐influenced (20 versus 24 nM N h−1) sites. Ammonium uptake rates were similar to regeneration rates in dark bottles. Dark uptake (attributed mainly to bacteria) accounted for ∼70% of total uptake (bacteria plus phytoplankton) in the light at most lake sites but only ∼30% of total uptake at river‐influenced sites in or near the St. Joseph River mouth (SJRM). Cluster analysis grouped stations having zero, average, or higher than average N‐cycling rates. Discriminant analysis indicated that chlorophyll concentration, oligotrich ciliate biomass, and total P concentration could explain 66% of N‐cycling rate variation on average. Heterotrophic bacterial N demand was about one third of the NH4+ regeneration rate. Results suggest that, with the exception of SJRM stations, bacterial uptake and protist grazing mediated much of the N dynamics during spring transition. Since NH4+ is more available to bacteria than NO3−, regenerated NH4+ may have a strong influence on spring, lake biochemical energetics by enhancing N‐poor organic matter degradation in this NO3− ‐replete ecosystem.

Nitrogen cycling in the southern North Sea: consequences for total nitrogen transport

Nitrate and ammonium uptake rates were measured during a series of cruises in the well-mixed region of the southern North Sea from February to September. Water column-integrated uptake rates ranged between 0.01 and 8.7 mmol N m −2 d −1 and 0.01 and 12.2 mmol N m −2 d −1 for nitrate and ammonium, respectively, with ammonium uptake dominating after the phytoplankton spring bloom in May. A moored buoy continuously measuring nitrate and chlorophyll a and seabed current meters were also deployed in the central southern North Sea in the region of the East Anglian plume—a permanent physical feature which transports nutrients towards continental Europe. This enabled the flux of water and hence of nutrients across the southern North Sea to be determined and an assessment of the contribution of freshwater nutrients to the flux to be made. A simple box model is developed to relate the phytoplankton uptake of nitrate and ammonium to the transport of nitrate, ammonium and particulate organic matter (POM) across the southern North Sea. This showed the importance of the plume region of the North Sea in the processing of nitrogen, with nitrate dominating total nitrogen transport prior to the spring bloom ( 10 340×10 3 kg N inflow to the plume in March) and transport of nitrogen as ammonium, nitrate and POM in approximately equivalent amounts during summer (2560, 2960 and 2151×10 3 kg N inflow to the plume, respectively, in July). The box model also demonstrates more generally the need to assess nitrogen transport as nitrate, ammonium and POM if an improved understanding of the impact of nutrient input in shelf seas is to be achieved.

Defoliation tolerance and ammonium uptake rate in perennial tussock grasses

Stipa clarazii, Ball. has been shown to be more tolerant to defoliation and a superior competitor to S. tenuis Philo and S. ambigua Speg. 3 perennial grasses native to semiarid rangelands in central Argentina. Mechanisms contributing to its great defoliation tolerance and competitive ability, however, are largely unexplored. We examined tolerance to defoliation and ammonium uptake rates on defoliated and undefoliated plants of those species at 10, 25, and 50 ppm NH4+ using (NH4)2SO4 solutions containing 60 atom %15N excess. By mid-spring, greater regrowth following defoliation in S. clarazii than in S. tenuis or S. ambigua indicated greater defoliation tolerance in the first than in the other 2 species. Stipa clarazii had similar of higher ammonium uptake rates than S. tenuis and S. ambigua. Higher ammonium uptake rates in S. clarazii thus appear to be one of the mechanisms most likely contributing to its greater competitive ability and defoliation tolerance when compared to the other 2 species. Defoliated plants of all 3 species had similar or greater ammonium uptake rates than undefoliated plants. These results suggest that photosynthetic canopy reestablishment may be achieved without sacrificing root function in these perennial grasses, at least as long as carbon reserves do not become a limiting factor. Ammonium uptake rates increased when NH4+ concentrations increased in the labeled solutions in S. clarazii, S. tenuis and S. ambigua. This result demonstrates the capacity of the root system for increasing nutrient acquisition during periods of high resource availability.

Defoliation tolerance and ammonium uptake rate in perennial tussock grasses

Stipa clarazii, Ball. has been shown to be more tolerant to defoliation and a superior competitor to S. tenuis Philo and S. ambigua Speg. 3 perennial grasses native to semiarid rangelands in central Argentina. Mechanisms contributing to its great defoliation tolerance and competitive ability, however, are largely unexplored. We examined tolerance to defoliation and ammonium uptake rates on defoliated and undefoliated plants of those species at 10, 25, and 50 ppm NH4+ using (NH4)2SO4 solutions containing 60 atom %15N excess. By mid-spring, greater regrowth following defoliation in S. clarazii than in S. tenuis or S. ambigua indicated greater defoliation tolerance in the first than in the other 2 species. Stipa clarazii had similar of higher ammonium uptake rates than S. tenuis and S. ambigua. Higher ammonium uptake rates in S. clarazii thus appear to be one of the mechanisms most likely contributing to its greater competitive ability and defoliation tolerance when compared to the other 2 species. Defoliated plants of all 3 species had similar or greater ammonium uptake rates than undefoliated plants. These results suggest that photosynthetic canopy reestablishment may be achieved without sacrificing root function in these perennial grasses, at least as long as carbon reserves do not become a limiting factor. Ammonium uptake rates increased when NH4+ concentrations increased in the labeled solutions in S. clarazii, S. tenuis and S. ambigua. This result demonstrates the capacity of the root system for increasing nutrient acquisition during periods of high resource availability.

High rates of ammonium recycling drive phytoplankton productivity in the offshore Mississippi River plume

As part of an integrated study of the regulation of carbon fixation in the offshore Mississippi River plume, we measured the rates of 15 N-labeled ammonium and nitrate uptake in the surface plume waters from offshore to nearshore along the plume axis towards the Mississippi Delta. Concentrations of nitrate in the plume ranged from 0.19 to 2.5 µM with the highest concentrations primarily in the shoreward stations, while ammonium ranged from 0.17 to 0.44 µM, showing little spatial variability. Rates of ammonium uptake ranged from 16.5 to 260 nM h -1 , and showed a strong trend of increasing values from offshore towards the Mississippi Delta. In contrast, nitrate uptake rates ranged from 3.2 to 25 nM h -1 . The high rates of ammonium uptake in the presence of low ammo- nium concentrations and elevated nitrate was made possible by elevated rates of ammonium re- generation that exceeded ammonium uptake by 1.7 to 5.7-fold in the plume. The plume exhibited relatively low f-ratios and also contained elevated levels of Synechococcus as determined by flow cytometry and high levels of form IA (α -cyanobacterial) rbcL transcripts. These data suggest that a major portion of the carbon fixation observed in the offshore Mississippi River plume represents recycled production supported by high rates of ammonium regeneration.

Defoliation Tolerance and Ammonium Uptake Rate in Perennial Tussock Grasses

Fil: Saint Pierre, Carolina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Bahia Blanca. Centro de Recursos Naturales Renovables de la Zona Semiarida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiarida; Argentina

A zinc-adapted fungus protects pines from zinc stress.

• Here we investigated zinc tolerance of ectomycorrhizal Scots pine (Pinus sylvestris) seedlings. An ectomycorrhizal genotype of Suillus bovinus, collected from a Zn-contaminated site and showing adaptive Zn tolerance in vitro, was compared with a nonadapted isolate from a nonpolluted area. • A dose-response experiment was performed. Dynamics of plant and fungal development, and phosphate and ammonium uptake capacity, were assessed under increasing Zn stress. Effects of Zn on transpiration, nutrient content and Zn accumulation were analysed. • Significant Zn-inoculation interaction effects were observed for several responses measured, including uptake rates of phosphate and ammonium; phosphorus, iron and Zn content in shoots; transpiration; biomass of external mycelia; and fungal biomass in roots. • The Zn-tolerant S.bovinus genotype was particularly efficient in protecting pines from Zn stress. The growth of a Zn-sensitive genotype from a normal wild-type population was inhibited at high Zn concentrations, and this isolate could not sustain the pines' acquisition of nutrients. This study shows that well adapted microbial root symbionts are a major component of the survival strategy of trees that colonize contaminated soils.

Nitrogen Use Efficiency of Crop Plants: Physiological Constraints upon Nitrogen Absorption

Current global nitrogen fertilizer use has reached approximately one hundred billion kg per annum. In many agricultural systems, a very substantial portion of this applied nitrogen fertilizer is lost from soil to groundwaters, rivers and oceans. While soil physicochemical properties play a significant part in these losses, there are several characteristic features of plant nitrogen transporter function that facilitate N losses. Nitrate and ammonium efflux from roots result in a reduction of net nitrogen uptake. As external nitrate and ammonium concentrations, respectively, are increased, particularly into the range of concentrations that are typical of agricultural soils, elevated rates of nitrate and ammonium efflux result. The rapid down-regulation of high-affinity influx as plants become nitrogen replete further reduces the root's capacity to acquire external nitrogen; only nitrogen-starved roots absorb with both high capacity and high affinity. The results of studies using molecular biology methods demonstrate that genes encoding nitrate and ammonium transporters are rapidly down-regulated when nitrogen is resupplied to nitrogen-starved plants. Provision of ammonium to roots of plants actively absorbing nitrate imposes a block on nitrate uptake, the extent of which depends on the ammonium concentration, thus further reducing the efficient utilization of soil nitrate. During the daily variation of incoming light and during periods of low incident irradiation (i.e. heavy cloud cover) the expression levels of genes encoding nitrate and ammonium transporters, and rates of nitrate and ammonium uptake, are substantially reduced. Low temperatures reduce growth and nitrogen demand, and appear to discriminate against high-affinity nitrogen influx. In sum, these several factors conspire to limit rates of plant nitrogen uptake to values that are well below capacity. These characteristics of the plant's nitrogen uptake systems facilitate nitrogen losses from soils.

Nitrogen Use Efficiency of Crop Plants: Physiological Constraints upon Nitrogen Absorption

Current global nitrogen fertilizer use has reached approximately one hundred billion kg per annum. In many agricultural systems, a very substantial portion of this applied nitrogen fertilizer is lost from soil to groundwaters, rivers and oceans. While soil physicochemical properties play a significant part in these losses, there are several characteristic features of plant nitrogen transporter function that facilitate N losses. Nitrate and ammonium efflux from roots result in a reduction of net nitrogen uptake. As external nitrate and ammonium concentrations, respectively, are increased, particularly into the range of concentrations that are typical of agricultural soils, elevated rates of nitrate and ammonium efflux result. The rapid down-regulation of high-affinity influx as plants become nitrogen replete further reduces the root's capacity to acquire external nitrogen; only nitrogen-starved roots absorb with both high capacity and high affinity. The results of studies using molecular biology methods demonstrate that genes encoding nitrate and ammonium transporters are rapidly down-regulated when nitrogen is resupplied to nitrogen-starved plants. Provision of ammonium to roots of plants actively absorbing nitrate imposes a block on nitrate uptake, the extent of which depends on the ammonium concentration, thus further reducing the efficient utilization of soil nitrate. During the daily variation of incoming light and during periods of low incident irradiation (i.e. heavy cloud cover) the expression levels of genes encoding nitrate and ammonium transporters, and rates of nitrate and ammonium uptake, are substantially reduced. Low temperatures reduce growth and nitrogen demand, and appear to discriminate against high-affinity nitrogen influx. In sum, these several factors conspire to limit rates of plant nitrogen uptake to values that are well below capacity. These characteristics of the plant's nitrogen uptake systems facilitate nitrogen losses from soils.

31 Effects of light and ammonium on growth, ammonium uptake and pigmentation of four porphyra (bangiales, rhodophyta) species native to the northwestern atlantic

The marketability and physiological characteristics of Porphyra (nori) make it a good candidate for integrated aquaculture. Four species were grown in separate three‐week greenhouse trials via tumble culture in small tanks (38L). Porphyra umbilicalis, P. linearis, P. leucosticta and P. amplissima were grown at 15, 8, 15 and 12 C, respectively. Plant growth, ammonium uptake, and pigment content were evaluated at three light levels (4%, 13%, and 58% of ambient sunlight), and two ammonium concentrations (25 and 250 ?M). The growth of Porphyra umbilicalis was higher at 250 ?M than at 25 ?M ammonium (P?0.002), while growth of the other three species were not affected by nitrogen concentration (P&gt;0.05). With the exception of P. leucosticta, plant growth was greatest in high light (58% of ambient). Overall, average growth rates ranged from 31.8 (±2.8)%·day‐1 for P. amplissima to 8.8 (±1.2)%·day‐1 for P. leucosticta. Ammonium uptake rates were consistently higher in 250 than 25 ?M ammonium (P?0.05), except for P. amplissima (P&gt;0.05). Uptake by P. umbilicalis was greatest at high light (P&lt;0.005), but light did not affect uptake rates in the other three species (P&gt;0.05). The phycoerythrin content of P. umbilicalis and P. leucosticta was greater at 250 ?M than 25 ?M ammonium, while it was not affected by nitrogen level in P. linearis and P. amplissima. The phycoerythrin content of all species decreased from low to high light (P&lt;0.05); however, light affected the net phycoerythrin production only in P. leucosticta.

Nutrient uptake by coral-reef microatolls

We measured rates of ammonium and phosphate uptake into 12 microatolls at One Tree reef lagoon, Australia, on 14 occasions over a range of seasonal and meteorological conditions from 1993 to 1995. Nine of the microatolls were spiked with ammonia and/or phosphate every low tide (2.5–4 h) during low- (15-month) and high- (13-month) loading periods. The remaining three microatolls that were not spiked with nutrient served as reference conditions. Ammonium concentrations were elevated from an average background of 0.7 to 11 µM NH4 during the low-loading period and 36 µM NH4 during the high-loading period, resulting in a loading to the benthos of 3.5 and 18 mmol NH4 m–2 low tide–1. Phosphate concentrations were elevated from an average background of 0.2 to 2.3 and 5.1 µM PO4 during low- and high-loading periods, respectively, resulting in a loading of 0.66 and 3.9 mmol PO4 m–2 low tide–1. Ammonium and phosphate concentrations decreased significantly over low-tides, and uptake rates were proportional to concentrations (first-order). The average uptake-rate constant, S (m s–1), for ammonium and phosphate did not differ between the two loading periods but was highly variable. Averaged over both loading periods, S for ammonium was 129±74×10–6 m s–1 and S for phosphate was 67±39×10–6 m s–1. At background nutrient concentrations, estimated nutrient-uptake rates were 7.8 mmol NH4 m–2 day–1 and 1.2 mmol PO4 m–2 day–1. Excretion rates—calculated from the mean difference in uptake rates measured in reference and nutrient-enriched microatolls—were estimated to be 4.3 mmol NH4 m–2 day–1 and 0.9 mmol PO4 m–2 day–1. We reason and suggest that nutrient uptake rates in these microatolls were close to mass-transfer limited rates. We conclude that nutrient uptake into coral reefs can be highly dynamic, varying 10-fold spatially and temporally.

Ammonium uptake by seagrass communities: effects of oscillatory versus unidirectional flow

Nutrient dynamics of aquatic communities are dependent on the flux of nutrients to organisms within the communities. Flux is dependent on water column concentration and hydro- dynamic factors that affect both advection of nutrients through the community and rates of diffusion at the surfaces of organisms. In this study, we measured rates of ammonium uptake for a seagrass community under various hydrodynamic conditions and determined the effects of water velocity and oscillatory flow on uptake rates. Experiments were conducted using a portable flume deployed in natural Thalassia testudinum communities. Uptake rate constants ranged from 9.9 to 25.4 × 10 -5 m s -1 and were ~1.5 times higher in oscillatory flow than in unidirectional flow. Uptake rate constants were positively dependent on both water velocity and turbulent energy in the water column. These results demonstrate the importance of hydrodynamics on biogeochemical cycling in seagrass beds and pro- vide evidence of the efficacy of merging research on hydrodynamics and biogeochemistry in under- standing nutrient processes in complex nearshore communities.