Nitrate reductase and urease enzyme activity in the marine diatom<Emphasis Type="Italic"> Thalassiosira weissflogii</Emphasis> (Bacillariophyceae): interactions among nitrogen substrates

The dissolved nitrogen pool in aquatic systems is comprised of many different nitrogen forms, both inorganic and organic. Interaction among these nitrogen forms at the level of uptake and enzyme activity is, with the exception of NH₄⁺ and NO₃^–, not completely understood. Nitrate reductase (NR) and urease (UA) activities in the marine diatom Thalassiosira weissflogii (Grunow) Fryxell et Hasle were measured in NO₃^–, NH₄⁺, and urea-sufficient cultures before and after challenge additions of NH₄⁺, NO₃^–, and urea in a factorial design. NR and UA were constitutively expressed during growth on NO₃^–, NH₄⁺, and urea. Growth on NH₄⁺ or urea resulted in NR activities that were <10% of the activity observed in the NO₃^–-grown culture, while growth on NO₃^– resulted in UA values that were ~35% of the activities during growth on either NH₄⁺ or urea. The addition of NH₄⁺ or urea to NO₃^–-grown cultures resulted in an immediate decrease in cellular NO₃^– uptake rate, which was not mirrored by an immediate repression of in vitro NR activity; however, the diel peak in NR was suppressed in these challenge experiments. The addition of NO₃^– or NH₄⁺ to urea-grown cultures resulted in non-significant decreases in the urea uptake rate. UA was not impacted by NO₃^– addition, but NH₄⁺ addition significantly decreased UA throughout the experiment. These studies demonstrate that the uptake and assimilation of NO₃^– and urea may not be subject to the same internal feedback mechanism when challenged with other nitrogen substrates.Communicated by J.P. Grassle, New Brunswick     

Characterization of spring phytoplankton communities in the Río de La Plata maritime front using pigment signatures and cell microscopy

The surface abundance and species composition of phytoplankton communities were studied in a section across the continental shelf between the Río de La Plata and the oceanic waters of the Subtropical Convergence, during late spring (November 1999). Algal communities were examined using light microscopy and HPLC-derived (high-performance liquid chromatography) pigment concentrations. The CHEMTAX program was used to estimate the chlorophyll a (chl a) biomass of different algal classes. The inclusion of the most abundant members of the chl c pigment family (chl c₁, chl c₂, chl c₃ and chl c₂ monogalactosyldiacylglyceride esters) in the pigment matrix improved the CHEMTAX interpretation of field data. Using this novel approach four haptophyte populations were distinguished across the studied section, even though they had qualitatively similar pigment signatures, although one subtype lacked 19-hexanoyloxyfucoxanthin (Hex-Fuco). Five different phytoplankton assemblages, spatially segregated by the prevailing environmental conditions, were distinguished during the studied period. All of them showed a complex community structure, formed by a background of small-sized cells such as cyanobacteria, cryptophytes, haptophytes and prasinophyceans, on which diatom, cryptophyte or some haptophyte blooms were overlapped. In the estuarine assemblage, where maximum chl a concentrations where found, diatoms were always the dominant group (30–60% of total chl a), but cryptophytes (10–40%), prasinophyceans (2–20%) and dinoflagellates (2–12%) were also relevant. In the coastal assemblage diatoms were also the dominant group (35–45%), but haptophytes lacking Hex-Fuco were subdominant (20–35%). The continental shelf assemblage showed an almost exclusive dominance (90%) of haptophytes resembling the coccolithophorid E. huxleyi. Another haptophyte (Phaeocystis sp.) was dominant (75–85%) in the Malvinas Current assemblage. The Brazil Current assemblage was characterized by the codominance of cyanobacteria (45%) and haptophytes (35%). These results are discussed in relationship to the complex hydrographic features of the area.Communicated by O. Kinne, Oldendorf/Luhe     

Atmospheric Deposition, Mineralization and Leaching of Nitrogen in Subtropical Forested Catchments, South China

In recent years, China has conducted considerable research focusing on the emission and effects of sulphur (S) on human health and ecosystems. By contrast, there has been little emphasis on anthropogenic nitrogen (N) so far, even though studies conducted abroad indicate that long-range atmospheric transport of N and ecological effects (e.g. acidification of soil and water) may be significant. The Sino-Norwegian project IMPACTS, launched in 1999, has established monitoring sites at five forest ecosystems in the southern part of PR China to collect comprehensive data on air quality, acidification status and ecological effects. Here we present initial results about N dynamics at two of the IMPACTS sites located near Chongqing and Changsha, including estimation of atmospheric deposition fluxes of NO_x and NH_x and soil N transformations. Nitrogen deposition is high at both sites when compared with values from Europe and North America (25–38 kg ha^–1 yr^–1). About 70% of the deposited N comes as NH₄, probably derived from agriculture. Leaching of N from soils is high and nearly all as NO₃ ^–1. Transformation of N to NO₃ ^–1 in soils results in acidification rates that are high compared to rates found elsewhere. Despite considerable leaching of NO₃ ^–1 from the root zone of the soils, little NO₃ ^–1 appears in streamwater. This indicates that N retention or denitrification, both causing acid neutralization, may be important and probably occur in the groundwater and groundwater discharge zones. The soil flux density of mineral N, which is the sum of N deposition and N mineralization, and which is dominated by the N mineralization flux, may be a good indicator for leaching of NO₃ ^–1 in soils. However, this indicator seems site specific probably due to differences in land-use history and current N requirement.     

Seasonal variations in distribution patterns of sympagic meiofauna in Arctic pack ice

During two expeditions of the R.V. Polarstern to the Arctic Ocean, pack ice and under-ice water samples were collected during two different seasons: late summer (September 2002) and late winter (March/April 2003). Physical and biological properties of the ice were investigated to explain seasonal differences in species composition, abundance and distribution patterns of sympagic meiofauna (in this case: heterotrophs >20 µm). In winter, the ice near the surface was characterized by extreme physical conditions (minimum ice temperature: –22°C, maximum brine salinity: 223, brine volume: 5%) and more moderate conditions in summer (minimum ice temperature: –5.6°C, maximum brine salinity: 94, most brine volumes: 5%). Conditions in the lowermost part of the ice did not differ to a high degree between summer and winter. Chlorophyll a concentrations (chl a) showed significant differences between summer and winter: during winter, concentrations were mostly <1.0 µg chl a l^–1, while chl a concentrations of up to 67.4 µmol l^–1 were measured during summer. The median of depth-integrated chl a concentration in summer was significantly higher than in winter. Integrated abundances of sympagic meiofauna were within the same range for both seasons and varied between 0.6 and 34.1×10³ organisms m^–2 in summer and between 3.7 and 24.8×10³ organisms m^–2 in winter. With regard to species composition, a comparison between the two seasons showed distinct differences: while copepods (42.7%) and rotifers (33.4%) were the most abundant sea-ice meiofaunal taxa during summer, copepod nauplii dominated the community, comprising 92.9% of the fauna, in winter. Low species abundances were found in the under-ice water, indicating that overwintering of the other sympagic organisms did not take place there, either. Therefore, their survival strategy over the polar winter remains unclear.Communicated by O. Kinne, Oldendorf/Luhe     

Marine diatoms grown in chemostats under silicate or ammonium limitation. III. Cellular chemical composition and morphology of Chaetoceros debilis,Skeletonema costatum,and Thalassiosira gravida

Three marine diatoms, Skeletonema costatum, Chaetoceros debilis, and Thalassiosira gravida were grown under no limitation and ammonium or silicate limitation or starvation. Changes in cell morphology were documented with photomicrographs of ammonium and silicate-limited and non-limited cells, and correlated with observed changes in chemical composition. Cultures grown under silicate starvation or limitation showed an increase in particulate carbon, nitrogen and phosporus and chlorophyll a per unit cell volume compared to non-limited cells; particulate silica per cell volume decreased. Si-starved cells were different from Si-limited cells in that the former contained more particulate carbon and silica per cell volume. The most sensitive indicator of silicate limitation or starvation was the ratio C:Si, being 3 to 5 times higher than the values for non-limited cells. The ratios Si:chlorophyll a and S:P were lower and N:Si was higher than non-limited cells by a factor of 2 to 3. The other ratios, C:N, C:P, C:chlorophyll a, N:chlorophyll a, P:chlorophyll a and N:P were considered not to be sensitive indicators of silicate limitation or starvation. Chlorophyll a, and particulate nitrogen per unit cell volume decreased under ammonium limitation and starvation. NH₄-starved cells contained more chlorophyll a, carbon, nitrogen, silica, and phosphorus per cell volume than NH₄-limited cells. N:Si was the most sensitive ratio to ammonium limitation or starvation, being 2 to 3 times lower than non-limited cells. Si:chlorophyll a, P:chlorophyll a and N:P were less sensitive, while the ratios C:N, C:chlorophyll a, N:chlorophyll a, C:Si, C:P and Si:P were the least sensitive. Limited cells had less of the limiting nutrient per unit cell volume than starved cells and more of the non-limiting nutrients (i.e., silica and phosphorus for NH₄-limited cells). This suggests that nutrient-limited cells rather than nutrient-starved cells should be used along with non-limited cells to measure the full range of potential change in cellular chemical composition for one species under nutrient limitation.Contribution No. 943 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Feeding and respiration rates of a planktonic copepod (<Emphasis Type="Italic">Calanus sinicus</Emphasis>) oversummering in Yellow Sea Cold Bottom Waters

The distribution, feeding and oxygen consumption of Calanus sinicus were studied in August 2001 on a transect across Yellow Sea Cold Bottom Waters (YSCBW) and two additional transects nearby. The distribution of C. sinicus adults and copepodites stage CV appeared to be well correlated with water temperature. They tended to concentrate in the YSCBW (>10,000 ind. m^–2) to avoid high surface temperature. Gut pigment contents varied from 0.44 to 2.53 ng chlorophyll a equivalents (chl a equiv.) ind.^–1 for adults, and from 0.24 to 2.24 ng chl a equiv. ind.^–1 for CV copepodites. We found no relationship between gut pigment contents and the ambient chl a concentrations. Although the gut evacuation rate constants are consistent with those measured for other copepods, their low gut pigment contents meant an estimated daily herbivorous ingestion of <3% of body carbon in the YSCBW and <10% outside the YSCBW. However, based on estimates of clearance rates, C. sinicus feeds actively whether in the YSCBW or not, so the low ingestion rates probably reflect shortage of food. Oxygen consumption rates of C. sinicus ranged from 0.21 to 0.84 l O₂ ind.^–1 h^–1, with high rates often associated with high temperature. From the oxygen consumption rates, daily loss of body carbon was estimated to be 4.0–13.7%, which exceeds our estimates of their carbon ingestion rates. C. sinicus was probably not in diapause, either within or outside the YSCBW, but this cold-water layer provides C. sinicus with a refuge to live through the hot, low-food summer.Communicated by T. Ikeda, Hakodate     

Interactions between NH+4 and NO−3 uptake and assimilation: comparison of diatoms and dinoflagellates at several growth temperatures

Ammonium concentrations of ∼1 M are commonly cited as being the threshold for inhibition of NO₃ ⁻ uptake, but the applicability of this threshold to phytoplankton from different taxonomic classes has rarely been examined. Additionally, little is known about the influence of environmental variables (e.g. growth temperature) on the interaction between ambient NH₄ ⁺ and NO₃ ⁻ uptake. Four species of estuarine phytoplankton, two diatom [Chaetoceros sp., and Thalassiosira weissflogii (Grunow) Fryxell et Hasle] and two dinoflagellate [Prorocentrum minimum (Pavillard) Schiller, and Gyrodinium uncatenum Hulburt], were grown on NO₃ ⁻ at several different temperatures (4, 10, 15, or 20 °C), and the impact of NH₄ ⁺ additions on NO₃ ⁻ uptake/assimilation (non-TCA-extracted) and assimilation (TCA-extracted) was assessed. For all species at all temperatures, NO₃ ⁻ uptake/assimilation and assimilation rates decreased in a roughly exponential manner with increasing NH₄ ⁺ concentrations but were not completely inhibited even at elevated NH₄ ⁺ concentrations of 200 μM. Estimated half-inhibition concentrations (K _i) were significantly greater in the diatom species (mean ± SE; 2.70 ± 0.67 μM) than in the dinoflagellate species (1.26 ± 0.55 μM). Half-inhibition constants were positively related to temperature-limited relative growth rate although not significantly. The observed inhibition of NO₃ ⁻ uptake and assimilation, as a percentage of NO₃ ⁻ uptake in the absence of NH₄ ⁺, averaged about 80% and ranged from 49 to 100%. For all species, a significant (P < 0.001) positive correlation was found between percent inhibition of NO₃ ⁻ assimilation and temperature-limited relative growth rate. Two experiments on Chesapeake Bay phytoplankton during an April 1998 diatom bloom showed that in short-term (∼1 h) temperature manipulation experiments, percent inhibition of NO₃ ⁻ uptake/assimilation was also positively related (P = 0.05) to experimental temperature. The observed relationships between temperature-limited relative growth rate and percent inhibition of NO₃ ⁻ assimilation rates for the species tested suggest that at the enzyme level, the inhibitory mechanism of NO₃ ⁻ assimilation is similar among species, but at the whole cell level may be regulated by species-specific differences in the accumulation of internal metabolites. These findings add not only to our understanding of species-specific variability and the role of growth temperature, but also provide additional data with which to evaluate current models of NH₄ ⁺ and NO₃ ⁻ interactions. Received: 31 August 1998 / Accepted: 7 December 1998     

Feeding habits of a pelagic amphipod,Themisto japonica

Vertical distribution, chlorophylla (chla) and phaeopigment concentrations in the gut, and natural nitrogen isotope ratio ( ¹⁵N) were investigated for pelagic amphipodsThemisto japonica (Bovallius) collected from the Sea of Japan in July 1987. Differences in diel vertical migration behavior were clearly observed between small and largeT. japonica. Many small (<5 mm body length) amphipods appeared in the phytoplankton-rich shallow layers. Their gut pigment concentrations were higher (mean 0.52 ± 0.15µg chla g^–1 amphipod) than those of large amphipods (mean 0.33±0.14µg g^–1); this implies that the amphipods fed on a large amount of phytoplankton during the early stage of life. The ¹⁵N values of small amphipods were lower (5.7 to 6.3) than those of large amphipods (6.8 to 11.7), reflecting the lower trophic level of small amphipods compared to large ones. The ¹⁵N values for small amphipods were similar to those of herbivorous zooplankton. The amphipods' feeding behavior thus changes from herbivorous to carnivorous as they grow.     

The influence of macrofauna on estuarine benthic community metabolism: a microcosm study

Effects of benthic macrofauna (Corophium volutator, Hydrobia sp., Nereis virens) on benthic community metabolism were studied over a 65-d period in microcosms kept in either light/dark cycle (L/D-system) or in continuous darkness (D-system). Sediment and animals were collected in January 1986 in the shallow mesohaline estuary, Norsminde Fjord, Denmark. The primary production in the L/D-system after 10 d acted as a stabilizing agent on the O₂ and CO₂ flux rates, whereas the D-system showed decreasing O₂ and CO₂ flux throughout the period. Mean O₂ uptake over the experimental period ranged from 0.38 to 1.24 mmol m^–2 h^–1 and CO₂ release varied from 0.80 to 1.63 mmol m^–2 h^–1 in both systems. The presence of macrofauna stimulated community respiration rates measured in darknes, 1.4 to 3.0 and 0.9 to 2.0 times for O₂ and CO₂, respectively. In contrast, macrofauna lowered primary production. Gross primary production varied from 1.06 to 2.26 mmol O₂ m^–2 h^–1 and from 1.26 to 2.62 mmol CO₂ m^–2 h^–1. The community respiratory quotient (CRQ, CO₂/O₂) was generally higher in the begining of the experiment (0–20 d, mean 1.89) than in the period from Days 20 to 65 (mean 1.38). The L/D-system exhibited lower CRQ (ca. 1) than the D-system. The community photosynthetic quotient varied for both net and gross primary production from 0.64 to 1.03, mean 0.81. The heterotrophic D-system revealed a sharp decrease in the sediment content of chlorophyll a as compared to the initial content. In the autotrophic L/D-system, a significant increase in chlorophyll a concentration was observed in cores lacking animals and cores with C. volutator (The latter species died during the experiment). Due to grazing and other macrofauna activities other cores of the L/D-system exhibited no significant change in chlorophyll a concentration. Community primary production was linearly correlated to the chlorophyll a content in the 0 to 0.5 cm layer. Fluxes of DIN (NH₄ ⁺+NO₂ ^–+NO₃ ^–) did not reveal significant temporal changes during the experiment. Highest rates were found for the cores containing animals, mainly because of an increased NH₄ ⁺ flux. The release of DIN decreased significantly due to uptake by benthic microalgae in the L/D-system. No effects of the added macrofauna were found on particulate organic carbon (POC), particulate organic nitrogen (PON), total carbon dioxide (TCO₂) and NH₄ ⁺ in the sediment. The ratio between POC and PON was nearly constant (9.69) in all sediment dephts. The relationship between TCO₂ and NH₄ ⁺ was more complex, with ratios below 2 cm depth similar to those for POC/PON, but with low ratios (3.46) at the sediment surface.     

Model of stomatal ammonia compensation point (STAMP) in relation to the plant nitrogen and carbon metabolisms and environmental conditions

Agricultural crops can be either a source or a sink of ammonia (NH₃). Most NH₃ exchange models developed so far do not account for the plants nitrogen (N) metabolism and use prescribed compensation points. We present here a leaf-scale simplified NH₃ stomatal compensation point model related to the plants N and carbon (C) metabolisms, for C₃ plants. Five compartments are considered: xylem, cytoplasm, apoplasm, vacuole and sub-stomatal cavity. The main processes accounted for are the transport of ammonium (NH₄⁺), NH₃ and nitrate (NO₃⁻) between the different compartments, NH₄⁺ production through photorespiration and NO₃⁻ reduction, NH₄⁺ assimilation, chemical and thermodynamic equilibriums in all the compartments, and stomatal transfer of NH₃.The simulated compensation point is sensitive to paramaters related to the apoplastic compartment: pH, volume and active transport rate. Determining factors are leaf temperature, stomatal conductance and NH₄⁺ flux to the leaf. Atmospheric NH₃ concentration seem to have very little effect on the compensation point in conditions of high N fertilization. Comparison of model outputs to experimental results show that the model underestimates the NH₃ compensation point for high N fertilization and that a better parametrisation of sensitive parameters especially active trasport rate of NH₄⁺ may be required.     

Photosynthetic acclimation and biochemical responses ofGelidium sesquipedale cultured in chemostats under different qualities of light

The red algaGelidium sesquipedale (Clem.) Born. et Thur. has been cultured in chemostats to assess the effects of light quality and photon-fluence rate (PFR) on growth, photosynthesis and biochemical composition. Plants under blue and red light (BL and RL) showed higher growth rates than under white light (WL) of the same PFR (40 mol m^–2 s^–1). The light-saturated rate of photosynthesis was higher for algae grown under BL and RL than for algae grown under WL. When algae were transferred to WL of moderate PFR (100 mol m^–2 s^–1), the light-saturated rate of photosynthesis decreased, being higher in previously RL-grown algae than in previously BL- and WL-grown algae. The initial slope of photosynthesis-irradiance (PI) curves () was affected by PFR but not by light quality. Pigment content was little affected by light quality. Light-quality treatments also affected the biochemical composition of the alga; previous exposure to various light treatments activate or repress several metabolical pathways that are fully expressed in the subsequent phase of WL of moderate PFR. Thus, phycobiliproteins and soluble proteins increased for previously BL- and RL-grown algae, whereas insoluble carbohydrate concentration was reduced, indicating a change of the C-partitioning between carbon compounds and organic nitrogen compounds. Inorganic nitrogen metabolism was also affected by light: under WL of moderate PFR, NO₃ ^– was totally depleted from sea water, and maximal values of NO₃ ^– uptake were recorded. In addition, neither NO₂ ^– nor NH₄ ⁺ was released. However, when algae were transferred to a low PFR, there was a drastic reduction of NO₃ ^– uptake under WL, which only partially recovered over time. It was accompanied by the release of NO₂ ^–, but not NH₄ ⁺, to the culture medium. Under BL and RL, however, there was a transient enhancement of NO₃ ^– uptake that was followed by a net release of NO₂ ^– and NH₄ ^–. Growth rates were not correlated with PFR. This could be due to the the dynamics of internal carbon mobilization and accumulation in the algae. When algae were exposed to a moderate PFR of WL, carbon requirements for growth were satisfied by photosynthesis. Thus, there was a net accumulation of carbon in the tissue. In contrast, when algae were exposed to low PFRs of either WL, BL or RL, observed growth rates could not be maintained by photosynthesis and carbon was mobilized.     

Effect of ammonia on survival and osmoregulation in different life stages of the lobsterHomarus americanus

Acute toxicity of ammonia was determined for cultured larval, postlarval, and wild adult lobsters (Homarus americanus) in 1988. Ammonia tolerance was found to increase with ontogenetic development. Based on 96-h LC₅₀ values of 58 mg l^–1 NH₄ ⁺ + NH₃ l^–1 seawater (0.72 mg NH₃ l^–1) for Stage I larvae, 87 mg NH₄ ⁺ + NH₃ l^–1 (1.7 mg NH₃ l^–1) for Stage II larvae, 125 mg NH₄ ⁺ + NH₃ l^–1 (2.13 mg NH₃) for Stage III larvae, 144 mg NH₄ ⁺ + NH₃ l^–1 (2.36 mg NH₃ l^–1) for Stage IV postlarvae, 377 mg NH₄ ⁺ + NH₃ l^–1 (5.12 mg NH₃ l^–1) for adult lobsters at 5°C and 219 mg NH₄ ⁺ + NH₃ l^–1 (3.25 mg NH₃ l^–1) for adult lobsters at 20°C, recommendations for safe levels of total ammonia and un-ionized ammonia were calculated using an application factor of 0.1. Effects of ammonia on osmoregulatory capacity were studied on postlarvae and adults. Ability of postlarvae and adults to hyper-regulate in low-salinity media decreased after exposure to ammonia. In postlarval lobsters, osmoregulatory capacity was significantly affected in ammonia concentrations exceeding 32 mg l^–1. Osmoregulatory capacity in adult lobsters (5 and 20°C) was affected at 150 mg l^–1. In postlarval lobsters, a minimum exposure time of 12 h was required to impair osmoregulatory capacity. The decrease in hemolymph osmotic pressure was caused by lower hemolymph sodium concentrations. The presence of ammonia in the external medium could markedly affect the Na⁺/NH₄ ⁺ transport mechanism by permanently, temporarily, or partially impairing the transport sites for sodium.     

A study on the landfill leachate and its impact on the groundwater quality of the greater area

Characterisation of the leachate originating from the Ano Liosia landfill (situated in Attica region, Greece) as well as assessment on the quality of the local aquifer were carried out. The experimental results showed that most of the parameters examined in the leachate samples such as colour, conductivity, TS, COD, NH₃–N, PO₄–P, SO₄ ^2–, Cl^–, K⁺, Fe and Pb were found in high levels. The organic load was quite high since the COD concentrations were in the range of 3250–6125mgL^–1. In addition, the low BOD/COD ratio (0.096–0.195), confirmed that the majority of this organic matter is not easily biodegradable. The groundwater near the landfill site was characterised as not potable and not suitable for irrigation water, since most of the physical and chemical parameters examined – such as colour, conductivity, DS, hardness, Cl^–, NH₃–N, COD, K⁺, Na⁺, Ca²⁺, Fe, Ni and Pb exceeded the permissible limits given by EE, EPA and the Greek Ministry of Agriculture. Furthermore, this study presents the application of the hydrologic evaluation of landfill performance (HELP) model for the determination of the yearly leakage from the base of the landfill after the final capping.     

Availability of ammonium influences photosynthesis and the accumulation of mycosporine-like amino acids in two<Emphasis Type="Italic"> Porphyra</Emphasis> species (Bangiales,Rhodophyta)

The effect of ammonium concentration on photosynthetic activity estimated as in vivo chlorophyll fluorescence, i.e. maximal quantum yield (F_v/F_m) and electron transport rate (ETR) and on the accumulation of mycosporine-like amino acids (MAAs), chlorophyll a (chl a), biliproteins (BP) and soluble proteins (SP) in the red algae Porphyra leucosticta Thuret in Le Jolis collected from Lagos (Málaga, Spain) and Porphyra umbilicalis (Linnaeus) J. Agardh from Helgoland (Germany) was evaluated. Discs of both species were incubated with three ammonium concentrations (0, 100 and 300 µM) under artificial PAR and UV radiation for 7 days. Photosynthetic activity decreased under the culture conditions due to UV radiation and ammonium availability. The decrease of both F_v/F_m and maximal ETR was related to ammonium supply, i.e. the lowest decrease occurred in algae growing with the highest concentration of ammonium. In both species, after 7 days of culture, the content of chl a, BP and SP was higher under 300 µM than that under 0 and 100 µM ammonium. In both species, the content of MAAs was increased under 300 µM ammonium compared to the initial value, whereas a decrease under 0 and 100 µM ammonium was observed only in P. leucosticta. The content of MAAs in P. umbilicalis did not present significant differences compared to the initial value, probably because of the high initial content of MAAs. In both Porphyra species, four MAAs were identified: shinorine, porphyra-334, palythine and asterina-330. However, P. leucosticta modified its MAA pattern during the incubation time, reaching the same percentages found for P. umbilicalis, which did not show any change during the experimental period. P. leucosticta exhibited a decrease in BP/SP and BP/chl a ratios through the incubation time and an increase in MAAs/BP. The ratio MAAs/chl a did not show any variation with time or treatment, as was also true for all ratios in P. umbilicalis. In summary, ammonium supply diminished the decrease of F_v/F_m, increased the content of photosynthetic pigments (chlorophyll and biliprotein) and soluble protein, and stimulated of the accumulation of MAAs in the red algae P. leucosticta and P. umbilicalis.Communicated by O. Kinne, Oldendorf/Luhe     

Diurnal gut pigment rhythm and metabolic rate of<Emphasis Type="Italic"> Calanus euxinus</Emphasis> in the Black Sea

The vertical distribution, diel gut pigment content and oxygen consumption of Calanus euxinus were studied in April and September 1995 in the Black Sea. Gut pigment content of C. euxinus females was associated with diel vertical migration of the individuals, and it varied with depth and time. Highest gut pigment content was observed during the nighttime, when females were in the chlorophyll a (chl a) rich surface waters, but significant feeding also occurred in the deep layer. Gut pigment content throughout the water column varied from 0.8 to 22.0 ng pigment female^–1 in April and from 0.2 to 21 ng pigment female^–1 in September 1995. From the diel vertical migration pattern, it was estimated that female C. euxinus spend 7.5 h day^–1 in April and 10.5 h day^–1 in September in the chl a rich surface waters. Daily consumption by female C. euxinus in chl a rich surface waters was estimated by taking into account the feeding duration and gut pigment concentrations. Daily carbon rations of female C. euxinus, derived from herbivorous feeding in the euphotic zone, ranged from 6% to 11% of their body carbon weight in April and from 15% to 35% in September. Oxygen consumption rates of female and copepodite stage V (CV) C. euxinus were measured at different temperatures and at different oxygen concentrations. Oxygen consumption rates at oxygen-saturated concentration ranged from an average of 0.67 g O₂ mg^–1 dry weight (DW) h^–1 at 5°C to 2.1 g O₂ mg^–1 DW h^–1 at 23°C for females, and ranged from 0.48 g O₂ mg^–1 DW h^–1 at 5°C to 1.5 g O₂ mg^–1 DW h^–1 at 23°C for CVs. The rate of oxygen consumption at 16°C varied from 0.62 g O₂ mg^–1 DW h^–1 at 0.65 mg O₂ l^–1 to 1.57 g O₂ mg^–1 DW h^–1 at 4.35 mg O₂ l^–1 for CVs, and from 0.74 g O₂ mg^–1 DW h^–1 at 0.57 mg O₂ l^–1 to 2.24 g O₂ mg^–1 DW h^–1 at 4.37 mg O₂ l^–1 for females. From the oxygen consumption rates, daily requirements for the routine metabolism of females were estimated, and our results indicate that the herbivorous daily ration was sufficient to meet the routine metabolic requirements of female C. euxinus in April and September in the Black Sea.Communicated by O. Kinne, Oldendorf/Luhe     

Primary production of phytoplankton at a frontal zone located at the northern slope of the Dogger Bank (North Sea)

In July 1988 a survey was made in the Dogger Bank area of the North Sea. As a result of wind stress the area was found to be frequently well mixed. At the northerly slope a transition zone was observed between the stratified central North Sea and the well-mixed Dogger Bank area. Low nutrient concentrations were observed in surface waters; especially for nitrate (<0,1µM). High concentrations of phosphate (>0,5µM), nitrate (>1µM), ammonium (>2µM) and silicate (>2µM) only prevailed below the thermocline. Chlorophylla values were below 1µg l^–1 near the surface. Enhanced values (up to 4µg l^–1) were observed in the deeper layer at the transition zone and just below the thermocline at well-stratified locations. At the transition zone high specific C-fixation rates (up to 100 mg C mg^–1 chla d^–1) at the surface indicated the presence of enhanced productivity. The compensation depth for primary production was found to coincide with a specific C-fixation rate of 5 mg C mg^–1 chla d^–1. At greater depths, phytoplankton was only found where tidally induced vertical mixing allowed a regular exposure to higher light intensities. Storms resulted in a rapid redistribution of chlorophylla and enhancement of the C-fixation rate in the upper layer of the water column.Publication No. 10 of the project Applied Scientific Research Netherlands Institute for Sea Research (BEWON)     

Influence of relative water motion on the growth,ammonium uptake and carbon and nitrogen composition of Ulva lactuca (Chlorophyta)

In a series of multifactorial laboratory experiments, Ulva lactuca discs were grown in an apparatus in which they were exposed simultaneously to 3 simulated current speeds (7.5, 15, 22.5 cm s^-1) and a still control, and either 3 ammonium concentrations (0–10, 35–45 and 115–145 M) under ample uniform light (ca 200 E m^-2 s^-1) or 3 light intensities (approximately 35, 90 and 270 E m^-2 s^-1) with uniform surplus, ammonium. Disc growth rates were determined in each experiment as well as tissue nitrogen and carbon composition and fluxes of NH₄, NO₃/NO₂ and PO₄ in media. In a supplementary series of field experiments, U. lactuca discs were simultaneously exposed to 2 different water motion regimes in adjacent chambers at several sites characterized by widely different ammonium concentrations. In field experiments, growth rates were calculated and analyzed as a function of water motion at the various sites. The application of simulated current consistently enhanced disc growth rates in the laboratory, except at the lowest light intensity. In most cases this enhancement was fully realized at the lowest applied simulated current (7.5 cm s^-1). Simulated current slightly enhanced ammonium uptake rates by U. lactuca discs, relative to rates in still water, except at the highest ammonium concentration. C:N ratios of discs generally declined with increases in simulated current, except at the highest ammonium concentration. This decline was primarily attributable to increases in per cent N and was, again, mainly realized at 7.5 cm s^-1. The results suggested that simulated current compensated for N limitation, except when light was sufficiently low to become the overriding limiting factor, but that the enhancement of growth by simulated current could not be explained in terms of N metabolism alone. Field experiments showed that the higher level of water motion consistently enhanced growth at sites with comparatively low ammonium concentrations, but not at sites with moderate or high ammonium concentrations.     

Preparation,characterization, and kinetics of nanoscale iron in nitrate nitrogen removal from polluted water

The aim of this study was to examine the production of nanoscale ions via the liquid phase reduction method and the effectiveness of the removal of nitrate nitrogen (NO₃^?–N) as well as measure the products and kinetics of the reactions. The nanoparticles obtained were approximately 50 nm in diameter and the main component was iron (Fe). This custom-made nanoscale Fe was highly positively charged, and reacted rapidly with NO₃^?–N in oxygen-free and neutral conditions at room temperature. A 90% removal rate was achieved when the reaction occurred for 30 min in simulation sample water with vigorous shaking at 250 r/min at NO₃^?–N concentrations of 30, 50, 80 or120 mg N/L. The nanometer Fe dosage was maintained throughout the experiment at 4 g/L. A first-order kinetics equation was applied to the obtained experimental data which followed a pseudo first-order reaction. Data demonstrated that the removal of nitrate nitrogen from polluted groundwater using a nanoscale Fe iron was effective and rapid.     

Bioturbation effects of the amphipod Corophium volutator on microbial nitrogen transformations in marine sediments

Microcosms containing different densities of Corophium volutator, ranging from 0 to 6000 ind m^-2, were incubated in a flow-through system. Benthic fluxes of CO₂, O₂, NO₃ ^- and NH₄ ⁺ were measured regularly. Thirteen days after setup the microcosms were sacrificed and sediment characteristics, pore water NO₃ ^-, NH₄ ⁺ and exchangeable NH₄ ⁺ concentrations, and potential nitrification activity were measured. The presence of C. volutator increased overall mineralization processes due to burrow construction and irrigation. The amphipods increased the ratio CO₂/O₂ fluxes from 0.73 to 0.86 in microcosms inhabited by 0 and 6000 ind m^-2, respectively. Burrow ventilation removed NH₄ ⁺ from the sediment, which was nitrified in the oxic layer and transported NO₃ ^- to the burrow sediment, where denitrification potential was enhanced. Nitrification and total denitrification rates (denitrification of NO₃ ^- coming from the overlying water and of NO₃ ^- generated within the sediment) were calculated and discussed. Bioturbation by C. volutator increased both nitrification and denitrification, but denitrification was stimulated more than nitrification. Denitrification of NO₃ ^- coming from the overlying water was stimulated 1.2- and 1.7-fold in microcosms containing 3000 and 6000 ind m^-2 relative to control microcosms. The presence of C. volutator (6000 ind m^-2) stimulated nitrogen removal from the system, as dinitrogen, 1.5-fold relative to non-bioturbated microcosms. C. volutator individuals used in our study were collected from Norsminde Fjord, Denmark, in 1990.     

Inorganic nitrogen metabolism inUlva rigida illuminated with blue light

Inorganic nitrogen metabolism in blue light was studied for the green algaUlva rigida C. Agardh collected in the south of Spain (Punta Carnero, Algeciras) in the winter of 1987. NH₄ ⁺ has been reported to inhibit NO₃ ^- uptake; however,U. rigida showed a net NO₃ ^- uptake even when the NH₄ ⁺ concentration of the external medium was three or four times greater than the concentration of NO₃ ^-. NO₃ ^- uptake rates were similar in both darkness and in blue light of various photon fluence rates (PFR) ranging from 17 to 160 mol m^-2 s^-1. Since NO₃ ^- uptake is an active mechanism involving the consumption of ATP, respiratory metabolism can provide enough ATP to maintain the energetic requirement of NO₃ ^- transport even in darkness. In contrast, NO₃ ^- reduction inU. rigida was highly dependent on the net photosynthetic rate. After 7 h in blue light, intracellular NO₃ ^- concentrations ([NO₃ ^-] _i ) were higher in specimens exposed to intensities below the light compensation point (LCP) than in those incubated at a PFR above the LCP. When PFR is below the light compensation point, NO₃ ^- reduction is low, probably because all the NADH produced by the cells is oxidized in the respiratory chain in order to produce ATP to maintain a steady NO₃ ^- transport rate. The total nitrogen (TN) and carbon (TC) contents decreased from darkness to 33 mol m^-2 s^-1 in blue light. In this range, catabolic processes prevailed over anabolic ones. In contrast, increases in TN and TC contents were observed above the light compensation point. The C : N ratio increased with light intensity, reaching a stable value of 17 at 78 mol m^-2 s^-1 in blue light. Intracellular NO₃ ^- concentration and NO₃ ^- reduction appear to be directly controlled by light intensity. This external control of [NO₃ ^-]_i and the small capacity ofU. rigida to retain incorporated NO₃ ^-, NO₂ ^- and NH₄ ⁺ ions may explain its nitrophilic character.