In situ measurements of uptake of inorganic carbon and nitrogen by the aquatic liverworts Jungermannia vulcanicola Steph. and Scapania undulata (L.) Dum. in an acid stream,Kashiranashigawa, Japan

In situ uptake of inorganic carbon and nitrogen by the aquatic liverworts Jungermannia vulcanicola Steph. and Scapania undulata (L.) Dum. was measured in an acid stream, Kashiranashigawa, Japan. The uptake activities were similar in the both species. The activities were highest at the tip of shoots, and decreased gradually towards the base. Carbon uptake at the tip in the light was 10.4 × 10^–4 for J. vulcanicola and 8.1 × 10^–4 g C g dry wt^–1 h^–1 1 for S. undulata. Ammonium was effectively incorporated into the shoots, and the uptake activity at the tip was between 1.9 × 10^–5 and 5.8 × 10^–5 g N g dry wt^–1 h^–1. Nitrate uptake was smaller than ammonium uptake. The ratio of dark to light uptake in ammonium uptake experiments was larger than that in carbon uptake experiments. These results suggest that the liverworts use ammonium as a major nitrogen source, and that ammonium uptake was less dependent on light than carbon uptake.     

Seasonal nitrogen assimilation and carbon fixation in a fjordic sea loch

Carbon (C) fixation and nitrogen (N) assimilation rates havebeen estimated from ¹⁴C and ¹⁵N techniques for a 12 month periodin a Scottish sea loch. The maximum rate of nitrogen assimilated(29.92 mmol N m^–2 day^–1) was in April at the mostseaward station; similar high rates were experienced duringMay at the other stations. Carbon fixation rates were maximal(488–4047 mg C m^–2day^–1) at the time of highphytoplankton biomass (maximum 8.3 mg m^–3 chlorophylla) during May, whilst nitrate concentrations remained >0.7µ.mol l^–1. C:N assimilation ratios suggest nitrogenlimitation only during the peak of the spring bloom, althoughat times nitrogen (nitrate and ammonium) concentration fellto 0.2 µmol l^–1 in the following months. The verticalstability of the water column, influenced by tidal and riverineflushing, varied along the axis of the loch, resulting in markeddifferences between sampling stations. Although ammonium waspreferentially assimilated by phytoplankton, >50% of productionwas supported by nitrate uptake and only during the summer monthswas the assimilation of ammonium quantitatively important.     

Diel variability in nitrogenous nutrient uptake at photic and subphotic depths

Diel patterns in nitrogen uptake were investigated at threepositions along an offshore orientated transect in the SouthernBenguela Upwelling System. Both photic and sub-photic depthswere targeted in the ¹⁵N tracer experiments. An appreciableproportion of nitrogen uptake was found to occur at sub-photicdepths during both the day and the night. The importance ofsub-photic uptake decreased offshore but remained non-trivialat all locations. Although NH₄ dominated this uptake, NO₃ removalat depth did account for an important fraction of the totalwater column NO₃ uptake, particularly over the shelf at night.For example, sub-photic NO₃ uptake at night for the inshorestations averaged 68% of the total for the water column decliningto 3% offshore. The impact of sub-photic NO₄ uptake on new productionwill depend on whether the NO₃ is incorporated into primaryassimilation pathways or is transported to the sediments ininorganic form. Whereas sub-photic/photic uptake decreased offshore,night/day uptake ratios increased with distance from the coast,approaching unity at the offshore location. This effect wasmost obvious for the photic communities This finding supportsthe belief that dark nitrogen uptake assumes increased importanceas nutrients become less available. Photic zone f-ratios weresmaller at night (mean = 0.14) than during the day (mean = 0.28)indicating a greater importance of reduced nitrogen uptake duringthe dark hours This emphasis on NH₄ utilization at night canbe explained purely in terms of energy efficiency. Such die1phasing in f-ratios needs to be accounted for in scaling totalproduction to new proifudon.     

Daily changes of uptake of inorganic carbon and nitrogen,and their relation to phytoplankton blooms in late spring-early summer in Lake Nakanuma,Japan

Daily changes of inorganic carbon and nitrogen uptake were measured in May in 1986 in Lake Nakanuma, Japan. Uptake of inorganic carbon and ammonium in the light-bottle experiments in the 1 m layers, showed daily changes similar to chlorophyll a changes, though the uptake activities peaked before chlorophyll a peaks (phytoplankton blooms) appeared. Potential growth rates of phytoplankton and observed growth rates were calculated from the uptake rates and chlorophyll a changes. The potential growth rates did not always correspond to the observed growth rates. The potential growth rates did not correlate with the loss rates. The correlation between the observed growth rates and the loss rates was better. These results suggest that though the increase of uptake activities may be necessary for occurrence of phytoplankton blooms, loss processes may affect the occurrence of blooms.     

Diel variability in nitrogenous nutrient uptake by phytoplankton in the Chesapeake Bay plume

Diel patterns in the uptake of nitrogenous nutrients were observedin the coastal plume of the Chesapeake Bay system, but the specificpatterns varied with season. During the winter months, ratesof NH₄⁺ and urea uptake were significantly higher at night thanduring the day, and rates of NO₃^– uptake were higher duringthe day. During the summer, rates of NH₄⁺ and urea uptake weresignificantly higher at night only during half the studies conducted;during the remaining studies, there was either no significantdifference or rates of uptake of NH₄⁺ were higher during theday. Rates of NO₃^– uptake during the summer months werealso higher during the day than at night. Seasonal differenceswere also apparent in the time of day at which maximum observeduptake rates of each nitrogen nutrient occurred. During thewinter-spring months, maximum observed rates of NO₃^– uptakeoccurred between first light and noon, whereas during the summermonths, maximum observed uptake rates of NO₃^– occurredboth morning and afternoon, and consistently 9–16 h afterthe maximum observed peak in the uptake of reduced nitrogen.We interpret these findings in terms of seasonal shifts in nitrogennutritional status of the assemblages, as well as species-specificdifferences in the effect of a given stimulus (e.g. a nitrogenpulse at the mouth of the Bay) to entrain an uptake response,and we suggest that the extent of this variability must be understoodbefore generalizations about the use of f-ratios as characteristicsof specific populations or water masses can be drawn.     

Microplanktonic regeneration of ammonium and dissolved organic nitrogen in the upwelling area of the NW of Spain: relationships with dissolved organic carbon production and phytoplankton size-structure

Ammonium regeneration and dissolved organic nitrogen (DON) releasewere studied experimentally in the euphotic zone of shelf andoceanic waters of NW Spain in relation to coastal upwellingdynamics and the size-structure of phytoplankton communities.Incubations of plankton labelled with [¹⁵N]ammonium were madeduring four cruises, two of which also included size-fractionateddeterminations of chlorophyll a and primary production, andexperimental determinations of production rates of dissolvedorganic carbon (DOC) using ¹⁴C. Inorganic nitrogen concentrationswere mainly related to nitrate enrichment by upwelling pulses,while ammonium concentrations were generally low in all situations.Ammonium did not accumulate in the study area, suggesting adaily time scale coupling between regeneration and uptake. Incontrast, DON largely exceeded inorganic nitrogen in all situationsand generally increased from spring to autumn. Ammonium regenerationwas positively correlated with DON release and both rates showedthe largest variation in summer, with minimum values duringactive upwelling and maximum values when upwelling relaxed.Comparison of DON stocks and rates in different shelf areassuggests that DON release near the coast during summer was morepersistent in the water than DON release in off-shelf and oceanicareas. The carbon:nitrogen ratio of DOC and DON release rateswas highly variable, revealing a large excess of DOC comparedwith DON. This excess can be attributed to either an underestimateof total DON release (as only release from ammonium was measured)or to an enhanced production of carbon-rich organic substancesby diatoms in coastal areas. By considering a broad range oftrophic situations, this study reveals a fundamental differencebetween short term release of DOC and DON by plankton. Physiologicalprocesses (such as carbohydrate exudation by diatoms) seem tobe the cause of large DOC excess, whereas trophic processes(such as grazing) are more likely the cause of enhanced DONrelease.     

On the compatibility of carbon uptake rates calculated from stable and radioactive isotope data: implications for the design of experimental protocols in aquatic primary productivity

Basic differences between the ¹³C and the ¹⁴C techniques ofmeasuring carbon uptake by phytoplankton exist at the levelof the isotopic analysis. With the first method, a ratio ofisotopic abundances is measured on the sample, whereas an absoluteamount of isotope is estimated with the second method. If acarbon source other than the labeled one is utilized by thephytoplankton during incubation, the stable isotope method maybe biased. Specific uptake values will be underestimated bythis effect. It is, however, possible to obtain unbiased estimatesof the ¹³C-labeled carbon uptake by using an equation containinga term describing the final particulate carbon concentration.Only under this condition will carbon uptake rates derived from¹³C isotope data be always compatible with the ¹³C method ofmeasuring primary production ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP), Second International Workshop heldat the National Oceanographic Institute, Haifa, Israel in April–May1984.     

Daylight ultraviolet radiation and the photoinhibition of phytoplankton carbon uptake

¹⁴C carbonate uptake by phytoplankton in quartz and laborator4yglass vessels was compared in full daylight. Carbon uptake inthe Laboratory glass vessels was –25% higher during 1.5and 3.7 h incubations. Covering the quartz vessels with 6 rumthick soda-lime glass led to carbon uptake rates 80% higherthan in unshaded quartz bottles. The differences in carbon uptakerates can be explained by the transmittance characteristicsof the different glass types (approximate cut-off wavelengthfor laboratory glass = 320 mm and for soda-lime glass = 350mm). Additional expenrnents yielded preliminary data on theeffective photoinhibiting spectrum in daylight (the productof the action spectrum of photoinhibition and the downwellingspectral irradiance at noon). It is concluded that at Latitudesof central Europe in the summer the donunant photoinhibitionwavelength, during noon hours on a bright day, is between 320nm and 360 nm. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982.     

Interactions of light, temperature and inorganic nitrogen in controlling planktonic nitrogen utilisation in the Tagus estuary

The effect of light, temperature and ammonium on inorganic nitrogen uptake by phytoplankton was investigated from June 1994 through December 1995 at three sites in the Tagus estuary (Portugal), during high tide of neap tides. Ammonium concentrations higher than 10 M reduced nitrate uptake down to 24% but never prevented it. Below this threshold concentration, nitrate uptake was neither inhibited nor changed. Uptake of both nitrate and ammonium as a function of light intensity exhibited a saturation response. Uptake reduction occurred in the near bottom phytoplankton populations, particularly for nitrate. The ammonium uptake system was less limited by light than the nitrate uptake system, indicating the importance of ammonium as a nitrogen source for the phytoplankton which is likely to experience high changes in light in the well-mixed water column of this estuarine environment. Ammonium uptake was exponentially related to temperature in the upper estuary whereas in the mid and lower estuary this relationship was linear. The effect of temperature on nitrate uptake was linear but far less marked than for ammonium uptake.     

Coral uptake of inorganic phosphorus and nitrogen negatively affected by simultaneous changes in temperature and pH

The effects of ocean acidification and elevated seawater temperature on coral calcification and photosynthesis have been extensively investigated over the last two decades, whereas they are still unknown on nutrient uptake, despite their importance for coral energetics. We therefore studied the separate and combined impacts of increases in temperature and pCO₂ on phosphate, ammonium, and nitrate uptake rates by the scleractinian coral S. pistillata. Three experiments were performed, during 10 days i) at three pH_T conditions (8.1, 7.8, and 7.5) and normal temperature (26°C), ii) at three temperature conditions (26°, 29°C, and 33°C) and normal pH_T (8.1), and iii) at three pH_T conditions (8.1, 7.8, and 7.5) and elevated temperature (33°C). After 10 days of incubation, corals had not bleached, as protein, chlorophyll, and zooxanthellae contents were the same in all treatments. However, photosynthetic rates significantly decreased at 33°C, and were further reduced for the pH_T 7.5. The photosynthetic efficiency of PSII was only decreased by elevated temperature. Nutrient uptake rates were not affected by a change in pH alone. Conversely, elevated temperature (33°C) alone induced an increase in phosphate uptake but a severe decrease in nitrate and ammonium uptake rates, even leading to a release of nitrogen into seawater. Combination of high temperature (33°C) and low pH_T (7.5) resulted in a significant decrease in phosphate and nitrate uptake rates compared to control corals (26°C, pH_T = 8.1). These results indicate that both inorganic nitrogen and phosphorus metabolism may be negatively affected by the cumulative effects of ocean warming and acidification.     

植被退化对温带典型草原根系-土壤系统碳氮分配的影响

以3种不同退化程度的温带典型草原(大针茅轻度退化、中度退化和重度退化)为研究对象,研究植被退化对温带典型草原土壤及根系碳氮含量及储量的影响。结果显示:(1)植被退化对地下根系碳含量影响不显著(P0.05),而对地下根系氮含量的影响显著(P0.05),中度退化样地根系氮含量显著高于轻度退化和重度退化样地(P0.05)。(2)植被退化对根系碳氮储量影响显著(P0.05),根系碳氮储量随着土层深度增加而减少,总根系碳氮储量随退化程度加剧而降低。(3)土壤有机碳、总碳和总氮含量及储量均受退化程度和采样深度的影响显著(P0.05),其含量随着土壤深度的增加而显著减少,随退化程度加剧而显著降低(P0.05)。(4)土壤是根系-土壤系统碳氮储存的最主要场所,储量占比90%以上。虽然土壤碳氮储量均存在表层聚集现象,但表层储量所占比例在各样地间差异显著(P0.05)。     

Summer nutrient dynamics of the Middle Atlantic Bight: nitrogen uptake and regeneration

Nitrate and ammonium uptake and ammonium regeneration rates(by zooplankton, microplankton and benthos) were measured onthe Atlantic continental shelf (Middle Atlantic Bight) duringsummer, 1980. Euphotic zone profiles of NO₃^– and NH₄⁺uptake rates were similar in magnitude and vertical structureover a large geographical area. Microplankton NH₄⁺ regenerationrates, although measured less frequently, also showed a relativelyconsistent vertical structure; rates were positively correlatedwith uptake rates. Nitrate assimilation (‘new’ production)was used to estimate vertical eddy diffusivity and paniculatesinking rates. Eddy diffusion estimates ranged from <0.1to >2.0 cm² s^–1 and were positively related to arealprimary production. Estimated particulate sinking rates averaged5 mg at Nm^–2d^–1 and compared favorably with sedimentationrates measured from free-floating and moored sediment traps.Benthic nitrogen regeneration rates represented <10% of thispaniculate nitrogen flux. Within the mixed layer, NH₄⁺ assimilation(‘regenerated’ production) represented 50–80%of the total (NO₃^– + NH₄⁺ ) nitrogen productivity and33% for the euphotic zone. Of this, 30% was attributed to zooplankton,63% to microplankton (<100 µm) and 7% to benthos. Onthe average, 74% of the microplankton NH₄⁺ regeneration wasassociated with organisms passing 1 µm filters.     

Effects of storage conditions on total carbon and nitrogen contents of soil and plant samples

Aims The storage of soil and plant samples has important significance for ecological studies, but has not been widely used. This study aims to compare total carbon/nitrogen content of soil and plant samples before and after long term storage, and further to investigate the feasibility of archiving samples for time series ecological studies at large temporal scales.Methods Soil and plant samples were collected in the growing season in 2011. Carbon/nitrogen mass fraction were analyzed after four years of storage, and were compared with the data obtained before storage using pairwise t-test and linear regression.Important findings Nitrogen mass fractions of stored samples were linearly correlated to the data before storage along the 1:1 line under different storage conditions, and the correlation coefficient r was greater than 0.98 (except for soil samples stored at temperature lower than 20 °C and with particle size <2 mm, r = 0.91). The carbon mass fraction after storage was changed by the storage conditions. Carbon mass fractions of stored samples with particle size <0.15 mm were linearly correlated to the data before storage along the 1:1 line (r > 0.98). Carbon mass fractions of samples with particle size <2 mm increased after storage, and the slope of the linear relationship was 1.26 and 1.04 for soil and plant samples respectively. These results indicated that, nitrogen content of stored samples was stable under different storage conditions, while the stability of carbon content was affected by sample particle size but by storage temperature. Archived samples used for carbon/nitrogen analysis were suggested to be ground to particle size <0.15 mm under fully dry and completely sealed conditions.     

Day-night changes in production of carbohydrate and protein by natural phytoplankton population from Lake Biwa, Japan

The ¹³C and gas chromatography-mass spectrometry (¹³C-GC-MS)method was applied to determine the day-night changes in thecomposition of photosynthetic products of the natural phytoplanktonpopulation from Lake Biwa, Japan. Glucose is the most abundantmonosaccharide in acid-hydrolyzable carbohydrate. The contributionof glucose was high in incubatesd samples in daytime and decreasedduring the night. Other monosaccharides (rhamnose, fucose, ribose,arabinose, xylose, mannose and galactose) and amino acids tendedto be produced throughout both day- and night-time. These resultssuggested that the carbon flows from glucose, which might constitutereserve glucan, to other monosaccharides and amino acids duringnight-time. The disproportionate production of glucose (reservedglucan) during daytime was thus partly cancelled out at night.     

氯嘧磺隆与尿素对土壤微生物生物量碳、氮及无机氮的影响

通过室内培养试验,研究了不同浓度氯嘧磺隆(20、200、2000 μg·kg-1土)单一施用及与尿素(120 mg· kg-1土)配合施用情况下,土壤微生物生物量碳、氮和土壤铵态氮、硝态氮随时间的动态变化规律.结果表明:各浓度氯嘧磺隆单独处理在整个培养期(60 d)中对微生物生物量碳、氮均有抑制作用,且浓度越高,后期抑制作用越强;各浓度氯嘧磺隆处理在培养前期对硝态氮、铵态氮没有明显影响,中期(15 d)能显著提高土壤中铵态氮的含量,后期(30 d后)显著提高了土壤中硝态氮的含量.尿素单独施用及与氯嘧磺隆配施均能在短时间内增加微生物生物量碳、氮,但随后配施处理的促进作用减弱;尿素单独和配施均能持久增加土壤中铵态氮、硝态氮含量.     

Flooding affects uptake and distribution of carbon and nitrogen in citrus seedlings

Soil flooding has been widely reported to affect large areas of the world. In this work, we investigated the effect of waterlogging on citrus carbon and nitrogen pools and partitioning. Influence on their uptake and translocation was also studied through 1?N and 13C labeling to provide insight into the physiological mechanisms underlying the responses. The data indicated that flooding severely reduced photosynthetic activity and affected growth and biomass partitioning. Total nitrogen content and concentration in the plant also progressively decreased throughout the course of the experiment. After 36 days of treatment, nitrogen content of flooded plants had decreased more than 2.3-fold compared to control seedlings, and reductions in nitrogen concentration ranged from 21 to 55% (in roots and leaves, respectively). Specific absorption rate and transport were also affected, leading to important changes in the distribution of this element inside the plant. Additionally, experiments involving labeled nitrogen revealed that 1?N uptake rate and accumulation were drastically decreased at the end of the experiment (93% and 54%, respectively). 13CO? assimilation into the plant was strongly reduced by flooding, with δ13C reductions ranging from 22 to 37% in leaves and roots, respectively. After 36 days, the relative distribution of absorbed 13C was also altered. Thus, 13C recovery in flooded leaves increased compared to controls, whereas roots exhibited the opposite pattern. Interestingly, when carbohydrate partitioning was examined, the data revealed that sucrose concentration was augmented significantly in roots (37-56%), whereas starch was reduced. In leaves, a marked increase in sucrose was detected from the first sampling onwards (36-66%), and the same patter was observed for starch. Taken together, these results indicate that flooding altered carbon and nitrogen pools and partitioning in citrus. On one hand, reduced nitrogen concentration appears to be a consequence of impaired uptake and transport. On the other hand, the observed changes in carbohydrate distribution suggest that translocation from leaves to roots was reduced, leading to significant starch accumulation in leaves and further decreases in roots.     

Planktonic nitrogen transformations during a declining cyanobacteria bloom in the Baltic Sea

The uptake of ¹⁵N-labelled nitrogen nutrients (ammonium, urea,nitrate) was studied during the decline of a bloom of nitrogen-fixingcyanobacteria in the Baltic Sea. This was done by sampling anorth-south transect of stations, representing different stagesof the bloom. Comparison with nitrogen fixation data showedthat this process was of minor importance, and that the nitrogenuptake was dominated by regenerated nitrogen, mainly ammonium.From time series incubations for studying nutrient uptake, itappears that the regeneration of ammonium was substantial, butthat the production of urea or nitrate was slow. The integrateddaily uptake was calculated for the 0–15 m interval atfour stations and values ranged between 6 and 21 mmol N m^–2day^–1, of which the regenerated nutrients, ammonium andurea, constituted 71–93%. Nitrate was of minor importanceand the highest nitrate uptake rates were found close to thethermocline (at 15 m) and in the southern part of the Baltic.Comparison with carbon fixation data reported from simultaneousmeasurements at two stations gave C/N uptake ratios of 4.9 and2.1 for integrated daily uptake. Contrary to earlier findings,the concentration of DON increased with increasing salinity(from 15 to 17 µmol l^–1). This was correlated withthe declination of the bloom and is suggested to be a resultof a gradual release of less easily utilized DON from the degradationof cyanobacteria. The C/N ratio of DOM was high, 21–23.     

Dark uptake of inorganic14C in oligotrophic oceanic waters

Dark uptake of inorganic ¹⁴C by offshore plankton was measuredat two depths at 36 stations in the Atlantic Ocean from 52°Sto 26°N, mainly along 30°W. The samples were incubatedfor 2 h with and without inhibition of biological activity withHgCl₂. In addition, six time course experiments were performed.The mean dark uptake rate varied from 0.68 to 4.82 (µmolC m^–3 h^–1 over the transect and showed a significantpositive relationship with chlorophyll a. The dark uptake wasusually >5% of the maximum photosynthetic capacity (P^m),and higher values relative to P^m were associated with low valuesof P_m and not with high absolute dark values. A linear relationshipbetween dark uptake and P_m was found with a background value(y-axis intercept) of 0.51 (µmol C m^–3 h^–1and a slope of 0.77% of P^m. A major fraction of the dark signal,66–80% of the total signal, persisted in bottles treatedwith HgCl2, indicating that most of the dark signal was independentof biological activity. Time course experiments showed a lineardark uptake with time for the first hours, whereafter the uptakeceased. At stations with low concentrations of inorganic nitrogen[>1 (µmol (NH₄⁺+NO₃^–)], a second stage was observedafter 3–8 h, probably due to an increase in bacterialactivity. The results suggest three mechanisms for the darkvalue in short-term incubations in oligotrophic waters. A backgroundvalue independent of biomass and incubation time which was thedominant part of the dark signal in samples with very low phytoplanktonbiomass (>0.3 p-g Chi a 1"). Another important part was residualsof ¹⁴C associated with plankton, probably adsorbed to compoundsinside the cells. This fraction was dominant in short-term incubationsat chlorophyll concentrations >0.3 p.g Chi a H. Active uptakeby living cells (total minus ‘HgCl2 uptake‘) wasonly a minor part of the dark signal in short-term incubations,but dominated at longer incubation time (>3–9 h), probablydriven by an increase in bacterial activity. A significant enhancementof the non-photosynthetic uptake of ¹⁴C was observed in light,probably associated with a carbon-concentrating mechanism inphytoplankton or light stimulation of ß-carboxylationactivity. The results strongly suggest that dark values shouldbe subtracted from the light uptake. This correction is particularlyimportant when photosynthetic rates are low, e.g. at low lightor in short-term incubations where a time-zero background becomesa significant part of the total uptake in light. Present address: National Environmental Research Institute,Department of Marine Ecology and Microbiology, Frederiksborgvej399, PO Box 358, DK-4000 Roskilde, Denmark     

Uptake and regeneration of inorganic nitrogen in coastal waters influenced by the Mississippi River spatial and seasonal variations

The Mississippi and Atchafalaya Rivers introduce large amountsof nutrients to surface waters of the northern Gulf of Mexico.This paper reports the most complete data to date on inorganicnitrogen uptake and regeneration in a broad range of coastalenvironments influenced by the river water, along with informationon nutrient concentrations and including pico-, nano-, and microplanktonspecies composition. Nitrate in surface waters is greatly reducednear the river plume, at salinities between 5 and 25 PSU, wherethe largest variance in uptake rates was observed, and was coincidentwith peaks in surface chlorophyll. Despite the depletion ofnitrate, nitrogen limitation was a rare event during the study,because of relatively high ammonium concentrations (>1 µmolNH₄⁺ I^–1 and regeneration rates. Two contrasting situationscharacterize the seasonal nitrogen dynamics in surface shelfwaters. High nitrate input during the spring caused a largebloom in which the cells were well adapted to use nitrate.Thedominant phytoplankton species were chain forming diatoms, alsoreported in sediment-trap studies in the area. Ammonium regenerationonly accounted for a small fraction of the nitrogen requirementsduring the bloom. In contrast, the low flow of river water duringsummer resulted in low nitrate concentrations in surface water.In this case phytoplankton productivity was highly reduced andmay depend greatly on ‘in sita’ ammonium regeneration.