Effect of nitrogen source on short-term light and dark CO2 uptake by a marine diatom

Based on a series of short-term incubations involving the marine diatom Chaetoceros simplex (Bbsm), precultured in NH ₄ ⁺ -, NO ₃ ^- -and urea-limited continuous cultures at several dilution rates, we found that both the short-term specific rate of ¹⁴CO₂ uptake and the amount of CO₂ fixed after 8- and 16-min incubations were unaffected by enrichment with NH ₄ ⁺ , urea, or NO ₃ ^- when NH ₄ ⁺ or urea were the preconditioning forms of N, but were slightly suppressed when the cells were first grown on NO ₃ ^- . Similar enrichments in the dark, however, led to significant CO₂ uptake under all conditions of NH ₄ ⁺ enrichment and to similarly enhanced CO₂ uptake, but only at high growth rates, when urea was the source of enrichment nitrogen. Our light results are contrary to some contemporary findings, but there does seem to be agreement that photosynthetic rates of rapidly growing phytoplankton will not be affected by exposure to pulses of nitrogen. Enhanced dark uptake, in contrast, appears to be characteristic of phytoplankton under all degrees of N limitation, and, as such, may be useful as an “all or nothing” index of the nitrogen status of natural waters. There is some indication that the index may be useful in determining both the form of and the degree of N limitation as well.     

Uptake and assimilation of 15NH 4 + by a variety of corals

Corals from the Swain Reefs of the Great Barrier Reef were exposed to ¹⁵NH ₄ ⁺ in sea water, and the disappearance of NH ₄ ⁺ from the medium and the appearance of ¹⁵N in the corals was followed. Disappearance of NH ₄ ⁺ usually followed a reasonably smooth curve with rate of disappearance decreasing as the concentration of NH ₄ ⁺ decreased; the decrease in rate was not the result of damage to the corals. The rate of uptake of ¹⁵NH ₄ ⁺ vs concentration did not describe a normal substrate saturation curve, as uptake may have been diffusion limited. ¹⁵N was assimilated into the corals in the form of stable organic nitrogenous compounds. Its assimilation usually exhibited an initial lag. An ahermatypic coral did not remove NH ₄ ⁺ from the medium. Differences in NH ₄ ⁺ uptake with a 1-h cycle of light and dark were minor. Isolated zooxanthellae assimilated ¹⁵NH ₄ ⁺ as indicated by accumulation of ¹⁵N with no lag.     

Different patterns of growth and nitrogen uptake in two clones of marineSynechococcus spp.

We grew marineSynechococcus Clones WH7803 and WH8018 at 150µE m^–2 s^–1 in dilute batch cultures with NH ₄ ⁺ as the limiting nutrient. The maximal uptake capacities for NH ₄ ⁺ and NO ₃ ^- were measured in frequent experiments during log and stationary phases of growth. Clone WH7803, originally isolated from oceanic waters, had a specific uptake rate of NH ₄ ⁺ that approximated the maximum (log phase) specific growth rate (ca ~ 0.025 h^–1). NO ₃ ^- uptake was observed only after nitrogen in the media was depleted; the NO ₃ ^– uptake capacity was ca 12% the capacity for NH ₄ ⁺ uptake throughout the nitrogen depleted period. Growth was arrested upon nitrogen depletion, but resumed soon after reinoculation into fresh media, even after 5 d of starvation. Clone WH8018, originally isolated from coastal waters, revealed a five-fold enhancement in the NH ₄ ⁺ uptake rate relative to growth rate at the time of nitrogen depletion. As nitrogen starvation proceeded, this enhancement was reduced. This clone, too, was able to take up NO ₃ ^- once nitrogen in the media was depleted, but only after ca 20 h. Growth continued for a limited period during nitrogen depletion, but nitrogen-starved cells were slow to recover upon reinoculation into fresh media. We speculate that clonal differences may reflect differences in the molecular regulation of nitrogen assimilation.     

Ammonium regeneration by microzooplankton in the Oslofjord

Concentrations of dissolved inorganic nitrogen compounds above the pycnocline in the Oslofjord are very low in the summer, with turnover times of the inorganic N pools of no more than a few hours. To investigate the possibility that continued phytoplankton growth in the summer depends on ammonium excretion by microzooplankton, rates of NH ₄ ⁺ regeneration and assimilation were measured by a ¹⁵N isotope dilution method. Daytime regeneration rates at 0–2 m depth were 0–28% of the calculated assimilation rates at ambient NH ₄ ⁺ concentrations. Regeneration was faster during a dinoflagellate bloom in August than in mixed diatom-dinoflagellate blooms in June and September. Most of the NH ₄ ⁺ appeared to be produced by juvenile copepods, rotifers, tintinnids, and heterotrophic dinoflagellates in the size fraction 45–200 m.     

Application of a 15N tracer method to the study of dissolved organic nitrogen uptake during spring and summer in Chesapeake Bay

The dissolved organic nitrogen (DON) pool in marine waters contains a diverse mixture of compounds. It is therefore difficult to accurately estimate planktonic uptake of DON using the limited number of radiolabeled compounds commercially available. We describe a method to estimate DON uptake rates using ¹⁵N-labeled DON recently released from phytoplankton. To make ¹⁵N-labeled DON, we incubated surface water with ¹⁵NH ₄ ⁺ and then isolated the DON, including any recently released DO¹⁵N, with ion retardation resin. This DON was then added to a freshly collected water sample from the same environment to quantify the rate of DON uptake. The technique was applied to investigate rates of DON uptake relative to inorganic nitrogen in the mesohaline Chesapeake Bay during May 1990 and August 1991. The May experiment took place after the spring bloom, and rates of DON uptake [ranging from 0.31 to 0.53 g-atom (g-at) Nl^-1 h^-1] often exceeded rates of NH ₄ ⁺ and NO ₃ ^- uptake combined. The rates of DON uptake at this time were higher than estimated bacterial productivity and were not correlated with bacterial abundance or bacterial productivity. They were, however, correlated with rates of NO ₃ ^- uptake. In May, we estimate that only 7 to 32% of DON uptake was a result of urea utilization. In contrast, in August, when regenerated nutrients predominate in Chesapeake Bay, rates of DON uptake (ranging from 0.14 to 0.51 g-atom Nl^-1 h^-1) were an average of 50% of the observed rates of NH ₄ ⁺ uptake. Consistent with the May experiment, rates of DON uptake were not correlated with bacterial production. A sizable fraction of DON uptake, however, appeared to be due to urea utilization; rates of urea uptake, measured independently, were equivalent to an average of 74% of the measured rates of DON uptake. These findings suggest that, during both periods of study, at least a fraction of the measured DON uptake may have been due to utilization by phytoplankton.     

Effect of nitrogen supply on nitrogen uptake,accumulation and assimilation in Porphyra perforata (Rhodophyta)

Porphyra perforata J. Ag. was collected from a rocky land-fill site near Kitsilano Beach, Vancouver, British Columbia, Canada and was grown for 4 d in media with one of the following forms of inorganic nitrogen: NO ₃ ^- , NH ₄ ⁺ and NO ₃ ^- plus NH ₄ ⁺ and for 10 d in nitrogen-free media. Internal nitrogen accumulation (nitrate, ammonium, amino acids and soluble protein), nitrate and ammonium uptake rates, and nitrate reductase activity were measured daily. Short initial periods (10 to 20 min) of rapid ammonium uptake were common in nitrogen-deficient plants. In the case of nitrate uptake, initial uptake rates were low, increasing after 10 to 20 min. Ammonium inhibited nitrate uptake for only the first 10 to 20 min and then nitrate uptake rates were independent of ammonium concentration. Nitrogen starvation for 8 d overcame this initial suppression of nitrate uptake by ammonium. Nitrogen starvation also resulted in a decrease in soluble internal nitrate content and a transient increase in nitrate reductase activity. Little or no decrease was observed in internal ammonium, total amino acids and soluble protein. The cultures grown on nitrate only, maintained high ammonium uptake rates also. The rate of nitrate reduction may have limited the supply of nitrogen available for further assimilation. Internal nitrate concentrations were inversely correlated with nitrate uptake rates. Except for ammonium-grown cultures, internal total amino acids and soluble protein showed no correlation with uptake rates. Both internal pool concentrations and enzyme activities are required to interpret changes in uptake rate during growth.     

Relative growth of different species of marine algae in wasterwater-seawater mixtures

In recent studies, we developed a combined nutrient removal-marine aquaculture process for the tertiary treatment of wastewater and the production of commercially important shellfish. Part of this process consists of an outdoor mass cultivation system for marine algae. During our previous experiments we observed that marine diatoms almost exclusively are the dominant algal species in our outdoor cultures. To better understand this phenomenon of diatom dominance we grew 16 species of marine algae in continuous monoculture under laboratory conditions simulating to some degree the conditions prevailing in our outdoor experiments. Species such as Skeletonema costatum, Monochrysis lutheri and Tetraselmis sp., which were never dominant in our outdoor cultures, grew as well in monoculture, as Phaeodactylum tricornutum, frequently, the prevalent species outdoors. However, when monocultures of Dunaliella tertiolecta and Thalassiosira pseudonana (3H) were purposely contaminated with P. tricornutum, the latter species quickly became dominant. It is suggested that a complex interaction of environmental factors is usually responsible for the dominance of a particular species; one such factor may be the nitrogen source in the growth media. Under conditions of virtually, complete nitrogen assimilation, the carbon: nitrogen ratio in the algae was high (7 to 8) when NO ₃ ^- –N was the source of nitrogen, and low (4 to 6) when NH ₄ ⁺ –N was the prime form of nitrogen. When algal growth was low, resulting in a large inorganic nitrogen residue, the carbon:nitrogen ratio was low regardless of whether NO ₃ ^- –N or NH ₄ ⁺ –N was the main nitrogen source.Contribution No. 3297 from the Woods Hole Oceanographic Institution.     

Regional studies of daily,seasonal and size fraction variability in ammonium remineralization

Rates of ammonium remineralization were determined using a ¹⁵N isotope dilution technique for two oceanic regions, one coastal region, and one estuarine region, covering a wide range of ambient nutrient, light, and temperature conditions. Results showed that NH ₄ ⁺ assimilative and regenerative fluxes were primarily in balance, even when the ambient nitrogenous pool was completely dominated by NO ₃ ^- . Variations in uptake and remineralization rates relative to time of day and season were also determined. Size fraction studies at several of the sites showed that the smallest size fraction (<10 m) was usually the most important in remineralizing NH ₄ ⁺ , and the importance of the apparent bacterial fraction (<1 m) may increase following blooms. The results support the concept that, over a wide variety of conditions, the fluxes of NH ₄ ⁺ remineralization and uptake are tightly coupled; phytoplankton are able to utilize NH ₄ ⁺ at the rate that it is produced by heterotrophic processes.     

Light-dependency of nitrate uptake by phytoplankton over the spring bloom in Auke Bay,Alaska

The effect of light intensity on nitrate uptake by natural populations of phytoplankton was examined by ¹⁵N traceruptake experiments during the spring (March–May 1987) in Auke Bay, Alaska. The data were fit to a rectangular hyperbolic model which included a term for dark uptake. Three types of curves described nitrate uptake as a function of light intensity. The first (Type I) had a low half-saturation light intensity (K _I), low chlorophyll-specific uptakes rates, no dark uptake and occasional photoinhibition. These were observed during a period of biomass decrease, accompanied by low daily light and strong wind, prior to the major bloom. The second type (Type II) had relatively high K _I, high chlorophyll-specific uptake rates, and no dark uptake. Type II curves were observed during most of the period prior to nitrate depletion in the surface waters. Types I and II both appeared prior to nitrate depletion in the water and reflected variations in the light history of the phytoplankton population. The third type (Type III) occurred in nitrate-deplete conditions, when nitrate uptake was less dependent on light intensity (i.e., high rates of dark uptake and lower K _I). Decreased light-dependency during this period was coupled with physiological nitrogen deficiency in the population. Comparing these parameters to those of photosynthetic carbon fixation, K _Ivalues of nitrate uptake were generally higher than those of photosynthesis prior to nitrate depletion, and lower during nutrient-deplete conditions.     

Effect of algal bloom deposition on sediment respiration and fluxes

Using sediment cores collected in November 1989 from Aarhus Bight, Denmark, the fluxes of O₂, CO₂ (total CO₂), NH ₄ ⁺ , NO ₃ ^– +NO ₂ ^– and DON (dissolved organic nitrogen) across the sediment-water interface were followed for 20 d in an experimental continous flow system. On day 7, phytoplankton was added to the sediment surface, to see the result of simulated algal bloom sedimentation. Benthic O₂ consumption and CO₂ efflux, 38 to 41 mmol O₂ m^-2 d^-1 and 25 to 30 mmol CO₂ m^-2 d^-1, respectively, immediately increased by 39% and 100% after the algal addition, but gradually declined to the orginal level. Fluxes of NH ₄ ⁺ (1.0 to 1.2 mmol m^-2 d^-1) and DON (0.3 to 0.9 mmol m^-2 d^-1) increased due to the organic substrate addition. NH ₄ ⁺ and NO ₃ ^– flux changed direction, becoming an efflux and influx, respectively, for a few days and a large amount of DON (max. rate 4.0 mmol m^-2 d^-1) was immediately produced either by bacterial hydrolytic activity or from autolysis of the algae. DON was the most significant nitrogen component in pore water and in terms of N-flux from sediment. A temporary stimulation of anaerobic respiration processes (sulfate reduction and denitrification) and a decrease in nitrification were indicated. After the effect of the organic addition had declined, the fluxes gradually reverted to the original rates. The halflife of the added algal material, of which 20 to 30% was very labile, was estimated to be 2 to 3 wk.     

Decomposition of urea associated with photosynthesis of phytoplankton in coastal waters

Decomposition of urea in seawater was studied in Mikawa Bay, a shallow eutrophic bay on the southern coast of central Japan. The urea concentration in seawater ranged from 1.3 to 5.9 μg-at. N/1 and comprised 12 to 40% of the dissolved organic nitrogen. Using ¹⁴C labelled urea, the rate of CO₂ liberation from urea and the incorporation rate of urea carbon into the particulate organic matter were determined. For the surface samples, high rates of CO₂ liberation from urea as well as the incorporation of urea carbon into the particulate organic matter were observed in the light, while much lower rates were obtained in the dark. Incubation experiments with exposure to different light intensities revealed that the rate of CO₂ liberation from urea and the incorporation of urea carbon into particulate organic matter changed with light intensity, showing a pattern similar to that of photosynthesis. The highest liberation and incorporation rates were observed at 12,000 lux. Incubation in light and in dark produced marked decreases and increases, respectively, in urea and ammonia, while no appreciable changes were observed for nitrate and nitrite. It is suggested that urea decomposition associated with photosynthetic activity of phytoplankton is one of the major processes of urea decomposition, and that it plays a significant role in the nitrogen supply for phytoplankton in coastal waters.     

Impact of UV-B radiation on uptake of 15N-ammonia and 15N-nitrate by phytoplankton of the Wadden Sea

Natural marine phytoplankton populations from the German Wadden Sea and unialgal cultures of the haptophycean Phaeocystis pouchetii were tested in 1989 under controlled UV-B stress conditions. Assimilation of ¹⁵N-nitrate in phytoplankton consisting mainly of P. pouchetii, or in pure cultures of this alga, was found to be very sensitive to enhanced UV-B dosage in comparison ¹⁵N-ammonia uptake. In contrast, in phytoplankton samples containing Ceratium spp., Coscinodiscus sp., Noctiluca sp. or others, rate of ¹⁵NO₃ ^- uptake was higher and only slightly affected by UV-B irradiance compared to the P. pouchetii sample. UV-B inhibitory effect on uptake of inorganic nitrogen by P. pouchetii was more pronounced under strong white-light conditions and after a UV-B pre-illumination period of several hours than under low white light. Pools of glutamine and alanine decreased after UV-B exposure. Results are discussed with reference to the damaging effects of white light and UV-B on nitrogen metabolism.     

Urea as a nitrogen source for the phytoplankton in the Oslofjord

Ambient concentrations of urea in the inner Oslofjord, Norway, showed a pronounced yearly cycle in 1980, with values in the range 0.1 to 10.0 μg-at N l^-1; this cycle resemble that of ammonia although urea concentrations were usually lower. The uptake of urea by phytoplankton was investigated using ¹⁵N. Urea was usually a less important N source than NH ₄ ⁺ , and accounted for 0 to 53% (mean 19%) of summed NH ₄ ⁺ +NO ₃ ^- + urea uptake rates from April to October. Absolute as well as relative (specific) uptake rates of urea were higher in the summer (June–August) than at other times. Uptake of urea was inhibited by NH ₄ ⁺ concentrations higher than 1 to 2 μg-at N l^-1. The summed NH ₄ ⁺ +NO ₃ ^- + urea uptake rate was exponentially related to temperature.     

Phosphorus- and nitrogen-limited photosynthesis and growth of Gracilaria tikvahiae (Rhodophyceae) in the Florida Keys: an experimental field study

The relative effects of NH ₄ ⁺ (N) and PO ₄ ^3- (P) on growth rate, photosynthetic capacity (P_max), and levels of chemical constituents of the red macroalga Gracilaria tikvahiae McLachlan were assayed during winter and summer, 1983 in inshore waters of the Florida Keys by using in-situ cage cultures. During winter, both N and P enrichment enhanced growth over that of ambient seawater; however, P rather than N accounted for more (60%) of the increased winter growth. During summer, P, but not N, enhanced growth over ambient seawater and accounted for 80% of increased growth. Similarly, P_max was enhanced by both P and N during winter (but mostly by P) and only by P during summer. Elevated C:P, C:N and N:P ratios of G. tikvahiae tissue during winter, but only C:P and N:P ratios during summer, support the pattern of winter N and P limitation and summer P-limitation. This seasonal pattern of N vs P limited growth of G. tikvahiae appears to be a response to seasonally variable dissolved inorganic N (twofold greater concentrations of NH ₄ ⁺ and NO ₃ ^- during summer compared to winter) and constantly low to undetectable concentrations of PO ₄ ^3- . Mean C:P and N:P ratios of G. tikvahiae tissue during the study were 1 818 and 124, respectively, values among the highest reported for macroalgae.     

Sediment ammonium availability and eelgrass (Zostera marina) growth

The interaction of sediment ammonium (NH ₄ ⁺ ) availability and eelgrass (Zostera marina L.) growth, biomass and photosynthesis was investigated using controlled environment and in-situ manipulations of pore water ammonium concentrations. Sediment diffusers were used to create pore water diffusion gradients to fertilize and deplete ammonium levels in sediments with intact eelgrass rhizospheres. Between October, 1982 and September, 1983 controlled environment experiments using plants from shallow (1.3 m) and deep (5.5 m) stations in a Great Harbor, Woods Hole, Massachusetts, USA eelgrass meadow along with in-situ experiments at these stations provided a range of sediment ammonium concentrations between 0.1 and 10 mM (adsorbed+interstitial NH ₄ ⁺ ). The results of the in-situ experiments indicate that nitrogen limitation of eelgrass growth does not occur in the Great Harbor eelgrass meadow. A comparison of NH ₄ ⁺ regeneration rates and eelgrass nitrogen requirements indicates an excess of nitrogen supply over demand and provides an explanation for the lack of response to the manipulations. Results of controlled environment experiments combined with in-situ results suggest that sediment ammonium pool concentrations above approximately 100 mol NH ₄ ⁺ per liter of sediment (interstitial only) saturate the growth response of Zostera marina.     

Composition chimique élémentaire (C,H, N) et équivalent énergie d'Acartia clausi (Crustacea: Copepoda), espèce importante dans la bioénergétique des écosystèmes côtiers de Méditerranée nord-occidentale

Fixed-point sampling of a shallow tidal estuary was performed hourly for 14 d in summer of 1982 and again in winter of 1983. This sampling regime was of statistically appropriate duration to allow characterization of the variability between periods of 2 to 96 h by spectral analysis of the time-series. The project (PULSE) took place in the Newport River Estuary, located behind the Outer Banks of North Carolina, USA. In all, twentyeight parameters were monitored, encompassing the meteorology, hydrology, water chemistry and phytoplankton-production physiology. Although the annual cycle was monitored, only the winter and summer seasons are compared here, i.e., the lowest water temperatures with the highest water temperatures. The physics, chemistry and biology of the estuary at the hourly scale were highly variable and non-random. The estuary is riverine in winter; growth-limiting nitrogen is supplied as nitrate (NO ₃ ^- ) and ammonium (NH ₄ ⁺ ) by runoff from the drainage basin. In summer, the estuary is lagoonal; nitrogen is supplied as NH ₄ ⁺ by biological regeneration. Chlorophyll a biomass varies primarily at the 4 d period in winter and at the diel period in summer. Although finely tuned to environmental variability, phytoplankton abundances were at equilibrium insofar as daily chlorophyll production was balanced by losses, i.e., grazing, export and deposition. Most important, high-frequency processes, here periods at the scale of cell-division times, can be very important in phytoplankton ecology.     

Inhibition of diatom photosynthesis by germanic acid: Separation of diatom productivity from total marine primary productivity

Germanic acid was shown to inhibit photosynthetic ¹⁴CO₂ uptake in marine diatoms. Inhibition was not complete even at concentrations of 20 mg Ge/1 nor in cultures incubated for extended periods of time (up to 24 h). The decrease in photosynthesis due to Ge(OH)₄ was independent of the stage of growth of the diatom culture. At 0.5 and 1.0 mg Ge/1, the degree of inhibition was dependent on the concentration of Si(OH)₄ in the medium. At 5 and 10 mg Ge/1, inhibition was not affected by Si(OH)₄ concentrations as high as those found in the sea-120 g-at Si/1. The effect of Ge(OH)₄ on photosynthesis is specific for diatoms; other marine phytoplankton were not inhibited. In mixed cultures of diatoms and marine flagellates, the reduction in ¹⁴CO₂ fixation upon addition of Ge(OH)₄ was used to calculated the proportion of diatom photosynthesis to total photosynthesis, and calculated proportions agreed well with actual proportions. Inhibition by Ge(OH)₄ was also used to estimate the percent of diatom photosynthesis in a natural marine community, and this was compared with the diatom portion of the crop. Diatom photosynthesis was higher than one might expect from crop figures, although both diatom photosynthesis and diatom numbers in the crop were low.     

Comparisons of nutrient uptake rates for Baltic macroalgae with different thallus morphologies

In-situ experiments were performed during different seasons to determine uptake rates of PO _3- ⁴ , NH ₊ ⁴ and NO _- ³ within ecologically representative ranges of nutrient concentrations, of dominant macroalgae in the Baltic Sea. Uptake rates were governed by nutrient concentrations, water temperature and thallus morphology, but not by the phylogenetic affinity of the species. Nitrogen uptake rates were always higher than those of phosphorus at the same concentrations, and NH ₊ ⁴ –N uptake rates exceeded those of NO _- ³ –N. The lowest uptake rates occurred among the late successional, long-lived, coarse species with low surface: volume ratios (Fucus vesiculosus, Furcellaria lumbricalis andPhyllophora truncata). The highest uptake rates were measured for short-lived, opportunistic algae, filamentous or with numerous hairs, (Cladophora glomerata, Enteromorpha ahlneriana, Scytosiphon lomentaria, Dictyosiphon foeniculaceus andCeramium tenuicorne). The latter group also had the highest V_max:k_max ratios, which indicates a more competitive advantage for nutrient uptake at low concentrations.     

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     

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.