Nitrogen regeneration by the surf zone penaeid prawn Macropetasma africanus

Nitrogen excretion of individual Macropetasma africanus (Balss) from an exposed beach/surf zone in Algoa Bay, South Africa was monitored under laboratory and field conditions in relation to body mass, temperature and feeding during 1985. Excretion rate experiments were performed on starved prawns at 15°, 18°, 20° and 25°C, as well as on individuals fed on four different diets (mussel, fish, shrimp and natural diet) at 15° and 20°C. The ratios of the excreted compounds to total nitrogen excreted were similar for the four diets despite differences in their nitrogen content and in the amount of food consumed. At 15° and 20°C, ammonia excretion rates of fed individuals were four to seven times higher than in starved prawns. the excretion rates were not correlated with nitrogen content of diets. M. africanus recycles 1 557 g NH₄–N per metre strip per year or 1 832 g total nitrogen m^-1 yr^-1, which constitute 12 and 14%, respectively, of total phytoplankton requirements of the surf zone. This study indicates that large motile crustaceans, when abundant, can play an important role in nutrient recycling in turbulent marine environments.     

Nitrogen and energy loss via nonfaecal and faecal excretion in the marine teleost Lithognathus lithognathus

Nitrogen excretion and assimilation efficiencies of individual Lithognathus lithognathus (Cuvier 1830), a marine teleost from high energy surf zones in Algoa Bay, South Africa, were determined under laboratory conditions in 1985. Nonfaecal-nitrogen excreted by starved and fed L. lithognathus consists mainly of ammonia with urea and amino acids as secondary excretory products. Ammonia excretion rates were temperature dependant with the excretion rate of starved fish significantly lower than those of fed fish, at all three experimental temperatures. The mass component b of the mass/ammonia excretion equation was temperature independent and ranged from 0.651 to 0.700 and 0.589 to 0.635 for starved and fed fish respectively. The mean percentage of food energy lost via dissolved nonfaecal excretory products (exogenous plus endogenous) was 6.11±6.07%. Assimilation efficiencies ranged from 70.75 to 99.29% for dry matter and from 95.72 to 99.58% on an energy basis. The combined nonfaecal and faecal energy loss was calculated at 11.87% of the ingested energy. The benthic feeding ichthyofauna recycle 255 g total nitrogen per metre strip per year which constitutes 2% of total phytoplankton nitrogen requirements of the surf zone.     

Excretion of ammonia by zooplankton and its potential contribution to nitrogen requirements for primary production in the Catalan Sea (NW Mediterranean)

Excretion of ammonia by mesozooplankton (>200 m zooplankton) and its potential contribution to the nitrogen requirement for phytoplankton growth has been estimated for different hydrographical situations along a transect across the Catalan Sea (Northwestern Mediterranean). The nitrogen excreted as ammonia was estimated from mesozooplankton biomass and specific excretion rates. Nitrogen requirements of phytoplankton were estimated by means of carbon fixation rates and C:N ratios of <200 m particulate organic matter. Minimum C:N ratios and maximum primary production, zooplankton biomass, phytoplankton nitrogen requirements, and nitrogen excretion of zooplankton occurred near the Catalan density front. On average, the nitrogen regenerated by the mesozooplankton accounted for 43% of the nitrogen requirements of the phytoplankton. The specific excretion rates of ammonia and the percentage of phytoplanktonnitrogen requirements supplied by excreted nitrogen were higher at coastal stations. In some coastal and frontal stations, the ammonia excreted exceeded the phytoplanktonnitrogen demand. Bacteria competing for nutrient supply and the possible uncoupling between rate processes and standing stocks of phyto- and zooplankton could explain the apparent excess of regenerated ammonia.     

Metabolic characteristics of midwater zooplankton: Ammonia excretion,O:N rations,and the effect of starvation

The nature of protein catabolism in a wide range of species of midwater zooplankton was investigated. The weight-specific ammonia excretion rates (g NH₃–N g^–1 dry wt h^–1, y) decline exponentially with minimum depth of occurreece (MDO, x), y=163.4 x^{–0.479±0.212} (95%ci) (CI=confidence interval), when temperature is held constant. The change in ammonia excretion can be partially explained by the decrease in percent protein (%P) with MDO, %P=80.17 MDO^{–0.148±0.122 (95%ci)} The atomic O:N ratio of freshly caught zooplankters ranged from 9.1 to 91, with most measurements between 9 and 25. Detailed studies were carried out on the response of one of the species studied (Gnathophausia ingens) to starvation (28 d). After 14 d of starvation the average ammonia excretion rate declined by more than 75% to less than 1 g NH₃–N g^–1 wet wt h^–1, although the average oxygen consumption declined by only 13% within the first 7 d of starvation and then remained stable. This differential response of oxygen consumption and ammonia excretion to starvation resulted in an increase in the average O:N ratio of starved animals from an initial 33 to 165 after 21 d. The average O:N ratios of fed mysids remained below 38 during the experiment. G. ingens maintains a relatively uniform metabolic rate during starvation by relying more heavily on its large lipid stores than when being fed.     

Effects of salinity on respiration and nitrogen excretion in two species of echinoderms

The 30-d survival limit of Eupentacta quinquesemita and Strongylocentrotus droebachiensis is 12–13 S. The activity coefficient (1 000/righting time in seconds) of stepwise acclimated sea urchins declined from 16.3 at 30 S to 3.5 at 15 S. Oxygen consumption rates (QO₂) of both species held at 30 S and 13°C were highest in June and lowest in December. During the summer, when environmental salinity is most variable in southeastern Alaska, the QO₂ of both species held at 30, 20 and 15 S varied directly with salinity. Perivisceral fluid PO₂ varied directly with acclimation salinity in sea urchins, but not in sea cucumbers. Perivisceral fluid oxygen content of acclimated sea urchins was significantly lower at 15 and 20 S than at 30 S due to reduced PO₂ and extracellular fluid volume at the lower salinities. The QO₂ of both species varied directly with ambient salinity during a 30-10-30. semidiurnal pattern of fluctuating salinity. No change occurred in the average QO₂ of either species over a 15-30-15. semidiurnal pattern of fluctuating salinity. Sea urchin perivisceral fluid PO₂ declined as ambient salinity fluctuated away from the acclimation salinity in both cycles and increased as ambient salinity returned to the acclimation salinity. Total nitrogen excretion of stepwise acclimated sea cucumbers declined significantly from 30 to 15 S, but there was no salinity effect on total nitrogen excretion in sea urchins. Ammonia excretion varied directly with salinity in stepwise acclimated sea cucumbers (67–96% of total nitrogen excreted), but there was no salinity effect on ammonia excretion (89–95% of total nitrogen excreted) of sea urchins. Urea excretion did not vary with salinity in sea cucumbers (2–4% of total nitrogen excreted) or sea urchins (2–9% of total nitrogen excreted). Primary amines varied inversely with salinity in sea cucumbers (2–30% of total nitrogen excreted), but did not vary with salinity in sea urchins (2–4% of total nitrogen excreted). The oxygen: nitrogen ratio of both species indicated that carbohydrate and/or lipid form the primary catabolic substrate. The O:N ratio did not vary as a function of salinity. Both species are more tolerant to reduced salinity than previously reported, however, rates of oxygen consumption and/or nitrogen excretion are modified by salinity as well as season.     

Physiological and biochemical aspects of embryonic and larval development of the winter flounder Pseudopleuronectes americanus

Eggs and larvae of the winter flounder Pseudopleuronectes americanus Walbaum were hatched and raised in the laboratory under controlled conditions. Biochemical composition was measured during development and found to be similar to that of other species: 65 to 80 percent protein, 15 to 30 percent fat, and 0 to 5 percent carbohydrate. Ash content was 7 to 10 percent of dry weight. The chorion comprised more than half of the weight of an egg and the data suggested that it was possibly a source of nutrition to the developing embryo. The sequence of utilization appeared to be carbohydrate and then protein to hatching, lipid, mixed lipid and protein, the predominantly protein until feeding began. Carbohydrate was accumulated at first feeding and depleted when growth began. Protein and lipid were deposited in approximately constant proportions. Respiration rates of eggs were low, 0.002–0.015 l O₂ egg^-1 h^-1, but rose gradually from fertilization to hatching. Respiration rates of early larvae were from two to eight times that of eggs (0.033–0.131 l O₂ larva^-1 h^-1). Variation in larval respiration rates indicated a three-fold difference in rate according to level of activity. Eggs excreted ammonia at an increasing rate from fertilization to hatching. Larvae excreted ammonia, primary amines, and other unidentified organic nitrogenous substances. Rates of excretion and proportions of excretory products varied with stage of development. Primary amine excretion was variable and a major component in early stages. Ammonia-N excreted was two to 20 times primary amine N excreted. Unidentified substances were the predominant form of N excretion during early feeding. Ammonia accounted for most of the N excreted in older larvae. Early specific growth rates were 2.1 and 5.5%. Net caloric conversion and net and gross nitrogen efficiencies were low in first feeding larvae compared to adult fishes (32.2, 27.7, and 10.7% respectively).Contribution no. 5071 from the Woods Hole Oceanographic Institution     

Grazing,defecation and excretion rates of copepods from inter-island channels of the Canadian Arctic archipelago

Zooplankton (>800 m) and water samples were collected at night at eleven stations in Parry Channel and adjacent waters, between 28 August and 14 September 1986. Chlorophyll concentrations varied between 17 g l^-1 at the surface at one station in Wellington Channel, Canada, and 1.5 g l^-1 throughout the top 30 m at one station in Byam Martin Channel, Canada. In tows from 0 to 50 m the zooplankton community at all stations was dominated by varying proportions of three species,Calanus hyperboreus, C. glacialis andMetridia longa. Levels of chlorophyllderived pigments inC. hyperboreus Stages V, IV and III were correlated with the concentration of chlorophyll at the chlorophyll maximum. Defecation rate constants, measured for the different stages where present, were variable but not correlated with ambient chlorophyll concentrations. Ammonia excretion was measured simultaneously for the communities in which defecation was being measured in the invividual species and stages. In these experiments the rate of ammonia accumulation decreased significantly over the period during which the copepods were actively defecating (usually the first 3 h) and then tended to a constant level (over the next 18 to 20 h). The time courses of ammonia accumulation could be described by a model comprised of the sum of a straight line and a saturating curve. For seven experiments the ammonia release given by the asymptote of the saturating component was correlated with the estimate of community defecation, obtained by summing the individual defecations, suggesting that the two processes were closely associated. Weight specific defecation and weight specific defecation-associated ammonia excretion were both correlated with ambient chlorophyll concentration. The ratio of initial to basal ammonia excretion rate varied between 2 and 20, so that defecation-associated ammonia release may be important in the estimation of in vivo nitrogen excretion or regeneration rates.     

Metabolism and elemental composition of zooplankton from the Barents Sea during early Arctic summer

Rates of oxygen consumption, ammonia excretion and phosphate excretion were measured on a hydromedusae (Aglantha digitale), pteropods (Limacia helicina, Clione limacina), copepods (Calanus finmarchicus, C. glacialis, C. hyperboreus, Metridia longa), an amphipod (Parathemisto libellula), a euphausiid (Thysanoessa inermis) and a chaetognath (Sagitta elegans), all of which were dominant species in the Barents Sea during early summer 1987. Water and ash contents and elemental composition (C and N) were also analysed on the specimens used in these metabolic experiments. Between species variations were 67.8% to 94.7% of wet weight in water content, 6.4% to 56.5% of dry weight in ash content, 16.7% to 61.0% of dry weight in carbon content, and 4.3% to 11.2% of dry weight in nitrogen content. Oxygen consumption rates ranged from 0.33 to 13.8 l O₂ individual^-1 h^-1, ammonia excretion rates, from 0.0072 to 0.885 gN individual^-1 h^-1 and phosphate excretion rates, from 0.0036 to 0.33 g P individual^-1 h^-1. In general, higher rates were associated with larger species, but considerable differences were also seen between species. The ratios between the rates (O : N, N : P, O : P) exhibited a wide species-specific variation, indicating differences in dominant metabolic substrates. Typical protein oriented metabolism was identified only in S. elegans. From the results of metabolic rate measurements and elemental analyses, daily losses of body carbon and nitrogen were estimated to be 0.50 to 4.15% and 0.084 to 1.87%, respectively, showing faster turnover rates of carbon than that of nitrogen. Comparison of daily loss of body carbon of the Barents Sea zooplankton with that of the Antarctic zooplankton indicated reduced rates of the former (63% on average).     

Persistent blooms of surf diatoms along the Pacific coast,USA

Productivity was studied in two diatom species, Chaetoceros armatum T. West and Asterionella socialis Lewin and Norris, which form persistent dense blooms in the surf zone along the Pacific coast of Washington and Oregon, USA. Past observations have shown that surf-diatom standing stock usually declines in summer along with concentrations of nitrate and ammonium. Using the ¹⁴C method, photosynthetic rates in natural surf samples were measured monthly for one year (October 1981 through September 1982) at a study site on the Washington coast. Also measured were temperature, salinity, dissolved nutrients, particulate carbon and nitrogen (used as estimates of phytoplankton C and N), and chlorophyll a. Assimilation numbers (P _max) were higher in summer (5 to 8 g C g^-1 chl a h^-1) than in winter (3 to 4gC). Specific carbon incorporation rates (µ_max) showed no obvious seasonality, mostly falling within the range of 0.09 to 0.13 g C g^-1 C(POC) h^-1. The discrepancy between the seasonal trends for chlorophyll-specific and carbon-specific rates reflects a change in the carbon-to-chlorophyll ratio. Because of seasonal differences in daylength and light intensity, daily specific growth rates () are thought to be higher in summer than in winter. Neither ammonium enrichment assays nor particulate carbon-to-nitrogen ratios provided convincing evidence for nitrogen limitation during summer, and the observed changes in diatom abundance cannot be explained on this basis. Both the high diatom concentrations and their seasonal variations probably are due mainly to factors other than growth rates; two factors considered important are diatom flotation and seasonal changes in wind-driven water transport. C. armatum usually dominates the phytoplankton biomass in the surf zone, and evidence suggests that this species is strongly dominant in terms of primary production.Contribution No. 1391 of the School of Oceanography, University of Washington, Seattle, Washington, USA     

The common jellyfish Aurelia aurita: standing stock,excretion and nutrient regeneration in the Kiel Bight,Western Baltic

The population dynamics, ammonia and inorganic phosphate excretion, and nutrient regeneration of the common jellyfish Aurelia aurita was investigated from 1982 to 1984 in the Kiel Bight, western Baltic Sea. During summer 1982, medusae abundance ranged between 14 and 23 individuals 100 m^-3, biomass was estimated at about 5 g C 100 m^-3 and the mean final diameter of individuals was 22 cm. Abundance, based on numbers, in 1983 and 1984 was an order of magnitude lower; biomass was less than 2 g C 100 m^-3 and jellyfish grew to 30 cm. During the summers of 1983 and 1984, A. aurita biomass constituted roughly 40% of that of the total zooplankton>200 m. In 1982, for which zooplankton data were lacking, it was assumed that medusae biomass was greater than that of all other zooplankton groups. Total ammonia excretion ranged between 6.5 and 36 mol h^-1 individual^-1, whereas inorganic phosphate release was 1.4 to 5.7 mol h^-1 individual^-1. Allometric equations were calculated and exponents of 0.93 for NH₄–N release and 0.87 for PO₄–P excretion were determined. Nitrogen and phosphorus turnover rates were 5.4 and 14.6% d^-1, respectively. In 1982, the medusae population released 1 100 mol NH₄–N m^-2 d^-1, about 11% of the nitrogen requirements of the phytoplankton. The inorganic phosphate excretion (150 mol m^-2 d^-1) sustained 23% of the nutrient demands of the primary producers. In the other two years the nutrient cycling of the medusae was much less important, and satisfied only 3 to 6% of the nutrient demands. It is suggested that in some years A. aurita is the second most important source of regenerated nutrients in Kiel Bight, next to sediment.     

Nitrogenous excretion by the sediment-living bivalve Nucula tenuis from the Oslofjord,Norway

The resting rate of ammonia excretion for the sediment living bivalve Nucula tenuis (Montagu) was found to be 38.8 gN mg^-1 dw h^-1×10^-4 in August and November 1985 in the Oslofjord. The excretion rate of experimental individuals was 37% higher when placed in artificial glass bead sediment. The regression between dry weight and excretion was logN excretion=1.338+1.192 log x, where excretion is gN individual^-1 h^-1×10^-4 and log x=mg dry weight.     

Ammonium excretion by the ophiuridOphiothrix fragilis as a function of season and tide

Ammonium excretion of a dense population (~1 500 individuals m^–2) of the ophiuridOphiothrix fragilis (Abildgaard) was measured in the Dover Straits (French coast) between May 1989 and March 1990: the excretion rate varied from 4.8 µg N g^–1 dry wt h^–1 in November to 12.8 µg N g^–1 dry wt h^–1 in June. Mean individual ammonium excretion,E, wasE=0.019t +1.26 (whereE=µg N individual^–1 andt=time in min;r=0.80;N=81). Variations in the ammonium excretion rate during a tidal cycle appeared to arise from variations in the duration of the suspension-feeding activity ofO. fragilis, which was governed by the strength of the tidal current. During short-term starvation, excretion was low (E=0.009t+1.47;r=0.91;N=17), increasing with increasing length of starvation [E=4.62lnt–2.5;r=0.95;N=17], as observed for other echinoderms; this could be due to catabolism of tissue. The daily ammonia flux from thisO. fragilis population to the water column was estimated at 41 mg N m^–2 d^–1.     

Respiration and excretion by the ctenophore Mnepiopsis leidyi

Respiration (dissolved oxygen and carbon dioxide) and excretion (dissolved organic carbon, inorganic and organic nitrogen and phosphorus) rates were measured for a variety of sizes of Mnemiopsis leidyi over a temperature range of 10.3° to 24.5°C. Both respiration and excretion rates were a direct linear function of animal weight and very temperature sensitive (Q₁₀4). Oxygen uptake ranged from 155 to 489 g at O/(g dry weight) day^-1 and carbon dioxide release from 43 to 166 M. Organic carbon made up about 38% of the total carbon released. Inorganic nitrogen excretion, exclusively in the form of ammonium, comprised 54% of the total nitrogen release and ranged from 10 to 36 M NH₄/(g dry weight) day^-1. Average release of dissolved primary amines (expressed as glycine equivalents) equaled 43% of the organic nitrogen fraction. Inorganic phosphorus release ranged from 2.0 to 4.9 M/(g dry weight) day^-1 and made up about 72% of the total phosphorus loss. The turnover of elements in the body was calculated as 5 to 19% per day for carbon and nitrogen, depending on the temperature, and an even higher 20 to 48% per day for phosphorus. These values are comparable to rates observed for small, active zooplankton.     

Variation in urea excretion in the gulf toadfishOpsanus beta

Under long-term (24 h) control measurements, significant urea was excreted (15 to 90% of excreted nitrogen) byOpsanus beta (Goode and Bean) collected in 1989 from Biscayne Bay, Florida, USA. Urea excretion rates and plasma urea concentrations were not affected by antibiotic treatments which decreased intestinal microbe populations. These results suggest that nitrogen recycling by gut microbe urease is probably not significant in this species. Urea excretion rates increased significantly following 8 h of air-exposure and in response to high levels of NH₄Cl. These results suggest that urea is synthesized and excreted by the toadfish primarily in situations that limit ammonia excretion. Thus, the ornithineurea cycle in the toadfish liver primarily maintains low concentrations of ammonia within the fish. High levels of variability in urea excretion rates and variation in response to air-exposure suggest that urea synthetic rates are affected by immediate past environmental conditions.     

Ammonia excretion by the benthic estuarine shrimp Crangon franciscorum (Crustacea: Crangonidae) in relation to diet

Ammonia excretion of individual Crangon franciscorum Stimpson was monitored in response to ingestion of single meals. The three experimental diets were tubificids, mysids and fish. Ammonia excretion was also monitored for individual shrimp which had been starved. The rate of ammonia excretion was higher for fed than for starved individuals in all cases. Ammonia excretion rates were higher for shrimp which were fed tubificids than those fed the other diets. The rate of excretion was influenced by both weight of the individual and the amount ingested of each diet. Ammonia excretion was influenced by dietary factors other than nitrogen content of the diet or the quantity ingested. The data suggest that field estimates of ammonia excretion based on the excretion rates of starved animals may be underestimates. The recent feeding history of an organism influences the rate of ammonia excretion as well as the relationship between the rate of excretion and weight.     

Metabolism of epipelagic tropical ctenophores

Measurements of respiration and excretion at 25°C were made for five species of ctenophores collected during five cruises to the Bahamas (1982–1984). The mean element-specific respiration and ammonium excretion rates of freshly collected specimens of all species ranged from 4 to 16% d^-1, the mean atomic O:N ratios were 10 to 16, and ammonium averaged 60 to 90% of the total dissolved nitrogen excreted. For adult ctenophores, the carbon content ranged from 0.6% carbon (as percent of dry weight) for Bolinopsis vitrea to 3.7% carbon for Beroë ovata. There was a marked increase in the organic content (% carbon of dry weight) of small Bolinopsis vitrea with tentacles compared to fully lobate adults. B. vitrea had increasingly higher metabolic rates when held at food concentrations up to 100 copepods 1^-1 (about 250 g C 1^-1). The overall range between starved and well-fed B. vitrea was about two times for respiration and a factor of three for ammonium excretion. B. vitrea decreased from well-fed to a starved metabolic rate in about a day after removal from food. The metabolic rate of Eurhamphaea vexilligera was not measurably affected by short-term starvation or feeding (maximum 25 copepods 1^-1). In feeding experiments, E. vexilligera of 20 to 56 mm length fed at rates equivalent to clearance rates of 250 to 1 800 ml h^-1.     

Effects of prolonged starvation on O2 consumption,NH 4 + excretion,and chemical composition of the bathypelagic mysid Gnathophausia ingens

The large bathypelagic mysid Gnathophausia ingens was collected in January 1980 at 400 to 700 m depth from the San Clemente Basin off southern California. Instars 7-8 and Instars 10-12 were starved in the laboratory for up to 19 wk. Oxygen consumption and ammonia excretion rates, and water, protein, lipid, and ash contents were determined periodically during starvation. Protein and lipid were metabolized in approximately equal amounts by starved individuals after the initial weeks of food deprivation. Unidentified components (probably non-protein nitrogenous compounds) apparently were oxidized within the first 7 wk of starvation. Oxygen consumption and ammonia excretion by Instars 7-8 decreased steadily during 19 wk of starvation. In contrast, stable or increasing respiration and excretion rates were observed for fed mysids. The mean respiration rate of Instars 10-12 did not change significantly during 13 wk of starvation, although ammonia excretion rates decreased. Low metabolic rates and large lipid reserves probably help G. ingens to withstand long periods of starvation in the mesopelagic environment. Calculations based on the laboratory data demonstrate that small, infrequent meals could account for the rates of metabolism and growth observed for G. ingens in the field.     

In-situ measurements of nitrogenous nutrient uptake by kelp (Ecklonia maxima) and phytoplankton in a nitrate-rich upwelling environment

Nitrogen uptake by the kelp Ecklonia maxima Osbeck and phytoplankton was examined under different conditions of nutrient availability in a kelp bed off the Cape of Good Hope by measuring nutrient depletion in large plastic bags by the kelp and ¹⁵N uptake by phytoplankton. E. maxima took up nitrate and ammonia, but not urea, and showed only a weak preference for reduced nitrogen. Phytoplankton absorbed all three forms of nitrogen available, with a preference for ammonia and urea. Ambient nitrate concentration exhibited a marked and rapid decrease with northerly winds and an increase in response to offshore southerly winds. Nitrogen uptake by E. maxima was linearly related to ambient concentration and did not saturate even at nitrate concentrations >20g-at N l^-1, resulting in a significantly higher tissue nitrogen content under upwelling conditions. Nitrate imported by upwelling was the chief source of nitrogen utilised within the kelp bed. Locally regenerated nitrogen (ammonia and urea) was calculated to contribute only ca 4% of total nitrogen uptake during upwelling and 30% during the relaxation or downwelling phase.     

Ingestion and assimilation of nitrogen from benthic sediments by three species of coral

We quantified the nitrogen and enzyme hydrolyzable amino acid (EHAA) concentrations of sediments prior to and after corals sloughed, ingested, and egested sediments layered onto their surfaces, for the three coral species Siderastrea siderea, Agaricia agaricites, and Porites astreoides in Jamaica. The percent nitrogen of the sediments egested by all three species was lower than in the sediments available to the corals. Additionally, the sediments sloughed (not ingested) by A. agaricites and P. astreoides were lower in percent nitrogen, while the sediments sloughed by S. siderea had the same percent nitrogen as that of the available sediments. The percent nitrogen of the sediments sloughed and egested by P. astreoides showed significant negative and positive relationships, respectively, to increasing sediment loads, while the percent nitrogen of the sediments sloughed and egested by both S. siderea and A. agaricites showed no relationship to sediment load. EHAA concentrations were not significantly different between the sloughed and available sediments but were significantly lower in the sediments egested by S. siderea and A. agaricites (EHAA concentrations were not measured for P. astreodies sediment fractions). Comparisons of the nitrogen and EHAA concentrations in the sloughed and egested sediments to what was available prior to coral processing show that maximum ingestion was between 0.1 and 0.2 µg N µg^–1 coral N cm^–2 and between 0.5 and 0.6 µg EHAA·cm^–2. Maximum assimilation efficiencies were estimated to be 30–60% of the available nitrogen. The data show that corals ingest and alter the nitrogen concentration of particles that land on their surfaces. The corals abilities to process these sediments, and the sediments possible contributions to coral nutrition, are discussed based on these results.Communicated by O. Kinne, Oldendorf/Luhe     

Lethal effects of ammonia and nitrite onPenaeus chinensis juveniles

Juveniles of the prawnPenaeus chinensis (3.96 ±0.18 cm, 0.36±0.06 g) reared in Taiwan in 1989 were exposed to different concentrations of ammonia and nitrite, by a static renewal method in 33 seawater at pH 7.94 and at 26 °C. The 24, 48, 96 and 120 h LC₅₀ (median lethal concentration) of ammonia were 3.29, 2.10, 1.53 and 1.44 mg l^–1 for NH₃-N (un-ionized ammonia as nitrogen) and 79.97, 51.14, 37.00 and 35.09 mg l^–1 for ammonia-N (un-ionized plus ionized ammonia as nitrogen). The 24, 96, 120, 144 and 192 h LC₅₀ of nitrite-N were 339, 37.71, 29.18, 26.98 and 22.95 mg l^–1. The LC₅₀ decreased with increasing exposure time. During the first 96 h,P. chinesis juveniles were more susceptible to ammonia than nitrite. However, prawns were less tolerant to nitrite than ammonia when exposed for more than 96 h. The threshold was found at 120 and 192 h for ammonia and nitrite, respectively, on the toxicity curves. Incipient LC₅₀ was 1.44 mg l^–1 for NH₃-N, 35.09 mg l^–1 for ammonia-N and 22.95 mg l^–1 for nitrite-N. The safe value forP. chinensis juveniles was 0.14, 3.51 and 2.30 mg l^–1, respectively.