Respiration and ammonia excretion by marine metazooplankton taxa: synthesis toward a global-bathymetric model

For thirteen representative taxa of metazooplankton from various depth horizons (<4,200 m) of the world’s oceans, respiration rate (681 datasets on 390 species) and ammonia excretion rate (266 datasets on 190 species) are compiled and analyzed as a function of body mass (dry mass, carbon or nitrogen), habitat temperature, habitat depth and taxon. Stepwise-regression analyses reveal that body mass is the most important parameter, followed by habitat temperature and habitat depth, whereas taxon is of lesser importance for both rates. The resultant multiple regression equations show that both respiration rate and ammonia excretion rate (per individual) increase with increase in body mass and habitat temperature, but decrease with habitat depth. Some taxa are characterized by significantly higher or lower rates of respiration or ammonia excretion than the others. Overall, the global-bathymetric models explain 93.4–94.2 % of the variance of respiration data and 80.8–89.7 % of the variance of ammonia excretion data. The atomic O:N ratios (respiration/ammonia excretion) are largely independent of body mass, habitat temperature, habitat depth and taxon, with a median of 17.8. The present results are discussed in light of the methodological constraints and the standing hypotheses for the relationship between metabolic rate and temperature. Perspectives for model improvement and possible application of it to plankton-imaging systems for rapid assessment of the role of metazooplankton in C or N cycles in the pelagic ecosystem are briefly discussed.     

The effect of laboratory conditions on the extrapolation of experimental measurements to the ecology of marine zooplankton. IV. Changes in respiration and excretion rates of boreal zooplankton species maintained under fed and starved conditions

Herbivorous zooplankton species (Calanus plumchrus, Paracalanus parvus and Euphausia pacifica) and carnivorous species (Parathemisto pacifica and Pleurobrachia pileus) collected from Saanich Inlet, British Columbia, Canada, were maintained in the laboratory under fed and starved conditions. Respiration rate and excretion rates of ammonia and inorganic phosphate were measured successively on the same batch populations of each species in different feeding conditions. Respiration rate remained at a constant level or increased during the feeding experiment but decreased progressively in starved individuals. Herbivorous, but not carnivorous, species showed a rapid decrease in both excretion rates for the first few days of an experiment irrespective of feeding conditions. However, the general level of excretion rates of fed specimens was higher than that of starved ones. The O:N, N:P and O:P ratios were calculated from respiration, ammonia excretion and phosphate excretion and discussed in relation to metabolic substrates of animals during the experiment. A marked difference was shown in the O:N ratio between fed hervivores (>16) and fed carnivores (7 to 19), suggesting highly protein-oriented metabolism in the latter. One unknown factor causing variation in excretion rates is speculated to be the physiological stress on animals during sampling from the field. It is suggested that the laboratory measurement of realistic excretion rates of zooplankton is difficult owing to their large fluctuations, but this is not the case with respiration rate.     

Metabolic rates of epipelagic marine copepods as a function of body mass and temperature

Metabolic rates (oxygen consumption, ammonia excretion, phosphate excretion) have been calculated as a function of body mass (dry, carbon, nitrogen and phosphorus weights) and habitat temperature, using multiple regression. The metabolic data used for this analysis were species structured, collected from Arctic to Antarctic seas (temperature range: -1.7°C to 29.0°C). The data were further divided into geographical and/or seasonal groups (35 species and 43 data sets for oxygen consumption; 38 species and 58 data sets for ammonia excretion; 22 species and 31 data sets for phosphate excretion). The results revealed that the variance attributed to body mass and temperature was highest (93-96%) for oxygen consumption rates, followed by ammonia excretion rates (74-80%) and phosphate excretion rates (46-56%). Among the various body mass units, the best correlation was provided by the nitrogen unit, followed by the dry weight unit. The calculated Q₁₀ values varied slightly according to the choice of body mass units; overall ranges were 1.8-2.1 for oxygen consumption rates, 1.8-2.0 for ammonia excretion rates and 1.6-1.9 for phosphate excretion rates. The effects of body mass and temperature on the metabolic quotients (O:N, N:P, O:P) were insignificant in most cases. Although the copepod metabolic data used in the present analysis were for adult and pre-adult stages, possible applications of the resultant regression equations to predict the metabolic rates of naupliar and early copepodite stages are discussed. Finally, global patterns of net growth efficiency [growth (growth+metabolism)^-1] of copepods were deduced by combining the present metabolic equation with Hirst and Lampitt's global growth equation for epipelagic marine copepods.     

Influence of temperature changes on oxygen uptake and ammonia and phosphate excretion,in relation to body size and weight,in Oikopleura dioica (Appendicularia)

The relationship between the rates of oxygen consumption, ammonia and phosphate excretion of a pelagic tunicate, the larvacean Oikopleura dioica Fol, 1872 were assessed as a function of size, dry weight and ash-free dry weight at 15°, 20° and 24°C. O. dioica has higher respiration and excretion rates than copepods of similar weight, but the weight exponent of the allometric power function: Y=aX ^b is similar to that of other poikilotherms. Temperatures above 20°C have a depressing effect on respiration and ammonia excretion. 90% of the variance in metabolic rates is explainable by body mass and temperatures Q₁₀ values for oxygen consumption, ammonia and phosphate excretion, respectively, are 2.45, 1.86 and 1.75 between 15° and 20°C, and 3.75, 2.90 and 3.60 between 20° and 24°C. Metabolic quotients (O:N, O:P, N:P) indicate a protein-oriented diet. The results of this study suggest weak metabolic regulation in O. dioica, an energetic strategy which allows an immediate response to favourable changes in feeding conditions.     

Experimental evaluation of the energy balance in Octopus vulgaris, fed ad libitum on a high-lipid diet

A complete energy balance equation was estimated for the common octopus Octopus vulgaris at a constant temperature of 20°C, fed ad libitum on anchovy fillet (Engraulis encrasicolus). Energy used for growth and respiration or lost with faeces and excreted ammonia was estimated, along with total energy consumption through food, for six specimens of O. vulgaris (with masses between 114 and 662 g). The energy balance equation was estimated for the specimens at 10-day intervals. During each 10-day interval, food consumed, body mass increase and quantity of faeces voided were measured. The calorific values of octopus flesh, anchovy flesh and faeces were measured by bomb calorimetry. Oxygen consumption and ammonia excretion rates were monitored for each specimen during three 24-h experiments and daily oxygen consumption and ammonia excretion were estimated. It was found that 58% of the energy consumed was used for respiration. The amount of energy invested in somatic and gonadal growth represented 26% of the total energy budget. The energy discarded through faeces was 13% of consumed energy. The estimated assimilation efficiency (AE) values of O. vulgaris feeding on anchovy (80.9–90.7%) were lower than the AE values estimated for other cephalopod species with different diets of lower lipid content such as crabs or mussels. Specific growth rates (SGR) ranged 0.43–0.95 and were similar to those reported for other high-lipid diets (bogue, sardine) and lower than SGR values found for low-lipid, high-protein diets (squid, crab, natural diet). Ammonia excretion peak (6 h after feeding) followed the one of oxygen consumption (1 h after feeding). The values of atomic oxygen-to-nitrogen (O:N) ratio indicated a protein-dominated metabolism for O. vulgaris.     

Metabolic rates of epipelagic marine zooplankton as a function of body mass and temperature

The metabolic rates (oxygen uptake, ammonia excretion, phosphate excretion) of epipelagic marine zooplankton have been expressed as a function of body mass (dry, carbon, nitrogen and phosphorus weights) and habitat temperature, using the multiple-regression method. Zooplankton data used for this analysis are from phylogenetically mixed groups (56 to 143 species, representing 7 to 8 phyla, body mass range: 6 orders of magnitude) from various latitudes (habitat temperature range:-1.4° to 30°C). The results revealed that 84 to 96% of variation in metabolic rates is due to body mass and habitat temperature. Among the various body-mass units, the best correlation was provided by carbon and nitrogen units for all three metabolic rates. Oxygen uptake, ammonia excretion and phosphate excretion are all similar in terms of body-mass effect, but differ in terms of temperature effect. With carbon or nitrogen body-mass units, calculated Q₁₀ values are 1.82 to 1.89 for oxygen uptake, 1.91 to 1.93 for ammonia excretion and 1.55 for phosphate excretion. The effects of body mass and habitat temperature on the metabolic quotients (O:N, N:P, O:P) are insignificant. The present results for oxygen-uptake rate vs body mass do not differ significantly from those reported for general poikilotherms by Hemmingsen and for crustaceans by Ivleva at a comparable temperature (20°C). The importance of a body-mass measure for meaningful comparison is suggested by the evaluation of the habitat-temperature effect between mixed taxonomic groups and selected ones. Considering the dominant effects of body mass and temperature on zooplankton metabolic rates, the latitudinal gradient of community metabolic rate for net zooplankton in the ocean is estimated, emphasizing the non-parallelism between community metabolic rates and the standing stock of net zooplankton.     

Metabolic activity and elemental composition of krill and other zooplankton from Prydz Bay,Antarctica, during early summer (November–December)

Oxygen uptake and ammonia excretion rates, and body carbon and nitrogen contents were measured in krill (Euphausia superba) and eight other zooplankton species collected during November–December 1982 in the Prydz Bay, Antarctica. From these data, metabolic O:N ratios (by atoms), body C:N ratios (by weight) and daily metabolic losses of body carbon and nitrogen were calculated as a basis from which to evaluate seasonal differences in metabolism and nutritional condition. Comparison of the present data with mid-summer (January) data revealed that early-summer E. superba were characterized by higher metabolic O:N ratios (58.7 to 103, compared with 15.9 to 17.5 for mid-summer individuals). Higher O:N ratios of early-summer E. superba resulted largely from reduced ammonia excretion rates and, to a lesser degree, from slightly increased oxygen uptake rates. Body C:N ratios of E. superba were low in early-summer (3.8 to 4.2) compared with mid-summer krill (4.1 to 4.7) due to lowered body-carbon contents in the former (42.6 to 43.6% compared with 43.2 to 47.5% dry weight of midsummer individuals); gravid females formed an exception, since no seasonal differences in body elemental composition were detected for these. No significant changes in water content (75.3 to 81.4% wet wt) and nitrogen content (9.9 to 11.1% dry wt) in E. superba were evident between the two seasons. Seasonal differences in metabolic rates and elemental composition were less pronounced in a salp (Salpa thompsoni), but a higher metabolic O:N ratio occurred in early-summer individuals. Interspecific comparison of the seven remaining zooplankton species studied with twelve species from mid-summer zooplankton investigated in an earlier study indicated that higher metabolic O:N ratios in early-summer are characteristic not only of herbivore/filter-feeders, but also of some carnivores/omnivores. No relationship between metabolic O:N ratios and body C:N ratios was apparent either intraspecifically or interspecifically, within or between early-summer and mid-summer seasons.     

Etude de la respiration et de l'excrétion d'azote et de phosphore des populations zooplanctoniques de l'upwelling mauritanien (mars–avril, 1972)

Fiftysix experiments were run for 22 h on 200 μ-net zooplankton in non-filtered sea water. The equations of orthogonal regression lines between respiration, and mineral, total nitrogen, and phosphorus excretion rates have been computed for 14 stations in the upwelling Mauritanian area. Correlation coefficients are high: excretion values may be estimated from respiration values in order to follow the N and P flux through the Mauritanian zooplanktonic populations. O:N, O:P, N:P, mineral:total excretion ratios are calculated for each station and are not significantly different in the 3 areas of the upwelling area studied. On the average, 48% of excreted phosphorus is thoroughly oxidized into phosphate and needs 142.4 atoms in respiration per P atom. The remainder, excreted as organic phosphorus, requires the same amount of oxygen for its later mineralization. The O:P ratio thus obtained is close to the theoretical -276. Fiftyfour percent of the nitrogen excreted is mineral and the O:N-NH ₊ ⁴ ratio shows a dominant carbohydrate and fat catabolism. The N:P ratio is constant, and close to 10 for both mineral and total excretion.     

Relation between respiration,excretion (ammonia and inorganic phosphorus) and activity of amylase and trypsin in different species of pelagic copepods from an Indian Ocean equatorial area

The relationship between respiration, nitrogen and phosphorus excretion and specific activity of amylase and trypsin was investigated from shipboard experiments with several species of copepods from the equatorial divergence area of the Indian Ocean, in June 1978. Statistical analysis of O:N ratios in 6 of the most common species revealed three main groups: a group displaying low O:N (Pontella fera, Candacia pachydactyla); a group with high O:N (Undinula darwini, Euchaeta marina, Temora discaudata); and a species displaying an intermediate O:N ratio: Scolecithrix bradyi. O:P and N:P did not differ significantly between species. There was a direct relationship between average O:N and the ratios of specific activity of the digestive enzymes amylase and trypsin. Species displaying low O:N and/or A:T ratios such as P. fera, C. pachydactyla and Oncaea venusta probably metabolize proteins more efficiently than they do plant carbohydrates (high nitrogen excretion and low amylase activity). Species displaying high O:N and A:T ratios, such as U. darwini, E. marina and T. dicaudata (low nitrogen excretion and high amylase activity) were assumed to use carbohydrates (starch) and proteins with equal efficiency. S. bradyi showed a large range of variations in trypsin activity and low amylase activity, resulting in a low average A:T ratio, but its O:N ratio was intermediate. Variations in O:N and O:P ratios were related to differences in the nutritional strategy of the different species, based on literature data concerning the anatomy of their mouth parts and their selectivity for animal material in mixed-food experiments.     

Oxygen uptake,ammonia excretion and phosphate excretion by krill and other Antarctic zooplankton in relation to their body size and chemical composition

Oxygen uptake, ammonia excretion and phosphate excretion were measured in 14 Antarctic zooplankton species, including various size classes of krill (Euphausia superba), during a cruise to the Antarctic Ocean adjacent to Wilkes Land in the summer of 1980. Elemental composition (C, N and P) was also determined on the specimens used in these metabolic rate measurements. The values obtained for C, N and P were 4.7 to 47.5%, 1.2 to 12.5% and 0.09 to 1.23% of dry weight, respectively. Regression analyses of metabolic rates on different measures of body weight (fresh, dry, C, N and P) were made on krill, salps and other zooplankton as arbitrarily defined groups and also on the combined groups to determine the best measure of body weight for intra- and interspecific comparison of metabolic rates. The correlations were highly significant in all regressions, although no common measure of body weight provided the best correlation for the three groups of animals. Except for the regression of ammonia excretion on C and N weight, all other regressions of metabolic rates and body weights were significantly different within these three groups. In the combined group, oxygen uptake and ammonia excretion were better correlated to C and N weights than to dry and P weights. For phosphate excretion in the combined groups, dry weight gave the best correlation. Despite these results, the choice of a particular measure of body weight was shown to be important in a comparison of the rates between krill and salps because of their widely different chemical compositions. Our results of rate measurements are compared with those of previous workers for some Antarctic zooplankton, particularly krill. Some of the previous data are in good agreement with ours, while others are not. Possible contributing factors are considered in the latter case. The ratios between the rates (O:N, N:P and O:P) fell within the general ranges reported for zooplankton from different seas. The O:N ratio was consistently low (7.0 to 19.8, by atoms) in all species, suggesting the importance of protein in their metabolic substrates. Protein-oriented metabolism was also supported by the results of C and N analyses which indicated no large deposition of lipid in these animals. From the results of metabolic rate measurements and elemental analyses, daily losses in bodily C, N and P for Antarctic zooplankton in summer were estimated as 0.4 to 2.8%, 0.6 to 2.5% and 1.3 to 19.4%, respectively. These values are approximately one order of magnitude lower than those reported for subtropical and tropical zooplankton.     

温度、体重对皱纹盘鲍耗氧量和排氨量的影响

皱纹盘鲍的耗氧量和排氨量的实验结果表明：温度和体重对鲍的耗氧量和排氨量有明显的影响,且两固子间存在着交互作用,皱纹盘鲍的个体耗氧量「Ｑ（Ｏ）／ｍｇｈ＾－１」和排氧量「Ｑ（Ｎ）／μｍｏｌｈ＾－１」与温度（θ／℃）和湿重（ｍ／ｇ）存在下列关系：Ｑ（Ｏ）＝０．０１２４ｍ＾０．８５５６１．０８５３＾θ,Ｑ（Ｎ）＝０．０２０７ｍ＾０．８５５６１．０８５３＾θ。２０℃皱纹盘鲍存在代谢的转变点。     

Metabolism and elemental composition of a giant chaetognath Sagitta gazellae from the Southern Ocean

Oxygen consumption, ammonia excretion and phosphate excretion rates were measured on Sagitta gazellae Ritter-Zahony, in conjunction with body composition analyses (water, ash, carbon, hydrogen, nitrogen and phosphorus). Both water content (94.7% of wet weight) and ash content (53.0% of dry weight) recorded on S. gazellae were the highest and the lowest, respectively, among the chaetognath data being reported. Contents of carbon, hydrogen and nitrogen of S. gazellae were the lowest among published values of chaetognaths. Metabolic comparison with other chaetognaths living in similar subzero water temperature revealed reduced rates in S. gazellae, while no appreciable differences were seen in the metabolic quotients (O:N, N:P and O:P ratios). The O:N atomic ratios were 10.5 to 15.9 indicating protein oriented metabolism. Reduced metabolic activity of S. gazellae is not due to their body composition as calculated daily metabolic losses of body carbon (0.50%), body nitrogen (0.38) and body phosphorus (1.6%) were also found to be lower than respective values reported on other congeners and even those of other zooplankton living in the Antarctic waters.     

Moderate stoichiometric homeostasis in the sea urchin Lytechinus variegatus: effects of diet and growth on C:N:P ratios

The influence of dietary elemental contents on consumer stoichiometry was investigated in selected and combined soft tissues (as a proxy of the whole individual) of the omnivorous sea urchin, Lytechinus variegatus. We raised urchins for 4 months in controlled seawater tanks using three different diets with different nutritional contents (from lower to higher: seagrass, red macroalgae, and a formulated diet). Individuals fed the different diets varied an average of 19.7, 19.4, and 38 % in C:N, C:P, and N:P ratios, respectively, with stronger temporal variability for C:P and N:P ratios across tissues and whole individuals. This resulted in homeostasis parameters (1/H) of ?0.45, 0.09, and 0.38, respectively, for C:N, C:P, and N:P, indicative of homeostatic to weakly homeostatic organisms, at least for C:P and N:P ratios. Individuals fed the nutrient-rich formulated diet had higher growth rates (14 ± 0.83 g WW month^?1) than those fed macroalgae or seagrass (9.3 ± 0.57 and 3.4 ± 0.33 g WW month^?1, respectively). However, rapid body increments in more nutritional diets caused both a decrease in the %N and an increase in the %P of soft tissues, which resulted in significant but opposite effects of diet stoichiometry and growth in sea urchin C:N (R = ?0.74 and R = 0.93, for diet and growth effects, respectively) and N:P ratios (R = 0.60 and R = ?0.63, also, respectively, for diet and growth effects). Among potential compensatory mechanisms helping to preserve certain levels of homeostasis, ingestion rates (g WW diet per g WW of urchin) were higher for seagrass and macroalgae diets than for the nutrient-rich formulated diet. In contrast, absorption and growth efficiencies displayed significant negative associations with nutrient contents in diets and did not exhibit nutritional compensation. Overall, our results suggest that resource stoichiometry strongly determines the growth rate of individuals (R = 0.88, P < 0.01), and moderate variability in C:N:P ratios of sea urchins possibly arise from differences in the allocation of proteins and RNA to body components, similarly to what has been proposed by the growth rate hypothesis.     

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.     

Respiration and excretion rates of Calanus glacialis in arctic waters of the Barents Sea

The respiration and excretion rates of Calanus glacialis (Jaschnov) Copepodite Stages III, IV, V, and adult females from the drift-ice area east of Svalbard (Barents Sea) were measured in shipboard experiments during the period from 27 May to 13 June, 1983. The phytoplankton biomass and abundance varied considerably between localities, but these variations were not generally reflected in the respiration and excretion rates of the copepod. The respiration and excretion rates of C. glacialis at the ambient temperature of-1.8°C (average respiration rates of 0.95, 0.73, 0.57, and 0.60 l O₂ mg^-1 dry wt h^-1 for Copepodite Stage III, IV, V, and adult females, respectively) were similar to those previously reported for other large-sized copepods from cold or temperate areas. Average respiration and excretion rates tended to decrease with incubation time or time after capture. Measurements on ten occasions within a period of 27 h after capture revealed excretion rates of ammonium ranging between 2.9 and 16.8 for C III, 3.7 and 21.1 for C IV, 1.3 and 28.4 for C V, and 1.6 and 18.7 for adult females, all expressed as nmol mg^-1 dry wt h^-1. In all experiments, excretion rates of inorganic phosphate varied between 0.7 and 1.5 (C III), 0.5 and 1.1 (C IV), 0.2 and 0.8 (C V), and 0.3 and 1.0 (adult females) nmol mg^-1 dry wt h^-1. Ratios of O:N, O:P, and N:P indicated that much of the metabolic energy was derived from catabolism of proteins. Comparison of the turnover rate of carbon and nitrogen showed, however, that nitrogen turnover was between 2.6 and 8.9 times higher than that of carbon. This may indicate that the copepods deaminate ingested protein, with the carbon skeleton of the amino acids subsequently being used in the synthesis of lipid compounds, possibly wax esters.     

Respiration and nitrogen excretion of zooplankton. IV. The influence of starvation on the metabolism and the biochemical composition of some species

Changes in the respiration, ammonia excretion and biochemical composition were studied for three species of starving zooplankton (Calanus finmarchicus, Sagitta elegans, and Acartia clausi). Over the period of starvation, the respiration rate of all three species followed the same pattern of an initial decrease followed by a more or less constant level. A similar pattern was observed for the ammonia excretion rate of S. elegans and A. clausi, whereas C. finmarchicus excretion appeared to oscillate between high and low levels of protein catabolism. Study of the biochemical changes showed that C. finmarchicus consumed primarily lipids, and at times proteins, to meet its energy requirement whereas S. elegans and A. clausi primarily used protein. Variations in the elemental composition as well as the O:N ratio confirmed that C. finmarchicus alternated between periods of protein-dominant catabolism and lipid-dominant catabolism during starvation. No similar change in catabolism was observed in the two other species. The results are discussed in terms of physiological mechanisms of resistance to starvation and were used to calculate the energy budget of S. elegans and C. finmarchicus during the period of total starvation. The significance of such budgets is discussed and some of the sources of error examined.Bedford Institute of Oceanography Contribution.     

The use of metabolic ratios for determining the catabolic substrates of a solitary ascidian

In a previous study, Hatcher (1989: Mar. Biol. 102: 445–452) found that variations in CO₂ and O₂ respiration rates in individual marine invertebrates led to RQ (respiratory quotient) values which were variable and often outside theoretical limits. The present study was designed to examine the variability in several excretionbased metabolic ratios which are often used as alternatives for the RQ in qualitative predictions of catabolic substrates. The experimental organism was a solitary ascidian,Herdmania momus (Savigny), collected near Perth, Western Australia, between January and July 1984. Respiration and excretion rates ofH. momus were examined as a function of a progressive nutritional stress, and covariation was examined. Nutritional stress accounted for more of the variation over time in respiration rates (40 to 50%) than in NH ₄ ⁺ excretion rates (20%). Significant net exchanges of dissolved organic compounds were measured. Qualitative predictions of catabolic substrates were based on a comparison of metabolic ratios with theoretical limits. The O:PO ₄ ^3- ratios were lower than the theoretical limits. The values of the O:NH ₄ ⁺ and NH ₄ ⁺ :PO ₄ ^3- ratios were not influenced by nutritional conditions but changed as a function of reproductive condition of the ascidians. Based on the results of this study, it was concluded that metabolic ratios measured on individual ascidians cannot be reliable predictors of catabolic substrates.     

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.     

The cladoceran trophic status in the nitrogen limited ecosystem of Lake Kinneret (Israel)

The impactof cladocerans metabolic activities on the carbon (C), nitrogen (N) and phosphorus (P) dynamics in Lake Kinneret (Israel) is presented. The study, is based on the incorporation of field data and experimental measurements. Grazing, respiration and production rates of Diaphanosoma spp., Ceriodaphnia spp. and Bosmina spp. were experimentally measured at three temperatures, and the results were extrapolated to the field biomass distribution atthese respective temperatures, and the total lake capacity was calculated using the following equation: consumption = respiration + production + excretion. The field capacity of consumption, respiration and production were found to be mostly correlated with biomass density, but the temporal fluctuation of the percent of excretion from consumed energy differed. The increase in P, decline in N and decrease in the TN/TP mass ratio in the epilimnion of Lake Kinneret during 1969-2004 created N limitation. An increase in C and dedine in TN, with a consequential increase in the C/TN ratio were documented. TP was augmented but the C/TP ratio was only slightly increased. During 1975-2004, P was probably, a minor limitation for cladoceran growth. The positive impact of recycled P by cladocerans underthe N limitation in Lake Kinneret is discussed. There is a current threat on the water quality, derived from N limitation (mostly in summer-fall) and consequent Cyanophyta blooms. Thus, the role of recycled P bygrazers may be significant.     

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.