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.     

The measurement of respiratory electron-transport-system activity in marine zooplankton

A modification of the tetrazolium reduction method (Packard, 1971) for measurement of respiratory electron transport system activity in marine zooplankton is described. A major modification is the addition of Triton X-100 to the reaction mixture to solubilize formazan produced, thus eliminating an organic extraction step and increasing the sensitivity and versatility of the method. The kinetic parameters of the assay are presented and the apparent effects of Triton X-100 are discussed. The correlation between electron-transport-system (ETS) activity and in vivo respiration (ETS/R) for Calanus pacificus has been found to be 2.02±0.29 (standard deviation). The method is presented as providing a reliable estimate of the zooplankton component of oxygen consumption in the sea.     

Post-bloom grazing byCalanus glacialis,C. finmarchicus andC. hyperboreus in the region of the Polar Front,Barents Sea

The plankton community in the Polar Front area of the Barents Sea was investigated during a cruise from 14 to 28 July 1987. The colonial algaePhaeocystis pouchetii andDinobryon pellucidum dominated the phytoplankton. Depth integrated carbon assimilation rates varied from 190 to 810 mg C m^–2 d^–1. A high carbon:chlorophyll ratio (which varied from 123 to 352) prevailed at the three stations investigated, which may relate to facultative heterotrophic behaviour byD. pellucidum. The herbivorous zooplankton community was dominated byCalanus glacialis, C. finmarchicus, andC. hyperboreus. Maximum zooplankton biomass was found in the same depth strata as phytoplankton chlorophyll maximum. The herbivorous copepod populations did not display consistent day-night vertical migration patterns. Phytoplankton consumption rates of the various life stages were estimated from the turnover rate of plant pigments in the gut. The gut defecation rate constant (R) varied from 0.014 to 0.027 min^–1 at 0°C in copepodites (Stage II to adult female) ofC. glacialis, independent of developmental stage.Calanus spp. community carbon ingestion rates calculated from particulate carbon:chlorophyll ratios, were 10, 65 and 400% of daily phytoplankton carbon fixation rates at Stations 1, 2 and 3, respectively.     

Metabolism and body composition of a copepod (<Emphasis Type="Italic">Neocalanus cristatus</Emphasis>: Crustacea) from the bathypelagic zone of the Oyashio region,western subarctic Pacific

Metabolism [respiratory oxygen consumption, electron-transfer-system (ETS) activity] and body composition [water, ash, carbon (C), nitrogen (N), carbon/nitrogen (C/N) ratio] of stage C5/C6 Neocalanus cristatus from 1000 to 2000 m depth of the Oyashio region, western subarctic Pacific, were determined during the period of July 2000 through June 2003. Compared with the C5 specimens from shallow depths (<250 m), those from 1000 to 2000 m were characterized by quiescent behavior, reduced respiration rates (30% of the rates at active feeding), very low water content (61–70% of wet weight), but high C content (56–64% of dry weight) and C/N ratios (7.2–10.6, by weight). Artifacts due to the recovery of live specimens from the bathypelagic zone appeared to be unlikely in this study, as judged by the consistent results between re-compression (100 atm) and non-compression (1 atm) respiration experiments, and between ETS activities and respiration rates directly measured. In addition, the respiration rates of C6 males and females of N. cristatus from the same 1000–2000 m depth were two to three times higher than the rates of C5 individuals, but were similar to the rates of a bathypelagic copepod, Paraeuchaeta rubra. Combining these results with literature data, C budgets of: (1) diapausing C5 specimens, weighing 6–10 mg dry weight; (2) molt to C6 females; and (3) the complete the life span were established, taking into account assorted losses in respiration during diapause at stages C5 and C6, molt production and egg production. Respiratory C losses by C5 and C6 specimens were estimated on the basis of body N as adjusted metabolic rates [AMR; µl O₂ (mg body N)^–0.843 h^–1], then N budgets were also computed subtracting N lost in the form of cast molts and eggs from the initial stock. Calculations revealed that allocation of the C stock was greatest to egg production (34–57%), followed by respiration (27%) and cast molts (3%), leaving residual C of 13–36% in spent C6 females. The present results for N. cristatus from the North Pacific are compared with those of Calanus spp. in the North Atlantic.Communicated by O. Kinne, Oldendorf/Luhe     

Mitochondrial DNA sequence variation and phylogeography of oceanic insects (Hemiptera: Gerridae: Halobates spp.)

Relatively few insects have invaded the marine environment, and only five species of sea skaters, Halobates Eschscholtz (Hemiptera: Gerridae), have successfully colonized the surface of the open ocean. All five species occur in the Pacific Ocean, H. germanus White also occurs in the Indian Ocean, whereas H. micans Esch- scholtz is the only species found in the Atlantic Ocean. We sequenced a 780 bp long region of the mitochondrial cytochrome oxidase subunit I gene (COI) for a total of 66 specimens of the five oceanic Halobates species. Our purpose was to investigate the genetic variation within species and estimate the amount of gene flow between populations. We defined 27 haplotypes for H. micans and found that haplotype lineages from each of the major oceans occupied by this species are significantly different, having sequences containing five to seven unique base substitutions. We conclude that gene flow between populations of H. micans inhabiting the Atlantic, Pacific, and Indian Ocean is limited and hypothesize that these populations have been separated for 1 to 3 million years. Similarly, there may be limited gene flow between H. germanus populations found in the Pacific and Indian Ocean and between H. sericeus populations inhabiting the northern and southern parts of the Pacific Ocean. Finally, we discuss our findings in relation to recent hypotheses about the influence of oceanic diffusion on the distribution and population structure of oceanic Halobates spp. Received: 29 July 1999 / Accepted: 23 November 1999     

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.     

Scavenging deep-sea amphipods: Effects of food odor on oxygen consumption and a proposed metabolic strategy

In situ respiration rates as a response to the odor of food were measured for two species of scavenging amphipods, Paralicella caperesca from 3 650 m in the western North Atlantic Ocean and Orchomene sp. B from 1 300 m in the Santa Catalina Basin off southern California (USA). In addition, complementary laboratory starvation/respiration rates for a shallow-water species, Orchomene sp. A, were determined. Initial elevated O₂ consumption rates were found for up to 8 h in all deep-sea amphipods exposed to bait odor, followed by a period of lowered respiration equivalent to rates observed in individuals not exposed to bait. Orchomene sp. A revealed a response similar to that observed in the deep-sea species. A metabolic strategy is proposed whereby scavenging amphipods efficiently utilize large episodic organic falls in the food-limited environment of the deep sea. This strategy involves (1) the ability to withstand long periods of starvation, (2) rapid response to an organic fall, (3) rapid location of the organic fall, (4) maximal rate of food consumption with maximal quantity ingested, and (5) efficient utilization of the consumed food. Each of these attributes are explored with the expected and observed mechanisms employed to achieve them.     

The effect of laboratory conditions on the extrapolation of experimental measurements to the ecology of marine zooplankton VI. Changes in physiological activities and biochemical components of Acetes sibogae australis and Acartia australis after capture

An investigation was made into the possible causes for the decline of physiological activities in Acetes sibogae australis Colefax and Acartia australis Farran during their maintenance for a period of 2 d following capture. Physiological activities comprising respiration, electron transport system (ETS) activity and excretion of ammonia and phosphate, and the biochemical components protein, RNA, ATP and adenylate energy charge (EC) were measured. Excluding fluctuation in the ammonia excretion rate, physiological rates and biochemical components decreased on an individual basis with time after capture for both species for this period. These decreases were generally moderated if expressed on a protein basis. Notwithstanding these changes, the adenylate EC of both species was constant at the normal level. From the results of stable EC and other physiological and biochemical consequences, it was concluded that food shortage rather than capture stress was the major factor contributing to decreases in the various physiological and biochemical properties of the two zooplankton species studied.     

Respiration and excretion by oceanic salps

Rates of oxygen consumption and ammonium nitrogen excretion were measured on the solitary and/or aggregate generations of ten species of oceanic salps collected by SCUBA divers during cruises in the Atlantic Ocean (1982–1985). Species that were visibly more active had higher metabolic rates than did less active species. Rates were 1.5 to 2 times lower and O:N ratios were lower when salps were held before incubation than when incubation began at the time of collection. Respiration rate showed a better relationship to length than to weight, suggesting that metabolic activity may be connected mainly with swimming. O:N ratios were between 13 and 28 for most species and generations, but higher and more variable in Pegea spp. Exretion of urea was low or undetectable. Rates of metabolic demand (turnover) ranged from 9.7 to 99% body carbon d^-1 and 6.4 to 55.6% body nitrogen d^-1.Contribution No. 5988 from the Woods Hole Oceanographic Institution and No. 412 from the Allan Hancock Foundation     

Photosynthesis,photorespiration, and dark respiration in eight species of algae

The rates of photosynthesis and dark respiration for 7 marine algae and 1 fresh-water alga were measured and compared. The dinoflagellates Glenodinium sp. and zooxanthellae have high dark respiration rates relative to photosynthetic rates, which may decrease their net growth rates. Photorespiration in the 8 algal species was studied by examining the effects of the concentration of oxygen on the rates of photosynthesis, on the incorporation of ¹⁴CO₂ into the photorespiratory pathway intermediates glycine and serine, and on the postillumination burst of carbon dioxide production and oxygen consumption. A combination of these results indicates that all the algae tested can photorespire, but that Glenodinium sp., Thalassiosira pseudonana, and zooxanthellae either have a photorespiratory pathway different from that proposed for freshwater algae (Tolbert, 1974), or an additional pathway for glycolate metabolism.     

Population structure of the black tiger prawn, Penaeus monodon, among western Indian Ocean and western Pacific populations

We examined the population structure of the black tiger prawn, Penaeus monodon Fabricius, 1798, in the Indo-West Pacific by analyzing the geographic distribution of elongation factor 1-alpha intron sequences from specimens collected during the winter and spring of 1997. Both the molecular phylogeny of alleles and F-statistics indicated very strong differentiation between populations from the western Indian Ocean and western Pacific. This pattern is concordant with other recent studies of marine species in this region, implying that the Indo-Australian Archipelago represents a biogeographic break between populations in the Indo-West Pacific. F _ST-values among populations in the western Indian Ocean also indicate structure within this region, whereas no structure was found among western Pacific populations. Nucleotide diversity was significantly lower in the western Indian Ocean populations than in the western Pacific, implying that the populations have regional differences in demographic history. Received: 16 November 1998 / Accepted: 26 May 1999     

A regularity in trace element contents of marine zooplankton species

Four zooplankton species, three Arthropoda and one Chaetognata, from the Sea of Japan were analyzed in 1984 for eight major and fifteen trace elements, mainly by instrumental neutron activation analysis. Major and trace element contents on a dry weight basis varied little within a factor of 3.7, except for Ca (a factor of 5.7) among the four species. A log-log linearity with a slope of almost-1 was observed between mean oceanic residence time calculated from the mean dissolved river-water input _R and the concentration factor with respect to mean seawater concentration (CF _SW )for each species. The products of _R and CF _SW were nearly constant within a factor of 10, except for Br and Sb over seven orders of variation of CF _SW for each zooplankton species. It demonstrates a new regularity in trace element contents of marine zooplankton species. This relationship leads to the conclusion that the concentration factors of elements for these zooplankton species with respect to elements for these zooplankton species with respect to mean dissolved river-water concentration (CF _RW )are nearly constant within a factor of 10 with average values of 2.98 to 3.43 in logarithm.     

Optimization of exergy and implications of body sizes of phytoplankton and zooplankton in an aquatic ecosystem model

Size appears to be an important parameter in ecological processes. All physiological processes vary with body size ranging from small microorganisms to higher mammals. In this model, five state variables — phosphorus, detritus, phytoplankton, zooplankton and fish are considered. We study the implications of body sizes of phytoplankton and zooplankton for total system dynamics by optimizing exergy as a goal function for system performance indicator. The rates of different sub-processes of phytoplankton and zooplankton are calculated, by means of allometric relationships of their body sizes. We run the model with different combinations of body sizes of phytoplankton and zooplankton and observe the overall biomass of phytoplankton, zooplankton and fish. The highest exergy values in different combinations of phytoplankton and zooplankton size indicate the maximum biomass of fish with relative proportions of phytoplankton and zooplankton. We also test the effect of phosphorus input conditions corresponding to oligotrophic, mesotrophic, eutrophic system on its dynamics. The average exergy to be maximized over phytoplankton and zooplankton size was computed when the system reached a steady state. Since this state is often a limit cycle, and the exergy copies this behaviour, we averaged the exergy computed for 365 days (duration of 1 year) in the stable period of the run. In mesotrophic condition, maximum fish biomass with relative proportional ratio of phytoplankton, zooplankton is recorded for phytoplankton size class 3.12 (log V μm³ volume) and zooplankton size 4 (log V μm³ volume). In oligotrophic condition the highest average exergy is obtained in between phytoplankton size 1.48 (log V μm³ volume) and zooplankton size 4 (log V μm³ volume), whereas in eutrophic condition the result shows the highest exergy in the combination of phytoplankton size 5.25 (log V μm³ volume) and zooplankton size 4 (log V μm³ volume).     

Root respiration and oxygen flux in salt marsh grasses from different elevational zones

Plants growing in waterlogged environments are subjected to low oxygen levels around submerged tissues. While internal oxygen transport has been postulated as an important factor governing flooding tolerance, respiration rates and abilities to take up oxygen under hypoxic conditions have been largely ignored in plant studies. In this study, physiological characteristics related to internal oxygen transport, respiration, and oxygen affinity were studied in low intertidal marsh species (Spartina alterniflora and S. anglica) and middle to high intertidal species (S. densiflora, S. patens, S. foliosa, a S. alterniflora × S. foliosa hybrid, S. spartinae, and Distichlis spicata). These marsh plants were compared to the inland species S. pectinata and the crop species rice (Oryza sativa), corn (Zea mays), and oat (Avena sativa). Plants were grown in a greenhouse under simulated estuarine conditions. The low marsh species S. anglica was found to transport oxygen internally at rates up to 2.2 μmol O₂ g fresh root weight⁻¹ h⁻¹. In contrast, marsh species from higher zones and crop species were found to transport significantly less oxygen internally, although rice plants were able to transport 1.4 μmol g⁻¹ h⁻¹. Under hypoxic conditions, low marsh species were better able to remove dissolved oxygen from the medium compared to higher marsh species and crops. The oxygen concentration at which respiration rates declined due to limited oxygen (P _crit) was significantly lower in low marsh species compared to inland and crop species; P _crit ranged from <4 μM O₂ in the low marsh species S. anglica up to 20 μM in the inland species corn. Flooding-sensitive crop species had significantly higher aerobic respiration rates compared to flooding-tolerant species in this study. Crop species took up 3.6–6.7 μmol O₂ g⁻¹ h⁻¹ while all but one marsh species took up <3.5 μmol O₂ g⁻¹ h⁻¹. We conclude that oxygen transport, aerobic demand, and oxygen affinity all play important and interrelated roles in flood tolerance and salt marsh zonation.     

How size relates to oxygen consumption,ammonia excretion,and ingestion rates in cold (<Emphasis Type="Italic">Enteroctopus megalocyathus</Emphasis>) and tropical (<Emphasis Type="Italic">Octopus maya</Emphasis>) octopus species

The scaling of metabolic rates with body mass is one of the best known and most studied characteristics of aquatic animals. Herein, we studied how size is related to oxygen consumption, ammonia excretion, and ingestion rates in tropical (Octopus maya) and cold-water (Enteroctopus megalocyathus) cephalopod species in an attempt to understand how size affects their metabolism. We also looked at how cephalopod metabolisms are modulated by temperature by constructing the relationship between metabolism and temperature for some benthic octopod species. Finally, we estimated the energy balance for O. maya and E. megalocyathus in order to validate the use of this information for aquaculture or fisheries management. In both species, oxygen consumption and ammonia excretion increased allometrically with increasing body weight (BW) expressed as Y = aBW ^b . For oxygen consumption, b was 0.71 and 0.69 for E. megalocyathus and O. maya, respectively, and for ammonia excretion it was 0.37 and 0.43. Both species had low O/N ratios, indicating an apparent dependence on protein energy. The mean ingestion rates for E. megalocyathus (3.1 ± 0.2% its BW day⁻¹) and O. maya (2.9 ± 0.5% its BW day⁻¹) indicate that voracity, which is characteristic of cephalopods, could be independent of species. The scope for growth (P = I − (H + U + R) estimated for E. megalocyathus was 28% higher than that observed in O. maya (320 vs. 249 kJ day⁻¹ kg⁻¹). Thus, cold-water cephalopod species could be more efficient than tropical species. The protein and respiratory metabolisms of O. maya, E. megalocyathus, and other octopod species are directly dependent on temperature. Our results offer complementary evidence that, as Clarke (2004) stated, the metabolic response (R and U) cannot be determined mechanistically by temperature, as previously proposed (Gillooly et al. 2002).     

Oxygen consumption of midwater fishes and crustaceans from the eastern Gulf of Mexico

Oxygen consumption was measured as a function of temperature, oxygen partial-pressure (P_O2)and species depth of occurrence for twenty-three species of midwater fishes and crustaceans collected from the eastern Gulf of Mexico from June 1981 to July 1985. Q₁₀s (7° to 20°C) of 3.90 and 3.24 were recorded for myctophid and non-myctophid fish groups, respectively, while values of 2.22, 2.19, 2.19 and 2.54 were calculated for sergestid, penaeid, carid and euphausiid crustacean groups, respectively. Q₁₀s were consistent for species within each group. All of the species tested regulated their oxygen consumption to P_O2levels normally encountered within the eastern Gulf. Values of critical partial pressure (P_c) ranged from 20 to 40mm Hg and increased slightly with increasing temperature and respiration rate. Declining respiration with increasing minimum depth of occurrence was primarily a function of temperature alone. Changes in size, dry weight and water content contributed only a small fraction to the observed decrease. This finding contrasts with studies from the eastern Pacific Ocean, where temperature is a minor contributor to changes in respiration rate with depth.     

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.     

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.     

Carbon and nitrogen metabolism by Monochrysis lutheri: Measurement of growth-rate-dependent respiration rates

Dark respiration rates were measured and carbon-excretion rates calculated for a nitrate-limited population of the marine chrysophyte Monochrysis lutheri grown in continuous culture at 20°C on a 12 h light-12 h dark cycle of illumination and over a series of 4 growth rates. A significant (P<0.05) positive correlation was found between dark respiration rate and growth rate. From a simple linear fit to the data, the respiration rate at maximum growth rate was estimated to be roughly 10.5% of the maximum gross-carbon-production rate, and more than three times higher than the extrapolated respiration rate at zero net-growth rate. Carbon-excretion rates showed no significant correlation with growth rate, and averaged less than 5% of the maximum gross-carbon-production rate. Mean cell nitrogen to carbon ratios were correlated in a virtually linear manner (r=0.994) with growth rate, and at a given growth rate were consistently higher than nitrogen to carbon ratios for the same species grown on continuous light. A comparison of carbon and nitrogen quotas as a function of growth rate for M. lutheri and other species suggests that the increase of cellular nitrogen at high growth rates under nitrate-limited growth conditions may be associated with the storage of cellular protein or amino acids rather than the presence of an inorganic nitrogen reservoir. The maximum nitrate uptake rate per cell during the day changed very little over the range of growth rates studied, and was comparable to the maximum uptake rate found for cells grown on continuous light. However, the cell nitrogen quota increased steadily with growth rate, causing a reduction in the maximum specific-uptake rate of nitrate during the day at high growth rates. The dark nitrate-uptake capacity of the population was clearly exceeded by the supply rate at the two higher growth rates, leading to a buildup of nitrate during the night which amounted to as much as 21% of the particulate nitrogen in the growth chamber by morning.Hawaii Institute of Marine Biology Contribution No. 478.