Sinking rates of fecal pellets from gelatinous zooplankton (Salps,Pteropods, Doliolids)

Sinking rates were determined for fecal pellets produced by gelatinous zooplankton (salps, Salpa fusiformis and Pegea socia; pteropods, Corolla spectabilis; and doliolids, Dolioletta gegenbaurii) feeding in surface waters of the California Current. Pellets from the salps and pteropods sank at rates up to 2 700 and 1 800 m d^-1, respectively; such speeds exceed any yet recorded for zooplankton fecal pellets. Fecal pellets of salps were rich in organic material, with C:N ratios from 5.4 to 6.2, close to values for living plankton. The relation between volume and sinking rate indicates that salp and pteropod pellets are slightly less dense than those of pelagic Crustacea; moreover, pellet density varied between different collection dates, probably because of differences in composition. In contrast, doliolid pellets sank at rates up to 208 m d^-1, a rate much lower than would be expected from pellet size. Thus, density and sinking rates of pellets are much more variable in zooplankton than would be expected from studies of crustaceans alone. Moreover, the extraordinarily high sinking rates of fecal pellets of salps indicates that these tunicates may be disproportionately important in the flux of biogenic materials during periods when they form dense population blooms.     

Effects of ambient ammonia levels on blood ammonia, ammonia excretion and heart and scaphognathite rates of Nephrops norvegicus

During commercial handling of Nephropsnorvegicus (L.) there are a number of situations when the prawns may be exposed to very high ambient ammonia levels. These experiments evaluated the effects of increased levels of ambient total ammonia (TA = NH₃ + NH₄ ⁺) on␣blood ammonia, ammonia efflux rates and on the cardio-ventilatory performance of N. norvegicus. When prawns were taken from <1 to 2000 μmol TA l⁻¹ medium, blood TA concentrations increased rapidly for the first 2 h but tended to drop thereafter. Original blood TA levels were restored 6 h after the prawns were transferred back from seawater containing 2000 to <1 μmol TA l⁻¹. Sudden exposure to 500, 1000, 2000 or 4000 μmol TA l⁻¹ medium induced blood TA concentrations to increase respectively to 50, 30, 33 and 36% of external concentrations (normally, internal TA values are much higher than external levels). Immediately after transfer back to seawater with low ammonia concentration ( <1 μmol TA l⁻¹), excretion rates were higher than those of control prawns, and the absolute amounts of TA excreted were considerably higher than those calculated to have accumulated in the haemolymph. Heart rate (HR) and scaphognathite rate (SR) were not altered when prawns were subjected to sudden alterations in ambient ammonia ( <1 to 2000 to <1 μmol TA l⁻¹). When water ammonia concentrations were altered more gradually, both rates increased, but only at 4000 μmol TA l⁻¹. These results show that N. norvegicus is able to remove ammonia from the haemolymph and/or transform ammonia into some other substance when subjected to increased levels of ambient ammonia. Possible mechanisms involved (e.g. active transport across the gills; storage in some other tissue; glutamate synthe sis) are discussed. Received: 20 May 1996 / Accepted: 1 July 1996     

Bioluminescence spectra of shallow and deep-sea gelatinous zooplankton: ctenophores, medusae and siphonophores

We have examined the variability and potential adaptive significance of the wavelengths of light produced by gelatinous zooplankton. Bioluminescence spectra were measured from 100 species of planktonic cnidarians and ctenophores collected between 1 and 3500 m depth. Species averages of maximal wavelengths for all groups ranged from 440 to 506 nm. Ctenophores (41 species) had characteristically longer wavelengths than medusae (34 species), and the wavelengths from siphonophores (25 species) had a bimodal distribution across species. Four species each produced two different wavelengths of light, and in the siphonophore Abylopsistetragona these differences were associated with specific body regions. Light from deep-dwelling species had significantly shorter wavelengths than light from shallow species in both ctenophores (p = 0.010) and medusae (p = 0.009). Although light production in these organisms was limited to the blue-green wavelengths, it appears that within this range, colors are well-adapted to the particular environment which the species inhabit. Received: 27 April 1998 / Accepted: 27 October 1998     

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.     

Distribution and trophic links of gelatinous zooplankton on Dogger Bank,North Sea

The ecology of small, gelatinous zooplankton is not integrated into management of Dogger Bank (54° 00′ N, 3° 25′ E to 55° 35′ N, 2° 20′ E). In pursuit of this goal, gelatinous zooplankton and their potential prey were sampled along a transect across the bank on June 10–16, 2007. Eleven species of small medusae and ctenophores were collected, with six abundant taxa occurring in greater numbers below the thermocline and in the shallower, southeastern portion of the bank. There were no statistically significant diel changes in distribution. In contrast, potential prey were distributed more evenly across the bank and throughout the water column. Isotopic analyses revealed that gelatinous zooplankton fed on both smaller (100–300 μm) and larger (>300 μm) mesozooplankton, but also potentially on each other. These ecological insights suggest that small medusae and ctenophores should be integrated into sustainable management of Dogger Bank.     

Differences between cell division and carbon fixation rates associated with light intensity and oxygen concentration: Implications in the cultivation of an estuarine diatom

The effect of light intensity and oxygen concentration on the growth of an estuarine diatom was investigated. Differences between rates of cell division and net carbon fixation were found to be dependent upon light intensity and oxygen concentration. Under conditions favoring large differences between cell division and net carbon fixation cultures of Thalassiosira pseudonana clone 3H depart from exponential and enter stationary phase at low cell concentrations. It is suggested that single cell algae may not be able to balance maintenance, growth, and division outside a fairly narrow range of environmental conditions.     

Digestion rates and assimilation efficiencies of siphonophores fed zooplankton prey

Digestion times and assimilation efficiencies are critical factors used in calculations of carbon and nitrogen budgets. Digestion times of natural copepod prey differed significantly among 4 genera of siphonophores (P>0.001), from a minimum of 1.6 h to a maximum of 9.6 h. Assimilation efficiencies, in contrast, were uniformly high; 87 to 94% for carbon and 90 to 96% for nitrogen. Nitrogen assimilation was consistently greater than carbon assimilation. Assimilation efficiencies calculated according to dry weight substantially underestimated assimilation of carbon and nitrogen, while calculations using ash-free dry weights and ash-free to dry weight ratios approached values for carbon assimilation. These values are appreciably higher than most of the assimilation efficiencies previously measured for a few other planktonic carnivores. These results indicate very efficient digestion of food by siphonophores in oceanic environments where prey densities are low.     

Respiration and ammonia excretion of euphausiid crustaceans: synthesis toward a global-bathymetric model

Respiration and ammonia excretion rates of 19–24 euphausiids from the epipelagic through bathypelagic zones of the world’s oceans were compiled. Body mass (expressed in terms of dry mass, carbon or nitrogen), habitat temperature and sampling depth were designated as parameters in multiple regression analysis. Results suggested that the three parameters were highly significant, contributing 71–89 % of the variance in respiration rates and 69–81 % of the variance in ammonia excretion rates. Atomic O:N ratios derived from simultaneous measurements of respiration and ammonia excretion rates ranged from 11 to 90 (median: 27), and no appreciable effects of the three parameters on O:N ratios were detected. If global-bathymetric models for the metabolism and chemical composition of copepods and chaetognaths are compared with those of euphausiids, it becomes evident that euphausiids are unique in that they maintain high metabolic rates and accumulate moderate amounts of energy reserves (lipids).     

A comparison of zooplankton patterns in the California Current and North Pacific Central Gyre

This study compares the small-scale (100's to 1000's of meters) horizontal spatial distribution of zooplankton from stations in the California Current (29°N; 118°W) and in the North Pacific Central Gyre (31°N; 155°W). Patterns were inferred from the spacing of vertical tows and the variability in abundance of 15 taxonomic categories caught in these tows. In the California Current, 4 tows were taken at random positions within 2000 m of a drogue or fixed geographic position and 4 replicate tows were taken at the drougue or the fixed point. Four series of these 8 tows were taken around noon and midnight during two days. At the Central Gyre station, two similar series (one day and one night) were taken while following a drogue. Estimates of the scale of structure based on comparisons of replicate and random tow variability suggest that aggregated patterns in the Central Gyre are larger than a few hundred meters, while the California Current structures may be less than 100 m in size. Day-night changes in patch size were not apparent in either area. Evidence for multispecies patches was found only for the Central Gyre station. Replication samples in both areas gave more consistent measures of community structure (relative proportions of species) than the random tows. Similarity of community structure decreased with increasing distance between tows being compared. The decrease was much greater for the California Current station, a further indication of smaller patch size in this area. Diurnal changes in community structure occurred in both areas.Based in part on a dissertation submitted in partial fulfillment of the requirements of the Ph.D. degree at the University of California, San Diego. Portions of this research were supported by the Marine Life Research Program of the Scripps Institution of Oceanography.     

Habitat specialization and the exploitation of allochthonous carbon by zooplankton

The significance of spatial subsidies depends on consumer resource interactions in the recipient habitat. Lakes are subsidized by terrestrial carbon sources, but the pathways of allochthonous carbon through lake food webs are complex and not well understood. Zooplankton vertically partition resources within stratified lakes in response to life history trade-offs that are governed by predators, the quantity and quality of food, and abiotic conditions (e.g., UV, temperature, and viscosity). We measured habitat specialization of zooplankton in an oligotrophic lake where allochthonous and autochthonous resources varied with depth. During stratification, the quantity and quality of zooplankton food was highest in the hypolimnion. We used a yearlong time series of the delta13C of zooplankton and particulate organic matter (POM) to determine which zooplankton species exploited hypolimnetic rather than epilimnetic resources. Because the delta13C of POM decreased with depth, we used the delta13C of zooplankton to detect inter- and intraspecific variation in habitat selection. We incubated Daphnia pulex at discrete depths in the water column to confirm that the delta13C of zooplankton can indicate habitat specialization. Zooplankton that specialized in the epilimnion relied more on allochthonous carbon sources than those that specialized in the hypolimnion. Therefore, the fate of allochthonous carbon subsidies to lakes depends on spatially explicit consumer-resource interactions.     

Numerical modelling of diel carbon production and zooplankton grazing on the Scotian shelf based on observational data

Using high resolution vertical distributions of chlorophyll and zooplankton, and field observations of photosynthetic parameters, it is shown that on the Scotian Shelf the peak in the vertical profile of primary production generally lies shallower than the chlorophyll maximum, but coincides with the peak in the vertical profile of copepods. A simple numerical model shows that the 24th carbon budget can be balanced using the best available estimates of the rate constants for phytoplankton growth, zooplankton grazing and vertical migration. This calculation is very sensitive to the size of the weight-specific ration and favors values of ～40% d^?1 for it.     

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 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.     

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.     

Some effects of hypoxia and medium ammonia enrichment on efflux rates and circulating levels of ammonia inNephrops norvegicus

Eutrophication has been reported for autumn months in regions of the Kattegat/Skagerrak, causing stress to bottom-living organisms. The present studies, undertaken in April (1989), investigated the effects of hypoxia and high ammonia levels in the burrowing decapodNephrops norvegicus (L.). The net ammonia efflux rates and circulating ammonia levels at 6 and 12°C, at normoxia [partial pressure of O₂ in the water (torr),P _wO₂ = 155 torr)] and hypoxiaP _wO₂ = 24 torr) in normal seawater and ammonia-enriched (300µmol ammonia l^–1) seawater were examined. The hourly weight-specific efflux rates were very variable and in all groups included some individuals which showed periods of no net efflux, or even a net uptake of ammonia. At each temperature, net efflux-rate differences due to treatments were not significant (P>0.05; ANOVA, in all cases) and only the differences between the net efflux rates of the normoxic groups were significantly affected by temperature (P<0.05; ANOVA). Circulating ammonia levels were also variable, and at 6°C the ammonia-enriched groups had significantly higher weight-specific blood ammonia content values than the normoxic group (P<0.05 in both cases). A net uptake of ammonia occurred in ammoniaenriched conditions — probably along a reversed NH ₄ ⁺ gradient, as downhill pNH₃ gradients were maintained in all groups — and may represent the only means by which some branchial efflux of ammonia could proceed.     

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.     

A new method for estimating phytoplankton growth rates and carbon biomass

A new method is described for the determination of phytoplankton growth rates and carbon biomass. This procedure is easy to apply and utilizes the labeling of chlorophyll a (chl a) with ¹⁴C. Pure chl a is isolated using two-way thin-layer chromatography, and the specific activity of chl a carbon is determined. Data from laboratory cultures indicate that the specific activity of chl a carbon becomes nearly equal to that of total phytoplankton carbon in incubations lasting 6 to 12 h and can be used to calculate phytoplankton growth rates and carbon biomass. Application of the method to the phytoplankton community in an eutrophic estuary in Hawaii indicates that the cells are growing with a doubling time of about 2 d and that about 85% of the particulate carbon consists of phytoplankton carbon.     

Haemolymph constituent levels and ammonia efflux rates of Nephrops norvegicus during emersion

Nephropsnorvegicus (L.) were subjected to 8 h of emersion, either between layers of seawater-soaked hessian with periodical (20 min) flushes of seawater (high humidity, HH) or to unprotected emersion (low humidity, LH). Blood ammonia levels rose during emersion in both groups but reached higher levels under LH conditions. Ammonia efflux rates after re-immersion were higher than those of control prawns, and amounts of ammonia excreted at such times were considerably higher than those calculated to have accumulated in the blood during emersion. Possible explanations for such differences are discussed. C _aO2 and C _vO2 decreased rapidly to ca. 10% normoxia values within 2 h of HH and LH emersion and remained low throughout the remaining emersion time. Emersion-induced tissue hypoxia increased blood concentrations of glucose and lactate. Lactate accumulation was higher during LH emersion, compared with HH emersion. Blood pH dropped ca. 0.40 units but increased again after 2 h of re-immersion. Acidosis was probably related more to respiratory difficulties (CO₂ accumulation) than to lactate accumulation, as blood lactate values remained high after 2 h of re-immersion. The ability of N. norvegicus to cope with emersion appears to be little influenced by high humidity conditions. Received: 26 June 1996 / Accepted: 5 August 1996     

Respiration and nitrogen excretion of zooplankton. II. Studies of the metabolic characteristics of starved animals

Respiration and nitrogen-excretion studies were carried out on several species of zooplankton (Meganyctiphanes norvegica, Phronima sedentaria, Acartia clausi and Sagitta setosa) under starvation. Although all the species were mainly ammonotelic, apparently a significant amount of organic nitrogen was excreted; the validity of the measurements and their significance are discussed. The effect of duration of starvation showed for M. norvegica and A. clausi two different patterns of behaviour, which were chiefly a function of the rate of biomass turnover of the species studied. The rates of metabolism, chemical composition, and reaction to starvation varied with season in M. norvegica. The physiological balance of the experimental animals was examined by calculating the protein carbon equivalent to respiratory and excretory catabolism, and by use of atomic O:N ratios. Starved individuals catabolized more protein carbon than can be accounted for by the amount of respiratory oxygen utilized. A hypothesis, which suggests that there are three levels of resistance to starvation, is proposed to explain this paradox, and its metabolic basis is discussed.