Accumulation and release of petroleum-derived aromatic hydrocarbons by four species of marine animals

Survival of Ctenodiscus crispatus during exposure to hypoxia (P _{O 2}<3 mm Hg) at 5°C is greater than that of any echinoderm reported in the literature, the LT₅₀ being 248 h; this is reduced to 236 h in the presence of hydrogen sulfide. Unlike Asterias vulgaris and A. forbesi, both of which lose the tube foot response to tactile stimulation long before death from hypoxia occurs, C. crispatus remains responsive until death. The extension of the highly protrusible epiproctal cone, which occurs in 75% of the mud stars simultaneously exposed to hypoxia and H₂S, serves to maintain burrow contact with the overlying water. The rate of oxygen consumption remains constant down to an ambient oxygen partial pressure of 10 to 25 mm Hg, becoming more oxygen-dependent after prior exposure of the asteroids to hypoxia. C. crispatus exhibits a clear oxygen-debt phenomenon as well as a compensatory reduction in the residual P _{O 2} (oxygen partial pressure at which oxygen consumption ceases) from 2.4 to 0.2 mm Hg after hypoxic exposure.     

The hypoxia avoidance behaviour of juvenile Atlantic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis> L.) depends on the provision and pressure level of an O<Subscript>2</Subscript> refuge

The frequency of low O₂ (hypoxia) has increased in coastal marine areas but how fish avoid deleterious water masses is not yet clear. To assess whether the presence and oxygen pressure (PO₂) level of an O₂ refuge affects the hypoxia avoidance behaviour of fish, individual Atlantic cod (Gadus morhua L.) were exposed to a range of O₂ choices in a 2-way choice chamber at 11.4°C over two different experiments. Cod in the first experiment were allowed access to a fixed O₂ refuge (fully air-saturated seawater) whilst oxygen pressure (PO₂) on the other side was reduced in steps to a critically low level, i.e. 4.3 kPa—a point where cod can no longer regulate O₂ consumption. Under these conditions, cod did not avoid any level of hypoxia and fish swimming speed also remained unchanged. In contrast, strong avoidance reactions were exhibited in a second experiment when fish were again exposed to 4.3 kPa but the safety, i.e. PO₂, of the refuge was reduced. Fish not only spent less time at 4.3 kPa as a result of fewer sampling visits but they also swam at considerably slower speeds. The presence of an avoidance response was thus strongly related to refuge PO₂ and it is unlikely that cod, and possibly other fish species, would enter low O₂ to feed in the wild if a sufficiently safe O₂ refuge was not available. It is therefore hypothesized that the feeding range of fish may be heavily compressed if hypoxia expands and intensifies in future years.     

Oxygen and acid-base status of the sea urchin Psammechinus miliaris during environmental hypoxia

The respiratory and acid-base responses to hypoxia of the sea urchin Psammechinus miliaris Gmelin have been studied both in the laboratory and in the field. Sea urchins were collected from the Clyde Sea area, Scotland, between March and July 1993. Individual urchins were unable to regulate their oxygen uptake during hypoxia. The partial pressure of oxygen (P_{O 2}) in the coelomic fluid was lower than ambient P_{O 2} during normoxia but still decreased with a decrease in environmental P_{O 2}. There was a significant increase in pH and in the concentrations of bicarbonate, magnesium and calcium in the coelomic fluid, but a significant decrease in the partial pressure of carbon dioxide (P_{CO 2}), in response to declining P_{O 2}s. The disturbance of respiratory parameters in urchins accompanying hypoxic exposure in rock pools was similar to that observed in the laboratory, although not as severe as might have been expected on the basis of the laboratory experiments.     

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.     

Respiration rate and cost of swimming for Antarctic krill,<Emphasis Type="Italic"> Euphausia superba</Emphasis>, in large groups in the laboratory

Constructing realistic energy budgets for Antarctic krill, Euphausia superba, is hampered by the lack of data on the metabolic costs associated with swimming. In this study respiration rates and pleopod beating rates were measured at six current speeds. Pleopod beating rates increased linearly with current speed, reaching a maximum of 6 beats s^–1 at 17 cm s^–1. There was a concomitant linear increase in respiration rate, from 1.8 mg O₂ g_D^–1 h^–1 at 3 cm s^–1 to 8.0 mg O₂ g_D^–1 h^–1 at 17 cm s^–1. The size of the group tested (50, 100 and 300 krill) did not have a significant effect on pleopod beating rates or oxygen consumption (ANCOVA, F=0.264; P>0.05). The cost of transport reached a maximum of 75 J g^–1 km^–1 at 5 cm s^–1, and then decreased with increasing current speed to 29 J g^–1 km^–1. When considered in light of energy budgets for E. superba, these data indicate that the cost of swimming could account for up to 73% of total daily metabolic expenditure during early summer.Communicated by G.F. Humphrey, Sydney     

Effects of hypoxia on the respiratory and circulatory regulation of Nephrops norvegicus

The burrowing decapod Nephrops norvegicus (L.) was kept under various degrees of hypoxia in order to measure respiration, heart rate, scaphognathite rate, haemolymph oxygen content and pH. An emergence reaction to hypoxia occurred only in dim light (<10^-2 m-c) or darkness, but after 10 d of moderate hypoxia the decapods showed no emergence response at all. The weight specific respiration of quiescent individuals was relatively low and increased only slightly in hypoxia (P_wO₂=40 torr). Heart rate, about 50 beats min^-1, changed little during hypoxia, down to P_wO₂=40 torr, whereas scaphognathite rates rose from about 60 beats min^-1 at normoxia to peak at 120 beats min^-1 at P_wO₂=40 torr. The oxygen extraction efficiency (E) remained at 20 to 30% during the first hour of hypoxia then rose gradually to maximum values of 30 to 40%. A small respiratory alkalosis of the blood became evident only after 4h of hypoxia (P_wO₂=50 torr). Normoxic postbranchial O₂ tensions (P_aO₂) were low (25–30 torr) and showed only a small decline during hypoxia. Over 10 to 13 d in moderate hypoxia an effective biosynthesis of 0.024 mM haemocyanin individual^-1 d^-1 occurred in fed decapods, whereas controls (normoxic) showed no significant change in pigment levels. A linear relationship between oxygen carrying capacity and haemocyanin concentration was found. It is contended that N. norvegicus is better able to cope with periodic exposure to hypoxia when food of sufficient quantity and quality is available.     

Effects of environmental parameters on the oxygen consumption of four marine invertebrates: a comparative factorial study

The oxygen consumption curves of two decapod crustaceans (Palaemon serratus, Penaeus monodon) and two prosobranch molluscs (Trunculariopsis trunculus, Nassarius mutabilis) have been detected in the entire pO₂ interval from 0 to 160 mmHg, under different conditions of temperature and salinity. From the experimental curves, physiological parameters such as the initial oxygen consumption velocity, the Q ₁₀ values and the oxygen independence indices have been measured. The latter parameters have been obtained using normalised plots which allow their better evaluation. The effects of temperature, salinity and oxygen partial pressure on the oxygen-consumption features have been studied using a factorial experimental plan which allows measurement of the effects of each experimental variable as well as the effects of synergistic interactions between different variables. Received: 27 March 2000 / Accepted: 13 November 2000     

Effects of body size and temperature on the metabolic rate of Penaeus monodon

Laboratory measurements of oxygen consumption were made on Penaeus monodon (Fabricius) from protozoea to adult stage at temperatures between 15° and 35°C. The logarithmic relationship between weight-specific respiration rate (WRt) and temperature (T) for two size groups, Protozoea 1 (PZ1) to Postlarva 1 (PL1) and PL to adult, are given as; WRt=10^{0.431+0.0146 (T)} (ml O₂ g^-1 h^-1) and WRt=10^{-0.948+0.0338 (T)} (ml O₂ g^-1 h^-1), respectively. Additionally, equations relating metabolic rate, temperature and size for the two size groups are; PZ1-PL1: log M=0.431+0.0146T+(1.25 (log TL)+0.579), and PL1-adult: log M=-0.948+0.0338T+(2.60(log CL)-0.683), where M=oxygen consumption in ml O₂ individual ^-1h^-1, T=temperature in °C, TL=total length in cm, and CL=carapace length in cm. Activation energies of 6 186.75 J for PZ1-PL1 and 14 066.62 J for PL-adults point to different metabolic pathways or to differences in the ratio between the metabolic pathways used.     

Effects of O2, pH and DIC on photosynthetic net-O2 evolution by marine macroalgae

Net photosynthetic O₂ evolution by five marine macroalgae:Ulva lactuca L.,Enteromorpha sp.,Ceramium strictum Harvey,Fucus serratus L., andF. vesiculosus L., collected from Danish waters in the summer of 1983 was followed at increasing O₂ and with pH either fixed close to pH 7, 8 or 9, or drifting upwards during photosynthesis in a closed chamber to determine the effects of changing O₂, pH and DIC (dissolved inorganic carbon) on photosynthesis. Increasing O₂, increasing pH and decreasing DIC together limited O₂ evolution. Raising the O₂ concentration with pH and DIC held constant resulted in less inhibition of net-O₂ evolution than when all three factors acted together. The O₂ inhibition of photosynthesis was similar to the reported O₂ inhibition of ribulose 1,5-bisphosphate carboxylase isolated from lower and higher plants. Net-O₂ evolution as a function of the molar ratio of O₂ to HCO^– ₃ + CO₂ in solution provided a general, linear relationship (r ² = 0.72 to 0.84), predicting inhibition of photosynthesis based on O₂ pH and DIC changing together. Slopes of this relationship, representing competition between O₂ and carbon based on external concentrations, were similar for the five taxonomically different algae, suggesting that similar processes act to reduce net-O₂ evolution.     

Respiratory quotient and ammonia excretion in Tilapia mossambica

Tilapia mossambica (Peters), acclimated to and tested in fresh water at 30°C, maintained a routine respiratory quotient (R Q) of about unity and an ammonia quotient (A Q) (Vol. NH₂/Vol. O₂) of about 0.2 at high ambient oxygen concentrations. At low oxygen concentrations (below 2 ppm) R Q and A Q increased sharply to values of 8 and 1, respectively (at 0.6 ppm), indicating a close relationship of increase in anaerobic energy utilization and increase in protein metabolism at inadequate oxygen concentrations. T. mossambica (8 cm), exercised continuously with intervening sampling and flushing stops for 6 h, at a swimming speed of about 2 body lengths/sec, derived some anaerobic energy throughout the exercise (R Q: 1.2), utilizing more protein the longer the exercise. The coupling of the increased protein metabolism and anaerobic energy utilization may be of advantage in preventing acidosis and also in conserving sodium (Na⁺) in fish.Part of this work was included in a paper presented at the Centennial Meeting of The American Fisheries Society, September 13–16, 1970, New York.     

Blood oxygen-binding characteristics of bigeye tuna (Thunnus obesus), a high-energy-demand teleost that is tolerant of low ambient oxygen

 We found blood from bigeye tuna (Thunnus obesus) to have a significantly higher O₂ affinity than blood from other tunas. Its P₅₀ (partial pressure of oxygen, P_O2 required to reach 50% saturation) was 1.6 to 2.0 kPa (12 to 15 mmHg) when equilibrated with 0.5% CO₂. Previous studies employing similar methodologies found blood from yellowfin tuna (T. albacares), skipjack tuna (Katsuwonus pelamis), and kawakawa (Euthynnus affinis) to have a P₅₀ of 2.8 to 3.1 kPa (21 to 23 mmHg). These observations suggest that bigeye tuna are more tolerant of low ambient oxygen than other tuna species, and support similar conclusions derived from laboratory whole-animal studies, depth-of-capture data, and directly-recorded vertical movements of fish in the open ocean. We also found the O₂ affinity of bigeye tuna blood to be essentially unaffected by a 10 C° open-system temperature change (as is the blood of all tuna species studied to date). The O₂ affinity of bigeye tuna blood was, however, more affected by a 10 C° closed-system temperature change than the blood of any tuna species yet examined. In other words, bigeye tuna blood displayed a significantly enhanced Bohr effect (change in log P₅₀ per unit change in plasma pH at P₅₀) when subjected to the inevitable changes in partial pressure of carbon dioxide (P_CO2) and plasma pH that accompany closed-system temperature shifts, than when subjected to changes in plasma pH accomplished by changing P_CO2 alone. In vivo, the resultant large decrease in O₂ affinity (i.e. the increase in P₅₀) that occurs as the blood of bigeye tuna is warmed during its passage through the vascular counter-current heat exchangers ensures adequate rates of O₂ off-loading in the swimming muscles of this high-energy-demand teleost. Received: 12 March 1999 / Accepted: 18 December 1999     

The effect of ration size, temperature and body weight on specific dynamic action of the common cuttlefish Sepia officinalis

The effect of meal size (shrimp Crangon crangon) [0.83–18.82% dry body weight (Dw)] on specific dynamic action (SDA) was assessed in cuttlefish Sepia officinalis (1.03–6.25 g Dw) held at 15 and 20°C. Cuttlefish <2 g significantly expended less energy in feeding and digesting their meal than cuttlefish >2 g when given the same quantity of food. Handling, eating and digesting a shrimp meal was temperature dependent with cuttlefish processing and digesting a similar sized shrimp meal faster at 20°C than at 15°C. The proportional increase in oxygen consumption (2.07 ± 0.02) was not correlated with feeding rate (FR) and was independent of temperature and cuttlefish size. The SDA peak was not correlated with FRs, and increased as cuttlefish size and temperature increased. The mean SDA coefficient was 0.87 ± 0.07% of the ingested energy; one of the lowest SDA values recorded amongst vertebrates and invertebrates. Daily energy requirements (KJ day⁻¹) for S. officinalis were calculated from laboratory estimates of energy losses due to standard (MO_{2 Standard}), routine (MO_{2 Routine}) and feeding (MO_{2 SDA}) oxygen consumption. Laboratory estimates of daily metabolic expenditures were combined with results from previous investigations to construct an energy budget for 1 and 5 g cuttlefish consuming a meal of 5 and 15% Dw at 20°C and the amount of energy available for growth was estimated to be between 35 and 80.3% of the ingested energy.     

Does oxygen deficiency modify the functional response of Saduria entomon (Isopoda) to Bathyporeia pilosa (Amphipoda)?

The functional response of the predatory isopod Saduria entomon to the prey amphipod Bathyporeia pilosa was measured in normoxia (95% O₂ saturation), moderate hypoxia (45% O₂ saturation) and hypoxia (35% O₂ saturation) in aquarium experiments. The prey densities tested ranged from 400 to 8000 ind m⁻². Prey density influenced consumption rates of S. entomon in normoxia and 45% O₂ saturation, but there was no difference between consumption rates at these two oxygen levels. Nevertheless the form of functional response differed. In normoxia S. entomon showed a positively density-dependent functional response to B. pilosa, indicating a potentially stabilizing effect on the prey population. In moderate hypoxia the variance in consumption increased, decreasing the statistical power to distinguish between response models. The functional response of S. entomon in moderate hypoxia was best described with a density-independent response, characterized as destabilizing for the prey population. In hypoxia (35% O₂) predation by S. entomon did not respond to increasing prey density, as almost no amphipods were eaten at this oxygen level. The results are discussed in terms of the usability of theoretical models to examine predator–prey relationships in stressful environments. Received: 26 April 1997 / Accepted: 20 May 1997     

Automatic potentiometric determination of dissolved oxygen

Commercially available automatic titration systems were tested in 1988 for potentiometric titration of liberated iodine by the Winkler method of oxygen determination. The potentiometric equivalence point was also compared to the manual starch end point. Finally the automatic method was used in order to estimate belowhalocline respiration in the Kattegat, Sweden. Standard deviations of 0.007 ml O₂ l^–1 or 0.1 to 0.3% coefficients of variation (% standard deviation of the mean) were achieved when titrating 25 ml from replicate 60-ml oxygen bottles using the automatic method, or 50 ml manually. The precision for replicate titrations of 50-ml aliquots of 0.001N KIO₃ was <0.05% (0.002 ml 0.01N Na₂S₂O₃) for the automatic method. Titration time for 25-ml aliquots was 2 to 4 min, somewhat longer than for manual titrations (1 to 1.5 min). However, during titration the operator is free to perform other tasks. It is not possible to use automatic sample changers, due to rapid iodine volatilization. The equipment can be handled by relatively unskilled analysts and is suitable for use on board research vessels or in field stations [weight for a Mettler^TM titrator (Mettler Instrumente AG, Greifensee, Switzerland) <10 kg, volume <0.1 m³]. Below-halocline oxygen consumption in the SE Kattegat ranged from 0 to 6 ml O₂ m^–3 h^–1 (mean values for September and October 1988=1.69 and 0.66 ml O₂ m^–3 h^–1, respectively, with 95% confidence limits of ca. ±0.6 ml O₂ m^–3 h^–1).     

Abiotic conditions in cephalopod (Sepia officinalis) eggs: embryonic development at low pH and high pCO2

Low pO₂ values have been measured in the perivitelline fluids (PVF) of marine animal eggs on several occasions, especially towards the end of development, when embryonic oxygen consumption is at its peak and the egg case acts as a massive barrier to diffusion. Several authors have therefore suggested that oxygen availability is the key factor leading to hatching. However, there have been no measurements of PVF pCO₂ so far. This is surprising, as elevated pCO₂ could also constitute a major abiotic stressor for the developing embryo. As a first attempt to fill this gap in knowledge, we measured pO₂, pCO₂ and pH in the PVF of late cephalopod (Sepia officinalis) eggs. We found linear relationships between embryo wet mass and pO₂, pCO₂ and pH. pO₂ declined from >12 kPa to less than 5 kPa, while pCO₂ increased from 0.13 to 0.41 kPa. In the absence of active accumulation of bicarbonate in the PVF, pH decreased from 7.7 to 7.2. Our study supports the idea that oxygen becomes limiting in cephalopod eggs towards the end of development; however, pCO₂ and pH shift to levels that have caused significant physiological disturbances in other marine ectothermic animals. Future research needs to address the physiological adaptations that enable the embryo to cope with the adverse abiotic conditions in their egg environment. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Life on the edge: physiological problems in penaeid prawns <Emphasis Type="Italic">Litopenaeus stylirostris</Emphasis>, living on the low side of their thermopreferendum

The underlying physiological mechanisms explaining why the adult penaeid prawn Litopenaeus stylirostris cannot successfully face heavy stressful events on the low edge of its thermopreferendum (20–22°C) were studied during the austral winter. Prawns were studied during recovery from net fishing and rapid transfer from outdoor earthen ponds into indoor facilities. This was assimilated to a predator–prey interaction. O₂-consumption, hemolymph osmotic pressure (OP_h), arterial O₂ partial pressure (PO₂), a–v O₂-capacitance and mortality rates were analysed. Data were compared to similar challenges performed at 28°C during the austral summer. At 20–22°C, mortality of up to 70% was observed after 2 days whereas at 28°C, maximum mortality was 3–5%. Mortality occurred when OP_h shifted towards equilibrium with seawater, the resting O₂-consumption, the a–v O₂-capacitance and the arterial PO₂ went down to minimal values. These events can be counterbalanced by transiently hyper-oxygenating the hemolymph or by blocking the OP_h shift in isosmotic water (Wabete et al. in Aquaculture 260:181–193, 2006): both led to a dramatic decrease in mortality. It is concluded that in penaeid prawns L. stylirostris, a mismatch between O₂-demand and O₂-supply contributes to setting the geographical limits for this animal species through an impairment of their hemolymph O₂-carrying capacity during heavy stressful events like chasing by predators.     

Determination of photosynthetic rates for the marine algae Fucus vesiculosus and Laminaria digitata

A continuously recording, flow-through oxygen electrode system for the measurement of oxygen exchange is described and applied to an investigation of photosynthetic rates in the marine algae Fucus vesiculosus L. and Laminaria digitata (Huds.) Lam. The photosynthetic rate (mg O₂.g dry weight^-1.h^-1) at 15°C and 21.5 mW.cm^-2 (usually just saturating) ranges in F. vesiculosus from 1.20 in basal portions of the thallus to 9.27 at the apices and in L. digitata from 1.19 mg O₂ at the thallus base to 3.97 mg O₂ at distances of several centimetres behind the upper thallus margin. This variation is reduced when the photosynthetic rate is expressed in terms of fresh weight or surface area.This research was carried out while one of us (R.J.K.) was an Alexander von Humboldt fellow at the University of Kiel, and is part of the programme Sonderforschungsbereich 95, Wechselwirkung Meer-Meeresboden, Universität Kiel.     

Chemical composition and physiological properties of fucoids under conditions of reduced salinity

Thallus segments of Fucus serratus L. and F. vesiculosus L. (Phaeophyceae, Fucales) were transferred into seawater media with a salinity range from 32.65 to 2.25 and maintained for at least 2 weeks. Several parameters of chemical composition as well as rates of photosynthetic and respiratory oxygen exchange, ¹⁴C-assimilate patterns, and release of ¹⁴C-assimilates into the culture medium have been investigated. Compared to controls, in both species dry weight, ash, chloride, and mannitol contents distinctly decline proportionally to reduction of salinity in the incubation media, whereas content of total N (in terms of protein content) remarkably increase. Respiratory O₂-consumption is markedly increased at lower salinities, whereas rate of photosynthetic O₂-evolution shows some depression. Relatively little effects of salinity changes are observed in distribution of photosynthetically assimilated ¹⁴C among the major groups of photosynthates. Release of ¹⁴C-assimilates into the incubation medium never exceeds 2% of total ¹⁴C-uptake, but is stimulated in media of reduced salt content. The results are discussed with emphasis on phenomena of long-term adaptation and osmoregulation in the marine fucoid species.     

Primary production of sands in the lagoon of an atoll and the role of foraminiferan symbionts

Chlorophyll content and in situ oxygen production of sands were measured in the lagoon of Takapoto Atoll, Tuamotu Islands (French Polynesia). Stations were spaced from a depth of 17 m to the water limit on the beach. Photosynthetic pigments and net primary production for the top 3 cm of sediment ranged from 56 to 907 mg chlorophyll a m^-2 and from 115 to 354 mg O₂ m^-2 h^-1, respectively. Responsible organisms were mostly Foraminifera, among which Amphistegina lessoni, containing unicellular symbionts, dominated. The sands are thus significant contributors to the lagoon's primary production, while the role of phytoplankton is comparatively negligible.     

Carbon-dioxide production and metabolic parameters in the ophiurid Ophiothrix fragilis

As part of the evaluation of fluxes between the water column and a rich benthic community of the Dover Strait (Eastern English Channel), laboratory measurements of oxygen consumption were carried out on a common ophiurid, Ophiothrix fragilis (Abildgaard), from February 1993 to February 1995. The mean O₂-consumption rate was evaluated at 0.31 mg O₂ g⁻¹ h⁻¹ (ash-free dry weight). Simultaneous measurements of O₂ consumption and CO₂ production using the pH-alkalinity method revealed an average respiratory quotient of 0.69 proved suitable for converting oxygen demand to carbon flux. A seasonal trend in respiration data was demonstrated by sinusoidal curves fitted to O₂-uptake and CO₂-release data as a function of time. The influence on respiration rate of two seasonal parameters (temperature and food availability) is discussed; linear regression indicated a highly significant relationship between O₂ consumption (or CO₂ production) and temperature; both O₂-consumption and CO₂-production rates decreased with starvation. The average O:N ratio was estimated at 8.46, close to the theoretical value when proteins constitute the catabolic substrate. The annual carbon respired by the O. fragilis community examined and the estimated annual primary production by phytoplankton indicate that the respiration of the O. fragilis community could supply 35% of phytoplankton carbon requirements. Received: 1 August 1996 / Accepted: 4 September 1996