Rates of clearance, ingestion, ammonia excretion, respiration and egg production were measured in food-acclimated (0 to 1700 W C 1-l) planktonic copepods Acartia tonsa in relation to food concentration. Carbon and nitrogen budgets were constructed. Clearance peaked at a food concentration of 150 W C 1-l, and decreased at both higher and lower concentrations. Ingestion and egg production rates increased sigmoidally with food concentration approaching plateaus equivalent to 180 and 64 % body C d-l, respectively. Rates of ammonia excretion and respiration increased with algal concentration in a decelerating manner. Respiration and excretion rates of copepods fed at saturation food concentration were more than 4 times higher than those for starved individuals. The causality of the increased respiration rate in association with feeding (specific dynamic action, SDA) is discussed by considering the physiology and biochemistry of the processes that potentially contribute to SDA. The theoretical biochemical minimum costs of biosynthesis accounted for between 50 and 116 % of observed SDA, while assimilation costs equalled 18 to 28 %. Costs of feeding, digestion and excretion (-1 % of SDA), and the mechanical work required to transport food down the gut, contributed insignificantly to SDA. It is concluded that the increment in metabolic rate of feeding A. tonsa largely relates to biosynthesis ('growth') and transport, and that the efficiency of egg production in this species is near its theoretical maximum.

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Growth and glycogen content of Chironomus anthracinus in Lake Esrom, Denmark was examined during summer stratification in 1992 and 1993 . Simultaneously, effects of oxygen deficiency on glycogen utilization and survival were experimentally studied . The population consisted of almost fullgrown 4th instar larvae in 1992 and 2nd and 3rd instar larvae in 1993 . Growth rate and glycogen content changed as hypolimnetic oxygen deficiency increased . During a 1st phase of stratification dry weight and glycogen content increased (2nd and 3rd instars) or was almost constant (4th instar) but decreased significantly during the following 2nd phase . This change from growth to degrowth and utilization of endogenous glycogen reserves correlated with a change in the thickness of the microxic layer (<0 .2 mg 021 -') above the sediment surface . The layer increased from 2-3 m in phase 1 to 4-5 m in phase 2, and we suggest that this deteriorated the oxygen conditions and resulted in a change in larval energy metabolism from fully aerobic during the 1st phase to partly anaerobic in the 2nd phase . During the 2nd phase larval metabolism was estimated at less than 20% of normoxic rate . Experimental exposure of the larvae to anoxia indicated highly different survival of young larvae (2nd and 3rd instars) and older larvae (large 4th instars) . The mortality of young larvae was 50% after three days in anoxia at 10 °C, whereas only 25% of the older larvae had died after 3-4 weeks under similar conditions . Extending the treatment, however, resulted in increased death rate of the 4th instar larvae with only 10% surviving after seven weeks . The anaerobic metabolism of 4th instar larvae as estimated from glycogen degradation at 10°C was 5% of normoxia in the interval from 0-5 days but 1 .5% in the interval from 20-25 days . It is concluded that survival of C. anthracinus in anoxia is very limited, but traces of oxygen in the environment allowing for faint aerobic metabolism prolong the survival time of the larvae from a few days (2nd and 3rd instars) or a few weeks (4th instar) to probably 3-4 months .

The profundal zone of Lake Esrom, Denmark has a dense population of Chironomus anthracinus, which survives 2-4 months of oxygen depletion each summer during stratification . The metabolism of 3rd and 4th instar larvae was examined in regard to variation in biomass and temperature . Respiration at air saturation was described by a curvilinear multiple regression relating oxygen consumption to individual AFDW and temperature . At 10 °C and varying oxygen regimes the 02 consumption and CO2 production of 4th instar larvae were almost unaltered from saturation to about 3 mg 02 1 -l , but decreased steeply below this level . The respiratory quotient increased from 0 .82 at saturation to about 3 .4 at oxygen concentrations near 0 .5 mg 02 1 -1 . This implied a shift from aerobic to partially anaerobic metabolism . At 0 .5 mg 021 -1 the total energy production equalled 20% of the rate at saturation of which more than one third was accounted for by anaerobic degradation of glycogen . This corresponded to a daily loss of 12 pg mg AFDW -1 or approximately 5% of the body reserves . At unchanged metabolic rate the glycogen store would last three weeks, but long term oxygen deficiency causes a further suppression of the energy metabolism in C. anthracinus . 43