Non-Redfield carbon and nitrogen cycling in the Arctic: Effects of ecosystem structure and dynamics

The C:N ratio is a critical parameter used in both global ocean carbon models and field studies to understand carbon and nutrient cycling as well as to estimate exported carbon from the euphotic zone. The so-called Redfield ratio (C:N = 6.6 by atoms) [Redfield et al., 1963] is widely used for such calculations. Here we present data from the NE Greenland continental shelf that show that most of the C:N ratios for particulate (autotrophic and heterotrophic) and dissolved pools and rates of transformation among them exceed Redfield proportions from June to August, owing to species composition, size, and biological interactions. The ecosystem components that likely comprised sinking particles and had relatively high C:N ratios (geometric means) included (1) the particulate organic matter (C:N = 8.9) dominated by nutrient-deficient diatoms, resulting from low initial nitrate concentrations (approximately 4 μM) in Arctic surface waters; (2) the dominant zooplankton, herbivorous copepods (C:N = 9.6), having lipid storage typical of Arctic copepods; and (3) copepod fecal pellets (C:N = 33.2). Relatively high dissolved organic carbon concentrations (median 105 μM) were approximately 25 to 45 μM higher than reported for other systems and may be broadly characteristic of Arctic waters. A carbon-rich dissolved organic carbon pool also was generated during summer. Since the magnitude of carbon and nitrogen uncoupling in the surface mixed layer appeared to be greater than in other regions and occurred throughout the productive season, the C:N ratio of particulate organic matter may be a better conversion factor than the Redfield ratio to estimate carbon export for broad application in northern high-latitude systems

PURIFICATION OF ENDOLAMINARINASE AND EXOLAMINARINASE AND PARTIAL CHARACTERIZATION OF THE EXOACTING FORM FROM THE COPEPOD ACARTIA-CLAUSI (GIESBRECHT, 1889)

International audienc

MEDIUM-TERM ACCLIMATION OF FEEDING AND OF DIGESTIVE AND METABOLIC ENZYME-ACTIVITY IN THE NERITIC COPEPOD ACARTIA-CLAUSI .1. EVIDENCE FROM LABORATORY EXPERIMENTS

International audienceThe time-relationship between food supply, intermediate metabolism (ETS, GDH activities) and nutritional processes (ingestion, digestive enzyme activities) was established experimentally for the small neritic copepod Acartia clausi. In the first 3 experiments, food-limiting conditions (suboptimal growth) were imposed and the food supply simulated: (1) different concentrations of a single pulse of phytoplankton; (2) different declining phytoplankton concentrations; or (3) a double pulse of phytoplankton. In all 3 cases positive acclimation of ingestion rates and digestive enzyme activities (amylase, laminarinase and trypsin) was observed. Trypsin appeared to have a low threshold of sensitivity to food changes suggesting that the copepods may enhance their potential for protein digestion at low food concentrations. In the fourth experiment we confirmed that under saturating food concentrations, copepods showed increased food intake and intermediate metabolism but no acclimation of their digestive enzyme systems. Except during starvation, day-to-day patterns of ETS and GDH activities did not covary with food quantity, although ETS varied with food quality and food type. Contradictions between previously published and present results are discussed and confirm that the time acclimation hypothesis of Mayzaud & Poulet (1978; Limnol. Oceanogr. 23: 1144-1154) applies only for food-limiting conditions. Under the present experimental conditions, A. clausi appeared to require 48 h to significantly acclimate to changes in food ration exceeding a threshold of 1.5 to 2-mu-gC ind.-1. The time acclimation hypothesis should be revised to emphasize that nutrition is likely controlled by feedback mechanisms which operate to meet the metabolic requirements of individuals and that acclimation of the different aspects of nutrition (ingestion and digestion) take place over different time scales