We describe the application of inverse methods to the estimation of food web fluxes in undersampled ocean environments. The general objective is to deduce the flow networks that conserve mass, satisfy basic biological constraints, and are compatible with the observed structure of the food web. Given inevitable gaps in the observational data, a number of different networks will fit these requirements. In this paper, we estimate the simplest possible flow structure, that is, the network that minimizes (1) the sum total of the rate constants that relate flux to stock and (2) the differences among the constants. We present a general framework for the application of the inverse algorithm: (1) a linear compartmental model of the oceanic food web that initially includes all the possible inter-compartmental fluxes, and (2) a set of constraints on the flow estimates that reflect contemporary knowledge of the limits and efficiencies of ecophysiological processes. The methodology is applied to detailed observations of food web structure and dynamics in 2 areas off the English coast. The inverse solutions are discussed in terms of current concepts of the role of the microbial loop in the pelagic marine ecosystem.
The dependence of growth and photosythesis on cell size in diatoms is evaluated in terms of an energy-balance model of microalgal physiology. Based on a review of available observations it appears that cell size can account for much of the interspecific variability in maximum growth rate ( p, , , ) and in optical absorption cross-section (ach,). Both CL, and a,,, decrease with increases in cell size. Cell size does not appear to influence the chlorophyll a:carbon ratio, the quantum efficiency of photosynthesis nor the photon flux density (PFD) at which growth rate is llght-saturated. Available observations do not allow an evaluation of the size dependence of maintenance metabolic rates or the PFD at which light compensation of growth occurs. This analysis confirms the competitive advantage of small cell size of microalgae under nutrient-sufficient conditions at both light-saturating and light-limiting growth rates to the extent that small-celled diatoms have enhanced catalytic efficiencies of growth and light absorption.
Biological dynamics in the pelagic ocean are intermittent rather than steady. In oceanic regimes, where nitrogen is limiting to phytoplankton growth, an important fraction of the annual, primary production depends on transient episodes of increased nitrate supply: at such times the role of locally-regenerated nitrogen is correspondingly less. Proper averaging of these variable rates, in time and space, is the key to reconciliation of existing data on the biogenic fluxes of oxygen and carbon in the ocean. The magnitude of oceanic production supported by nitrate (the new production) is higher than previously thought.
The temperature response of photosynthetic capacity was compared to that of 3 carboxylating enzymes in Arctic marine phytoplankton. Only the activity of ribulose-1.5-bisphosphate carboxylase (RuBPC) consistently exhibited an apparent activation energy equivalent to that of photosynthetic capacity. This is the first indication from field samples of phytoplankton that changes in the activity of RuBPC may be closely associated with changes in photosynthetic capacity. The maximum photosynthetic capacity attained by Arctic phytoplankton at a given in situ temperature appeared to increase from about 0.4 mg C mg Chl a-' h-' at -1.5 ' C to an apparently constant value of about 2.0 mg C mg Chl a-' h-' at temperatures equal to and greater than 0.0 "C.
Water-column primary production was determined by the I4C in situ method during the spring bloom in the North Atlantic Ocean. For the same samples, the parameters of the photosynthesislight (P-I) curve were determined in broad-band light, and in narrow spectral bands for construction of the action spectrum. Using these parameters, with information on the vertical distribution of chlorophyll, measurements of light absorption by particulate materials, and data on surface irradiance, watercolumn production was calculated using 4 different production models. When compared to in situ primary production measurements, the results show that the spectral model, Model 1, is the best estimator of water-column primary production. Model 2 which used broad-band crB (the initial slope of P-I curve, normalized to biomass B) with light integrated over wavelength, and Model 4 (broad-band a* and broad-band light), consistently underestimated production by about 25 % and 60 % respectively. However, Model 3 (in which light is computed using a depth-averaged attenuation coefficient, R and in which a* is assumed to be wavelength-independent) gave water-column primary production estimates not significantly different from in situ values. It is recommended that the spectral model should be applied, whenever possible, in the computations of water-column primary production. If, however, broad-band crB has to be used in the calculations, it is suggested that light at depth be computed if possible using K The use of the fully broad-band model, Model 4, is not recommended. This is because the model gave strongly biased estimates of water-column primary production relative to the observed values.
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