Gross Excretion Rates of Natural Marine Phytoplankton and Heterotrophic Uptake of Excreted Products in the Southern North Sea, as Determined by Short-Term Kinetics

Abstract

Phytoplankton extracellular production has been determined kinetically during the spring phytoplankton bloom in three marine biotopes representative of the Southern North Sea (Scheldt estuary, Belgian coast and English Channel), differing greatly in their biological activities. Short-term kinetics of phytoplankton extracellular compounds produced during 14C incubations have been shown to increase our present capability for estimating gross phytoplankton extracellular production, if compared with a single measure after 4-h incubation. Underestimates of up to 60 O/O were found when a fixed incubation of 4 h was used. This underestimate can be attributed either to heterotrophic uptake of excreted products or to the presence of a lagphase in 14C extracellular compounds prod.uction, related to physiological properties of phytoplankton. The data indicate that the phytoplankton extracellular release percentage, expressed with respect to the total photo-assimilated carbon, ranges from 0 to 62 O/O with lower values in the estuarine area. Seasonal variations show, in Belgian waters and in the English Channel, an increase in phytoplankton excretion percentage during the bloom from about 20 to 60 "/o. Heterotrophic uptake of phytoplankton extracellular products has been calculated from the evolution curve of I4C phytoplankton excretion products by means of a simple mathematical model. Results indicate that heterotrophs can occasionally remove up to 39 V" of the total amount of excreted matter per hour indicating that phytoplankton excretion can sometimes produce substances wlth high turnover

Carbon exchange and 14C tracer methods in a nitrogen-limited diatom, Thalassiosira pseudonana

“…Using Equation 10, excretion estimated from a 6 h incubation would range from 69 to 100 % of true among steady-state cultures (with error largest at low growth rates), and 59 to 54 % for the perturbed cultures. For a 2 h incubation, as sometimes suggested (Nalewajko et al, 1976;Lancelot, 1979), the corresponding estimates would be 32 to 100 % for steady-state, and 23 to 26 % for perturbed, cultures. These errors are much more substantial than those in production estimates, and pose a difficult problem for field measurements.…”

“…Such utilization can be rapid enough to influence apparent excretion rates after several hours of incubation (Nalewajko et al, 1976;Lancelot, 1979;Billen et al, 1980). It has generally been assumed that excreted carbon equilibrates so quickly with added tracer that underestimates due to tracer disequilibrium would not be an important problem in short incubat i o n~, and there is experimental support for this view (Wiebe and Smith, 1977;Wolter, 1982).…”

Factors affecting phytoplankton extracellular release in the Southern Bight of the North Sea

“…light intensities as described before. Phytoplankton exudation rates were given by the initial slope of the kinetic curves (Lancelot, 1979). Fig.…”

“…In addition, samples were collected along a transect from eutrophic Belgian coastal waters to the more oligotrophic English Channel during a whole annual cycle The classical radiocarbon method for measuring primary production (Steeman-Nielsen, 1952) was adapted to measure gross production rate of extracellular organic compounds, as a function of incubation time (Lancelot, 1979). Different sets of short-term kinetics -in vitro, in sib, in semi-situ light intensitieswere performed in order to examine possible effects of light intensity.…”

“…Dissolved organic material released by phytoplankton may constitute an important direct source of food for microheterotrophs (Itumaga and Hoppe, 1977;Wiebe and Smith 1977;Chrost, 1978;Lancelot, 1979;Hagstrom, 1979, 1982;Wolter, 1982). It is therefore important, not only to determine accurately the rate of the excretion process, but also to understand the main environmental factors controlling it.…”