GROWTH RATE VARIATION IN THE N:P REQUIREMENT RATIO OF PHYTOPLANKTON¹

Abstract

Theoretical considerations predict that the cell N:P ratio at transition from nitrogen limitation to phosphorus limitation of phytoplankton growth (critical ratio, Rc) varies, as a function of population growth rate. This prediction is confirmed by experimental, data from the literature along with new experimental data for the marine, prymnesiophyte Pavlova lutheri (Droop) Green. Rc passes through a maximum at intermediate growth rates for the three phytoplankton species for which data, are available, but there is significant interspecific variability in its value. There is no theoretical or experimental evidence to support the idea that the ratio of subsistence N and P cell quotas is equal to Rc over the range of growth rates, or that the subsistence quota ratio equals the ratio of the N and P cell quotas minus a storage fraction. Examination of N:P composition ratios can be used to determine which nutrient is limiting, but cannot be used to determine relative growth rates or competitive advantage between species limited by the same nutrient. Growth rates are determined by environmental conditions and by the cell quota of the limiting nutrient, without reference to the cell quota of the non‐limiting nutrient.

Responses of elemental and biochemical composition of Chaetoceros muelleri to growth under varying light and nitrate : phosphate supply ratios and their influence on critical N: P

“…Droop growth theory, under the assumption of a threshold transition between N and P limitation, predicts that the critical N : P will vary with the growth rate. Thus, only at zero growth rate will the critical N : P, as defined by Terry et al (1985), have the same value as the optimal N : P employed by Rhee and coworkers.…”

“…Although limnologists have long recognized that there is a wide gray area in the N : P ratio between N-and P-limited conditions (Hecky and Kilham 1988), geochemists have often assumed that the Redfield ratio defines the transition between N and P limitation for the ocean in its entirety, regardless of local variations. Despite widespread use of the Redfield N : P of 16 mol N : mol P to differentiate N-limiting from P-limiting conditions, there is growing evidence that the transition point between N and P limitation occurs at a particulate N : P ratio that exceeds 16 : 1 mol N : mol P for phytoplankton cultures in balanced growth (Terry et al 1985;Wynne and Rhee 1986;Liu et al 2001). …”

“…This is a very convenient ratio to measure operationally but is of less value for assessing the switchover point between N and P limitation when cells are actively growing. Terry et al (1985) used the term ''critical N : P'' to designate the transition point between N and P limitation at any nutrient-lim-ited growth rate. Droop growth theory, under the assumption of a threshold transition between N and P limitation, predicts that the critical N : P will vary with the growth rate.…”

“…The critical N : P at which N and P are colimiting will have a value that falls between the optimal N : P ratios for N-limiting/P-replete conditions and P-limiting/N-replete conditions. As with the critical N : P (Terry et al 1985), the optimal N : P is expected to depend on the growth rate (Klausmeier et al 2004), as well as other environmental conditions, including temperature and photon flux density (PFD).…”

Accumulation of DOC in Low Phosphate Low Chlorophyll (LPLC) area: is it related to higher production under high N:P ratio?

“…Also, we assumed thatμ is the same regardless of the organisms considered. We can suspect this assumption is not always verified (Goldman and McCarthy, 1978;Terry et al, 1985), but to keep the model as simple as possible, we have used it in our simulations. Hence, for phytoplankton and bacteria, the maximum growth rateμ is set to 2.3 10 −5 s −1 (2 d −1 ), thusμ is equal to 1.39 10 −5 s −1 (1.2 d −1 ).…”