Phytoplankton nutrient competition under dynamic light regimes

Many physiological processes in phytoplankton, including nutrient uptake, vary on a number of temporal scales. Experiments show that the daily cycle in irradiance affects nutrient uptake rates. We used a Droop-based model of resource competition to investigate how diel variability in nutrient uptake influences phytoplankton competition and community structure. The analytical approximation we derive shows that if nutrient uptake is light dependent, the minimum nutrient requirements and, hence, nutrient competitive abilities depend on light regime in a species-specific way. Consequently, daily variations in irradiance may slow rates of competitive exclusion or reverse the identity of the superior competitor but not allow stable coexistence. Irradiance-induced fluctuations in the maximum nutrient uptake rate of the superior competitor can lead to fluctuations in ambient nutrient concentration and an increase in the average nutrient concentration compared to constant light conditions. This can enhance nutrient use by inferior competitors. These results may be applicable to bacteria–phytoplankton nutrient competition as well. Depending on the costs and benefits of maintaining nutrient uptake in the dark, different strategies of nutrient use are optimal under different light regimes. Our results suggest that by mediating limiting nutrient use, fluctuations in irradiance may alter the structure of phytoplankton communities. In aquatic systems, physical forcing and biotic responses are tightly coupled (Steele and Henderson 1994). Major ecological interactions such as competition and predation are affected by environmental fluctuations. Light is an important environmental factor and an essential resource for phytoplankton that fluctuates on multiple temporal scales. Previously we have demonstrated that daily and seasonal light fluctuations significantly affect phytoplankton competition for light (Litchman and Klausmeier 2001). However, in many aquatic systems phytoplankton compete not for light but for nutrients. A substantial body of literature indicates that nutrient uptake rates depend on many environmental factors including light. It is, therefore, likely that light fluctuations can mediate nutrient competition among phytoplankton. Here we explore how variation in the uptake rates of the limiting nutrient modifies resource competition in phytoplankton. Numerous experimental studies have shown that uptake of many essential nutrients by phytoplankton depends on irradiance. Frequently, the uptake rates are lower in the dark. Riegman et al. (2000) showed that nitrogen (N) uptake of Emiliania huxleyi in the dark was 30% of the uptake rate in the light. Phosphorus (P) uptake in the dark was also lower than in the light, and the greatest difference between the uptake rates was for the lowest growth rate. The maximum nutrient uptake rate (Vmax) usually exhibits a much stronger light dependence than the half-saturation 1 To whom correspondence should be addressed. Present address: School of Biology, 310 Ferst Drive, Georgia Institute of Technology, Atlanta, Georgia 30332 ([email protected]). 2 Present address: School of Biology, 310 Ferst Drive, Georgia Institute of Technology, Atlanta, Georgia 30332.