Abstract
Light (20-450 μmol photons m-2 s-1), temperature (3-11°C) and inorganic nutrient composition (nutrient replete and N, P and Si limitation) were manipulated to study their combined influence on growth, stoichiometry (C:N:P:Chl a) and primary production of the cold water diatom Chaetoceros wighamii. During exponential growth, the maximum growth rate (~0.8 d-1) was observed at high temperture and light; at 3°C the growth rate was ~30% lower under similar light conditions. The interaction effect of light and temperature were clearly visible from growth and cellular stoichiometry. The average C:N:P molar ratio was 80:13:1 during exponential growth, but the range, due to different light acclimation, was widest at the lowest temperature, reaching very low C:P (~50) and N:P ratios (~8) at low light and temperature. The C:Chl a ratio had also a wider range at the lowest temperature during exponential growth, ranging 16-48 (weight ratio) at 3°C compared with 17-33 at 11°C. During exponential growth, there was no clear trend in the Chl a normalized, initial slope (α*) of the photosynthesis-irradiance (PE) curve, but the maximum photosynthetic production (Pm) was highest for cultures acclimated to the highest light and temperature. During the stationary growth phase, the stoichiometric relationship depended on the limiting nutrient, but with generally increasing C:N:P ratio. The average photosynthetic quotient (PQ) during exponential growth was 1.26 but decreased to <1 under nutrient and light limitation, probably due to photorespiration. The results clearly demonstrate that there are interaction effects between light, temperature and nutrient limitation, and the data suggests greater variability of key parameters at low temperature. Understanding these dynamics will be important for improving models of aquatic primary production and biogeochemical cycles in a warming climate.
Highlights
Models of phytoplankton growth are important for understanding aquatic production and ecosystem-scale biogeochemistry
The first set of measurements of particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP), chlorophyll a (Chl a) and photosynthesis-irradiance (PE) curves were obtained during exponential growth
The maximum growth rate was observed at 11°C in high light (~0.8 d-1) and at 3°C the growth rate was ~30% lower under similar light conditions
Summary
Models of phytoplankton growth are important for understanding aquatic production and ecosystem-scale biogeochemistry. Abiotic variables such as light, temperature and nutrient availability are the most important aspects regulating productivity and growth in phytoplankton. The influence of these parameters on production is most often studied independently, whereas interaction effects such as the temperature dependent nature of light utilization for photosynthesis, may be expected [1]. The Redfield C:N:P ratio of 106:16:1 is widely used as an average composition of these elements in phytoplankton [2]. Nutrient limitation will typically move the stoichiometric ratio even further away from the Redfield ratio as the limiting nutrient is at a minimum and non-limiting nutrients are taken up and stored in excess [2]
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