Photophysiology of nitrate limited phytoplankton communities in Kongsfjorden, Spitsbergen

Gemma Kulk,Anita G J Buma,Willem H Van De Poll

doi:10.1002/lno.10963

Abstract

AbstractIn Arctic coastal regions, the phytoplankton bloom is often initiated by meltwater induced stratification in spring, while subsequent nutrient depletion is believed to drive phytoplankton succession in summer. The associated changes in photophysiology are difficult to identify, because these can be governed by acclimation to light and nutrient availability as well as variations in phytoplankton biomass and taxonomic composition. In the present study, the consequences of nutrient limitation for photophysiology and growth were assessed in natural phytoplankton communities from Kongsfjorden, Spitsbergen. A series of nutrient addition experiments demonstrated N‐limitation from mid‐June onwards and possible co‐limitation with P later in summer. The onset of N‐limitation was associated with a pronounced change in taxonomic composition from a dictyochophytes to a haptophytes dominated community. Fast Repetition Rate fluorometry measurements of photosystem II (PSII) photophysiology showed that the dictyochophytes dominated community was characterized by high PSII efficiency and electron transport rates which were efficiently used for growth. Marked changes in PSII photophysiology were observed later in summer, with decreasing efficiencies, lower connectivity between reaction centers, and slower turnover rates. Simultaneously, alternative electron requirements downstream of PSII became more important and energy was likely allocated to the uptake of nutrients rather than carbon fixation and growth. Relief of nutrient limitation during the nutrient addition experiments did not lead to pronounced changes in PSII photophysiology. It is, therefore, concluded that PSII photophysiology of the phytoplankton community in Kongsfjorden is associated with changes in species composition rather than a direct effect of nutrient availability or nutrient limitation.

Highlights

While changes in photophysiology are often difficult to observe in natural phytoplankton communities due to the combined effects of environmental conditions and phytoplankton community structure (Suggett et al 2009), the constant taxonomic composition during the short-term incubations of this study allowed for the assessment of the effects of nutrient limitation and taxonomic composition on photophysiology and electron transport rates
Changes in photophysiology are often difficult to observe in natural phytoplankton communities, with the variability in photophysiology driven by both light and nutrient availability and phytoplankton biomass and taxonomic composition
The phytoplankton community in central Kongsfjorden became nitrate limited from mid-June onwards and the onset of nitrate limitation was followed by a pronounced change in taxonomic composition with a shift in dominance from dictyochophytes to haptophytes and a decrease in phytoplankton cell size

Summary

Introduction

Nutrient starvation affects photochemical energy conversion by a decrease in photosynthetically important proteins such as D1 and Rubisco (Geider et al 1993; Steglich et al 2001), while energy derived from the light reactions of photosynthesis may be used for the rapid uptake of nutrients at the expense of carbon fixation (Beardall et al 2001) Many of these short-term stress related variations in photophysiology appear to decrease as phytoplankton acclimate to low nutrient availability over longer periods of time (Cullen et al 1992; Parkhill et al 2001; Suggett et al 2009). The results are discussed in the context of photophysiological acclimation in response to environmental conditions and the consequences for phytoplankton primary production and growth in polar regions

Methods

Results

Discussion

Conclusion