Photophysiology of nitrate limited phytoplankton communities in Kongsfjorden, Spitsbergen

In Arctic coastal regions, the phytoplanktonbloom is often initiated by meltwaterinduced stratification in spring, while subsequent nutrientdepletion is believed to drive phytoplanktonsuccession in summer. The associated changes in photophysiology are difficult to identify, because these can be governed by acclimation to light and nutrientavailability as well as variations in phytoplanktonbiomass and taxonomic composition. In the present study, the consequences of nutrientlimitation for photophysiology and growth were assessed in natural phytoplanktoncommunities from Kongsfjorden, Spitsbergen. A series of nutrientaddition 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 haptophytesdominated 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 nutrientsrather than carbon fixationand growth. Relief of nutrientlimitation during the nutrientaddition experiments did not lead to pronounced changes in PSII photophysiology. It is, therefore, concluded that PSII photophysiology of the phytoplanktoncommunity in Kongsfjorden is associated with changes in species composition rather than a direct effect of nutrientavailability or nutrientlimitation.