Future climate scenarios for a coastal productive planktonic food web resulting in microplankton phenology changes and decreased trophic transfer efficiency.

Albert Calbet,Stamatina Isari,Jens C Nejstgaard,Rodrigo A Martínez,Zachary S Tait,Andrey F Sazhin,Ulf Båmstedt,Despoina Sousoni,Stella A Berger,Jean-Marie Bouquet,Lorena Olmos,Hans H Jakobsen,Eric M Thompson

doi:10.1371/journal.pone.0094388

Abstract

We studied the effects of future climate change scenarios on plankton communities of a Norwegian fjord using a mesocosm approach. After the spring bloom, natural plankton were enclosed and treated in duplicates with inorganic nutrients elevated to pre-bloom conditions (N, P, Si; eutrophication), lowering of 0.4 pH units (acidification), and rising 3°C temperature (warming). All nutrient-amended treatments resulted in phytoplankton blooms dominated by chain-forming diatoms, and reached 13–16 μg chlorophyll (chl) a l−1. In the control mesocosms, chl a remained below 1 μg l−1. Acidification and warming had contrasting effects on the phenology and bloom-dynamics of autotrophic and heterotrophic microplankton. Bacillariophyceae, prymnesiophyceae, cryptophyta, and Protoperidinium spp. peaked earlier at higher temperature and lower pH. Chlorophyta showed lower peak abundances with acidification, but higher peak abundances with increased temperature. The peak magnitude of autotrophic dinophyceae and ciliates was, on the other hand, lowered with combined warming and acidification. Over time, the plankton communities shifted from autotrophic phytoplankton blooms to a more heterotrophic system in all mesocosms, especially in the control unaltered mesocosms. The development of mass balance and proportion of heterotrophic/autotrophic biomass predict a shift towards a more autotrophic community and less-efficient food web transfer when temperature, nutrients and acidification are combined in a future climate-change scenario. We suggest that this result may be related to a lower food quality for microzooplankton under acidification and warming scenarios and to an increase of catabolic processes compared to anabolic ones at higher temperatures.

Highlights

Global change may impact marine and freshwater plankton dynamics by altering many different factors and mechanisms
Mesocosm M2 was maintained at a similar pH to M1 during most of the period
Focussing on single-stressors, such as acidification or temperature, in ecological research is unrealistic; it may approximate the response of the system to the chosen factor, but does not mimic expected climate change scenarios

Summary

Introduction

Global change may impact marine and freshwater plankton dynamics by altering many different factors and mechanisms. Different future scenarios have been suggested for the distribution of nutrients in the upper mixed layer in coastal systems. It has been argued that the predicted rise in temperature will increase the stability of the upper mixed layer of the open oceans and reduce primary production by decreasing available nutrients [5,6,7]. This oligotrophication of the ocean would favour small and flagellated cells compared to diatoms [8]. Increased upwelling and human-related discharges of nutrients, should be an advantage for larger phytoplankters (e.g., diatoms), and could result in increased planktonic food web productivity in coastal zones

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Discussion

Conclusion