Shift towards larger diatoms in a natural phytoplankton assemblage under combined high-CO2 and warming conditions

Abstract

An indoor mesocosm experiment was carried out to investigate the combined effects of ocean acidification and warming on the species composition and biogeochemical element cycling during a winter/spring bloom with a natural phytoplankton assemblage from the Kiel fjord, Germany. The experimental setup consisted of a "Control" (ambient temperature of ~4.8 °C and ~535 ± 25 μatm pCO₂), a "High-CO₂" treatment (ambient temperature and initially 1020 ± 45 μatm pCO₂) and a "Greenhouse" treatment (~8.5 °C and initially 990 ± 60 μatm pCO₂). Nutrient replete conditions prevailed at the beginning of the experiment and light was provided at in situ levels upon reaching pCO₂ target levels. A diatom-dominated bloom developed in all treatments with Skeletonema costatum as the dominant species but with an increased abundance and biomass contribution of larger diatom species in the Greenhouse treatment. Conditions in the Greenhouse treatment accelerated bloom development with faster utilization of inorganic nutrients and an earlier peak in phytoplankton biomass compared to the Control and High CO₂ but no difference in maximum concentration of particulate organic matter (POM) between treatments. Loss of POM in the Greenhouse treatment, however, was twice as high as in the Control and High CO₂ treatment at the end of the experiment, most likely due to an increased proportion of larger diatom species in that treatment. We hypothesize that the combination of warming and acidification can induce shifts in diatom species composition with potential feedbacks on biogeochemical element cycling.

Keywords: diatoms; mesocosms; ocean acidification; spring bloom; warming.

Fig. 1.

Development of (A) CO₂ partial pressure (pCO₂), (B) pH_totalscale and (C) TA during the experiment. pCO₂ and pH were calculated from measured TA and DIC (see Methods for details). Vertical lines indicate time of nitrate depletion; red line corresponds to Greenhouse treatment, green line to High CO₂ treatment and the black line to the Control.

Fig. 2.

Development of major dissolved inorganic nutrients and POM concentrations during the experiment. (A) nitrate, (B) phosphate, (C) silicate, (D) particulate organic nitrogen, (E) particulate organic phosphorus and (F) biogenic silica. Vertical lines indicate time of nitrate depletion; red line corresponds to Greenhouse treatment, green line to High CO₂ treatment and black to Control.

Fig. 3.

Temporal development of total Chl a during the experiment. Vertical lines indicate time of nitrate depletion; red line corresponds to Greenhouse treatment, green line to High CO₂ treatment and black to Control.

Fig. 4.

Carbon dynamics during the experiment for (A) DIC (corrected for gas exchange), (B) particulate organic carbon and (C) DOC. Colour coding according to Fig. 1. Vertical lines indicate time of nitrate depletion; red line corresponds to Greenhouse treatment, green line to High CO₂ treatment and black to Control. Please note that concentrations of one of the Controls deviate from its duplicate after the bloom peak due to a defective lid (see section Carbon cycling in the Results section for details).

Fig. 5.

Elemental stoichiometry of POM during the experiment. Colour coding according to Fig. 1. Vertical lines indicate time of nitrate depletion; red line corresponds to Greenhouse treatment, green line to High CO₂ treatment and black to Control. Grey horizontal lines represent Redfield proportions of (A) 6.6, (B) 106:1 and (C) 16:1.

Fig. 6.

Chl a equivalent concentrations of: (A) Diatoms, (B) Chlorophytes, (C) Dinophytes, (D) Cryptophytes and (E) Prasinophytes analysed by CHEMTAX. Vertical lines indicate time of nitrate depletion; red line corresponds to Greenhouse treatment, green line to High CO₂ treatment and black to Control.

Fig. 7.

Cell counts according to group sizes from flow cytometry (left panel): (A) Group I (<2 μm), (B) Group II (2–20 μm) and (C) Group III (>20 μm) and percent contribution to total Chlorophyll fluorescence (right panel): (D) Group I, (E) Group II and (F) Group III. Colour coding according to Fig. 1. Vertical lines indicate time of nitrate depletion; red line corresponds to Greenhouse treatment, green line to High CO₂ treatment and black to Control.