Phytoplankton responses and associated carbon cycling during shipboard carbonate chemistry manipulation experiments conducted around Northwest European shelf seas

Abstract

Abstract. The ongoing oceanic uptake of anthropogenic carbon dioxide (CO 2 ) is significantly altering the carbonate chemistry of seawater, a phenomenon referred to as ocean acidification. Experimental manipulations have been increasingly used to gauge how continued ocean acidification will potentially impact marine ecosystems and their associated biogeochemical cycles in the future; however, results amongst studies, particularly when performed on natural communities, are highly variable, which may reflect community/environment-specific responses or inconsistencies in experimental approach. To investigate the potential for identification of more generic responses and greater experimentally reproducibility, we devised and implemented a series (n = 8) of short-term (2-4 days) multi-level (≥ 4 conditions) carbonate chemistry/nutrient manipulation experiments on a range of natural microbial communities sampled in Northwest European shelf seas. Carbonate chemistry manipulations and resulting biological responses were found to be highly reproducible within individual experiments and to a lesser extent between geographically separated experiments. Statistically robust reproducible physiological responses of phytoplankton to increasing pCO 2 , characterised by a suppression of net growth for small-sized cells (< 10 µm), were observed in the majority of the experiments, irrespective of natural or manipulated nutrient status. Remaining between-experiment variability was potentially linked to initial community structure and/or other site-specific environmental factors. Analysis of carbon cycling within the experiments revealed the expected increased sensitivity of carbonate chemistry to biological processes at higher pCO 2 and hence lower buffer capacity. The results thus emphasise how biogeochemical feedbacks may be altered in the future ocean.

Impact of ocean acidification on Arctic phytoplankton blooms and dimethyl sulfide concentration under simulated ice-free and under-ice conditions

“…Since the dissolution of atmospheric CO 2 increases its concentration in seawater, low pH-high pCO 2 environments could enhance phytoplankton growth rate by facilitating CO 2 uptake and reducing the energy cost of the Carbon Concentrating Mechanism (CCM, cellular processes used by algae to overcome CO 2 limitation in water) in some phytoplankton species (Gao and Campbell, 2014 and references therein). Accordingly, a decrease in pH has been reported to stimulate carbon fixation by phytoplankton Gao and Campbell, 2014;Wu et al, 2014;Mackey et al, 2015;Thoisen et al, 2015), but negative impacts of decreasing pH on phytoplankton growth have also been reported and attributed to pH-induced alterations in algal cell physiology, acid-base chemistry, trace metal availability, ion transport, protein functions, and nutrient uptake Gao and Campbell, 2014;Richier et al, 2014;Mackay et al, 2015;Thoisen et al, 2015). Due to these potential antagonist effects, it is still difficult to predict how a specific bloom in a given area will respond to the projected decrease of ocean pH.…”

Preface to special issue (Impacts of surface ocean acidification in polar seas and globally: A field-based approach)

“…As can be seen in the tables, a large number of different parameters were measured (processes examined) (Richier et al, 2014). For the majority of parameters measured, in the majority of experiments there were no significant detectable responses to deliberate short term severe manipulation of the carbonate system.…”

“…The stations encompassed a wide range of environmental conditions, including in temperature, ice-cover, carbonate chemistry, nutrients, productivity, and plankton composition. They included: (i) North Sea waters in which previous in situ observations and a carbonate chemistry bioassay experiment (Richier et al, 2014) had been conducted during the UKOA north-west European shelf seas cruise D366; (ii) North Atlantic waters south of Iceland characterised by relatively high coccolithophore abundance; (iii) north-south and east-west transects across Barents, Greenland and Norwegian Seas encompassing strong gradients of the carbonate system, nutrient concentration and ecosystem productivity; (iv) ice-edge waters of the Greenland Sea encompassing strong changes in the carbonate system; and (v) Svalbard fjord waters characterised by relatively high pteropod abundance.A total of 70 CTD casts were conducted throughout the cruise and at most stations a wide range of environmental observations were undertaken on discrete water samples collected from depth profiles. Underway measurements were also conducted throughout the cruise except in ice-covered or shallow coastal waters.…”

“…Or in other words it is of interest to have an overview of the likely impacts. Tables 2 and 3 All experiments were set up and run according to the methods previously described for the first cruise to the NW European Shelf (Richier et al 2014), with a few exceptions. In particular, extensive cleaning procedures and trace metal clean sampling techniques were applied in the iron-limited waters of the Southern Ocean.…”

“…Irradiance (100 µmol quanta. m -2 s -2 ) was provided by daylight simulation LED panels (Powerpax, UK) over a light-dark cycle approximating the ambient photoperiod: 24 h continuous light in the Arctic (except for experiment E1 with 18-6 h) and 18-6 h in the Southern Ocean.As can be seen in the tables, a large number of different parameters were measured (processes examined) (Richier et al, 2014). For the majority of parameters measured, in the majority of experiments there were no significant detectable responses to deliberate short term severe manipulation of the carbonate system.…”