Seawater carbonate chemistry and phytoplankton and eubacterial community compositions in the northwest subarctic Pacific

On-deck CO2-Fe-manipulated incubation experiments were conducted using surface seawater collected from the Western Subarctic Gyre of the NW Pacific in the summer of 2008 to elucidate the impacts of ocean acidification and Fe enrichment on the abundance and community composition of phytoplankton and eubacteria in the study area. During the incubation, excluding the initial period, the mean partial pressures of CO2 in non-Fe-added bottles were 230, 419, 843, and 1124 µatm, whereas those in Fe-added treatments were 152, 394, 791, and 1008 µatm. Changes in the abundance and community composition of phytoplankton were estimated using HPLC pigment signatures with the program CHEMTAX and flow cytometry. A DGGE fingerprint technique targeting 16S rRNA gene fragments was also used to estimate changes in eubacterial phylotypes during incubation. The Fe addition induced diatom blooms, and subsequently stimulated the growth of heterotrophic bacteria such as Roseobacter, Phaeobacter, and Alteromonas in the post-bloom phase. In both the Fe-limited and Fe-replete treatments, concentrations of 19'-hexanoyloxyfucoxanthin, a haptophyte marker, and the cell abundance of coccolithophores decreased at higher CO2 levels (750 and 1000 ppm), whereas diatoms exhibited little response to the changes in CO2 availability. The abundances of Synechococcus and small eukaryotic phytoplankton (<10 µm) increased at the higher CO2 levels. DGGE band positions revealed that Methylobacterium of Alphaproteobacteria occurred solely at lower CO2 levels (180 and 380 ppm) during the post-bloom phase. These results suggest that increases in CO2 level could affect not only the community composition of phytoplankton but also that of eubacteria. As these microorganisms play critical roles in the biological carbon pump and microbial loop, our results indicate that the progression of ocean acidification can alter the biogeochemical processes in the study area.

Ocean acidification induces multi-generational decline in copepod naupliar production with possible conflict for reproductive resource allocation

Climate change, including ocean acidification (OA), presents fundamental challenges to marine biodiversity and sustained ecosystem health. We determined reproductive response (measured as naupliar production), cuticle composition and stage specific growth of the copepod Tisbe battagliai over three generations at four pH conditions (pH 7.67, 7.82, 7.95, and 8.06). Naupliar production increased significantly at pH 7.95 compared with pH 8.06 followed by a decline at pH 7.82. Naupliar production at pH 7.67 was higher than pH 7.82. We attribute the increase at pH 7.95 to an initial stress response which was succeeded by a hormesis-like response at pH 7.67. A multi-generational modelling approach predicted a gradual decline in naupliar production over the next 100 years (equivalent to approximately 2430 generations). There was a significant growth reduction (mean length integrated across developmental stage) relative to controls. There was a significant increase in the proportion of carbon relative to oxygen within the cuticle as seawater pH decreased. Changes in growth, cuticle composition and naupliar production strongly suggest that copepods subjected to OA-induced stress preferentially reallocate resources towards maintaining reproductive output at the expense of somatic growth and cuticle composition. These responses may drive shifts in life history strategies that favour smaller brood sizes, females and perhaps later maturing females, with the potential to profoundly destabilise marine trophodynamics.

The photo-physiological response of a model cnidarian-dinoflagellate symbiosis to CO2-induced acidification at the cellular level

We measured the relationship between CO2-induced seawater acidification, photo-physiological performance and intracellular pH (pHi) in a model cnidarian-dinoflagellate symbiosis - the sea anemone Aiptasia sp. -under ambient (289.94 ± 12.54 µatm), intermediate (687.40 ± 25.10 µatm) and high (1459.92 ± 65.51 µatm) CO2 conditions. These treatments represented current CO2 levels, in addition to CO2 stabilisation scenarios IV and VI provided by the Intergovernmental Panel on Climate Change (IPCC). Anemones were exposed to each treatment for two months and sampled at regular intervals. At each time-point we measured a series of physiological responses: maximum dark-adapted fluorescent yield of PSII (Fv/Fm), gross photosynthetic rate, respiration rate, symbiont population density, and light-adapted pHi of both the dinoflagellate symbiont and isolated host anemone cell. We observed increases in all but one photo-physiological parameter (Pgross:R ratio). At the cellular level, increases in light-adapted symbiont pHi were observed under both intermediate and high CO2 treatments, relative to control conditions (pHi 7.35 and 7.46 versus pHi 7.25, respectively). The response of light-adapted host pHi was more complex, however, with no change observed under the intermediate CO2 treatment, but a 0.3 pH-unit increase under the high CO2 treatment (pHi 7.19 and 7.48, respectively). This difference is likely a result of a disproportionate increase in photosynthesis relative to respiration at the higher CO2 concentration. Our results suggest that, rather than causing cellular acidosis, the addition of CO2 will enhance photosynthetic performance, enabling both the symbiont and host cell to withstand predicted ocean acidification scenarios.

Ingestion and clearance rates of Copepods

Feeding activity, selective grazing and the potential grazing impact of two dominant grazers of the Polar Frontal Zone, Calanus simillimus and Rhincalanus gigas, and of copepods < 2 mm were investigated with incubation experiments in the course of an iron fertilized diatom bloom in November 2000. All grazers were already actively feeding in the low chlorophyll waters prior to the onset of the bloom. C. simillimus maintained constant clearance rates and fed predominantly on diatoms. R. gigas and the small copepods strongly increased clearance and ingestion of diatoms in response to their enhanced availability. All grazers preyed on microzooplankton, most steadily on ciliates, confirming the view that pure herbivory appears to be the exception rather than the rule in copepod feeding. The grazers exhibited differences in feeding behavior based on selectivity indices. C. simillimus and R. gigas showed prey switching from dinoflagellates to diatoms in response to the phytoplankton bloom. All grazers most efficiently grazed on large diatoms leading to differences in daily losses for large and small species, e.g. Corethron sp. or Thalassionema nitzschioides. Species-specific diatom mortality rates due to grazing suggest that the high feeding activity of C. simillimus prior to and during the bloom played a role in shaping diatom population dynamics