Size-dependent photosynthetic variability in the North Pacific Subtropical Gyre

Abstract

Photosynthesis-irradiance (P-E) relationships and in situ measurements of primary production for 2 phytoplankton size fractions (> 2 µm and 0.2 to 2 µm) were used to evaluate variability in photophysiology over a 5 yr period (2004 to 2009) in the North Pacific Subtropical Gyre (NPSG). Picophytoplankton (0.2 to 2 µm) were dominant contributors to euphotic zone chlorophyll a (chl a) concentrations (averaging 91 ± 2% [mean ± SD] of the 0 to 125 m depth-integrated inventories) and accounted for a major fraction (averaging 74 ± 7%) of the in situ, depth-integrated, 14 Cbased primary production. Short-term in vitro P-E experiments were conducted to examine the photophysiology of both phytoplankton size classes. Results from these experiments demonstrated that in the well-lit ocean (0 to 45 m) chl a normalized maximum rates of photosynthesis, P chl max , were significantly greater among the larger phytoplankton size class than in the smaller size fraction (1-way ANOVA, p < 0.01), while in the dimly lit region (125 m) there were no significant sizedependent differences in P chl max (1-way ANOVA, p > 0.05). Neither the initial slope of the P-E relationships, α, nor the light intensities required to saturate photosynthesis, E k , varied significantly between the 2 size fractions. Although larger phytoplankton appear to constitute a relatively small fraction of phytoplankton biomass and production in this ecosystem, the photophysiological responses of plankton in this size class demonstrated considerable variability, suggesting these larger size phytoplankton experience time-variable changes in growth despite persistently oligotrophic habitat conditions. KEY WORDS: Photosynthesis · Photophysiology · Phytoplankton · Primary production· North Pacific · Oligotrophic Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 440: 27-40, 2011 28 Photosynthesis-irradiance (P-E) relationships demonstrate several well-defined characteristics, which are typically described by using hyperbolic or negative exponential models (Jassby & Platt 1976. Various photophysiological parameters can be derived from such relationships, including the maximum rate of carbon fixation (termed P max ), the initial slope (termed α) of the P-E response, which provides a measure of light harvesting efficiency by photosynthesis, the susceptibility of photosynthesis to inhibition at elevated light flux (termed β) and the irradiance required to saturate photosynthesis (termed E k ). When used in P-E relationships, rates of photosynthesis are frequently normalized to concentrations of chlorophyll a (chl a), where the resulting normalized maximum photosynthetic rate is termed P chl max . When combined with information on temperaturedependent phytoplankton growth (Eppley 1972), P-E derived parameters can be incorporated into biooptical models and used to estimate areal rates of primary production from remotely sensed determinations of phytoplankton pigments, temperature and light (Bidigare et al. 1992, Pla...

Microbial response to enhanced phosphorus cycling in the North Pacific Subtropical Gyre

“…2D & 4). Li et al (2011) showed that although large phytoplankton (> 2 µm) represent a relatively small fraction of chl a and 14 C-PP in the NPSG, their highly variable photophysiological responses suggest that they experience time-variable changes in growth despite constant oligotrophic habitat conditions, which could be one of the reasons for their summer blooms in the NPSG. While the contribution of the > 2 µm fraction to 14 C-PP did not change during our experiment, likely because of co-limitation with another element or vitamin, its contribution to Pi and ATP uptake increased.…”

Phytoplankton response to deep seawater nutrient addition in the North Pacific Subtropical Gyre

“…Finally, it is well know that SYN possess nitrate re ductase, the gene that encodes for the utilization of nitrate, and recent evidence reveals that some strains of PRO also have the gene (Casey et al 2007, Johnson & Lin, 2009, Martiny et al 2009) and therefore have the potential to up-regulate nitrate reductase in response to an injection of nutrients. Thus, with the predominance of cyanobacteria in surface waters at Station ALOHA with the ability to assimilate nitrate, as well as a background community of larger cells (> 2 µm) that are highly photosynthetically efficient (see discussion below on community structure, Li et al 2011), we would expect the nutrient-impoverished phytoplankton community to assimilate nitrate injected into surface waters rapidly due to chronic nitrate limitation (but not phosphate limitation) and thus mask the signal of the physical pulse of nutrients. The fact that nitrate pulses are observed mostly in winter and spring, with fewer occurrences in summer and fall, means that there is uncoupling between the physical supply of nutrients and its assimilation by phytoplankton at Station ALOHA (Karl et al 2001), thus providing evidence that the ecosystem is not poised to assimilate new nutrients in winter.…”

“…Small cells (< 2 µm) numerically dominate the microbial community in oligotrophic open ocean gyres due to their large surface area to volume ratio and thus their ability to outcompete larger cells for nutrients at low concentrations (Chisholm 1992, Li et al 2011). Blooms of < 2 µm phytoplankton cells, such as Synechococcus spp., do occur in the open ocean (Glover et al 1988, McGillicuddy Jr. et al 2007).…”

Distribution of ultraphytoplankton in the western part of the North Pacific subtropical gyre during a strong La Niña condition: relationship with the hydrological conditions

“…In the subtropical Pacific Ocean, phytoplankton assemblages are mainly dominated by small-sized organisms from 0.2 µm to 2 µm in diameter (Li et al, 2011). These microorganisms have been reported to significantly contribute to the biological pump and export of carbon in oligotrophic conditions (Richardson and Jackson, 2007).…”