Light limitation of summer primary production in the coastal Gulf of Alaska: physiological and environmental causes

Abstract

We examined photosynthesis-irradiance (P-E) relationships for coastal Gulf of Alaska phytoplankton communities during summer 2003. Phytoplankton were adapted to high light levels, with elevated light saturation parameters (average Ek 390 µmol photons m -2 s -1 ), low photosynthetic efficiencies (α), and an absence of photoinhibition. Maximum photosynthesis rates (Pmax) ranged from 0.9 to 12.9 µg C µg chl a -1 h -1 and were consistently higher in the nearshore Alaska Coastal Current than elsewhere in the region. Differences between photosynthetic responses of phytoplankton com- munities from near surface versus the subsurface chlorophyll maximum layer were minor. Small phytoplankton ( 20 µm), a difference associated with the higher C:chlorophyll ratios of the small phytoplankton. Acclimation to high light probably arose because of the shallow pycnocline and the highly variable light environment, the latter generated by internal waves and variable cloud cover. Simulations showed that estimated water column productivity, which ranged from 359 to 909 mg C m -2 d -1 , was sensitive to modeled changes in cloud cover and pycnocline depth, through both direct effects on light availability and indirect effects on phytoplankton physiology. Climate change could therefore affect summer primary production rates in the coastal Gulf of Alaska through changes in stratifica- tion, wind mixing energy, and cloud cover. However, summer phytoplankton growth in this region is also limited by nutrient availability and microzooplankton grazing, indicating a potentially complex relationship between phytoplankton production and environmental variability.