Light and nutrient control of photosynthesis in natural phytoplankton populations from the Chukchi and Beaufort seas, Arctic Ocean

Abstract

During the 2010-2011 'Impacts of Climate Change on the EcoSystems and Chemistry of the Arctic Pacific Environment' project, we measured photosynthetic parameters in natural Arctic phytoplankton assemblages from the Chukchi and Beaufort seas. Water-column samples were taken from the near surface (3.1 6 0.9 m) and subsurface (28 6 10.3 m) at , 85 stations each year representing a wide range of ecological conditions, including under sea ice (UI) and in open water (OW). The physiological response of phytoplankton to light was used to assess photo-acclimation, photosynthetic efficiency, and maximum chlorophyll a (Chl a) normalized rates of carbon fixation. Phytoplankton from the subsurface were acclimated to lower irradiance, as evidenced by higher photosynthetic efficiencies (a * ), reduced mean absorption spectra (ā * ) associated with heavy pigment packaging, higher maximum quantum yields of photosynthesis (W m ), increased Chl a content (Chl a : POC), and higher potential growth rates (m m ) than surface samples. In addition, phytoplankton growing in the UI subsurface had higher m m , increased W m , and higher Chl a content, as well as reduced ā * compared with those found in OW. P * m did not vary between habitats despite vastly different nutrient and light conditions (averaging , 1 mg C mg 21 Chl a h 21 ), except where nitrate exceeded . 10 mg m 23 , in which case P * m averaged 5-6 mg C mg 21 Chl a h 21 . Results from a stepwise regression analysis of photosynthetic parameters vs. environmental factors indicate that the concentration of inorganic nitrogen (significant relationships with P * m , a * , W m , m m , and Chl a : POC) and temperature at sample depth (a strong indicator of habitat type; significant relationship with b * , ā * , W m , m m , and Chl a : POC) are the best predictors of photosynthetic variables. In addition, the amount of light available at sample depth significantly predicted both E k and Chl a : POC. Our results suggest that it is the balance between light and nutrient availability in the various environments encountered in the seasonal sea ice zone that result in the pattern of photo-physiological data presented here. A significant proportion of primary production has now been observed to occur under the sea ice; therefore, our results may be a change from prior conditions in the region.

Photoacclimation of Arctic Ocean phytoplankton to shifting light and nutrient limitation

“…As the ice receded and incoming PAR increased, would be expected to increase even further as long as nutrients were available in surface waters (Nymark et al ). However, after the seasonal peak in phytoplankton biomass, NO 3 − limitation ultimately controlled photosynthetic capacity (Moore et al ; Hill et al ; Tremblay et al ; Palmer et al ). Despite higher available irradiance, severe NO 3 − depletion after the peak bloom constrained to relatively low rates during the HLLN season (Tables and ), comparable to other measurements made under nutrient limiting conditions (< 1 mg C mg −1 Chl a h −1 ) (Hill and Cota ).…”

Sea surfacepCO2and O2dynamics in the partially ice-covered Arctic Ocean

“…The physiological model [e.g., Platt et al ., ] used the irradiance (PAR) and biomass (Chl‐ a ) dependence of photosynthesis, where GPP is gross primary production in mg C m −3 h −1 , is the Chl‐a ‐specific maximum rate of photosynthesis (mg C mg Chl‐ a −1 h −1 ), α is the Chl ‐ a ‐specific rate of light‐limited photosynthesis (mg C mg Chl‐ a −1 h −1 (µmol quanta m −2 s −1 ) −1 ), I is the PAR irradiance (µmol quanta m −2 s −1 ), β (same units as α ) accounts for photoinhibition, and Chl‐ a is the concentration of chlorophyll‐ a in mg m −3 converted to carbon using 0.008 g Chl‐ a per gram C [ Palmer et al ., ]. The model used a value of 0.20 mg C mg Chl‐ a −1 h −1 , an α value of 0.017 mg C mg Chl‐ a −1 h −1 (µmol quanta m −2 s −1 ) −1 , and β value of 0.0006 mg C mg Chl‐ a −1 h −1 (µmol quanta m −2 s −1 ) −1 [ Palmer et al ., ].…”

Contrasting optical properties of surface waters across the Fram Strait and its potential biological implications

“…However, the response of phytoplankton to increased light levels is a complex interplay between light, temperature, nutrient availability, stratification and light-adaptation and acclimation of the phytoplankton themselves (e.g. Palmer et al, 2013), and thus not only dependent on changes in light availability. When sea ice is present, as currently is the case, euphotic depths will be obviously even shallower in the EGC, than presented here.…”