Abstract:

Phytoplankton potential primary production was calculated using photosynthesis-irradiance (P-E) response curves. Depth-discrete samples were collected during nighttime using a CTD-rosette system aboard the R/V Point Sur and incubated (< 12 h) on deck in an incubation pool until P-E experimentation ensued the following solar noon. Sample bottles were shaded to approximate the average irradiance at sample depth. The purpose of this was to maintain similar irradiance exposure to phytoplankton cells in order to preserve their photophysiology. The methods for estimating photosynthetic rates (i.e. carbon assimilation) followed the procedure for small volume (3 mL) short term (0.5 h) incubations outlined Lewis and Smith (1983). A known volume of natural seawater was spiked with radiolabeled 14-C sodium bicarbonate (final activity = 1 µCi mL-1) and incubated in a photosynthetron under a gradient of artificial light using a combination of neutral density filters. Each photosynthetron was equipped with a clear acrylic cooling plate filled with a solution (40 g L-1) of copper sulfate (CuSO¬4) to simulate irradiance spectral quality of natural oceanic waters. Photosynthesis (P, mg C h-1 m-3) was calculated using the relationship by Parsons et al. (1984). The specific photosynthetic rates normalized to biomass (PB , gC (g Chl a)-1 h-1), saturated rate of photosynthesis (PBs , gC (g Chl a)-1 h-1), photosynthetic efficiency (αB , (gC (g Chl a)-1 h-1)(µmol photon m-2 s-1)-1), rate of photoinhibition (ßB , (gC (g Chl a)-1 h-1)(µmol photon m-2 s-1)-1), and maximum rate of photosynthesis (PBmax , gC (g Chl a)-1 h-1) were calculated from resulting scintillation counts (disintegrations per minute, dpm) using a MATLAB program and empirical relationships outlined by Platt et al. (1980). Total alkalinity (ΣCO2) was assumed to be 2.0 mM and chlorophyll a concentration was determined fluorometrically using methanol extraction (Welschmeyer 1994).