Diurnal patterns in photosynthetic capacity and depth-dependent photosynthesis-irradiance relationships in Synechococcus spp. and larger phytoplankton in three water masses in the Northwest Atlantic Ocean

The photosynthetic characteristics of prokaryotic phycoerythrin-rich populations of cyanobacteriaSynechococcus spp. and larger eukaryotic algae were compared at a neritic frontal station (Pl), in a warm-core eddy (P2), and at Wilkinson's Basin (P3) during a cruise in the Northwest Atlantic Ocean in the summer of 1984.Synechococcus spp. numerically dominated the 0.6 to 1 μm fraction, and to a lesser extent the 1 to 5 μm size fractions, at most depths at all stations. At P2 and P3, all three size categories of phytoplankton (0.6 to 1 μm, 1 to 5 μm, and >5 μm) exhibited similar depth-dependent chages in both the timing and amplitude of diurnal periodicities of chlorophyllbased and cell-based photosynthetic capacity. Midday maxima in photosynthesis were observed in the upper watercolumn which damped-out in all size fractions sampled just below the thermocline. For all size fractions sampled near the bottom of the euphotic zone, the highest photosynthetic capacity was observed at dawn. At all depths, theSynechococcus spp.-dominated size fractions had lower assimilation rates than larger phytoplankton size fractions. This observation takes exception with the view that there is an inverse size-dependency in algal photosynthesis. Results also indicated that the size-specific contribution to potential primary production in surface waters did not vary appreciably over the day. However, estimates of the percent contribution ofSynechococcus spp. to total primary productivity in surface waters at the neritic front were significantly higher when derived from short-term incubator measurements of photosynthetic capacity rather than from dawn-to-duskin situ measurements of carbon fixation. The discrepancy was not due to photoinhibitory effects on photosynthesis, but appeared to reflect increased selective grazing pressure onSynechococcus spp. in dawn-to-dusk samples. Low-light photoadaptation was evident in analyses of the depth-dependency ofP-I parameters (photosynthetic capacity,P max; light-limited slope, alpha;P max alpha,I k ; light-intensity beyond which photoinhibition occurs,I b ) of the > 0.6 μm communities at all three stations and was attributable to stratification of the water column. There was a decrease in assimilation rates andI k with depth that was associated with increases in light-limited rates of photosynthesis. No midday photoinhibition ofP max orI b was observed in any surface station. Marked photoinhibition was detected only in the chlorophyll maximum at the neritic front and below the surface mixed-layer at Wilkinson's Basin, where susceptibility to photoinhibition increased with the depth of the collected sample. The 0.6 to 1 μm fraction always had lower light requirements for light-saturated photosynthesis than the > 5 μm size fraction within the same sample. Saturation intensities for the 1 to 5 μm and 0.6 to 1 μm size fractions were more similar whenSynechococcus spp. abundances were high in the 1 to 5 μm fraction. The > 5 μm fraction appeared to be the prime contributor to photoinhibitory features displayed in mixed samples (> 0.6 μm) taken from the chlorophyll maxima. InSynechococcus spp.-dominated 0.6 to 1 and 1 to 5 μm size fractions, cellular chlorophylla content increased 50- to 100-fold with depth and could be related to increases in maximum daytime rates of cellularP max at the base of the euphotic zone. Furthermore, the 0.6 to 1 μm and > 5 μm fractions sampled at the chlorophyll maximum in the warm-core eddy had lower light requirements for photosynthesis than comparable surface samples from the same station. Results suggest that photoadaptation in natural populations ofSynechococcus spp. is accomplished primarily by changing photosynthetic unit number, occuring in conjuction with other accommodations in the efficiency of photosynthetic light reactions.