Diurnal photosynthetic performance of seaweeds measured under natural conditions

Abstract

Statements comparing photosynthetic performance characteristics of species rely upon empirical data, usually light-saturation curves (photosynthesis, P, versus incidentlight flux-density, I o, relationships) derived from instantaneous measurements. The specific comparative parameters are initial slope and maximum photosynthesis, P max. For phytoplankton, diurnal variation in specific productivity at maximum incident light, I max is typically asymmetrical, i.e., there is a morning maximum followed by an afternoon depression. Five seaweed species, numerical dominants from the Outer Banks of North Carolina, were examined for patterns of diurnal photosynthetic performance in sunlight of habitat equivalence. These were Codium decorticatum (Woodw.) Howe and Ulva curvata (Kutz.) De Toni in the Chlorophyceae, Dictyota dichotoma (Huds.) Lamour. and Petalonia fascia (O.F. Mull.) Kuntze in the Phaeophyceae, and Gracilaria foliifera (Forssk.) Borg. in the Rhodophyceae. Diurnal patterns of oxygen exchange were varied, some symmetrical about the midday axis, others asymmetrical, and were specific for (1) species, (2) derived habitat, (3) thallus absorptance (1-I/I o, where I is the transmitted light), (4) developmental stage, and (5) diurnal photosynthetically active radiation (PAR) history. All species show a depression in oxygen exchange rates at less than 0.1 I max, and show varying degrees of recovery when I o decreases from that value. Diurnal photosynthetic performance of some species at 0.03 I o (total diurnal maximum) exceeds several times that at 0.70 I o (total diurnal maximum), an observation not predicted by instantaneous measurements. Specific day-rate integrals of I o vary, producing transient initial slope and P max values. Thus, initial slope and P max values derived from instantaneous measurements in the laboratory bear little relationship to actual diurnal production. At this time there appears to be no substitute for direct measurement of diurnal photosynthesis.