Photosynthetic and respiratory characteristics of the shade-tolerant kelp Agarum clathratum

To clarify the photosynthetic and respiratory characteristics of Agarum clathratum in relation to shade tolerance, parameters of photosynthetic light-response curves of A. clathratum growing at depths of 5–16 m were compared with those of Saccharina coriacea growing at shallower depths of 1–10 m. Moreover, using the photosynthetic parameters and continuous data on underwater photosynthetically active radiation (PAR), daily photosynthesis and respiration in the underwater light environment for the two species were estimated and compared. The lower values of respiration rate and light compensation point (LCP) of A. clathratum (1.8–2.5 µL O 2 cm –2 h –1 , 2.0–2.4 µmol m –2 s –1 ), compared with those of S. coriacea (5.2–5.6 µL O 2 cm –2 h –1 , 4.9–8.3 µmol m –2 s –1 ), indicate that it reduces daily respiration and maintains a positive daily net photosynthesis at low light intensities, enabling it to inhabit deeper depths than S. coriacea . Lower ratios of daily respiration to daily gross photosynthesis (14.7–35.2%) estimated in A. clathratum , compared with those (28.1–84.0%) in S. coriacea , also suggest a shade-tolerant strategy for achieving greater net photosynthesis by reducing respiration.