Light requirements for photosynthesis and growth in several macroalgae from a small soft-water stream in the Spessart Mountains, Germany

Fluence rate-response curves (P vs I curves) were measured in several species of stream macroalgae from laboratory culture and from a small soft-water stream. Photosynthesis of laboratory-grown algae was light saturated at 24 μmol photons m−2 s−1 of white light in Klebsormidium rivulare (Kützing) Morison et Sheath, at 25.51 μmol m−2 s−1 in Batrachospermum helminthosum Bory emend. Sheath et al., at 35 μmol m−2 s−1 in Balbiania investiens (Lenormand) Sirodot (cultivated at 4 μmol m−2 s−1), at 42 μmol m−2 s−1 in Microspora quadrata Hazen, at 60 μmol m−2 s −1 in Vaucheria bursata (O.F. Müller) C. Agardh (= V. sessilis De Candolle), Stigeoclonium tenue (C. Agardh) Kützing, Audouinella hermannii (Roth) Duby in De Candolle, Balbiania investiens (cultivated at 40 μmol m −2 s−1) and at 67 μmol m−2 s−1 in Cladophora glomerata (Linnaeus) Kützing. Photosynthesis of algae taken from the Bieber stream was light saturated at significantly higher photon irradiances of 115 μmol m−2 s−1 in Batrachospermum helminthosum and Vaucheria bursata and at 190 μmol m−2 s−1 in Cladophora glomerata. IK-values (photon fluence rate at which the linear branch of the P vs I curves and the saturation plateau intersect) ranged from 12.5 to 29.4 μmol m−2 s−1 in algae from laboratory cultures and from 13.3 to 32.5 μmol m−2 s−1 in algae taken from the Bieber stream. P vs I curves of Vaucheria bursata and Cladophora glomerata collected directly from the stream proved to have the lowest α-values (initial slopes). Growth in the laboratory was light saturated at 20 μmol m−2 s−1 of white fluorescent light in Batrachospermum helminthosum and at 20–50 μmol m−2 s−1 in Vaucheria bursata. In Cladophora glomerata growth was not saturated at photon f1uence rates up to 100 μmol m−2 s−1 The different light requirements for growth and photosynthesis reflect the light conditions at the locations in the Bieber stream where the species studied are growing.