Topographic shading and wave exposure influence morphology and ecophysiology of Ecklonia radiata (C. Agardh 1817) in Fiordland, New Zealand

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Patterns in morphology, pigment concentration, and light saturation kinetics of Ecklonia radiata reveal great morphological and physiological variability among individuals from sites spanning strong gradients in topographic shading and wave exposure among the 14 fjords in southwestern New Zealand. Morphology of E. radiata varies from relatively narrow (85 ± 4.7 mm) (mean ± standard error), thick (3.2 ± 0.30 mm) blades from the well‐illuminated, wave‐exposed outer coast sites to wide, undulate (460 ± 36.8 mm,) and thin (0.46 ± 0.059 mm) blades from quiescent, topographically shaded inner fjord sites. Chlorophyll a (Chl a) concentration of blades (0.084‐1.34 µg g−1 of tissue) and the ratio of fucoxanthin to Chl a (0.33 to 0.56) also increased along this gradient, indicating photoacclimation within the inner fjord populations. In situ measurements of light saturation kinetics indicate maximum photosynthetic rates at lower irradiance (Imax = 43.7 vs. 257 µmol quanta m−2 s−1) for algae at inner fjord sites relative to well‐lit outer fjord locations. Individuals exhibiting characteristically photoacclimated relative electron transfer rate curves had more deplete δ13C (‐13.35% to ‐22.35%) than individuals with higher Imax. There was no significant association between the kelp morphology or geographic location and the observed recombinant DNA diversity of ITS sequences that would indicate the presence of two Ecklonia species in the fjords. E. radiata occupies a wide range of habitats in Fiordland and displays variability in morphology and photo‐physiological responses to low light that coincide with gradients in wave exposure and topographically shaded light conditions.