Geographical variation in morphology of the two dominant kelp species, Ecklonia maxima and Laminaria pallida (Phaeophyceae, Laminariales), on the west coast of Southern Africa

Laminaria pallida and Ecklonia maxima are large, commercially valuable kelps that co-dominate inshore waters of the west coast of Southern Africa in a geographically changing pattern. In the south, E. maxima dominates and forms a canopy in shallow waters (< about 5 m deep), with L. pallida forming a sub-canopy and extending down to 20 m or more. Northward along the Southern African coast and into Namibia, E. maxima is progressively replaced by L. pallida. Corresponding with this change in dominance, L. pallida shows certain morphological changes along the south-to-north geographical gradient. To explain these phenomena, we examined a range of morphological characters in both kelps (stipe length, stipe weight, stipe outer diameter, stipe inner diameter, length of hollow section in the stipe, and frond weight), and various environmental factors (seawater temperature, seawater turbidity, cloud/fog data, daylength, and wave and wind data). Our results, based on measurements at seven sites along 1600 km of coast between Cape Town and Swakopmund (Namibia), quantified and confirmed the change in dominance and the northward increase in stipe hollowness in L. pallida. The morphology of E. maxima did not change with latitude. Water turbidity, wind speed, and wave height differed significantly along the coast. However, only turbidity showed a steady trend, increasing northward in terms of all indicators (chlorophyll a, particulate inorganic carbon, particulate organic carbon) while wind speed and wave height showed a generally decreasing trend. Furthermore, the hollowness of L. pallida was also not related to the flexibility of the stipes. Our results suggest that L. pallida sporophytes may progressively outcompete E. maxima northward, perhaps because they are more low-light tolerant, and we suggest that by developing a hollow stipe, the sporophytes may grow faster in length, potentially increasing their competitive advantage in the shallow water where they must compete with sporophytes of E. maxima.