Photoacclimation of Ecklonia radiata (Laminariales, Heterokontophyta) in Doubtful Sound, Fjordland, Southern New Zealand

The kelp Ecklonia radiata is found in the sublittoral zone in the Doubtful Sound complex, and is most abundant at the coastal entrance with abundance declining toward the head of the Sound. A feature of the fjord is a persistent tannin-stained, low-salinity layer (LSL) on the surface of the underlying salt water. In this unique marine environment there is an inverse relationship between the thickness of the LSL and light levels – at the head of Doubtful Sound the LSL is thicker and light reaching E. radiata at depth can be extremely low; the inverse occurs at the coastal entrance. The mountainous terrain reduces light further, particularly on the northern aspect of the fjord. To assess whether E. radiata showed evidence of photoacclimation across this light gradient, sporophytes were collected from five sites within the complex in November 2000. Pigment contents [chlorophyll (chl) a, c and fucoxanthin], photosynthetic parameters [maximum photosynthetic rate (P_max), alpha (α), respiration rate (R_d), saturation photon-flux density (PFD) (E_k), compensation PFD (E_c) and photoinhibition (β)] and morphological parameters of E. radiata were measured. Results indicate that E. radiata acclimates to the low light at sites near the head of the fjord by increasing pigment contents, particularly chl a, as well as the surface area of the thallus. Furthermore, E. radiata increases its efficiency at capturing and transferring energy at low light by increasing α and decreasing E_k and E_c values. In doing so, E. radiata maintains similar dry weight–normalized P_max at the five sites. It is suggested that photoacclimation may therefore be a process by which E. radiata overcomes the reduced light imposed by the LSL and extends its distribution towards the inner reaches of the fjord.