Response of Ecklonia radiata (Laminariales) to light at 15°C with reference to the field light budget at Goat Island Bay,New Zealand

Gametophytes of Ecklonia radiata (C.Ag.)J.Ag. grew in culture at 15°C under daily quantum doses ranging from 0.86 to 360 cE m^-2. Growth rates increased with quantum doses up to 40 cE m^-2 d^-1, then reproduction began and the relative growth constant declined while ovum release came earlier with increasing light up to about 100 cE m^-2 d^-1. Above 100 cE m^-2 d^-1 there were no consistent trends with increasing light, except that at the higher quantum doses, fertile female plants had fewer and larger cells and therefore fewer potential ova. Reproduction varied with daily quantum dose rather than with daylength. Given the same daily dose, plants grew fastest in low photon flux density, long daylength conditions. Gametophytes grown in the field developed at similar rates to those in culture. Gametophytes survived seven months of darkness at 10°C but died after one week of darkness at 20° to 23°C. Sunlight of 1 000 E m^-2 s^-1 was fatal to gametophytes and to sporophytes under 2 mm long after 10 to 15 min. Light budgets were prepared for plants growing at 7-and 15-m depths from 1976 to May 1980 in Goat Island Bay, New Zealand (Lat.36° 16S, Long. 174° 48E). Underwater light was measured under various environmental conditions. Relationships between transmission of light through the sea, data from diving visibility records and continuous surface meteorological records were studied. Approximations were made of the average percentage of surface light transmitted to 7 and 15 m over half-monthly periods. By applying these average transmittance values to the records of surface incident light, the average daily quantum doses were calculated. Light on open bottom in Goat Island Bay may sometimes be limiting for gametophyte reproduction in winter at 15-m depth. At depths less than 7 m, summer photon flux densities may reach damaging levels.