Water motion affects the growth rates of Kappaphycus alvarezii and related red seaweeds

Abstract

Tagged thalli of three Eucheumatoid seaweed species were grown in five locations comprising a natural gradient of water motion on a reef flat in Kaneohe Bay, Oahu. The gradient of water motion was related to the wind-shadow effect of a small island on the reef which sheltered part of the reef from the prevailing trade winds. Three experiments were conducted at different times of year in which growth rates, water motion, and other environmental factors were measured weekly in the pens for 8 to 12 weeks. In all three experiments significant ( P < 0.05) correlations were found between the growth rates of the seaweeds and the amount of water motion measured by the dissolution rate of calcium sulfate clodcards placed in the pens. Regression equations of growth rates vs. water motion were significant for all experiments at P < 0.05 but the slopes of the equations differed among the different seaweed species, for the same species at different times of year, and for the same species attached loosely or rigidly to the substratum. The growth response of Kappaphycus alvarezii to water motion was greater than the responses of K. striatum or Eucheuma denticulatum. With K. alvarezii, growth response to water motion was greater in summer than winter, and in thalli that were loosely held to the substratum vs. thalli that were held rigidly. Coefficients of determination indicated that water motion could account for 81–98% of the variation in growth rates among pens in the different experiments. Growth rates increased up to the highest water motion velocities measured in the pens, 15 cm s −1. It was concluded that culture of these species requires high levels of water motion provided by strong and consistent trade winds under reef farming conditions. The results support the multifactorial hypothesis of seaweed growth regulation.