Abstract
Undaria pinnatifida farming is a large industry in Japan and Korea, with a market of over $NZ600 million per annum; U. pinnatifida was introduced to New Zealand in 1987. One method of improving crop yield and quality is to use fertilisers. Before fertilisers can be used efficiently however, information on the nutrient uptake kinetics of U. pinnatifida is required. This study examined inorganic N, and to a lesser extent the PO₄⁻³ uptake of U. pinnatifida in Otago Harbour, New Zealand, over a season. Many other biological and environmental factors have been found to interact with nutrient uptake, and were examined on a seasonal basis: photosynthesis, soluble tissue NO₃⁻,% tissue N and C, pigment levels, seawater nutrients, light and temperature.
Short term (hours), multiple flask, tissue disk uptake experiments were used to estimate the nutrient uptake kinetics of Undaria pinnatifida for each month from May to November 1996. A new data analysis method to accurately attain the initial slopes from nutrient uptake curves fitted to the Michaelis-Menten model of uptake is described. Photosynthesis vs irradiance curves for each month were also obtained, along with data on pigment levels so that the photosynthetic physiology of U. pinnatifida could be evaluated. Long term (2 month) culture experiments, examining growth and nutrient removal by whole sporophytes in different levels of nutrients, near the end of the growth season were also conducted.
The V(max) for NO₃⁻, NH₄⁺, and PO₄⁻³ uptake by Undaria pinnatifida was among the highest of any seaweeds reported in the literature. In contrast the initial slopes of nutrient uptake were low compared to those reported for other seaweeds. NO₃⁻ V(max) had an inverse relationship with soluble tissue NO₃⁻ (ca. 0.7 - 28 µmole g⁻¹ wet wt),% tissue N levels (1.47 - 2.89 %, dry weight), and environmental N levels (ca. 0.5 - 7.4 µM) over most of the season. C:N ratios (atom:atom) ranged from 12.5 to 23.6 over the season. There was no inhibition or suppression of NO₃⁻ uptake by NH₄⁺ in multiple nutrient solutions. Pigments had little role as N storage pools in U. pinnatifida, whereas soluble tissue NO₃⁻ pools had a moderate N storage role (in relation to other kelps). It was concluded that short term experiments (duration of hours) only provide information on nutrient uptake applicable to short term uptake. To attain information on long term uptake rates, long term (duration of weeks) experiments need to be conducted. The culture of sporophytes in seawater media with increased N availability increased growth rates markedly over those in the field. Results from the long term experiments and the short term nutrient uptake rates were used to model fertiliser uptake by a seaweed farm, to determine the economic potential of fertilisation. Further experimentation and information is required to make a definitive conclusion on the potential of fertilisation in U. pinnatifida farms, but there appears to be promise.