Exposure to waves enhances the growth rate and nitrogen status of the giant kelp Macrocystis pyrifera

The influence of wave-exposure on the growth and erosion rates of <i>Macrocystis pyrifera</i> was determined in Paterson Inlet, Stewart Island, New Zealand, for each season of 2002. During autumn, following a period of low seawater nitrogen, sites that received significant amounts of oscillatory flow from waves exhibited higher frond growth rates than at wave-sheltered sites. Blade and stipe growth rates at wave-exposed sites were at a yearly maximum during this period, and were 4.3 and 1.6 times higher respectively than at wave-sheltered sites where growth was at a yearly minimum. During this time C:N ratios and nitrogen concentrations of blade tissues indicated that the nitrogen status of frond apices, where the majority of growth occurs, was greater at wave-exposed sites than at wave-sheltered sites. During periods when kelp tissue nitrogen levels are low, but some inorganic nitrogen is available in the water column, oscillatory flow may enhance nutrient uptake by <i>M. pyrifera</i> by increasing the flux of nutrients into kelp canopies and by reducing the size of diffusion boundary layers at the kelp surface. Exposure to waves also modified the seasonal pattern of <i>M. pyrifera</i> growth by ameliorating the negative effect of low seawater nitrogen concentrations during summer and autumn. Blade and stipe growth was relatively steady at wave-exposed sites during the study period, while wave-sheltered sites had distinct seasonal patterns with low growth during summer and autumn, increasing through winter to spring maxima. In this system, water motion influences the growth rates of <i>M. pyrifera</i> only at specific times of year; when tissue nitrogen levels are low but some inorganic nitrogen is available in the water column.