Abstract
The influence of wave-exposure on the growth and erosion rates of Macrocystis pyrifera 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 M. pyrifera 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 M. pyrifera 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 M. pyrifera only at specific times of year; when tissue nitrogen levels are low but some inorganic nitrogen is available in the water column.