Seasonal growth, erosion rates, and nitrogen and photosynthetic ecophysiology of Undaria pinnatifida (Heterokontophyta) in southern New Zealand

The goal of this study was to examine growth, erosion rates, and the photosynthetic and nitrogen ecophysiology of the invasive seaweed Undaria pinnatifida (Harv.) Suringar. Sporophytes of U. pinnatifida that appeared in Otago Harbour, southeastern New Zealand, in late autumn (May) 1996 were tagged, and their growth rates followed until the onset of senescence in early summer (November 1996). Blade growth rates were maximal between May and August when they ranged from 0.77 ± 0.05 to 0.93 ± 0.05 cm · d⁻¹ and declined from September onward. In laboratory experiments, U. pinnatifida took up nitrate and ammonium simultaneously at rates ranging from 21.3 ± 2.1 to 179.3 ± 65.1 μmol · g⁻¹ dry weight (dwt) · h−1. When monthly patterns of growth rate, seawater inorganic nitrogen, nitrogen uptake kinetics, soluble tissue nitrate, % tissue carbon (C) and nitrogen (N), and C:N ratio were considered together, there was no evidence that N limited the growth of U. pinnatifida. Furthermore, the photosynthetic parameters P_max, α, E_k, E_c, and R_d derived from P versus E curves indicated that the growth of U. pinnatifida was not light limited, and that the population could potentially grow deeper than its observed location at 4 m depth. Nitrogen and light ecophysiological parameters of U. pinnatifida more closely resemble those of small, ephemeral seaweeds, such as Ulva, than other members of the Laminariales. We suggest that a “plastic” physiology may allow U. pinnatifida sporophytes to match their physiology to a range of environments, which is one reason for its success as an invasive seaweed.