Seasonal Patterns in the Photosynthetic Capacity, Tissue Pigment and Nutrient Content of Different Developmental Stages of Undaria pinnatifida (Phaeophyta: Laminariales) in Port Phillip Bay, South-Eastern Australia

Abstract

Photosynthetic performance, dark respiration, pigment concentration, tissue nutrient concentration and fresh : dry weight ratios were measured in juvenile and adult sporophytes of Undaria pinnatifida throughout the growing season from Port Phillip Bay. The photosynthetic rates (15 to 42 mg O₂ g⁻¹ dw h⁻¹) of Undaria pinnatifida sporophyte stages were higher on a dry weight basis in spring compared to summer coinciding with rapid growth of juvenile sporophytes in spring. Higher dw:fw ratios of adult Undaria sporophytes were found compared to young sporophytes. The seasonal trends in P_max and α on a Chl a basis in juvenile plants are explained by higher pigment (Chl a, c, fucoxanthin) concentrations in spring than summer. Differences in pigment content and their ratios between sporophyte life stages may be indicative of light adaptation by juvenile plants. Lower saturated light requirement (I_k) and compensation points (I_c) were observed in spring compared to summer plants and lower I_k values of juvenile sporophytes compared to adult sporophytes was also found during spring. Spring and summer compensation points in this study mostly ranged from 7.63 to 15.49 μmol m⁻² s⁻¹. Low I_k and I_c, and high P_max, α, and pigment content may enhance the capacity of juvenile Undaria to utilise low photon flux rates. No seasonal differences were found between respiration rates on a dry weight basis or between respiration in young and adult sporophytes. Respiration rates normalised to Chl a, were 2–3 times higher in summer in both young and adult stages. High C:N ratios (by atom) in juvenile (spring mean = 20.2; summer mean = 27.9) and adult sporophytes (spring mean = 11.32; summer mean = 18.6) indicate a higher accumulation of reserve carbohydrates by juvenile plants and N limitation in summer. Tissue C:N ratios increased in both stages from winter through to summer. Low N:P ratios in spring and summer for both stages (< 5) is also suggestive of N limitation. Overall the high N availability in Port Phillip Bay and the low light adapted physiological characteristics provide Undaria with a competitive advantage over other fast growing macroalgae.