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

Photosynthetic performance, dark respiration, pigment concentration, tissue nutrient concentration and fresh:dry weight ratios were measured in juveni le and adult sporophytes of Undaria pinnatifida throughout the growing seas on from Port Phillip Bay. The photosynthetic rates (15 to 42 mg O-2 g(-1) d w h(-1)) of Undaria pinnatifida sporophyte stages were higher on a dry weig ht basis in spring compared to summer coinciding with rapid growth of juven ile sporophytes in spring. Higher dw:fw ratios of adult Undaria sporophytes were found compared to young sporophytes. The seasonal trends in P-max and alpha on a Chl a basis in juvenile plants are explained by higher pigment (Chl a, c, fucoxanthin) concentrations in spring than summer. Differences i n pigment content and their ratios between sporophyte life stages may be in dicative of light adaptation by juvenile plants. Lower saturated light requ irement (Ik) and compensation points (I,) were observed in spring compared to summer plants and lower Ik values of juvenile sporophytes compared to ad ult sporophytes was also found during spring. Spring and summer compensatio n points in this study mostly ranged from 7.63 to 15.49 mu mol m(-2) s(-1). Low I-k and I-c, and high P-max, alpha, and pigment content may enhance th e capacity of juvenile Undaria to utilise low photon flux rates. No seasona l differences were found between respiration rates on a dry weight basis or between respiration in young and adult sporophytes. Respiration rates norm alised to Chl a, were 2-3 times higher in summer in both young and adult st ages. High C:N ratios (by atom) in juvenile (spring mean = 20.2; summer mea n = 27.9) and adult sporophytes (spring mean = 11.32; summer mean = 18.6) i ndicate a higher accumulation of reserve carbohydrates by juvenile plants a nd 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 stag es (< 5) is also suggestive of N limitation. Overall the high N availabilit y in Port Phillip Bay and the low light adapted physiological characteristi cs provide Undaria with a competitive advantage over other fast growing mac roalgae.