| Summary: | This is the first study to quantify temporal and spatial variations in growth rates and productivity, inorganic N uptake and photosynthetic performance of the subtidal brown macroalga, Ecklonia radiata, in relation to an environmental gradient in Doubtful Sound, Fiordland. Doubtful Sound has a unique marine environment created by a low-salinity layer (LSL) that reduces light transmission through the water column. Three sites were chosen from across the environmental gradient; two sites near the entrance of the sound, Causet Cove and Thompson Sound, and a site further in towards the head of the fiord, Seymour Island. Blade and stipe growth rates, erosion rates, morphological parameters and productivity of Ecklonia radiata were measured at the three sites between September 1998 and March 2000. E. radiata displayed a temporal variation in growth rates, with maximum growth rates in September and October 1998 and minimum rates between June and July 1999. Blade growth rates in September and October 1998 were double those for the same period the following year. Blade growth rates at Seymour Island were lower than rates at Causet Cove and Thompson Sound on five occasions. Stipe growth rates also displayed a temporal pattern with higher rates between October and February and lower rates between June and August. Erosion rates were generally similar between sites throughout the study. Eight morphological parameters were measured but only stipe length and blade thickness were significantly different between sites. Stipes of E. radiata at Seymour Island were longer and the primary blades thinner than those at Causet Cove and Thompson Sound. Daily individual sporophyte productivity (g DW d-1 and g C d-1) exhibited a similar temporal pattern to growth rates. E. radiata at Seymour Island exhibited the lowest daily population productivity rates (g DW m-2 d-1). The population density of E. radiata was low and reached a maximum of 11 individuals per m2 at Causet Cove and Thompson Sound. The low population productivity was attributed to the low population density of E. radiata in Doubtful Sound. C:N ratios and low tissue N indicated that E. radiata was N-limited year-round, although more strongly between February and April 1999. Inorganic N pools showed a temporal variation, with less inorganic N accumulated between November 1999 and March 2000. Ecklonia radiata showed a preference for NH4+ over NO3- this was reflected in the larger intracellular NH4+ pools. The low intracellular NO3- pools reflected the low ambient seawater inorganic NO3- concentrations. Time-course experiments examined the ability of field collected Ecklonia radiata tissue to remove NO3- and NH4+ at 5 and 30 µM from media over 300 minutes. E. radiata showed three patterns of depletion: linear, surge and no depletion. High rates of NO3- depletion occurred at both 5 and 30 µM between March and May, while NH4+ depletion rates were higher at 5 µM than at 30 µM and showed no temporal pattern. Multiple-flask experiments were conducted between June 1999 and May 2000 to determine uptake kinetic parameters for E. radiata. Uptake rates were higher for NH4+ than for NO3- uptake rates. There was no consistent temporal or spatial variation in Ks and α indicating that E. radiata had no preference for either form of inorganic N investigated in this study. The ability of E. radiate to take up inorganic N at an ecologically (relevant) low concentration was evaluated using the uptake rate at 2 µM (V2). V2 values were low overall and this may reflect a low N requirement as a result of low growth rates, due in part to low ambient seawater inorganic N concentrations throughout the study period. The temporal photosynthetic performance of E. radiata was investigated using photosynthesis versus photon flux density (PFD) curves (PE) and pigment content between September 1998 and March 2000. Pmax at all study sites was highest between June and August 1999 and ranged from 0.8- 1.2 mg O2 gDW-1 h-1. E. radiata at Causet Cove and Thompson Sound also exhibited higher α p compared to kelp at Seymour Island during these months. Chl α and c contents displayed a temporal pattern with contents maximal in November and December 1999 while no defined temporal pattern for fucoxanthin content was exhibited. E. radiata at Seymour Island exhibited greater photoinhibition compared to E. radiata at Causet Cove or Thompson Sound, with inhibition occurring at PFD ≥ 100 µmol photons m-2 s-1. The interactive effect of N availability and light on growth rates and productivity of Ecklonia radiata is summarized and the implications of the low productivity exhibited by E. radiate are discussed in relation to the nearshore marine ecosystem.
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