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Demographic Studies of the Annual Kelps Nereocystis luetkeana and Costaria costata (Laminariales, Phaeophyta) in Puget Sound, Washington

Demographic studies of subtidal algae, particularly annuals, are rare. For a period of 16 months, we documented recruitment, growth, reproduction and mortality of the sporophytes of the canopy-forming kelp Nereocystis luetkeana and the understory kelp Costaria costata, and monitored the densities of three other brown seaweeds, in a subtidal community in the southern Puget Sound. The study site, located on the west side of the Tacoma Narrows, Washington (47°15'N, 122°33f W), is a shallow, estuarian environment with a mixed sand and cobble substrate that is exposed to swift tidally generated currents. Juvenile Nereocystis and Costaria were first observed in early March 1993 and 1994 and continued to appear until mid-June in 1993. Patterns of growth, reproduction and survivorship in Nereocystis and Costaria are distinctly different. Nereocystis stipe growth rates peaked (~ 5 cm/day) at the end of June and then declined, while relative blade growth rates increased (to 6 cm/day) after the period of maximum stipe elongation. Relative blade growth in Costaria declined from ~ 2 cm/day in late May until growth ceased in mid-August. Reproductive Nereocystis plants were first observed in early May; 85% of the population was fertile by early July. Reproductive Costaria plants were first observed on 22 June; 83% were fertile by early August. Both Nereocystis and Costaria were strict annuals at this site.