A study of growth, reproduction, and competition in populations of Iridaea cordata (Turner) Bory (Rhodophyta) in Georgia Strait, B.C.
Date
1979
Authors
Adams, Robert W.
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Abstract
Observations were made on Iridaea cordata communities in northern Georgia Strait during a 15 month period to investigate the in situ population dynamics of this species.
The standing crop of I. cordata populations sampled at three locations where it dominated, ranged from lows of 0.00±0.000 to 0.03± 0.009 kg fresh/0.5 m² in December through February, to highs of 0.7±0.05 to 2.0±0.24 kg fresh/0.5 m² which occurred variably in May through August. Increase and decline in both water temperature and solar radiation were generally coincident with seasonal variation in I. cordata standing crop. Nitrate-nitrogen depletion may limit growth in some locations during summer. I. cordata standing crop may also be limited by competition for light with laminarian species and for space by Plocamium coccineum.
Standing crop and density of I. cordata life history phases varied differentially with season. Gametangial fronds were dominant in May and June and tetrasporangial fronds were dominant in August through December. There was no consistent relation between I. cordata population standing crop and life phase composition.
Patterns of macroscopic development of cystocarpic and tetrasporic soral cover on I. cordata fronds are described. Maximum potential spore production of cystocarpic fronds occurred in July and of tetrasporic fronds occurred in late August through September.
The growth rate of transplanted I. cordata fronds was highest at shallow depths, in mid rather than late summer and for cystocarpic rather than tetrasporangial fronds.
The settlement of spores was high on Plexiglas plates immersed for short periods adjacent to I. cordata populations. However the survival of spores and growth to upright fronds was very limited on the horizontal surfaces of plates attached to platforms in the I. cordata community. Nearly monotypic, dense stands of I. cordata did colonize the vertical surfaces (sides) of concrete blocks, 12-14 months after immersion in June through September. A greater biomass of I. cordata was achieved if substrate was immersed from mid rather than late summer of the previous year or was immersed for two or more years. Uprights of I. cordata juveniles were observed on blocks three months after immersion in June.
Artificial populations of I. cordata were constructed at various densities on natural cobble and rope substrates. Population growth rates were maximum at an optimum frond area index (FAI) which was less than that in naturally recruited dense populations in mid summer. Over a period of 45-60 days, high density artificial populations exhibited a loss of large fronds, while low density populations added large fronds which were generated from the numerous frond initials on the basal crust. Calculation of potential biomass production in one summer using growth rates obtained at an optimum FAI yielded estimates approximately 1.6-2.0 times the maximum standing crops attained in adjacent natural populations.
Some implications of these observations to macroalgal cultivation are discussed.