Regulation of a Fucus djstichus L. emend. Powell population is probably mediated through a density-dependent mortality of the recruits. Blocks seeded with different densities of germlings of E distichusand cleared plots with naturally occurring recruits of different densities and with artificially thinned densities were used to monitor the effect of density on mortality and growth among recruits in a populat~on of F: distjchus in False Creek. Vancouver, British Columbia. Canada. In the first 2 mo of development, germlings on the high density blocks experienced a lower mortality than those on the low density blocks. At later stages (> 2 mo), the effect of density on mortality was reversed. Plants recruited into the cleared plots were > 1 mm in length and they also experienced a positive effect of density on mortality. In both the density blocks and cleared plots, plant mortality eventually became independent of plant density. Plant growth rate was generally not related to density but was related to plant size (length). There was a large variation in the sizes of the plants. All 3 measures of si7c inequality: zero-centered skewness, coefficient of variation, and Gini coefficient are significantly correlated. Plants in both the density blocks and cleared plots showed a slight tendency towards a more normal or positively skewed size distribution from fall 1986 to spring 1987, probably due to an increased mortality of the smaller plants during this time. The mechanism of dominance-suppression and difference in the intrinsic growth rate or timing of settlement probably all contributed to the inequality in size distribution.

ABSTRACT. Various phenomena related to reproduction and recruitment in a population of Fucus distichus L. emend. Powell in Vancouver, British Columbia. Canada were evaluated. Using log linear analysis and tests for simple, multiple and partial associations, a g e and size were both found to b e significant, but size slightly more so than age, as descriptors of reproductive events. Reproductive plants were found throughout the sampling period, from September 1985 to November 1987, but peaked in fall and winter of each year. Potential egg production, based on number of eggs produced per conceptacle and number of conceptacles per unit area of receptacle, is size-dependent. However, estimated monthly egg production, calculated by observed number of eggs in clusters extruded from the receptacle, is independent of plant size. Two types of recruits were monitored Microrecruits (< 1 mo-old of microscopic size) are germlings developed from fertilized eggs. Their numbers were assessed using settling blocks. Macrorecruits are detectable by the unaided eye and are plants appearing in the permanent quadrats for the first time. They can first be detected when about 3 to 4 mo old. The recruitment pattern of microrecruits is significantly correlated with reproductive phenology and patterns of potential and estimated monthly egg production. The pattern of recruitment of macrorecruits is negatively correlated with reproductive phenology and that of the estimated monthly egg production. Peaks in microrecruitment are not always followed by peaks in macro-recruitment. This apparent discrepancy is probably due to a differential survivorship of microrecruits over time or to the possible existence of a 'germling bank'. Patterns of survival and emergence of macrorecruits may b e independent of those of microrecruits or may be unrelated to the prevailing reproductive phenology.

Reproductive effort (ratio of fertile to total plant dry weight) was measured in Fucus distichus L. emend. Powell. On average, 12.7 % of plant dry weight is in reproductive biomass and this amount is relatively constant over most months. Reproductive effort does not vary among plants of different sizes and dry weights. However, fertile plants are significantly (p ~0 . 0 5 ) bigger than the nonfertile plants, and have a better survival rate. There is no clear indication of cost of reproduction with respect to the longevity of fertile vs non-fertile plants. Fertile plants, especially those > 17 cm in length, tend to exhibit negative or zero growth much more often than non-fertile plants, suggesting that the cost of reproduction may be manifested in the form of reduced growth, rather than in greater mortality or shorter longevity of the fertile plants. The failure to detect cost of reproduction with respect to mortality and longevity may be due to the modular nature of the plants, where cost occurs at the level of the modules (branches) rather than at the level of the whole plant.

A Fucus dlstichus L. emend Powell population at False Creek, Vancouver, Canada, showed seasonal variations In plant mean length and growth rates. Mean length was greater in wlnter (4.2 to 5 3 cm) and lower In summers of 1986 and 1987 (2.7 to 4 3 cm). Absolute growth rates showed a significantly opposite trend, being hlgher In spnng and summer (0.24 to 1.17 cm mo-') and lower in fall to winter (-0 5 to 0.4 cm mo-l) Spearman rank-order correlation indicated that, with some except~ons, monthly patterns of change In plant mean length for plants of different ages were slgnlflcantly correlated, but growth rates were not slgniflcantly correlated among ages. Patterns of change In length among cohorts were sign~f~cantly correlated, but those of growth rates were not, suggesting a low dependence of growth rate on age Growth rate was significantly correlated (Spearman rank-order correlation) wlth plant length, and the patterns of change in growth rates among plants of different size classes were also significantly correlated (Pearson's correlation), suggesting that growth rate IS strongly size-dependent. A seasonal pattern became more obvious only lf plants were grouped by slze rather than by age. Plants from different cohorts generally had constant rates of mortality Mortality was generally hlgher for younger and smaller plants durlng winter and sprlng, and for older and larger plants during summer Older plants also exhibited hlgher mortallty in the fall. Log linear analysis indicated that the effects of both size (length) and age on growth rate, wlth or without the effect of mortality, and on mortallty considered alone, were s~gnificant. However, when the contr~bution of size was known, the effect of age on mortallty became insignificant. Size is shown by assoclation analyses to be a better predictor of growth and rnortallty than age.

Matnx models based on size and stage, corresponding to the monophasic, biphasic and triphasic life histones of algae have been constructed. Similarities and differences of these models with the Leslie matnx model, as well as with other extensions and modif~cations of it, are discussed. Two examples are given to show the utility of these models: one based on field data for Sargassum siliquosurn J. Ag. and a second, based on data for Laminaria longicruris Pyle., taken from the literature. Stable size distribution, reproductive values, sensitivity and elasticity of these models are analyzed. Many algae show discrete episodes of growth, reproduction, die back and regeneration. Hence, discrete matrix models appear to realistically describe the life history processes of these algae.

A general 9 X 9 matrix model based on recruit stages and plant size is proposed for a population of the brown alga Fucus distichus L. emend. Powell in False Creek, Vancouver, Canada. Twenty-six matrices were constructed each representing a monthly time interval covering the period from July 1985 to November 1987. Yearly matrices were also constructed as a product of monthly matrices covering periods of 1 yr. The characteristics of these matrices were evaluated by comparing the dominant eigenvalue (i.e. the population growth rate, A), the stable distribution, and reproductive values for each matrix. The relative contribution of each matrix parameter to population growth was assessed using elasticity analysis. The survival and transition of the plants among size classes contribute at least 50 % to the population growth rate. However, because E distichus plants do not exhibit vegetative regeneration, the population can only experience positive growth in the presence of recruitment. Absence of a germling bank has little effect on monthly projected population growth, but can reduce the population growth rate projected from the yearly matrix by as much as 83%. Current population size structure is very different from the projected stable distribution. The stable distribution generally has a large proportion of the plants in the recruit stage. Reproductive values are positively related to plant size but are comparable among the largest 4 size classes. Random combinations of monthly, seasonal, or yearly matrices were used to simulate population growth under various fluctuating environmental conditions. A negative growth rate was obtained in > 6 0 % of the simulation runs. In reality within the sampling period covered, the population is on the decline. It is likely that this population may recover by occasional pulses of a large number of recruits, as was observed in 1986.

Populations of Iridaea splendens at Brockton Point, Stanley Park, Vancouver, Canada were observed to alternate in dominance between the gametophytic phase in summer and tetrasporophytic phase in winter . The mechanism regulating this alternation is not clear . Using a matrix projection model to simulate population growth, we show that this alternation is possible if there are differential survival and recruitment rates of the two phases in summer and winter . Sensitivity and elasticity analyses indicate the relative importance of perennation vs . recruitment . Recruitment from tetrasporophytes and from gametophytes both contribute about 25 % to the population growth . Perennation among gametophytes is more important than among tetrasporophytes . The implication of this is that if this population is to be harvested, more tetrasporophytes can be harvested than gametophytes without resulting in the depletion of the resource . This is simulated in the matrix model by comparing the relative effects on population growth of increasing the mortality rate of the perennation phase of tetrasporophyte and gametophyte by 50 to 75 %, and increasing recruitment rate in either phase, from summer to winter or from winter to summer .

A zonation study of Sargassum siliquosum J. Ag., S. paniculatum J. Ag. and other Sargassum spp. was conducted using line transects. The structure of the community was analyzed using Principal Component Analysis (PCA) and the relation between percentage cover of the different dom~nant species and environmental parameters was extracted using Canonical Correlation Analysis. Results show that variation in the community structure was primarily accounted for by the less dominant algae taken together. Individually, however, contributions of these algae were e a s~l y masked by the contribution from the most dominant Sargassum spp. Significant canonical correlation (p <0.05) was extracted between algal cover, primarily that of Sargassum, and temperature and tidal level. Significant correlation was also extracted between algal cover and the antecedent events of minimum temperature, mean lowest low tide levels and water movement.