Crowding in clonal seaweeds: Does self-thinning occur in Mastocarpus papillatus shortly before stand biomass peaks?

Fronds from crowded stands of clonal seaweeds, particularly those in which holdfasts are mostly perennial and are the major source of new fronds every year, are thought not to undergo self-thinning during the growth season, unlike those from crowded stands of unitary seaweeds. For clonal seaweeds, it is not known, however, what happens at the very end of the growth season, when crowding is highest for the year. By sampling twice more frequently than previously done for similar species, the possible occurrence of frond self-thinning was tested for Mastocarpus papillatus (Rhodophyta, Gigartinales, Petrocelidaceae) from western Canada during the growth season (spring) of 2003. Initially, stand biomass increased together with frond density, as found previously for similar clonal seaweeds. Shortly before stand biomass peaked for the year (June), frond density remained statistically unchanged. Thus, the increased sampling precision of this study confirms that fronds of these clonal seaweeds do not undergo self-thinning, not even shortly before crowding is highest. Frond size inequality for M. papillatus remained statistically similar during the growth season, which is also consistent with a model of no self-thinning. There are similarities in biomass–density dynamics and in size inequality dynamics between clonal seaweeds and clonal vascular plants.     

SEASONAL CHANGES IN SIZE STRUCTURE OF SARGASSUM AND TURBINARIA POPULATIONS (PHAEOPHYCEAE) ON TROPICAL REEF FLATS IN THE SOUTHERN RED SEA1

Seasonal variation in density, thallus length and biomass, population size structure, and allometric length‐biomass relationships was investigated in populations of Sargassum ilicifolium (Turner) C. Agardh, Sargassum subrepandum (Forssk.) C. Agardh, and Turbinaria triquetra (J. Agardh) Kütz. (Phaeophyceae) on shallow reef flats in the southern Red Sea. Thallus length and biomass varied strongly with season, with the highest values occurring in the cooler months. Thallus densities showed no significant temporal variation. Log‐total biomass versus log‐density relationships were positive throughout the growth season without any decrease in the slope of the relationship. In two populations, biomass‐density combinations approached the interspecific biomass‐density line, but the massive annual shedding of modules occurred before self‐thinning would set in. Allometric length‐biomass relationships varied with season in all populations and were associated with seasonal module initiation, growth, and shedding. Evidence of a strong asymmetric competition was found in two high‐density populations. These populations showed a predominance of small thalli during peak development, asymmetrical Lorenz curves, increasing Gini coefficients, and increasing thallus length relative to biomass during the main growth phase. In two other less crowded populations, small thalli were absent during peak development, Lorenz curves were symmetrical, and Gini coefficients decreased during the main growth phase. In these populations, size equalization appears to be due to responses at the modular level rather than size‐dependent mortality. We conclude that changes in size structure in this highly seasonal environment are determined by module dynamics, modified by asymmetric competition in some populations, with a minor role of recruitment and no regulatory effect of self‐thinning.     

A discrete-time logistic model of frond dynamics for Mazzaella parksii (Rhodophyta, Gigartinales)

Mathematical modelling is useful in population ecology and resource management. Logistic models have traditionally been applied to unitary organisms, but it is unclear whether they could be used at the frond (ramet) level for clonal seaweeds. This study shows that frond dynamics for the clonal seaweed Mazzaella parksii (=M. cornucopiae) can be described by a discrete-time logistic model. The model is realistic in that it includes density-dependence, which was previously demonstrated experimentally for this species, and only necessitates data on frond density measured at discrete time intervals. This may constitute a useful tool for the management of clonal seaweeds of economic importance that occur in dense stands. This revised version was published online in August 2006 with corrections to the Cover Date.     

THE IMPACT OF FROND CROWDING ON FROND BLEACHING IN THE CLONAL INTERTIDAL ALGA MAZZAELLA CORNUCOPIAE (RHODOPHYTA, GIGARTINACEAE) FROM BRITISH COLUMBIA, CANADA

The importance that frond crowding represents for the survival of fronds of the clonal intertidal alga Mazzaella cornucopiae (Postels et Ruprecht) Hommersand (Rhodophyta, Gigartinaceae) was investigated in Barkley Sound, British Columbia, Canada. Frond density is high for this species, up to 20 fronds·cm^?2 in the most crowded stands. Frond crowding imposes a cost in the form of reduced net photosynthetic rates when fronds are fully hydrated as a result of reduced irradiance compared with experimental (not found naturally) low-density stands. However, the interaction between desiccation and irradiance alters this relationship between net photosynthetic rates and frond density. During a typical daytime low tide in spring, irradiance is 10–30 μmol·m^?2·s^?1 below the canopy of fronds, and frond desiccation (relative to total water content) can reach 43% at the end of the low tide. In contrast to natural stands, fronds from experimentally thinned stands are subjected to irradiances up to 2000 μmol·m^?2·s^?1 because of the spatial separation among fronds and can desiccate up to 81% at the end of the same low tide. Laboratory experiments showed that negative net photosynthetic rates occur between 40% and 80% desiccation at an irradiance of 515 μmol·m^?2·s^?1, and the literature suggests that strong bleaching could occur as a result. At 20 μmol·m^?2·s^?1 of irradiance and desiccation levels up to 40%, simulating understory conditions of natural stands, net photosynthetic rates are never negative. Experimental thinning of stands of M. cornucopiae done during spring effectively resulted in a stronger extent of frond bleaching compared with natural stands. Therefore, the cost of reduced net photosynthetic rates at high frond densities when fronds are fully hydrated is counterbalanced by the protective effects of frond crowding against extensive bleaching, essential for survival at the intertidal zone. Future research will have to demonstrate the possible relationship between the frequency and duration of negative net photosynthetic rates and the extent of frond bleaching.     

Review of studies on biomass-density relationships (including self-thinning lines) in seaweeds: Main contributions and persisting misconceptions

Ecological models relating biomass and density are relatively simple to calculate and offer information on, for example, the interactions among organisms and size constraints. Biomass‐density relationships have mostly been studied for terrestrial plants, but recently they have also been increasingly investigated for seaweeds. Unfortunately, a number of misconceptions have limited the overall contribution of algal studies to biomass‐density theory in general. Aiming to improve this situation, the present paper first summarizes the current knowledge on biomass‐density theory, particularly focusing on the main concepts that, with varying degrees of validity, exist in the published literature: the self‐thinning rule (in its boundary and dynamic interpretations), the interspecific biomass‐density relationship, and the ultimate biomass‐density line. Afterwards, the present paper provides a critical review of past biomass‐density studies on seaweeds. The main contributions of studies on clonal and unitary species are discussed, while the misconceptions that persist to these days are identified in order to help future studies to be based on solid grounds.     

The relationship between stand biomass and frond density in the clonal alga Mazzaella cornucopiae (Rhodophyta,Gigartinaceae)

The negative relationship between stand biomass and plant density observed in terrestrial plants was tested among fronds of a clonal red alga, Mazzaella cornucopiae. Stand biomass and frond density were estimated bimonthly for 1 year on 7 permanent quadrats. A positive linear correlation was found between biomass and density for the whole data set, suggesting the lack of self-thinning among fronds of this intertidal alga at natural densities. Higher frond densities could favor increased water retention among fronds, thus minimizing desiccation during low tides. In this way, stands could maintain higher production of biomass. Fronds may also be cushioned better against wave action at higher frond densities, thus decreasing the detachment of larger fronds. The temporal variation of the relationship between biomass and density was plotted separately for these 7 quadrats. Four quadrats showed positive linear correlation between both variables (the other three quadrats showed non-significant positive linear correlation). Their four slopes are statistically similar to that found for the entire data set. It is possible, then, that there is only one positive slope for the biomass-density relationship in this population. If this is true, standing biomass could be estimated from the density of fronds.     

Regeneration and reproduction of Mazzaella cornucopiae (Rhodophyta, Gigartinaceae) after frond harvesting

The effects that different intensities of frond harvesting have on frond regeneration and subsequent production of reproductive structures were investigated for the red intertidal alga Mazzaella cornucopiae (Postels & Ruprecht) Hommersand from British Columbia, Canada. Harvesting was done by pruning fronds in the late spring (when stand biomass is highest) of 1993 at two intensities: total and partial collection of fronds, in this second case leaving all frond biomass less than 1 cm high in place. Holdfasts were not damaged. Total percent cover of thalli, frond density, mean frond length, and stand biomass for these experimental quadrats were statistically similar to values for control quadrats in the spring of 1994. These results suggest that one total harvest of fronds per year, done in late spring without damaging holdfasts, may give the highest sustainable yield of biomass. The effects of harvesting intensity on reproduction were variable and difficult to explain. Neither the appearance nor the abundance of cystocarpic fronds were affected by frond pruning, compared with control areas, but pruning did affect the appearance and the abundance of tetrasporic fronds. Partial pruning resulted in a longer presence of tetrasporic fronds, whereas total pruning was associated with their complete absence. Results are compared with those for the few other species of the Gigartinaceae for which experimental harvesting has been done. This revised version was published online in August 2006 with corrections to the Cover Date.     

DEMOGRAPHY OF FRONDS OF SARGASSUM LAPAZEANUM FROM SOUTHERN BAJA CALIFORNIA, MEXICO, DURING EARLY STAGES

In terms of biomass, Sargassum lapazeanum (Phaeophyceae, Fucales) is one of the most important seaweeds in La Paz Bay, on the southeastern coast of the Baja California Peninsula, Mexico. This species can be found year-round in shallow subtidal sites. Standing biomass is lowest between fall and winter and highest between spring and summer. We are currently studying the annual demography of fronds as a necessary step to understanding the mechanisms of population regulation. Work is being done on a bed of about 90 m in length and three m in width. At this stage of abstract submission, we can report results for winter only. Random samples were collected in February and in March 2000 to estimate stand biomass, frond density, and size (frond length) structure. In addition, we labeled fronds with numbered plastic tags to estimate their rates of growth and of mortality during this period. Rates of recruitment were estimated from a combined analysis of the above. In February, fronds were all shorter than 6.5 cm. Between February and March, mean total frond density increased from 122 to 776 fronds m⁻² (n = 30 quadrats). The mean recruitment rate was 667 fronds m⁻², the mean growth rate was 0.5 cm day⁻¹ (n = 60 fronds), and the mean mortality rate was 43 fronds m⁻². From last year's preliminary observations, we expect to observe peaks of biomass and of reproduction in late spring, followed by negative growth rates and high mortality rates during summer.     

RAMET DYNAMICS FOR THE CLONAL SEAWEED PTEROCLADIELLA CAPILLACEA (RHODOPHYTA): A COMPARISON WITH CHONDRUS CRISPUS AND WITH MAZZAELLA CORNUCOPIAE (GIGARTINALES)

Little is known about the dynamics and the ecological interactions among ramets (fronds) from populations of clonal red seaweeds. Small ramets are very difficult to tag, so their growth cannot be monitored directly. The temporal variation of the relationship between stand biomass and ramet density offers information on ramet performance. We calculated this relationship for an intertidal population of Pterocladiella capillacea (Gmelin) Santelices et Hommersand (Gelidiales) from Baja California, Mexico. Biomass and density were positively correlated on an annual basis, indicating that biomass accumulated without involving self-thinning among ramets. This contrasts with nonclonal seaweeds, for which self-thinning among individuals occurs during growth, but agrees with other clonal red seaweeds, such as Chondrus crispus Stackhouse and Mazzaella cornucopiae (Postels et Ruprecht) Hommersand (both Gigartinales). The growth pattern for these members of the Gelidiales and of the Gigartinales holds despite differences in holdfast morphology and ramet branching degree and despite differences in the capacity of coalescence during early stages, known only for the Gigartinales. The positive slope for the dynamic biomass–density relationship, on a bilogarithmic scale, was statistically steeper for M. cornucopiae than for P. capillacea and for C. crispus. This suggests that the addition of new ramets during the growth season may be relatively more beneficial for biomass accumulation rates for M. cornucopiae. This would be expected for high-intertidal species subjected to strong abiotic stress, for which ramet crowding constitutes a key protection. Pterocladiella capillacea occurs at the mid-intertidal zone and C. crispus at the subtidal zone, so ramets would be relatively less important in that respect.     

STRUCTURE AND DYNAMICS OF A POPULATION OF PALMARIA PALMATA (RHODOPHYTA) IN NORTHERN SPAIN1

Palmaria palmata (Linnaeus) O. Kuntze (Rhodophyta, Palmariaceae) is a seaweed commercially harvested for human consumption. Its population density, size structure, and frond dynamics were investigated from May 1999 to May 2001 at one intertidal locality in the northern coast of Spain, which is within the southern distributional boundary of the species in the eastern Atlantic coasts. The effect of size, age, and the life‐history phase (haploid vs. diploid) on frond growth and mortality were also evaluated. The study was carried out by mapping and monitoring fronds in the field. New fronds (macroscopic recruits or sprouts) appeared in spring, but subsequent mortality of these young fronds and detachment of the host plant led to lower density values in January. Palmaria palmata exhibited a distinctive seasonal growth cycle, with positive net growth from March to August and breakage from August to March. Interannual differences were also detected, with higher net growth in 2000 than in 1999. Net growth was apparently independent of age, reproductive status (fertile vs. reproductive), and life‐history phase (haploid vs. diploid) but was dependent on size, as longer fronds showed minor growth or greater breakage than small ones. Mortality, on the other hand, was more dependent on age than on size in the period analyzed (March–May 2000). Results of the study indicate that both size and age should be included as state variables and temporal changes in transition probabilities considered in the development of demographic models of the species.     

The interspecific biomass–density relationship for terrestrial plants: where do clonal red seaweeds stand and why?

For crowded stands of terrestrial plants, ranging from mosses to trees, plant (or ramet, for clonal plants) density is negatively related to stand biomass. Stand biomass and ramet density were determined for Mazzaella cornucopiae and for Pterocladiella capillacea , two morphologically distinct intertidal clonal red seaweeds, to compare them with terrestrial plants. For these seaweeds, ramet densities were similar to the highest values reported for terrestrial plants (mosses, specifically). Stand biomass was higher than average values expected from the terrestrial interspecific biomass–density relationship, but lower than the limits expected from the terrestrial ultimate biomass–density line. These seaweeds show unexpectedly low ramet slenderness and high biomass packing per unit of volume, compared with the trend observed for terrestrial plants. Possible explanations for these differences are related to the particular physiology and habitat of intertidal clonal seaweeds.     

Scaling of Frond Form in Hawaiian Tree Fern Cibotium glaucum: Compliance with Global Trends and Application for Field Estimation

Large‐fronded tree ferns are critical components of many tropical forests. We investigated frond and whole‐plant allometries for Hawaiian keystone species Cibotium glaucum, for prediction and to compare with global scaling relationships. We found that C. glaucum fronds maintain geometric proportionality across a wide range of plant and frond sizes. These relationships result in strong allometries that permit rapid field estimation of frond size from simple linear dimensions. C. glaucum frond allometries complied with intra‐ and interspecific global trends for leaf area versus mass established for much smaller‐leafed species, indicating ‘diminishing returns’ in photosynthetic area per investment in mass for larger fronds. The intraspecific trend was related to declining water content in larger fronds, but not to a significantly larger investment in stipe or rachis relative to lamina. However, C. glaucum complied with the global interspecific trends for greater allocation to support structures in larger leaves. Allometries for frond number and size versus plant height showed that as plants increase in height, frond production and/or retention progressively declines, and the increases of leaf size tend to level off. These frond and whole plant‐level relationships indicate the potential for estimating frond area and mass at landscape scale to enrich studies of forest dynamics.     

The effects of control on the biomass, carbohydrate content and bud reserves of bracken (Pteridium aquilinum (L.) Kuhn), and an evaluation of a bracken growth model

This paper examines the initial effects of bracken control on frond numbers and biomass, and the biomass, carbohydrate reserves and bud densities of bracken stands cut once per year, twice per year, subject to a single application of asulam or left untreated. The seasonal dynamics of these parameters are displayed; carbohydrate and biomass are both removed from the rhizome system to produce frond tissue, and are replenished at the end of the growing season. Asulam application reduced densities of both active and dormant buds, and both frond biomass and density. It did not significantly reduce rhizome biomass or carbohydrate reserves in the two years after treatment. Cutting, either once or twice per year reduced both rhizome biomass and rhizome carbohydrate reserves, as well as bud densities, though the latter were reduced in proportion to biomass. Cutting twice a year reduced the production of fronds, both in numbers and biomass. The collected data were used to evaluate a model of bracken growth, and subsequently to improve estimates of some of the model parameters. The model simulations of control treatments were compared to field data. The effects of cutting once per year and spraying with asulam were predicted accurately, but the bracken stand was more resilient to cutting twice per year than would be expected from model predictions. The combination of cutting and spraying is discussed as a potential tool in land management and the deficiencies of the model are discussed in relation to the need for future research into the biology of bracken.     

Population dynamics and culture studies of the edible red alga Callophyllis variegata (Kallymeniaceae)

Callophyllis variegata is a red alga that has been exported to Japan as an edible seaweed over the past few years. Available data strongly suggest that after a few years of exploitation of the C. variegata stands in southern Chile, a decline of its abundance seems to be occurring. However, there is not sufficient knowledge available to sustain harvesting strategies or to develop cultivation techniques. This study describes the C. variegata landings in the south of Chile and also provides some basic data on the biology of this species. Experiments related to the cultivation of early developmental stages, frond cultivation under controlled conditions and the regeneration capacity of the holdfast of this red alga are also presented. Spore production occurs mainly in autumn and winter and survival of carpospores decreased as the temperature increased from 8 to 12°C. Survival of tetraspores increased significantly, from 50 to 60% to over 80%, when temperature was raised from 8 to 12°C. Carpospore survival was also significantly affected by the photon flux density. This effect was mainly at 8°C, whereas at 33 µmol m^?2 s^?1 the survival of the spores is always low. The cultivation of apical portions of C. variegata under laboratory conditions showed that lower temperatures (8°C) significantly increased growth. Salinity and photon flux density did not have an effect on specific growth rate. Laboratory experiments demonstrated that holdfasts can regenerate fronds and that these fronds can be excised and cultivated, and are far more tolerant to environmental factors than the apical portions.     

Plant response to overcrowding – Lemna minor example

Plants adopt various strategies in response to increasing density. We tested that response in two populations of Lemna minor L. – a free floating aquatic plant that frequently experiences intraspecific competition for space. Surface area of fronds and colonies, colony size (the number of fronds per colony), the rate of reproduction (based on the number of produced fronds) and growth rate (enlargement of surface area of all colonies) were the analysed factors presumably affected by density. The study was performed in natural stands and in experimental conditions with the use of two contrasting plant densities. Plants growing in natural conditions produced fronds of smaller and less variable surface area as a response to overcrowding but the number of fronds per colony was unrelated to plant density. Stable experimental conditions facilitated formation of fronds and colonies larger than in the field but frond detachment decreasing colony size was more intensive at high than at low density. This strategy allowed plants to more efficiently occupy limited available space. No self-thinning was observed during experimental cultures. Due to increasing frond area in cultures, growth rate was always higher than the rate of plant reproduction. Both were strongly negatively affected by high density. Performed calculations indicate that density-dependent growth inhibition starts when L. minor colonies cover the available water surface with a mono-layer mat. Some types of responses were found to significantly differ between analysed populations, which was also shown by genetic differences tested with he ISSR-PCR technique. Possible causal relationship between plant strategies and their genomic structure needs, however, further studies.     

纯林自然稀疏研究综述

自然稀疏是林分内的个体由于部分有限的资源而引起的一部分个体死亡的现象。评述40a来世界上有关纯林自然稀疏的研究内容,包括了自然稀疏法则,异速生长模型、平均个体重与密度的时间轨线,种的自然稀疏线和自然稀疏动力线的关系,自然稀疏线的斜率与生物量的生物量的关系,自然稀疏的机理。     

AN EXPERIMENTAL EVALUATION OF DENSITY AND PLANT SIZE IN TWO LARGE BROWN SEAWEEDS

The effects of density on the growth rate and survival of individual plants as well as changes in population structure (hierarchy) and biomass accumulation (self-thinning) were experimentally evaluated in two brown macroalgae. Laminaria digitata (Hudson) Lamouroux and Fucus serratus Linnaeus populations were constructed at five (650–5156 plants·m^?2) and four densities (650–2668 plants·m^?2), respectively, and were cultivated in tanks. The relative growth rates and survivorship of individuals and the populations’ biomass and density (estimated dry mass) were periodically measured. To investigate how plant population size hierarchies influence conspecifics, single density populations of L. digitata were constructed of up to three sizes of plants in equal proportions, and these parts of the populations were compared through time with plants of the three sizes grown singly. At higher density, L. digitata plants grew more slowly, while F. serratus populations showed a similar trend that was never statistically significant. Survival of plants of both species was lower at high densities, and mortality selectively removed smaller plants. Plants of both species exhibited zero growth rates before death, when parts of the fronds were lost, but meristems (apical in F. serratus, at the base of the frond in L. digitata) were preserved until the death of the plant. All singly grown L. digitata plants survived, but survivorship was low in the fractions of small plants in mixed-size populations compared with that of the largest size plant fractions. Small L. digitata plants grew relatively faster than did large ones singly, but in mixed-size populations, small plants grew relatively slowly. Plant sizes became progressively more unequal (Gini and skewness coefficients) until self-thinning started reducing the size variability. The seaweeds followed self-thinning (density-biomass) trajectories predicted by the self-thinning “rule”, and self-thinning appeared to be seasonal- rather than species-dependent, as it occurred at a time of year when ambient light levels start to fall in the Isle of Man. Culture studies of this kind, despite their considerable potential, are a tool as yet underexploited by marine ecologists as a means of assessing intraspecific competitive interaction among seaweeds.     

Comparative spatial distribution, size, biomass and growth rate of two varieties of bracken fern (Pteridium aquilinum L. Kuhn) in a neotropical montane habitat

A comparative field study of caudatum and arachnoideum, the two Pteridium aquilinum varieties of the caudatum subspecies known to grow in the neotropics, was performed in a montane savanna habitat of the Venezuelan Andes that was affected by wildfire. Frond size, ramet density and spatial distribution, blade and rhizome biomass, and frond elongation and expansion rates were measured in separate, isolated stands each containing only one bracken variety and covering approximately the same area (540 m²). In addition to clearly discernible morphological differences, caudatum and arachnoideum were found to possess distinct features: caudatum tends to develop open stands of relatively shorter blades of 76.6±0.89 cm (±SE) of rachis length and lower ramet density (1.6 fronds m^-2, max.=7 fronds m^-2) whereas arachnoideum grows into longer, more expanded fronds 124±1.6 cm tall and significantly higher ramet density (5.1 fronds/m², max.=14.6 fronds m^-2). The sum of aerial and underground biomass was found to be notably larger for arachnoideum (8522±614 Kg/ha) than for caudatum (1929±131 Kg ha^-1) in stands growing under the same habitat conditions. Therefore the spatial distribution of arachnoideum appeared considerably more compact than that of caudatum. Blade growth rates and development time were also very different. Newly emerged caudatum croziers developed into mature blades within 42 to 48 days following an inverse exponential curve whereas arachnoideum blades required 70 to 75 days to reach maturity following a linear development. All the above dissimilarities are interpreted as the hitherto unreported indication of diverging growth strategies of two cohabitant bracken varieties following fire.     

Non-timber forest product harvesting in alien-dominated forests: effects of frond-harvest and rainfall on the demography of two native Hawaiian ferns

Non-timber forest products (NTFP) represent culturally and economically important resources for millions of people worldwide. Although many NTFP are harvested from disturbed habitats and therefore subject to multiple pressures, few quantitative studies have addressed this issue. Similarly few NTFP studies have assessed seasonal variation in demographic rates even though this can confound harvest effects. In Hawaiȁ8i, the wild-gathered ferns, Microlepia strigosa and Sphenomeris chinensis, represent highly important cultural resources but declining populations have led to conservation concerns. Both ferns are harvested from disturbed, alien-dominated forests and contemporary Hawaiian gathering practices often consist of harvest and concurrent weeding of alien invasive species. We assessed the effects of concurrent frond-harvest and alien species weeding on frond structure, density, and rates of production by comparing experimentally harvested vs. control plots, and documented relationships between frond demographic patterns and precipitation. Gathering practices had no impact on frond density of either species or on most other demographic parameters over the short term. Exceptions included a significant decrease in the density of the longest S. chinensis fronds and a significant decrease in M. strigosa frond production when fronds were gathered without alien weeding. However, seasonal and annual changes in frond density and production occurred across all plots of both species and were significantly correlated with precipitation. The relatively low harvest effects for both species are likely due to several factors including short frond longevity and the strict criteria used by gatherers to select harvestable fronds. The potential for sustainable harvest in the context of alien-dominated forests is discussed.     

Seasonal variation in epifaunal communities associated with giant kelp (Macrocystis pyrifera) at an upwelling‐dominated site

Kelp forests are highly productive and species‐rich benthic ecosystems in temperate regions that provide biogenic habitat for numerous associated species. Diverse epifaunal communities inhabit kelp sporophytes and are subject to variations in the physical environment and to changes experienced by the kelp habitat itself. We assessed seasonal variations in epifaunal invertebrate communities inhabiting giant kelps, Macrocystis pyrifera, and their effects on this seaweed. Six seasonal samplings were conducted over a year at an upwelling‐dominated site in northern‐central Chile where physical conditions are known to fluctuate temporally. More than 30 taxa were identified, among which peracarid crustaceans stood out in both diversity and abundance. Species richness and abundance differed among sporophyte sections (holdfast and fronds) and throughout the year. The frond community was dominated by two grazers (the amphipod Peramphithoe femorata and the isopod Amphoroidea typa), while suspension feeders, grazers, and omnivores (the amphipod Aora typica, the isopod Limnoria quadripunctata, and polychaetes) dominated the holdfasts. Abundances of the dominant species fluctuated throughout the year but patterns of variation differed among species. The most abundant grazer (P. femorata) had highest densities in summer, while the less abundant grazer (A. typa) reached its peak densities in winter. Interestingly, the area of kelp damaged by grazers was highest in autumn and early winter, suggesting that grazing impacts accumulate during periods of low kelp growth, which can thus be considered as ‘vestiges of herbivory past.’ Among the factors determining the observed seasonal patterns, strong variability of environmental conditions, reproductive cycles of associated fauna, and predation by fishes vary in importance. Our results suggest that during spring and early summer, bottom‐up processes shape the community structure of organisms inhabiting large perennial seaweeds, whereas during late summer and autumn, top‐down processes are more important.