PHOTOSYNTHETIC RESPONSE OF THE GIANT KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) TO ULTRAVIOLET RADIATION1

Solar ultraviolet radiation (UVA + UVB) impairs photosynthesis in marine algae. Canopy blades of the giant kelp Macrocystis pyrifera (L.) C. Agardh are exposed to high levels of solar UV in the field. To determine the effects of UV radiation on photosynthesis in the giant kelp and to identify sites of UV damage, O₂ evolution, reaction center organization, light harvesting, and energy transfer efficiency were measured in canopy blades that had been exposed to elevated levels of UV in the laboratory. UV treatment reduced both the light-saturated rate and the light-limited rate of photosynthesis by 50% but produced no significant change in the rate of dark respiration. A significant impairment of photosystem II (PSII) reaction center function was observed, suggesting that PSII is a major site of damage in chromophytes. Reduced quantum efficiency of photosynthesis and loss of energy transfer from light-harvesting pigments (fucoxanthin, chlorophyll a, and chlorophyll c) to PSII indicate that the major light-harvesting complex of M. pyrifera, the fucoxanthin-chlorophyll protein complex (FCPC), was another site of UV damage. These measures provide the first evidence of a direct effect of UV radiation on specific sites in the photosynthetic apparatus of chromophytes and indicate that in situ fluorescence excitation analysis may be a simple means to detect UV stress in algae.     

SPORE SUPPLY AND HABITAT AVAILABILITY AS SOURCES OF RECRUITMENT LIMITATION IN THE GIANT KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)1

The causes of spatial variation in the recruitment of benthic marine algae are frequently misunderstood because of difficulties in distinguishing among the many factors that influence the supply and establishment of microscopic propagules. We used the recently constructed San Clemente Artificial Reef (SCAR) experiment to examine the roles of dispersal distance, size of spore source, and habitat availability as sources of variation in the recruitment of the giant kelp Macrocystis pyrifera (L.) C. Ag., a species whose recruitment has often been considered to be dispersal limited. Sparse colonization on SCAR by adult Macrocystis occurred within 6 months after reef construction via drifters (i.e. individuals from neighboring kelp beds that became dislodged and set adrift). The abundance of drifters on SCAR declined exponentially with distance from the nearest source population (San Mateo), suggesting that San Mateo was the likely source of drifters. Dense recruitment of small Macrocystis sporophytes was observed within 8 months of reef construction. The density of recruits on SCAR showed an initial increase with distance from San Mateo before declining exponentially. Nonetheless, substantial recruitment was observed at the most distant locations on SCAR located 3.5 km from San Mateo. In contrast to drifters, the density of recruits was positively correlated to the bottom cover of artificial reef substrate. Importantly, no correlation was found between the local density or fecundity of drifters and the local density of kelp recruits suggesting that recruitment on SCAR resulted from widespread spore dispersal rather than from the local dispersal of spores from drifters.     

NEUTRAL LIPIDS AS MAJOR STORAGE PRODUCTS IN ZOOSPORES OF THE GIANT KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)1

Neutral lipids, consisting primarily of triacylglycerols, were found to be a major form of carbon reserve in zoospores of the giant kelp Macrocystis pyrifera (L.) C. Ag. The fluorescent stain Nile Red revealed large lipid droplets in the posterior end of the cell, which comprised 20–41% of cellular carbon in newly released spores. Flow cytometric analyses of newly released spores stained with Nile Red revealed considerable variation in the neutral lipid content among spores that was independent of spore size. Lipid droplets were consumed during germination in spores maintained either under constant light or in continual darkness. The availability of light appeared to delay, but did not preclude, lipid use. The rate of lipid use during germination varied considerably among germlings with some cells consuming all of their lipid reserves within 5 h after release. In addition to zoospores, lipid droplets were observed in both male and female gametes. Numerous droplets were observed in eggs, while single lipid droplets were observed in sperm. Neutral lipid droplets were not observed in gametophytes or sporophytes except in developing gametes and spores. Large lipid reserves thus seem to be confined to the microscopic life history stages that presumably have relatively high energy demands. By serving as a supplemental fuel reserve, neutral lipids may be important in extending the effective range of zoospore dispersal.     

EFFECTS OF THE ENCRUSTING BRYOZOAN,MEMBRANIPORA MEMBRANACEA,ON THE LOSS OF BLADES AND FRONDS BY THE GIANT KELP,MACROCYSTIS PYRIFERA (LAMINARIALES)1

Juvenile sporophytes of the giant kelp, Macrocystis pyrifera (L.) C. A. Agardh, were transplanted from local kelp beds to stations located various distances from the outfall from an electrical generating station that was known to cause an increase in the settlement of fouling organisms. Plants near the outfall became heavily fouled by the encrusting bryozoan, Membranipora membranacea (L.), and lost about one-third of their blades during the course of the experiment. Blade loss was significantly correlated with amount of fouling. To test the hypothesis that fouling causes blade loss, we paired fouled and unfouled plants of about the same age, overall length, and number of fronds and placed them at stations in nearby kelp beds and near the outfall. At the stations in the kelp beds, the fouled plants lost blades more rapidly than the unfouled controls. However, at the station near the outfall the “control” plants quickly became fouled so there was little difference in treatments and there was no significant difference in blade loss. Plants fouled by Membranipora suffered greater blade loss than clean plants probably because fouled blades are fragile and break off easily and because fish bite off chunks of blade while foraging on the attached bryozoans.     

PHYSIOLOGICAL PERFORMANCE OF FLOATING GIANT KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE): LATITUDINAL VARIABILITY IN THE EFFECTS OF TEMPERATURE AND GRAZING1

Rafts of Macrocystis pyrifera (L.) C. Agardh can act as an important dispersal vehicle for a multitude of organisms, but this mechanism requires prolonged persistence of floating kelps at the sea surface. When detached, kelps become transferred into higher temperature and irradiance regimes at the sea surface, which may negatively affect kelp physiology and thus their ability to persist for long periods after detachment. To examine the effect of water temperature and herbivory on the photosynthetic performance, pigment composition, carbonic anhydrase (CA) activity, and the nitrogen (N) and carbon (C) content of floating M. pyrifera, experiments were conducted at three sites (20° S, 30° S, 40° S) along the Chilean Pacific coast. Sporophytes of M. pyrifera were maintained at three different temperatures (ambient, ambient ? 4°C, ambient + 4°C) and in presence or absence of the amphipod Peramphithoe femorata for 14 d. CA activity decreased at 20° S and 30° S, where water temperatures and irradiances were highest. At both sites, pigment contents were substantially lower in the experimental algae than in the initial algae, an effect that was enhanced by grazers. Floating kelps at 20° S could not withstand water temperatures >24°C and sank at day 5 of experimentation. Maximal quantum yield decreased at 20° S and 30° S but remained high at 40° S. It is concluded that environmental stress is low for kelps floating under moderate temperature and irradiance conditions (i.e., at 40° S), ensuring their physiological integrity at the sea surface and, consequently, a high dispersal potential for associated biota.     

NORTHERN AND SOUTHERN HEMISPHERE HYBRIDS OF MACROCYSTIS (PHAEOPHYCEAE)1

All combinations of individuals of Macrocystis pyrifera (L.) C. Agardh, M. integrifolia Bory, and M. angustifolia Bory hybridized. Gametophyte isolates obtained from 18 individuals were used, including M. pyrifera and M. integrifolia from the extremes of their Northern Hemisphere ranges along the Pacific Coast of North America and M. pyrifera and M. angustifolia from Tasmania, Australia. All combinations of gametophytes produced sporophytes of normal morphology, with the exception of crosses involving three gametophyte isolates. One female (M. integrifolia) and two male (M. pyrifera and M. angustifolia)gametophyte isolates were unable to produce normal sporophytes in combination with gametophytes of the opposite sex. Some cultures of female gametophytes produced abnormally shaped parthenogenetic sporophytes. Gametophytes and sporangia of M. pyrifera had n= 16 chromosomes. The M. integrifolia female gametophyte that was unable to produce normal sporophytes had n = ca. 32 chromosomes. These results show that these species of Macrocystis have not become reproductively isolated. Although these species may be considered conspecific according to the biological species concept, we recommend that they continue to be recognized as separate species based on morphological differences.     

EFFECT OF HIGH IRRADIANCE ON RECRUITMENT OF THE GIANT KELP MACROCYSTIS (PHAEOPHYTA) IN SHALLOW WATER1

Laboratory and field experiments were done hi Still-water Cove, Carmel Bay, California, and Monterey Harbor, California, to determine the effect of photosynthetically active radiation (PAR) on the shallow (upper) limit of giant kelp, Macrocystis pyrifera (L.) C. Agardh. At shallow depths, M. pyrifera did not recruit or grow to macroscopic size from gametophytes or embryonic sporophytes transplanted to vertical buoy lines; sharp decreases in PAR with depth coincided with observed recruitment and sporophyte distributions. Shade manipulations indicated that settlement of M. pyrifera zoospores was decreased, but not prohibited, by high PAR. Postsettlement stages (gametophytes and embryonic sporophytes), however, survived only under shade. These results suggest that high PAR can inhibit the recruitment of M. pyrifera to shallow water by killing its postsettlement stages; whether or not ultraviolet (UV) radiation also inhibits recruitment was not tested. In either case, however, it appears that high irradiance (PAR and/or UV) regulates the shallow limit of M. pyrifera prior to temperature and desiccation stresses inherent to intertidal regions. In an additional experiment, recruitment or growth of transplanted gametophytes or embryonic sporophytes of Macrocystis integrifolia Bory also did not occur at shallow depths, suggesting that this shallow water species accesses high irradiance regions via a method other than sexual reproduction.     

VARIATION IN NITROGEN PHYSIOLOGY AND GROWTH AMONG GEOGRAPHICALLY ISOLATED POPULATIONS OF THE GIANT KELP,MACROCYSTIS PYRIFERA (PHAEOPHYTA)1

Three geographically isolated populations of the giant kelp, Macrocystis pyrifera (L.) C. Ag., were examined for responses to nitrate availability in batch culture experiments using juvenile sporophytes reared from spores in the laboratory. Although maximum rates of nitrate-saturated growth were similar among groups, there were significant quantitative differences in the response to nitrate limitation that can be related to natural patterns of nutrient availability at these sites. Plants from Santa Catalina Island (most oligotrophic) achieved maximum growth rates at ambient nitrate concentrations that were lower than those for plants from Monterey Bay, California (most eutrophic), or Refugio State Beach (near Santa Barbara, California). Tissue nitrogen and amino acid concentrations were highest in plants cultured from Santa Catalina Island populations at all external nitrate concentrations, suggesting that differences in nitrate requirements for growth may reflect the efficiency of nitrate uptake and assimilation at subsaturating nitrate concentrations. Given the different physical environments from which these plants came, the data suggest that geographically isolated populations of M. pyrifera have undergone genetic divergence that can be explained by ecotypic adaptation to unique habitat conditions at these sites.     

CRITICAL IRRADIANCE LEVELS AND THE INTERACTIVE EFFECTS OF QUANTUM IRRADIANCE AND DOSE ON GAMETOGENESIS IN THE GIANT KELP,MACROCYSTIS PYRIFERA1

Gametophytes of Macrocystis pyrifera (L.) C. Ag. were cultured under a series of quantum irradiances in three photoperiod regimes. The quantum irradiances in each photoperiod were adjusted to provide equal daily irradiation dosages between photoperiods which allowed a critical examination of the interactions between quantum irradiance and quantum dose in determining gametophyte fertility. The lowest quantum irradiance which stimulated gametogenesis in more than 50% of the female gametophytes was 5 μE·m^?2·s^?1. The saturating irradiance was ca. 10 μE·m^?2·s^?1 at photoperiods of 12 h or greater. In terms of daily quantum dose, the lowest dose at which greater than 50% gametogenesis occurred was 0.2 E·m^?2·d^?1. However, this critical quantum dose was higher (0.4 E·m^?2·d^?1) when instantaneous irradiances were less than 5 μE·m^?2·s^?1. The saturation quantum dose was also affected by the rate at which the quantum dose was received and varied from 0.4 to 0.8 E·m^?2·d^?1. Gametophytes in all three photoperiods reached 100% fertility at quantum irradiances above 5 μE·m^?2·s^?1. Photoperiod effects were small and could be accounted for by quantum dosage effects.     

PHOTOSYNTHETIC PERFORMANCE,LIGHT ABSORPTION,AND PIGMENT COMPOSITION OF MACROCYSTIS PYRIFERA (LAMINARIALES,PHAEOPHYCEAE) BLADES FROM DIFFERENT DEPTHS1

Macrocystis pyrifera (L.) C. Agardh is a canopy‐forming species that occupies the entire water column. The photosynthetic tissue of this alga is exposed to a broad range of environmental factors, particularly related to light quantity and quality. In the present work, photosynthetic performance, light absorption, pigment composition, and thermal dissipation were measured in blades collected from different depths to characterize the photoacclimation and photoprotection responses of M. pyrifera according to the position of its photosynthetic tissue in the water column. The most important response of M. pyrifera was the enhancement of photoprotection in surface and near‐surface blades. The size of the xanthophyll cycle pigment pool (XC) was correlated to the nonphotochemical quenching (NPQ) of chl a fluorescence capacity of the blades. In surface blades, we detected the highest accumulation of UV‐absorbing compounds, photoprotective carotenoids, ΣXC, and NPQ. These characteristics were important responses that allowed surface blades to present the highest maximum photosynthetic rate and the highest PSII electron transport rate. Therefore, surface blades made the highest contribution to algae production. In contrast, basal blades presented the opposite trend. These blades do not to contribute significantly to photosynthetate production of the whole organism, but they might be important for other functions, like nutrient uptake.     

ARRESTED DEVELOPMENT OF GIANT KELP (MACROCYSTIS PYRIFERA,PHAEOPHYCEAE) EMBRYONIC SPOROPHYTES: A MECHANISM FOR DELAYED RECRUITMENT IN PERENNIAL KELPS?1

Delayed recruitment of microscopic stages in response to cyclical cues is critical to the population dynamics of many annual and seasonally reproducing perennial seaweeds. Microscopic stages may play a similar role in continuously reproducing perennials in which adult sporophytes are subject to episodic mortality, if they can respond directly to the unpredictable onset and relaxation of unfavorable conditions. We experimentally evaluated the potential for temporary reduction in limiting resources (light, nutrients) to directly delay recruitment of giant kelp (Macrocystis pyrifera (L.) C.A. Agardh) gametophytes and embryonic sporophytes. Laboratory cultures were subjected to limiting conditions of light and nutrients for 1 month and then exposed to nonlimiting conditions for 10 days. Gametophytes in all treatments failed to recruit to sporophytes after 2 weeks, suggesting they are not a source of delayed recruitment in giant kelp. Sporophytes in light‐limited treatments, however, survived and grew significantly slower than non–light‐limited controls. When stimulated with light, light‐limited sporophytes grew from 2 to>10 times faster than unstimulated controls depending on nutrient availability. These results suggest that limiting resources can delay recruitment of embryonic giant kelp sporophytes for at least 1 month. Flexible timing of recruitment from embryonic sporophytes may enhance persistence of continuously reproducing perennial species when mac‐ roscopic adults are subject to episodic large‐scale removals.     

PRESENCE OF SPOROPHYLLS IN FLOATING KELP RAFTS OF MACROCYSTIS SPP. (PHAEOPHYCEAE) ALONG THE CHILEAN PACIFIC COAST1

Some species of macroalgae continue to live for extended periods of time after detachment and may even maintain reproductive structures, yet very little is known about this process. Here, we describe the presence of sporophylls (with sporogenous tissues) on floating kelp rafts of Macrocystis spp. along the coast of Chile. Surveys were conducted at nine sites (18–50° S) during austral summer 2002, and floating kelp rafts were seen and collected at seven of these nine sites (between 22 and 50° S). Fifteen (26.8%) of the 56 samples had sporophylls, indicating maintenance of sporophylls after detachment. Some of the kelp sporophytes with reproductive blades showed signs of having been afloat for long periods (indicated by the large size of attached stalked barnacles). Additionally, experiments showed that floating kelps released viable zoospores. To understand the reproductive dynamics of floating kelps, we compared these results with information from attached populations of Macrocystis spp. at nearby coastal sites. In general, attached kelp had higher proportions of sporophylls than floating rafts, suggesting that detachment may negatively affect reproductive status. Nevertheless, floating kelps remained functionally reproductive, suggesting that zoospores may be dispersed via floating rafts. Published reports on other macroalgae indicate that some species (Lessoniaceae, Fucaceae, and Sargassaceae) are fertile and probably release zoospores or zygotes while floating or drifting in ocean currents. Because dispersal distances achieved by spores of most macroalgae are relatively short, release of spores from floating algae may be an alternative mechanism of long‐distance dispersal.     

CONTRIBUTION OF TEMPORAL AND SPATIAL COMPONENTS TO MORPHOLOGICAL VARIATION IN THE KELP ECKLONIA (LAMINARIALES)1

Environmental conditions that are known to cause morphological variation in algae (e.g., wave exposure) often vary in both space and time and are superimposed onto the distinct seasonal growth cycles of most temperate macroalgae. We tested the hypothesis that the morphology of the small kelp Ecklonia radiata (C. Agardh) J. Agardh is the product of an interaction between site (five reefs of different wave exposure) and the time of year that sampling occurs (summer vs. winter 2004). We determined that wave exposure had a strong directional effect on kelp morphology, with “Reefs” accounting for up to 43.4% of variation in individual morphological characters. “Times” had a narrowly nonsignificant effect on overall morphology but accounted for up to 31% of variation in individual characters. Many characters were affected by wave exposure, whereas only a few were (strongly) affected by time (e.g., thallus biomass). Interactive effects between “Reefs” and “Times” were generally small, accounting for 15.8% of variation in lamina thickness, but much less for most other characters. We conclude that wave exposure exerts a strong control over the morphology of E. radiata, but that the nature of the effect depends on the magnitude of wave exposure. We also conclude that most of the effects of wave exposure are consistent through time and do not interact with cycles of growth and pruning in any major way.     

REPRODUCTIVE PLASTICITY IN THE KELP ALARIA NANA (PHAEOPHYCEAE)1

Unlike terrestrial plants, reproductive control in algae has not been attributed to biotic factors or internal chemical signals. However, experimental removal of reproductive structures (sporophylls) from the kelp Alaria nana Shrader resulted in the development of functional reproducing structures on the vegetative frond in 45% (10/22) of the experimental plants but not on the control plants where sporophylls were left intact (n = 35). In naturally occurring Alaria nana, this compensatory reproductive response occurred at a low incidence and was restricted primarily to larger individuals in the population. Thus, compensation and developmental plasticity, mediated In plant size, may play an important role in how algae respond to reproductive tissue loss.     

FLOW‐INDUCED MORPHOLOGICAL VARIATIONS AFFECT DIFFUSION BOUNDARY‐LAYER THICKNESS OF MACROCYSTIS PYRIFERA (HETEROKONTOPHYTA,LAMINARIALES)1

In slow mainstream flows (<4–6 cm · s^?1), the transport of dissolved nutrients to seaweed blade surfaces is reduced due to the formation of thicker diffusion boundary layers (DBLs). The blade morphology of Macrocystis pyrifera (L.) C. Agardh varies with the hydrodynamic environment in which it grows; wave‐exposed blades are narrow and thick with small surface corrugations (1 mm tall), whereas wave‐sheltered blades are wider and thinner with large (2–5 cm) edge undulations. Within the surface corrugations of wave‐exposed blades, the DBL thickness, measured using an O₂ micro‐optode, ranged from 0.67 to 0.80 mm and did not vary with mainstream velocities between 0.8 and 4.5 cm · s^?1. At the corrugation apex, DBL thickness decreased with increasing seawater velocity, from 0.4 mm at 0.8 cm · s^?1 to being undetectable at 4.5 cm · s^?1. Results show how the wave‐exposed blades trap fluid within the corrugations at their surface. For wave‐sheltered blades at 0.8 cm · s^?1, a DBL thickness of 0.73 ± 0.31 mm within the edge undulation was 10‐fold greater than at the undulation apex, while at 2.1 cm · s^?1, DBL thicknesses were similar at <0.07 mm. Relative turbulence intensity was measured using an acoustic Doppler velocimeter (ADV), and overall, there was little evidence to support our hypothesis that the edge undulations of wave‐sheltered blades increased turbulence intensity compared to wave‐exposed blades. We discuss the positive and negative effects of thick DBLs at seaweed surfaces.     

REPRODUCTION AND MORPHOLOGICAL VARIATION IN SOUTHERN CALIFORNIA POPULATIONS OF THE LOWER INTERTIDAL KELP EGREGIA MENZIESII (LAMINARIALES)1

Intertidal Egregia menziesii (Turner) Aresch. populations were studied at three Southern California sites to determine temporal and spatial patterns of reproduction and morphology. The timing of sporophyll production and sporophyte recruitment was similar at all sites. Sporophyll production was much greater during winter periods of colder seawater temperatures and shorter day lengths. Sporophyte recruitment occurred from spring through midsummer, ～5 months following maximal sporophyll production. Lateral blade morphologies varied in a consistent manner, suggesting a developmental mechanism for form variation in Egregia thalli. Spatulate blades dominated shorter axes and the bases of longer axes, whereas filiform laterals became abundant toward the tips of longer axes. Filiform laterals (9.8 mg O₂·g^?1·h^?1) had higher light‐saturated net photosynthetic rates than spatulate laterals (6.8 mg O₂·g^?1·h^?1), resulting in a 12% increase in the productivity of Egregia per meter of filiform frond.     

GRAZING ON GIANT KELP MICROSCOPIC PHASES AND THE RECRUITMENT SUCCESS OF ANNUAL POPULATIONS OF MACROCYSTIS PYRIFERA (LAMINARIALES,PHAEOPHYTA) IN SOUTHERN CHILE1

The giant kelp Macrocystis pyrifera (L.) C. Agardh is widely distributed in the Northern Hemisphere and Southern Hemisphere, yet it exhibits distinct population dynamics at local to regional spatial scales. Giant kelp populations are typically perennial with the potential for year‐round reproduction and recruitment. In southern Chile, however, annual giant kelp populations exist and often persist entirely on secondary substrata (e.g., shells of the slipper limpet Crepipatella fecunda [Gastropoda, Calyptraeidae]) that can cover up to 90% of the rocky bottom. In these populations, the macroscopic sporophyte phase disappears annually during winter and early spring, leaving a 3–4 month period in which a persistent microscopic phase remains to support the subsequent year’s recruitment. We tested the effects of a suite of grazers on the recruitment success of this critical microscopic phase at two sites in southern Chile. Field experiments indicated that the snail Tegula atra negatively impacted M. pyrifera sporophyte recruitment, but that recruitment was highest in the presence of sessile female limpets, C. fecunda. Conversely, small male C. fecunda (biofilm grazers) did not regulate kelp recruitment. Laboratory observations showed that C. fecunda males only grazed on microscopic kelp gametophytes and small (<250 μm) sporophytes, rejecting larger sporophytes, whereas T. atra grazed on all the kelp stages. Recruitment to the C. fecunda treatments far exceeded that to bare rock in the absence of grazers but was not due to the physical presence of C. fecunda shells. We concluded that the key to M. pyrifera recruitment success in southern Chile is its capacity to colonize secondary substrates provided by the slipper limpet C. fecunda.     

PHENOTYPIC PLASTICITY INDUCED IN TRANSPLANT EXPERIMENTS IN A MUTUALISTIC ASSOCIATION BETWEEN THE RED ALGA JANIA ADHAERENS (RHODOPHYTA,CORALLINALES) AND THE SPONGE HALICLONA CAERULEA (PORIFERA: HAPLOSCLERIDA): MORPHOLOGICAL RESPONSES OF THE ALGA1

The association between the red macroalga Jania adhaerens J. V. Lamour. and the sponge Haliclona caerulea is the most successful life‐form between 2 and 4 m depth in Mazatlán Bay (Mexican Pacific). J. adhaerens colonizes the rocky intertidal area and penetrates into deeper areas only when it lives in association with H. caerulea. The aposymbiotic form of the sponge has not been reported in the bay. To understand the ecological success of this association, we examined the capacity of J. adhaerens to acclimate in Mazatlán Bay using transplant experiments. The transplanted aposymbiotic J. adhaerens did not survive the first 2 weeks; however, J. adhaerens when living in association with H. caerulea, acclimated easily to depth, showing no sign of mortality during the 103 d of the experiment. We conclude that the ability of J. adhaerens to colonize in deeper areas in this hydrodynamic environment may in part rely on the protection provided by the sponge to the algal canopy. Both species contribute to the shape of the associated form. Nevertheless, the morphological variation in the association appears to be dominated by the variation in J. adhaerens canopy to regulate pigment self‐shading under light‐limited conditions and/or tissue resistance under high hydrodynamics. Consequently, our results are consistent with light as the abiotic controlling factor, which regulates the lower depth distribution of the association in Mazatlán Bay, through limiting the growth rate of J. adhaerens. Hydrodynamics may determine the upper limit of the association by imposing high mass losses.