Short-term dispersal of kelp fauna to cleared (kelp-harvested) areas

The kelp Laminaria hyperborea forms large forests and houses a numerous and diverse fauna, especially in the kelp holdfast and stipe epiphytes. Kelp harvesting creates cleared areas and fragmentizes the kelp forest. We investigated the dispersal ability of kelp fauna to cleared, harvested areas by studying their colonization pattern to artificial substrata (kelp mimics) exposed for a short (3 days) and longer time period (35 days) at different sites within the kelp forest (one site) and at a cleared area (two sites). Most of the kelp fauna (111 species) showed a rapid dispersal and colonized the artificial substrata within the cleared area. The similarity of the faunal community in the mimics with the natural kelp holdfast community increased with the length of the exposure period. During the experiments, 87% of the mobile species in the kelp plants were found in the kelp mimics, indicating good dispersal for slow-moving animals like gastropods, polychaetes and tube-building crustaceans. Relating the frequency of the different faunal groups in the untrawled kelp forest to their frequency in the kelp mimics, showed gastropods, amphipods and decapods to have relatively high dispersal rates, whereas isopods, bivalves, polychaetes and tanaids showed a lower dispersal rate than expected. Amphipods dispersed as juveniles and adults. No significant differences were found between the faunal composition and number of species in the mimics placed inside the kelp forest and in the cleared area. Remaining holdfasts and pebbles were identified as refuges/alternative habitats in the harvested area, and may together with the nearest kelp vegetation, serve as sources for colonization to new substrata. The high dispersal ability of most of the kelp fauna provides maintenance of the faunal composition of disturbed habitats and ensures colonization of recovering algal habitats regardless of reproduction strategy.     

Patterns of abundance and assemblage structure of epifauna inhabiting two morphologically different kelp holdfasts

The mobile fauna associated with two sympatric kelp species with different holdfast morphology (Saccorhiza polyschides and Laminaria hyperborea) was compared to test for differences in the assemblage structure of holdfast-associated mobile epifauna. A total of 24,140 epifaunal individuals were counted from 30 holdfasts of each kelp species. Overall epifaunal abundances exceeded faunal abundances previously reported from holdfasts of other kelps. Three taxonomic groups, Amphipoda, Mollusca, and Polychaeta, accounted for ca. 85% of all individuals. Total abundances increased with the amount of habitat available, quantified either as the volume or the area provided by the holdfasts. The multivariate structure of the epifaunal assemblage did not differ between holdfasts of the two kelp species. However, epifaunal assemblages responded differentially to the habitat attributes provided by each type of kelp holdfast: multivariate variation in the assemblage structure of epifauna was mostly explained by holdfast area and volume for L. hyperborea, and by the surface-to-volume ratio for S. polyschides holdfasts. Therefore, the physical attributes of biogenic habitats, here kelp holdfasts that better predict patterns in the assemblage structure of associated fauna can differ according to their different physical morphology, even though the overall assemblage structure of associated fauna was similar.     

Changes in the fish fauna associated with a sub-Antarctic <Emphasis Type="Italic">Macrocystis pyrifera</Emphasis> kelp forest in response to canopy removal

The fish fauna associated with a Macrocystis pyrifera forest and the effects of the canopy removal on this fauna were studied in the Beagle Channel, Tierra del Fuego. Seasonal changes and differences in the fish communities that inhabit the canopy and the holdfast were also studied. Two patches of kelp forest separated by 200 m were selected. In one, the canopy was removed periodically while the other was used as a control. Samples were collected seasonally, from autumn 1999 to 2001. Fish fauna in the water column was sampled using trammel nets and holdfast fish fauna was sampled by removing the complete holdfast. Different assemblages of fish species were captured in the water column (surface and bottom) and in the holdfast. In the former there were mainly pelagic and benthopelagic species and the latter predominantly demersal species. The principal effect of the canopy removal was a drop in the abundance (and total weight) of Paranotothenia magellanica (doradito), the species with the strongest relationship with the canopy, principally at the surface of the treated patch probably due to a reduction in the availability of refuges. A high degree of seasonality was observed for fish species’ total weight, abundance and diversity, with higher values in summer and autumn. Taking into account our data and available data on kelp growth in the same locality, we suggest of cutting the kelp forest (preferably those farther away from the coast) once a year in winter or early spring, in order to minimize impact on the fish community.     

Seasonal changes in taxon richness and abundance of mobile invertebrates inhabiting holdfast of annual kelp Ecklonia radicosa (Phaeophyceae,Lessoniaceae) at the central Pacific coast of Japan

To investigate seasonal changes in the taxon richness and abundance of mobile invertebrates inhabiting holdfasts of the warm temperate annual kelp Ecklonia radicosa, five holdfasts were collected monthly at the central Pacific coast of Japan from April to November 2014. During the study period, there was little variation in holdfast height and diameter, which ranged from 5.9 to 8.5 cm and from 7.1 to 10.8 cm, respectively. In total 7087 animals were collected from 40 holdfasts (177.2 individuals inds./holdfast, on average). The number of mobile invertebrates gradually increased from May (15 ± 9.9 inds./holdfast) to August (346 ± 152.5 inds./holdfast), with over 300 inds./holdfast until October before rapidly decreasing in November (110 ± 85.6 inds./holdfast). Similarly, taxon richness increased gradually from April (4.3 ± 1.0 taxa/holdfast) to August (11.0 ± 3.7 taxa/holdfast), and decreased in November (8.6 ± 2.3 taxa/holdfast). Interestingly, hundreds of mobile invertebrates inhabited holdfasts of kelp plants that had shed their blade in October and November. Taxon composition from August to October and the number of invertebrates from July to November were comparable to data previously reported for perennial kelps. In this study, the importance of annual Ecklonian species as biogenic habitats was demonstrated for the first time.     

Ecological effects of harvesting Lessonia (Laminariales,Phaeophyta) in central Chile

Lessonia nigrescens and L. trabeculata are economically important canopy-forming kelps in Chile. Experimental harvesting of stipes above the first dichotomy reduces stipe movement and inter-stipe friction, allowing the development of a heavy epiphytic load and increased grazing. Complete stipe removal leads to holdfast death as neither species is able to simultaneously regenerate all stipes. The invertebrate fauna inside the holdfast does not respond to upper canopy changes, but mortality does occur in partial or complete plant removals. Kelp removal also affects inter-plant distances, results in increased access of grazers to the outside and inside of kelp holdfasts, reduces recruitment of other algal species, and modifies the morphology of L. trabeculata such that the plants become more susceptible to removal by water movement.     

Evaluating stress in rocky shore and shallow reef habitats using the macrofauna of kelp holdfasts

Kelp often dominates hard rocky substrata in the coastal watersof temperate, boreal and subantarctic regions of the world's oceans. The habitat providedby kelp holdfasts supports high species richness and high abundance of a range of invertebratetaxa. Studies conducted on the effects of stress on these communities have indicated that theyare sensitive to stressors and that the response is often predictable. With the growing body ofliterature on natural variation in holdfast community structure, studies of the impacts of stressare becoming more readily interpretable making the macrofauna of this habitat a potentiallyimportant tool for evaluating anthropogenic effects on hard substrata in coastal environments. Limitations of the use of holdfast fauna in studies of stress include the fact that brownseaweeds may disappear from polluted areas of reef, and where they persist, holdfaststructure may be altered directly by the stressor. An additional problem is that manipulative,experimental studies are difficult. Artificial holdfasts may provide a means by which thedemonstrated sensitivity of the holdfast community can be combined with a manipulative approach to testspecific hypotheses related to effects of stress in shallow coastal environments.     

Artificial substrata in a shallow sublittoral habitat: do they adequately represent natural habitats or the local species pool?

Artificial substrata have been advocated as tools which have considerable potential for monitoring both natural and anthropogenic effects on invertebrate communities of shallow coastal environments. In this experiment, community structure was compared between two dominant natural algal habitats (kelp holdfasts and algal turf) and artificial substratum units (ASUs; nests of pan scourers) deployed in close contact with, and 20 cm above the substratum. Univariate and multivariate statistical analyses were applied to the data to determine the similarity of community structure between the four different habitats. In addition, recently developed measures of taxonomic distinctness were applied to the data from both sets of artificial substrata to determine if they provided a representative sample of the local epifaunal species pool and thus have the potential to be used as surrogate samples for this important faunal group. There were marked differences between community structure in each of the habitats. Both sets of artificial substrata were dominated by tubicolous polychaetes with abundances that were more than an order of magnitude greater than in the holdfast and turf samples. The fauna recruiting to the artificial substrata deployed above the substratum showed the lowest values in the univariate summaries of diversity and evenness and were unrepresentative of the local species pool. Artificial samples deployed in contact with the substratum showed greater diversity and evenness but were still mostly unrepresentative of the local species pool. The tendency for both sets of artificial substrata to under-sample amphipods and to be dominated by suspension-feeding polychaetes suggests that methods using these units may be relatively insensitive to the effects of anthropogenic impacts (e.g. sewage outfalls) where shifts in community structure including increased dominance of suspension-feeders and polychaetes and a reduced dominance of amphipods have been observed. Further studies, including the evaluation of temporal variation in community structure related to the time at which the ASUs are deployed and duration of deployment, are needed to test the wider utility of artificial substrata as tools for monitoring shallow, sublittoral, epifaunal communities.     

Kelp rafts in the Southern Ocean

Kelp rafts were surveyed during three summer traverses between Hobart and Macquarie Island (subAntarctic) to determine the potential for dispersal of kelp‐associated macroinvertebrates across the latitudes of 46–53°S. Rafts of Durvillaea antarctica dominated the sightings (94% of rafts) and extrapolations to the full Southern Ocean between these latitudes indicate a figure of over 70 million rafts afloat at any one time, 20 million of which support a holdfast, the habitat supporting the highest faunal diversity in attached kelp plants. In contrast, few, small rafts of Macrocystis pyrifera were observed. The potential for dispersal of fauna is presumed to be related to the species of kelp with which they are associated. Empirical studies of survival of animals while drifting at sea, and also on making land‐fall, are required to allow fuller interpretation of the significance of these findings.     

Artificial structures influence fouling on habitat-forming kelps

The addition of artificial structures along urbanised shorelines is a global phenomenon. Such modifications of habitats have important consequences to the abundance of fouling organisms on primary substrata, but the influence on fouling of habitat-formers living on these structures is poorly understood. Fouling of habitat-forming kelps Ecklonia radiata on pier-pilings was compared to that on rocky reefs at three locations in Sydney Harbour. Kelps on pilings supported different assemblages of bryozoans from those on reefs. The abundances of bryozoans on kelps, in particular of the non-indigenous species Membranipora membranacea, were significantly greater on pilings. Differences were consistent in time and space. This indicates that the addition of artificial structures also affects fouling on secondary biogenic substrata, altering biodiversity and potentially facilitating the introduction and dispersal of non-indigenous epibiota. Understanding the processes that cause these patterns is necessary to allow sensible predictions about ecological effects of built structures.     

Artificial structures influence fouling on habitat-forming kelps

The addition of artificial structures along urbanised shorelines is a global phenomenon. Such modifications of habitats have important consequences to the abundance of fouling organisms on primary substrata, but the influence on fouling of habitat-formers living on these structures is poorly understood. Fouling of habitat-forming kelps Ecklonia radiata on pier-pilings was compared to that on rocky reefs at three locations in Sydney Harbour. Kelps on pilings supported different assemblages of bryozoans from those on reefs. The abundances of bryozoans on kelps, inparticular of the non-indigenous species Membranipora membranacea, were significantly greater on pilings. Differences were consistent in time and space. This indicates that the addition of artificial structures also affects fouling on secondary biogenic substrata, altering biodiversity and potentially facilitating the introduction and dispersal of non-indigenous epibiota. Understanding the processes that cause these patterns is necessary to allow sensible predictions about ecological effects of built structures.     

Settlement and post-settlement processes limit the abundance of the geniculate coralline alga Calliarthron on subtidal walls

Cover of geniculate coralline algae is significantly greater on subtidal platforms than on vertical walls in a central California kelp forest. To determine the cause(s) of this pattern, settlement and growth of Calliarthron were compared on horizontal and vertical substrata attached to platforms or walls. Recruitment on horizontal substrata attached to reef walls indicated that spores were abundant along the walls. Densities of recruits were significantly smaller on vertical substrata attached to walls than on horizontal substrata attached to platforms, indicating that Calliarthron settlement is reduced on vertical substrata. Growth of basal crusts and the densities of fronds were also significantly greater on horizontal substrata attached to platforms than on vertical substrata attached to walls. Irradiance reaching wall surfaces was less than irradiance reaching platform surfaces. We suggest that light reaching wall surface is low enough to affect Calliarthron growth. These results indicate that the low abundance of Calliarthron on subtidal walls is a result of poor recruitment and slow growth.     

Benthic macroalgae as a dispersal mechanism for fauna: influence of a marine tumbleweed

Effective dispersal is problematic for benthic organisms without planktonic larvae; rafting and vertical migrations are mechanisms that can potentially be employed by such fauna, but these strategies entail considerable predation risk as well as other disadvantages. Unattached, but non-floating, “drift” algae harbor large numbers of fauna and may serve as an alternative dispersal mechanism in some systems. This paper reports field manipulations in Florida Bay, Florida, USA designed to determine (1) if such algae can disperse benthic animals, and (2) if dispersal efficiency varies as a function of two common substrata types: seagrass and bare sediment. A live immersion stain was used to mark faunal associates of Laurencia spp. algal clumps in situ. The fidelity of molluscs, decapods, ophiuroids, and fishes to stationary algal clumps was then compared with the fidelity of these animals to clumps that were forced to tumble over a given distance with a blower apparatus; these experiments were performed over both sand and seagrass substrata. Measurements of frequency, spatial extent, and rate of algal drift were made to aid in assessing the potential importance of benthic algae as a dispersal mechanism.

Algal clumps often rolled in a manner similar to that of terrestrial tumbleweeds; mark-recapture work showed that algal clumps can move up to 0.5 km/day and that algal drift is a frequent phenomenon. The algal masses were effective transporters of benthic fauna, including mobile shrimps and fishes; dispersal was more efficient over sand than over seagrass. Dispersal of fauna via this mobile habitat should entail lower risk than other adult dispersal stratagems such as vertical migration or rafting; this mechanism would be most advantageous for brooding species or those with limited planktonic phases. Differential fidelity to clumps tumbling across seagrass versus sand suggests that the algae could facilitate exchange of fauna between isolated seagrass patches.     

Kelp and sea urchin settlement mediated by biotic interactions with benthic coralline algal species

Species interactions can influence key ecological processes that support community assembly and composition. For example, coralline algae encompass extensive diversity and may play a major role in regime shifts from kelp forests to urchin-dominated barrens through their role in inducing invertebrate larval metamorphosis and influencing kelp spore settlement. In a series of laboratory experiments, we tested the hypothesis that different coralline communities facilitate the maintenance of either ecosystem state by either promoting or inhibiting early recruitment of kelps or urchins. Coralline algae significantly increased red urchin metamorphosis compared with a control, while they had varying effects on kelp settlement. Urchin metamorphosis and density of juvenile canopy kelps did not differ significantly across coralline species abundant in both kelp forests and urchin barrens, suggesting that recruitment of urchin and canopy kelps does not depend on specific corallines. Non-calcified fleshy red algal crusts promoted the highest mean urchin metamorphosis percentage and showed some of the lowest canopy kelp settlement. In contrast, settlement of one subcanopy kelp species was reduced on crustose corallines, but elevated on articulated corallines, suggesting that articulated corallines, typically absent in urchin barrens, may need to recover before this subcanopy kelp could return. Coralline species differed in surface bacterial microbiome composition; however, urchin metamorphosis was not significantly different when microbiomes were removed with antibiotics. Our results clarify the role played by coralline algal species in kelp forest community assembly and could have important implications for kelp forest recovery.     

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.     

Modelling secondary succession of neotropical mangroves: Causes and consequences of growth reduction in pioneer species

Mangrove forest structure is the result of interactions between species responses to abiotic stress factors, disturbance, dispersal and competition. The combination of abiotic conditions and disturbance history may determine the growth potential of the species, whereas dispersal, competition and external or biogenic changes in abiotic conditions may tune their succession. Even in forests with only a few species, this set of factors can create multiple species compositions. For example, pure stands of the mangrove Laguncularia are common in newly colonized areas, but they may evolve into mixed stands dominated by Rhizophora or Avicennia. We demonstrate such an evolution with a field study of mangroves established in abandoned rice fields. We use an individual-based simulation model to understand how species-dependent dispersal, growth rate and shade tolerance lead to the observed patterns in horizontal and vertical forest structure. We find that the initially dominating species will be gradually replaced in the canopy. This alteration cannot be explained exclusively by shade tolerance of the succeeding species. Rather, it is essential to assume that the height growth rate of the pioneer species slows down relative to those of the later species. A decrease in nutrient availability may be responsible for these relative changes in the growth rates of different species.     

Assessing species richness of macrofauna associated with macroalgae in Arctic kelp forests (Hornsund,Svalbard)

Kelp beds support diverse and productive benthic coastal ecosystems and are often perceived as cold water analogs of tropical coral reefs, yet the levels of species richness in polar regions have remained largely unexplored. The present study aims to assess the magnitude of macrozoobenthic species richness associated with the macroalgae in an Arctic kelp forest. The study was conducted in Hornsund, a high latitude Arctic fjord off west Spitsbergen (76–77°N). A total of 403 samples (i.e., individual algae) were collected by scuba divers at three sites located along the fjord axis at varying depths (5–10 m). The most common invertebrate species and the sample species richness (mean 11.5 species per sample) were consistent among the most common algal species (Laminaria digitata, Saccharina latissima, Alaria esculenta, Desmarestia aculeta, Odonthalia dentata, Phycodrys rubens). Fauna associated with overstory kelps was concentrated on the holdfasts. A total of 208 species were identified; Bryozoa, Polychaeta and Hydrozoa yielded the highest numbers of species (70, 52 and 37, respectively). The non-parametric Chao2 estimator of true species richness gave an estimate of 259 species (with 95% confidence intervals from 234 to 308). Despite the high sampling effort, the total species richness was not captured. This may stem from the high level of rarity within the fauna associated with kelps; 38% of the species occurred only in one or two samples. The studied fauna was much less diverse than similar assemblages surveyed at lower latitudes. The high numbers of species and the common occurrence of colonial forms (Bryozoa, Hydrozoa) in the current study show that these taxa cannot be omitted in macrobenthic diversity surveys performed within kelp habitats.     

Long distance kelp rafting impacts seaweed biogeography in the Northeast Pacific: the kelp conveyor hypothesis

Routine DNA barcoding of the Haida Gwaii seaweed flora revealed “endemic species” attributed initially to this region's past as a glacial refugium. However, subsequent barcode records from central California rapidly eroded this list leaving species characterized by disjunct distributions (DD) between California and Haida Gwaii. This observation prompted a more detailed look at species for California and British Columbia and revealed that 33 of 180 DNA‐barcoded genetic groups in common between these regions (~18%) predominantly displayed DD between California and northern British Columbia. A previous discovery that a red abalone shell found in Haida Gwaii (far north of its range) had a float‐bearing kelp (Nereocystis luetkeana) holdfast attached to it prompted a closer consideration of the COI‐5P barcode data in support of a “kelp conveyor hypothesis.” The hypothesis posits that there has been a net migration of Californian species to northern British Columbia the vector being species growing on substrata carried along with kelp rafts on the winter Davidson Current.     

The variable routes of rafting: stranding dynamics of floating bull kelp Durvillaea antarctica (Fucales,Phaeophyceae) on beaches in the SE Pacific

Dispersal on floating seaweeds depends on availability, viability, and trajectories of the rafts. In the southern hemisphere, the bull kelp Durvillaea antarctica is one of the most common floating seaweeds, but phylogeographic studies had shown low connectivity between populations from continental Chile, which could be due to limitations in local supply and dispersal of floating kelps. To test this hypothesis, the spatiotemporal dynamics of kelp strandings were examined in four biogeographic districts along the Chilean coast (28°–42°S). We determined the biomass and demography of stranded individuals on 33 beaches for three subsequent years (2013, 2014, 2015) to examine whether rafting is restricted to certain districts and seasons (winter or summer). Stranded kelps were found on all beaches. Most kelps had only one stipe (one individual), although we also frequently found coalesced holdfasts with mature males and females, which would facilitate successful rafting dispersal, gamete release, and reproduction upon arrival. High biomasses of stranded kelps occurred in the northern‐central (30°S–33°S) and southernmost districts (37°S–42°S), and lower biomasses in the northernmost (28°S–30°S) and southern‐central districts (33°S–37°S). The highest percentages and sizes of epibionts (Lepas spp.), indicative of prolonged floating periods, were found on stranded kelps in the northernmost and southernmost districts. Based on these results, we conclude that rafting dispersal can vary regionally, being more common in the northernmost and southernmost districts, depending on intrinsic (seaweed biology) and extrinsic factors (shore morphology and oceanography) that affect local supply of kelps and regional hydrodynamics.     

Morphological,ultrastructural, and genetic characterization of coalescence in the intertidal and shallow subtidal kelps Lessonia spicata and L. berteroana (Laminariales,Heterokontophyta)

Coalescing macroalgae may fuse with conspecifics, forming genetically heterogeneous entities known as chimera. This process has been shown in taxa from roughly half the red algal orders and in the Codium species, a green alga. Field observations indicate that common and dominant kelps along central Chile exhibit a fused holdfast. We evaluated whether such fusions are true coalescence processes in Lessonia spicata and Lessonia berteroana. To this end, we characterized the ultrastructural event involved in holdfast fusion in the laboratory. Additionally, coalescence in natural populations was quantified by measuring the frequency of individuals with genetically heterogenic stipes within the same holdfast. Results indicate that coalescence appears as a frequent process in laboratory, mostly restricted to intraspecific fusions. During fusion, the meristodermatic cells located in the contact area modify their morphology and reduce the number of plastids, mitochondria, and cell inclusions. The cell wall becomes much thinner and develops plasmodesmata, enhancing communication with equivalent cells of the other coalescencing individual. Stipe genotyping indicates that there is a widespread occurrence of chimerism in both species and genetic heterogeneity is increasing directly with the increasing number of stipes. The combination of results suggests that kelp frequently coalesce in the field, and the histological response observed approaches that of red algae. Since kelps are part of the dominant vegetation in low intertidal and shallow subtidal beds, the adaptive values of coalescence in these species should be evaluated. It is concluded that coalescence and chimerism are evolutionary convergent processes, occurring in all three major groups of seaweeds.     

Life and death beneath macrophyte canopies: effects of understory kelps on growth rates and survival of marine,benthic suspension feeders

Summary Experiments conducted on rocky bottoms at 7–11 m depth in the San Juan Archipelago, Washington assessed effects of canopies of understory kelps on growth of benthic suspension feeders, determined the mechanisms responsible for effects, and assessed the influence of kelp canopies on survivorship of benthic fauna. Kelp canopics influenced growth rates of diverse suspension feeders. At several sites the musselMytilus edulis, the barnacleBalanus glandula, and the serpulid polychaetePseudochitinopoma occidentalis grew faster on the bottom beneath kelp canopies than on nearby exposed substrata. The cheilostome bryozoanMembranipora membranacea showed a mixed response to kelp canopies, growing faster in exposed regions at one site, but faster beneath canopies at another. There were no differences in growth of 2 other species (the cheilostome bryozoanCheilopora praelonga and the spongeMyxilla incrustans) between kelp and no-kelp treatments; however, some processes influenced by plant canopies affected their growth. Specific mechanisms responsible for kelp effects on growth were assessed in a series of field experiments usingPseudochitinopoma, Membranipora, Cheilopora andMyxilla. Particulate deposition on the bottom, which is more intense beneath canopies, negatively affected growth of all 4 species. Kelps also reduced rates of flow and prevented devented development of microalgal turfs beneath the canopy.Pseudochitinopoma grew faster in the weaker flows below canopies and bothCheilopora andMyxilla grew faster where there were no microalgal turfs. These other effects of kelp canopies were at least as important to growth (in the cases ofCheilopora andMyxilla) or more important to growth (in the case ofPseudochitinopoma) than were the general, deleterious effects of higher sedimentation beneath canopies. The lower growth rates caused by higher sedimentation beneath kelp canopies did not result in higher rates of animal mortality. Surprisingly, kelp canopies typically did not influence mortality due to predation. For 7 of 12 taxa, mortality rates did not differ between kelp-covered and exposed treatments. Significantly higher mortality occurred outside canopies for only 4 of 12 taxa, and for at least 2 of these 4 differences probably were not related to predation.Mytilus, a species rare at these depths, exhibited higher mortality beneath kelp canopies due to predation by crabs. Other macrophytes in fresh and salt water, as well as some benthic animals that create complex, 3-dimensional habitats, should influence benthic organisms and assemblages in ways analogous to the kelps acting through their effects on flow, particle transport, and shading.