David O. Duggins

SEA OTTERS AND KELP FORESTS IN ALASKA: GENERALITY AND VARIATION IN A COMMUNITY ECOLOGICAL PARADIGM'

Multiscale patterns of spatial and temporal variation in density and popu- lation structure were used to evaluate the generality of a three-trophic-level cascade among sea otters (Enhydra lutris), invertebrate herbivores, and macroalgae in Alaska. The paradigm holds that where sea otters occur herbivores are rare and plants are abundant, whereas when sea otters are absent herbivores are relatively common and plants are rare. Spatial patterns were based on 20 randomly placed quadrats at 153 randomly selected sites distributed among five locations with and four locations without sea otters. Both sea urchin and kelp abundance differed significantly among locations with vs. without sea otters in the Aleutian Islands and southeast Alaska. There was little (Aleutian Islands) or no (southeast Alaska) overlap between sites with and without sea otters, in plots of kelp density against urchin biomass. Despite intersite variation in the abundance of kelps and herbivores, these analyses demonstrate that sea otter predation has a predictable and broadly generalizable influence on the structure of Alaskan kelp forests. The percent cover of algal turf and suspension feeder assemblages also differed significantly (although less dramatically) between locations with and without sea otters. Temporal variation in community structure was assessed over periods of from 3 to 15 yr at sites in the Aleutian Islands and southeast Alaska where sea otters were 1) continuously present, 2) continuously absent, or 3) becoming reestablished because of natural range expansion. Kelp and sea urchin abundance remained largely unchanged at most sites where sea otters were continuously present or absent, the one exception being at Torch Bay (southeast Alaska), where kelp abundance varied significantly through time and urchin abundance varied significantly among sites because of episodic and patchy disturbances. In contrast, kelp and sea urchin abundances changed significantly, and in the expected directions, at sites that were being recolonized by sea otters. Sea urchin biomass declined by 50% in the Aleutian Islands and by nearly 100% in southeast Alaska following the spread of sea otters into previously unoccupied habitats. In response to these different rates and magnitudes of urchin reduction by sea otter predation, increases in kelp abundance were abrupt and highly significant in southeast Alaska but much smaller and slower over similar time periods in the Aleutian Islands. The different kelp colonization rates between southeast Alaska and the Aleutian Islands appear to be caused by large-scale differences in echinoid recruitment coupled with size- selective predation by sea otters for larger urchins. The length of urchin jaws (correlated with test diameter, r2 = 0.968) in sea otter scats indicates that sea urchins <15-20 mm test diameter are rarely eaten by foraging sea otters. Sea urchin populations in the Aleutian Islands included high densities of small individuals (<20 mm test diameter) at all sites and during all years sampled, whereas in southeast Alaska similarly sized urchins were absent from most populations during most years. Small (<30-35 mm test diameter) tetracycline- marked urchins in the Aleutian Islands grew at a maximum rate of -10 mm/yr; thus the population must have significant recruitment annually, or at least every several years. In contrast, echinoid recruitment in southeast Alaska was more episodic, with many years to perhaps decades separating significant events. Our findings help explain regional differences in recovery rates of kelp forests following recolonization by sea otters.

Magnification of secondary production by kelp detritus in coastal marine ecosystems.

Kelps are highly productive seaweeds found along most temperate latitude coastlines, but the fate and importance of kelp production to nearshore ecosystems are largely unknown. The trophic role of kelp-derived carbon in a wide range of marine organisms was assessed by a natural experiment. Growth rates of benthic suspension feeders were greatly increased in the presence of organic detritus (particulate and dissolved) originating from large benthic seaweeds (kelps). Stable carbon isotope analysis confirmed that kelp-derived carbon is found throughout the nearshore food web.

Kelp beds and sea otters: an experimental approach

In Torch Bay, Alaska, USA, sea urchins structure shallow subtidal communities by consuming most macroscopic algae. Experimental removal of urchins leads to the development, within 1 yr, of a kelp association of high biomass and diversity. In the 2nd yr, however, these attributes decrease as a single kelp species, Laminaria groenlandica, becomes dominant. Additional field experiments confirm the competitive superiority of this kelp. These manipulations lead to predictions regarding the ecological role of sea otters, a once common (but now absent) sea urchin predator. The predictions are tested and verified by examining areas into which sea otters have recently been transplanted. As expected, a rapid and extensive modification of algal species composition and a dramatic increase in kelp biomass follow the return of sea otters. See full-text article at JSTOR

Ecology of under story kelp environments. I. Effects of kelps on flow and particle transport near the bottom

Abstract Because of their likely ecological importance, the effects of understory kelps on fluid and paniculate transport near the bottom were assessed in waters of the San Juan Archipelago, Washington, U.S.A. Relative to more exposed rocky substrata at identical depths, bottoms beneath kelp canopies were exposed to weak fluid transport and were characterized by greater rates of deposition of particulates. A tracer particle experiment demonstrated that kelps inhibited transport of suspended particles from the overlying water column to the bottom. Thus, the higher rates of paniculate deposition beneath kelp canopies probably were caused by longer particle residence times and higher probabilities of paniculate redeposition beneath canopies and not by higher rates of particle import. These hydrodynamic effects may play important roles in the ecology of animals that inhabit understory kelp environments. We propose several potential effects of flow modification by kelps on larval recruitment and dispersal and growth of suspension-feeders.

Ecology of understory kelp environments. II, Effects of kelps on recruitment of benthic invertebrates

Previous studies demonstrated that understory kelps altered rates of mass and particulate transport beneath kelp canopies. This study examined the implications of these physical effects, plus shading by canopies, on the recruitment of four taxa of benthic invertebrates. Using field manipulations of understory kelp density and substratum orientation, we examined the effect of kelps on invertebrate recruitment and assessed the mechanisms involved. These mechanisms included reduced flow, increased sedimentation, reduced light intensity and reduced microalgal cover. All effects were significant to recruitment, but their relative importance varied among taxa. Two species examined, the serpulid polychaete Pseudochitinopoma occidental (Bush) and the cheilostome bryozoan Membranipora membranacea (Linnaeus) have long-lived planktonic larvae. Kelp canopies significantly affected recruitment of these two species, but in different ways. Pseudochitinopoma recruitment was reduced by sedimentation and light but was unaffected by ambient flow directly. Recruitment of Membranipora exhibited patterns consistent with that of passive transport of larvae with higher recruitment in high flow, no-kelp environments. Its recruitment was reduced by higher sedimentation beneath canopies, but showed no response to changes in light intensity. Spirorbid polychaetes have short planktonic phases of unknown durations. Kelps significantly affected their recruitment but difference in recruitment rates between kelp-covered and exposed substrata varied in direction with season. Spirorbid recruitment was reduced by light and high levels of sedimentation, but their recruitment patterns were unaffected by ambient flow directly. The cyclostome bryozoan Tubulipora sp. has a very short planktonic period (seconds to minutes) and factors affecting its recruitment were consequently difficult to decipher. It apparently responded negatively to higher light intensity and increased sedimentation. The effects of kelp canopies on recruitment depend on the complex interaction of these variables.

Starfish Predation and the Creation of Mosaic Patterns in a Kelp‐Dominated Community

Subtidal kelps in Torch Bay, Alaska, resemble a mosaic of discrete algal patch types. This pattern reflects the patchy distribution of herbivorous sea urchins (Strongylocentrotus spp.) and the rapid successional dynamics of the plant association. Factors potentially causing the aggregated distribution of urchins include physical parameters, food availability, interspecific competition, and starfish predation. Each of these factors is considered, with particular emphasis on predation. The starfish Pycnopodia helianthoides is the only predator observed feeding on sea urchins in Torch Bay, and it elicits a strong escape response in two of the three urchin species (and the third at small size). Manipulations of Pycnopodia and urchins indicate that predation, by creating even short—lived herbivore—free patches, can significantly affect subtidal algal assemblages, and thus primary productivity, species richness, and species distribution in this important marine community. See full-text article at JSTOR

Interspecific facilitation in a guild of benthic marine herbivores

SummaryInterspecific relationships and trophic function within the urchin guild are considered in light of experiments and observations performed in situ. Two conclusions are reached: 1) interactions between members of the guild contribute to its persistence, and 2) the plant-herbivore interactions so important in structuring this community can best be evaluated when guild members are treated as a unit. As with other mixed species trophic units, the mechanisms of interspecies facilitation operate to reduce the effects of predation and increase foraging efficiency.

Effects of Flow Speed on Growth of Benthic Suspension Feeders

In separate experiments in 1991 and 1992, the sensitivities of growth rates of six species of benthic suspension feeder to flow speed were tested in a series of turbulent pipe flows. Species examined were the cheilostome bryozoan Membranipora membranacea (1991); the serpulid polychaete Pseudochitinopoma occidentalis (1991); and the barnacles Balanus glandula (1991), B. crenatus (1992), Semibalanus cariosus (1992), and Pollicipes polymerus (1992). In both experiments, animals were exposed to one of five constant, narrow ranges of speed that varied from about 2-15 cm s-1. Growth rates of Membranipora and Pseudochitinopoma in 1991 declined significantly and monotonically with increasing flow speed, despite evidence that at faster flows there were greater concentrations of suspended food available and higher particulate fluxes. In contrast, there was no detectable relationship between speed and growth of B. glandula over the same range of flow speeds in 1991. Results of the 1992 experiment indicated variability in growth responses among three species of barnacle. Growth rates of S. cariosus and P. polymerus were insensitive to flow speed, whereas growth rates of B. crenatus increased from low speeds to a maximum at an intermediate speed of about 8 cm s-1, and then tended to decrease at higher speeds. Combined results of the two experiments indicate that the growth response of animals to flow was most obviously related to the relative flow energy of the animals natural habitat. Growth rates of animals that typically experience relatively weak flows (P. occidentalis, M. membranacea, and B. crenatus) were affected significantly by flow speed, whereas growth rates of animals from comparatively high-energy environments (B. glandula, S. cariosus, and P. polymerus) were relatively insensitive to flow speed. In contrast, animal morphology and behavior were not obviously related to growth responses to flow. A key to understanding the impact of flow on a suspension-feeders growth may be the animals ability to handle and process particles that have impacted the feeding apparatus and to deliver them to the point of ingestion.

Experimental studies of herbivory and algal competition in a low intertidal habitat

SummaryEcological factors regulating the species composition and abundance of the plant assemblage in the low intertidal zone were studied in the semiprotected San Juan Islands in Washington state. In particular, the roles of an abundant herbivore (the chiton Katharina tunicata) and competition among plants were evaluated experimentally. Densities of Katharina were manipulated in large areas to establish 3 treatments: Katharina removals (N=2), controls (N=2), and Katharina addition (N=1). These sites were monitored for 4 years. Over this time, algal abundance and diversity increased in the Katharina-removal areas; algae in most functional groups proliferated, and a multi-storied intertidal kelp bed eventually developed. In the Katharina-addition, the abundance of all plants except crusts, diatoms, and surfgrass decreased, and overall diversity declined. Control sites underwent year-to-year fluctuation in the abundance of the most conspicuous alga, Hedophyllum sessile, but remained otherwise unchanged.At the end of the third year, the dominant space occupiers (the large brown algae) were removed from half of each of the Katharina removal and control areas. These kelps positively affect abundances of Katharina but are negatively correlated with limpet numbers. In the absence of competition from the kelps, the abundance and diversity of most other algal groups increased. This change was especially dramatic in the areas lacking both chitons and kelps, indicating that herbivory and competition have negative, additive effects on smaller algae. However, even in the absence of Katharina, abiotic disturbance (desiccation and storms) apparently mediates the competitive dominance of the kelps.

FATTY ACID SIGNATURES DIFFERENTIATE MARINE MACROPHYTES AT ORDINAL AND FAMILY RANKS 1

Primary productivity by plants and algae is the fundamental source of energy in virtually all food webs. Furthermore, photosynthetic organisms are the sole source for ω‐3 and ω‐6 essential fatty acids (EFA) to upper trophic levels. Because animals cannot synthesize EFA, these molecules may be useful as trophic markers for tracking sources of primary production through food webs if different primary producer groups have different EFA signatures. We tested the hypothesis that different marine macrophyte groups have distinct fatty acid (FA) signatures by conducting a phylogenetic survey of 40 marine macrophytes (seaweeds and seagrasses) representing 36 families, 21 orders, and four phyla in the San Juan Archipelago, WA, USA. We used multivariate statistics to show that FA composition differed significantly (P < 0.001) among phyla, orders, and families using 44 FA and a subset of seven EFA (P < 0.001). A second analysis of published EFA data of 123 additional macrophytes confirmed that this pattern was robust on a global scale (P < 0.001). This phylogenetic differentiation of macrophyte taxa shows a clear relationship between macrophyte phylogeny and FA content and strongly suggests that FA signature analyses can offer a viable approach to clarifying fundamental questions about the contribution of different basal resources to food webs. Moreover, these results imply that taxa with commercially valuable EFA signatures will likely share such characteristics with other closely related taxa that have not yet been evaluated for FA content.