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 animal's 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-feeder's growth may be the animal's ability to handle and process particles that have impacted the feeding apparatus and to deliver them to the point of ingestion.

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

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.

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.