Abundance Of Sessile Invertebrates Research Articles

Impact assessment of fish cages on coralligenous reefs through the use of the STAR sampling procedure

The study aimed at contributing to the development of methods to assessing the effects of human disturbance on coralligenous reefs. The effects of the presence of a fish farm on coralligenous reefs were evaluated using the STAR (STAndaRdized coralligenous evaluation procedure) sampling procedure. An asymmetrical sampling design was used to compare the aquaculture site with two reference sites in areas unaffected by human pressure. The response of different ecological indices (ESCA, Ecological Status of Coralligenous Assemblages; ISLA, Integrated Sensitivity Level of coralligenous Assemblages; COARSE, COralligenous Assessment by Reef Scape Estimate) and descriptors (α-diversity, β-diversity and Sensitivity Level) of this kind of disturbance was compared. Results indicate that coralligenous reefs are vulnerable to aquaculture fish cages, and differences in the structure of coralligenous assemblages between the disturbed and the reference sites were mostly due to the decrease in β-diversity. On the contrary, no significant differences in the number of taxa/groups were highlighted. Encrusting Corallinales, erect Rhodophyta, Dictyotales, Fucales and Halimeda tuna were more abundant in reference sites than in disturbed site, while Peyssonnelia spp. and algal turfs had the opposite trend. Conversely, no significant differences between conditions were found in the abundance of sessile invertebrates. The study supports the suitability of the STAR approach to be employed in impact evaluation assessments, such as in monitoring programs. The present study is a first attempt to combine three different ecological indices (ESCA, ISLA and COARSE) within a unified approach, in order to assess the status of coralligenous reefs subjected to a moderate human-induced disturbance. The inconsistent response of the different indices highlights the advantage of applying different indices and descriptors to evaluate the variable human pressures on natural systems.

Direct and indirect effects of giant kelp determine benthic community structure and dynamics.

Indirect facilitation can occur when a species positively affects another via the suppression of a shared competitor. In giant kelp forests, shade from the canopy of the giant kelp, Macrocystis pyrifera, negatively affects understory algae, which compete with sessile invertebrates for space. This raises the possibility that giant kelp indirectly facilitates sessile invertebrates, via suppression of understory algae. We evaluated the effect of giant kelp on the relative abundance of algae and invertebrates by experimentally manipulating kelp abundance on large artificial reefs located off San Clemente, California, USA. The experiments revealed a negative effect of giant kelp on both light availability and understory algal abundance and a positive effect on the abundance of sessile invertebrates, which was consistent with an indirect effect mediated by shade from the kelp canopy. The importance of these processes to temporal variability in benthic community structure was evaluated at 16 locations on natural reefs off Santa Barbara, California, over an eight-year period. Interannual variability in the abundance of understory algae and in the abundance of sessile invertebrates was significantly and positively related to interannual variability in the abundance of giant kelp. Analysis of these observational data using Structural Equation Modeling (SEM) indicated that the magnitude of the indirect effect of giant kelp on invertebrates was six times larger than the direct effect on invertebrates. Results suggest that the dynamics of this system are driven by variability in the abundance of a single structure-forming species that has indirect positive, as well as direct negative, effects on associated species.

Benthic-pelagic links and rocky intertidal communities: bottom-up effects on top-down control?

Insight into the dependence of benthic communities on biological and physical processes in nearshore pelagic environments, long considered a "black box," has eluded ecologists. In rocky intertidal communities at Oregon coastal sites 80 km apart, differences in abundance of sessile invertebrates, herbivores, carnivores, and macrophytes in the low zone were not readily explained by local scale differences in hydrodynamic or physical conditions (wave forces, surge flow, or air temperature during low tide). Field experiments employing predator and herbivore manipulations and prey transplants suggested top-down (predation, grazing) processes varied positively with bottom-up processes (growth of filter-feeders, prey recruitment), but the basis for these differences was unknown. Shore-based sampling revealed that between-site differences were associated with nearshore oceanographic conditions, including phytoplankton concentration and productivity, particulates, and water temperature during upwelling. Further, samples taken at 19 sites along 380 km of coastline suggested that the differences documented between two sites reflect broader scale gradients of phytoplankton concentration. Among several alternative explanations, a coastal hydrodynamics hypothesis, reflecting mesoscale (tens to hundreds of kilometers) variation in the interaction between offshore currents and winds and continental shelf bathymetry, was inferred to be the primary underlying cause. Satellite imagery and offshore chlorophyll-a samples are consistent with the postulated mechanism. Our results suggest that benthic community dynamics can be coupled to pelagic ecosystems by both trophic and transport linkages.

Dynamics of the Epifauna of the Bivalve Pinna Bicolor: Interactions Among Recruitment, Predation, and Competition

The epifauna on the shells of a subtidal population of the bivalve Pinna bicolor is composed mainly of encrusting bryozoans and serpulid polychaetes, with tunicates and sponges relatively uncommon. Only a small subset of the observed interactions between sessile organisms were capable of changing the patterns of abundance on individual Pinna shells. Didemnid tunicates were capable of excluding most other species, but the most common competitive interactions, those between encrusting bryozoans, had variable outcomes and frequently did not result in the exclusion of the loser from a patch (shell). Sponges were also capable of excluding bryozoans and serpulids, although at slower rates than tunicates. Serpulids were overgrown by all other species, although overgrowth usually did not result in their exclusion from patches. Predators were, in general, not capable of influencing the abundance of any species greatly. The exception involved two species of monacanthid fish, juveniles of which ate newly metamorphosed didemnid ascidians. These fish thus could prevent monopolization of some patches by tunicates. The effects of predation by juvenile fish and overgrowth by tunicates were variable in space and time. The interaction between variable tunicate recruitment and predation was important. Didemnids recruited in low numbers and, even in a year of high recruitment, did not colonize a large number of patches; in addition, most recruits were eaten by fish. Predation by fish played no role in another year, because few tunicates recruited. It is likely that the abundance of fish was also variable, since their recruitment is also likely to be variable in time. The combination of these factors has resulted in few patches being colonized by tunicates. In the 2 yr of the study, none of the fifty patches under observation was colonized successfully by didemnids, and observations on other Pinna shells suggest that no substantial establishment occurred in the 2—3 yr preceding this study. Thus, in most patches, predation and competition did not strongly influence the abundance of sessile organisms. The patterns of occupation of individual patches, and the overall abundance of sessile species in the community, could be explained adequately by the observed patterns of recruitment. This situation contrasts with the occupation of patches on the pilings of a nearby pier, where competitive interactions between occupants of a patch are the most important factor. The results of these experiments provide a mechanism to explain the previously noted difference in the abundance of sessile invertebrates on the shells of Pinna bicolor and on the pilings of the pier.