The role of Argopecten purpuratus shells structuring the community composition in Paracas Bay, Peru

SALAZAR, A. G.; BETINA JUDITH LOMOVASKY; OSCAR IRIBARNE,; MENDO, J.

Concepción, Chile

Congreso; CENSOR Midterm Symposium; 2006

The shells of mollusc (alive and/or dead) can generate an Ecosystem Engineering (EE) effect (Jones et al., 1994, 1997) in coastal communities affecting the habitat structures by modification its complexity and heterogeneity changing the availability of resources to other organisms (Gutierrez et al., 2000, 2003). They control: (a) substrate availability for the settlement of epibenthic sessile organisms that could not settle on soft bottom and for the larval settlement; (b) refuge availability where the influence of predator or abiotic factors producing stress conditions are lower; and (c) particle transportation and solutes by changes produced by the flux and the water infiltration (Gutiérrez et al., 2003).        In the Peruvian upwelling system a high population increase of the scallops Argopecten purpuratus in the subtidal of soft bottom, as positive effect during El Niño (EN) periods has been observed (Artnz and Farbach, 1996; Mendo and Wolff, 2003). The shell beds of dead animals and the new habitats are re-colonized during EN periods, next, in the transition toward non-EN periods, important number of new individuals dead (Wolff and Mendo, 2000). Thus, EE effect in the community structure can be happening in the marine bottom by the accumulation of valves. Inclusion experiments in the subtidal of Bahía Paracas, Peru, are been used to evaluate the effect of different densities of valves of Argopecten purpuratus on the (i) settlement of other species on the shell beds (epifaunal) and, (ii) on the infaunal organisms.  Preliminary results, of bimonthly samplings (February to June 2006), indicated that a total of 30 species of invertebrates marines was registered in the different treatments (area with100%, 50%, 0% of cover by shell) and the natural means. The representative groups were Polychaetes, Crustacean and Mollusks in epibenthic organisms and Polychaete in the infaunal. The number of species in epifaunal and infaunal not showed significant differences between the treatments (one way-ANOVA, F(6,170)=0.75, p>0.05). The treatments and the natural means had like abundant species of the epifaunal: Balanus laevis, Actiniaria sp., Nassarius gayi, Eurypanopeus transversus, Nereis callaona and Nereis sp., whereas the species with greater biomass were: Nassarius gayi, Eurypanopeus transversus and Actiniaria sp. Also, infaunal had like species abundant to: Nereis sp., Ophiodromus sp. and Lumbrineris sp., whereas the species with greater biomass were: Lumbrineris sp., Nassarius gayi and Eurypanopeus transversus. The abundance and biomass of epifaunal organisms not showed significant differences between treatments (one way-ANOVA, F (6,170) =1.10, p>0.05); however,  differences were observed in the abundance and biomass of infaunal organisms  between treatments (one way-ANOVA, F (6,170) =4,85, p<0.05): the abundance from substrate covered 100% by shells was higher than others. The infaunal biomass from the substrate covered the 100% by shells was higher than other treatments and the natural means (Tukey’s test p<0.05). The number of species, abundance and biomass of epifaunal and infaunal organism had a gradual increase later 120 days the experiment was installed (one-way-ANOVA, F (5,164) =5,58, p<0.05). Also, 5 macroalgae species were collected inside the experiments. The presence of macroalgae was different between the treatments (one way-ANOVA, F (3,86) =24,91, p<0.05, posterior Tukey`s test, p<0.05) with greater values inside treatments than natural means.   In the treatments with presence of macroalgae, the number of epibenthic species was greater (one way-ANOVA, F (5,81) =6,89, p<0.05). The organic matter (OM) content do not vary significantly between treatments (Kruskal-Wallis; p>0.05), nevertheless, we observed greater amount of OM in natural means in contrast to the experiment.  A characteristic of this bay is the continuous contribution of organic matter (OM) in the sediment due to the internal upwelling that is generated by the circulation of the currents in this bay (Maldonado, 1988).    Our preliminary results show that the beds of shells act as substrate to some epibenthic sessile organisms (i.e cirriped, crepidulas and actinia), and give refuge to mobile organisms (crabs and molluscs), nevertheless, the organisms of infauna (polychaetes) are favored. To lower organic to matter content in the sediment in each treatments permit the increase of organisms of infaunal in this first months of experimental time.