Understanding two of the major forces controlling the community structures and dynamics of intertidal rocky shore and sandy beach of Peru: possible effects under El Niño events

IRIBARNE O. O.; LOMOVASKY B. J.; FIRSTATER F. N.; HIDALGO F. J.; GAMARRA SALAZAR A.

Concepción

Simposio; Censor Midterm Symposium; 2006

COPAS; Alfred Wegener Institute for Polar and Marine Research; National Oceanography Centre

The relative influence of nutrient availability (?bottom-up?), predation/competition (?top-down?) and ecosystem engineering are some of the main forces controlling community structures and dynamics. The different primary producers supporting coastal food webs may be benthic micro algae (Sullivan and Moncreiff, 1990), macro algae or phytoplankton (Deegan and Garritt, 1997) or nutrient input through watersheds (Valiela et al., 1997). Studies of coastal community?s responses to nutrient enhancement showed inconsistent results, apparently because their trophic connections include a higher diversity of primary producers and consumers. Thus, coastal food webs may be more resilient to changes in sources of primary production or predation level (and hence harvesting). El Niño (EN) changes the availability of nutrients, and thus it is likely that community control (and production of commercial marine resources) switch from being top-down to be bottom-up controlled. Therefore basal information of the community in combination with an experimental approach are been used by our team to evaluate the extent of the relative importance of both processes in the coastal community structure. Combinations of herbivore/predator exclusion (e.g., gastropods, sea starts, urchins) and nutrient enrichment experiments are developing in the rocky shore benthos of Central Peru. Additionally, shorebirds observations, succession experiments, and isotopes analysis are been used to complete the information obtained and to understand the process acting in this system. Furthermore, possible changes in this system under EN conditions will be analysed. The other main force under study is the ecosystem engineering (EE, see Jones et al., 1994) effect, which refers to the changing in availability of resources to other organism produced by one organism (the ecosystem engineering) without using this resource. For example, shells and structures of macroalgae modify the mortality rate of many species (Gutierrez et al., 2003). Given that one characteristic manifestations of EN is the proliferation of molluscs (e.g., Argopecten purpuratus; e.g., Arntz et al., 1987, Wolff, 1987), and the mortality of macroalgae forest and intertidal mytilid beds it is expected EE effects locally important in EN main areas. The valve production can affect the habitat structures by modification its complexity and heterogeneity changing the availability of resources to other organisms. They control the substrate availability for the settlement of epibenthic sessile organisms that could not settle on soft bottom and for the larval settlement, refuge availability where the influence of predator or abiotic factors producing stress conditions are lower, and particle transportation and solutes by changes produced by the flux and the water infiltration (Gutiérrez et al., 2003). To study this process several subtidal field experiments with shells of A. purpuratus and community samples in Bahía Independencia and Bahía Paracas, and intertidal rocky shore experiments with shells of Semimytilus algosus y Perumitylus purpuratus and with algae Prionitis sp. in Bahía Ancón are been carried out. In the other hand, to study the effects of macroalgae stranding (Ulva sp, Gigartinoides) on macrobenthos soft bottom community seasonal samples and field experiments will be carry out in sandy beach of Bahía Ancón.