Succession and coastal nutrients concentrations in Ancón bay, Peru

FIRSTATER F. N.; HIDALGO F. J.; FLORES G.; LOMOVASKY B. J.; TARAZONA J.; IRIBARNE O. O.

Lima

Conferencia; Internacional Conference on the Humboldt Current System; 2006

Instituto del Mar del Perú; Institut de Recherche pour le Développement; Food and Agriculture Organization

Introduction- Anthropogenic nitrogen inputs to coastal zones from terrestrial sources have already resulted in the eutrophication and degradation of many coastal ecosystems (e.g., estuaries, bays, and inland seas) worldwide. This increase in nutrient loading is generally associated with shifts in the abundance and functional diversity of primary producers. Using an interdisciplinary approach (Menge, 2000), linking community processes to coastal oceanographic chemical parameters, we tested the hypothesis that even in rich nitrogen areas, anthropogenic inputs may exert a major influence in intertidal community structure, and this influence is associated to the amount of the impact. To evaluate this hypothesis we described and compared the succession of intertidal benthic communities in four areas in a bay subject to urban anthropogenic impact, and assessed its degree of eutrophication. Methods- Experiments were conducted in the rocky shore located at the south of Ancón Bay (11º46 S, 77º11 W), a protected bay in the central coast of Peru from November 2005 to February 2006. Part of sewage produced by the town of Ancón is discharged by a tube directly to the bay, in the intertidal rocky shore. Four sites were selected in the bay: Desagüe (Des), in the wastewater outfall, Punta Cruz (PC) _300 m west, Playa Hermosa (PH) _300 m east, and Muelle San Martin (SM) _600 m northeast. In each site, 18 plots (12 x 12 cm) were cleared in the mid intertidal. Plots in each site corresponded to 6 replicates of three treatments: one complete exclusion of motile herbivores (made by a barrier 1 cm width of epoxy putty, painted with antifouling paint), one half barrier (an L-shaped partial barrier, as a control for the presence of barrier) and one un-manipulated open area. The percent cover of sessile invertebrates and algae and the density of motile animals were determined at the end of the experiment. All plots were photographed with a digital camera. To determine percent cover we used a digital grid with 64 evenly spaced dots covering the central 8 x 8 cm area of the plots images. We recorded the sessile algae, barnacles and mussels occurring under each dot. For motile invertebrates, we estimated densities by censoring four randomly placed 2 x 2 cm subunits in each plot image. All organisms were identified to the lowest possible taxonomic level. Some algae and invertebrates could not be identified to the species level and were combined into larger groups for some analyses. For statistical comparisons we chose the 6 most abundant groups: the green algae Ulva spp., the barnacle Jehlius cirratus, mytilids, two red algae, Gelidium spp. and Chondracanthus chamissoi, and bare space. To evaluate differences in the amount of nutrients between sites, three replicates of seawater samples were taken weekly from November to January at each site. Water samples were filtered (50µm) in the field and frozen. Nitrate, phosphate, nitrite and silicate were later analyzed. Results- There were no differences in nutrients concentration among sites during the nine weeks sampled (Kruskal-Wallis test, P>0,05 for all comparisons). However, at each site, differences were found in nutrient concentration among weeks (Kruskal-Wallis test, P0,05 for both comparisons). No differences were found among sites neither treatments in the number of taxa established (two way Anova, P>0,05). At each site, there were no differences in percent cover of each group among treatments (Kruskal-Wallis test, P>0,05, data arcsin transformed). Differences were found in the percent cover among sites for all groups except for Gelidium spp. (Kruskal-Wallis test, data arcsin transformed, P