Succession and Coastal Nutrients Concentrations in Ancón Bay, Perú. En: International Conference of The Humboldt current system. Lima, Perú

FIRSTATER, F.; HIDALGO, F.; FLORES, G.; BETINA JUDITH LOMOVASKY; TARAZONA, J.; OSCAR IRIBARNE,

Lima, Peru

Congreso; International Conference of The Humboldt current system; 2006

Introduction Anthropogenic nitrogen inputs to coastal zones from terrestrial sources have already resulted in the eutrophication and degradation of many semienclosed coastal ecosystems (e.g., estuaries, bays, and inland seas) worldwide. This increase in nutrient loading is generally known to be associated with shifts in the abundance and functional diversity of primary producers. Using an interdisciplinary approach (Menge, 2000), linking community processes to coastal oceanographic conditions, 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 this we describe and compare the succession of intertidal benthic communities in four areas in a bay subject to urban anthropogenic impact.   Methods Experiments were conducted in the rocky shore in the south of Ancón Bay (11º46 S, 77º11 W), a protected bay 60 Km north of Lima, 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 (made by a barrier 1 cm width of epoxi putty, painted with antifouling paint), one half barrier (a 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 was 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 censusing 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, the 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, P<0,05 for all comparisons), except for silicate at Des and SM (Kruskal-Wallis test, P>0,05 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<0,05 for all comparisons). For Ulva spp. percent cover in PH was lower than other sites. For Mytilids, percent cover in PC was higher than other sites. For J. cirratus, the lowest percent cover in SM was different from PC and PH. Furthermore PH was different from Des, with intermediate percent cover. PH was the only site without C. chamissoi, this site was different from Des and PC. PH was the site with the lowest values of bare space, and was different from SM and Des (non parametrics Tukey-type test, P<0,05 for all comparisons). Three species of motile herbivores were found. Tegula atra was only present at Des, with a mean density of 12,21 ind/m2. Scurria spp. was found at all sites except Des, with mean densities of 17,36 ind/m2 at PH, 5,79 ind/m2 at PC, and 10,81 ind/m2 at SM. Siphonaria spp. was found at PH with a mean density of 3,86 ind/m2 and in the same density at PC. Most of these herbivores were found in un-manipulated and partial exclusions areas, although T. atra entered in excluded plots at Des, as Siphonaria at PC.   Conclusions and Discussion The lack of differences in nutrient concentrations obtained in the different sites may be due to water mixing that operate, in the scale of few hundreds meters, homogenizing water masses.  The bay is object of an intense use by residents who recollect several intertidal invertebrates and algae species for consumption. This may lead to a decrease in herbivory pressure, and would explain the lack of differences in cover between herbivores excluded and non-excluded plots within each site. Nevertheless interactions among organisms may explain variations between-sites. The direct effect of limpet grazing in limiting algal abundance would explain the lowest Ulva spp. cover and the absence of C. chamissoi at PH, where the density of Scurria spp. was the highest. This can lead to the dominance of J. cirratus, released from inter-specific competition for settling space. PC and Des are proportionally more exposed to wave action than the other sites, and in PC was find the highest cover for Mytilids. Wave exposure is a factor that have been pointed as beneficial for Mytilids settlement. Another physical factor that distinguish PC and Des from PH and SM is the substratum nature, while PC-Des are rocky shores, PH-SM are seawalls. Although it is not clear yet how the type of substratum affects early colonization. Some other factors that control key ecological processes, as larval transport and particle delivery should be assessed at small scale to fully explain the observed differences among sites. Present results indicate that the intertidal assemblage of Ancón Bay shows signs consistent with an intermediate degree of organic enrichment (Díaz et al 2000).