Bottom-up effects on rocky intertidal communities

FIRSTATER F. N.

Concepción

Simposio; Censor Midterm Symposium; 2006

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

Communities may be regulated by top-down factors such as herbivory and predation and bottom-up factors, the resources availability, such as nutrient. In rocky intertidal communities, bottom-up control factors have received relatively little attention. However, accumulating evidence indicates that productivity is limited by nutrient availability in these systems. El Niño has the capacity to impose major changes on nearshore benthic dynamics. ENSO causes an increase of the ocean temperature, which reduces nutrient mixing to the surface waters and therefore may reduce intertidal productivity. Thus, it is important to understand the response of communities to different nutrient regimes under EN conditions. Therefore, the general aim of my work is to investigate how nutrients affect rocky intertidal communities. In order to address this bjective, I investigated: (i) the effects of anthropogenic nitrogen enrichment on the succession in a eutrophicated bay, moreover I experimentally assessed the importance of nutrient addition in the mid intertidal (ii) epilithic microalgae and (iii) mussel bed communities. (i) Anthropogenic nitrogen inputs to coastal zones from terrestrial sources have already resulted in the eutrophication and degradation of many semienclosed coastal ecosystems worldwide. This increase in nutrient loading is generally known to be associated with shifts in the abundance and functional diversity of primary producers. We tested the hypothesis that even in rich nitrogen areas, anthropogenic inputs may exert a major influence in intertidal community structure. To evaluate this hypothesis I 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. Until now, I have found differences in the percent cover of several species among sites in the bay, nevertheless there were not differences among successions with herbivores excluded and non-excluded treatments. This could be explained by a reduced herbivory pressure as result of human recollection. Further experiments are necessary to assess the effect of herbivory. (ii) Biofilms consisting of mucus, bacteria and micro-algae form on any hard surface immersed in water, and are a key, but frequently overlooked, functional component of the benthos on rocky shores worldwide. The biomass and productivity of periphyton is affected by abiotic factors (e.g., light, waterflow, nutrients, substrata) and by biotic interactions (resource competition, herbivory). Despite the importance of this biofilm in community dynamics of rocky shores it has received relatively little attention, mainly because of technical difficulties in sampling and quantification. To test the relative influence of nutrient and grazing in the mid intertidal epilithic biofilm abundance, a nutrient supply experiment in combination with grazers exclusion was carried out. (iii) Mussels are the competitive dominants of the mid and low intertidal zones, the intersticial space created by these mussels harbors a great variety of organisms. Population or individual traits of a facilitator can affect whether it modifies the environment, determining the degree to which other species benefit. Therefore, factors controlling such traits can ultimately determine the relative balance of costs and benefits of interactions between species. Moreover, such facilitation can have important consequences for community regulation. By fertilizing water column in intertidal mesocosm I am studying the effect of nutrient enrichment on mussels growth and its community assemblage.