COMBINED EFFECTS OF CLIMATE-RELATED DISTURBANCE AND NUTRIENT ENRICHMENT ON ROCK POOL BENTHIC ASSEMBLAGES IN NORTH PORTUGAL

Investigating the effects of anthropogenic modifications to natural systems is a major goal of ecology. Current and predicted climate change and eutrophication due to human activities, in particular, are key drivers of patterns of distribution, abundance and diversity of marine populations and assemblages. The present thesis will be included in an ongoing research project (started in February 2012 and expected to end in September 2013) aimed at experimentally testing hypotheses on the interactive effects of climate-related events (i.e. changes in mean intensity and temporal variance of mechanical disturbance simulating the impact of waves during storms) and eutrophication (i.e. nutrient enrichment due to increased runoff in coastal areas caused by land use by humans) on assemblages of algae and invertebrates from tide pools along the rocky coast of Viana do Castelo (north Portugal). This is the first example of an experimental study involving simultaneous manipulations in the field of all these drivers of biodiversity. On the basis of preliminary data on the frequency of past occurrence of extreme storms at the study area, it was decided to perform a total of five events of disturbance over the period of the experiment. In order to examine the effects of changes in temporal patterns of occurrence of storms predicted by climate models, the five events were arranged according to two levels of temporal variance: one characterized by events regularly distributed (one every four months) over the period of the study, the other by events aggregated in shorter periods, separated by prolonged period with no experimental disturbance. Each level of variance is crossed with two levels of intensity of disturbance, performed by twice (high intensity) or once (low intensity) mechanically impacting the substratum (over three random 30 x 30 cm quadrates in each pool) with a metallic hand rake. Eutrophication is performed by applying in selected rock pools two PVC dispensers containing pellets of a slow-release fertilizer (200 g in each dispenser). Preliminary data indicated that this treatment is able to determine a significant increase in the water concentration of nitrate and phosphate in treated compared to control pools. The replacement of the fertilizer every two months assures the maintenance of the nutrient enrichment treatment over the duration of the study. Each combination of intensity, variance of disturbance and nutrient enrichment is repeated in three randomly chosen pools, while three more pools are left unmanipulated and used as control, for a total of 39 pools interspersed along about 1.5 km of coastline. The abundance (percentage cover of sessile organisms and number of individuals of mobile animals) of each species (or higher taxonomical group when necessary) is estimated in the field at each of eleven dates of sampling. The present thesis will include previously collected data and will add at least one date of sampling (February 2013), two events of disturbance (January and March 2013) and two nutrient replacements (February and April 2013). The collected data will be analyzed with multivariate (e.g. PERMANOVA) and univariate (ANOVA) statistical techniques that will provide preliminary data on the main and interactive effects of combined stressors on the structure of whole assemblages, the abundance of individual taxa and the total number of taxa (a proxy for species richness) from rock pools. The final findings from the experiment will contribute to the understanding and predicting of responses of natural assemblages to multiple anthropogenic factors and will provide relevant information for conservation and management options aimed at preventing or mitigating their potential impacts.

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