Impacts of Climate Variability and Anthropogenic Stressors on Estuarine Macrobenthic Assemblages - Socio-economic Implications and Human Health Risk Assessement

Abstract

Among the most productive, valuable and dynamic systems across the world are estuaries, supporting a wide variety of fauna and flora exposed to large natural fluctuations. The major aim of the present thesis was to understand the multiple stressors (e.g. eutrophication, climate variability and chemical pollution) impacting estuarine ecosystems, through direct or indirect effects on their macrobenthic assemblages and key species, and to integrate it in order to assess potential human health risks. The Mondego estuary and Ria de Aveiro served as a basis to perform this work, by constituting estuarine systems that have experienced distinct human disturbances over time, which made these areas of exceptional study interest. The first chapter explores the structural and functional changes occurred in the macrobenthic communities of the Mondego’s south arm in response to the eutrophication history, subsequent restoration measures implementation in 1998 and extreme climate episodes (floods, droughts and heat waves). Progressive environmental quality degradation characterized the pre-restoration. A replacement and rapid decline of Zostera noltii beds by opportunistic macroalgal blooms was observed, concomitant with significant reduction of macrobenthic abundance, biomass, production and species biodiversity. After restoration, macrobenthic communities showed some signs of recovery but a succession of extreme weather events appeared to delay it. The largest floods (2000/01) and the prolonged drought (2004/05) seemed to be the most negative events impacting macrobenthic assemblages of the Z. noltii beds, in opposition to the mud- and sandflat areas which were mostly affected by the heat wave (2003). All climate phenomena promoted a general abundance, production and biodiversity impoverishment and further distinct changes at trophic and taxonomic levels. On the other hand, total community biomass seemed to be less susceptible to climate stressors exhibiting a slight recovery tendency. The second chapter focuses on the seagrass Z. noltii recolonization, in the former most eutrophic area, after its disappearance for more than 20 years, and its subsequent implications on a mud snail Hydrobia ulvae population. A decade after restoration measures implementation, Z. noltii began to gradually recolonize the sandflat area, coinciding with significant increases in H. ulvae abundance, biomass and production. The seagrass provided long-term protection and abundant food resources for H. ulvae reproductive adults, contrarily to the ephemeral macroalgae, very common during the pre-restoration phase. Through time, large size individuals increased considerably, becoming the population more stable and structured, comparatively to the pre-restoration existing population, which was very unstable and juveniles dominated. In the third chapter, a different anthropogenic stress source (chemical pollution by polychlorinated biphenyls – PCBs – and hexachlorobenzene – HCB) was investigated in a nearby system, the Ria de Aveiro. Contamination by organochlorines varied along a spatial gradient, being mostly restricted to the entrance point of industrial effluents in the system. Water and SPM presented residual levels but deeper sediments exceeded quality guidelines, inducing toxic effects to biota. It is evident a PCBs bioaccumulation trend throughout Scrobicularia plana lifespan, being the bivalve able to incorporate annually almost half of a gram of PCBs from the sediments, which was consequently free for higher trophic levels. There is no danger by consuming S. plana, at least in a short-term, as they exhibited PCB concentrations complying the European regulation. However, consumption of contaminated bivalves over many years and after reaching a steady-state concentration in the body (bioaccumulation) could be dangerous to the human health. The fourth chapter addresses a mesocosms laboratorial experiment concerning PCB-153 bioaccumulation and detoxification by the edible and economic important shrimp Palaemonetes varians over a 30-day period, using the water as the contamination pathway. Depending on the exposure concentrations, different bioaccumulation kinetics and uptake rates were observed. For low PCB- 153 levels, accumulation followed a saturation model, reaching an apparent steady state after 15 days exposure, whilst for intermediate and high PCB-153 levels, accumulation was faster and best fitted using simple linear regressions. Even after decontamination, the shrimps were not able to recover completely background values, retaining into their tissues levels of contamination harmful to the environment and inclusively prohibited for human consumption. Overall, this study provides a valuable contribute towards estuarine sustainable management and human and ecological health risk assessment.