The main objective of the research was to gather information on the effects of climate changes and their consequent influence on the hydrological and dynamic characteristics of plankton in the estuaries.
The author of the thesis was Guillermo Aravena Cuevas and his work entitled Impact of climate variability on the pelagic ecosystem of two contrasting estuaries of the Basque coast (Bay of Biscay): a time series analysis. He has studied the estuaries from Bilbao and Urdaibai.
The data was obtained from the environmental monitoring programme that has been undertaken in these two estuaries since 1997. Over the last decade (1997-2006) an environmental monitoring plan of the pelagic ecosystem of the estuaries has been implemented at these estuaries. By using hydrological data (flow, temperature of water and dissolved oxygen) and biological ones (chlorophyll and mesozooplankton), together with meteorological variables (temperature of air and precipitations) and climatic data (Oscillation of the North Atlantic, ONA), the effects of climate changes on the hydrological conditions and the planktonic communities of these estuaries were evaluated.
Time series analysis
Mr Aravena applied a number of the most recent and specific time series analysis methodologies. Time series analysis involves methods that help to interpret a sequence of data measured over determined moments of time, ordered chronologically and, normally, spaced out between each other in a uniform manner. Representative information is extracted from these data, referring both to origins as to the possibility of extrapolating and predicting future behaviour.
In the words of the author of the thesis, the time series analysis methodologies used were suitable for evaluating the changes in the pelagic ecosystems of the Bilbao and Urdaibai estuaries and for determining their relationships with climate factors.
In general, the tendencies of these environmental and biological variables showed significant differences between estuaries and salt marshes. According to Mr Aravena, this demonstrates that the response of estuarial variables and climatic variability may be different due to, amongst other factors, the effects produced by human activity at each estuary.
Mr Aravena observed that the ONA plays an important role in the variations of air temperature and, consequently, in the recorded water temperatures along the Basque coast. As regards the water-atmosphere interaction, the direct effect of air temperature on that of water showed significant differences, depending on the physical and hydrological characteristics in both estuaries.
In the final part of his thesis, Mr Aravena analysed the effects of environmental variables on the dynamics of Acartia populations the dominant component of mesozooplankton in both estuaries. According to the author, the environmental factors and biological interactions appear to influence the distribution and abundance of important components of mesozooplankton such as Acartias. For the first time the presence of A. tonsa was recorded, plankton that has colonised the inner waters of the Bilbao estuary, moreover negatively affecting the abundance of A. clausi. This is due to the fact that the A. tonsa invasive species has displaced the autochthonous one (A. clausi) to the outer zone of the estuary. This has not been the case with the Urdaibai estuary where interior waters have been occupied by A. bifilosa, the more abundant estuarial species in recent decades.
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