One of the main objectives of marine biology is to understand the trophic food chains involving microbian plankton. That is, to study the sequence of organisms in relation to their nutrition, given that live organisms can be classified from the perspective of nutrition. In this classification each organism is at one level and all these levels are united one to the other, forming a chain. In this succession of stages or levels an organism feeds itself and is devoured. The structure of this food succession or chain is what defines the channelling of energy and organic material at the different levels. The goal of the research was to study the microbian plankton of the Urdaibai estuary. The author of the thesis is Ms Aitziber Sarobe Egiguren and her work entitled (The trophic dynamics of microbian plankton in the Urdaibai estuary). This is an estuary which has great variability, as the tides and river flow considerably affect its hydrological structure. Although a number of studies have been made of the Urdaibai estuary, Ms. Sarobe observed the need to investigate the principal trophic relations of the components of microbian plankton (bacteria, phytoplankton and protozoa) over a brief time scale. Moreover, such analyses or measurements were undertaken at both the interior and exterior of the estuary, as these present very different hydrographic characteristics and trophic situations. To this end, the researcher undertook two studies: one aimed at the annual cycle and the second at studying short-term changes. She studied the annual cycle at the exterior of the estuary. The short-term study, however, was undertaken at two areas: at the same exterior zone in which the annual study was carried out and at the interior of the estuary (more or less 7 km from Gernika). The long-term studies were undertaken between May 1998 and May 1999, with a monthly sampling (13 samples in total). The short-term studies, on the other hand, were done between 3 May 2001 and 8 of May of the same year, taking samples every other day (8 samples in total). Thanks to the samples taken in both zones, Ms Sarobe was able to compare the trophic relations that living plankton organisms have in areas that are clearly different in terms of both hydrographic and trophic conditions. From the results of these samples, the researcher came to certain conclusions. On the one hand, Ms Sarobe observed that the increase in phytoplankton in the annual cycle was related to temperature, light, availability of nutrients and the flow of the river, amongst other factors. With respect to the bacteria, the study of the annual cycle showed that the large biomasses of bacteria were observed in the summertime; with lesser amounts in winter. Moreover, the researcher concluded that the production of bacteria was limited by the sources of food rather than the temperature. The research on short-term changes indicated that the biomass and bacterial production is greater in the interior of the estuary than in the exterior. According to Ms Sarobe, this could all be due to the flow from the water purification plant located near to the interior channel of the estuary. Ms Aitziber Sarobe Egiguren (Donostia, 1974) is a biology graduate. The leaders of her PhD thesis were Ms Emma Orive Aguirre and Ms Arantza Iriarte Gabikagojeaskoa, from the Department of Plant Biology and Ecology of the Faculty of Science and Technology at the University of the Basque Country (UPV/EHU). In drawing up her thesis, she had the support of the UPV/EHU and, in preparing certain research projects, that of the Department of Education, Universities and Research of the Basque Government. She is currently a teacher at secondary school level.