Composició i estructura de la comunitat de zooplàncton de les llacunes de La Pletera

Abstract

La Pletera, located north of the river Ter’s mouth, is one of the few salt marshes that is kept nowadays in the Catalan coast. Its great biological richness and high productivity comes from the brackish lagoon system, in which there are: two natural lagoons (Bassa de Fra Ramon and Bassa del Pi) and a newly established lagoon (Pletera Nova). To preserve and restore this area partially constructed, the LIFE NATURA project of restoration is being carried out (LIFE Pletera Project, 2014). Framed within the LIFE Pletera project, a study about the ecological state of the marsh before restoration was performed. The objectives of this study have been: (i) to determine the composition and structure of the zooplankton community and to study its relationship with the temporal pattern of flooding and the degree of eutrophication of the lagoons; (ii) to determine the resilience of the aquatic community after a disturbance depending on the type of disturbance and the degree of eutrophication of the lagoons. (iii) to evaluate the applicability of different models described for temporary lagoons in the permanent lagoons of La Pletera. During the study, four flood-confinement periods were observed. In each of these situations two characteristic situations have been observed: an increase in the water level and a reduction in salinity after a disturbance were observed; afterwards, a progressive reduction of the water level and an increase of the saline concentration were noticed. In a nutrient multivariable analysis of the different lagoons two principal dimensions have been identified. The first dimension related to the accumulation of nutrients, whereas the second related to the nutrient input episodes. In the second dimension, significant differences between the different lagoons have been found. However, the pattern of succession in the composition of species seems to be the same for the different lagoons, determined by a temporal factor. The water level of the lagoon is the structuring and disturbing element that is related both to the size structure as well as to of the community´s resilience. In the analysis of the different disturbances, there is a greater resilience to winter disturbances than to summer disturbances. We conclude that the temporal pattern of flood-confinement of the different lagoons is the key factor to explain the species composition, the size structure and resilience after a disturbance of the lagoons. In constrast, other factors such as the age of the lagoon or the nutrient load are less important factors. In reference to the resilience, different trends between the different lagoons have been found. Interestingly, autumn-winter disturbances are more resilient on the three lagoons than spring-summer disturbances. Differences in nutrient load between Bassa del Pi and Pletera Nova, but not between Pletera Nova and Bassa de Fra Ramon, suggest that the size of the lagoon might be one of the causes for these differences, in addition to the antiquity of the lagoons. Finally, differences in the described models for temporary lagoons were probably caused by the presence of fish in the lagoons of La Pletera