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Natural and Anthropogenic Variations in a Channelized Water Course in Centre of Portugal

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Natural and Anthropogenic Variations in a Channelized Water Course in Centre of Portugal 257 Natural and anthropogenic variations in a channelized water course in Centre of Portugal Pedro M. Silva-Santos1, Simone V. Oliveira2, Rui M. V. Cortes2, António C. Albuquerque3 1 Lab. Ecologia Aplicada (LEA), Universidade de Tras-os-Montes e Alto Douro, Quinta de Prados, 5000- 911, Vila Real, Portugal. Email: [email protected] 2 Departamento Florestal, Universidade de Trás-os-Montes e Alto Douro, Quinta de Prados, 5000-102, Vila Real, Portugal. Email: [email protected] , [email protected] 3 Departamento Engenharia Florestal, Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017, Lisboa, Portugal. Email: [email protected] ABSTRACT The present study took place in the Mondego River, located in the Centre of Portugal. The lower sector of the river (Lower Mondego) is largely man-made due to regularization and rectification of the channel. The objective was to assess the impacts on the aquatic communities (fishes and benthic invertebrates). Fauna inventories were performed in June and September of 2000 and 2001, toge- ther with habitat characterization. Three sampling sites were selected in this segment and compared to a reference site located ups- tream. It is concluded that the presence of structures such as submerged weirs and riprap, promoted the diversity, due to the physical complexity, which they introduced into the system. The dramatic flood peaks that occurred in the winter of 2000/01 also caused substantial changes in the fluvial dynamics and in the habitats: the large amount of suspended solids transported resulted in a river- bed of unstable fine materials, and in a subsequent biological impoverishment. However, both communities displayed a high resi- lience to these changes; the inter-annual differences being obscured by the seasonal ones on macroinvertebrate communities. Keywords: benthic invertebrates, fishes, channelization, fluvial dynamics, floods. RESUMEN El presente trabajo tuvo lugar en el río Mondego, localizado en el Centro de Portugal. El tramo bajo del río (Bajo Mondego) se encuentra intensamente transformado debido al regularización y rectificación del cauce. El objetivo fue evaluar los impactos en las comunidades acuáticas (peces e invertebrados bentónicos). Se realizaron inventarios de fauna en Junio y Septiembre de 2000 y 2001, junto con la caracterización del hábitat. Se seleccionaron tres localidades en este segmento fluvial y se compararon con una localidad de referencia localizada aguas arriba. Se concluye que la presencia de varias estructuras como los azudes sumer- gidos y enrocamentos, promovió la diversidad, derivado de la complejidad física que ellos introdujeron en el sistema. Las riadas dramáticas que ocurrieron en el invierno de 2000/01 también causaron cambios sustanciales en la dinámica fluvial y en los hábi- tats: la gran cantidad de sólidos en suspensión transportada dio lugar a un substrato constituido por materiales finos inestables, y al empobrecimiento biológico consecuente. Sin embargo, ambas comunidades mostraron una elevada resiliencia a estos even- tos, quedando las diferencias interanuales ocultadas por las diferencias estaciónales en las comunidades de invertebrados. Palabras clave: invertebrados bentónicos, peces, canalización, dinámica fluvial, riadas. INTRODUCTION ze crop production through irrigation. In these cases, the riverbed has been lowered, while ripa- Unlike some other European countries such as rian land is periodically cleared of trees and Denmark and England (Brookes, 1988), Portugal riverbank edges are cleaned (Aguiar et al., 2001). has only few extensions of channelized river seg- The objectives of this study carried out in the ments, most of which are set in urban environ- terminal part of the Mondego River (Centre of ments (Aguiar et al., 2001). These segments are Portugal) were to characterize the macroinverte- located on large floodplains, in order to maximi- brate communities and fish (abundance and Limnetica 23(3-4): 257-270 (2004). DOI: 10.23818/limn.23.22 © Asociación Ibérica de Limnología, Madrid. Spain. ISSN: 0213-8409 258 Silva-Santos et al. diversity) in different sectors of the river, com- called the Lower Mondego and is fully regulated paring channelized segments with natural ones. and channelized. The Lower Mondego runs An additional objective was to evaluate the along an extensive alluvial plain of Cenozoic effects of the floods that occurred during the materials that extends from Coimbra to Figueira study period (January of 2001) in these same da Foz (37.5 km), with altitude levels between sectors, and which caused a large impact on the 0 m and 16 m (Vieira & Ferreira, 1997). river habitats. In channelized sectors, the The main purpose of the channelization of influence of hydraulic structures on the diver- Lower Mondego was flood control (to a limit sity of organisms was also assessed. of 1200 m3/s for a recurrence period of 100 years) and the enlargement of the agricultural area. The design of an entirely new river channel was cou- MATERIAL AND METHODS pled with two dams upstream of Coimbra, to regulate the flows from the main river and the tri- Study area and channelization activities butaries. Specifically the channelization of the Lower Mondego consisted in resection the river- The Mondego River flows in an E-W direction bed and in the construction of extensive longitudi- from the “Serra da Estrela” mountains (1547 m) nal dykes, constraining this river segment in a for about 227 km until “Figueira da Foz”. The entirely new channel with artificial banks, trape- influence of the Atlantic increases the relative zoidal shaped in cross section. Submerged weirs humidity of the air and affects temperatures and and riprap were also constructed to reduce the precipitation. The catchment area has 6671 km2 hydraulic energy, to settle the transported sedi- and a medium altitude of 375 m, with a mean ments and to stabilize the riverbanks. annual precipitation of 1130 mm. We may divi- In December and January of 2000/01 an de the longitudinal profile into different seg- extremely high precipitation in the NO and SE ments, according to the morphometric characte- slopes of the basin occured. Cumulative precipi- ristics. The segment located in the lowest part is tation for the two months registered return Figure 1. Location of the sampling sites within the Mondego Basin. M1– Reference site; M2, M3 and M4 - Sites on channelized sector. Localización de las estaciones de muestreo en la Cuenca de Mondego. M1– Localidad de referencia, M2, M3, M4 – locali- dades en el tramo canalizado. Natural and anthropogenic variations in a channelized water course 259 periods of 250 and 300 years respectively the substratum upstream of the net using a vigo- (Rodrigues et al., 2001). The high saturation rous kicking and/or feet action. The sampling level of the soil increased the superficial flow in was performed using a 350 µm mesh net with the catchment, which exceeded the storage 600 cm2 aperture and embraced all the existent capacity of the reservoirs. Consequently, the biotopes with an effort proportional to the rela- flood events of January 26 and 27 caused the tive importance of each habitat (Table 1). The rupture of the dykes downstream of Coimbra. nets were washed carefully between collections. The contents of the hand-net were placed in Sampling sites labeled containers, transported to the laboratory, and the specimens sorted alive. For preservation Three sampling sites were set in the regularized 70% ethanol was used for subsequent identifi- Central Bed of the Mondego River (M2, M3, cation. Wherever possible, identification was M4), between Coimbra and the river mouth done to species level using the available keys (Figueira da Foz), and one located approximately (except Diptera and Oligochaeta). 14 Km upstream of Coimbra (M1) - (Fig. 1). This last one, was a control site since it was not affec- Ichthyic communities ted by channelization, although it is located approximately 20 Km downstream of the Dam of In contrast with benthic collections, fish sam- Aguieira. This site has well developed riparian pling allowed the discrimination between habi- vegetation on the left bank. In contrast, the right tat types in order to detect the influence of bank presents scarce vegetation due to vegetation hydraulic structures and erosion sedimentation clear cutting and to the high deposition of mate- processes (Table 1). Fish were captured using an rials (small stones, gravel and sand). The sites in Electrocatch apparatus, model WFC7-HV, the artificial segment exhibit extensive riprap powered by a generator (Honda GX160) of 4.0 along the banks, with constant slope of 45º, par- KW. Electrofishing used DC current and the tially colonized by shrubs and trees (mainly voltage was set between 150 to 200 V in order to Populus spp.). Medium fraction materials (stones produce a current from 2.5 to 4 A. This fact, and gravel) dominated the riverbed, but were together with a constant capture effort of 7 replaced by a lower grain size fraction in the minutes in each meso-habitat (CPUE), allowed second part of the study after the flood events. comparable results among the different invento- M2 shows a semi-disaggregated submerged weir ried habitat types. Electrofishing captures were with resulting shear stress in the wet channel. A complemented by 30 x 2.5 m static multimesh more detailed description of the different charac- gill net (mesh types: 32 mm, 43 mm, 50 mm, 65 teristics of each meso-habitat is in table 1. mm and 85 mm), which was placed in the dee- Macroinvertebrate (M) and fish (F) sampling per pools for 3-4 hours. was carried out simultaneously in June (6) and September (9) of 2000 and 2001. Physical and Data treatment chemical parameters (dissolved oxygen, pH, temperature, conductivity, alkalinity and total To compare sites and periods for both sampled suspended solids) were also analysed at the communities, the total number of individuals and sampling sites for the same periods (Table 2). Shannon diversity index were computed through PRIMER 5.2.2 (Clarke & Gorley, 2001).
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