EDP Foz Tua: Bivalvia – Complementary Studies [2011]

Study Extent

The study was carried out in 31 fixed stations covering the whole length of river Tua (19 stations) and the lower valleys of rivers Tuela (6 stations) and Rabaçal (6 stations). Sampling took place between 5-14 July and 25-27 October 2011.

Sampling

Sampling was designed to obtain complementary data: (1) a general characterization of the freshwater bivalve communities (presence/absence and Capture per Unit Effort only), using 19 field stations located every 2 km in the lenthic zone (lower Tua) and every 5 km in the lothic zone (upper Tua and lower Tuela and Rabaçal), totalizing 31 field stations, and (2) a detailed characterization aiming to quantify the abundances at community and population levels, using a restricted number of field stations (5 in river Tua, 2 in river Tuela and 1 in river Rabaçal), roughly 10 km apart. The method used to perform the general characterization of the communities was random visual search of the river bottom in each field station, using “snorkeling” in clear water of less than 1 m deep. All bivalves found were counted, identified, measured (shell length) and weighted, before being returned to the habitat. A relative count (number/hour/observer) of all bivalves present was made in each field station. The characterization of the riverbed substrate was included, by estimating the relative proportion of rock, pebble, gravel, sand and mud present), as well as a characterization of the riparian vegetation, using a simplified version of the River Habitat Survey (RHS) methodology. To sample small bivalves fully buried in the substrate (sphaeriidae: genus Pisidium, Sphaerium and Musculium; Cyrenidae: genus Corbicula; Unionidae and Margaritiferidae: juveniles only) a modified kick-sampling method was applied using a hand net of 0.5 mm mesh in suitable habitats for these specimens (fine sediments near the margins, roots, underwater vegetation, etc). The captured specimens were sorted and preserved in 96% alcohol for subsequent identification in the lab. The results (presence/absence, total numbers and relative numbers) were used to define homogeneous sectors of rivers regarding the bivalve communities, either by using objective landmarks (e.g., small dams, orographic features, confluence of two water lines), or subjectively, using the station or the medium point between stations. For detailed characterization, 10 transects of 1 m wide perpendicular to the river (independently of the river’s width) were surveyed in each station by 2-3 technicians by scuba diving. All animals found were identified, counted and processed as above. These data were used to estimate the densities present in each station (expressed as number of bivalves/100 meters of river, by multiplying the number of captures for all transects combined by 10). The number of individual per sector of river (as defined above) was found by multiplying the average value of abundance per 100 m of river from the various stations of a given sector by the coefficient corresponding to the length of that sector (for example, for a sector of 10 km multiplied by the factor 100).

Quality Control

All individuals was identified by experienced researchers with a deep knowledge of the Iberian freshwater mussels

Method steps

1. To be completed

The taxonomic coverage of this dataset spans 1 class, 2 orders, 4 families and 8 species. All individuals were identified to the species level.

1. Animalia
   rank: kingdom
2. Mollusca
   rank: phylum
3. Bivalvia
   common name: Bivalves rank: class
4. Unionoida
   rank: order
5. Veneroida
   rank: order
6. Unionidae
   rank: family
7. Margaritiferidae
   rank: family
8. Cyrenidae
   rank: family
9. Sphaeriidae
   rank: family

The data was collected at 30 field stations along the valleys of rivers Tua, Tuela and Rabaçal (Northeast Portugal, Europe), within the municipalities of Alijó, Carrazeda de Ansiães, Mirandela, Macedo de Cavaleiros and Valpaços.