Diatom responses to Holocene environmental changes in a small lake in northwest Spain

Diatom responses to Holocene environmental changes in a small lake in northwest Spain Manel Leira Centre for the Environment, Department of Geography, University of Dublin, Trinity College, Dublin 2, Ireland (E-mail: le[email protected]) Keywords: Diatoms, Holocene, sedimentary environments, palaeolimnology, northwest Spain 1 Abstract The Holocene history of Lagoa Grande (1,360 m a.s.l; northwest Spain) has been investigated through the analyses of diatoms remains found in a sediment core. Changes in the lake have been interpreted in terms both of climatic variability and major changes in the catchment area. The diatom record begins at about 9800 yr BP and exhibits three marked changes at 8200, 6000, 4200 and at 2600 yr BP, which are interpreted as responses to climatic changes detected by marine and other terrestrial palaeoclimatic records. The initial lake was shallow and Aulacoseira species characteristic of acidic and oligotrophic waters were dominant. The early of Holocene is characterized by the presence of Aulacoseira ambigua, which suggests an increase in water depth and nutrient levels. Throughout the Holocene diatom assemblages underwent several changes linked to fluctuating limnological conditions. These changes in lake characteristics coincide with the regional succession of vegetation, suggesting that climate forcing initiated these limnological changes. During the last 2000 years catchment processes appear to be the major factor controlling lake characteristics in Lagoa Grande site. Diatom responses reflected the progressive peatland development and growth of littoral vegetation in the lake, and not only a change in the regional environment. 2 Introduction Changes in lake water chemistry can be driven by many different factors either natural or human induced. Palaeolimnological investigations have shown the links between acid deposition of anthropogenic origin and surface water acidity (Battarbee and Renberg, 1990; Davis, 1987; Psenner, 1994; Renberg et al., 1993). Peat development and soil acidification have also been shown to lead to surface water acidification, especially where surface waters have low alkalinity (Brugam and Swain, 2000). Furthermore changes in climate affect many physical and chemical limnic properties, including nutrient availability and pH, all of which affect diatom community composition (Moser et al. 1996). In recent years, detailed studies in marine and other terrestrial palaeoclimatic records have shown that the Holocene climate variability has been wider and punctuated by cooling oscillations (Bond et al. 1997; 2001). The timing and frequency of these oscillations have already been established in different archives at least in the northern North Atlantic suggesting that solar activity played a major role (Bond et al. 2001). Little is still known, however, about the environmental effect of these climate oscillations on Southwest Europe (Desprat et al. 2003). This climatic variability, although weaker in amplitude than the shifts of the glacial cycles, has been large enough to have significant effects on lake ecosystems as indicated by the large fluctuations in hydrological conditions (Magny 1993, 2004). In addition, climate changes can trigger different processes and have varied patterns on a regional scale (Davies et al 2003; Magny et al. 2003). But, while vegetation response to climate changes through the Holocene are relatively well studied in Spain, their effects on aquatic environments have received much less attention. Compositional changes in the diatom remains found in sediment cores have been shown to represent a valuable indicator of past changes on lacustrine ecosystems 3 and their catchment area (Prather and Hickman, 2000; Smol and Cumming, 2000). Observed variations of diatom-inferred pH with temperature (Psenner and Schmidt, 1992) represent a good example of how climate change indirectly affects diatom communities (Fritz, 1996). The major aim of this study has been to examine which environmental factors are responsible for the observed changes in diatom communities and providing information on lake history in relation to palaeoenvironmental changes in the catchment during the Holocene. Most of the pollen records studied in Spain have been collected from lakes, so that they reflect changes in the catchment vegetation of those lakes that can influence lake characteristics through the retention of nutrients in the accumulating catchment biomass (Prather and Hickman, 2000). Catchment vegetation history was first assessed using pollen records from the same lake (Maldonado Ruiz, 1994) and then diatom analyses were made to study the effect of vegetation dynamics and other changes on the aquatic ecosystem. pH is usually one of the most important parameters determining the diatom assemblage composition in freshwaters, and diatom-inferred pH reconstructions have been shown to be generally reliable (Birks et al. 1990; Birks, 1995). Furthermore, this information has been compared with other palaeoecological and palaeoclimatic data from surrounding areas in an attempt to present possible palaeolimnological inferences related to climate. Site description Lagoa Grande is located in the Serra de Queixa (Galicia, northwestern Iberian Peninsula) (UTM 29TPG4479) at an altitude of 1360 m a.s.l. (Figure 1). Serra de Queixa is an upland area in the southeast of Galicia (Figure 1) with a quite uniform morphology. The highest altitude is Cabeza de Manzaneda (1778 m a.s.l.) and the lowest is Baños de Molgas (500-600 m a.s.l.). Glacial deposits lie on the upper levels 4 of the range (Pérez Alberti et al. 1993; Vidal Romaní and Santos, 1994). Geology of the area is characterized by schist, sandstone and Cambrian-Ordovician quartzite southwards, and by Hercinian granite northwards (IGME, 1981). The common upland soils are ranker and lithosols (Aira Rodríguez, 1986). Vegetation is a mixed forest dominated by Betula pendula, Acer pseudoplatanus and Quercus pyrenaica, with a ground cover of Saxifraga spathularis, Holcus mollis, Doronicum carpetanum, Dryopteris expansa and Poa nemoralis. Erica arborea is also common (Izco Sevillano, 1987). Lagoa Grande is an elliptical, shallow basin formed during the last deglaciation, which began as early as 17 500 yr BP (Muñoz Sobrino et al. 2001). It has a small catchment area and is only drained by a small creek. The underlying geology is composed of granite and gneiss. Currently, the lake measures nearly 100 m in length and has a surface area of 0.5 ha. Most of the surface of the lake is less than 0.5 m deep. These shallow areas provide a good habitat for both submerged and emergent macrophytes, which are abundant throughout the lake. The vegetation in the surroundings of the lake is dominated by heathland, with Halimiun alyssoides and Chamaespartium tridentatum. Quercus pyrenaica is also abundant and in the near-shore areas Betula pendula is present. Salix atrocinera, Prunus avium and Frangula alnus are also present. Current diatom communities are chracterised by acidophilous taxa such as Eunotia, Cymbella and Pinnularia, which are common in peatbogs and occurr in oligotrophic and dystrophic waters. Climatic conditions are characterized by an average annual temperature below 8ºC. Summers are cool with monthly mean temperature below 15ºC. Precipitation is 5 concentrated in autumn and winter, and it ranges from 800 mm yr-1 in the lower region, to more than 2000 mm yr-1 in the upland area (Carballeira et al. 1982). Methods A 630 cm long sediment core was taken in summer 1995 using a Russian corer (Jowsey, 1966) close to the centre of the lake. Coring was done continuously in 50 cm sections. The lithology was described using the Troels-Smith system (1955). Subsamples of 1 cm3 were taken from the centre of the core for diatom and chrysophyte stomatocyst analyses at 10 cm intervals. The proportion of diatom frustules to chrysophyte cysts can be a useful index of trophic status in temperate lakes (Smol, 1985). Chrysophytes are generally associated with low total phytoplankton biomass and trophic status and oligotrophic phases in temperate lakes. Samples for diatom and total stomatocyst analyses were prepared according to Renberg (1990). Aliquots of evaporated suspensions were embedded in Naphrax (R.i. = 1.74). When possible, at least 500 diatom valves per sample were counted on random transects in the light microscope (Nikon Optiphot II) with phase-contrast oil immersion objectives at a magnification of x1000. At some levels, however, diatom preservation was very poor. Counting techniques followed Battarbee (1986). Diatom concentration was estimated using microsphere markers (Battarbee and Kneen, 1982). Taxonomic identifications were mainly based on Krammer and Lange-Bertalot (19861991). Critical identifications were confirmed using a scanning electron microscope (JEOL JSM-6400 SEM). Total stomatocysts were enumerated simultaneously with diatoms. The absolute chronology used is based on radiocarbon dating of 2 bulk samples taken at 55cm and 285cm depth (Table 1). At 500 and 610 cm depth, dates were estimated through correlation of a pollen profile of the studied core (Santos, unpublished data) 6 with the chronology obtained by Maldonado Ruiz (1994). Linear interpolation was used to develop the chronological model, assuming present day for the core top. Diatom stratigraphic diagrams were drawn using the PSIMPOLL computer program (Bennett, 1992). The main zones of change in the diatom data were identified by classification; the method applied was binary splitting by information content, as implemented in the PSIMPOLL program. Square root transformation was used in order to optimise the signal to noise ratio. The "broken-stick" model was used to assess the significance of the diatom assemblage zones (Bennett, 1996). Diatoms have often been used to reconstruct lake acidification in water quality studies (Battarbee et al. 1990). Although there are several published diatom-based transfer functions for reconstructing pH in Europe (e.g. Birks et al. 1990; Korsman and Birks, 1996; Juggins et al. 1996), none of these functions were derived from lakes located near the studied area. In this paper diatom-inferred pH was derived using EDDI software for palaeoenvironmental reconstructions (Juggins, 2001). Lake-water pH values were reconstructed using weighted averaging (WA) (Birks et al. 1990) with inverse de-shrinking because the reconstructed values lay closer to the mean of the training set values (Juggins, 2001). The reliability of each reconstructed pH value was assessed using the percentage of taxa in the fossil sample not present in the modern calibration dataset. Experiments with simulated data (Birks, 1998) suggest that a reconstruction with <25% of fossil taxa not found in the modern dataset is reliable. The diatom taxa present in the Lagoa Grande core are all common and well represented in the European combined pH training set. Multivariate ordination techniques were performed using CANOCO (version 3.10) (ter Braak, 1988, 1990). To establish whether species responses were linear or unimodal, floristic variations in the diatom data were assessed using detrended 7 correspondence analysis (DCA; ter Braak, 1987). Unimodal ordination analysis was used because the gradient on the first axis was longer than 2 standard deviation units (3.353). Therefore, DCA (Hill and Gauch, 1980), a unimodal technique ordination, was used to identify the main gradients in the species data. This technique assumes that any structure within the data is a response to changes in a few unknown environmental variables (latent variables). This method also maximizes species dispersion along a hypothetical environmental gradient (ter Braak, 1987). All taxa with more than 2% abundance in at least two samples were used in the statistical analyses (see species list and codes used in the ordination plot in Table 2). Results Core lithology and chronology The sediment core is homogeneous in colour with no abrupt transitions. A detailed sediment lithology of the Lagoa Grande sequence is shown in Figures 2 and 3. The upper sediment is predominantly organic, while the lower part contains silt and clay. The estimated age for the bottom of the sequence is ca. 9850 yr BP indicating that the record spans nearly the entire Holocene period. The boundaries of the zones in years BP were estimated from these dates by linear interpolation. Diatom and chrysophyte stomatocyst stratigraphy Diatom remains are absent between 190 and 220 cm depth, and badly preserved in the most superficial sediments of Lagoa Grande. One hundred thirty-three diatom taxa, belonging to 17 different genera, were identified from the core material. The most significant features in the diatom stratigraphy are the high percentages of Aulacoseira species, particularly Aulacoseira nygaardii; the rise of Aulacoseira valida at 510 cm; and the dominance of Aulacoseira tethera between 130 and 60 cm. Another conspicuous feature is the peak in abundance of Aulacoseira ambigua and Navicula 8 leptostriata between 450 and 260 cm. Eight diatom assemblage zones were distinguished using cluster analysis (Figure 2). A brief description of diatom assemblage zones is given in Table 3. Inferred pH The diatom-inferred pH value at the beginning of the profile is around 6 and increased slightly over zone GDE2 reaching values above 6 pH units (Figure 2). The inferred pH varies between 6 and 7 during zones GDE2 and GDE3. There is, however, a small drop below pH 6 at 450 cm. Diatom-inferred pH experiences small excursions along zone GDE4 and GDE5, but the overall pH value reconstructed for these zones keeps around 6. The lowest inferred pH values of the whole record occurs in the uppermost three zones, with a drop at the beginning of zone GDE7 to a value around 5 pH units. Statistics Indirect ordination, DCA detrended by segments, was used to analyze the species variation in Lagoa Grande core samples (Figure 4a, b). DCA axis 1 (λ1=0.333; 29.5%) and DCA axis 2 (λ2=0.160; 14.1%) explain a total of 43.6% of the variance in the diatom percentage data (Figure 4a). The underlying environmental variables are interpreted from the species distribution within the DCA first two axes (Figure 4b). The first axis represents a pH gradient, with increasing pH to the left. Species at the low end of the axis include alkaliphilous species such as Aulacoseira valida, Aulacoseira ambigua, and Navicula atomus. At the high end of the first axis Eunotia diodon, Eunotia paludosa, Eunotia incisa and Cymbella hebridica all typify acid conditions in the lake. The second DCA axis is interpreted to represent nutrient levels. The higher scoring species for the second axis include Aulacoseira lirata, Cymbella perpusilla, Aulacoseira alpigena and Aulacoseira distans. These diatom species are associated 9 sediment inputs and the development of littoral vegetation, and not only change in the regional climate characterised by an atmospheric climate cooling. After 2000 yr BP Aulacoseira tethera became very abundant along with periphytic assemblages. Aulacoseira tethera is a facultative planktonic diatom indicative of oligotrophic conditions, and has been reported from waters with pH values ranging between 4.4 and 5.3 in upland lakes of Cumbria (United Kingdom) (Haworth et al. 1986). This diatom assemblage indicates shallow water, but with enough open water habitats for seasonal phytoplankton development. Relatively high lake levels are indicated by pulses of Aulacoseira tethera, while dry seasonal episodes are indicated by assemblages dominated by Eunotia spp. These episodes are probably related to the filling of the lake basin and the seasonality of the water level. Simultaneous high values of Cyperaceae pollen (Maldonado Ruiz, 1994) probably also relate to fluctuating lake levels that made suitable habitats for littoral vegetation during the dry season. The most recent part of the record is characterized by low concentrations and poor diatom preservation. Epiphytic and acidophilous taxa such as Eunotia, Cymbella and Pinnularia are more abundant. These species are common in peatbogs and occurr in oligotrophic and dystrophic waters (Krammer and Lange-Bertalot, 1986-1991). This suggests that modern conditions have existed since ca. 1000 yr BP in Lagoa Grande. The chrysophyte cyst to diatom ratio is high during this zone, although this also may be related to the better preservation of these more heavily silicified algal remains. Conclusions The diatom record presented here can be directly correlated with pollen derived climate changes, and therefore it is possible to draw the following conclusions concerning the response of the lake to climatic changes. 16 The diatom record suggests that the lake was originally acidic and oligotrophic due to cold climatic conditions. Aulacoseira ambigua increase indicates periods of high water levels and increased productivity in response to a warmer and more humid climate. Milder conditions would enhance catchment processes, promoting leaching and nutrient availability. Changes in diatom assemblages during the Holocene indicate fluctuating limnological conditions. These oscillations in Lagoa Grande record have been dated at ca. 8200, 6000, 4200 and 2600 yr BP. At these times diatom assemblages reflect fairly low lake productivity and cold water. Similar events within the Holocene have been observed in pollen profiles from the region, but there is no chronological coincidence in all sequences. (Allen et al. 1996; Sánchez Goñi 1999; Sánchez Goñi and Hannon, 1999). At Lagoa Grande the changes in the diatom record were contemporaneous with the shifts in vegetation communities, indicating that external forcing factors initiated these changes. These external factors may be local catchment processes or regional climatic cooling. Catchment processes have influenced Lagoa Grande over most of the Holocene, but only during the Late Holcene catchment changes played a controlling role in the lake development and concealed the effects of climate. Despite that, at present it is not possible to exclude rigorously the influence of local catchment processes as an explanation of these oscillations in lake history. Acknowledgements The lake sedimentary core was taken by J.R. Vidal, L. Santos and M.L. Caeiro. I thank R. Bao and L. Santos for helpful discussions and comments on the manuscript. Radiocarbon dates on the sediment samples were provided by Beta Analytic Inc. Miami (Florida) and funded by R. Bao. The Electron Microscopy Service of the 17 University of A Coruña provided facilities and technical help. This paper profited from the reviews by M.F. Sánchez Goñi and H.J.B. Birks. References Aira Rodríguez, M.J., 1986. Contribución al estudio de suelos fósiles, de montaña y antropógenos de Galicia por análisis polínico. PhD Thesis, Universidade de Santiago de Compostela, Spain, 363 pp. Allen, J.R., Huntley, M.B., Watts, W.A., 1996. The vegetation and climate of northwest Iberia over the last 14,000 yr. Journal of Quaternary Science 11, 125-147. Anderson, D.S., Davis, R.B., Berge, F., 1986. Relationships between diatom assemblages in lake surface-sediments and limnological characteristics in southern Norway. In Smol, J.P., Battarbee, R.W., Merilainen J. (Eds.), Diatoms and lake acidity. Dr Junk, Dordrecht, 97-109. Battarbee, R.W. 1986. Diatom analysis. In Berglund, B.E. (Ed.), Handbook of Holocene Palaeocology and Palaeohydrology. John Wiley Sons Ltd, Chichester, 527570. Battarbee, R.W., Kneen, M.J., 1982. The use of electronically counted microspheres in absolute diatom analysis. Limnology and Oceanography 27, 184-188. Battarbee, R.W., Mason, J., Renberg, I., Talling, J. F., 1990. Palaeolimnology and lake acidification. The Royal Society, London, 219 pp. Battarbee, R.W., Renberg, I., 1990. The Surface Water Acidification Project (SWAP) paleolimnology programme. Phil. Trans. R. Soc. Lond. B. 327, 227-232. Bennet, K.D., 1996. Determination of the number zones in a biostratigraphical sequence. New Phytologists 132, 155-170. 18 Bennett, K.D., 1992. PSIMPOLLa QuickBASIC program that generates PostScript page description files of pollen diagrams. Newsletter (INQUA Commission for the study of the Holocene: Working group on data-handling methods) 8, 11-12. Berger, A., Tricot, C., 1986. Global climatic changes and astronomical theory of paleoclimates. In Cazenave, A. (Ed.), Earth Rotation: Solved and Unsolved Problems. Reidel Publishing Company, Dordrecht, 111-129. Birks, H.J.B., 1995. Quantitative environmental reconstructions. In: D. Maddy, Brew, J.S. (Eds.), Statistical modelling of quaternary science data, Vol. 5, pp. 161-254. Quaternary Research Association, Technical Guide, Cambridge. Birks, H.J.B., 1998. Numerical tools in paleolimnology - progress, potentialities, and problems. Journal of Paleolimnology 20, 307-332. Birks, H.J.B., Line, J.M., Juggins, S., Stevenson, A.C., ter Braak, C.J.F., 1990. Diatoms and pH reconstruction. Philosophical Transactions of the Royal Society of London B 327, 263-27. Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I., Bonani, G., 1997. A pervasive millenial-scale cycle in North Atlantic Holocene and Glacial climates. Science 278, 1257-1266. Brugam, R.B., 1983. The relationship between fossil diatom assemblages and limnological conditions. Hydrobiologia 98, 223-235. Brugam, R.B., Swain, P., 2000. Diatom indicators of peatland development at Pogonia Bog Pond, Minnesota, USA. The Holocene 10, 453-464. Carballeira, A., González, B., Juste, J., Molina, A.M., Retuerto, R., Ucieda, F., 1982. Ensayo de definición de regiones ecológicas en función del clima. Cuenca de los ríos Bibey, Jares y Navea. Trabajos Compostelanos de Biología 9, 139-157. 19 Davis, B.A.S., Brewer, S., Stevenson, A.C., Guiot, J., Data, 2003. The temperature of Europe during the Holocene reconstructed from pollen data. Quaternary Science Reviews 22, 1701-1716. Davis, R.B., 1987. Paleolimnological diatom studies of acidification of lakes by acid rain: an application of quaternary science. Quaternary Science Reviews 6, 147-163. Denys L., 1991-1992. A check-list of the diatoms in the Holocene deposits of the western Belgian coastal plain with a survey of their apparent ecological requirements. I. Introduction, ecological code and complete list. Belsgische Geolgische Dienst. Professional Paper 246, 1-41. Desprat, S., Sanchez Goni, M.F., Loutre, M.-F., 2003. Revealing climatic variability of the last three millennia in northwestern Iberia using pollen influx data. Earth and Planetary Science Review 213, 63-78. Duplessy, J.C., Labeyrie, L., Arnold, M., Paterne, M., Duprat, J., Van Weering, T.C.E., 1992. Changes in surface salinity of the North Atlantic Ocean during the last deglaciation. Nature 358, 485-488. Fritz, S., 1996. Paleolimnological records of climatic change in North America. Limnology and Oceanography 41, 882-889. Gasse, F., 1986. East African Diatoms: Taxonomy, Ecological Distribution. J. Cramer, Berlin, 292 pp. Gasse, F., Van Campo, E., 1998. A 40,000-yr pollen and diatom record from lake Tritivakely, Madagascar, in the Southern Tropics. Quaternary Research 49, 299-311. Harrison, S.P., Yu, G., Tarasov, P.E., 1996. Late Quaternary lake-level record from northern Eurasia. Quaternary Research 45, 138-159. 20 Haworth, E.Y., Atkinson, K.M., Newell, P.S., 1986. Distribution of certain diatom taxa in the water bodies of Cumbria, U.K. In Round, F. (Ed.) Proceedings of the 9th International Diatom Symposium. Biopress, Bristol, 17-27. Hill, M.O., Gauch, H.E.J., 1980. Detrended correspondence analysis: an improved ordination technique. Vegetatio 42, 47-58. Huntley, B., Prentice, I.C., 1988. July temperatures in Europe from pollen data, 6,000 years before present. Science 241, 687-690. Izco Sevillano, J., 1987. Galicia. In Lorca, M.P., Martínez, S.R., (Eds.) La Vegetación en España. Servicio de Publicaciones Universidad Alcalá de Henares, Madrid, 385418. Jowsey, P. C., 1966. An improved peat sampler. New Phytologists 65, 245-248. Juggins, S., 2001. The European Diatom Database. User Guide. Version 1.0. Department of Geography (University of Newcastle), 72 pp. Juggins, S., Flower, R.J., Battarbee, R.W., 1996. Paleolimnological evidence for recent chemical and biological changes in U.K. acid Waters Monitoring Network sites. Freshwater Biology 36, 203-219. Julià, R., Burjachs, F., Giralt, S., 1997. Redepositional or climatic event in Iberia during the Boreal. Journal of Quaternary Science 12, 73-74. Korsman, T., Birks, H.J.B., 1996. Diatom-based water chemistry reconstructions from northern Sweden: a comparison of reconstruction techniques. Journal of Paleolimnology 15, 65-77. Krammer, K., Lange-Bertalot, H., 1986-1991. Bacillariophyceae. In: Ettl, H., Gerloff, J., Heynig, H., Mollenhauer, D. (Eds.), Süßwasserflora von Mitteleuropa. Fischer, Stuttgart. 21 Leira, M., Santos, L., 2002. An early Holocene short climatic event in the northwest Iberian Peninsula inferred from pollen and diatoms. Quaternary International 93-94, 3-12. Lepistö, L., 1990. Some centric diatoms as indicators of water quality in Finnish lakes. In Simola, H. (Ed.) Proceedings of the 10th International Diatom Symposium,. Koeltz, Koenigstein, 499-504. Magny, M., 1993. Holocene fluctuations of lake levels in the French Jura and subAlpine ranges, and their implciations for past general circulation patterns. The Holocene 3, 306-313. Magny, M., 2004. Holocene climate variability as reflected by mid-European lakelevel fluctuations and its probable impact on prehistoric human settlements. Quaternary International 113, 65-79. Magny, M., Begeot, C., Guiot, J., Peyron, O., 2003. Contrasting patterns of hydrological changes in Europe in response to Holocene climate cooling phases. Quaternary Science Reviews 22, 1589-1596. Maldonado Ruiz, F.J., 1994. Evolución tardiglaciar y holocena de la vegetación en los macizos del Noroeste Peninsular. PhD Thesis. Universidad Politécnica de Madrid, Spain, 171 pp. Martínez-Cortizas, A., Pontevedra-Pombal, X., García-Rodeja, E., Nóvoa-Muñoz, J.C., Shotyk, W., 1999. Mercury in a Spanish peat bog: archive of climate change and atmospheric metal deposition. Science 284, 939-942. Mayewski, P.A., Rohling, E.E., Stager, J.C., Karlen, W., Maasch, K.A., Meeker, L.D., Meyerson, E.A., Gasse, F., van Kreveld, S., Holmgren, K., Lee-Thorp, J., Rosqvist, G., Rack, F., Staubwasser, M., Schneider, R.R., Steig, E.J., 2004. Holocene climate variability. Quaternary Research 62, 243-255. 22 Moser, K. A., MacDonald, G.M., Smol, J.P., 1996. Applications of freshwater diatoms to geographical research. Progress in Physical Geography 20, 21-52. Muñoz Sobrino, C., Ramil-Rego, P., Rodríguez Guitián, M.A., 2001. Vegetation in the mountains of northwest Iberia during the last glacial-interglacial transition. Vegetation History and Archaeobototany 10, 7-21. Peñalba, M.C., 1989. Dynamique de végétation tardiglaciaire et Holocène du CentreNord de l’Espagne d’après l’analyse pollinique. PhD Thesis. Université dÀix, Marseille, France, 165 pp. Peñalba, M.C., 1994. The history of the Holocene vegetation in northern Spain from pollen analysis. Journal of Ecolology 82, 815-832. Pérez Alberti, A., Rodríguez Guitián, M., Valcárcel Díaz, M., 1993. Las formas y los depósitos glaciares en las Sierras Orientales y Septentrionales de Galicia (NW Península Ibérica). In Pérez Alberti, A., Guitián Rivera, L., Ramil Rego, P. (Eds), La evolución del paisaje en el entorno de los caminos jacobeos. Cambios ambientales y actividad humana. Xunta de Galicia, Santiago de Compostela, pp 61-90. Pons, A., Reille, M., 1988. The Holocene and Upper Pleistocene pollen record from Padul (Granada, Spain): a new study. Palaeogeography, Palaeoclimatology, Palaeoecology 66, 243-263. Prather, C., Hickman, M., 2000. History of a presently slightly acidic lake in northeastern Alberta, Canada, as determined through analysis of the diatom record. Journal of Paleolimnology 24, 183-198. Psenner, R., 1994. Environmental impacts on freshwaters: acidification as a global problem. The Science of the Total Environment. 143, 56-61. Psenner, R., Schmidt, R., 1992. Climate-driven pH control of remote alpine lakes and effects of acid deposition. Nature 356, 781-783. 23 Ramil Rego, P., Muñoz Sobrino, C., Rodríguez Guitián, M., Gómez Orellana, L., 1998. Differences in the vegetation of North Iberian Peninsula during the last 16,000 years. Plant Ecology 138, 41-62. Renberg, I., 1990. A procedure for preparing large sets of diatom slides from sediment cores. Journal of Paleolimnology. 4, 87-90. Renberg, I., Korsman, T., Birks, H.J.B., 1993. Prehistoric increases in the pH of acidsensitive Swedish lakes caused by land-use changes. Nature. 362, 824-827. Sánchez Goñi, M.F., 1999. Holocene climate evolution: a high resolution pollen record from Las Pardillas Lake (Iberian Cordillera, Spain). Records of Environmental and Climate Change in the Mediterranean Region. The Lacustrine Contribution. 1st Workshop Southern Europe Working Group "European Lake Drilling Program" (ELDP). 26th-27th November, Pyrenean Institute of Ecology-CSIC, Zaragoza, Spain. Sánchez Goñi, M.F., Hannon, G., 1999. High altitude vegetational pattern on the Iberian Mountain Chain (North-Central Spain) during the Holocene. The Holocene 9, 39-57. Santos, L., 1996. Estudio de la deglaciación finicuaternaria en el NW de la Península Ibérica: datos paleobotánicos y geomorfológicos. PhD Thesis. Universidade da Coruña, Spain, 218 pp. Santos, L., Vidal Romani, J.R., Jalut, G., 2000. History of vegetation during the Holocene in the Courel and Queixa Sierras, Galicia, northwest Iberian Peninsula. Journal of Quaternary Science 15, 621-632. Siver, P.A., 1991. The Biology of Mallomonas: Morphology, Taxonomy and Ecology. Kluwer Academic, Dordrecht. 230 pp. Siver, P.A., Kling, H., 1997. Morphological observations of Aulacoseira using scanning electron microscopy. Canadian Journal of Botany 61, 2195-2204. 24 Smol, J.P., 1985. The ratio of diatom frustules to chrysophycean statospores: A useful paleolimnological index. Hydrobiologia 123, 199-208. Smol, J.P., Cumming, F., 2000. Tracking long-term changes in climate using algal indicators in lake sediments. Journal of Phycology 36, 986-1011. ter Braak, C.J.F., 1987. Ordination. In Jongman, R.H.G., ter Braak, C.J.F. (Eds.) Data Analysis in Community and Landscape Ecology. Centre for Agricultural Publishing and Documentation (Pudoc), Wageningen, 91-173. ter Braak, C.J.F., 1988. CANOCO-a FORTRAN program for canonical community ordination by [partial] [detrended] [canonical] correspondence analysis, principal analysis and redundancy analysis (version 2.1). TNO Institute of Applied Computer Science, Wageningen, The Netherlands, 95 pp. ter Braak, C.J.F., 1990. Update notes: CANOCO version 3.1. Agricultural Mathematics Group, 6700 AC Wageningen, The Netherlands, 35 pp. Troels-Smith, J., 1955. Karakterisaring af løse jordarter. Danmarks Geologiske Undersøgelse, Series IV 3, 39-73. Turner, C., Hannon, G.E., 1988. Vegetational evidence for late Quaternary climatic changes in southwest Europe in relation to the influence of the North Atlantic Ocean. Philosophical Transactions of the Royal Society of London B 318, 451-485. Vidal Romaní, J.R., Santos, L., 1994. La deglaciación finicuaternaria en el Noroeste Peninsular (Serra de Queixa-Invernadoiro, Ourense, Galicia): datos geomorfológicos y paleobotánicos. Cuaternario y Geomorfología 8, 33-44. Watts, W.A., 1986. Stages of climatic change from full glacial to Holocene in northwest Spain, southern France and Italy: a comparison of the atlantic coast and the mediterranean basin. In A. Ghazi R. Fantechi (eds.) Current Issues in Climate 25 FIGURE CAPTIONS Figure 1.- Map of Spain showing the geographical position of the Serra de Queixa. The detailed map shows the location of Lagoa Grande sequence and its catchment. Figure 2.- Diatom summary diagram of the core from Lagoa Grande. Only selected taxa are shown (taxa>2%) along with a plot showing the diatom-inferred pH. Diatom assemblages zones (DAZ) and lithology are also shown. Asterites indicate the ages attributed by correlation with another core analysed by Maldonado Ruiz (1994). Figure 3.- Diatom concentrations expressed as total number of diatom valves per gram of sediment and stomatocyst to diatom ratio (C/D). Percentage of euplanktonic, tychoplanktonic and benthic taxa in Lagoa Grande core are also shown. Asterites indicate the ages attributed by correlation with another core analysed by Maldonado Ruiz (1994). Figure 4.- Plot of the DCA scores for the first two ordination axis for Lagoa Grande record samples. The samples of the different diatom assemblages zones are indicated with different symbols (GDE1 z, GDE2 , GDE3 , GDE4 , GDE5 , GDE6 ¡, GDE7 , GDE8 ) (a).Plot of the DCA scores for the first two ordination axes for the Lagoa Grande record species. Corresponding taxa names are given in Table 2 (b). Figure 5.- Summarised Holocene pollen diagram from Lagoa Grande (Laguna de Las Lamas) of Maldonado Ruiz (1994), redrawn. 32 Table 1.- Radiocarbon dates from the Lagoa Grande sequence. The calibrated age range (Stuiver & Reimer, 1993) was calculated with a probability of 95.4% (two sigma). Depth (cm) Laboratory reference Years 14C (BP) Years cal. (BP) Years cal. AD(+) BC (-) δ13 C (‰) 55-56 ß-134913 1050±50 1060-910 890, 1040 -26.1 285-286 ß-134914 4010±70 4640-4280 -2690, -2330 -24.7 Table 2.- Diatom taxa used in the statistical analysis and the corresponding codes used in the ordination plot. Code Species Name ACH HEL Achnanthes helvetica (Hustedt) Lange-Bertalot AUL ALP Aulacoseira alpigena (Grunow) Krammer AUL AMB Aulacoseira ambigua (Grunow) Simonsen AUL DIS Aulacoseira distans (Ehrenberg) Simonsen AUL LIR Aulacoseira lirata (Ehrenberg) Ross AUL NIL Aulacoseira distans var. nivalis (W. Smith) Haworth AUL NIV Aulacoseira distans var. nivaloides Camburn AUL NYG Aulacoseira nygaardii Camburn AUL SP1 Aulacoseira sp 1 AUL SPP Aulacoseira spp. AUL TET Aulacoseira tethera Haworth AUL VAL Aulacoseira valida (Grunow) Krammer CYM GRA Cymbella gracilis (Ehrenberg) Kützing CYM HEB Cymbella hebridica (Grunow) Cleve CYM PER Cymbella perpusilla var. striator (Kalbe) Krammer EUN CAR Eunotia carolina Patrick EUN DIO Eunotia diodon Ehrenberg EUN EXI Eunotia exigua (Brébisson) Rabenhorst EUN INC Eunotia incisa Gregory EUN PAL Eunotia paludosa Grunow EUN SER Eunotia serra Ehrenberg FRA COv Fragilaria construens f. venter (Ehrenberg) Hustedt FRA VIR Fragilaria virescens Ralfs GOM GRA Gomphonema gracile Ehrenberg GOM PAR Gomphonema parvulum (Kützing) Kützing NAV ATO Navicula atomus (Kützing) Grunow NAV LEP Navicula leptostriata Jørgensen NEI ALP Neidium alpinum Hustedt PIN GIB Pinnularia gibba Ehrenberg PIN INT Pinnularia interrupta W. Smith PIN MIC Pinnularia microstauron (Ehrenberg) Cleve PIN SPP Pinnularia spp PIN SUB Pinnularia subcapitata Gregory TAB FLO Tabellaria flocculosa (Roth) Kützing 33 Table 3.- Description of the local diatom assemblage zones in Lagoa Grande. DAZ Depth (cm) Age (yr BP)/(cal yr BP) Description GD1 620-575 98509200 11300-10450 The first three zones represent the early Holocene. Diatom concentration is relatively low in the beginning (0.56 x 109 valves g-1 of dry matter) although it increases rapidly. Small acidophilous tychoplanktonic Aulacoseira including A. lirata (24%) A. alpigena (18%) A. tethera (17%) A. distans (9%) and A. nygaardii (8%) dominate the assemblage. The mesotrophic acidophilous taxon Tabellaria flocculosa is also well represented in this initial flora with an abundance of 16%. The chrysophyte cyst to diatom frustule (C/D) ratio ranges from 0.08 to 0.16. GDE2 575-515 92008300 10450-9250 Facultative planktonic species continue to dominate. Aulacoseira nygaardii and Aulacoseria distans var. nivaloides account for 50% of the assemblage at 530 cm depth. The euplanktonic Aulacoseira ambigua became more abundant (12%). This mesotrophic alkaliphilous species is the only euplanktonic taxon observed throughout the record. Diatom concentrations decrease sharply down to 0.7 x 109 valves g-1. GDE3 515-455 8300-7200 9250-8000 Aulacoseira distans var. nivaloides and Aulacoseira nygaardii continue to dominate the assemblage. There is significant change in the diatom flora with the decline of Aulacoseira ambigua which disappears while Aulacoseira valida increases up to 21% at 500 cm. Diatom concentrations drop to 0.2 x 109 valves g-1 at the beginning of this zone and increase again towards the end. The cyst to diatom ratio remains low and relatively constant ranging from 0.9 to 0.11. GDE4 455-255 7200-3600 8000-4100 This zone represents the mid-Holocene period. It is defined by an increase of the euplanktonic Aulacoseira ambigua, and the decrease and disappearance of Aulacoseira valida. Aulacoseira distans var. nivaloides and Aulacoseira nygaardii remain the dominant species. The acidophilous and mesooligotrophic species Navicula leptostriata is also abundant. After an initial increase at 450 cm depth (5.8 x 109 valves g-1) diatom concentrations decrease coinciding with the highest sediment accumulation rate. The cyst to diatom ratio is fairly constant throughout this zone peaking at 390 cm (0.21). GDE5 255-225 3600–3200 4100-3650 This zone represents the latter half of the mid-Holocene. Diatom abundances are very low. Aulacoseira ambigua has disappeared at the end of the previous zone and the benthic Navicula leptostriata decreases to less than 2% while the tychoplanktonic and oligomesotrophic Aulacoseira nygaardii increase to represent nearly 59% of the assemblage. 225-185 3200-2700 3650-3050 Diatoms are absent or too sparse too count. GDE6 185-145 2700-2200 3050-2400 Diatom abundance and preservation is very poor. The chrysophyte cyst to diatom ratio increases from 0.11 to 0.24. It is characterized by a relatively high diversity of periphytic acidophilous taxa (Eunotia spp. Cymbella spp. and Pinnularia spp.) while tychoplanktonic species decreased to 7% at the start. Epiphytic Tabellaria flocculosa and Eunotia serra dominate the assemblages. At the end of this zone a peak of Aulacoseira nygaardii is also noticeable. GDE7 145-55 2200-1000 2400-1000 The most abundant species is Aulacoseira tethera followed by Eunotia carolina Tabellaria flocculosa and Fragilaria virescens. A. distans var. distans A. distans var. nivaloides and A. nygaardii are also abundant in the uppermost part of this zone. Overall cyst to diatom ratio decreases. 34 GDE8 55-0 1000-0 100-0 Diatom concentration is the lowest of the whole record and diatom valves are badly preserved. The diatom flora is dominated by the acidophilous taxa Eunotia spp. Cymbella spp. and Pinnularia spp. The chrysophyte cyst to diatom valves ratio increases. 35