Effects of building up a dam in a shallow mountain lake (Baciver, Central Pyrenees)

Changes in the water, the sediment, and the structure and dynamics of the submerged macrophyte populations were studied after the building up of a dam in a high mountain Pyrenean lake in late summer 1990. The most outstanding variations in the physical and chemical features were: (a) a 5.5 m rise of the shore level, (b) a decrease in light transmittance into the water column, (c) an oxygen depletion in deep waters, coupled to a pH decrease and a rise in conductivity and cation concentration, (d) an increase in the total dissolved nitrogen (mainly ammonium) and phosphorus concentration in the water column, and (e) an increase in the reduced N-compounds (nitrite and ammonium) in the sediment pore-water of the Isoetes lacustris community, coupled to a decrease in nitrate. Changes in the chemical features seem to be related to the decomposition of organic matter provided by the flooded terrestrial subalpine meadows and shrubs, and the lake's submersed vegetation. Both the area occupied by the submersed vegetation and the number of macrophyte species suffered an important reduction with the daming of Lake Baciver. All the small, non dominant species (Eleocharis acicularis, Subularia aquatica and Isoetes setacea) dissappeared along the summer 1991. Sparganium angustifolium experienced a degradation (low density of shoots, short leaves) in 1991 which lead to its complete die off in spring 1992. The strong attenuation of the light regime could account for these losses. Isoetes lacustris individuals originally living below 0.6 m depth were completely dead in summer 1991, probably due to an oxygen depletion in winter time. The only surviving Isoetes plants (those originally living abo ye 0.6 m depth which were between 5.8 and 6.1 m depth after the dam) increased its leaf length as a response to the decrease in available irradiance. Although the number of produced leaves was slightly lower in 1991 than it was before the daming, the production of biomass increased, since leaves were longer. However, the lost of leaves was very high, resulting in a global decay of the population. In late spring 1992 surviving Isoetes lacustris had the remaining leaves very damaged, probably because of the winter anoxy, and no leaves were produced in summer 1992. This brought a complete dissapearence of the community in the lake.Efectos del represamiento de un lago somero de montaña (Baciver, Pirineos centrales). Se estudiaron los cambios en el agua, el sedimento y la estructura y dinámica de las poblaciones de macrófitos sumergidos después del represamiento de un lago de alta montaña de los Pirineos a finales del verano de 1990. Las variaciones más notables de las características físicas y químicas fueron: a) elevación de 5,5 m del nivel del litoral; b) disminución de la transmitancia de la luz en la columna de agua; c) depleción de oxígeno en las aguas profundas, junto a la reducción del pH y al aumento de la conductividad y de la concentración de cationes; d) aumento de la concentración de nitrógeno disuelto total (principalmente amonio) y de fósforo en la columna de agua, y e) aumento de los compuestos de N reducidos (nitrito y amonio) en el agua intersticial de la comunidad de Isoetes lacustris, ligada a una disminución del nitrato. Los cambios en las características químicas parecen estar relacionados con la descomposición de la materia orgánica aportada por los prados y matorrales subalpinos terrestres, así como por la vegetación sumergida del lago. Tanto el área ocupada por la vegetación sumergida como el número de especies de macrófitos experimentaron una importante reducción con el represamiento del lago Baciver. Todas las especies pequeñas, no dominantes (Eleocharis acicularis, Subularia aquatica e Isoetes setacea), desaparecieron a lo largo del verano de 1991. Sparganium angustifolium experimentó un proceso de degradación (baja densidad de pies, hojas cortas) en 1991 que condujo a su completa desaparición en la primavera de 1992. La fuerte atenuación del régimen de iluminación podría explicar dichas pérdidas. Los individuos de Isoetes lacustris que originalmente vivían por debajo de los 0,6 m de profundidad estaban completamente muertos en el verano de 1991, probablemente debido a la depleción invernal de oxígeno. Las únicas plantas de Isoetes que sobrevivieron (las que originalmente vivían por encima de los 0,6 m de profundidad y que después del represamiento se hallaban entre 5,8 y 6,1 m) aumentaron su longitud foliar como respuesta a la reducción de irradiancia disponible. Aunque el número de hojas producidas fue ligeramente menor en 1991 de lo que era antes del represamiento, la biomasa producida aumentó, pues las hojas eran más largas. Sin embargo, la pérdida de hojas fue muy grande, lo que produjo una reducción global de la población. A finales de la primavera de 1992, los Isoetes lacustris supervivientes tenían las hojas restantes muy dañadas, probablemente debido a la anoxia invernal, y no se produjo ninguna hoja nueva en el verano de 1992. Esto condujo a la desaparición total de la comunidad en el lago

Effects of seagrasses and algae of the Caulerpa family on hydrodynamics and particle-trapping rates

The widespread decline of seagrass beds within the Mediterranean often results in the replacement of seagrasses by opportunistic green algae of the Caulerpa family. Because Caulerpa beds have a different height, stiffness and density compared to seagrasses, these changes in habitat type modify the interaction of the seafloor with hydrodynamics, influencing key processes such as sediment resuspension and particle trapping. Here, we compare the effects on hydrodynamics and particle trapping of Caulerpa taxifolia, C. racemosa, and C. prolifera with the Mediterranean seagrasses Cymodocea nodosa and Posidonia oceanica. All macrophyte canopies reduced near-bed volumetric flow rates compared to bare sediment, vertical profiles of turbulent kinetic energy revealed peak values around the top of the canopies, and maximum values of Reynolds stress increased by a factor of between 1.4 (C. nodosa) and 324.1 (P. oceanica) when vegetation was present. All canopies enhanced particle retention rates compared to bare sediment. The experimental C. prolifera canopy was the most effective at particle retention (m2 habitat); however, C. racemosa had the largest particle retention capacity per structure surface area. Hence, in terms of enhancing particle trapping and reducing hydrodynamic forces at the sediment surface, Caulerpa beds provided a similar or enhanced function compared to P.oceanica and C. nodosa. However, strong seasonality in the leaf area index of C. racemosa and C. taxifolia within the Mediterranean, combined with a weak rhizome structure, suggests that sediments maybe unprotected during winter storms, when most erosion occurs. Hence, replacement of seagrass beds with Caulerpa is likely to have a major influence on annual sediment dynamics at ecosystem scales.This research was funded by the European Network of Excellence ‘‘Marine Biodiversity and Ecosystem Function’’ (MarBEF); FP6, EC contract no. 505446 and a grant from the Fundacio ´n BBVA. EPM was supported by a European Union Marie Curie host fellowship for transfer of knowledge, MTKD-CT-2004-509254, the Spanish national project EVAMARIA (CTM2005-00395/MAR) and the regional government of Andalusia project FUNDIV(P07-RNM-2516)

Habitat and Scale Shape the Demographic Fate of the Keystone Sea Urchin Paracentrotus lividus in Mediterranean Macrophyte Communities

Demographic processes exert different degrees of control as individuals grow, and in species that span several habitats and spatial scales, this can influence our ability to predict their population at a particular life-history stage given the previous life stage. In particular, when keystone species are involved, this relative coupling between demographic stages can have significant implications for the functioning of ecosystems. We examined benthic and pelagic abundances of the sea urchin Paracentrotus lividus in order to: 1) understand the main life-history bottlenecks by observing the degree of coupling between demographic stages; and 2) explore the processes driving these linkages. P. lividus is the dominant invertebrate herbivore in the Mediterranean Sea, and has been repeatedly observed to overgraze shallow beds of the seagrass Posidonia oceanica and rocky macroalgal communities. We used a hierarchical sampling design at different spatial scales (100 s, 10 s and <1 km) and habitats (seagrass and rocky macroalgae) to describe the spatial patterns in the abundance of different demographic stages (larvae, settlers, recruits and adults). Our results indicate that large-scale factors (potentially currents, nutrients, temperature, etc.) determine larval availability and settlement in the pelagic stages of urchin life history. In rocky macroalgal habitats, benthic processes (like predation) acting at large or medium scales drive adult abundances. In contrast, adult numbers in seagrass meadows are most likely influenced by factors like local migration (from adjoining rocky habitats) functioning at much smaller scales. The complexity of spatial and habitat-dependent processes shaping urchin populations demands a multiplicity of approaches when addressing habitat conservation actions, yet such actions are currently mostly aimed at managing predation processes and fish numbers. We argue that a more holistic ecosystem management also needs to incorporate the landscape and habitat-quality level processes (eutrophication, fragmentation, etc.) that together regulate the populations of this keystone herbivore

Seagrass Canopy Photosynthetic Response Is a Function of Canopy Density and Light Environment: A Model for Amphibolis griffithii

A three-dimensional computer model of canopies of the seagrass Amphibolis griffithii was used to investigate the consequences of variations in canopy structure and benthic light environment on leaf-level photosynthetic saturation state. The model was constructed using empirical data of plant morphometrics from a previously conducted shading experiment and validated well to in-situ data on light attenuation in canopies of different densities. Using published values of the leaf-level saturating irradiance for photosynthesis, results show that the interaction of canopy density and canopy-scale photosynthetic response is complex and non-linear, due to the combination of self-shading and the non-linearity of photosynthesis versus irradiance (P-I) curves near saturating irradiance. Therefore studies of light limitation in seagrasses should consider variation in canopy structure and density. Based on empirical work, we propose a number of possible measures for canopy scale photosynthetic response that can be plotted to yield isoclines in the space of canopy density and light environment. These plots can be used to interpret the significance of canopy changes induced as a response to decreases in the benthic light environment: in some cases canopy thinning can lead to an equivalent leaf level light environment, in others physiological changes may also be required but these alone may be inadequate for canopy survival. By providing insight to these processes the methods developed here could be a valuable management tool for seagrass conservation during dredging or other coastal developments

Positive Feedbacks in Seagrass Ecosystems – Evidence from Large-Scale Empirical Data

Positive feedbacks cause a nonlinear response of ecosystems to environmental change and may even cause bistability. Even though the importance of feedback mechanisms has been demonstrated for many types of ecosystems, their identification and quantification is still difficult. Here, we investigated whether positive feedbacks between seagrasses and light conditions are likely in seagrass ecosystems dominated by the temperate seagrass Zostera marina. We applied a combination of multiple linear regression and structural equation modeling (SEM) on a dataset containing 83 sites scattered across Western Europe. Results confirmed that a positive feedback between sediment conditions, light conditions and seagrass density is likely to exist in seagrass ecosystems. This feedback indicated that seagrasses are able to trap and stabilize suspended sediments, which in turn improves water clarity and seagrass growth conditions. Furthermore, our analyses demonstrated that effects of eutrophication on light conditions, as indicated by surface water total nitrogen, were on average at least as important as sediment conditions. This suggests that in general, eutrophication might be the most important factor controlling seagrasses in sheltered estuaries, while the seagrass-sediment-light feedback is a dominant mechanism in more exposed areas. Our study demonstrates the potentials of SEM to identify and quantify positive feedbacks mechanisms for ecosystems and other complex systems

Positive Feedbacks in Seagrass Ecosystems – Evidence from Large-Scale Empirical Data

Positive feedbacks cause a nonlinear response of ecosystems to environmental change and may even cause bistability. Even though the importance of feedback mechanisms has been demonstrated for many types of ecosystems, their identification and quantification is still difficult. Here, we investigated whether positive feedbacks between seagrasses and light conditions are likely in seagrass ecosystems dominated by the temperate seagrass Zostera marina. We applied a combination of multiple linear regression and structural equation modeling (SEM) on a dataset containing 83 sites scattered across Western Europe. Results confirmed that a positive feedback between sediment conditions, light conditions and seagrass density is likely to exist in seagrass ecosystems. This feedback indicated that seagrasses are able to trap and stabilize suspended sediments, which in turn improves water clarity and seagrass growth conditions. Furthermore, our analyses demonstrated that effects of eutrophication on light conditions, as indicated by surface water total nitrogen, were on average at least as important as sediment conditions. This suggests that in general, eutrophication might be the most important factor controlling seagrasses in sheltered estuaries, while the seagrass-sediment-light feedback is a dominant mechanism in more exposed areas. Our study demonstrates the potentials of SEM to identify and quantify positive feedbacks mechanisms for ecosystems and other complex systems

Lack of association between PRNP 1368 polymorphism and Alzheimer's disease or vascular dementia

<p>Abstract</p> <p>Background</p> <p>Polymorphisms of the prion protein gene (<it>PRNP</it>) at codons 129 and 219 play an important role in the susceptibility to Creutzfeldt-Jakob disease (CJD), and might be associated with other neurodegenerative disorders. Several recent reports indicate that polymorphisms outside the coding region of <it>PRNP </it>modulate the expression of prion protein and are associated with sporadic CJD, although other studies failed to show an association. These reports involved the polymorphism <it>PRNP </it>1368 which is located upstream from <it>PRNP </it>exon 1. In a case-controlled protocol, we assessed the possible association between the <it>PRNP </it>1368 polymorphism and either Alzheimer's disease (AD) or vascular dementia (VaD).</p> <p>Methods</p> <p>To investigate whether the <it>PRNP </it>1368 polymorphism is associated with the occurrence of AD or VaD in the Korean population, we compared the genotype, allele, and haplotype frequencies of the <it>PRNP </it>1368 polymorphism in 152 AD patients and 192 VaD patients with frequencies in 268 healthy Koreans.</p> <p>Results and conclusion</p> <p>Significant differences in genotype, allele and haplotype frequencies of <it>PRNP </it>1368 polymorphism were not observed between AD and normal controls. There were no significant differences in the genotype and allele frequencies of the <it>PRNP </it>1368 polymorphism between Korean VaD patients and normal controls. However, in the haplotype analysis, haplotype Ht5 was significantly over-represented in Korean VaD patients. This was the first genetic association study of a polymorphism outside the coding region of <it>PRNP </it>in relation to AD and VaD.</p

Genetic Cross-Interaction between APOE and PRNP in Sporadic Alzheimer's and Creutzfeldt-Jakob Diseases

Alzheimer's disease (AD) and Creutzfeldt-Jakob disease (CJD) represent two distinct clinical entities belonging to a wider group, generically named as conformational disorders that share common pathophysiologic mechanisms. It is well-established that the APOE ε4 allele and homozygosity at polymorphic codon 129 in the PRNP gene are the major genetic risk factors for AD and human prion diseases, respectively. However, the roles of PRNP in AD, and APOE in CJD are controversial. In this work, we investigated for the first time, APOE and PRNP genotypes simultaneously in 474 AD and 175 sporadic CJD (sCJD) patients compared to a common control population of 335 subjects. Differences in genotype distribution between patients and control subjects were studied by logistic regression analysis using age and gender as covariates. The effect size of risk association and synergy factors were calculated using the logistic odds ratio estimates. Our data confirmed that the presence of APOE ε4 allele is associated with a higher risk of developing AD, while homozygosity at PRNP gene constitutes a risk for sCJD. Opposite, we found no association for PRNP with AD, nor for APOE with sCJD. Interestingly, when AD and sCJD patients were stratified according to their respective main risk genes (APOE for AD, and PRNP for sCJD), we found statistically significant associations for the other gene in those strata at higher previous risk. Synergy factor analysis showed a synergistic age-dependent interaction between APOE and PRNP in both AD (SF = 3.59, p = 0.027), and sCJD (SF = 7.26, p = 0.005). We propose that this statistical epistasis can partially explain divergent data from different association studies. Moreover, these results suggest that the genetic interaction between APOE and PRNP may have a biological correlate that is indicative of shared neurodegenerative pathways involved in AD and sCJD