Diatom responses to warming, heavy rains and human impact in a Mediterranean lake since the preindustrial period

In the Mediterranean region, annual mean air temperature will continue to increase during the 21st century, while seasonal precipitation is expected to decrease and extreme events to be more frequent. Human-induced climate change will severely impact aquatic ecosystems. A subdecadal stratigraphic diatom record of Lake Montcortès (central Pyrenees) was investigated, focusing on the potential responses of diatoms to anthropogenic warming and catchment alteration. The study includes the end of the Little Ice Age (LIA), the transition to the industrial and postindustrial eras, and the recent global warming and its current acceleration. Sediment samples were treated and diatoms taxonomically identified. Relationships between diatom taxa abundances and climatic (temperature and precipitation) and environmental (land use, soil erosion, and eutrophication) variables were investigated using multivariate statistical methods. The results indicate that, from ca. 1716 to 1971 CE, the diatom community was dominated by Cyclotella cyclopuncta and showed small perturbations, despite the pressure of important stressors such as strong cooling episodes, droughts and an intense use of the lake for hemp retting during the 18th and 19th centuries. However, during the 20th century, other centric species gained relevance, and from the 1970s on, Cyclotella ocellata competed with C. cyclopuncta for dominance. These changes coincided with pulse-like disturbances in the form of extreme rainfall events along with the gradual 20th century increase in global temperature. These perturbations affected the planktonic diatom community and led to instability dynamics. The benthic diatom community did not reflect any comparable shifts under the effect of the same climatic and environmental variables. Because heavy rainfall episodes are likely to intensify with current climate change in the Mediterranean region, their importance as stressors of planktonic primary producers should be taken into account as potential disrupters of biogeochemical cycles and trophic networks of lakes and ponds.

Regional precipitation trends since 1500 CE reconstructed from calcite sublayers of a varved Mediterranean lake record (Central Pyrenees)

The Mediterranean region is expected to be highly impacted by global warming, although the uncertainty of future scenarios, particularly about precipitation patterns remains quite large. To better predict shifts in its current climate system and to test models, more regional climate records are needed spanning longer than the instrumental period. Here we provide a high-resolution reconstruction of autumn precipitation for the Central Pyrenees since 1500 CE based on annual calcite sublayer widths from Montcortès Lake (Central southern Pyrenees) varved sediments. The 500-yr calcite data series was detrended and calibrated with instrumental climate records by applying correlations and cross-correlations to regional precipitation anomalies. Highest relationships were obtained between a composite calcite series and autumn precipitation anomalies for the complete calibration period (1900-2002) and for the two halves of the full period. Applied statistical tests were significant, evidencing that the climatic signal could be reconstructed. The reconstructed precipitation anomalies show interdecadal shifts, and rainfall decrease within the coldest period of the LIA and during the second half of the 20th century, probably associated to current Global Warming. Neither increasing nor decreasing linear trends or periods of extreme precipitation events were identified. Our results are coherent with other palaeohydrological reconstructions for northern Iberian Peninsula. Correlations between the predicted autumn precipitation and the main teleconnections -NAO, ENSO and WEMO- were weak, although a potential relationship with the Atlantic Multidecadal Oscillation (AMO) pattern is suggested. The obtained reconstruction provides the first estimations of regional autumn precipitation shifts in the Central Pyrenees and is one of the few reconstructions that cover annual-to-century scale climate variability of precipitation in the Mediterranean region from the end of the Litte Ice Age (LIA) to the current period of Global Warming.

DNA metabarcoding reveals differences in distribution patterns and ecological preferences among genetic variants within some key freshwater diatom species

Our study evaluates differences in the distribution and ecology of genetic variants within several ecologically important diatom species that are also key for Water Framework Directive monitoring of European rivers: Fistulifera saprophila (FSAP), Achnanthidium minutissimum (ADMI), Nitzschia inconspicua (NINC) and Nitzschia soratensis (NSTS). We used DADA2 to infer amplicon sequence variants (ASVs) of a short rbcL barcode in 531 environmental samples from biomonitoring campaigns in Catalonia and France. ASVs within each species showed different distribution patterns. Threshold Indicator Taxa ANalysis revealed three ecological groupings of ASVs in both ADMI and FSAP. Two of these in each species were separated by opposite responses to calcium and conductivity. Boosted regression trees additionally showed that both variables greatly influenced the occurrence of these groupings. A third grouping in FSAP was characterized by a negative response to total organic carbon and hence was better represented in waters with higher ecological status than the other FSAP ASVs, contrasting with what is generally assumed for the species. In the two Nitzschia species, our analyses confirmed earlier studies: NINC preferred higher levels of calcium and conductivity. Our findings suggest that the broad ecological tolerance of some diatom species results from overlapping preferences among genetic variants, which individually show much more restricted preferences and distributions. This work shows the importance of studying the ecological preferences of genetic variants within species complexes, now possible with DNA metabarcoding. The results will help reveal and understand biogeographical distributions and facilitate the development of more accurate biological indexes for biomonitoring programmes.

Historical shifts in oxygenation regime as recorded in the laminated sediments of lake Montcortès (Central Pyrenees) support hypoxia as a continental-scale phenomenon

Recent expansion of anoxia has become a global issue and there is potential for worsening under global warming. At the same time, obtaining proper long-term instrumental oxygen records is difficult, thus reducing the possibility of recording long-term changes in oxygen shifts that can be related with climate or human influence. Varved lake sediments provide the better time frame to study this phenomenon at high resolution. We tracked the oxic/anoxic shifts of the varved Lake Montcortès since 1500CE, and tried to recognise anthropogenic and climatic influences combining biological and geochemical proxies. Four main scenarios emerged: 1) years with abrupt sediment inputs (A); 2) years with outstanding mixing and oxygenation of the water column (B); 3) years with strong stratification, anoxia, intense sulfur bacterial activity and increased biomass production (C); 4) years with stratification and anoxia, but relatively less biomass production (D). In line with current limnologic trends, high supra-annual variability in the occurrence of oxygenation events was observed. Interestingly, at least 45.3% of the years were mixing years and, like the meromictic ones, were mostly clustered into groups of consecutive years, thus alternating years of monomixis with years of meromixis. Most years of D belong to the period 1500-1820CE, when human activities were the most intense. Most years of A belonged to the climatic unstable period of 1850-1899CE. Years of B were irregularly distributed but were best represented in the period 1820-1849CE. Most years of C belonged to the 20th century. More than 90% of the years with climatic instrumental records belonged to B and C. Current climate warming seems to be taking control over the oxygenation capacity of the lake, especially since the second half of the 20th century. Our results support recent findings related to hypoxia spreading at the global scale.

Comparison of HOMER and ACMANT homogenization methods using a central Pyrenees temperature dataset

Abstract. The aim of this research is to compare the results of two modern multiple break point homogenization methods, namely ACMANT and HOMER, over a Pyrenees temperature dataset in order to detect differences between their outputs which can affect future studies. Both methods are applied to a dataset of 44 monthly maximum and minimum temperature series placed around central Pyrenees and covering the 1910–2013 period. The results indicate that the automatic method ACMANT produces credible results. While HOMER detects more breaks supported by metadata, this method is also more dependent on the user skill and thus sensitive to subjective errors.

RADIAÇÃO ULTRAVIOLETA, OZÔNIO TOTAL E AEROSSÓIS NA CIDADE DE NATAL-RN

A cidade de Natal é denominada pelos seus habitantes de “Cidade do Sol” em razão de sua elevada luminosidade, contudo a capital do Rio Grande do Norte possui taxas de câncer de pele não melanoma acima da média das capitais do Nordeste do Brasil. Neste cenário, o presente artigo apresenta um estudo da radiação ultravioleta (UV) e índice UV em Natal e suas relações com variáveis relevantes como ozônio total, aerossóis, radiação solar global (RSG) e nebulosidade. A metodologia realizou um estudo descritivo e estatístico com aplicação da Análise de Componentes Principais (ACP) e Gráfico Biplot. Os dados foram obtidos no Instituto Nacional de Pesquisas Espaciais/Centro Regional do Nordeste (INPE/CRN), nos instrumentos TOMS-OMI/AURA e MODIS/TERRA e no Instituto Nacional de Meteorologia do Brasil. Os resultados indicaram que o índice UV apresenta média anual igual a 11, classificado como extremo pela Organização Mundial de Saúde e a intensidade da UVB e índice UV no primeiro semestre do ano são mais intensos que no segundo. A variabilidade anual da UV possui uma característica nos meses de setembro e outubro que consiste em uma estabilização/redução do índice UV numa fase de crescimento da RSG em razão da maior concentração anual de ozônio total, sendo que a maior quantidade de aerossóis contribui em menor escala para este evento. A ACP e gráfico Biplot contribuíram para identificar o evento, pois indicaram que o ozônio total e aerossóis possuem uma forte correlação negativa com o índice UV nos referidos meses. Os autores chamaram esta característica de Evento Anual de Primavera da UV (EAP-UV), uma vez que ocorre no início e durante esta estação.

Connection between El Niño-Southern Oscillation events and river nitrate concentrations in a Mediterranean river

The causes of interannual nitrate variability in rivers remain uncertain, but extreme climatic events have been suggested as drivers of large nitrate inputs to rivers. Based on a 24-year data set (1983-2006), we suggest that El Niño-Southern Oscillation (ENSO) can affect nitrate behavior in a seasonal extra-tropical stream, the Llobregat (NE Iberian Peninsula), located thousands of kilometers away from the ENSO oscillating system via atmospheric teleconnections. Two commonly used indices, the Southern Oscillation Index (SOI) and the self-calibrating-Palmer Drought Severity Index (scPDSI) showed highly significant correlations with nitrate concentrations, which recurrently increased during La Niña phases, coinciding with severe droughts.