Ecosystem shift of a mountain lake under climate and human pressure: A move out from the safe operating space

A multiproxy approach including chironomid, diatom, pollen and geochemical analyses was applied on short gravitational cores retrieved from an alpine lake (Lacul Bâlea) in the Southern Carpathians (Romania) to unveil how this lake responded to natural and anthropogenic forcing over the past 500 years. On the basis of chironomid and diatom assemblage changes, and supported by sediment chemical data and historical information, we distinguished two main phases in lake evolution. Before 1926 the lake was dominated by chironomids belonging to Micropsectra insignilobus-type and benthic diatoms suggesting well-oxygenated oligotrophic environment with only small-scale disturbance. We considered this state as the lake's safe operational space. After 1926 significant changes occurred: Tanytarsus lugens-type and T. mendax-type chironomids took over dominance and collector filterers increased until 1970 pointing to an increase in available nutrients. The diatom community showed the most pronounced change between 1950 and 1992 when planktonic diatoms increased. The highest trophic level was reconstructed between 1970 and 1992, while the indicator species of increasing nutrient availability, Asterionella formosa spread from 1982 and decreased rapidly at 1992. Statistical analyses evidenced that the main driver of the diatom community change was atmospheric reactive nitrogen (Nr) fertilization that drastically moved the community towards planktonic diatom dominance from 1950. The transformation of the chironomid community was primarily driven by summer mean temperature increase that also changed the dominant feeding guild from collector gatherers to collector filterers. Our results overall suggest that the speed of ecosystem reorganisation showed an unprecedented increase over the last 100 years; biological systems in many cases underwent threshold type changes, while several system components displayed non-hysteretic change between alternating community composition. We conclude that Lake Bâlea is outside of its safe operating space today. The main trigger of changes since 1926 was climate change and human impact acting synergically.

Investigation of sedimentation rates and sediment dynamics in Danube Delta lake system (Romania) by 210Pb dating method

Being a dynamic environment associated with complex costal, fluvial and marine processes, only a few studies regarding the evolution of the Danube Delta and the human impacts on its ecosystem have been carried out. Being a sensible to all processes occurring in its catchment area, information is stored in the deposited sediments, which can be used as tracers for natural and anthropogenic processes. The aim of this study is to determine a detailed reconstruction of the sedimentation rates in the last century by applying the ²¹⁰Pb dating method validated by ¹³⁷Cs profiles. Additionally, the impacts of the construction of river-regulating structures (mainly the Iron Gates Hydro-Energetic Power Plants) are investigated, along with the assessment of natural phenomena (floods, storms etc.). To achieve this, 26 sediment cores from seven lakes were collected. ²¹⁰Pb_sup and ¹³⁷Cs were determined using gamma spectrometry, while ²¹⁰Pb_tot was measured via alpha spectrometry (²¹⁰Po), using the CRS model for age determination. From the assessed lakes, the most affected was the Matița Lake with a maximum sedimentation rate of 10.93 g cm^-2 yr^-1 and the least affected was the Isac Lake. Average sedimentation rates are: 0.95 g cm^-2 yr^-1 for Cruhlig Lake, 0.70 g cm^-2 yr^-1 for Uzlina Lake, 0.44 g cm^-2 yr^-1 for Isac Lake, 0.47 g cm^-2 yr^-1 for Cuibida Lake, 0.51 g cm^-2 yr^-1 for Iacob Lake, 1.00 g cm^-2 yr^-1 for Matița Lake and 0.76 g cm^-2 yr^-1 for Merhei Lake. Physical parameters (water content, porosity and bulk density) and LOI (organic matter and inorganic carbon content) were determined for each core to differentiate organic and non-organic sedimentation. Beside the natural influences, it is difficult to track the effects of the Iron Gates and not all analysed lakes were suitable for this task. The 1940-1970 period and the following ten years were compared in means of sedimentation: a decrease in sedimentation can be observed in four of the lakes: 59% in Cruhlig Lake, 16% in Uzlina Lake, 10% in Iacob Lake and 42% in Isac Lake, leading to an average 32% for the four lakes. The other three lakes show increasing tendencies of 39% in this period: 87% in Matița Lake, 6% in Merhei Lake and 24% in Cuibida Lake. Sedimentation rates show growths of 3 times after 1989, the most affected being the two northern lakes (3 times increase in both Matița Lake and Merhei Lake) and the four central lakes (2 times in case of Cuibida Lake, 3 times in Iacob Lake, 3 times in Isac Lake and 4 times in Uzlina Lake) with an average increase of 3 times, while the southern one (Cruhlig Lake) 2 times.

7Be, 210Pb and 40K depositions over 11 years in Málaga

The monthly bulk depositional fluxes of three natural radionuclides (⁷Be, ²¹⁰Pb and ⁴⁰K) were measured at a Mediterranean coastal station (Málaga) over an 11-year period from 2005 to 2015. The mean annual depositional fluxes of ⁷Be, ²¹⁰Pb and ⁴⁰K were 1215, 144 and 67 Bq m^-2 year^-1 respectively, showing a clear seasonal trend with minimum values recorded during summer and maximum values in winter. The rainfall regime with dry summers allows estimating the dry deposition. Assuming constant dry deposition through each year, ⁷Be, ²¹⁰Pb and ⁴⁰K would account for 12.5, 26.5 and 33% of the bulk fallout respectively which indicates that deposition for ²¹⁰Pb and ⁴⁰K are significantly higher than ⁷Be. The precipitation-normalized enrichment factor alpha used to explain seasonal variations in the depositional fluxes of radionuclides with respect the rainfall, indicates higher depositional fluxes during spring and summer than expected from the amount of rainfall. Despite their different origin, ²¹⁰Pb and ⁷Be monthly depositional fluxes have strong correlation. The atmospheric deposition fluxes of ⁷Be, ²¹⁰Pb and ⁴⁰K were controlled mainly by the amount of rainfall (r = 0.89, 0.91 and 0.66 respectively). Moreover, principal component analysis was applied to the datasets and deposition of radionuclides and rainfall in the same component highlighting the importance of the washout mechanism. The mean depositional velocity of aerosols evaluated using ⁷Be and ²¹⁰Pb are similar and are compared to other published values.