Automated Computed Tomography Based Parasitoid Detection in Mason Bee Rearings

In recent years, insect husbandry has seen an increased in- terest in order to supply in the production of raw materials, food or as biological/environmental control. Unfortunately, large insect rearings are susceptible to pathogens, pests and parasitoids which can spread rapidly due to the confined nature of a rearing system. Thus, it is of interest to quickly and efficiently monitor the spread of such manifesta- tions and the overall population size. Medical imaging techniques could be used for this purpose, as large volumes can be scanned non-invasively. Due to its 3D acquisition nature, computed tomography seems to be the most suitable for this task. This study presents an automated, computed tomography-based, counting method for bee rearings that performs com- parable/similar to identifying all Osmia cornuta cocoons manually. The proposed methodology achieves this in an average of 7 minutes per sam- ple, compared to 90 minutes per sample for the manual count over a total of 12 samples collected around lake Zurich in 2020. Such an automated bee population evaluation tool is a valuable in combating environmental influences on bee, and potentially other insect, rearings.

Inhibited vertical mixing and seasonal persistence of a thin cyanobacterial layer in a stratified lake

Harmful blooms of the filamentous cyanobacteria Planktothrix rubescens have become common in many lakes as they have recovered from eutrophication over the last decades. These cyanobacteria, capable of regulating their vertical position, often flourish at the thermocline to form a deep chlorophyll maximum. In Lake Zurich (Switzerland), they accumulate during stratified season (May–October) as a persistent metalimnetic thin layer (~2 m wide). This study investigated the role of turbulent mixing in springtime layer formation, its persistence over the summer, and its breakdown in autumn. We characterised seasonal variation of turbulence in Lake Zurich with four surveys conducted in April, July and October of 2018 and September of 2019. Surveys included microstructure profiles and high-resolution mooring measurements. In July and October, the thin layer occurred within a strong thermocline ($$N \gtrsim 0.05$$ N ≳ 0.05 s$$^{-1}$$ - 1 ) and withstood significant turbulence, observed as turbulent kinetic energy dissipation rates ($$\varepsilon \approx 10^{-8}$$ ε ≈ 10 - 8 W kg$$^{-1}$$ - 1 ). Vertical turbulent overturns –monitored by the Thorpe scale– went mostly undetected and on average fell below those estimated by the Ozmidov scale ($$L_O \approx 1$$ L O ≈ 1 cm). Consistently, vertical diffusivity was close to molecular values, indicating negligible turbulent fluxes. This reduced metalimnetic mixing explains the persistence of the thin layer, which disappears with the deepening of the surface mixed layer in autumn. Bi-weekly temperature profiles in 2018 and a nighttime microstructure sampling in September 2019 showed that nighttime convection serves as the main mechanism driving the breakdown of the cyanobacterial layer in autumn. These results highlight the importance of light winds and convective mixing in the seasonal cycling of P. rubescens communities within a strongly stratified medium-sized lake.

Lake Ecosystem Robustness and Resilience Inferred from a Climate-Stressed Protistan Plankton Network

Network analyses of biological communities allow for identifying potential consequences of climate change on the resilience of ecosystems and their robustness to resist stressors. Using DNA metabarcoding datasets from a three-year-sampling (73 samples), we constructed the protistan plankton co-occurrence network of Lake Zurich, a model lake ecosystem subjected to climate change. Despite several documentations of dramatic lake warming in Lake Zurich, our study provides an unprecedented perspective by linking changes in biotic association patterns to climate stress. Water temperature belonged to the strongest environmental parameters splitting the data into two distinct seasonal networks (October–April; May–September). The expected ecological niche of phytoplankton, weakened through nutrient depletion because of permanent thermal stratification and through parasitic fungi, was occupied by the cyanobacterium Planktothrix rubescens and mixotrophic nanoflagellates. Instead of phytoplankton, bacteria and nanoflagellates were the main prey organisms associated with key predators (ciliates), which contrasts traditional views of biological associations in lake plankton. In a species extinction scenario, the warm season network emerged as more vulnerable than the cold season network, indicating a time-lagged effect of warmer winter temperatures on the communities. We conclude that climate stressors compromise lake ecosystem robustness and resilience through species replacement, richness differences, and succession as indicated by key network properties.

3D FEM ANALYSIS OF THE CONSTRUCTION PIT FOR A TBM-DRIVEN FLOOD DISCHARGE GALLERY

As one speciality of the Swiss hydraulic engineering tradition, several torrent diversion schemes were undertaken dating from the 18^th century, with the aim of using the natural lake as a retention basin. The recent project of flood protection of the lower Sihl valley and the City of Zurich features a gallery of 2.1km length with a 6.6m inner diameter designed for a 330m³/s free flow, discharging into Lake Zurich in an HQ₅₀₀ event. After a short presentation of the main features of the project, the paper concentrates on the target construction pit of the TBM drive close to a major railway line at the built-up lakeside.

Strangled Waters: First Wave

On a balmy day in June 1955, George Anderson took his sailboat out on Lake Washington, the long stretch of fresh water that separates Seattle from its eastern suburbs. Anderson had recently finished his doctoral research on phytoplankton, and knew the lake well. The water that day looked odd; he noticed a strange brown tinge. So he collected a sample in an empty beer bottle and brought it back to the University of Washington lab where he worked with his mentor, W.T. Edmondson, the ranking authority on the lake. Under the microscope, Anderson and Edmondson found a life form they’d never seen before. It grew in long, narrow chains, striated with lines that separated one cell from the next. They thought this might be a species infamous among limnologists, the cyanobacterium Oscillatoria rubescens. (Cyanobacteria, popularly known as blue-green algae, are in fact distinct from and far more ancient than algae. They appeared more than 3 billion years ago, when the planet was inhabited only by microbes, and were the first organisms to evolve photosynthesis. Their proliferation and release of great volumes of oxygen profoundly changed the chemical makeup of Earth’s atmosphere, making the evolution of complex life possible.) The researchers needed to be sure, so they sent a sample off to an expert, who confirmed their suspicions. O. rubescens signaled deteriorating conditions in Lake Washington. To Edmondson, it also meant an unprecedented opportunity to track the impacts of nutrient overload. O. rubescens had been the harbinger of drastic change in a number of western European lakes. The best-known case was that of Lake Zurich in Switzerland. Fed by Alpine glaciers, Lake Zurich was, until the late 1800s, an expanse of blue known for its abundant populations of whitefish and lake trout, which thrive in deep water. The lake is made up of two basins separated by a narrow passage. In the late nineteenth century towns at the edge of the lower basin, the Untersee, abandoned privies for flush toilets, and began to release their raw sewage into the lake.

Quantitative characterization of subaqueous landslides in Lake Zurich (Switzerland) based on a high-resolution bathymetric dataset

New high-resolution surveying techniques allow subaqueous geomorphology to be investigated in great detail. Such analyses are important as the morphologies are often indicative of past processes, including mass movements. For peri-alpine Lake Zurich, many mass-wasting events have occurred in the past millennia. While the ages of these events are known from past studies on the respective deposits in the lake basin, the surface expressions and distribution of the respective features on the slopes have not been extensively described. Here we quantitatively characterize the morphologic features on the entire lake floor. A total of 50 subaqueous landslides are morphologically identified in a high-resolution digital bathymetric model (DBM), mapped and characterized using a geographic information system (GIS). Many slides show relatively small erosion areas (<0.05 km2) and are located in shallow water (<10 m water depth). The roughness of the individual landslide-translation areas is quantified using the standard deviation of a measure called bathymetric position index (BPI) and related to the slides ages. The DBM allows the detection of traces of mass-movements dating back to c. 5000 cal years BP. Our results demonstrate that morphometric analyses on a high-resolution DBM can contribute to a better understanding of sublacustrine mass movements.