Michael Schurter

Benthic chamber and profiling landers in oceanography - A review of design, technical solutions and functioning

We review and evaluate the design and operation of twenty-seven known autonomous benthic chamber and profiling lander instruments. We have made a detailed comparison of the different existing lander designs and discuss the relative strengths and weaknesses of each. Every aspect of a lander deployment, from preparation and launch to recovery and sample treatment is presented and compared. It is our intention that this publication will make it easier for future lander builders to choose a design suitable for their needs and to avoid unnecessary mistakes.

Lake Baikal deepwater renewal mystery solved

Deepwater renewal by intrusions and turbulent diffusion in Lake Baikal is very effective despite the enormous depth of up to 1642 m and the permanently stable stratification below similar to 300 m depth. Temperature time series recorded at the bottom of a mooring installed since March 2000 in the South Basin of the lake indicate recurrent freshwater intrusions with volumes of 50 to 100 km 3, about one order of magnitude larger than previously observed intrusions. Numerous mechanisms have been proposed to explain the advective deep water renewal. Here we present for the first time direct observations which prove that they are caused by coastal downwelling and subsequent thermobaric instability along the steep lake shores. Understanding these mechanisms is an important prerequisite for studying biogeochemical cycles, for predicting the effects of climate change on this unique ecosystem and for evaluating the local climate history from the extraordinary sedimentary record of Lake Baikal.

Origin And Size Of Hypolimnic Mixing In Urnersee, The Southern Basin Of Vierwaldstattersee (Lake Lucerne)

Urnersee and Gersauersee are two adjacent basins of Vierwaldstättersee (Lake Lucerne, Switzerland), seperated by a sill of 85 m depth, with similar topography (max. depth 195 and 213 m, respectively) but remarkably different exposure to “external forces”, such as wind and river input. Urnersee is exposed to diurnal winds and to occasional strong storms from the south (Föhn) whereas the wind over Gersauersee is moderate or weak. Two rivers, both having very large discharges during storms, replace the total water volume of Urnersee about once a year; in contrast, no large river flows directly into Gersauersee. Between March and October 1986, meteorological parameters, water temperatures and currents were measured quasi-continuously with the aim to quantify hypolimnic water exchange and mixing in Urnersee and to asses the relative importance of wind mixing versus river-induced water exchange for the renewal of the deep water layers. Three periods could be identified: (1) in April, weak stratification and strong episodic storms exchange about 50% of the deep hypolimnion (DH, defined as layer below 110 m depth) leading to a mean heat flux of 36 Wm2. Because of the large wind mixing the water of the exposed Urnersee below about 20 m depth becomes lighter than in the sheltered Gersauersee. (2) In May and June, the horizontal density gradient causes about 65% renewal of the Urnersee DH by the heavier Gersauersee intermediate water but does not affect the heart content. (3) Simultaneously with these processes are the episodic river floods adding another 20% to the DH water exchange and causing a heat flux of about 6 Wm2. During the rest of the summer, water exchange remains below 10% and is mainly due to episodic flood while wind mixing has little influence. Yet, during floods water input into the DH per unit time can still be very large and heat, fluxes reach 600 Wm2 or more. The influence of lateral density currents between the two adjacent basins on hypolimnic mixing is of great ecological significance and explains the oxygen saturation found in the deep water of Urnersee compared to Gersauersee.ZusammenfassungUrner-und Gersauersee, zwei benachbarte Becken des in der Zentralschweiz gelegenen Vierwaldstättersees, sind durch eine 85 m tiefe Unterwasserschwelle, getrennt. Sie besitzen sehr ähnliche Topographie (maximale Tiefen 195 bzw. 213 m), sind aber «äußeren Kräften» (Wind, Zuflüsse) sehr verschieden ausgesetzt. Der Urnersee wird sowohl durch ausgeprägte Hang/Tal-Winde als durch gelegentliche starke Föhnstürme beeinflußt, im Gersauersee hingegen sind die Winde meist schwach. Zwei relativ große Zuflüsse, in denen starke Hochwasser auftreten können, erneuern das Wasser des Urnersees in etwa 1,4 Jahren; in den Gersauersee direkt mündet kein großer Fluß. Vom März bis Oktober 1986 wurden meteorologische Parameter, Wassertemperaturen und Strömungen quasi-kontinuierlich gemessen mit dem Ziel, Wasseraustausch und Mischung im Urnersee sowie die relative Wichtigkeit der Wind-bzw. Zuflußinduzierten Mischung zu quantifizieren. Drei Perioden konnten identifiziert werden: (1) Im April führen bei noch schwacher Schichtung starke winde zu einer Wassererneuerung im tiefen Hypolimnion (definiert durch das Volumen unterhalb 110 m) von 50% und zu einem vertikalen Wärmefluß von 36 Wm2. Als Folge dieser Mischung wird etwa unterhalb 20 m Tiefe das Wasser im Urnersee wärmer als dasjenige im Gersauersee. (2) Im Mai und Juni führt der horizontale Dichtegradient zu einem Austausch von ca. 65% des Wassers in tiefen Hypolimnions des Urnersees durch das schwerere Wasser aus den mittleren Tiefen des Gersauersees. (3) Gleichzeitig mit diesen Vorgängen führen die gelegentlichen Hochwasser in den Zuflüssen zu einem weiteren Wasseraustausch von ca. 20% und einem Wärmefluß von etwa 6 Wm2. Während des restlichen Sommers bleibt die Wassererneuerung kleiner als 10%; sie kommt fast ausschließlich durch Hochwasser-induzierte Dichteströmungen zustande, der Einfluß des Windes bleibt schwach. Während der Hochwasser können aber immer noch große Wassermengen pro Zeit ins Hypolimnion transportiert werden, und der Wärmefluß kann 600 Wm2 und mehr ausmachen. Der Einfluß der lateralen Dichteströme auf Mischung und Wasseraustausch zwischen den beiden Seebecken ist ökologisch sehr wichtig; er erklärt, wieso die Sauerstoffsättigung im Hypolimnion des Urnersees meistens sehr viel größer ist als im Hypolimnion des Gersauersees.

Effects of upstream hydropower operation and oligotrophication on the light regime of a turbid peri-alpine lake

Abstract.Anthropogenic activities in catchments can alter the light regimes in downstream natural waters, affecting light attenuation and the perceived optical properties of the waters. We analyzed the effects of upstream hydropower operation and oligotrophication on light attenuation and reflectance in Lake Brienz (Switzerland). For this purpose, we reconstructed its light regime for the pre-dam condition and for periods of 4-fold increased primary productivity, based on direct observations of light and beam attenuation as well as concentrations of optically active compounds, especially observed and simulated mineral particle concentrations. Based on our assessment, light attenuation before the construction of upstream dams was double the current value during summer and nearly half in winter. This result is consistent with pre-dam measurements of Secchi depths in the early 1920s. Using a simple optical model, a significant increase in reflectance since the 1970s was estimated, assuming a 4-fold decrease of optical active organic compounds within the lake. As reflectance is perceived by human eyes as turbidity, this may explain subjective reports by local residents of increasing turbidity in recent years.

Interface structure and flux laws in a natural double‐diffusive layering

The diffusive regime of double-diffusive convection generates staircases consisting of thin high-gradient interfaces sandwiched between convectively mixed layers. Simultaneous microstructure measurements of both temperature and conductivity from the staircases in Lake Kivu are used to test flux laws and theoretical models for double diffusion. Density ratios in Lake Kivu are between one and ten and mixed layer thicknesses on average 0.7 m. The larger interface thickness of temperature (average 9 cm) compared to dissolved substances (6 cm) confirms the boundary-layer structure of the interface. Our observations suggest that the boundary-layer break-off cannot be characterized by a single critical boundary-layer Rayleigh number, but occurs within a range of O(10(2)) to O(10(4)). Heat flux parameterizations which assume that the Nusselt number follows a power law increase with the Rayleigh number Ra are tested for their exponent . In contrast to the standard estimate =1/3, we found =0.200.03 for density ratios between two and six. Therefore, we suggest a correction of heat flux estimates which are based on =1/3. The magnitude of the correction depends on Ra in the system of interest. For Lake Kivu (average heat flux 0.10 W m(-2)) with Ra=O(10(8)), corrections are marginal. In the Arctic Ocean with Ra=O(10(8)) to O(10(12)), however, heat fluxes can be overestimated by a factor of four.

MELIMEX, an experimental heavy metal pollution study: Vertical mixing in a large limno-corral

In a limno-corral (diameter 12 m, depth to sediments 10 m), located in Baldeggersee (Switzerland), vertical mixing has been measured during more than one year and compared to the conditions in the open lake (maximum depth 65 m, surface area 5.3 km2). The temperature method by McEwen and Hutchinson yields Kz values between 5×10−2 cm2s−1 at the upper boundary of the thermocline and 2×10−3 cm2s−1 at the bottom, a value near the molecular diffusion of heat at 4°C (1.36×10−3 cm2s−1). Kz calculated from profiles of excess radon-222 generally agree with those from the temperature data. Compared to the open lake, the corral has a more shallow epilimnion. However, during calm meteorological conditions, vertical mixing in the upper 10 m is similar outside and inside the corral.

Enhanced mixing in narrows: A case study at the Mainau sill (Lake Constance)

Abstract: Previous work has identified bottom currents as a significant source of turbulence in stratified lakes. Sills may therefore be a major factor determining overall turbulent diapycnal (vertical) exchange in lakes with multi-basin hypolimnia.¶In order to investigate the contribution of the Mainau sill (separating Upper Lake Constance from Lake Überlingen) to the overall diapycnal mixing in Lake Constance, a series of temperature microstructure profiles was taken at Mainau Island in October 1993. From these profiles, using Batchelors method, the rate of dissipation of turbulent kinetic energy was determined and related in an energy balance to the turbulent kinetic energy input from the wind, the energy content of internal seiches and the energy dissipation of bottom currents at the sill. Further, the vertical diffusivities were calculated using the dissipation method.¶The analysis shows that ≈ 5.5% of the wind energy flux was found in to the water column (below 2 m depth) and that energy dissipation was 8 times higher in the shear zone of the thermocline (≈ 8.4 ˙ 10-3 W m-2) than in the bottom boundary layer (≈ 1.1 ˙ 10-3 W m-2). Dissipation above the Mainau sill (≈ 9.5 ˙ 10-3 W m-2) exceeded the basin-wide average dissipation of internal seiche energy (≈ 0.3 ˙ 10-3 W m-2) by a factor of 34. However, since the areal extent of the sill is small (≈ 1% of the lake area), the sill contributes only about 40% to the basin-wide dissipation. Also vertical diffusivities within the thermocline were consistently enhanced by approximately the same amount over the sill. The synthesis of the observations implies that the sill plays a disproportionately large, but not dominant, role for small-scale diapycnal mixing in the hypolimnion of Lake Constance.