ALBERT

Description: The importance of carbonate precipitation by phytoplankton in fresh water lakes has not been sufficiently considered in global carbon cycles and climate change scenarios. The objective of this study was to determine the influence of the calcifying bivalved phytoflagellate Phacotus lenticularis (Ehrenberg) Deising 1866 on the total calcite precipitation in five European hard-water lakes. For this purpose, an accurate mass determination of single Phacotus lenticularis shells was required. We developed a novel methodological approach to precisely determine the volume and mass of the calcified shells. Focused ion beam (FIB) techniques were employed to investigate internal structural features. Thin layer cross-sections of the shell profiles were reproduced and perforation as well as the crystalline structure of the calcite plates were monitored. 3D-shell models were computed by 360° rotation of the shell cross-sections using a CAD 3D imaging software to calculate precise volumes and estimate realistic masses. In contrast to previous estimates, we determined a 2.8-fold higher shell mass of 0.86 ng CaCO3 (standard deviation SD = 0.18) for the highly massive shells at a mean volume per individual of 334.1 µm³ (SD = 70). An initial shell porosity of less than 5% was derived from thin layer cross-section images, resulting in a presumed mean shell density of 0.0026 ng µm−3. The shell diameter was significantly influenced by the lake’s origin. The shells from each lake displayed substantial variations in diameter and shape. The pores in the shells showed two variations. Wider pore canals penetrated the whole shell wall, whereas small, elongated pores were located along the interspaces between calcite crystals with tabular habit. The approximate average dimensions of these calcite plates were 1.0 × 1.6 × 0.2 µm. The mean lateral wall thickness at the rim and centre of the shell were 1.98 µm (SD = 0.42) and 0.79 µm (SD = 0.17), respectively. The average carbonate precipitation by Phacotus lenticularis in relation to the total epilimnetic suspended calcite precipitation was 6.0% in the oligotrophic lake Großer Ostersee (Bavaria, Germany). During the growing season, Phacotus lenticularis contributed up to 21% of the particulate calcium carbonate in the epilimnion. These findings suggest that Phacotus lenticularis should be considered in the assessment of hard-water lake carbon cycling.

Description: Inland Water Remote Sensing Validation Campaign 2017

Description: We simulated an experimental summer storm in large-volume (~1200 m3, ~16m depth) enclosures in Lake Stechlin (https://www.lake-lab.de) by mixing deeper water masses from the meta- and hypolimnion into the mixed layer (epilimnion). The mixing included the disturbance of a deep chlorophyll maximum (DCM) which was present at the same time of the experiment in Lake Stechlin and situated in the metalimnion of each enclosure during filling. Size-fractionated (0.2-3 µm and 〉3.0 µm) chlorophyll a (Chla) development was monitored for 42 days after the experimental disturbance event. Mixing disrupted the thermal stratification, increased concentrations of dissolved nutrients and CO2 and changed light conditions in the epilimnion. Thereby, mixing increased the concentration of Chla of the small size fraction 0.2-3.0 within one week after mixing. After 2-3 weeks, mixing resulted in increased concentrations of Chla also in the large size fraction, which was associated to a bloom of Dolichospermums sp.

Description: We simulated an experimental summer storm in large-volume (~1200 m3, ~16m depth) enclosures in Lake Stechlin by mixing deeper water masses from the meta- and hypolimnion into the mixed layer (epilimnion). The mixing included the disturbance of a deep chlorophyll maximum (DCM) which was present at the same time of the experiment in Lake Stechlin and situated in the metalimnion of each enclosure during filling. Water physical variables and water chemistry was monitored for 42 days after the experimental disturbance event. Mixing disrupted the thermal stratification, increasing concentrations of dissolved nutrients and CO2 and changing light conditions in the epilimnion. Mixing, thus, stimulated phytoplankton growth, resulting in higher particulate matter concentrations of carbon, nitrogen and phosphorous.

Description: In this measurement campaign of five water bodies (lakes and reservoirs) several German research groups organised a joint effort to collect a data set for testing, evaluating, and potentially improving the abilities of satellite-based monitoring of water quality in standing waters. The strategy of the campaign is summarised in Figure 1 (documentation "Conceptual design of Inland Water Remote Sensing Validation Campaign 2017") and consists of three independently measured categories of data: (i) satellite-based monitoring, (ii) in situ monitoring, and (iii) bio-optical characterisation. The latter aspect, in particular, was intended in order to go beyond classical comparison of satellite-based and in-situ observations and to enable a more process-oriented and physically-based assessment of the observations made during the satellite overcasts. We concentrated our work on one week in summer 2017 and organised a synoptically measurement campaign on five lakes in Central Germany (Lake Arendsee, Lake Geiseltalsee, Kelbra Reservoir, Rappbode Reservoir, Lake Süßer See, see Tab. 1 in documentation "Main physical and limnological characteristics of the five water bodies from Inland Water Remote Sensing Validation Campaign 2017") based on various field and lab methods. The synoptically approach required the equipment of five sampling teams that are able to work independently from each other. Field- instruments used during the campaign (which required to be available in five sets) had been compared with each other in a separate intercalibration day. All lab-based measurements took place at the central lab of the Helmholtz-Centre for Environmental Research in Magdeburg using methods as outlined in Friese et al. (2014). The five water bodies were intentionally chosen because they reflect a broad range of temperate standing waters with respect to size, depth, trophic state, and the occurrence of cyanobacterial blooms. In addition, also natural and artificial water bodies are reflected by this set of lakes/reservoirs. To our knowledge, this is one of the rare multiple-teams efforts in remote sensing research on water quality making the collection of data in terms of their synoptic evaluation and broad methodological basis particularly useful and valuable.