Figure 1.
Study site and experimental setup
A) Map of Lake Ammer including the position of the ADCP mooring with the thermistor chain (black star) and a North-South transect with 15 stations (gray dots) that had interspaces of ∼1 km. B) General configuration of the ADCP mooring with the thermistor chain and field devices used.
Figure 2.
Vertical distribution of temperature (black), dissolved oxygen (blue), the acoustic backscatter strength ABS-ADP (red) and ABS-ADV (magenta), fluorescence in terms of P. rubescens-MFP (green solid line), and the cell density of the cyanobacterium P. rubescens (green open circles connected by a dotted line).
The data were measured with a CTD-probe (temperature and diss. oxygen), two different acoustic backscatter probes (ADP - 2 MHz and ADV - 6 MHz), a Moldaenke FluoroProbe (P. rubescens-MFP), and microscope cell counts of water samples (density of P. rubescens), respectively, (A) on 21 August 2009 and (B) on 16 August 2011. The in-situ measurements had a vertical resolution of 0.02–0.10 m.
Figure 3.
Scatter plots of the P. rubescens cell densities and (A–B) the P. rubescens Chl-a equivalent concentration measured with the Moldaenke FluoroProbe (P. rubescens-MFP), (C–D) the acoustic backscatter strength ABS measured with the ADP (ABS-ADP), and (E) the acoustic backscatter strength ABS measured with the ADV (ABS-ADV), respectively.
The data stems from profiles collected along the North-South transect. For 2009 three profiles were available measured on the 21 and 24 August and for 2011 two profiles were available from the 16 August 2011. Water samples for the analysis of P. rubescens cell densities were taken at 1, 4, 8, 10, 11, 12, 13, 14, 15, 20, and 25 m water depth. The profiles of the P. rubescens-MFP, the ABS-ADP, and the ABS-ADV were linear interpolated to the water sample depths, respectively. The black line represents the results of the least-squares fit model (Eq. 1).
Table 1.
Least-squares fit parameters and significance (Eq. 1) of the in-situ calibration of P. rubescens Chl-a eq. concentration (µg L−1) and acoustic backscatter strength ABS (-) to P. rubescens cell densities (cells mL−1) measured with MFP, ADP, and ADV, respectively in 2009 and 2011.
Figure 4.
Spatial distribution of physical parameters and different algae groups along a North-South directed transect at Lake Ammer on 16 August 2011.
(A) Temperature (°C). (B) Dissolved oxygen (mg L−1). (C) Turbidity (FTU). (D, E, and G) Concentration (µg L−1) of total chlorophyll-a (TChl-a), diatoms, and P. rubescens (Moldaenke FluoroProbe, MFP). (F) Concentrations of chlorophyll-a (µg L−1) measured with the Seapoint Chlorophyll Fluorometer (SCF). (H–I) Acoustic backscatter strength (ABS) measured with the ADP (ABS-ADP) and the ADV (ABS-ADP). The black solid lines in panels G–I indicate the 50% deviation from the local peak concentration of the P. rubescens Chl-a equivalent concentration (panel G) at each station measured with the MFP (P. rubescens-MFP). The transect had a spatial resolution of 1 km along the center line of the lake.
Figure 5.
Temporal changes in the vertical distribution of P. rubescens and zooplankton between the 23 and 24 of August 2009.
(A) Contour plot of the acoustic backscatter strength (ABS-ADCP) and the 5, 10, 15, and 20°C isotherms (black solid lines) between 3.5 and 45 m water depth. (B) Temporal (24 August 2009 between 06:00 and 15:00 h) and spatial (depth range: 3.5 and 25 m) zoom in of panel A with additional profiles (black arrows) of the vertical distribution of P. rubescens in terms of Chl-a equivalent concentration (left hand side, (P. rubescens-MFP) and acoustic backscatter strength (right hand side, ABS-ADP). The ABS-ADCP was measured with a bottom-mounted 614 kHz ADCP. Isotherms were calculated from the data of the thermistor chain.