Determination of Key Hydrophysical Conditions

Among the hydrophysical conditions affecting cyanobacterial occurrence, the most important ones are turbidity, temperature profiles (stratification), pH, oxygen concentration and - for rivers or streams - flow rate (Chapter 3).

Turbidity

Turbidity is easily assessed with a Secchi disc. It is slowly submerged into the water at a line to the point where it is just still visible (or no longer visible) and this depth is termed “Secchi depth” (Figure 12.1). The depth down to which photosynthesis is possible in aquatic ecosystems, the euphotic depth, is 1.5-2.5 fold the Secchi depth (Preisendorfer, 1986), and in freshwater studies, the factor 2.3 is widely used (Chapter 4). More precise determinations of the euphotic depth are possible by photon flux measurements requiring a submersible quantum sensor (for photosynthetically active radiation; PAR). However, for the assessment of conditions favouring cyanobacterial proliferation, the much cheaper and simpler determination of Secchi depths is usually sufficient and allows reproducible measurements also by untrained persons after a brief introduction to the method (for an example, see Box 11.1).

Equipment

Secchi discs can be self-made, but convenient ones are available from companies that provide field-sampling equipment. They should be 25 cm in diameter, made of sufficiently heavy material to be readily submersible, may include holes to ensure easy horizontal sinking and be attached to a chain or cord of sufficient length with depth marks (Figure 12.1).

Figure 12.1 (a) Secchi depth measurement: the Secchi disc is lowered at the graduated rope to the depth where it is no longer visible. At this point, the disc is repeatedly lifted and lowered to determine accurately the depth at which the disc becomes visible, and this depth is read from the markings on the rope; the reading can be improved by using an underwater viewer to avoid reflection from the water surface (bathyscope, b). (c) Discontinuous depth- integrated sampling: with a water sampler, samples are taken at predefined (exemplary) depths and then combined.

Procedure

• • Lower the disc into the water in the shade of a boat (or a pier) as reflections from the surface may distort the reading.
• • Lower it to the depth at which it is just still visible; move it up and down several times to confirm that depth.
• • If the water surface is very turbulent (e.g., through strong wind), it may help to create a quiet surface with a box without a bottom.
• • Blooms may be very patchy, and immersing the disc will move them away from that spot. In such cases, wait a few seconds until they have redistributed.
• • Do not wear sunglasses during the procedure as that may distort (i.e., reduce) the reading.
• • Comparing readings between fieldworkers is an easy, but important exercise to reduce uncertainty, and it generally leads to remarkably similar results once the procedure has been discussed, understood and agreed.

For measuring transparency in shallow depth such as bathing sites, a Secchi disc with a smaller diameter can be mounted on a graduated rod instead of a rope. This allows rapid and precise measurements while wading in the water up to depth of about one meter.

For greater depths or under poor light conditions, the reading can be improved by using an underwater viewer or bathyscope (Figure 12.1) made of a wide box or tube with a transparent bottom on one side.

Temperature, Oxygen and pH Profiles

Whether a lake or reservoir is thermally stratified or totally or partially mixed can be determined from temperature, oxygen and pH depth profiles, usually measured at a central location. Modern fieldwork equipment includes multiprobes on long cables that can be lowered stepwise, taking readings at defined depths. A simplified approach is the measurement of temperatures in water samples taken at the defined depth, either directly after hauling the water sampler to the surface or with a thermometer mounted on the water sampler. For the latter approach, sufficient time needs to be allowed for an accurate reading and the haul to the surface has to be rapid enough to avoid errors through changes in the water’s temperature when moved from deep layers to the surface.

More precise and continuous data are obtained by installing thermistor chains permanently in the water column. This may be of interest when raw water offtake sites are located at a depth close to the thermocline (see Chapter 8).

From such depth profiles, thermal and chemical stratification can be determined as described in Chapter 4 and Box 4.3.

Additional Parameters Measured On Site

The availability of field-portable sensors enables quick data collection for a suite of informative water quality parameters. This includes multiparameter datasondes that are capable of simultaneously measuring chlorophyll-д, phy- cocyanin and turbidity along with the other parameters mentioned above. More sensitive multispectral sondes may also be able to discern between different types of phytoplankton and estimate their relative abundance through fluorescence measurements (see section 13.6). These tools can be used to help verify the presence of cyanobacteria while on site (through detecting phycocyanin/phycoerythrin) and can help direct sampling to locations of cyanobacteria maxima. For example, a datasonde profile can be collected throughout the water column or along a horizontal gradient, and samples can be collected at discrete depth or locations with elevated phyco- cyanin or chlorophyll-д concentrations.

Flow Rate and Discharge

In running waters such as rivers and streams, the determination of flow velocity and discharge is of interest for aspects such as estimates of nutrient input to a lake or reservoir, or turbulent mixing (see Chapter 7). Flow velocity is measured with a current meter. Current meters commonly can be mechanical with a propeller or based on Doppler acoustics. Since most running water show turbulent flow and pronounced gradients within the transversal section, a measurement of average flow velocity can be only achieved by measurements at multiple points in the profile. For some purposes, the temporal and seasonal variation at defined measurement points is more important than an exact determination of average flow velocity or discharge, and a measurement of flow velocity at a single, well-defined point in the middle of the stream or river may be sufficient for cyanobacterial monitoring and management purposes because in longer time series (frequent) data on relative changes in flow velocity are more meaningful than (a few) accurate measurements of absolute discharge. Correlating measured flow velocities with precipitation in the catchment may be helpful.

Discharge, the volume of water that flows through a transect per unit of time, usually in m³/sec, is estimated from measurements at multiple points in the profile. This may require additional expertise or training. Discharge data may be available from regional water authorities.