Photosynthetically Active Radiation, Attenuation Coefficient, Depth of the Euphotic Zone, and Water Turbidity in the Neva Estuary: Relationship with Environmental Factors

Evaluating Underwater Light Availability for Phytoplankton: Mean Light Intensity in the Mixed Layer versus Attenuation Coefficient

“…A common lapse in phytoplankton studies is the characterization of light availability for phytoplankton using, besides the light attenuation coefficient, the compensation depth that corresponds to the bottom of the euphotic zone and is defined as the depth where the intensity of photosynthetically active radiation is 1% of incident PAR. Given that this amount of PAR is sufficient to sustain photosynthesis, the compensation depth is another metric widely used in phytoplankton research (e.g., [31]). However, the euphotic layer and the mixed layer are usually neither coincident nor proportional, and whilst the euphotic depth depends on inherent optical properties of the water, the mixing depth is mainly affected by physical-meteorological forcing, bathymetry, and even tides.…”

Secchi Disk Depth or Turbidity, Which Is Better for Assessing Environmental Quality in Eutrophic Waters? A Case Study in a Shallow Hypereutrophic Reservoir

“…In addition to high concentrations of phytoplankton, water transparency is often reduced by suspended matter [2]. For example, in the upper part of the Neva Estuary, with an average depth of 5 m, the concentration of suspended mineral matter explained 91% of the turbidity variance [10]. In the Sestroretsky Razliv reservoir, the TSS concentration also reveals a negative relationship with D sd (Figure 4).…”

“…However, there is evidence that under high turbidity conditions, the Secchi disk can provide incorrect data, such as about the depth of the euphotic zone [25]. This can be especially pronounced at high concentrations of colored dissolved organic matter in the water, as shown, for instance, in the case of the Neva River estuary [10]. Although CDOM did not have a direct pronounced effect on Secchi disk depth in the Sestroretskiy Razliv reservoir (Figure 4), high CDOM concentrations, as an additional factor, can disrupt the contrast of the white disk and yield incorrect water transparency values [10,25] because photons reflected from the Secchi disk can be absorbed strongly by the CDOM on their way back to the water surface [25].…”

“…The measurement of water transparency is a widely used method for assessing the quality of the aquatic environment [1][2][3][4] and is included in the list of environmental indicators in the monitoring systems of many countries [5][6][7]. Water transparency plays a key role in the life of aquatic organisms because light is necessary for the creation of primary production by plankton [8][9][10], and changes in water transparency impact trophic interactions in aquatic ecosystems [11][12][13]. Moreover, water transparency serves as a simple and understandable indicator for assessing water quality in aquatic environments, even for non-specialists, as low transparency makes the water less attractive for swimming and boating and diminishes the aesthetic appeal of the reservoir [14].…”