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Umer, B. and V. Malačič (2022): Biexponential decrease of PAR in coastal waters (Northern Adriatic).
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The attenuation coefficients of photosynthetically active radiation (PAR) were extracted from the vertical profiles of PAR in coastal waters (the Gulf of Trieste, Northern Adriatic). The vertical profiles were collected roughly twice per month from July 2011 to December 2015, and the PAR values just above the sea surface were compared with the PAR data measured on a buoy.

This research supports the nonlinear fit with the biexponential expression
for the dependence of PAR with depth, yielding a much better match with the
data than the fit with a mono-exponential expression. However, another reasoning
for biexponential attenuation is because it functions as a solution for a homogeneous
differential equation of a second order. The method for estimating
the water type is offered with an analysis of the attenuation coefficients of PAR.
It was found that for a particular location (the Gulf of Trieste), the attenuation
coefficient in a mono-exponential decrease of PAR is 0.19 – 0.21 m^{-1}, while for a
biexponential decrease of PAR, the coefficient of the long-range attenuation is
0.12 – 0.14 m^{-1} and that of short-range attenuation is 0.8 – 0.9 m^{-1}. This leads to
the conclusion that most water columns match coastal water type 1, while the
surface layer is represented by coastal water types 7 or 9. From the estimate of
the water types, the coefficients of downward irradiation were inferred as
0.19 m^{-1} ± 0.01 m^{-1} (long-range attenuation) and 3.0 m^{-1} ± 0.7 m^{-1} (short-range
attenuation). These coefficients can deter-mine the heat source inside the water
column.

Keywords: PAR, attenuation coefficient, irradiation, Secchi disk depth, fitting methods

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