Prediction of the light attenuation coefficient through the Secchi disk depth: empirical modeling in two large Neotropical ecosystems

Abstract

The easiest way to evaluate water transparency is from the Secchi disk depth (SD). The behavior of radiation passing through water can also be quantified by the light attenuation coefficient (k) of photosynthetically active radiation (PAR), measured using quanta meters. Due to the high costs of quanta meters, k is usually predicted from SD. This prediction can be made using the index SD 9 k or using nonlinear models that correlate these variables. The aims of this study were to generate models predicting k through SD in two large Neotropical ecosystems (a reservoir and a floodplain) and to test whether 1.7, the index used worldwide, is adequate for predicting k in these ecosystems. The current paper also tested the effects of time (between seasons and among years) and space (between ecosystems and inside each ecosystem) upon such predictions. A standard 30-cm Secchi disk was used to measure SD. Simultaneously, PAR was measured at two depths to estimate k. A total data set of 2,136 measurements taken from 2001 to 2005 was used. The index generated, 2.26, was significantly different from 1.7 (t test, p \ 0.01). This suggests that 1.7 is not adequate for Neotropical ecosystems. In general, both the index and the models generated differed across ecosystems and over time. In addition, the analysis showed that a nonlinear model (k = 2.00 9 SD -0.76 , N = 2,136) was more reliable for predictions.

Modelling Upwelling Irradiance using Secchi disk depth in lake ecosystems

“…On the other hand the value of CONST found in this work was different from the value proposed by Padial & Thomaz (2008) for neotropical ecosystems. The differences between the results could be related by the evidence that no zenithal angle corrections were used in the work of Padial & Thomaz (2008) but also that the CONST could be dependant by the used data set. Thus, if we choose to represent K d,PAR ×Z d as a constant, we need to introduce an apparent reflectance to take account of the site specificities.…”

“…More recently, the same relationship was found using a quanta irradiance meter (Hojerslev & Aarup 2002). Padial & Thomaz (2008) found, analysing a large data set, a new value for the constant that links the inverse relationship between Photosynthetically Active Radiation diffuse attenuation coefficient (K d,PAR ) with the Secchi disk depth. Therefore, the importance of the Secchi disk measure is related also to the possibility to obtain an estimation of the K d,PAR , or in the visible band, without using an expensive quanta-meter or spectroradiometer.…”

Effects of the flood regime on the body condition of fish of different trophic guilds in the Upper Paraná River floodplain, Brazil

“…As a further result, these events reduce the primary productivity of phytoplankton (Train and Rodrigues, 1998;Rodrigues et al, 2002) and of submerged macrophytes (Padial and Thomaz, 2008;Thomaz et al, 2009), affecting the abundance and composition of the plankton (Velho et al, 2001;Ortlepp and Mürle, 2003) and the periphyton (Bielsa et al, 2003;Murakami et al, 2009). Thus, although the floods increase the availability of allochthonous food items from the flooded areas and drift, they reduce the autochtonous food supply.…”

Optimizing the Use of Secchi Depth as a Proxy for Euphotic Depth in Coastal Waters: An Empirical Study from the Baltic Sea

“…Consequently, the relationship between the optical parameters, or in this case the Z eu :Z SD ratio, varies according to the changes in the optical properties of water, e.g., [1,[4][5][6]. The correlation between the parameters is, in its simplest form, expressed as follows: (1) The reported values of the conversion coefficient, m, usually range from less than 1 up to 5, or even 10 ( [4] and references therein).…”