Relationship between Secchi disk visibility and chlorophyll a in aquaculture ponds

The potential of using Bannister's linear equation (k(t) = k(w) + k(c) c) ( where k(t) is the overall Light extinction coefficient, k(w) represents the non-phytoplankton light extinction, k(c) is the specific light extinction coefficient due to chlorophyll a (chla), c is the chla concentration, and k (c) c represents the light extinction due to chla) to partition sources of turbidity in Secchi disk visibility (SDV) measurements in aquaculture ponds was evaluated. Eight data sets from five sites around the world were used in the study. Chlorophyll a data were regressed against the overall light e xtinction coefficient determined from SDV measurements. The relationship be tween chla and overall Light extinction coefficient was linear for seven of the eight data sets. The contribution of non-phytoplankton turbidity to SD V measurements was estimated by the intercept of the linear regression line (equivalent to k(w)). The values obtained (range = 3.61-8.91 m(-1)) were v ariable and unpredictable between replicate ponds at all sites, but did not vary significantly over time (P < 0.05). Because chla concentration serves as an indicator of phytoplankton concentration, the contribution of phytop lankton turbidity to SDV measurements was estimated by the slope of the lin ear regression Line (equivalent to k(c)) multiplied by the chla concentrati on. The slope of the regression line (0.014 +/- 0.006 m(-1) (mg m(-3)) (-1) ) was similar to values reported for natural freshwater systems. The partit ioned light extinction coefficients and chla concentrations were also used to determine the threshold chla concentration above which SDV measurements are determined primarily by chla. The threshold chla concentrations (177-98 0 mg m(-3)) above which phytoplankton biomass becomes the primary determina nt of SDV were higher than observed chla concentrations. The results indica te that Bannister's linear equation can generally be used to partition and quantify the sources of turbidity in aquaculture ponds. The results also su ggest that the contribution of non-phytoplankton turbidity to SDV measureme nts in fertilized and fed aquaculture ponds can be more important than phyt oplankton turbidity. (C) 1999 Elsevier Science B.V. All rights reserved.