Comparing methods for measuring phytoplankton biomass in aquaculture ponds

Abstract

Algal biomass is important for aquaculture in terms of eutrophication, as a secondary metabolite threat, and as a food source for suspension feeders and benthos. In order to evaluate the applicability of an in situ FluoroProbe instrument, chlorophyll a (chl a) measured with this device was compared to chl a measured by a fluorometer and to algal densities and biovolumes determined by direct counts. Monthly surveys were conducted in 3 types of aquaculture ponds (tilapia, milk fish, and common orient clam) from October 2011 through April 2012 in the Cigu region of Tainan City, Taiwan. Results showed that green algae were the predominant phytoplankton group, followed by cyanobacteria, euglenoids, and diatoms. The 3 types of aquaculture ponds had wide ranges of chl a, pH, salinity, turbidity, PO4-3-P, and suspended solids. Chl a measured by the FluoroProbe had significant regressions with chl a values measured by a fluorometer, and with algal densities and biovolumes determined by counting; however, when chl a was >250 µg l-1, the correlation between the FluoroProbe and the fluorometer diminished. The difference in chl a reads between these 2 methods increased when chl a concentration exceeded 200 µg l-1. Of the 4 differently colored algal groups measured by the FluoroProbe, only the green and blue groups had significant regressions with their respective biovolumes, whereas the red and brown groups had no significant regression with their respective biovolumes. Our results show the applicability of the FluoroProbe in algal monitoring of aquaculture ponds, although caution is needed at higher chl a levels.