Water quality conditions and particulate chlorophyll a of new intensive seawater fishponds in Eilat, Israel: Daily and diel variations

Abstract

Water quality parameters were studied in new, intensive, intermediate-flow, marine fishponds in Eilat, Israel. The 100-m 3 ponds have a hard wall and differ from the earthen ponds studied here formerly in the continuous water aeration and stirring. This feature concentrates the organic detritus in the center to be flushed out daily, thus reducing the organic load in the pond. Dissolved oxygen, inorganic and organic nutrients, pH and particulate chlorophyll a concentrations were measured daily, and occasionally every 3–4 h over a 30-h period, during May 1987. The results showed two to three “bloom and crash” cycles of chlorophyll concentration in each of the two ponds observed. The concentrations of ammonia-N and inorganic P showed an inverse correlation with those of chlorophyll a whereas pH and dissolved oxygen levels showed daily changes whose magnitudes were proportional to chlorophyll a concentration. The large fluctuations in chlorophyll concentrations were found by epifluorescence microscopy to be related to microflagellate grazing of the phototrophic algae. Fish ammonia excretion and algal ammonia-N uptake accounted for most variations in water quality criteria, with only minor benthic contribution. Usually, chlorophyll a concentrations in the pond were medium to high, and the diel variation in water quality was phytoplankton-dominated. Only during times of low chlorophyll a concentrations was diel variation in water quality determined predominantly by fish metabolism. Water quality deteriorated significantly only during the maxima and minima of the phytoplankton bloom-and-crash cycles. In intermediate-flow ponds, temporary reduction of feeding is suggested as an effective water quality control measure. It should be used at the onset of both a bloom and a crash, in preference to increased water exchange.