A total nutrient budget for an experimental intensive fishpond with circularly moving seawater

Abstract

A total nutrient budget was measured in an experimental intensive fishpond with circularly moving seawater and a water retention time of 2 days. The pond was stocked with gilthead sea-bream, Sparus aurata. An important feature of this pond design was the significant reduction of the excess organic load of particulate matter by daily draining of the settled detritus. The major nutrient pathways consisted of fish excretion coupled with phytoplankton uptake. All inputs and outputs of phosphorus and nitrogen were measured daily over a 1-month period. Fish food accounted for more than 95% of the nutrient input, sustaining dense phytoplankton populations with up to 450 μg chlorophyll a l −1. Fish assimilated 21% of P and 26% of N inputs. On a monthly average, ca. 50% and 46% of the P and N inputs, respectively, were found in the particulate phase of the water column. This phase was dominated by a phytoplankton community which exhibited “bloom and crash” cycles due to microflagellate grazing. As the pond progressed from a bloom to a crash, the fractions of entering nutrients which ended in the particulate phase changed from 76% to 32% for P and from 63% to 34% for N. The drained settled detritus contained on average only 17% of the P and 10% of the N inputs. The outflow of dissolved matter contained 22% of P and 13% of N inputs. On a monthly average, surpluses (inputs minus outputs) of −10% for P and 5% for N were estimated in the budget, which balanced within experimental error. On a daily basis, however, the deviations from a balanced budget were occasionally larger, with surpluses ranging from −29% to 6% for P and from −3% to 8% for N. Attached macroalgal growth and decomposition are thought to be likely causes of these periodic deviations. The significant nonalgal conversion of dissolved total N into particulate N during phytoplankton crashes (as much as 20% of ammonia-N and 75% of dissolved organic-N) was probably due to uptake by bacteria and heterotrophic flagellates. Intensive bacterial activity, in particular sulphate reduction, was observed in the drained detritus.