Phytoplankton nutrient uptake dynamics in earthen marine fishponds under winter and summer conditions

Abstract

Chlorophyll and dissolved nutrients were measured daily in two adjacent marine fishponds during one summer and one winter month. The two ponds were supplied continuously with nutrient-rich water from a seawater well at a dilution rate of 0.38 day −1 and received daily inputs of fish food. The fishponds behave as hypertrophic systems with high primary productivity and phytoplankton biomass. They are unstable, with frequent plankton blooms and crashes. Three bloom and crash cycles of phytoplankton (diatoms) were recognised and described in detail. In general, the dissolved inorganic nutrients showed an inverse relationship to the chlorophyll- a concentration. The rate of regrowth of phytoplankton after a crash was temperature controlled, with a Q 10 of 2.3 which is similar to that found in laboratory studies. The phytoplankton took up ammonia-N until it was depleted and only then took up nitrate. There was a rapid uptake of nutrients which occurred on a timescale of minutes. Nutrient uptake occurred throughout the day and night. It was suggested that the crashes could not be caused by simple nutrient limitation resulting from changes in nutrient supply, and are most likely due to flagellate grazing. The ponds had a higher phytoplankton biomass and growth rate after a crash in summer than in winter. Both the pond which had been exposed to the air for 42 days the previous summer (dried) and the aged (non-dried) pond had similar nutrient-phytoplankton interactions; however, the timing of blooms and the species composition were different. The level of ammonia reached during crashes was higher in the aged pond due to in situ organic matter decomposition. For such intermediate flow ponds with a high plankton biomass, there are two critical periods when acute water quality problems may occur. During blooms, extreme values of dissolved oxygen can occur, and during crashes, high ammonia and low oxygen occur. The results explain why the water quality problems in this type of pond are only acute and not chronic.