Short-term nitrogen uptake and regeneration in early and late growth phaseshrimp ponds

Phytoplankton uptake of nitrogen and regeneration by the natural biota were measured over two 24-h periods in three commercial shrimp ponds [Penaeus j aponicus (Bate)] in subtropical Australia. Ponds were sampled on two occasi ons: early in the growth season when shrimp biomass was relatively low (100 0 kg ha(-1)): and late in the growth season when biomass was high (3000 kg ha(-1)). Ammonium-N, urea-N and chlorophyll a concentrations were higher in the second sampling period (1.68 +/- 0.61 mu M, 2.63 +/- 0.41 mu M and 115 .28 +/- 10.63 mu g L-1 respectively) than in the first (0.36 +/- 0.07 mu M, 0.40 +/- 0.12 mu M and 23.61 +/- 1.44 mu g L-1 respectively). Rates of phy toplankton uptake of urea, ammonium and nitrate were also higher in the sec ond sampling period(15.58 +/- 5.12, 1.90 +/- 0.37 and 1.02 +/- 0.19 mu M h- l respectively) than in the first (1.33 +/- 0.18, 0.69 +/- 0.04 and 0.06 +/ - 0.004 mu M h(-1) respectively), and there were no day-night differences i n rates, with the exception of higher urea uptake during the day in the fir st sampling period, The relative preference indices for urea and ammonium w ere higher than for nitrate during the first sampling period, whereas all t hree sources were taken up in equal proportion in the second period. Most o f the particulate nitrogen biomass and nitrogen uptake was in the < 10 mu m fraction (only measured during the second sampling period). The hulk of th is community was nanoflagellates. Urea was taken up more rapidly than the o ther nitrogen sources by both > 10 and < 10 mu m fractions, but the uptake of nitrate was proportionately higher in the > 10 mu m fraction. Ammonium a nd urea regeneration rates were higher in the second sampling period than i n the first (2.15 +/- 0.25 and 13.27 +/- 4.03 mu M h(-1), respectively, com pared with 0.67 +/- 0.04 and 1.39 +/- 0.15 mu M h(-1)), and both ammonium a nd urea regeneration appeared to be coupled to phytoplankton uptake. Shrimp excretion of ammonium and urea was a minor contributor to regeneration (9% in the first sampling period and 6% in the second). We hypothesize that mi crozooplankton and/or bacterial remineralization were the major contributor s to the regeneration. The shrimp ponds were therefore characterized by rap id turnover of urea and inorganic nitrogen.