THE SIGNIFICANCE OF REGENERATED NITROGEN FOR PHYTOPLANKTON PRODUCTIVITY IN THE RIO-DOCE VALLEY LAKES, BRAZIL

The in situ experiments for the measurement of nitrogen uptake rate by phytoplankton were carried out in the euphotic layer of Lakes Dom Hel vecio, Jacare and Carioca, located in the Rio Doce Valley Lake System, Brazil, during rainy and dry seasons. The daily uptake rates of ammon ia, nitrate and urea nitrogen were 1.0 to 12.3, 0.0 to 1.1 and 0.1 to 1.3 mg N . m(-3) . day(-1) in the surface water of the three lakes. Th ese values decreased with depth. The greater part of the nitrogen sour ce for phytoplankton was ammonia. The contributions of nitrate, on the other hand, were almost negligible. The phytoplankton preferentially utilized ammonia and urea. The vertical profile of the total nitrogen uptake rate was somewhat similar to that of the daily photosynthetic c arbon uptake rate. The primary production in both rainy and dry season s was estimated as 340 and 160 mg C . m(-2) . day(-1) and 31 and 19 mg N . m(-2) . day(-1) in Lake Dom Helvecio, 150 and 73 mg C . m(-2) . d ay(-1) and 21 and 14 mg N . m(-2) . day(-1) in Lake Jacare, and 370 an d 570 mg C . m(-2) . day(-1) and 53 and 63 mg N . m(-2) . day(-1) in L ake Carioca, respectively. A brief turnover time for ammonia and urea was obtained, whereas nitrate required longer periods. The present stu dy indicates that the regenerated forms of nitrogenous nutrients play significant roles as nitrogen sources for phytoplankton and in the bio geochemical nitrogen cycle in the euphotic layer of the Rio Doce Valle y Lakes.