Eutrophication effects on phytoplankton size-fractioned biomass and production at a tropical estuary

Size-fractioned phytoplankton (pico, nano and microplankton) biomass and production were estimated throughout a year at Recife harbor (NE Brazil), a shallow well mixed tropical hypereutrophic estuary with short residence times but restricted water renewal. Intense loads of P-PO4 (maximum 14 mu M) resulted in low N:P ratios (around 2:1), high phytoplankton biomass (B = 7.1-72 mu g chl-a L-1), production (PP = 102657 mu g C L-1 h(-1)) and photosynthetic efficiency (P-B = 0.5-45 mu g C mu g chl-a(-1)), but no oxygen depletion (average O-2 saturation: 109.6%). Nanoplankton dominated phytoplankton biomass (66%) but micro and nanoplankton performed equivalent primary production rates (47% each). Production-biomass models indicate an export of the exceeding microplankton biomass during most of the year, possibly through grazing. The intense and constant nutrient and organic matter loading at Recife harbor is thus supporting the high microplankton productivity that is not accumulating on the system nor contributing to oxygen depletion, but supporting the whole system's trophic web. (C) 2014 Elsevier Ltd. All rights reserved.