Effect of N:P ratios on response of Mediterranean picophytoplankton to experimental nutrient inputs

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 34:57-67 (2004) - doi:10.3354/ame034057 Effect of N:P ratios on response of Mediterranean picophytoplankton to experimental nutrient inputs Nona S. R. Agawin1,3,*, Carlos M. Duarte1, Susana Agustí1, Dolors Vaqué2 1Grupo de Oceanografía Interdisciplinar, Instituto Mediterraneo de Estudios Avanzados (CSIC-UIB), C/ Miquel Marqués 21, 07190 Esporles, Mallorca, Spain 2Institut de Ciències del Mar-CMIMA (CSIC), Departament de Biologia Marina i Oceanografía, Passeig Marítim de la Barceloneta, 08003 Barcelona, Spain 3Present address: Institute of Biodiversity and Ecosystem Dynamics-Aquatic Microbiology, Universiteit Van Amsterdam, Nieuwe Achtergracht 127, 1018 WS Amsterdam, The Netherlands *Email: agawin@science.uva.nl ABSTRACT: The effect of variations in N:P ratios on Mediterranean picoplankton Synechococcus sp. was tested with a nutrient enrichment experiment in large-scale mesocosms in a coastal Mediterranean bay community during summer 1998. By adding either N or P in excess of 42 mmol m-2 d-1 N or 2.1 mmol m-2 d-1, the mesocosm units (16 m3) received nutrient ratios varying between 2.5 to 160, i.e. 8-fold lower to 8-fold higher than background levels (N:P 20). The total phytoplankton had increased significantly after 1 wk of the experiment on mesocosms receiving high N:P ratios (excess N), with diatoms being mainly responsible for this increase. In contrast, Synechococcus sp., which along with small flagellates initially dominated the water column, rapidly increased in biomass, abundance and primary production, with higher abundance in mesocosms receiving excess N and P, and lower abundance in mesocosms receiving the background N:P load of 20 that is normal for the Bay of Blanes. Our study indicated that this complex pattern resulted from the response of the grazing community, which was highest at the Œbackground¹ ratio. Indeed, the potential grazers of Synechococcus sp. (heterotrophic nanoflagellates and phagotrophic ciliates) were highly abundant at the background ratio. Our results also showed that primary production of Synechococcus sp. was significantly inversely correlated (r = -0.98, p < 0.05) with specific grazing rates, suggesting that picophytoplankton responses to N:P loads arose from top-down effects related to the responses of the grazing community to the various N:P levels. The top-down effects were, however, obscured during the early part of the experiment, when there were transient imbalances between growth and loss rates of picophytoplankton owing to time lags between the growth of picophytoplankton and the growth of their grazers (heterotrophic nanoflagellates and phagotrophic ciliates). This resulted in a general net increase in Synechococcus sp. abundance in all mesocosms. KEY WORDS: Picophytoplankton · N:P ratios · Mediterranean Sea · Nutrient Full article in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 34, No. 1. Online publication date: January 16, 2004 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2004 Inter-Research.