The effects of nutrient additions on particulate and dissolved primary production and metabolic state in surface waters of three Mediterranean eddies

Biogeosciences, 8, 2595–2607, 2011 www.biogeosciences.net/8/2595/2011/ doi:10.5194/bg-8-2595-2011 © Author(s) 2011. CC Attribution 3.0 License. Biogeosciences The effects of nutrient additions on particulate and dissolved primary production and metabolic state in surface waters of three Mediterranean eddies A. Lagaria1,2 , S. Psarra2 , D. Lefèvre3 , F. Van Wambeke3 , C. Courties4,5 , M. Pujo-Pay6,7 , L. Oriol6,7 , T. Tanaka8,*,** , and U. Christaki1 1 Laboratoire d’Océanologie et des Géosciences, UMR8187, INSU-CNRS – Université Lille Nord de France, ULCO, 62930 Wimereux, France 2 Hellenic Centre for Marine Research, Institute of Oceanography, 71003 Heraklion, Crete, Greece 3 LMGEM, Laboratoire de Microbiologie, Géochimie et Ecologie Marines, UMR6117, INSU-CNRS – Université de la Méditerranée, Centre d’Océanologie de Marseille, Campus de Luminy Case 901, 13288 Marseille cedex 9, France 4 Observatoire Océanologique de Banyuls, UMS2348, INSU-CNRS, 66650 Banyuls/mer, France 5 Université Pierre et Marie Curie-Paris VI, UMS2348, Observatoire Océanologique de Banyuls, 66650 Banyuls/mer, France 6 Laboratoire d’Océanographie Microbienne, UMR7621, INSU-CNRS – Observatoire Océanologique, 66650 Banyuls/mer, France 7 Université Pierre et Marie Curie-Paris VI, UMR7621, Laboratoire d’Océanographie Microbienne, Observatoire Océanologique, 66650 Banyuls/mer, France 8 LOPB, Laboratoire d’Océanographie Physique et Biogéochimique, UMR6535, INSU-CNRS – Université de la Méditerranée, Centre d’Océanologie de Marseille, Campus de Luminy Case 901, 13288 Marseille cedex 9, France * present address: Laboratoire d’Océanographie de Villefranche, UMR7093, INSU-CNRS, 06230 Villefranche-sur-Mer, France ** present address: Université Pierre et Marie Curie-Paris VI, Laboratoire d’Océanographie de Villefranche, UMR7093, 06230 Villefranche-sur-Mer, France Received: 12 November 2010 – Published in Biogeosciences Discuss.: 9 December 2010 Revised: 7 June 2011 – Accepted: 6 September 2011 – Published: 14 September 2011 Abstract. We examined the effects of nutrient additions on rates of 14 C-based particulate and dissolved primary production as well as O2 -based metabolic rates in surface waters (8 m) of three anticyclonic eddies, located in the Western, Central and Eastern Mediterranean. Ship-board microcosm experiments employing additions of inorganic nitrogen (+N) and phosphorus (+P), alone and in combination (+NP), were conducted in June/July 2008 during the BOUM (Biogeochemistry from the Oligotrophic to the Ultra-oligotrophic Mediterranean) cruise. In all three experiments, particulate primary production was significantly stimulated by the additions of nitrogen (+N, +NP) while no effect was observed with the addition of phosphorus alone (+P). Percent extracellular release of photosynthate (PER) displayed the lowest values (4–8 %) in the +NP treatment. Among the three Correspondence to: S. Psarra () treatments (+N, +P, +NP), the +NP had the strongest effect on oxygen metabolic rates, leading to positive values of net community production (NCP > 0). These changes of NCP were mainly due to enhanced gross primary production (GPP) rather than reduced dark community respiration rates (DCR). In all three sites, in +NP treatment autotrophic production (whether expressed as GPP or PPtotal ) was sufficient to fulfil the estimated carbon requirements of heterotrophic prokaryotes, while addition of nitrogen alone (+N) had a weaker effect on GPP, resulting in metabolically balanced systems. At the three sites, in treatments with N (+N, +NP), phytoplankton and heterotrophic prokaryote production were positively correlated. Heterotrophic conditions were observed in the Control and +P treatment at the central and eastern sites, and autotrophic production was not sufficient to supply estimated bacterial carbon demand, evidence of a decoupling of phytoplankton production and consumption by heterotrophic prokaryotes. Published by Copernicus Publications on behalf of the European Geosciences Union. 2596 1 A. Lagaria et al.: The effects of nutrient additions on particulate and dissolved primary production Introduction In the ocean, the bulk of organic matter produced by photosynthesis is remineralised through respiration (del Giorgio and Duarte, 2002). The amount respired relative to the amount produced describes the net metabolism of the ecosystem. Net community production (NCP) is then the balance between gross primary production (GPP) and dark community respiration (DCR). When NCP > 0, more organic carbon is produced than respired, so the ecosystem is in a state of net autotrophy. When NCP < 0, the ecosystem is heterotrophic, in situ respiration exceeds in situ carbon fixation. Heterotrophic prokaryotes (Eubacteria and Archaea) are responsible for a significant portion of total respiration in the water column (Robinson, 2008). In the least productive areas, their contribution may even exceed 50 % of total respiration (Lemée et al., 2002; Gonzalez et al., 2003; Reinthaler et al., 2006). Respiration of heterotrophic prokaryotes, the sum of maintenance and growth costs, is supported by the uptake of dissolved organic carbon. A variety of mechanisms within planktonic food webs produce dissolved organic matter, through phytoplankton exudation, viral lysis, excretion/egestion and grazing processes by zooplankton and microzooplankton (Jumars et al., 1989; Nagata, 2008). The dissolved component of primary production (PPd) can represent a significant amount of total primary production (Marañón et al., 2004; Morán and Estrada, 2001, 2002) though it is often neglected in primary production measurements which typically estimate only particulate primary production. The portion of total primary production which is excreted as PPd is termed the Percentage of Extracellular Release (PER) and varies greatly depending on environmental conditions, including nutrient limitation (see review by Baines and Pace, 1991). PER reportedly increases when the phytoplankton are dominated by small-sized cells, most probably because their elevated surface/volume ratio promotes passive diffusion of small metabolites through the cell membrane (Bjørnsen, 1988). Dissolved primary production (PPd) furnishes a labile, easily assimilated carbon source for heterotrophic prokaryotes (Carlson, 2002; Nagata, 2008) and its relative contribution increases under conditions of mineral nutrient limitation (Baines and Pace, 1991). Thus, under conditions of nutrient limitation, phytoplankton produce, through PPd, substrate for heterotrophic prokaryotes whose growth is then potentially limited by the same mineral nutrient restricting phytoplankton growth. As the uptake of organic matter by heterotrophic prokaryotes forms a major carbon-flow pathway, factors controlling this uptake and its variability can dominate overall carbon fluxes and determine the metabolic status of a system (Thingstad and Rassoulzadegan, 1995). The coupling (...truncated)