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

A Lagaria,U Christaki,F Van Wambeke,C Courties,D Lefèvre,S Psarra,M Pujo-Pay,L Oriol,T Tanaka

doi:10.5194/bg-8-2595-2011

Abstract

Abstract. We examined the effects of nutrient additions on rates of 14C-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 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.

Highlights

In the ocean, the bulk of organic matter produced by photosynthesis is remineralised through respiration (del Giorgio and Duarte, 2002)
We examined the effects of nutrient additions on rates of 14C-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
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)

Summary

Introduction

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 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). 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)

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