MEPS 301:23-41 (2005)  -  doi:10.3354/meps301023

Net ecosystem metabolism in a micro-tidal estuary (Randers Fjord, Denmark): evaluation of methods

Frédéric Gazeau1,2,6,*, Alberto Vieira Borges2, Cristina Barrón3, Carlos M. Duarte3, Niels Iversen4, Jack J. Middelburg5, Bruno Delille2, Marie-Dominique Pizay1, Michel Frankignoulle2,†, Jean-Pierre Gattuso1

1Laboratoire d’Océanographie de Villefranche, CNRS-Université de Paris 6, BP 28, 06234 Villefranche-sur-Mer, Cedex, France
2Unité d’Océanographie Chimique (B5), Interfaculty Centre for Marine Research, Université de Liège, 4000 Sart Tilman, Belgium
3Institut Mediterrani d’Estudis Avançats (CSIC-UIB), C/Miquel Marquès 21, 07190 Esporles, Spain
4Aalborg Universitet (AU), Sohngnardsholmsvej 57, Aalborg, Denmark
5Netherlands Institute of Ecology (NIOO-KNAW), Centre for Estuarine and Marine Ecology, Postbus 140, 4400 AC Yerseke, The Netherlands
6Present address: Netherlands Institute of Ecology (NIOO-KNAW), Centre for Estuarine and Marine Ecology, Postbus 140, 4400 AC Yerseke, The Netherlands
*Email: Deceased on 13 March 2005

ABSTRACT: The metabolic status, the difference between organic matter production and consumption of an estuary (Randers Fjord, Denmark) has been assessed based on 2 field cruises in April and August 2001 and a number of approaches: (1) the oxygen (O2) incubation method, (2) dissolved inorganic carbon (DIC) budgets, (3) the response surface difference (RSD) method based on diel O2 changes and (4) land–ocean interaction in the coastal zone (LOICZ) budgets based on dissolved inorganic phosphorus (DIP). Although each method has its own associated limitations and uncertainties, the above approaches converged most of the time in consistent metabolic estimates, both in sign and magnitude, and revealed that this system was near metabolic balance in spring (net ecosystem production: NEP ~ 0) and net heterotrophic in summer (NEP ~ –50 mmol C m–2 d–1). In this shallow estuary (mean depth = 1.6 m), the benthic compartment was very active and represented 70 and 30% of the total gross primary production in April and August, respectively. NEP rates measured during this study are in the range of previously reported rates in estuaries.

KEY WORDS: Ecosystem metabolism · Estuaries · Primary production · Respiration · Randers Fjord · Oxygen · Carbon dynamics

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