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

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 (02) 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 similar to 0) and net heterotrophic in summer (NEP similar to -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