Abstract :
[en] The coastal zone, the transition area between land and the open ocean, receives considerable amounts of freshwater, nutrients as well as dissolved and particulate organic matter. These inputs make the coastal zone a very biogeochemically active area. The trophic status of the coastal zone, that is whether it is autotrophic or heterotrophic, is still a matter of debate. The present work had three main objectives: (1) to compile and analyse a bibliographic database of benthic and planktonic primary production (GPP) and community respiration (CR) measured by incubation methods and by the Land-Ocean Interaction in the Coastal Zone (LOICZ) budgeting approach in the European coastal zone, (2) to test different methods to estimate the functioning of three European coastal sites (Randers Fjord, Palma Bay and Scheldt Estuary). These methods are the classical O2 incubation technique, the LOICZ budgeting procedure and other open-system techniques such as the Response Surface Difference (RSD) method and daily cycles of dissolved inorganic carbon (DIC) and O2. Moreover, as the estimation of the gas exchange coefficient of the air-sea interface (k) is critical when applying open-system methods, this parameter and its relationship with wind speed were studied in two of the sites (Randers Fjord and Scheldt estuary) and (3) to study how the metabolic parameters such as GPP or CR both in the benthic and planktonic compartment are controlled by environmental conditions. More than 190 papers have been analysed among which 129 were integrated in a database available on the Internet (http://www.obs-vlfr.fr/eurotroph/index.php). The analysis of the bibliographic database allowed pinpointing a severe lack of knowledge in many areas of Europe such as the Eastern Mediterranean Sea, the Northern Baltic Sea and the Northern North Sea. An assessment of the net ecosystem production (NEP = GPP + CR) of the European coastal zone was not possible based on this study. Indeed, several confident estimates of parameters such as the mineralization of organic matter in the water column, the decrease of benthic production and respiration with depth and the distribution of macrophytes are lacking. The different methods tested during this study generally provided consistent results although each of these is associated with significant errors and uncertainties which limit their use to specific sites and conditions. Particularly, the LOICZ procedure applied to dissolved inorganic phosphorus (DIP) failed to provide a realistic estimate of the trophic status in the Scheldt estuary. This is due to a strong impact of abiotic processes such a sorption and desorption to and from suspended matter and sediments of the non-conservative DIP flux in this highly turbid system. Open-system methods suffer from the difficulty to estimate with accuracy the gas exchange coefficient. Although the relation linking this parameter to the wind speed was shown to be site-specific, the validation of this relationship in the Scheldt estuary using the LOICZ approach applied to DIC was not possible due to strong associated errors and uncertainties. Finally, significant differences observed between GPP and CR estimated by incubation and open-system methods confirmed previous results and suggest that future works should focus on this matter. The three sites investigated were well contrasted: from the nutrient-rich and turbid Scheldt estuary to the oligotrophic seagrass-dominated Bay of Palma. The metabolic parameters were well related to these environmental differences. Indeed, the highest CR rates were measured in the Scheldt estuary while GPP was strongly limited by light availability. The highest GPP was measured in the Randers Fjord where nutrient concentrations as well as water column stratification and a relatively low turbidity allowed the riverine phytoplankton to remain very active in the estuarine zone. While the Scheldt estuary was clearly the most heterotrophic site in this study, it was not possible to conclude whether the Randers Fjord or the Bay of Palma is the most productive ecosystem on an annual scale.
