Pelagic metabolism of the Douro estuary (Portugal) e Factors controlling primary production Isabel C. Azevedo a,b, * , Pedro M. Duarte c , Adriano A. Bordalo a,b a Institute of Biomedical Sciences, University of Porto, Lg. Abel Salazar, 2, P-4099-003 Porto, Portugal b Institute of Marine Research (CIIMAR), Portugal c Centre for Modelling and Analysis of Environmental Systems, Faculty of Science and Technology, Fernando Pessoa University, Prac ¸a 9 de Abril 349, P-4249-004 Porto, Portugal Received 6 August 2005; accepted 3 April 2006 Available online 9 June 2006 Abstract The pelagic metabolism of the Douro estuary (Portugal) and the factors influencing primary production (PP) and community respiration (CR) in this system were studied during an annual cycle (December 2002eDecember 2003). Sampling surveys were conducted twice a month during ebb and flood spring tides and water samples were collected for PP and CR assessments at three stations along the estuary (lower, middle and upper stretches). During the study period, PP values were in the range of 4.7e1878.5 mg C m ÿ2 d ÿ1 (average, 319.9 mg C m ÿ2 d ÿ1 ). River dis- charge controlled phytoplankton biomass inputs into the estuary as well as residence time. A decreasing trend in water column PP from the upper to the lower estuary related to higher nitrogen concentrations and phytoplankton biomass from riverine origin was observed. An inverse trend was found for CR, i.e., higher values were found in the lower, more urbanized stretch. During the study period, averaged CR values reached 1154 mg C m ÿ2 d ÿ1 . In general, heterotrophy dominated the entire estuary, except in the upper stretch from May through July, when increased PP, but also lower CR values were recorded. A positive correlation between chlorophyll a and P max was found which is unusual in coastal ecosystems, where a decreasing trend of the P/B ratio as a function of net primary production is generally observed. This could be explained by the relatively low phytoplankton biomass, preventing intraspecific competition from lowering photosynthetic capacity, on one hand, and the physiology of phytoplankton related to their origin in a semi-lotic (reservoir) ecosystem. No significant differences between tides were observed for all variables, except for the water light extinction coefficient (k) values, reflecting higher turbidity during the ebb. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: primary production; pelagic metabolism; respiration; phytoplankton; photosynthetic parameters; Douro estuary 1. Introduction The metabolic balance of a given system depends upon its pri- mary production and community respiration. Primary production is dependent on physical (light availability and temperature), chemical (nutrients) and biological factors, like phytoplankton biomass, species composition, size structure and grazing (Stearns et al., 1987; Cloern, 1991; Landry et al., 1995; Gallegos and Jordan, 1997; Calbet and Landry, 2004; Cermeno et al., 2006), as well as viral control (Proctor and Fuhrman, 1991). In estuaries, these factors may be influenced by freshwater inflow, since it carries nutrients, phytoplankton and suspended matter, which determines light availability in the water col- umn. River flow magnitude also controls residence time and, hence, the susceptibility of ecosystems to algal blooms, with effects propagating throughout the food web to higher trophic levels (Kimmerer, 2002). Seasonal shifts from auto- to hetero- trophy according to river flow variations have also been re- ported. For example, heterotrophy occurs during monsoon periods in tropical estuaries, when increased allochthonous or- ganic input leads to enhanced respiration (Ram et al., 2003). * Corresponding author. Institute of Biomedical Sciences, Lg. Abel Salazar, 2, P-4099-003 Porto, Portugal. E-mail address: iazevedo@icbas.up.pt (I.C. Azevedo). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.04.002 Estuarine, Coastal and Shelf Science 69 (2006) 133e146 www.elsevier.com/locate/ecss