Primary productivity off western and southern Portuguese waters
Bulletin of the Sea Fisheries Institute 1 (158) 2003, pp. 49-59

Maria Helena Cavaco and Graça Cabeçadas
Instituto Portugues de Investigaçao das Pescas e do Mar
Av. Brasília, 1449-006 Lisboa, Portugal

Key words: primary productivity, 14C method, assimilation number, Portuguese coastal waters.

Spatial and seasonal variations in phytoplankton productivity (PP) (measured with the 14C method) and chlorophyll a (Chl a) were analysed using data obtained from several transects along the Portuguese coast during five cruises carried out in August and November 1985, January and March 1986 and April 1987. The variation of several parameters was examined, including surface and integrated chlorophyll a, surface and integrated primary productivity and assimilation number. The average values of PP and Chl a (integrated down to the 1% light level) along the coast were higher for inshore regions, values ranging from 4.1 to 8.3 mgC m-2h-1 and 9.5 to 30.1 mg m-2, respectively, than for offshore regions where values ranged from 1.5 to 6.6 mgC m-2h-1 and 4.8 to 10.3 mg m-2. No significant geographical differences were detected between the western and southern coast. However, variations of vertically integrated estimates of PP occurred over the study period. The highest photosynthetic production was measured in March inshore (27.2 mgC m-2h-1) and in April offshore (1.0 mgC m-2h-1), while the chlorophyll a maximum was attained in summer with values as high as 66.1 mg m-2 inshore and 2.9 mg m-2 offshore. The assimilation numbers estimated along the coast were between 1.3 and 7.7 mgC mgChl a-1h-1. During most of the surveys the water column was nearly depleted of nutrients which indicated that conditions for algal growth were unfavourable. Significant positive correlations (p < 0.01) were found between integrated PP and surface Chl a on only two of the five cruises. Attempts were made to interpret these relationships by taking into account upwelling conditions. It was concluded that primary productivity and phytoplankton biomass measured along the Portuguese coast was within ranges reported for other regions of the Atlantic Ocean which are considered moderately productive.