In situ measurements of benthic primary production during emersion:: seasonal variations and annual production in the Bay of Somme (eastern English Channel, France)

A survey of benthic primary production during periods of emersion was performed in a muddy-sand station of the Bay of Somme. Primary production and respiration were estimated by in situ measurements of carbon dioxide fluxes using infra-red analysis. Photosynthetic response of the community to incident light and temperature was analysed at different periods of the year. Seasonal variations of the photosynthetic parameters were estimated using the photosynthesis versus irradiance (P-I) curves constructed in February, April, July, August and October. The rate of maximum gross community primary production (P-m), highly correlated to sediment chlorophyll a (Chl a) content, was low in February (6.7 mg C m(-2) h(-1)) and high in July (97.7 mg C m(-2) h-1). Photosynthetic efficiency at low light intensity (a) was positively correlated to P-m. The very high production (P-m=126.8mg Cm-2 h(-1)) and productivity (ratio of P-m, and sediment Chl a content) measured in March may be related to the set down of active planktonic microalgae. At five dates, the effects of temperature on primary production seemed to overshadow the role of light. The Q(10) for primary production varied from 1.2 in August to 3.0 in December. Daily potential primary production was calculated as a function of theoretical and measured irradiance for the period of superimposition of day and emersion. At the annual scale, the potential gross community primary production was 147g Cm-2 with theoretical irradiance and 110g Cm-2 with measured irradiances. The annual community respiration was 188g Cm-2, leading to a heterotrophic annual budget. The annual pattern of daily production can be largely explained by changes in day length. It is also characterized by a fortnightly variability due to the variation of the total daily irradiance available for photosynthesis caused by the superimposition of the tidal and day/night cycles. Finally, sharp variations occurring with nebulosity can overshadow this fortnightly variability. (C) 2004 Elsevier Ltd. All rights reserved.