Development and assessment of novel techniques to measure primary production in the Celtic Sea and English Channel

Marine primary production is an essential process in the global carbon cycle, controlling the flux of elements from the surface layer to the deep ocean waters. This research involved the characterisation of photosynthetic performance and primary production over different scales by three different techniques: incubations with 14C, fast repetition rate fluorometry (FRRF) and satellite remote sensing. The objectives were to establish the most appropriate way to estimate primary production, to understand the limitations of techniques and to obtain seasonal estimates of production. These were addressed by a cruise in the Celtic Sea in May 2000 and a one-year times series of measurements at a coastal site in the Western English Channel in 2001. The standard 14C method gave variable results with different incubation procedures. Corrections could be made to account for different light sources used during incubations but not to compensate for incubations of different durations. However, changes in measured values often followed patterns and only the inclusion of a dark period in experiments led to significant changes in derived parameter values. The quality of the FRRF data was dependent on slow profiling and high ambient light. Under these conditions, FRRF estimates of photosynthetic parameters were comparable to those from 14C incubations but showed slight day-to-day variations. Models using FRRF data always led to higher production estimates that those from the 14C method. Fifty percent of the variance in 14C estimates was explained by the variance in FRRF data. The FRRF offers the potential for high-resolution measurements over large horizontal scales but further research is needed to understand how these data relate to depth-integrated production.