Diurnal variability in chlorophyll fluorescence: observations and modeling

ABSTRACT Examples of the diel variability in chlorophyll are presented from data from moored in situ fluorometers and fromprofiles of beam aUenuation coefficient plotted against fluorescence. The data are discussed in terms of the three primaryprocesses thought to influence the die! variability: (1) fluorescence yield per unit chlorophyll .(2)chlorophyll changes percell (or, biomass), and (3) changes in phytoplankton biomass. A provisional model is presented which incorporates thesethree processes and its shortcomings are discussed. Finally, the problem of incorporating diel variability in plankton modelsis considered. For the model given, it is found that there is no consistent set of parameters which will describe the observeddiel chlorophyll variability for pre-bloom and bloom conditions in the Northeast Atlantic (spring, 1989).1 .INTRODUCTIONThe variation in solar irradiance, from day to night, has profound influence not only on planktonic organisms1 but ontheir environment as well.2 The diel variability in chlorophyll in the ocean was apparent early on.3'4'- In these olderstudies the thta on chlorophyll variability was usually not well-resolved because of the difficulty of sampling. Repeatedhydrocasts are often impractical, although Le Bouteillier and Herbland6 achieved a 13-day time series (sampling four times perday) of chlorophyll as a function of depth in the equatorial Atlantic.Ships are not ordinarily stationed to observe the evolution of events in the ocean. Furthermore, for the study of dielvariability, automated sampling and measurement are indispensable, and sampling programs have only recently beenconfigured for this. There has been some success in doing repeated casts for profiles of bio-optical properties,7 or casts atspecified times over a period of da89'10 However, the magnitude and significance of the variability was not revealeduntil the Biowatt Mooring experiment in the Sargasso Here I will discuss some of that data, and other data werecently collected as part of the Marine Light-Mixed Layers (ML-ML) program in the North Atlantic. These latter data arepreliminary, and represent on-going progress in studying the diurnal properties of optics and plankton in the ocean. I will alsodiscuss modeling avenues to the problem of diel variability, and how models might attempt to parameterize diel variabilitywithin longer time-scale phenomena.Soumia12 has reviewed the evidence for diel variations of chlorophyll in the ocean. Historically, the variations wereascribed to effects from irradiance,4'13 or, assuming balanced growth in the phytoplankton, from changes in biomass.6Increasingly, in the last five to seven years, there has been a reliance on in situ fluorometry to provide estimates ofchlorophyll. The benefit of this method is the essentially continuous measurement over depth or time, which fulfills thecondition mentioned above about automating the measurement of the variable of interest in diel studies. The disadavantage isthat in vivo fluorescence adds another component to the variability to the chlorophyll changes, making the signalsnotoriously difficult to interpret.14'15 Thus, in addition to the changes in chlorophyll induced by irradiance or from changesin biomass, there must be added a third effect, changes in fluorescence per unit chlorophyll, or the fluorescence yield.Variation in fluorescence yield forces constraints, but also grants opportunities in estimating photosynthesis andphysiological properties,15 and, indirectly, the study of mechanisms by which phytoplankton are mixed through the euphoticzone. 16Thus, it is possible to subdivide the diel variability in chlorophyll into three sources. First, there may be fluorescencechanges per unit chlorophyll. These are primarily caused by irradiance, are noted especially near the surface where irradiancesare high17J8 and they are rapid (time scale of minutes )