The Rapid Spatial and Temporal Mapping of Primary Production Using Natural Fluorescence

Abstract

ABSTRACT Natural fluorescence is the flux of fluoresced light emitted from chlorophyll a during photosynthesis. This signal is centered in the red region of the spectrum around 683 nm and a number of studies have indicated that it is sufficiently strong to be detected throughout the euphotic zone. Since it was first identified in the late 1970's, natural fluorescence has been investigated as a means for rapidly determining the distribution and abundance of marine phytoplankton, particularly since it is one of the few measurements that can be made without perturbing the crop. Moreover, this in situ measurement is rapid, yields information that is of high spatial and temporal resolution and may made from a number of different platforms. Most recently, research on natural fluorescence has determined that this signal can be related not only to the standing crop of phytoplankton, but to instantaneous rates of gross primary production. The first natural fluorescence field instruments were introduced in the mid-1980's. Since that time, natural fluorescence has been used to monitor temporal and spatial primary production in a variety of applications. In this paper, three examples of temporal and spatial mapping of phytoplankton productions are presented. Synoptic coverage is shown using data from measurements of natural fluorescence and 14C incorporation along a transect from Tahiti to New Zealand, from the western Sargasso Sea, and Friday Harbor, Washington. Long-term temporal monitoring is described using data from a mooring located in an urban reservoir in Los Angeles. Lastly, data is presented demonstrating the use of natural fluorescence to map the impact of a kraft mill effluent plume on local rates of primary production and chlorophyll a concentration. INTRODUCTION Natural fluorescence in the sea is the solar-stimulated emission of chlorophyll a in a narrow band centered at 683 nm. It results from the absorption of light by the phytoplankton crop. Because red light is strongly absorbed by water, solar radiation at the wavelengths of chlorophyll a emission disappears within the upper few meters of the water column, and the remaining upwelling radiance in the red comes almost entirely from the phytoplankton crop. Since the origins of this signal were first identified (Morel and Prieur, 1977; Neville and Gower, 1977), natural fluorescence has been widely investigated as a means for rapidly assessing the distribution and biomass of oceanic phytoplankton. Numerous mathematical descriptions have been presented of the relationship between natural fluorescence, the concentration of chlorophyll-a and the ambient light which excites the fluorescence (Gordon, 1979; Kattawar and Vastano, 1979; Kishino et al. 1984 a, b; Topliss and Platt, 1986;Kiefer et al. 1989). Recently, it has been also proposed that rates of primary production maybe estimated from measurements of natural fluorescence (Kiefer et al. 1989; Chamberlain et al. 1990). Because measurements of natural fluorescence are rapid and can be performed without perturbation of the phytoplankton crop, estimates of photosynthetic rate from natural fluorescence have certain advantages over in vitro measurements of photosynthesis. The rapidity and detailed temporal and spatial resolution of the fluorescence measurement make it extremely useful in regions where hydrographic conditions are complex.