Contributions of phytoplankton and other particles to inherent optical properties in New England continental shelf waters

Abstract

Variability in upper ocean optical properties is often driven by changes in the particle pool. We investigated the effects of such changes by characterizing individual particles. For particles in natural assemblages, we used a combination of Mie theory and flow cytometry to determine diameter (D), complex refractive index (n ϩ inЈ), and optical cross-sections at 488 nm. Particles were grouped into categories of eukaryotic pico/nanophytoplankton, Synechococcus, heterotrophic prokaryotes, detritus, and minerals to interpret variability in concurrently measured bulk inherent optical properties (IOPs) in New England continental shelf waters during two seasons. The summed contributions of individual particles to phytoplankton absorption and particle scattering were close to values for these properties measured independently using bulk methods (87% and 107%, respectively). In surface waters during both seasons, eukaryotic phytoplankton were responsible for the majority of both total particle absorption and total particle scattering. Mineral particles contributed the most to backscattering (b b ) in the spring, whereas in the summer both mineral and detrital particles were important. Synechococcus and heterotrophic prokaryotes never contributed more than 14% to IOPs. Our findings emphasize that the measurement of nonliving particles, including detritus and minerals, is necessary for understanding variability in b b in the ocean, an important quantity in the interpretation of satellite ocean color.

Significant contribution of large particles to optical backscattering in the open ocean

“…5). These findings add to previous studies that have 1) demonstrated in the laboratory that the carbonspecific scattering cross-section of different phytoplankton groups grown under different conditions is rather constrained (Stramski and Morel, 1990;Stramski and Reynolds, 1993;DuRand et al, 2002); 2) suggested that c p can track phytoplankton biomass (Durand and Olson, 1996;Chung et al, 1996Chung et al, , 1998Green et al, 2003;Oubelkheir et al, 2005); 3) shown that chl-a:c p ratios are closely correlated to variations in phytoplankton physiological parameters in the field (Behrenfeld and Boss, 2003); 4) demonstrated that the ratio of chl-a to a function of b bp retrieved from ocean color data tracks physiological trends expected from laboratory experiments over vast oceanic regions ; and 5) shown that the seasonal cycles of satellite-based chla:b bp follow closely, in the oligotrophic ocean, those of the in-situ cellular fluorescence of Prochlorococcus (Westberry et al, 2008). Together, these different lines of evidence add support to the hypothesis that b bp can provide an alternative to chl-a for monitoring open-ocean phytoplankton biomass from space Westberry et al, 2008).…”

“…A validation of the above estimates was only presented by Green et al (2003) who favorably compared the reconstructed total particulate scattering signals to independently measured bulk b p (which dominates c p ). Indeed, most of the aforementioned investigations adopted different hypotheses to which the calculated scattering cross-sections are strongly dependent: for example, some studies employed forward scattering measurements to obtain particle sizes (Durand and Olson, 1996;Chung et al, 1998;Green et al, 2003;Grob et al, 2007), while others assumed fixed sizes for a given particle type (Chung et al, 1996;Claustre et al, 1999;Oubelkheir et al, 2005).…”

“…Indeed, most of the aforementioned investigations adopted different hypotheses to which the calculated scattering cross-sections are strongly dependent: for example, some studies employed forward scattering measurements to obtain particle sizes (Durand and Olson, 1996;Chung et al, 1998;Green et al, 2003;Grob et al, 2007), while others assumed fixed sizes for a given particle type (Chung et al, 1996;Claustre et al, 1999;Oubelkheir et al, 2005). Thus, it is unclear if the differences in the reported contributions of microorganisms to bulk c p were real and due to variations in the geographic regions, or biased because of the the assumptions regarding size and scattering theory used to retrieve c p .…”

“…Other studies estimated that phytoplankton accounted for up to 50-60% of c p (Durand and Olson, 1996;Chung et al, 1996;Green et al, 2003). In addition, Durand and Olson (1996) showed that most of the diel variations in bulk c p could be attributed to variations in phytoplankton c p .…”

“…The contribution of oceanic microorganisms to IOPs such as the particle beam-attenuation and scattering coefficients (c p and b p , respectively) have been investigated rather extensively by means of flow cytometry (Durand and Olson, 1996;Chung et al, 1996Chung et al, , 1998Claustre et al, 1999;Green et al, 2003;Oubelkheir et al, 2005;Grob et al, 2007). Typically, the concentrations, sizes, and refractive indices of specific groups of microorganisms were estimated and then their contribution to c p calculated.…”