No abstract

The index of refraction (n) of particles is an important parameter in optical models that aims to extract particle size and carbon concentrations from light scattering measurements. An inadequate choice of n can critically affect the characterization and interpretation of optically-derived parameters, including those from satellite-based models which provide the current view of how biogeochemical processes vary over the global ocean. Yet, little is known about how n varies over time and space to inform such models. Particularly, in situ estimates of n for bulk water samples and at diel-resolving time scales are rare. Here, we demonstrate a method to estimate n using simultaneously and independently collected particulate beam attenuation coefficients, particle size distribution data, and a Mie theory model. We apply this method to surface waters of the North Pacific Subtropical Gyre (NPSG) at hourly resolution. Clear diel cycles in n were observed, marked by minima around local sunrise and maxima around sunset, qualitatively consistent with several laboratory-based estimates of n for specific phytoplankton species. A sensitivity analysis showed that the daily oscillation in n amplitude was somewhat insensitive to broad variations in method assumptions, ranging from 11.3 ± 4.3% to 16.9 ± 2.9%. Such estimates are crucial for improvement of algorithms that extract the particle size and production from bulk optical measurements, and could potentially help establish a link between n variations and changes in cellular composition of in situ particles.

Photosynthesis acts as a fundamental control in the cycling of biologically reactive elements in the ocean. Modeling photosynthesis requires an understanding of its response to light, specifically the maximum rate of photosynthesis per photon absorbed and the irradiance level at which it becomes light-saturated (E k ), though field measurements of these parameters are both time and labor intensive. As absorbed light either drives photosynthesis, is re-emitted as fluorescence, or is converted to heat, fluorescence can be related to the photosynthetic response to light in that, as light increases, there exists an inflection point where the probability that excess absorbed energy is dissipated as heat increases and fluorescence yield is decreased. Accordingly, we use a combination of in vivo chlorophyll fluorescence, particulate matter absorption spectra, and photosynthetically active radiation measurements to approximate this inflection irradiance (termed E FT ) and relate it to modeled E k along a transect from the oligotrophic North Pacific Subtropical Gyre to the edge of the more eutrophic subpolar gyre (~45 N). We find that E FT declines by a factor of 4Â from values of 200-300 μmol photons m À2 s À1 in the oligotrophic gyre to 50-100 μmol photons m À2 s À1 north of the transition zone and correlates well with E k from traditional data and models. This latitudinal pattern is associated with changes in biomass concentrations and the phytoplankton carbon to chlorophyll ratio, as well as with changes in particulate carbon to nitrogen ratios. Collectively, these results demonstrate a promising framework to capture high-resolution variability in a key photosynthetic parameter.

No abstract