David T. Drapeau

Optical backscattering by calcifying algae : Separating the contribution of particulate inorganic and organic carbon fractions

Light scattering properties of biogenic CaCO3 particles [particulate inorganic carbon (PIC)] were determined on cultured calcifying algae and field-derived CaCO3 particles. The particles were separated from particulate organic carbon (POC) with a flow cytometer, volume-scattering functions were measured with a laser light-scattering photometer, and particle composition was measured using atomic absorption spectrometry. Small calcite coccoliths were best sorted by gating on the ratio of horizontally polarized forward light scattering and vertically polarized forward light scattering; plated coccolithophores could be sorted by gating on side scattering and forward angle light scattering. Normalized volume-scattering functions for the culture-derived calcite particles varied by a factor of 2 for the different species. Backscattering cross sections (m2 particle−1) for calcite particles varied by ∼35 times and were generally a function of size. Backscattering efficiencies were ∼2–4 times higher for cells with CaCO3 than without it. CaCO3-specific backscattering showed much less variability across various species; the calcite-specific backscattering coefficient varied by only ∼38% for both cultured coccolithophores and field-derived CaCO3 particles. Organic carbon-specific backscattering of “naked” coccolithophores was highly consistent within all coccolithophores used in our experiments, as well as with values in the literature. Our results suggest that both POC and PIC can be optically estimated, the former by measuring backscattering of decalcified phytoplankton as well as their size distribution, and the latter is proportional to acid-labile backscattering. These results show the feasibility of a rapid optical technique for measuring two biogeochemically important carbon fractions in the sea.

Factors regulating the Great Calcite Belt in the Southern Ocean and its biogeochemical significance

The Great Calcite Belt (GCB) is a region of elevated surface reflectance in the Southern Ocean (SO) covering ~16% of the global ocean and is thought to result from elevated, seasonal concentrations of coccolithophores. Here we describe field observations and experiments from two cruises that crossed the GCB in the Atlantic and Indian sectors of the SO. We confirm the presence of coccolithophores, their coccoliths, and associated optical scattering, located primarily in the region of the subtropical, Agulhas, and Subantarctic frontal regions. Coccolithophore-rich regions were typically associated with high-velocity frontal regions with higher seawater partial pressures of CO2 (pCO2) than the atmosphere, sufficient to reverse the direction of gas exchange to a CO2 source. There was no calcium carbonate (CaCO3) enhancement of particulate organic carbon (POC) export, but there were increased POC transfer efficiencies in high-flux particulate inorganic carbon regions. Contemporaneous observations are synthesized with results of trace-metal incubation experiments, 234Th-based flux estimates, and remotely sensed observations to generate a mandala that summarizes our understanding about the factors that regulate the location of the GCB.

Toward a quantitative and empirical dissolved organic carbon budget for the Gulf of Maine, a semienclosed shelf sea

A time series of organic carbon export from Gulf of Maine (GoM) watersheds was compared to a time series of biological, chemical, bio-optical, and hydrographic properties, measured across the GoMbetween Yarmouth, NS, Canada, and Portland, ME, U.S. Optical proxies were used to quantify the dissolved organic carbon (DOC) and particulate organic carbon in the GoM. The Load Estimator regression model applied to river discharge data demonstrated that riverine DOC export (and its decadal variance) has increased over the last 80 years. Several extraordinarily wet years (2006–2010) resulted in a massive pulse of chromophoric dissolved organic matter (CDOM; proxy for DOC) into the western GoM along with unidentified optically scattering material (<0.2μmdiameter). A survey of DOC in the GoM and Scotian Shelf showed the strong influence of the Gulf of Saint Lawrence on the DOC that enters the GoM. A deep plume of CDOM-rich water was observed near the coast of Maine which decreased in concentration eastward. The Forel-Ule color scale was derived and compared to the same measurements made in 1912–1913 by Henry Bigelow. Results show that the GoM has yellowed in the last century, particularly in the region of the extension of the Eastern Maine Coastal Current. Time lags between DOC discharge and its appearance in the GoM increased with distance from the river mouths. Algae were also a significant source of DOC but not CDOM. Gulf-wide algal primary production has decreased. Increases in precipitation and DOC discharge to the GoM are predicted over the next century.

Estimating Particulate Inorganic Carbon Concentrations of the Global Ocean From Ocean Color Measurements Using a Reflectance Difference Approach

A new algorithm for estimating particulate inorganic carbon (PIC) concentrations from ocean color measurements is presented. PIC plays an important role in the global carbon cycle through the oceanic carbonate pump, therefore accurate estimations of PIC concentrations from satellite remote sensing are crucial for observing changes on a global scale. An extensive global dataset was created from field and satellite observations for investigating the relationship between PIC concentrations and differences in the remote sensing reflectance (Rrs) at green, red and near-infrared (NIR) wavebands. Three color indices were defined: two as the relative height of Rrs(667) above a baseline running between Rrs(547) and an Rrs in the the NIR (either 748 nm or 869 nm), and one as the difference between Rrs(547) and Rrs(667). All three color indices were found to explain over 90% of the variance in field-measured PIC. But, due to the lack of availability of Rrs(NIR) in the standard ocean color data products, most of the further analysis presented here was done using the color index determined from only two bands. The new two-band color index algorithm was found to retrieve PIC concentrations more accurately than the current standard algorithm used in generating global PIC data products. Application of the new algorithm to satellite imagery showed patterns on the global scale as revealed from field measurements. The new algorithm was more resistant to atmospheric correction errors and residual errors in sun glint corrections, as seen by a reduction in the speckling and patchiness in the satellite-derived PIC images.

Vertical Distributions of Coccolithophores, PIC, POC, Biogenic Silica, and Chlorophyll a Throughout the Global Ocean

Abstract Coccolithophores are a critical component of global biogeochemistry, export fluxes, and seawater optical properties. We derive globally significant relationships to estimate integrated coccolithophore and coccolith concentrations as well as integrated concentrations of particulate inorganic carbon (PIC) from their respective surface concentration. We also examine surface versus integral relationships for other biogeochemical variables contributed by all phytoplankton (e.g., chlorophyll a and particulate organic carbon) or diatoms (biogenic silica). Integrals are calculated using both 100 m integrals and euphotic zone integrals (depth of 1% surface photosynthetically available radiation). Surface concentrations are parameterized in either volumetric units (e.g., m−3) or values integrated over the top optical depth. Various relationships between surface concentrations and integrated values demonstrate that when surface concentrations are above a specific threshold, the vertical distribution of the property is biased to the surface layer, and when surface concentrations are below a specific threshold, the vertical distributions of the properties are biased to subsurface maxima. Results also show a highly predictable decrease in explained‐variance as vertical distributions become more vertically heterogeneous. These relationships have fundamental utility for extrapolating surface ocean color remote sensing measurements to 100 m depth or to the base of the euphotic zone, well beyond the depths of detection for passive ocean color remote sensors. Greatest integrated concentrations of PIC, coccoliths, and coccolithophores are found when there is moderate stratification at the base of the euphotic zone.

Coccolith dissolution within copepod guts affects fecal pellet density and sinking rate

The most common biomineral produced in the contemporary ocean is calcium carbonate, including the polymorph calcite produced by coccolithophores. The surface waters of the ocean are supersaturated with respect to calcium carbonate. As a result, particulate inorganic carbon (PIC), such as calcite coccoliths, is not expected thermodynamically to dissolve in waters above the lysocline (~4500–6000 m). However, observations indicate that up to 60–80% of calcium carbonate is lost in the upper 500–1000 m of the ocean. This is hypothesized to occur in microenvironments with reduced saturation states, such as zooplankton guts. Using a new application of the highly precise 14C microdiffusion technique, we show that following a period of starvation, up to 38% of ingested calcite dissolves in copepod guts. After continued feeding, our data show the gut becomes increasingly buffered, which limits further dissolution; this has been termed the Tums hypothesis (after the drugstore remedy for stomach acid). As less calcite dissolves in the gut and is instead egested in fecal pellets, the fecal pellet sinking rates double, with corresponding increases in pellet density. Our results empirically demonstrate that zooplankton guts can facilitate calcite dissolution above the chemical lysocline, and that carbon export through fecal pellet production is variable, based on the feeding history of the copepod.