William M. Balch

A biogeochemical study of the coccolithophore, Emiliania huxleyi, in the North Atlantic

The biogeochemical properties of an extensive bloom (∼250,000 km2) of the coccolithophore, Emiliania huxleyi, in the north east Atlantic Ocean were investigated in June 1991. Satellite (NOAA-AVHRR) imagery showed that the bloom was centered initially at 60°–63°N by 13°–28°W and lasted approximately 3 weeks. Spatial variations in satellite-measured reflectance were well correlated with surface measurements of the beam attenuation coefficient, levels of particulate inorganic carbon, and coccolith density. Rates of both photosynthesis and calcification were typically relatively low within the coccolithophore-rich waters, suggesting the population was in a late stage of development at the time of the field observations. Levels of dimethyl sulphide (DMS) in surface waters were high compared to average ocean values, with the greatest concentrations in localized areas characterized by relatively high rates of photosynthesis, calcification, and grazing by microzooplankton. The estimated spatially averaged flux of DMS to the atmosphere was 1122 nmol m−2 h−1, somewhat greater than that determined for the same region in June-July 1987. Coccolith production (1 × 106 tonnes calcite-C) had a significant impact on the state of the CO2 system, causing relative increases of up to 50 μatm in surface pCO2 in association with alkalinity and water temperature changes. Gradients in pCO2 were as great as 100 μatm over horizontal distances of 20–40 km. The environmental implications of these findings are discussed in relation to the spatial and temporal distributions of E. huxleyi.

Massive phytoplankton blooms under Arctic Sea ice

In midsummer, diatoms have taken advantage of thinning ice cover to feed in nutrient-rich waters. Phytoplankton blooms over Arctic Ocean continental shelves are thought to be restricted to waters free of sea ice. Here, we document a massive phytoplankton bloom beneath fully consolidated pack ice far from the ice edge in the Chukchi Sea, where light transmission has increased in recent decades because of thinning ice cover and proliferation of melt ponds. The bloom was characterized by high diatom biomass and rates of growth and primary production. Evidence suggests that under-ice phytoplankton blooms may be more widespread over nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in these waters may be underestimated by up to 10-fold.

The remote sensing of ocean primary productivity: Use of a new data compilation to test satellite algorithms

We tested global pigment and primary productivity algorithms based on a new data compilation of over 12,000 stations occupied mostly in the northern hemisphere, from the late 1950s to 1988. The results showed high variability of the fraction of total pigment contributed by chlorophyll a (ρ), which is required for subsequent predictions of primary productivity. Two models, which predict pigment concentration normalized to attenuation length or euphotic depth, were checked against 2,800 vertical profiles of pigments (chlorophyll a, phaeopigment and total pigment). Phaeopigments consistently showed maxima at about one optical depth below the chlorophyll maxima. We also checked the global Coastal Zone Color Scanner (CZCS; daily 20km resolution) archive for data coincident with the sea truth data. A regression of satellite-derived pigment versus ship-derived pigment had a coefficient of determination (r2) of 0.40 (n=731 stations). The satellite underestimated the true pigment concentration in mesotrophic and oligotrophic waters ( 1 mg pigment m-3). The error in the satellite estimate showed no trends with time between 1978 and 1985. In general the variability of the satellite retrievals increased with pigment concentration. Several productivity algorithms were tested which utilize information on the photoadaptive parameters, biomass and optical parameters for predicting integral production. The most reliable algorithm which explained 67% of the variance in integral production for 1676 stations suggested that future success in deriving primary productivity from remotely sensed data will rely on accurate retrievals of “living” biomass from satellite data, as well as the prediction of at least one photoadaptive parameter such as maximum photosynthesis.

Retrieval of coccolithophore calcite concentration from SeaWiFS Imagery

We examined blooms of the coccolithophorid E. huxleyi, observed in SeaWiFS imagery, with a new algorithm for the retrieval of detached coccolith concentration. The algorithm uses only SeaWiFS bands in the red and near infrared (NIR) to minimize the influence of the absorption by chlorophyll and dissolved organic material. We used published experimental determinations of the calcite specific backscattering and its spectral dependence, and assumed that the absorption coefficient of the medium was that of pure water, to estimate the marine contribution to the SeaWiFS radiance. The aerosol (and Rayleigh-aerosol interaction) contribution to the radiance was modeled as an exponential function of wavelength. These allow derivation of the coccolith concentration on a pixel-by-pixel basis from SeaWiFS imagery. Application to a July 30, 1999 SeaWiFS image of a bloom south of Plymouth, UK indicates that the SeaWiFS estimates are in good agreement with surface measurements of coccolith concentration.

Calcification rates in the equatorial Pacific along 140°W

Abstract The calcite standing stock, calcification rate, concentrations of detached coccoliths and plated coccolithophore cells were determined in the equatorial Pacific along 140°W, between 12°N and 12°S latitude, during August and September 1992. Continuous surface optical and fluorescence measurements were also taken along this transect. Integrated calcification ranged between 3 and 12% of the total carbon fixed into particulate matter. Calcification exceeded 50% of the total fixed carbon (per unit volume) at specific depths from the northern-most oligotrophic stations. A pronounced subsurface peak in suspended calcite was noted near the equator. Calcification was considerably more patchy than photosynthesis. Normalizing the calcification rates to the surface area of calcite-producing species provided an estimate of the extracellular calcite flux rates. These results showed that the populations from the equator to 3°N at 60 m depth, and near the surface from the equator to 9°S were the most active calcite producers. Underway estimates of light scattering showed the importance of upwelling for bringing cold, clear, relatively particle-free water to the surface, followed by growth and calcite production as the water warmed. When temperatures reached their upper range (about 28.8°C), light scattering decreased again, presumably as growth slowed and particles sank. Integrated calcification estimates averaged over the equatorial region were compared to sediment trap data: the results suggest significant disappearance of calcite particles in the top 1000 m, above the lysocline. One hypothesis to explain this is that dissolution occurred in microzones where decomposition of reduced organic matter lowered the pH sufficiently to dissolve calcite.

Factors affecting the estimate of primary production from space

Remote sensing of primary production in the euphotic zone has been based mostly on visible-band water-leaving radiance measured with the coastal zone color scanner. There are some robust, simple relationships for calculating integral production based on surface measurements, but they also require knowledge of photoadaptive parameters such as maximum photosynthesis which currently cannot be obtained from space. A 17,000-station data set is used to show that space-based estimates of maximum photosynthesis could improve predictions of ψ, the water column light utilization index, which is an important term in many primary productivity models. Temperature is also examined as a factor for predicting hydrographic structure and primary production. A simple model is used to relate temperature and maximum photosynthesis; the model incorporates (1) the positive relationship between maximum photosynthesis and temperature and (2) the strongly negative relationship between temperature and nitrate in the ocean (which directly affects maximum growth rates via nitrogen limitation). Since these two factors relate to carbon and nitrogen, “balanced carbon/nitrogen assimilation” was calculated assuming the Redfield ratio. It is expected that the relationship between maximum balanced carbon assimilation versus temperature is concave-down, with the peak dependent on nitrate uptake kinetics, temperature-nitrate relationships, and the carbon/chlorophyll ratio. These predictions were compared with sea truth data. The minimum turnover time for nitrate was also calculated using this approach. Lastly, sea surface temperature gradients were used to predict the slope of isotherms (a proxy for the slope of isopycnals in many waters). Sea truth data show that at size scales of several hundred kilometers, surface temperature gradients can provide information on the slope of isotherms in the top 200 m of the water column. This is directly relevant to the supply of nutrients into the surface mixed layer, which is useful for predicting integral biomass and primary production.

Re-evaluation of the physiological ecology of coccolithophores

Recent data on coccolithophore abundance and calcification from the Atlantic, Pacific and Indian Oceans is interpreted within the traditional physiological ecology paradigm known as the “Margalef Mandala” (Margalef 1978). As predicted by Margalef, coccolithophores should be most abundant in conditions of moderate turbulence and nutrient concentrations. However, blooms of coccolithophores (and other algal groups, too) show day length preferences, suggesting that the Margalef Mandala might be expanded to include day length as a third dimension. Variability in calcification per cell makes it difficult to extrapolate coccolithophore abundance (in the Margalef Mandala) to carbon fixation and vertical carbon fluxes. Because of this, there is a discrepancy between biologists and geologists concerning when coccolithophores should be most abundant. Biologists contend that coccolithophores are found in stratified, mesotrophic, environments while geologists contend that they are most abundant in highly productive environments. A conceptual model is presented which attempts to bridge the two schools based on a) variable calcification per cell, and b) grazing differences between mesotrophic and eutrophic environments that will alter the vertical flux of the coccolithophores to the sediments.

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.

A meeting place of great ocean currents: shipboard observations of a convergent front at 2°N in the Pacific

Abstract We present a synthesis of physical, chemical and biological shipboard observations of a convergent front at 2°N, 140°W and its surrounding environment. The front was a component of a tropical instability wave generated by shear between westward-flowing equatorial waters to the south and warmer equatorial counter current water to the north. Surface waters on the cold side were undersaturated with oxygen, which suggests that the water had only been exposed at the sea surface for a period of a few weeks. Although the atmospheric exposure time was short, the effects of biological activity could be detected in enhanced concentrations of total (dissolved plus suspended particulate) organic carbon concentration, proving that TOC can be produced in the top centimeters of the changing environmental conditions. The front itself was dominated by the accumulation of a “patch” of buoyant diatoms Rhizosolenia castracanei concentrated in the top centimeters of the warm surface water north of the front, and elevated chlorophyll concentrations were observed from the air over a spatial scale of order 10–20 km northward from the front. The nitrogen budget and thorium data suggest that a significant fraction of the elevated POC, and virtually all of the PON, arrived in the patch waters as imported particles rather than in situ photosynthesis. Photosynthetic uptake of carbon appears to have occurred in patch waters, but without corresponding uptake of fixed nitrogen (an uncoupling of the usual Redfield stoichiometry). Solute chemistry of the patch appears to be controlled by turbulent mixing, which flushes out patch waters on a time scale of days

Response of water-leaving radiance to particulate calcite and chlorophyll a concentrations: A model for Gulf of Maine coccolithophore blooms

A coupled atmosphere and ocean radiative transfer model, the Gulf of Maine (GOM) model, was developed to simulate water-leaving radiance from a vertically stratified ocean containing a bloom of the coccolithophore Emiliania huxleyi. The model is based largely on atmospheric and ocean data representing the Gulf of Maine. The atmospheric submodel simulates direct sunlight and diffuse skylight illuminating the sea surface and is adjusted to account for seasonal changes in atmospheric aerosols. The optical properties of E. huxleyi, required by the ocean submodel, are derived from measurements collected in Gulf of Maine coccolithophore blooms occurring in 1989 and 1990. The modeled response of volume reflectance to the combined effects of chlorophyll and particulate calcite compares favorably with field measurements of E. huxleyi cell abundance and coastal zone color scanner (CZCS)-derived volume reflectance representing a coccolithophore bloom in the northeast Atlantic Ocean. The GOM model was used to investigate the response of normalized water-leaving radiance, modeled for visible CZCS and sea viewing wide field of view sensor (SeaWiFS) bands, to particulate calcite and chlorophyll a. Ranges in the concentrations of particulate calcite, chlorophyll a, and colored dissolved organic material (CDOM) are selected to represent conditions reported for coccolithophore blooms. Water-leaving radiance increases with increasing particulate calcite concentration, primarily because of a disproportionately large amount of backscatter from detached coccoliths (about an order of magnitude larger than is predicted using Mie theory). As a result, CZCS plant pigment algorithms based upon radiance ratios may be corrupted more severely than previously estimated. As an alternative to radiance ratio-based algorithms, an iterative procedure (also referred to as optimization) is used to invert the GOM model in order to simultaneously retrieve particulate calcite and chlorophyll a concentrations. The approach uses normalized water-leaving radiance computed for all visible CZCS or SeaWiFS bands. Tests of the approach suggest that independent of errors associated with instrument calibration and atmospheric correction, errors in the retrieved concentrations are small, even when high concentrations of CDOM and vertical structure within the water column are neglected, i.e., with the assumptions that CDOM concentration is small and the water is vertically homogeneous. However, since there are no data sets of contemporaneous chlorophyll a concentration, particulate calcite concentration, and CZCS imagery, a rigorous test of the model and inversion technique must wait for the launch of new ocean color scanners such as the NASA SeaWiFS.