Diatoms are unicellular or chain-forming phytoplankton that use silicon (Si) in cell wall construction. Their survival during periods of apparent nutrient exhaustion enhances carbon sequestration in frontal regions of the northern North Atlantic. These regions may therefore have a more important role in the 'biological pump' than they have previously been attributed, but how this is achieved is unknown. Diatom growth depends on silicate availability, in addition to nitrate and phosphate, but northern Atlantic waters are richer in nitrate than silicate. Following the spring stratification, diatoms are the first phytoplankton to bloom. Once silicate is exhausted, diatom blooms subside in a major export event. Here we show that, with nitrate still available for new production, the diatom bloom is prolonged where there is a periodic supply of new silicate: specifically, diatoms thrive by 'mining' deep-water silicate brought to the surface by an unstable ocean front. The mechanism we present here is not limited to silicate fertilization; similar mechanisms could support nitrate-, phosphate- or iron-limited frontal regions in oceans elsewhere.

[1] We present results of the optimization of near-real time on-board sampling strategy in the Iceland-Faroes oceanic frontal area, based on the outputs of a mesoscale 3D operational data assimilation forecasting experiment. By minimizing a root mean square error cost function, we show that in this example an along-front sampling strategy, i.e. with transects parallel to the front, produces smaller errors in temperature, salinity, nitrate, phytoplankton, and zooplankton fields, as a result of a combination of the direction of the sampling of the front and errors associated with the asynopticy of observations (Doppler effect). This is contrary to the classic across-front sampling strategies that are used in most field experiments reported in the literature, i.e. where transects are perpendicular to the front. A control model shows that at these spatio-temporal scales, the along front sampling strategy is optimal when the frontal instability has sufficiently developed.

Three methods of calculating the biovolume of particles from their shadows as recorded by and optical plankton counter (OPC), based on optical geometry, are presented. In the first method (Vsphere), particles are assumed to be opaque spheres. In the other two methods, particles are represented as opaque spheroids, oriented with their major axes either parallel to the flow thus presenting maximum shadow area (Vmax), or randomly orientated relative to the flow (Vran). The models were tested by comparing with net biovolume, measured from samples of a zooplankton assemblage dominated by Calanus finmarchicus collected during a cruise to the northeast Atlantic during 2001. The randomly orientated spheroidal model (Vran) provided the best fit with the net data: on average the ratio of OPC biovolume to net biovolume was 1.02, compared to ratios of 0.84 when calculating OPC biovolume as Vmax and 1.50 when calculating as Vsphere. The Vran and Vmax methods gave reasonable estimates of net biovolume from OPC measurements without recourse to the use of empirical tuning parameters that are otherwise required. This success was enhanced by the fact that the community chosen for validation purposes was dominated by a single species, C. finmarchicus, which could be approximated by spheroids of known dimension. The calibration methods are less likely to be effective when applied to zooplankton communities incorporating a diverse range of organisms.

Water can exhibit many different colors due to a variety of physical properties.Here, we focus on some observable colors within very pure freshwater. We only treat the absorption of light due to electronic and ro-vibrational excitations and scattering due to refractive index fluctuations of the water and the respective consequences for the appearance of colors.

The seasonal, and in particular, winter distribution of Calanus finmarchicus and its predators were investigated using multifrequency (38–600 kHz) acoustic backscatter measurements in the Irminger Sea, North Atlantic, as part of Marine Productivity, a UK contribution to GLOBEC. The distribution of Calanus, which over-winters deeper than 500 m, well below the acoustic penetration depth of a frequency suitable for detecting targets of approximately 0.2 cm from the surface, was examined using a 600-kHz RDI Acoustic Doppler Current Profiler (ADCP) mounted on a lowered CTD frame. A net system, for species information, and an optical plankton counter (OPC), for numerical abundance, was deployed at each station to ground-truth the 600-kHz acoustic backscatter data. Model-predicted profiles of acoustic backscatter, calculated using OPC derived Calanus abundances and a fluid-filled cylinder acoustic scattering model, were consistent with the observed 600-kHz profiles. Calanus predators, euphausiids, e.g., Meganyctiphanes norvegica and fish, e.g., Myctophids, were observed using a surface towed Simrad EK500 multifrequency echosounder. Seasonal variations in the depth of sound scattering layers at each frequency were observed within the Irminger Sea, which suggests that the Calanus predators may be deepening their winter depth to coincide with the maximum over-wintering Calanus abundances.