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Changes in the population sizes of bacteria and of heterotrophic microflagellates in seawater during the first 30–60 h after sampling indicate that these protozoa control bacterial numbers in situ. The observations allow crude estimates of in situ grazing rates and of the minimum bacterial concentration which sustains protozoan growth. In the water samples studied, an average flagellate will clear 1–2 × 10−5 ml h−1 (15°C). If this result is extrapolated to other areas, typical concentrations of microflagellates in the sea suggest that between 5 and 250% of the water column is cleared of bacteria per day. Bacterial numbers above about 106 ml−1 will sustain flagellate growth.

Microalgae colonizing the underside of sea ice in spring are a key component of the Arctic foodweb as they drive early primary production and transport of carbon from the atmosphere to the ocean interior. Onset of the spring bloom of ice algae is typically limited by the availability of light, and the current consensus is that a few tens‐of‐centimeters of snow is enough to prevent sufficient solar radiation to reach underneath the sea ice. We challenge this consensus, and investigated the onset and the light requirement of an ice algae spring bloom, and the importance of snow optical properties for light penetration. Colonization by ice algae began in May under >1 m of first‐year sea ice with ∼1 m thick snow cover on top, in NE Greenland. The initial growth of ice algae began at extremely low irradiance (<0.17 μmol photons m−2 s−1) and was documented as an increase in Chlorophyll a concentration, an increase in algal cell number, and a viable photosynthetic activity. Snow thickness changed little during May (from 110 to 91 cm), however the snow temperature increased steadily, as observed from automated high‐frequency temperature profiles. We propose that changes in snow optical properties, caused by temperature‐driven snow metamorphosis, was the primary driver for allowing sufficient light to penetrate through the thick snow and initiate algae growth below the sea ice. This was supported by radiative‐transfer modeling of light attenuation. Implications are an earlier productivity by ice algae in Arctic sea ice than recognized previously.

Absolute cross section for photodetachment of Li − ions near the 1s threshold leading to Li + formation has been measured by overlapping a beam of Li − ions with synchrotron radiation from an undulator. Multiple excitation to the coreexcited Li 1s(2s2p 3 P) 2 P and Li 1s(2p3s 3 P) 2 P states, i.e. conjugate shakeup, is found to be much stronger than observed for any neutral atom or positive ion, reflecting the dominant influence of electron correlation for negative ions. The cross section for multiple excitation is significantly larger than for single excitation.Negative ions represent a special class of atomic systems with properties markedly different from neutral atoms and positive ions. In recent years, weakly bound negative atomic ions have attracted considerable interest, and such ions are of fundamental importance in atomic physics. Their structure and dynamics form a critical test for theoretical calculations since the additional electron is bound by means of electron correlation. Recently theoretical and experimental studies of atomic negative ions have been reviewed focusing on their binding energies (Andersen et al 1999), resonances (Buckmann and Clark 1994), and photodetachment cross sections (Ivanov 1999). The experimental photodetachment studies have only been concerned with photodetachment of outer-shell electrons, with lasers being used as the photon source, whereas experimental cross sections for photodetachment of inner-shell electrons have never been reported. The reason may be a combination of the very low densities which can be obtained for negative atomic ions, the limiting photon flux available from bending magnets at synchrotron-radiation facilities and relatively small cross sections. However, with access to synchrotron-radiation beam lines equipped with insertion devices, the increased photon flux allows inner-shell photodetachment studies of atomic negative ions to be performed.This experimental study was initiated with the aim of exploring the competition between photoinduced single and multiple excitation of a weakly bound negative ion exhibiting strong correlation in the ground state. The negative ions of the alkali metals or the alkaline earth metals are suitable candidates for such an investigation (see Andersen et al 1999). We have chosen the Li − system due to its simplicity as a closed-shell ion with only 4 electrons and because the binding energy of its 1s electrons is convinient for the undulator beamline of the ASTRID storage ring. Li − is a highly correlated system-without introducing electron correlation the system is not bound. Thus, correlation is not a small perturbation on the system, rather a dominant factor of the behaviour of the system.Following the negative hydrogen ion, the negative lithium ion is the most important closedshell negative ion which is simple enough to be theoretically tractable in a rigorous manner.

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