The current database on benthic microalgal production in Arctic waters comprises 10 peer-reviewed and three unpublished studies. Here, we compile and discuss these datasets, along with the applied measurement approaches used. The latter is essential for robust comparative analysis and to clarify the often very confusing terminology in the existing literature. Our compilation demonstrates that i) benthic microalgae contribute significantly to coastal ecosystem production in the Arctic, and ii) benthic microalgal production on average exceeds pelagic productivity by a factor of 1.5 for water depths down to 30 m. We have established relationships between irradiance, water depth and benthic microalgal productivity that can be used to extrapolate results from quantitative experimental studies to the entire Arctic region. Two different approaches estimated that current benthic microalgal production in the Arctic is between 1.1 and 1.6=10 7 tons C year -1 . Climate change is expected to increase the overall primary production and affect the balance between pelagic and benthic productivity in the Arctic. It is therefore imperative to get better quantitative understanding of the relationship between increased freshwater run-off, shrinking sea-ice cover, light availability and benthic primary production to assess future impact on the Arctic food web and trophic coupling.

A bstract: Three benthic diatom taxa Navicula perminuta, Melosira moniliformis and Nanofrustulum shiloi were isolated from sublittoral sandy sediments from the brackish southern Baltic Sea and established as unialgal cultures. Growth rates were determined under controlled conditions at different incubation temperatures (7-27°C), irradiances (10-600 pmol photons m 2 s ') and salinities (1-50). The diatoms exhibited a wide range of growth tolerance. All of them grew well with growth rates of 0.3-1.5 divisons (p) d-i under the given gradients of parameters, indicating a classification as euryhaline and eurythermal species. In accordance with these results, photosynthesis was characterised at optimal with suboptimal growth conditions of temperature and irradiance, using the methodological approach of oxygen production. Maximum oxygen production rates after preincubation under 150 pmol photons m~2 s-' reached values of 120 to 360 pmol 0 2 mg chlorophyll a h 1. All three benthic diatoms from the Baltic Sea are physiologically well adapted to the fluctuating environmental conditions in shallow-water habitats without production loss under suboptimal conditions.

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During summer 2007, Arctic microphytobenthic potential primary production was measured at several stations around the coastline of Kongsfjorden (Svalbard, Norway) at B5 m water depth and at two stations at five different water depths (5, 10, 15, 20, 30 m). Oxygen planar optode sensor spots were used ex situ to determine oxygen exchange in the overlying water of intact sediment cores under controlled light (ca. 100 lmol photons m -2 s -1 ) and temperature (2-4°C) conditions. Patches of microalgae (mainly diatoms) covering sandy sediments at water depths down to 30 m showed high biomass of up to 317 mg chl a m -2 . In spite of increasing water depth, no significant trend in ''photoautotrophic active biomass'' (chl a, ratio living/dead cells, cell sizes) and, thus, in primary production was measured at both stations. All sites from B5 to 30 m water depth exhibited variable rates of net production from -19 to ?40 mg O 2 m -2 h -1 (-168 to ?360 mg C m -2 day -1 ) and gross production of about 2-62 mg O 2 m -2 h -1 (17-554 mg C m -2 day -1

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In contrast to numerous studies on the biomass of marine microphytobenthos from temperate coastal ecosystems, little is known from polar regions. Therefore, microphytobenthos biomass was measured at several coastal sites in Arctic Kongsfjorden (Spitsbergen) during the polar summer (June-August 2006). On sandy sediments, chla varied between 8 and 200 mg m -2 and was related to water depth, current/wave exposure and geographical location. Biomass was rather independent of abiotic parameters such as sediment properties, salinity, temperature or light availability. At three stations, sediments at water depths of 3-4, 10, 15, 20 and 30 m were investigated to evaluate the effect of light availability on microalgae. Significant differences in distribution patterns of biomass in relation to deeper waters )10 m were found. The productive periods were not as distinct as phytoplankton blooms. Only at 3-4 m water depth at all three stations were two-to threefold increases of biomass measured during the investigation period. Hydrodynamic conditions seemed to be the driving force for differences in sediment colonisation by benthic microalgae. In spite of the extreme Arctic environmental conditions for algal growth, microphytobenthos biomass was comparable to marine temperate waters.

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