Jana Woelfel

Benthic microalgal production in the Arctic: applied methods and status of the current database.

The current database on benthic microalgal production in Arctic waters comprises 10 peer-reviewed and three unpublished studies.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.

Growth and photosynthesis characteristics of three benthic diatoms from the brackish southern Baltic Sea in relation to varying environmental conditions

Abstract: 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 μmol photons m−2 s−1) 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 (μ) d−1 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 μmol photons m−2 s−1 reached values of 120 to 360 μmol O2 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.

Microbial Mats from Wind Flats of the Southern Baltic Sea

The Baltic Sea covers an area of 377,000 km² and is the world’s largest brackish-water ecosystem. In geological terms, the Baltic Sea is quite young and its development began with the thawing of the Weichselian ice sheet after the last glaciation 15,000 years BP (before present). Because the connection to the North Sea was mostly temporary, the salinity conditions changed often. For the last 8,000 years BP, the salinity regime remained more or less unchanged and brackish (Bjorck, 1995). The catchment area is 1.6 million square kilometers. The annual input of riverine and precipitation freshwater is about 660 km3, while further 475 km3 of saline water flows in from the North Sea. The discharge of brackish water into the North Sea over the small straits between Denmark and Sweden averages 950 km3 (Bjorck, 1995). Apart from these small connections, the Baltic Sea is surrounded by land. This has a great impact on the salinity regime. Through the inflow of saline bottom water from the North Sea, salty and oxygen-rich water is delivered. Thus, the salinity of the freshwater-influenced surface water decreases from 25–15 PSU in the western part, to 8–6 PSU in the central Baltic Sea and down to 2 PSU in the Bottnian Gulf in the north–east (Matthaus, 1996). Therefore, the Baltic Sea can be characterized as a very large estuary. Despite this horizontal salinity gradient, the conditions in the Baltic Sea are very different from those in estuaries because of missing tides. The tidal range is extremely low with 12–15 cm, but wind direction and wind speed may temporarily induce high waves and change sea water levels (Brosin, 1965; Lass and Magaard, 1996). Consequently, salinity levels at any point do not vary much, resulting in a rather stable vertical salinity gradient, but strong horizontal salinity gradients along the shore line structure the benthic fauna and flora profoundly.

Ecophysiological Performance of Benthic Diatoms from Arctic Waters

The benthos of shallow water coastal zones in the Arctic region consists of a consortium of eukaryotic and prokaryotic microorganisms of high biodiversity. The dominant organisms are benthic diatoms, which form an assemblage referred to as microphytobenthos. This phototrophic community is generally known from temperate marine regions as being highly productive and providing a major food source for benthic suspension or deposit feeders (Cahoon, 1999), as filter for oxygen and other elemental fluxes at the sediment/water interface (Risgaard-Petersen et al., 1994) and as stabiliser of sediment surfaces by the excretion of extracellular polymeric substances (DeBrouwer et al., 2005). Consequently, microphytobenthos represents a key component in the functioning of trophic webs in many coastal regions. Some marine ecosystems, such as the German Wadden Sea, are mainly controlled by the production biology of benthic diatoms. However, structure and function of microphytobenthic communities are badly studied in the Arctic regions (Glud et al., 2009).