Expedition to study impact of inflow on life in the Baltic Sea

From the 16- 30 of September 2015, DTU's research vessel Dana will be in the Baltic Sea on a two week research expedition led DTU Aqua.

Researchers from DTU Aqua and German colleagues from Geomar Kiel and Leibniz Institute for Baltic Sea Research in Warnemünde will investigate the interactions between several Baltic key species -- ranging from phytoplankton, zooplankton such as copepods (small crustaceans), fish eggs and larvae over jellyfish to adult fish. Special focus will be on the Baltic cod, which spawns at this time of year.

"The stock size of cod in the Eastern Baltic has been low for decades, partly because of adverse oxygen conditions on the spawning grounds. Since the mid 1980's only one of the cod's traditional spawning areas has had sufficient oxygen and high enough salinity for cod eggs to survive in larger scale, namely the Bornholm Basin. In recent years the stock size has slightly increased, but the cod have been relatively thin and in poor nutritional condition. However, after the recent inflow of oxygen rich water we might have a different situation," says leader of the research cruise Bastian Huwer, DTU Aqua.

The Baltic Sea's salinity is much lower than that of ocean water as a result of abundant freshwater runoff from the surrounding land, combined with the shallowness of the sea itself. An important source of salty water are infrequent inflows of North Sea water into the Baltic. Such inflows, important to the Baltic ecosystem because of the oxygen they transport into the Baltic deeps, used to happen frequently until the 1980s. In contrast, during the past decades they have become much less frequent.

Yet in December 2014 a large inflow of oxygen rich salty water flowed in from the North Sea.This makes 2015 a particularly interesting year to study the Baltic eco system interactions and the cod's spawning.

"The inflow may have improved the environmental conditions for growth and reproduction of cod adults, and for the survival of eggs and larvae both in the Bornholm Basin and in areas further to the East. This could potentially contribute to a healthier and more abundant stock, even though the condition and development of the stock also depends on other factors" says Bastian Huwer, DTU Aqua.

Because regular monitoring cruises by Danish, German and Polish research vessels do not traditionally cover the month of September, not much is presently known about the seasonal dynamics of the distribution and interactions between key species in the Baltic at this time of year. In contrast to most other cod stocks which spawn during a relatively short period, the Baltic cod spawns from early spring to late fall, and the present Dana cruise in September is expected to provide new knowledge about the later part of the spawning season.

The cruise will also monitor for the comb jelly Mnemiopsis leidyi, an invasive species, which was introduced to Northern Europe in 2005 and reappeared in the Baltic in the fall of 2014 after having been absent for three years.

Using the contrasting environments of the Arkona, Bornholm, Gdansk and Gotland Basins the cruise will cover a variety of different depth, salinity, temperature and oxygen gradients to describe the distribution and behaviour of key plankton and fish species and their various life stages, and to investigate the associated feeding interactions and the related community patterns.

The major scientific goals of the cruise are to resolve:

· The hydrographic situation after inflow in a broad area from Arkona to Gotland Basin.

· physiological preferences and tolerances of key meso-zooplankton species through controlled experiments on board

· abundance, distribution, nutritional condition and phenology of key zooplankton, including fish eggs and larvae, as well as gelatinous plankton species in different areas of the central Baltic.

· abundance and survival of herring and cod eggs and larvae

· individual condition, abundance and distribution of spawning cod and herring

· predation pressure on copepods and fish early life stages by herring and sprat as well as gelatinous plankton

· distribution (vertical and horizontal) of sprat and herring in relation to hydrography, zooplankton prey and predator (cod) abundance