Time Lapse of the Sea
Changes in the North Sea and Baltic Sea have - for example due to toxic algal blooms - an influence also on humans. Photo: HZG/ Sabine Billerbeck / HZG
The North and Baltic Seas are habitats that are always changing over the course of time—such changes are also occurring even today. Currents, temperatures and winds change and with them the living conditions for sea animals and plants. To understand how intense this variability is and how it is triggered, researchers from the Institute of Coastal Research at the Helmholtz-Zentrum Geesthacht (HZG) have run a sixty-year computer simulation for the first time for the North and Baltic Seas. The results are, in part, astonishing and not least vital in understanding the consequences of climate change.
Habitats change. This is an entirely natural process. Even without human endeavours, the climate, for example, can change. There are ice and warm ages as well as long or short-term climatic fluctuations. Over the north-eastern Atlantic, for example, every ten years, large-scale changes occur in air pressure conditions that also characterise weather in Europe. These changes crucially influence the North Sea and the Baltic Sea habitat. If changes in wind or ocean currents occur, salinity and water temperatures can also change. On this, in turn, depends how much phytoplankton thrives in the water or how well the fish eggs and larvae develop.
Sixty years worth of data sets
Understanding how and why the marine habitat changes requires more than heading out on a ship and taking water samples or collecting data from individual weather stations. Such a database is too incomplete to be able provide an overview of the changes across the entire North and Baltic Seas. Using complex mathematical models, scientists from the Institute of Coastal Research at the HZG have for the first time carried out a computer simulation over a long period of sixty years – from 1948 to the year 2008 – of changes in the North and Baltic Seas. The notable aspect here is that the researchers have linked different simulation models with one another for their analysis: first, a mathematical model that simulates the physical environmental parameters, water temperatures, ocean currents or salinity in the sea; second, an ecosystem model that mathematically describes the biological and chemical processes.
Simulated average (sixty years) phytoplankton production in the North Sea and Baltic Sea from the coupled ecosystem model ECOSMO. Lower production occurs in the seasonally layered central marine areas. High production occurs in the partially mixed, coastal areas influenced by riverine input.
These include the nutrient cycles that promote growth of phytoplankton as well as the zooplankton, which feed on the plants, and in the end, the fish that eat the zooplankton or each other. The model calculates the actual physical values—that is, the changes in water temperature or the movement of water masses— every twenty minutes for the entire sixty-year period as well as the biological reaction of the organisms to these changes. A tremendous quantity of data emerges. It takes approximately thirty-six hours to simulate the sixty-year time lapse.
Examining physics and biology together
Such a coupling of biology and physics in one simulation and a joint analysis for the North and Baltic Seas over so long a period are as yet unique in this combination. “The North and Baltic Seas are often examined separately in simulations,” says Ute Daewel, the oceanographer who carried out the simulations together with her fellow oceanographer Corinna Schrum at the HZG. “But the water mass exchange between both seas is extremely important.” The situation off of southern Norway, for example, can be simulated correctly only if both the North and Baltic Seas are examined together.” Every few years, if the weather conditions are favourable, a strong influx of saltwater from the North Sea to the Baltic Sea occurs. This is how vital oxygen reaches the deep water layers of the Baltic Sea. Water from the Baltic Sea, on the other hand, flows on the sea surface into the North Sea, influencing the habitat there.
The art of reading mass data
The simulations initially provide the researchers with an enormous quantity of data, whereby the twenty-minute values are averaged for every day of the past. For all days combined, there are twenty thousand data sets, which in turn contain numerous values for physical and biological parameters. “Such a volume of data is initially of little use; the art is in summarizing the values,” says Daewel. The scientists must examine the data sets more closely, for example, for certain marine regions or time periods. But which areas or periods are interesting? Which of these regions allow statements about changes? To find out, Ute Daewel subjected the results of her simulation to a statistical method known as an empirical orthogonal function (EOF) analysis. This analysis recognizes certain remarkable changes in the data known as variability.
The EOF analysis of the surface currents in the Baltic Sea revealed that the most striking change occurred after 1988, when the current component from north-westerly directions had strongly increased. This leads to the fact that the water at the sea surface is increasingly being pushed from the Swedish mainland to the Baltic Sea. Thus a kind of upwelling process is initiated. This causes water to rise from the depths on the coast and enter the upper water layers. This deep water contains nutrients such as phosphates, which stimulate the growth of phytoplankton near the water surface.
Harmful algal blooms explained
Cyanobacteria, which were earlier known as blue-green algae, profit, amongst other organisms, from this nutrient supply. They can form considerable algae blooms during the upwelling phases. These blooms can be problematic if cyanobacteria that produce poisonous substances proliferate. Bathing will be forbidden in those blooms, and fish or mussels can enrich the toxins or even die from them. In general, Daewel says, the growth of phytoplankton in the Baltic Sea has increased – as has the fish biomass. On the one hand, this was due to the fact that many nutrients came from land—for example, from sewage or fields—to the Baltic Sea.
Algal bloom in the North Sea. Photo: HZG/ Sabine Billerbeck
As the current simulation shows, the wind regime change over the Baltic Sea plays an increasingly decisive role over the years, which drives the upwelling process on the Swedish coast. This finding is new and surprising.
Complex situation in the North Sea
For the North Sea, which is spatially more complex than the Baltic Sea due to strong tides and the influence of ocean currents, the results of the simulations are even more complex. There are thus clear differences between the northern and southern North Sea. The northern North Sea is more dominated by the water influx from the Atlantic. The water masses here in summer are clearly layered in the warm surface water and cold deep water. In the southern North Sea, the water is much more heavily mixed – particularly by the interaction of tides with the coast and the seafloor. Furthermore, the influence of the English Channel is stronger here. The EOF analysis shows several striking types of variability for the North Sea. A water temperature increase in the entire North Sea, whereby the north and south differ, is interesting for example. The entire water column became warmer in the more strongly mixed southern North Sea. In the northern North Sea, the considerable heating is rather limited to the upper water layers.
The change in the ocean currents is also striking. These have increased in the past three decades as a result of the variable wind regime. An exception is in the deep water of the northern North Sea, where the current velocities of the 1970s to the 1990s were especially low. The consequences of the phenomena uncovered in the simulation for the organisms in the North Sea are still unclear. Ute Daewel, however, suspects that, as a result of the higher current velocities in the North Sea, there is an increased influx of nutrients from the North Atlantic, which leads to an increase in the phytoplankton in the central North Sea. Nevertheless, it is clear from the simulations that, above all, the change in wind conditions over the region is responsible for longer-term fluctuations in the phytoplankton of the North Sea. Here, too, closer detailed investigations must more precisely clarify what this means for the habitat.
Evaluating the influence of climate change
Ute Daewel’s simulations over long periods are important in understanding the variability of the North and Baltic Seas. This is also vital in recognising variability caused by climate change. The simulations have since been extended by a few years and now cover the period until 2015. “We want to understand which changes are due to natural variability and which ones are not,” says the researcher. Science has come a good deal closer to this aim with the sixty-year simulation.
Text: Tim Schröder / science journalist