A presentation by Prof. Åsa Danielsson
Professor Åsa Danielsson is from the Swedish Linköping University, Department of Thematic Studies (TEMA), Water and Environmental Studies (WES). She is the head of WES which has been defined as a centre for multi- and interdisciplinary research and education, where natural and cultural scientists collaborate in search for knowledge across the boundaries of traditional disciplines.
This interesting and highly informative talk presented on the 22nd February 2013, focused on Silica in the Baltic Sea, a nutrient that had previously been overlooked.
The Baltic Sea has experienced significant changes in nutrient loads, due to increased agricultural and industrial production, which has resulted in several deleterious impacts to the Baltic Sea ecosystem. These include decreased water quality, increased algal blooms and a changing fish population. Dissolved silicate (DSi) is an essential nutrient for the growth of diatoms and thus its role in the Baltic Sea is crucial. The impact due to increased nitrogen (N) and phosphorus (P) loads has been studied over the past century; however, what is less known is the impact that the decreased load and concentration in DSi will have.
DSi load is believed to have decreased due climate change, as well as increased retention due to river regulation. Diatoms are part of an intricate species succession, which begins in spring and with an autumn bloom. The Diatoms are believed to grow rapidly during spring, which is when they scavenge available nutrients. During this phase, they are followed by dinoflagellates, which are mobile enough to bring up nutrients. When conditions are favourable such as a large stock of P, dinoflagellates are replaced by Cyanobacteria, which can fix Nitrogen from dissolved N2. Plankton composition in the southern Baltic Sea varies seasonally and annually, and the dominance of either diatoms or dinoflagellates varies from Spring to Spring in the northern Baltic. With a decrease in DSi, more P and N will be left to Dinoflagellates. With an increase in recycled organic material in the upper water column, more phosphorus is retained for the summer period, and this could lead to high Cyanobacteria blooms.
Riverine loads of DSi were previously assumed to be unaffected by anthropogenic impacts. However a comparison of heavily dammed and regulated rivers with undammed ones shows that an artificial water reservoir and increased river regulation leads to decreased silica loads. 1900 represents conditions with small human impacts, while the main changes in nutrient loads began correlate with river regulation and the onset of eutrophication in the 1950s. The trend has seen a decrease of DSI, while N and P remain high. If this trend continues, we may expect a complete depletion of DSi, as well as DSi limitation of diatoms within the next decades in the Baltic Proper. Species composition and amount is expected to change drastically as well.
Therefore, the ratios between the nutrients, Si, N and P are extremely important in the “Baltic Cocktail” as they determine the development of several phytoplankton such as diatoms, dinoflagellates and cyanobacteria. It is challenging to decide whether to reduce N or P, as the Baltic will then switch from a “summer cyanobacteria problem” or a “spring dinoflagellate problem” respectively.
Conclusively, it would probably be best to reduce both N and P levels, and increase DSi.
See the SIBER Homepage: http://siber.ecology.su.se/
Catchment Research Group 