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Aranda’s monitoring cruise reveals: Last summer’s algal bloom in the Baltic Sea was the result of a surge of nutrient-rich water from the Baltic Proper

Press release 2018-09-28 at 14:43
Aaltopoijua nostetaan Arandan peräkannelle huoltoa varten. Kuvaaja Ilkka Lastumäki
A wave buoy is lifted on board of the Aranda for maintenance. © Ilkka Lastumäki
Research vessel Aranda’s first monitoring cruise after its renovation was a success despite challenging weather conditions and a tight schedule. The results show that salty and oxygen-depleted water reaching as far as the Eastern Gulf of Finland could spell difficult cyanobacterial bloom conditions next year as well.

“The challenging circumstances were a good test for the performance of the vessel, the laboratories and our sampling equipment. We were able to collect samples in the Gulf of Finland, the Northern Baltic Sea and the Southern Bothnian Sea essential for the assessment of the status of the sea”, says Leading Researcher Maiju Lehtiniemi from the Finnish Environment Institute.

Lehtiniemi and Senior Researcher Pekka Kotilainen from the Finnish Environment Institute were the chief scientists of the latest monitoring cruise, which began on 11 September.

“The renovated Aranda proved functional, and the modifications allow us to make even more efficient use of the vessel. Aranda is now fit for unrestricted international voyages, and it can be used in all kinds of weather conditions in the Baltic Sea”, explains the Finnish Environment Institute’s Director-General Lea Kauppi. “Both the results of Aranda’s voyage and other monitoring data clearly show that fast actions are still needed to improve the state of the Baltic Sea.”

Water in the Gulf of Finland highly stratified at the moment

Old, salty, oxygen-depleted and phosphorus-rich water from the deeps of the Baltic Proper, which became displaced as a result of the major Baltic inflows of 2014–2016, has been flowing into the Gulf of Finland since the autumn of 2016. As is typical in persistent high-pressure conditions, surface water from the Gulf of Finland flowed into the Baltic Proper over the summer and was replaced by salty and oxygen-depleted deep water as far as the easternmost parts of the Gulf of Finland.

The whole of the Gulf of Finland is highly stratified at the moment. The halocline and thermocline are located at depths of 20–30 metres. Beneath, there are considerable volumes of salty water with elevated phosphate levels. How much of the phosphorus will end up in the surface layer, depends on wind conditions during the rest of the autumn and ice conditions over the coming winter. This, in turn, is of crucial importance from the perspective of next summer’s cyanobacteria blooms.

Salty water has also been pushed into the Southern Bothnian Sea through the Sea of Åland. However, there has been no decrease in benthic oxygen levels and there are no signs of elevated phosphate levels compared to the situation two years ago.

Conditions in the Baltic Proper crucial for algae levels in the Gulf of Finland

The cyanobacterial blooms of the summer of 2018 were the heaviest this decade. Almost the entire Gulf of Finland was covered in algae at times.

“The situation was not due to phosphorus emissions in the Gulf of Finland’s own catchment area. Water quality in offshore areas and also in the archipelago was affected by the anoxic and phosphate-rich deep water flowing in from the Baltic Proper. Emissions during the current millennium have decreased more in the drainage basin of the Gulf of Finland than anywhere else in the Baltic Sea Region”, explains Senior Research Scientist Seppo Knuuttila.

The Finnish Environment Institute has not surveyed the coastal waters along the Gulf of Finland this year. Instead, water quality and chlorophyll a levels have been measured at fortnightly intervals at the intensive coastal monitoring stations operated by the Southeast Finland and Uusimaa Centres for Economic Development, Transport and the Environment. Information on the marine environment was also collected by means of satellite imagery and merchant vessels’ Algaline surveying. Short offshore monitoring cruises were conducted with the help of the Finnish Border Guard in January and June. The halocline elevation observed by the Aranda cruise in September was first detected in the Gulf of Finland back in June.

Summertime cyanobacterial blooms have been declining especially in the Eastern Gulf of Finland throughout the last decade. However, algae levels in the outer archipelago began to rise once more in the summer of 2017, and the trend continued this summer. The high algae levels in the Eastern Gulf of Finland towards the end of the summer were due to strong easterly winds in late July and early August, which caused phosphate-rich water from the deeps to mix with the surface layer. Chlorophyll a levels equivalent to the early-August cyanobacterial bloom were last measured in the area in the summer of 2005.

The reasons behind the high salinity and nutrient levels in the Gulf of Finland relate to currents determined by climatic factors and conditions in the Baltic Proper. Changes in conditions in the Baltic Proper will also be crucial for the situation in the Gulf of Finland in the future. This highlights the need to cut emissions further in the entire drainage basin of the Baltic Sea as well as the important role that state-of-the-art offshore monitoring in the Baltic Sea sub-basins plays in evaluating the state of the Gulf of Finland and forecasting changes.

Rising temperatures in the Baltic Sea Region will increase the risk of cyanobacterial blooms in the future. Higher water temperatures and stronger stratification may increase the magnitude of anoxic areas in the sea, which in turn will accelerate the release of phosphorus from the seabed. The increasing probability of wintry rainfall will also cause more and more nutrients to drain into the sea.

Plans for more extensive use of Aranda

The funding of marine research and especially the use of the renovated research vessel have been subject to public debate.

According to Director-General Kauppi, ensuring funding for research infrastructure such as Aranda is a challenging task in today’s project-orientated operating environment. “The financing of research infrastructure in the long term is a particular concern in many areas of research. Our goal is to make more extensive use of Aranda both nationally and internationally. We are constantly in negotiations with potential partners.”

Thanks to Aranda’s renovation, the prospects are greater than ever. The vessel satisfies the requirements of modern marine research, its ecological impacts have been substantially reduced, and Aranda now provides an even safer working environment.

More information

  • Director General Lea Kauppi, Finnish Environment Institute SYKE
    tfn 0295 251 700, firstname.lastname@ymparisto.fi
  • Leading research scientist Maiju Lehtiniemi, Finnish Environment Institute SYKE
    tfn 0295 251 356, firstname.lastname@ymparisto.fi
  • Senior research scientist Seppo Knuuttila, Finnish Environment Institute SYKE
    tfn 0295 251 286, firstname.lastname@ymparisto.fi
  • Senior researcher Pekka Kotilainen, Finnish Environment Institute SYKE
    tfn 0295 251 317, firstname.lastname@ymparisto.fi
  • Communication specialist Eija Järvinen, Finnish Environment Institute SYKE
    tfn 295 251 242, firstname.lastname@ymparisto.fi

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