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News|21 May, 2009

The Invisible Threat to the Baltic Sea: Spring Algae Blooms

Annotation 2019-11-21 152557

WWF today released a report on the distribution of so called “spring blooms” in the Baltic Sea. In 2009, the northern Baltic Proper witnessed the strongest spring bloom of the past 15 years, with production peaking just a few weeks ago. The spring blooms are well known to scientists, but as they take place from March to May when not so many people are out on (or in) the sea, their impact on the environment is not well known or discussed outside of scientific circles.

“The algae produced during an average spring bloom in the Baltic Proper equals 1 000 000 truckloads of algae that would constitute a line of trucks travelling from the North Cape to Gibraltar and back – or 12 000 km! Only the strongest blue-green algal blooms in the summer can come close to producing similar amounts of algae”, explains Dr. Sampsa Vilhunen, Marine Expert with WWF Finland.

When the dead and decomposing algae sink to the sea floor, they start to consume oxygen from the surrounding water layer. As a result of an average spring bloom in the Gulf of Finland and Gulf of Riga, oxygen is theoretically consumed to zero in a layer 2.5 meters above the sea floor, contributing to so called “dead zones”. The corresponding figure for the Baltic Proper is 1.6 m. The dead zones decrease reproductive success of commercial fish stocks such as flatfish and cod. They also add to the vicious cycle of eutrophication by re-releasing phosphorus from the sea floor, a phenomenon called “internal loading”.

“Because oxygen levels are already low in the bottom layers, massive spring blooms can effectively prevent the recovery of bottom ecosystems. The spring bloom therefore contributes to the problem of dead zones and internal nutrient loading in the Baltic Sea”, says Dr. Sampsa Vilhunen.

The intensity of the spring bloom is mainly governed by the amount of nitrogen entering the sea from sources like farming, waste water and the combustion of fossil fuels. Dead zones created by decomposing spring bloom algae can release more phosphorus through internal loading. Phosphorus is the limiting factor for the summer blue-green algae blooms, and these algae have the capacity to enrich the water with nitrogen. The vicious cycle is closed.

“The mechanisms of this ‘vicious cycle’ of spring and summer algal blooms clearly show the need to reduce both phosphorus and nitrogen emissions to the Baltic Sea”, says Pauli Merriman, Director of the WWF Baltic Ecoregion Programme. “While several quick and cost-efficient measures can be taken to improve the quality of waste water, including a ban on phosphates in detergents and improved municipal waste water facilities, we cannot expect to solve the eutrophication problem without a radical change in European agricultural policy. Subsidies that are still used to promote production should instead be used to support environmentally sustainable rural development.”

For more information, please contact:

Dr. Sampsa Vilhunen, Marine Expert, WWF Finland
Tel. +358 40 550 3854 Email.

Pauli Merriman, Director, WWF Baltic Ecoregion Programme
Tel. +46 767 886 185 Email.

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