Low water & no water

Summer 2019 field season brought a strange and new problem for marine biologists and boaters alike. Usually the wind is blowing from southwest, but during the summer months of 2019, most of the wind blew from the north. This meant that water was running towards south and the water level was more than half a meter lower than the mean water level for many weeks. The lowest water levels were approximately -65 cm during the low waters of July and August, which means many hundreds of meters of escaping sea at shallow shores.

Low water level in Isonkivenletto-island near Ulkokrunni, photo by Suvi Saarnio, Metsähallitus.

Some of the docks were left on dry land and Metsähallitus marine team could do wading points in places where you usually have to snorkel or use a drop video.

This wading point was supposed to be in water, photo by Teemu Uutela, Metsähallitus.

Water level decline started only after aquatic macrophytes had already started their growing season. This meant, that for many weeks of low water level aquatic plants were stranded on dry land and started dying. Some smaller and more rigid plants like Subularia aquatica and Limosella aquatica struggled for a long time before succumbing to the dry environment but some died earlier, for example Stuckenia pectinata and Stuckenia filiformis. They have no rigid stem or leaves and are only kept upright with the surrounding water.

Mudwort (Limosella aquatica) on dry land, photo by Suvi Saarnio, Metsähallitus.

The most striking change happened in green algae Charales. They turned completely white and were obviously dead. When the water level rose again close to normal level, these ghostly white Chara meadows could be seen from the boat at the bottom of shallow bays.

White Charales in the shallow beaches of Krunnit, photo by Suvi Saarnio, Metsähallitus.
White Charales in the shallow beaches of Krunnit, photo by Suvi Saarnio, Metsähallitus.

It is difficult to know for sure what this low water summer meant for the underwater nature of the Bothnian Bay in general but at least locally this lowered the macrophyte biomass significantly. Some of the species might have had time to migrate deeper but especially the ones which were already forming meadows by the time the water level went down, faced an unpredictable and bleak future.

Water escaping the shallow beach of Ulkokrunni, photo by Jaakko Haapamäki, Metsähallitus.

Written by Essi Keskinen

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