Dive transect on the Swedish west coast

During the summer, the BalticSeaWeed blog did al ot of fieldwork, both at Askö on the east coast and Tjärnö on the west coast.

Among other things, we performed an inventory of algae populations along two transects (laid out measuring tape) outside Tjärnö on the salty west coast.

The scuba diver swims from the beach with a tape measure that has been attached at the waterline down to the depth where no more algae grow. Depending on water clarity, this may vary from a few meters to more than 20 meters depth.

Once the algae end, the diver takes out her slate (the single most important tool for any marine biologist) and begins by noting the depth and how much of the tape measure that’s been rolled out. Subsequently, the diver notes down all the algal species she sees and appreciates how much of them there are, on a 7-point scale (1, 5, 10, 25, 50, 75 and 100%).

When the diver has recorded all of this about the starting point, she swims slowly along the transect (tape measure) and continues to note the depth, length and species when it becomes a visible difference in the species that dominates, in order to produce a map of different “algal belts”.

Each “belt” is also sampled, using frames and bags. The diver uses a fixed size frame, which can be loose or attached to a bag, of a size usually 20×20 or 50×50 cm, depending on how many species and how much algae it is.

The diver puts the frame on the bottom, picks the largest algae by hand and puts them into the bag and then use a scraper to get off all the algae that grows within the frame and whisk them into the bag. It’s harder than it looks to work under water when everything is floating around.

For you to get an idea of how it works, Joakim Hansen, who helped out as dive buddy this summer, shared what he was filming with the BalticSeaWeed blog. Here’s how it looks when you scrape a frame.

Why, then have we done this, except that it’s very nice to go for a dive?

On these two sites, these inventories have been conducted for several years. In ecology, it is very important to have measurements that extend over a long period of time in order to see if there is a genuine change in the environment, or if it is just normal variations between years.

So during the cold, dark months, we will pick up our bags with frozen algae out of the freezer (there were over 30 of them), thaw them, sort them into piles according to species, dry and weigh and record in the protocols, thus getting the number of grams dry weight of each species that grew in each frame. By comparing our data with previous protocols, we can then see if it has become more or less of any species, and if any new species have appeared or if any have disappeared over the years.

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