Measuring the seagrass beds posed a number of challenges for the two researchers. For one thing, the photographs from the 1930s were all in black and white, and it took some time before the researchers learned how to recognise the seagrass amid the various types of grey. They then compared these images with photographs that Dolch and his colleagues took during research flights. Here the difficulty was telling the difference between the green seagrass and areas covered with green algae. As Tobias Dolch explains: “We visited a number of sites in the Wadden Sea on foot to see first-hand whether it was really seagrass or green algae – this helped us to gradually get a better feel for the photos, and to better interpret the different shades of green.” The onsite work also helped the researchers estimate how thickly the grass grows, since it is also a key indicator of the beds’ health. The rule of thumb is “the thicker, the better”, but that’s hard to judge from an aeroplane.
It remains unclear whether improved water quality alone is responsible for the resurgence of seagrass, though it is certainly a major factor. That gives grounds for optimism with regard to other seagrass beds. In many regions, particularly in newly industrialised countries, thanks to intensive agricultural practices, as well as aquaculture installations, surplus nutrients are landing in the ocean. The situation is very similar to that in Western Europe in the 1980s. According to Dolch, “What we’re seeing now is that conditions improve when there is less pressure on coastal waters and the seagrass has time to catch its breath – but apparently it takes quite a bit of time for the ecosystem to recover.”
Another positive development is that experts around the world are now keeping a closer watch on the seagrass and on seawater quality. Back in the 1970s, things were very different: only a handful of experts noted and documented the loss of seagrass. One of them was Karsten Reise, who directed the Wadden Sea Station on Sylt for many years. “I made an initial flyover of the Wadden Sea in 1978 and noticed that the beds were in extremely poor shape,” the biologist explains. “Back then hardly anybody was interested in seagrass. The focus was on mussel banks or life in the mudflat soil, not the pitiful state of the seagrass beds.”
While the seagrass dwindled, various species of green algae flourished in the 1980s, as Karsten Reise observed. In some cases, they also overgrew the seagrass beds, which Reise took to be an alarming trend. “It became clear that the green algae were profiting from the nutrients and growing rapidly, while the seagrass slowly disappeared.” Supported by Germany’s Federal Environment Agency, in the 1990s Reise began mapping the mudflats – for both the green algae and the seagrass.