Dr Tobias Dolch
Schleswig-Holstein Wadden Sea
The seagrass is back
For several decades, Schleswig-Holstein’s Wadden Sea produced very little seagrass, a plant that is especially important for juvenile fish and migratory birds. It was especially hard hit by the surplus nutrient levels in the North Sea. But AWI researchers’ latest studies indicate that the seagrass beds have recently begun expanding. That’s good news, and could have a far-reaching message – after all, it shows that, with the right environmental protection measures, marine ecosystems can successfully bounce back.
Seagrass, really? Yes, you read that right: grasses also grow in the sea. Of course they’re not the same plants as those growing on land, but they are quite similar: they form roots, anchoring them firmly in the ground; and, just like a lawn, they can form dense carpets that cover large areas. Seagrasses grow in many shallow and sandy coastal waters around the globe – in the South China Sea, the USA, and the Wadden Sea, part of the North Sea. Though seagrass beds aren’t as well known as other coastal ecosystems like coral reefs or mangrove forests, they are extremely important. They are home to countless snails, which graze on the algae covering the grass; juvenile fish use seagrass to hide from predators; and they provide a key food source for migratory birds like the brant. Given their importance, many researchers are now concerned, as the seagrass beds on many coasts are shrinking – and with them, a unique ecosystem.
Good news from the Wadden Sea
Thankfully, the latest news from the North Frisian Wadden Sea is quite different: though the seagrass beds between Sylt and the Eiderstedt peninsula shrank to only a fourth of their former size between the 1970s and 1990s, they’ve recently made a comeback and look larger and fuller than they have in years. “That really is good news for once, and it shows that habitats can recover if you give them a fighting chance,” says geographer Tobias Dolch from the AWI’s Wadden Sea Station in List on Sylt. Together with AWI biologist Christian Buschbaum, he’s spent the last several years comprehensively investigating the seagrass beds in the Wadden Sea – and compared their observations with aerial photography from the 1930s and 1950s.
Their findings are cause for relief: today the seagrass is doing just as well as it did in the first half of the twentieth century. Dolch and Buschbaum believe this is mainly due to the improved water quality in the German Bight – especially when it comes to the plant nutrients phosphate and nitrogen. In the water, they promote the growth of algae, which is bad for seagrass: small algae thrive under higher nutrient concentrations, overgrowing the blades of seagrass. As a result, the seagrass doesn’t get enough light, and wastes away.
Too many nutrients
Particularly in the 1970s and 1980s, huge quantities of phosphorus and nitrogen found their way to the Elbe, Weser and Rhine rivers, and eventually to the North Sea. The nitrogen mostly came from farming, in the form of manure and artificial fertilizers. Some of the phosphate also came from these fertilizers, but also from detergents, where it was used as a water softener. Following a ban on phosphate in detergents and improvements in water-treatment facilities, the phosphate levels in wastewater have been significantly reduced since the late 1980s. Further, thanks to new regulations for agriculture and better fertilizers, the nitrogen levels have also declined. As Dolch explains, “It took more than a decade before the effects of these measures could be seen in the environment. But our estimates clearly show that the seagrass beds have started bouncing back since the end of the 1990s.” Whereas the seagrass coverage in the North Frisian Wadden Sea was only 30 to 40 square kilometres from the 1970s to the 1990s – roughly the size of the island Pellworm – it had climbed back to 142 square kilometres by 2010.
The researchers believe the current seagrass beds in Northern Frisia may even be slightly larger than in the past. Compared with the aerial photography from the 1930s and 1950s, the seagrass has now reclaimed the same areas and, in some cases, expanded beyond them. Amazingly, today’s beds have once again grown in the same shape. “We assume this is due to the currents at work in the Wadden Sea,” says Buschbaum. “Seagrass tends to grow in areas protected by islands and large sandbars, and its position hasn’t changed substantially over the decades.” In contrast, seagrass grows only poorly in open areas subject to heavy waves and greater sediment movement.
Why are seagrass beds important?
Seagrass beds perform essential functions in the Wadden Sea. For example, their interlacing roots and rhizomes hold together the sandy soil and offer a safe place for fish like herrings, needlefish and the sand smelt to lay their eggs. Even at low tide, their dense carpet of leaves retains a few-centimetre-thick layer of water, ensuring the eggs don’t dry out and can develop without being found by predators. The seagrass beds are also home to various small crustaceans and opossum shrimps, which in turn attract many small and juvenile fish like the small goby – a species much more commonly found in the protection of the seagrass beds than the grass-free mudflats.
Painstaking image analysis
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.
Measuring water quality
The Water Framework Directive, which was first proposed in the mid-1990s and finally ratified in 2000, gave this research new impetus. It was the first legislation to set objectives for sustainable water use and water protection – which also made it necessary to define indicators for assessing water quality. The duo green algae / seagrass is a perfect fit, since the green algae point to a surplus of nutrients, while healthy seagrass indicates low nutrient levels. Over the years, the AWI’s Wadden Sea Station gathered extensive data on seagrass, which Tobias Dolch’s diligent efforts have now supplemented. “It wasn’t all that easy to find historical photographs of the Wadden Sea in suitable quality – we had to sift through the stacks in a range of libraries and archives,” says Dolch. “And I’m sure that many more are still out there somewhere.”
For Tobias Dolch, Christian Buschbaum and Karsten Reise, the story of the seagrass in Schleswig-Holstein is an encouraging one. “When it comes to the Wadden Sea and the North Sea, for many years the focus has mostly been on their problems,” says Karsten Reise. But it also makes perfectly good sense to view the region as a valuable environment, and to present it as one. “At some point I came up with idea of comparing the seagrass beds on all European coasts – and found that we have the largest ones, which is really something special.” Nevertheless he recognises that there is still much to do – though the amount of nutrients that make their way to the North Sea has declined, things are still far from perfect. As Reise relates, “The nitrogen levels are still four times higher than before industrialisation. These numbers could sink even further if we succeeded in reducing inputs from farming.” The seagrass shows what the results can be achieved – and will hopefully serve as an example for others to follow.