Where the water is cleaner, there are more animals-_– one would think.A team of researchers from the Senckenberg am Meer Research Institute and the University of Oldenburg has now shown that this is not always and everywhere the case. The group led by Anja Singer investigated in the East Frisian Wadden Sea how biodiversity and total biomass have changed compared to the 1980s. She noted a clear downward trend: the populations of the then predominant species common watt snail, tree tube worm and silt canker were around more than 80 percent. The total biomass has almost halved, the number of individuals per square meter by 31 percent, according to the study, which has now been published in the journal»Frontiers in Marine Science».
The experts see the reason for the decrease in the reduced food supply. Restrictions on the use of fertilizers and higher requirements for wastewater treatment plants have affected water quality over the past 40 years and at the same time reduces the nutrient content in rivers such as the Elbe, Weser and Rhine.As a result, algal blooms, for example, became rarer-_– at the same time, the food supply for many once widespread common species shrank.
However, this does not mean impoverishment of the ecosystem. «What may be a disadvantage for the waddling snail is a significant gain for other organisms: The better water quality has a positive effect on seagrass beds and oyster reefs, for example,» says study co-author Ingrid Kröncke in a Press release of the Senckenberg Gesellschaft. The total number of species found in the study area has remained almost constant since the 1980s.
Climate change also has an impact on the Wadden Sea habitat, the team summarizes: The surface water, which is warmer on average two degrees Celsius, makes it easier for some invasive species to prevail against native species adapted to colder water.For example, the Pacific rock oyster benefits from climate change, write Singer and colleagues. Sea level rise has also already affected the wattworm Arenicola marina, whose biomass has increased by about 75  compared to 1980; percent. The higher water level changes the pattern of sand accumulation, in the sediments the silt content decreases. Both together improve the living conditions for the characteristic mudflat dweller, explains Singer.
For their study, Singer, Kröncke and the other team members used data from around 500 Measuring stations from 2018 compared with a similarly large, historical data set from the 1980s. The Wadden Sea, which is protected as a UNESCO World Heritage Site, extends over an approximately 11 500 Square kilometer area. With its sand and mudflats, mussel beds, dense seagrass meadows, salt marshes, sandy beaches and dunes, it forms a variety of interconnected habitats, according to the UN organization. The coastal region is home to more than 10 000 Animal and plant species and offer ten to twelve million migratory birds a resting and feeding place every year.
Source — https://www.spektrum.de/news/sauberes-wasser-im-wattenmeer-lebt-heute-weniger/2117661