Less and less microalgae in the Wadden Sea

Researchers at the Uni­ver­si­ty of Old­en­burg are ana­lyz­ing data from the Low­er Sax­ony State Office for Water Man­age­ment, Coastal Pro­tec­tion and Nature Con­ser­va­tion. They found that high­er tem­per­a­tures could be the cause of small­er cells.

The cell vol­ume of microal­gae in the Low­er Sax­ony Wad­den Sea has decreased by 30 per­cent between 2006 and 2019. Diatoms in par­tic­u­lar, known as diatoms, were affect­ed by this shrink­age. This was report­ed by a team led by bio­di­ver­si­ty expert Prof. Dr. Hel­mut Hille­brand from the Insti­tute of Chem­istry and Biol­o­gy of the Sea at the Uni­ver­si­ty of Old­en­burg in the jour­nal Lim­nol­o­gy and Oceanog­ra­phy. Accord­ing to the team, the long-term trend could be due to ris­ing tem­per­a­tures and a decreas­ing lev­el of the nutri­ent phos­pho­rus in the North Sea. The decreas­ing cell size of uni­cel­lu­lar algae could be an indi­ca­tor of changes in the biot­ic com­mu­ni­ty as a whole, the researchers said.

Phy­to­plank­ton — the extreme­ly diverse group of uni­cel­lu­lar algae that pop­u­late the light-filled upper­most lay­ers of oceans, lakes and rivers — fix­es about as much car­bon diox­ide per year as land plants and there­fore releas­es half of all oxy­gen pro­duced by pho­to­syn­the­sis on Earth. It there­fore plays a fun­da­men­tal role in the ecosys­tems of all bod­ies of water.

“The diver­si­ty of these organ­isms is remark­able, reflect­ed for exam­ple in the extreme dif­fer­ence in size between the small­est and largest algal cells.”

- Prof. Dr. Hel­mut Hille­brand, direc­tor of the Helmholtz Institute

On the one hand, the size of the plank­ton cells is char­ac­ter­is­tic for each species. On the oth­er hand, cell vol­ume is also relat­ed to envi­ron­men­tal con­di­tions. “So far, how­ev­er, we have only a few data sets on how the cell sizes of indi­vid­ual species, but also of entire com­mu­ni­ties, change over longer peri­ods of time,” says co-author Dr. Lena Rönn from the Low­er Sax­ony State Agency for Water Man­age­ment, Coastal and Nature Con­ser­va­tion (NLWKN).

To find out whether the size of the pro­to­zoa changes at all, how dif­fer­ent species devel­op and whether there is a con­nec­tion with chang­ing envi­ron­men­tal con­di­tions, Hille­brand and his col­leagues eval­u­at­ed data from more than 1,700 sam­ples. The NLWKN had col­lect­ed and count­ed these sam­ples at 20 dif­fer­ent sites in the Low­er Sax­ony Wad­den Sea between 2006 and 2019. The team did not sim­ply look at the aver­age size of all cells with­in a sam­ple — as most pre­vi­ous stud­ies have done — but ana­lyzed the devel­op­ment of each species indi­vid­u­al­ly. This allowed Hille­brand’s team to inves­ti­gate whether a tem­po­ral trend can be attrib­uted more to a dis­place­ment of cer­tain species or to an adap­ta­tion of the indi­vid­ual species to chang­ing envi­ron­men­tal conditions.

The analy­sis showed that over the observed peri­od of 14 years, the aver­age cell size decreased by about one third. Both the indi­vid­u­als of the indi­vid­ual species and the cell vol­ume with­in the com­mu­ni­ty as a whole shrank. “We were able to detect the reduc­tion in cell size in 63 of 73 species stud­ied, and uni­cel­lu­lar algae with a shell of sil­i­ca, known as diatoms, were par­tic­u­lar­ly affect­ed,” says Rönn. The researchers also observed sea­son­al fluc­tu­a­tions: In sum­mer, the aver­age vol­ume of the cells was sig­nif­i­cant­ly small­er than in winter.

Accord­ing to the analy­ses of the researchers around Hille­brand, the reduc­tion in size can prob­a­bly be explained by increased tem­per­a­tures and low­er nutri­ent avail­abil­i­ty. As the team writes, many organ­isms form small­er indi­vid­u­als at high­er tem­per­a­tures than under cool con­di­tions. “If you com­pare trop­i­cal, tem­per­ate and polar habi­tats, you can find many exam­ples of this,” Hille­brand says. This cor­re­la­tion was also known for phy­to­plank­ton. In trop­i­cal seas, the trend is exac­er­bat­ed by low­er nutri­ent levels.

The researchers expect that the reduc­tion in algae will also affect oth­er organ­isms in the Wad­den Sea habi­tat, espe­cial­ly those that feed on them, includ­ing ani­mal plank­ton and mus­sels. The study also demon­strates the impor­tance of main­tain­ing and sus­tain­ing time series for eco­log­i­cal research, Hille­brand said. It was only because the team re-deter­mined the cell size of the algal species in each sam­ple that the researchers were able to deter­mine their shrink­age in the first place. The result also makes it pos­si­ble to cal­cu­late more pre­cise­ly the car­bon con­tained in the plank­ton, an impor­tant input vari­able for mod­els in envi­ron­men­tal and cli­mate impact research.