In step with climate change: the deep sea

Led by Dr. Karl-Michael Wern­er of the Thü­nen Insti­tute of Sea Fish­eries and Mar­grete Emblemsvåg of the Møre­forsk­ing AS Insti­tute and the Arc­tic Uni­ver­si­ty of Nor­way, an inter­na­tion­al research team has dis­cov­ered an unusu­al link between bot­tom­fish com­mu­ni­ties in East Green­land and the effects of cli­mate change. Eval­u­at­ing long time series, they observed that ecosys­tems across the entire depth range of 150 to1500 meters of water depth respond­ed simul­ta­ne­ous­ly to changes in the atmos­phere, sea ice cov­er, and sur­face tem­per­a­ture — sur­pris­ing­ly, most clear­ly in deep-sea fish­es liv­ing below 400 meters of water depth.

Over near­ly 20 years, from 1998–2016, researchers from the Thü­nen Insti­tute of Sea Fish­eries in Bre­mer­haven and the Green­land Insti­tute of Nat­ur­al Resources in Nuuk col­lect­ed data on changes in the fish com­mu­ni­ty at depths from 150 to 1500 meters. Dur­ing this peri­od, they sailed research ves­sels to sim­i­lar loca­tions off East Green­land each year to take sam­ples with a sci­en­tif­ic bot­tom trawl. This brought togeth­er infor­ma­tion from near­ly 1400 net catches.

“As we ana­lyzed the fish abun­dance data, we real­ized that the changes in depth regions between 350 and 1000 meters were greater than the changes in shal­low­er regions. When it was also sta­tis­ti­cal­ly con­firmed that these obser­va­tions at depth cor­re­lat­ed with changes in the atmos­phere, the biggest chal­lenge was to devel­op hypothe­ses about how these things were related.”

- Mar­grete Emblemsvåg

Ecological changes at depth

For their data analy­sis, the sci­en­tists used the sta­tis­ti­cal eval­u­a­tion method “ten­sor decom­po­si­tion,” which allowed them to exam­ine spa­tial and tem­po­ral changes in fish com­mu­ni­ties simul­ta­ne­ous­ly. “This method has not been used for eco­log­i­cal ques­tions for long, but it was recent­ly rec­og­nized that it is very well suit­ed for ana­lyz­ing spa­tiotem­po­ral process­es in fish com­mu­ni­ties,” explains Karl-Michael Wern­er of the Thü­nen Insti­tute. “The most strik­ing result was that the changes occurred at much greater depths than expect­ed.” For exam­ple, even at depths of more than 400 meters, the researchers observed an increase in bore­al species, i.e. species more adapt­ed to warmth, such as tusk (Brosme brosme) and blue ling (Mol­va dyptery­gia), while the occur­rence of Arc­tic and sub-Arc­tic species such as Green­land hal­ibut (Rein­hardtius hip­poglos­soides) and blue wolff­ish (Anarhichas den­tic­u­la­tus) decreased.

Slow change and extreme events

Ten­sor decom­po­si­tion results showed that the dis­tri­b­u­tion of fish com­mu­ni­ties at greater depths changed rapid­ly between the years 2005 and 2010. At the same time, both air tem­per­a­ture and sur­face lay­er salin­i­ty increased and sea ice extent decreased. How­ev­er, the results also showed short-term changes in deep-sea fish com­mu­ni­ties in par­al­lel with extreme envi­ron­men­tal events, such as in 2003 when air tem­per­a­ture was excep­tion­al­ly high. Fish com­mu­ni­ties in the deep sea thus appear to be sen­si­tive to both slow changes and extreme events that lead to changes in eco­log­i­cal con­di­tions with­in a very short time.

An elevator to the deep?

Since sta­tis­ti­cal analy­ses clear­ly demon­strate that bot­tom fish dis­tri­b­u­tions are relat­ed to changes in the atmos­phere and sur­face, it stands to rea­son that these phys­i­cal changes cre­ate a bio­log­i­cal cas­cade that extends from the sur­face to the deep sea. Some­thing sim­i­lar was observedhttps://www.industry24h.com/ at the same time in oth­er areas of the North Atlantic. “Changes at the sur­face can affect sink­ing rates of organ­ic mate­r­i­al, includ­ing dying plank­ton, to the deep with­in weeks and months. This sink­ing bio­mass is an impor­tant food base for the seafloor species com­mu­ni­ty on which fish feed,” explains Karl-Michael Werner.

The sec­ond hypoth­e­sis as to why species on the con­ti­nen­tal slope showed greater changes than in the shal­low­er water lay­ers could be due to the dis­tri­b­u­tion of dif­fer­ent water bod­ies. An ear­li­er study by author Mar­grete Emblemsvåg showed that deep­er lay­ers, where water mass­es of Atlantic ori­gin dom­i­nate, warmed slow­ly over the study peri­od, while this was not the case in Arc­tic water bod­ies in the shal­lows. This grad­ual increase in tem­per­a­ture at depth may be the clock for the fun­da­men­tal changes in fish dis­tri­b­u­tions, while the extreme events are more like­ly respon­si­ble for the tem­po­ral swings in fish fau­na via short-term changes in seafloor food avail­abil­i­ty. Clar­i­fi­ca­tion of these ques­tions is the sub­ject of fur­ther investigation.