The ocean wa­ter col­umn is pro­foundly dark...

Pic­tures Con­sid­ered #53

by Christoph

...for the most part. And it's vastly deeper than the di­a­gram Roberto showed in his re­cent post "The Bi­o­log­i­cal Pump". The eu­photic and mesopelagic zone sizes are cor­rect, but do not even ap­proximately cor­re­late with their ver­ti­cal rep­re­sen­ta­tion in the di­a­gram. Why is this a point worth men­tion­ing? No one ex­pects scale ac­cu­racy from a chart or di­a­gram, right? It's worth it, though, be­cause hu­mans pre­fer to think vi­su­ally. We un­der­stand pro­por­tions bet­ter if in a di­a­gram the dis­tances are shown to scale.

Lay­ers of the ocean. Source

Take a very prac­ti­cal ex­am­ple. If a di­a­gram shows the oceans "shal­low" to fit on a page (web or print, it doesn't mat­ter), the pho­to­syn­thetic cyanobac­te­ria at or deep in the sed­i­ment of the ocean floor are not par­tic­u­larly sur­pris­ing. They could have just some­how fallen down and even­tu­ally been buried. With cor­rect scal­ing how­ever, it looks quite dif­fer­ent when cyanos are now re­pro­ducibly found >3,000 m(!) be­low the "eu­photic zone", that is, in ab­solute dark­ness, and thrive there in sur­pris­ingly high num­bers. What the heck are they do­ing there? Def­i­nitely not photosynthe­sizing...

I'd like to share with you a "wow" mo­ment I had re­cently (see Fig­ure 1). You may know di­a­grams of the pelagic zone as you find them in Wikipedia. For ex­am­ple this one with the won­der­ful, yet sur­pris­ing, sky-blue for the "Hadalpela­gic" at a depth of 10 km, or this one, which at least shows the dark­ness 'down there'. En­ter sci­ence il­lus­tra­tor Nico­las Bezio, pre­sent­ing the ocean wa­ter col­umn in all its spendid dark­ness. While you take a deep dive let me point out a few flaws. Sure enough, no tech­ni­cal trick, not even Vantablack, can re­al­is­ti­cally re­pro­duce the dark­ness in the ocean's depth in print or on screen, the max­i­mum is "black" (hex color code #000000). The abyssal wa­ter and its sur­round­ing basaltic rock are equally dark − the lat­ter be­ing just less wet − and one needs a min­i­mum of dif­fer­ence in con­trast and color to vi­su­al­ize a wa­ter-filled abyssal trench. Also, the not‑really‑dramatic dif­fer­ences in den­sity of the abyssal wa­ter (~1 g/cm³) and its sur­round­ing basaltic rock (~3.5 g/cm³) can hardly be vi­su­al­ized. So, here Nico­las Bezio made a rea­son­able com­pro­mise.

More prob­lem­atic, ac­tu­ally de­ceiv­ing, is the ad­di­tion of ma­rine life. The sperm whale, the deep-sea squid in the ba­thy­pelagic zone, and the starfish loi­ter­ing on the abyssal plain are cer­tainly not de­picted nearly to scale (eas­ily off by a fac­tor of 100, men­tioned by Nico­las Bezio in his dis­claimer by­line.) This "artis­tic free­dom" makes the wa­ter col­umn more vivid but be­lit­tles it. On the other hand, the an­i­mals would only be a few pix­els at the right scale. Think of emo­jis the size of pe­ri­ods or com­mas. Lastly, the ~300 m long shore­line and coast, and the boat on the left, are like­wise "off scale" with re­spect to the ver­ti­cal ex­ten­sion of the wa­ter col­umn. So, the more fa­mil­iar vi­sual cues for the viewer, the "cal­i­bra­tion points", are in a way mis­lead­ing. En­joy the im­age de­spite these lim­i­ta­tions. An aside: the same li­mitations of awk­ward scales ap­ply to the fa­mous draw­ing "Geo­graphie der Pflanzen in den Tropen‑Ländern" of Alexan­der von Hum­boldt.

How­ever, Nico­las Bezio has solved one as­pect very well: the mon­strous steep­ness of the walls of the abyssal trench. There are few ex­am­ples above sea level of ~1 km high ver­ti­cal cliffs − El Ca­pi­tan in Yosemite Na­tional Park comes to mind. It is dif­fer­ent un­der wa­ter. Ero­sion and rock slides can't pro­ceed as we're used to, let­ting the rub­ble slip down­wards by grav­ity due to the im­mense den­sity dif­fer­ences of air (~0.0013 g/cm3) and wa­ter (~1 g/cm³). You see such steep abysses eas­ily when peek­ing on a nau­ti­cal map of the coast of El Hi­erro is­land − with a high­est el­e­va­tion of 1350 m at Mal­paso, go­ing to 2400 m be­low sealevel within ~1000 m from the coast­line of Las Playas − or on a sil­hou­ette of the el­e­va­tion above the seabed of the is­land of St. He­lena.

This blog goes back to tweet by @NickBezioArt and the in­ter­est­ing dis­cus­sion in the ac­com­pa­ny­ing thread.