Back to the Sea: the first oceanic post: photic zone


I just reread my first ‘what this blog is to be about’ post, and noticed that I haven’t really posted anything about the ocean yet!!

So I have decided to launch, along with the word of the day and video of the week programming (oh and look out for a plant/vegetable of the week too!!), an oceanic concept of the week, so that we can all get to know the oceans better together!! (and you can point out all the stuff I get wrong along the way!!).

I will post a link to wikipedia, but otherwise this will all be off the top of my head, to avoid the usual temptation to just paraphase the wiki article!

Let’s kick this off with the Photic Zone!!

The photic zone of a body of water is the surface portion that recieves enough light to support photosynthesis.

As light enters the ocean, or a lake or whatever, a couple of things happen. The light can be reflected, which sends a LOT of the light straight back to the atmosphere. Or if it actually makes it into the water column, it can then be bounced around by suspended particles, absorbed by suspended particles, reemitted by suspended particles. Think of the light as a sad child being buffeted around the jumping castle by the slings and arrows of fate. Or it can be attenuated by the water itself. In the simplest system, where there are no organisms or suspended particles in the water, the water itself reflects back blue light (hence water generally looking bluish when all alone) and absorbs red light (hence when you bleed when on SCUBA at any depth, your blood looks kinda purple-grey). So you can see how, even in wonderfully clear waters like pelagic waters in the middle of the ocean (in the absence of an algal bloom etc), light has to run a gauntlet of factors to make it to any depth at all.

In nice clear waters, the light only ever really gets a couple hundred meters down. Add in some algae, or any other realistic particles, and in reality, the light only gets to say 150 meters depth in enough quantity to support photosynthesis. So all your algae can only take care of business in the top 150 meters or so of the ocean. In lakes, with their suspended sediments due to shallow edges and weird swamp monsters stirring up the bottom, this depth can be WAY less.  A lot of the time the photic zone in lakes can be half a meter or less.

Things get weird when you start trying to get at the enough light ‘to support photosynthesis’ thing. I think that traditionally photosynthesis was thought not to be ‘supported’ when light levels drop below 1% of what made it through the surface (1%SI). But my buddy Al, who knows everything about algae tells me that algae do just fine at about 0.06%SI, so where does the photic zone really end?

But anyhow, the photic zone is important because it is the place where solar energy enters the oceanic food webs. The little algae sit there taking the solar energy, and using it to fix carbon into sugars, and hence proteins and everything else you ever wanted to eat. And stuff eats them. Big things, little things, and then things eat them, and we all get confused and scared at everything eating each other- where does it all end?? Energy sources in the oceans are much more diverse and cool than we thought they were even when I went to college (more in later posts!), but the solar imputs remain super important in driving life in the oceans!! And a lot of it happens in the photic zone!

And if things don’t eat them (unlikely, you’d think, but it happens all the time), the dead and dying algae sink down to the crushing depths of the ocean as ‘marine snow’ where all the wonderful carbon and nitrogen they have fixed feed deep sea life. Isn’t it wonderful!!

The world out there is teeming with life. There is life everywhere! It makes me realise that there are no ‘deserts’ on earth, we just think there are!!


~ by maoctopus on November 5, 2010.

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