Thursday, 11 September 2008

Of trawls and titration

Apologies for the quiet of the last day or so - life aboard the Nansen is more or less falling into a routine and I wasn't feeling particularly inspired to write something! In any case, the survey continues and we do nevertheless continue find interesting things and have novel experiences. So, on to the New and Interesting!




Trawls the other day netted some rather interesting beasts, the most captivating of which was probably a zebra shark, Stegostoma fasciatum. These sluggish sharks spend most of their life lying around on the sea bottom - and a lifestyle like that is good news in terms of surviving a trawl - indeed, after a few moments in a bucket of water, this fish was quite keen to start moving around again - which made measuring it quite tricky! We made the measurements, took a few photos of it and returned it to its watery domain.

Some more pictures of this rather endearing creature:









Now if a zebra shark isn't cute enough for you, we somehow managed to catch a minute postlarval cowfish, Lactoria fornasini in a trawl. Those little black tick marks on the ruler at the bottom are millimeters.









This rather toothy fish is Saurida undosquamis. If you think it looks toothy from the outside, a closely related species, Saurida tumbil, has about twice as many teeth as this on the inside of its mouth too...!





This fish may not look particularly inspiring, but it would fill Arrie with great joy - it is one of two Argyrosomus hololepidotus we caught in a trawl. This species appears to be more or less endemic to southeastern Madagascar, and he specifically wanted DNA samples with a corresponding voucher specimen (one of his primary reasons for coming on the part of this leg he participated in). Unfortunately, someone designated them as eminently suited for the dinner table and they got gutted and bled out. I think we managed to wrestle them away from the galley in the end... Incidentally, this trawl happened over rather dodgy trawling ground, and the bottom trawl got seriously trashed. The entire thing had to be re-made!

We've spent the last day or so doing a CTD line out into the Indian Ocean; we've come back inshore again, and we're now doing an acoustic survey along the coast to the next CTD line. The last station on the line was around here.

Of course, we don't like sitting around not doing anything - and we generally always have something to do on the ship, whether it is catching up on the paperwork, entering data into spreadsheets and/or databases, keeping our office inboxes clear, sorting samples, labelling pictures or processing water or chlorophyll samples. Or in my case, blogging!

One of the regular things one does on an oceanographic expedition is check how accurate the dissolved oxygen sensor on the CTD is. They have a tendency to drift over time as the silver inside them dissolves away and their sensitivity can change with depth (indeed, the extreme pressure of deep CTD casts [particularly over 1,000m] changes the physical structure of the membranes in them to the point that you get different readings on the downcast [going towards the bottom] than on the upcast [coming back up again] - this is termed hysteresis. This pdf explains in more detail). To the left, Roger with the titration equipment.

In any case, we decided it would be valuable to teach everyone on the science team how to do the Winkler titration. Of course, we don't use burettes on a ship; they're actually quite fiddly to use, and, if you try and accurately read a meniscus on a moving ship... well, good luck with that! Instead, there is an electronic "dosimat" which dispenses very accurate volumes of liquid (millilitres to 3 decimal places, i.e. a microlitre at a time, and far more accurate than a burette!); essentially, you push a button and it measures the liquid until you're happy your titration has reached its endpoint. Raymond told us that at his department at UCT, they have a machine that does the entire process for you at the touch of a button. No judging of endpoints! To the right, Jaques Phillipe tries his hand at titration whilst Raymond Roman looks on.

Of course, deciding to show someone a method and actually explaining it can be made considerably more challenging if the two parties are not particularly good at each other's language! Fortunately, John Bemiasa (who speaks very good English - infinitely better than my French - and of course fluent French and Malagasy) was on hand to make sure that the information could flow both ways! To the left, John explains the finer points of titration to Thomas Razafimanambina.

We worked up the 24 oxygen samples that were collected at the end of the CTD line. I will be interested to see what Raymond has to say about the data when he works up the values we got during the titrations - techniques like this take a bit of practice before you get them consistently right.

Raymond and I decided to run a little test earlier; we tested a bottle of water which used the chemicals from the Nansen, and another using the chemicals loaded on the ship specifically for the expedition - they both gave *exactly* the same reading, which is very encouraging. That is rather the point of the Winkler method - consistent results! Both samples were take from the same tap on the ship at more or less the same time. To the right, Thomas practices his titration.

Latest news is that there will be a trawl around 4:30am. I think they're secretly trying to break us!

Photos courtesy Carel Oosthuizen (3), Magne Olsen (1), James Stapley (7).

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