aus dem MARUM und dem Fachbereich Geowissenschaften der ...

MARIA S. MERIAN-Berichte, Cruise 20, Leg 4, Bridgetown – Freeport, 15.3.2012 – 7.4.2012
West- to Southwest-Florida Slope, in the Florida Straits and along the Great Bahama Bank.
Moreover, we wanted to get an overview of the variability of water masses in the ultimate vicinity of
these CWC habitats in space (locally-regionally) and time (tidal cycles). Bottom water samples were
taken at all localities to get an overview of the geochemical characteristics of these water masses. In
addition, sound velocity data were provided for hydroacoustic mapping (see chapter 5.2.3).
5.3.2 Sampling and methods
The CTD profiler used during MSM 20-4 was a Seabird "SBE 9 plus" underwater unit and a
Seabird "SBE 11plus V2" deck unit. Additionally, it was equipped with two dissolved oxygen
sensors, a chlorophyll-a sensor and a Seabird bottle release unit including a rosette water sampler.
For the analysis and interpretation of the measurements, the downcast raw data were processed
with "SBE Data Processing" software. For the visualisation of the data we used "Ocean Data View
(mp-Version 3.3.2)". Measured O2 values were verified by using the Winkler titration method
(Winkler, 1888). We performed single casts, one transect, and three Yoyo-CTDs with repeated casts
over 13 hours covering one complete tidal cycle. A total of 49 CTD profiles were measured during
the cruise. In addition, we received bottom water samples collected during the ROV dives in close
vicinity to living CWC.
5.3.3 Shipboard Analyses
5.3.3.1 Seawater Oxygen Analyses
The measurements of the CTD oxygen sensors were validated on board with water samples by
iodometric WINKLER-Titration after Grasshoff (1983). The measurements were performed on
all 49 CTD casts. Water samples were taken during upcasts only. When a designated sampler
bottle was released, the oxygen sensor readings were noted and later compared to the titration
results. Immediately after collection, the water samples were filled into volume-calibrated
WINKLER-bottles. Two parallel samples were taken, and we paid particular attention of not
having any air in the WINKLER-bottles. The oxygen was fixed with 0.5 cm 3 manganese-IIchloride
and 0.5 cm 3 alkaline iodide. Then the bottles were shaken and stored cool for several
hours. Before titration, the manganese hydroxide was solved with 1 cm 3 H2SO4 (9M) and the
bottles were shaken again. The samples were each transferred into a 250 ml beaker, where they
were titrated with 0.02 M sodium thiosulfate until the solution turned into yellow. After adding
1 cm 3 of zinc iodide solution, the titration was continued until the blue colour of the sample
disappeared. The factor of the thiosulfate solution was determined with a standard, which was
performed after each CTD station. The oxygen content was calculated from the thiosulfate
consumption by using the following standard formula:
O2 = (a * f * 0.112 * 103) / (b-1) [ml/l]
where a is the consumption of thiosulfate solution [ml], b is the volume of the WINKLER bottle
[ml], and f is the factor of the thiosulfate solution. A total number of 42 titrations were performed.
The oxygen contents range from 5.14 to 2.42 ml/l. The accuracy of our titrations is 0.5%, which is
in the range of values reported in the literature (0.06 to 0.89 %; Furuya & Harda, 1995). The two
oxygen sensors of the CTD, however, recorded different oxygen contents at shallow depths (