2.3 Bases Teóricas
2.3.5 Gobiernos Locales
2.3.5.1 Municipalidad Distrital de Combapata
G
CTD 11C
CTD 22Fig. 21. DMS vertical profiles fora)CTD 7 (19.5233◦W,35.4450◦N), CTD 11 (20.5283◦W,19.7200◦N) in the Mauritanian upwelling, CTD 22 (40.9783◦W,27.6900◦S); andb)DMS, DMSPp, and DMSPd vertical profiles for CTD 7 (19.5233◦W,35.4450◦N).
consisted of a single cast of the double net (200µm) to 200 m. One net sample was used for gut content analysis, and one for OPC and carbon analysis (the latter was split using a Folsom splitter).
3.15.1 Size-Fractionated Carbon
Half of the 200 m net (200µm) sample was used for carbon analysis. The zooplankton were size fractionated by screening the sample through 2,000, 1,000, 500, and 200µm sieves to create fractions of 200–500, 500–1,000, 1,000–2,000, and greater than 2,000µm. The greater than 2,000µm fraction was preserved. The other size fraction was made up to 500 or 1,000 ml depending on the den- sity of zooplankton, and 50 ml aliquots of each size frac- tion were filtered onto pre-ashed Whatman GF/F filters (in triplicate). Filters were dried for 48 h in a 60◦C oven and then compacted in aluminium foil for subsequent carbon- hydrogen-nitrogen (CHN) analysis at PML. The remainder of the sample was preserved with borax buffered formalde- hyde (4%) for taxonomic identification.
3.15.2 OPC
Half of the 200 m net was passed through the OPC to give an estimate of the size structure. This sample was then collected and preserved in 4% formalin for subsequent taxonomic analysis. The 20 m net zooplankton sample was processed through the OPC in a similar manner and pre- served.
The OPC was used in continuous flow-through mode during most of the cruise using the uncontaminated sea- water supply. This was interrupted only briefly at local
dusk and dawn to change data files, and for about 2 h each day on station to process the net samples. Its use was also prevented around Madeira, because of the ship’s speed. In-line samples were collected from the OPC outflow using 200µm mesh collection. This was preserved for subsequent analysis to validate the OPC data. A log of all zooplank- ton samples taken and samples for analysis are given in Appendix L.
3.15.3 Copepod Ingestion Rates
The gut fluorescence-evacuation method was used to obtain ingestion rates. At each station, one WP2 plankton net (200µm) was deployed to 200 m. The sample was im- mediately screened to obtain three different size fractions (200–500, 500–1000, and more than 1,000µm. Subsam- ples of each fraction were filtered and frozen for further determination of initial gut contents in each fraction. The remainder of one of the size fractions (one different frac- tion each day) was used for gut evacuation experiments. Copepods were kept in a cold box filled with filtered sea- water from the station (7 m) and subsamples were taken every 5 min for half an hour. Extra subsamples, at 45 and 60 min, were taken if copepod abundance was enough. Subsamples were also filtered and frozen for further gut content analysis.
In some stations along the transect, particularly in the Azores area, night stations were used to compare night and day gut contents. A fixed number of herbivorous copepods were taken from the frozen filters to extract gut contents. The copepod number used was 25 for the large fraction, 50 for the medium, and 75 for the small. Between 1–3 repli- cates were taken each time. Chlorophyll was extracted us-
ing 5 ml of acetone (90%) in 20 ml vials for 24 hours at 4◦C. Copepod gut fluorescence was determined using a Turner fluorometer before and after acidification, and expressed as chlorophyllaequivalents (in nanograms). Copepod gut content was plotted against time to obtain gut evacuation curves. Data were fitted to an exponential curve to calcu- late gut evacuation rate (slope of the curve).
3.15.4 Particulates
Samples for carbon–nitrogen analyses were obtained from two different depths: near surface (7 m) and the chlo- rophyll maximum, as determined by the fluorometer on the CTD. Water from the two depths was filtered through a membrane filter of 5µm and a 200µm gauze. The filtrate from each size fraction was filtered in triplicate onto pre- ashed Whatman GF/F filters to produce a series of repli- cate samples of the two size fractions (less than 5µm and less than 200µm). Filters were maintained for 48 h in an oven (60◦C) and then compacted in pre-ashed aluminium foil for CHN analysis (Appendix M).
3.15.5 Preliminary Results
The total biovolume of zooplankton in the top 200 m, and the proportion represented by each size fraction chang- es markedly across the transect (Fig. 22a). In compari- son to previous AMT transects at the same time of year, the AMT-5 pattern is slightly different (Fig. 22b). In the North, streamers of high productivity associated with up- welling, were crossed. The zooplankton in this area was higher than in previous years. The peak in biovolume at the southern end of the transect was smaller for the last two stations.
The underway data, shows marked diel changes. In general, the total biovolume is higher at night, and is char- acterized by an increase in the larger size fractions, e.g., days 274–276 (Fig. 22c).
3.16 Microzooplankton
Samples were taken to assess the latitudinal variabil- ity in composition and biomass of microzooplankton as- semblages, and the potential relationship of these param- eters with phytoplankton composition and environmental variables. The second objective was to carry out experi- ments to determine microzooplankton grazing activity at the DCM within the latitudinal range of the survey.
3.16.1 Methods
Microzooplankton samples were taken from Niskin bot- tles at every main daily CTD station (Appendix N). Four depths were sampled along the first part of the cruise until the beginning of CANIGO region, and six depths there- after. The three sampling depths that were always chosen were: surface water, the DCM, and a sample beneath the
DCM. The other sampling depths varied according to the available range of Niskin bottles between the surface and the DCM.
In order to obtain estimates of microzooplankton bio- mass, 500 ml water samples were fixed in 3% [final con- centration (f.c.)] pre-added acid Lugol solution and stored dark in the cold room for subsequent analysis using the Uthermol sedimentation technique, an inverted microscope and a video-image analysis system.
Between 40–20◦N, two replicates of 50 ml water samples were taken from six depths, as above, to assess the trophic status of the microzooplanktonic community. The samples were fixed in 1% (f.c.) glutaraldehyde, filtered through 0.8µm black polycarbonate membrane filters, mounted on- to slides and stored frozen (−80◦C) for subsequent analysis under an epifluorescence microscope.
3.16.2 Grazing
Water incubations to determine microzooplankton graz- ing were carried out at alternate CTD stations following the standard dilution method (Landry 1993).
Water from the chlorophyllamaximum was taken from the CTD into an acid-washed polycarbonate carboy and combined with 0.2µm filtered water to obtain four levels of natural water concentrations (20, 40, 70, and 100%). Filtered water was obtained by passing a sample through sequential 3.0, 0.6, and 0.2µm Gelman filters. This process presumably removes all phytoplankton including prochloro- phytes (Verity et al. 1996) and was always carried out im- mediately before use, to avoid the growth of bacteria in the filtered water. Filtered water (100 ml) from the 250 m depth Niskin bottle was added to each incubation bottle to avoid nutrient depletion and, then, growth limitation. One of the 100% level bottles was kept as a control, with no added nutrients.
Three replicate 2 l and 3 l polycarbonate bottles were slowly filled for each experimental concentration, thereby, avoiding bubbles. Silicon tubing was used for all the trans- fer procedures. Water was incubated on deck for 24 h, and cooled to ambient levels using underway flow, while a cal- ibrated neutral mesh was used to simulate in situ light intensity.
At the beginning of the incubations, three replicates of 250 ml of natural water were filtered onto GF/F filters to assess the initial chlorophyll concentration. After incuba- tion, chlorophylla concentration was determined for each bottle (two replicates per bottle) using a Turner 10-005 R fluorometer. To control for changes in the abundance of grazers during the experiment, a 500 ml water sample from the 100% level bottle was taken at the end of the incuba- tion period and fixed in 3% (f.c.) pre-added acid Lugol solution. The fluorometer used in the experiments and the one used for chlorophyll analysis were intercalibrated during the cruise.