The Ron Brown arrived at Station #5 around 0830 on 25 April with the decks wet from some early morning rain showers. Unlike the previous few days, the skies were cloud-covered. Still the winds were light (5 to 8 kts from the east southeast) and the air warm 25.9 C. The sea surface temperature was the warmest experienced so far on this cruise (27.147 C).
Almost immediately there was a net in the water. Russ Hopcroft deployed his Reeve net for a tow to collect larvaceans. In this tow were several larger species of larvaceans that he was expecting to encounter, but had not done so until now. The 1-m MOCNESS went in about 0915.
In the area of station 5, there are physical structures in the water column between 200 and 500 m known as “staircases”. When a plot of temperature or salinity versus depth is made, distinct steps in the profile are visible wherein there are zones of ten meters or more of isothermal and isohaline water and then narrow transition zones where both temperature and salinity change abruptly until there is another step. The zones of constant temperature and salinity are active mixing zones as demonstrated by work that Ray Schmitt and colleagues had done in this area some years ago. The staircase structure might also be an area of unique biology, but this has not been previously studied. Knowing that there was the possibility that staircase structure might be present, we looked carefully at the data from the first tow as it was in progress. The tow went smoothly and during the downcast, a plot of the temperature and salinity structure showed that indeed, there was a staircase structure in the same depth zone that Ray had observed in 1985.
At the end of the tow, while the net was approaching 25 m depth, the winch overheated and shut down. It took a relatively short time for the engineers to investigate the problem and to start the winch operating again. The last net then sampled the upper 25 meters and the net came on board about 1238.
The divers went in around 1300 with winds in the 12 to 15 kts range. They came back with a real beauty of a collection of jelly fish, siphonophores, and salps. This station was very different from the previous two in the rich surface life they encountered.
In the late afternoon, ring net tows primarily for larvaceans and foraminifera, and water collection for microzooplankton (tintinnids) were carried out. At dusk, the second 1-m MOCNESS was started. This tow was very successful. All the nets were opened and closed where intended and the catches were quite good. There were no problems with the winch this time. Once the 1-m net was secured, the cable termination and underwater unit from the 1-m system were moved to the MOC-10, the buckets installed, and the nets arranged for deployment. This took about an hour. By 2200 the MOC-10 was heading down to depth.
Some twelve hours later at 1000 on 26 April, the trawl re-appeared at the surface. It had caught a wonderful assortment of animals, especially fish. Tracey Sutton was especially pleased. And Martin Angel was busy trying to find more species of ostracods than he did on a cruise to the eastern tropical Atlantic some years ago. At the start of the station, he had about a quarter of all known ostracods identified on this cruise. Net one, the first to be fished as the system returned from 5000 m to the surface, was ripped up a bit on the starboard side and a support rope was ripped off the tape to which it had been attached. It is a mystery how this happened.
Shortly after, the 1/4- m MOCNESS was returned to service by using an old 16-bit electronics unit that was brought on the cruise in case other units failed to operate properly. This tow to 500 m went OK, except that the flowmeter stopped working during the net system's return to the surface. Water filtered for some nets will thus have to be calculated by time the net was open and distance traveled These samples caught with very fine mesh nets (64 um) were primarily used by those investigators interested in microzooplankton.
The second 10-m MOCNESS tow of station #5 started in the early afternoon under fair skies with winds a steady 12 to 15 kts from the east northeast (82 deg) and tropical air (26.22 C) and water (27.19 C) temperatures. Around 1900, the MOC-10 reached within 200 m of the bottom (~5200 m) with more than 7000 meters of wire out. The rest of the evening was spent flying the MOC-10 up from 5000 m. The tow took longer than expected - the net came up too fast to haul wire in at a fast rate, so a good portion of the tow was spent coming in at 10 or 15 m/min. The first net fished from 5000 to 4000 m. On this last tow, it was decided to use the last net to fish the upper 1000 m and use the other three to cover the 5000 to 1000 m range. The group was really interested to see what large-ish animals might be captured in the near surface zone with the big fine meshed nets. At midnight on the 26th, when the science watch changed, Larry Madin came into to take over the "flying" of the MOC-10. There was some discussion about whether the nets had opened and closed where intended because no significant angle change had been observed when the net system was sent the command to close one net and open the next. When the net system finally came on board about 0330 on the 27th, the concern disappeared. All evidence suggested that the nets opened and closed where intended and the catches were pretty spectacular. Contamination was minimal. Francesc Pages was excited because previously he had been catching a particular transparent jellyfish about 5 cm in diameter in the 1-m MOCNESS collections that had very few distinguishing characteristics and was an undescribed species as far as he could tell. In the 1000 to 0 net, a much larger individual was caught and it had characteristics that he could now use to make a description. Also Dhugal Lindsay was happy with the squid collection. Several species that had not been caught earlier were in the sample including a Vampyroteuthis. Martin Angel also found specimens of a species that he had been looking for. In addition, Russ Hopcroft gave Martin another ostracod not yet seen on the cruise from a Reeve net collection made soon after the trawl was on board (from 0415 to 0445). So the last station was ending with a flourish.
The quarter-meter tow began around 0600, under cloudy conditions and winds 13 to 18 kts from the east. A few rain squalls moved through the area. This tow was used to sample a special series of depths for Colomban de Vargas and Yurika Ujiie to look at how the vertical salinity and temperature structure in the upper 300 m might be affecting the Foraminifera. This targeted depth sampling was based on the information from the down trace and from earlier tows. The quarter-meter tow came back on board about 0900 with samples that were a bit disappointing to Colomban because the catches were fairly sparse and there were few Foraminifera.
The last tow of the cruise was one made with the 1-m system. It was targeted at the staircase structure mentioned above, which showed prominently on every tow at this station that went below 300 m (Figure 1). Nets were opened and closed so that one fished in an isothermal/isohaline (mixed) zone and then the next in the interface between it and the next mixed zone just above. We successfully fished 4 mixed zones and 4 transition regions starting about 550 m below the surface and ending about 400 m.
Over the last three days, Team DNA received more identified specimens for sequencing and also a few more unidentified forms that might be undescribed. The total number of identified specimens in the bank topped 1000. More than 400 sequences had been run and they anticipate there will be more than 100 good species sequences by the end of the cruise. Many of the other sequences may turn out to be usable or may need more work to make them good. The nature of the sequence would determine what additional steps might need to be taken, like running an additional PCR under different conditions to optimize the sequence reaction. There is more work to sequencing than simply extracting the DNA, amplifying it, and then running it through the sequencer to get the sequence. Having to repeat steps with different conditions is typical.
With the last over-the-side sampling completed around 1500 on the 27th of April, the R/V Ron Brown set sail for San Juan, Puerto Rico where the cruise will end. During the two day trip, the activity of the science party will focus on packing up all the gear and getting it ready for off-loading and shipping on the day the ship reaches port. In addition, the investigators will be writing sections of the cruise report so that it will be largely completed by the end of the cruise.
Figure1. The temperature profile from the last 1-m MOCNESS tow of the CMarZ cruise showing the staircase structure.