Night Diving

Photos from UAB in Antarctica group on Flickr, related to this post
By Chuck
Posted on 05/01/07

Boating at Palmer Station is normally restricted to the period of time between one hour after sunrise to one hour before sunset. Earlier in the summer, that did not pose a problem. When we arrived at Palmer on 21 February, the sun rose at 5:48 AM and set at 9:13 PM. Today (1 May), the sun rose at 8:24 AM and will set at 4:02 PM. So the dive we have going out after lunch needs to be back at Palmer by 3:02 PM. By the time Philip and I leave on 30 May, it will be rising at 9:51 AM and setting at 2:36 PM. And when Maggie and Craig sail north on 18 June, the sun will rise at 10:25 AM and set at 2:10 PM.

This shorter day length will soon mean that we can really go out diving only once per day and if we want to make more than one dive, we will have to do them all on the same trip instead of coming back to station in between. That is fine. We have obviously known this was coming and it has been part of our planning. We have a lot of experiments to be working on in the lab, and they are becoming even more of a focus now.

The shorter days also give us an opportunity. One thing which we learned from the work of Yusheng Huang, who is one of Jim’s former PhD students, is that the algal-associated amphipods here prefer to eat some species of macroalgae but tend to live on the algal species that they do not like to eat (such as the two most common species of Desmarestia that are shown in one of the Flickr photos). That may seem odd, but we have some ideas of why this may be.

An important part of this is that there are fish that eat macroalgae and also eat amphipods. They prefer to eat the same algal species as the amphipods do. So an amphipod eating one of these algae could find itself inside the stomach of a fish that likes to eat both.

Moreover, the fish do not like to eat the macroalgae that the amphipods live on because they are chemically defended. So an amphipod benefits from the alga’s defenses in getting a relatively safe place to live.

Still, the amphipods have to eat and clearly they like to eat some of the algae such as Palmaria that Philip has been talking about in some of his journals. All of Yusheng’s collections were done during the daytime. Our hypothesis is that at night when the fish cannot see them, the amphipods move from the unpalatable macroalgae like Desmarestia to the palatable ones like Palmaria. This is known to occur with some other amphipod-algal relationships in other parts of the world.

To test that hypothesis, we need to be able to collect macroalgae and their associated amphipods at night. We use the special bags that Maggie described in her Sponge Pod entry.  But of course, we need to get out diving at night to do that.

To dive at night we needed be able to go boating at night. That took special permission and we had to have lots of discussions with the station managers and others to make sure that it would be safe. But everyone was convinced that the way we have it set up, it would be quite safe.

We were fortunate that we can find all the species we need for this project right off the station. It is a little too far to safely swim at night, but very close for boating.

Because we wanted to give the amphipods several hours to get into any night time behavioral patterns they may have, we have waited until now when it gets dark fairly early to do this. That way the people on station who have to support the dive like the boating coordinator, Ryan Wallace, do not have to work too late into the evening.

Because of the darkness, we want to only do this on fairly calm conditions. So if the weather is good and appears stable during the day, we prepare for diving that night. Maggie and I make a dive early in the afternoon to collect daytime samples of algae with their amphipods to compare with the night samples. Then we put up a small buoy at a spot where there are individuals of the algal species we want to collect that night and I try to memorize where each alga is relative to the buoy anchor. We have a red chemical light stick attached to the buoy and before releasing the buoy I bend the stick to mix the two chemicals in it that make the light when they are combined.

Later, after it has been dark for several hours, we head out. Because we are diving so close to station, instead of putting most of our gear on in the boat, Maggie and I get into everything other than our tanks,  weight belts, and fins in the dive locker. Philip, Craig, and Bill help us with that and then as soon as we get into the boat we put on our weights, tanks, and fins. Then we leave the dock and head to the buoy with its red light stick attached.

Although we usually have only two dive tenders in the boat during daytime dives, we use three for the night dives so that we can have an extra pair of eyes and an extra light to make sure that no leopard seals are in the area.

Once we are comfortable that we won’t have company on the dive, Maggie and I drop into the water and head down the buoy line. Although I have a regular, strong dive light with me, I only use a tiny flashlight that has a red filter on it. This provides a very small amount of light and most underwater animals do not see red very well if at all. So I can see well enough to find the plants, but only because I memorized where they were that afternoon relative to the buoy anchor. And hopefully, my light is not disturbing the amphipods enough to cause them to move.

Maggie has a regular dive light and she stays back away from the buoy line about 10-15 feet, making sure not to shine her light on the plants. When I’ve collected a plant I swim over to her and she lights the collecting bags while I close up the full one and exchange it with her for a new empty one. Then I head back to the buoy anchor and find the next plant.

We have done this twice now. The first time was not as smooth as we’d hoped, but the second one went very well. We plan to do this two or three more times over the next week or two. And with this special night boating and night diving, we expect to be able to know if our hypothesis is correct and the animals are moving to the plants they like to eat at night.

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