May 6 2012
Spring is always a fascinating time here in the north. When we were out diving in April the surface layer had become a soup of zooplankton. Not nice for freediving, but very interesting. When I filled the refugium with 85 liters of seawater recently I saw several species of medusae, other types of jellyfish, fish larvae, polychaete worms, several species of calanoid copepods including a few large, possibly Calanus finmarchicus, and many unidentified crustacean nauplii. Later I have been out regularly catching plankton, and the myriad of animals is truly fascinating.
I restarted the refugium in mid April. It had then been running for 3.5 months. I noticed that no new harpacticoid bloom had occured, though I could always observe some of them swimming in there. The only animals that seemed to consistently grow in numbers were the amphipods. But they never got into such numbers that I was convinced that they could be a potential food source. But I must add that the corals and hydroids had their polyps out all the time. I decided to do a restart, mostly to try to recreate a harpactoid bloom, but also to get out the mud that started gathering at the bottom. While emptying I managed to catch an estimated 50%-70% of the fully adult amphipod specimens with a net with 3 mm mesh size. The estimated total number of such adults was 300. Below the trays, mud had gathered and oxygen was completely blocked out from some of it. It had turned black and smelled terribly of hydrogen sulfide. It was a beyond gross to empty that part of the refugium. I rinsed everything with freshwater. On the trays I used a high pressure washer outside. But rubbing with the coarse side of a kitchen sponge was more effective. After good rinsing and airing the smell quickly went away. I am not worried about the hydrogen sulfide poisoning anything in the refugium or DT. The reasoning I use is that if oxygen can't reach the HS, then the HS can't reach the refugium.
The refugium after 3.5 months.
After having sucked out most of the water and mud on the bottom. The stripes are from the hose scraping the bottom. You can see amphipods, mysids and hydroids. Lots of hydroids growing on the bottom, but to a large degree smothered by bacterial mud.
Black hydrogen sulphide containing mud from under the trays. Horrible smell!
Trays ready for cleanining outside. I am not sure if it would be easier to just drop these trays completely.
Adult amphipods collected from the refugium.
The number and biomass of blue mussels (Mytilus edulis) is now reaching such levels that I need to start thinking about how to get rid of them. At some point they will be too dominating in the tank. The coolest way to do that would be to introduce some predators. I have a number of very tempting candidates. At my favourite collection location there are lots of truly charming young specimens of common whelk (Buccinum undatum). The common sea star (Asteria rubens), is also, naturally, common around here. There are other sea stars and whelks too, but these are the best candidates. There are of course some issues with these animals. The first, and actually easiest, is if they will take other prey than blue mussels. If they do, then they can't be kept. But the biggest worry is simply that they will disturb many of my delicate organisms while moving around like bulldozers. I have many delicate hydroids, sea squirts, worms, and anemones that I don't want disturbed more than necessary. But I guess I'll have to make a trade off at some point.
Just some pictures from a rock pool
I am currently pursuing two strategies for growing zooplankton, none of which I have too much faith in, but I want to test them. On one hand I am waiting for the harpacticoid copepods to bloom, on the other hand I am steadily introducing zooplankton from the blooming ocean to see if there is something that can survive and reproduce in the refugium. I am also working on sketches for further experiemnts with zooplankton. One is to keep them in a separate refugium between the display tank and the phytoplankton refugium. This refugium will have a higher throughput from the main DT in addition to the supply from the other refugium. This will allow for much colder and cleaner water. But it requires wash out straining, for example by a net that will allow small individuals to escape, but not large ones.