By Iain Anderson
I was having second thoughts about the night-dive as I climbed into my cold and clammy wetsuit. We were sheltered in calm water on the western side of the Poor Knights Islands and the thought of relaxing over a glass of wine in the warm and dry boat cabin began to take hold in my mind. I had made a commitment to my fellows to join them on the night dive so there was no going back. I jumped into the water and flashed my torch around. The torch illuminated a swarm of insect-like creatures. I was witnessing the night-time surface migration of zooplankton.
After sunset miniature crustaceans such as copepods and other zooplankton rise to the surface from depths that can exceed 100 metres. If you go night diving under these conditions you can be surrounded by swarms of creatures. Turn your dive-light off and wave your arms about and you will see the seawater around you light up from bioluminescent phytoplankton. Larger animals such as squid feed on the plankton.
On this dive I encountered a squid (Sepioteuthis australis) and as I shone my torch on him he pointed his arms upward like a skydiver and began a slow freefall towards the bottom.
I followed him down and took several photographs. Occasionally he would stop, open his tentacles and lung at something in the plankton. As I descended, the water pressure compressed my buoyancy vest causing me to sink faster. I had to kick hard to slow my descent and almost lost sight of him as I stopped to inflate my buoyancy vest. The ease of freefall of the squid suggested that he too was negatively buoyant. I asked Dr Steve OâShea, a marine biologist with the Earth and Oceanic Sciences Research Institute at the Auckland University of Technology if this was so. Dr OâShea confirmed that these squids do not possess a mechanism such as an air cavity to maintain neutral buoyancy so that when they stop swimming they sink. They can maintain their depth by hydroplaning like a submarine, or they can hover by using their fins and by directing their exhaust funnel downward.
At 10 metres depth I gave up my pursuit and returned to the surface. Larger predators were in the water that night; towards the end of our dive one of my colleagues observed a shark, probably a bronze whaler circling the boat. Later that evening we witnessed something resembling âan electrical stormâ as luminescent phytoplankton gave off bursts of light, probably the result of collisions with fish. The next morning the plankton creatures had returned to the depths.
Why does zooplankton follow this daily rhythm? Several reasons have been put forward. Perhaps the zooplankton retreat to deep water to avoid predators that feed visually. The metabolic rate of each of these little creatures would also be much reduced in the colder water, and this would provide a mode for energy conservation. The zooplankton feeds on phytoplankton, such as diatoms. Diatoms are single-celled organisms that secrete hard and intricate silica skeletons. These skeletons are a wonder to behold but you need a microscope to see them. Most are much less than half a millimetre in size. They come in a myriad of shapes: some are long and cylindrical and others are round with long radiating spines. Like plants they contain chlorophyll and use energy from sunlight to convert water and carbon dioxide into food energy (sugars). Thus there might be a nutritional advantage for the zooplankton to leave the phytoplankton alone during the day, allowing food reserves to build up at the surface.
While we eat our dinner in our warm and dry boat cabin the critters below are having a feast. Night is when the sea surface really comes alive and you can experience this but be prepared to leave your comfort zone behind.
Acknowledgements: The author would like to thank Dr Steve OâShea of the Earth and Oceanic Sciences Research Institute, Auckland University of Technology for his helpful advice.
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