By Furu Mienis, Research Scientist, NIOZ - Royal Netherlands Institute for Sea Research
Cold-water coral reefs mainly occur in the deep, dark, cold, and food-limited parts of the ocean. How these ecosystems thrive in these hostile environments is still not well established. Short- and long-term monitoring of near-bottom environmental conditions is crucial to understand the preferred environmental setting needed to sustain cold-water coral reef growth.
For this reason, we not only collect water column data around and above the reefs, but we also measure the near-bottom environmental conditions through time with bottom landers. These stand-alone platforms can be deployed on the deep-sea floor for several days up to a year, during which they can measure a large variety of environmental variables. Not only do we want to know what the favorable conditions (e.g. temperature or current speed) are for coral growth, but we also like to know what they feed on. Unlike their tropical counterparts, cold-water corals do not have photosymbionts so they rely on food particles they can capture from the water column with their tentacles. Previous studies have shown that cold-water corals preferentially live in areas with strong currents, which will prevent them from getting buried by sediments and will increase the amount of food supplied to the corals.
The lander that we are using for the DEEP SEARCH project consists of an aluminum frame with glass spheres for floatation. We have equipped the frame with a current sensor, oxygen sensor, turbidity/fluorescence sensor to measure the amount of particles in the water column, a hydrophone to listen to underwater sound, and a sediment trap. In addition, the lander is equipped with a sophisticated camera system and a carousel, which releases bait at set intervals. The bait will attract fish and other mobile organisms that may be not be observed during our remotely operated vehicle (ROV) dives, which will help us improve species diversity estimates for the area. The camera system utilizes infrared light (invisible to deep-water animals) to ensure that conditions in the deep sea are as natural as possible.
During this cruise, we have deployed the bottom lander near the cold-water coral mounds of Richardson Hills. This is a very interesting study site, since the area is influenced by the occasional presence of the Gulf Stream. When the Gulf Stream is present, the corals are bathed in water characterized by increased temperatures (jumping from 39⁰F up to 50⁰F) and strong currents. By deploying the lander near the cold-water corals for several months, we can look at hourly to seasonal variability of environmental parameters. We will be able to determine how often the Gulf Stream is present and if blooms of plankton at the sea surface actually reach the reef, providing fresh food to the corals and associated fauna. All these measurements will help us to define why corals are growing here and to make predictions about the effect a changing environment will have on their distribution in the future.
We deployed the lander for a two day test period and filmed bait (whole tilapia) attached to the lander with highly-scientific equipment—sheer women’s tights. When the lander came back on deck, we discovered that the bait was entirely eaten. After downloading hundreds of videos, we enjoyed spectacular footage of crabs, fish and amphipods feeding on the bait. Today we have deployed the lander again for the long term. Hopefully we’ll have more to share with you this autumn when the lander returns to the surface!