The world’s oceans are teeming with the goo produced by microorganisms. Scientists refer to this detritus by the improbable name “marine snow.” Unlike the trash humans put in the water, marine snow is vitally important to the habitat: it links surface waters to the deep ocean, forms a component of the biological pump, and provides habitat for diverse microbial communities.
“Marine snow” encompasses a range of sinking, mostly organic, matter including dead plankton, plankton feces, and aggregates of tiny organisms. These carbon-rich globs are degraded by microbes and eaten by plankton as they drift toward the ocean bottom, providing energy to denizens of the deep. Bacteria decompose some of the nutrients in marine snow and, in the process, recycle it into a form usable by primary producers. The rest ends up on the sea floor and, eventually, in ocean sediments.
The details of this process, such as how quickly the snow gets processed by organisms, are difficult to study because snow is very fragile and often breaks up when collected with bottles, nets, and sediment traps. To get around that problem, a group from the Japanese Fisheries Research Agency (JFRA) led by Yuichiro Nishibe recently set out to examine one part of the cycle using a special in situ imaging system called the Video Plankton Recorder (VPR). The group also preformed experiments in the lab to compare to the data.
Nishibe and his team set out to measure how quickly copepods, a common type of plankton, eat the discarded houses of appendicularians, a planktonic filter feeder (fig. 1). “House” is the genteel term given to the ball of mucus appendicularians produce to collect food. When the house gets clogged up, the small zooplankton discard it and build another, repeating the process as often as 40 times a day. Some studies have found that the number of discarded houses per cubic meter of surface water can reach into the thousands. Scientists know that plankton eat the discarded houses, and they estimate that between 20 and 70% of the carbon in marine snow could be degraded by feeding. This rate needs to be pinned down to better evaluate how much carbon ends up sinking to the deep ocean.
To estimate the rate of consumption, the JFRA group towed the VPR behind a research vessel off the coast of Shikoku, Japan, raising and lowering it seven times from the surface to a depth of 100m. This process was repeated over two transects. The VPR was equipped with a camera and programmed to snap 15 pictures per second. In-focus objects were automatically extracted from the full images using real-time image segmentation software. These objects were then hand-sorted by an expert to identify appendicularian houses and copepods. The copepods were then further separated into three categories: individuals attached to houses, those attached to other particles, and those flying solo (fig. 2). The group then estimated the abundance of organisms in each of these classes at a variety of depths and averaged the abundances over each profile.
The JFRA scientists were able to use these numbers to tease out the relationship between the copepods and the appendicularian houses. They found there is a tight coupling between the two groups. The VPR images showed that the proportion of copepods attached to the houses increased with the concentration of available houses up to about 2000 houses per cubic meter. Even when there were many houses in the water, copepods were only associated with a house about 35% of the time. Further analysis revealed that most of the consumption by the copepods took place in the upper 50 m of the water column. This result was consistent with the group’s observation that copepods did not attach themselves to houses as frequently at depths below 50 m.
To corroborate and better understand the field data, Nishibe and his team performed feeding studies in the lab. They grew copepods and fed them appendicularian houses. Behavioral observations were also conducted by video taping the feeding habits of an individual copepod in a chamber with a single house. Eight, one hour videos were made of different copepods eating new houses.
The video recordings from the lab revealed that the copepods spent about 6 minutes feeding on a house in a single sitting. The individual would swim away from the house and then return, paying many short visits to the house over the duration of the recording. This suggests that the in situ data could underestimate how often copepods eat the houses since they spend so much time swimming from place to place.
The estimated rate of house consumption by copepods from the field data came in at just under 10%. This may seem like a small number, but as Nishibe points out, it can make a big difference on a larger scale. Other studies have found that appendicularians can account for the vast majority of all organic carbon getting fluxed to the deep ocean. In that global context, 10% degredation represents a huge break down of material. It is certainly important enough to consider when discussing and studying oceanic carbon cycles.