How to Find Particles in the Sea

One of our biggest challenges in marine biology is measuring populations of organisms, whether they are whales, fish, krill, or bacteria.  We want to know how many, what kind, over what area they occur, and ultimately what controls their distribution….why are they there?  For this cruise, we have the bird and marine mammal team using binoculars, while the zooplankton team uses underwater optics and acoustics to seek out their target organisms.  But each system for detection has its strengths and weaknesses, which is why we use so many instruments to measure the distribution of marine organisms.  Furthermore, like a lawyer trying to win a case, we try to provide numerous lines of evidence that ‘prove’ our estimate of population size.

Recovering the VPR mounted in a CTD rosette on board the R/V Endeavor

I have been using an optical instrument for many years now that Kaylyn described in an earlier blog post: the Video Plankton Recorder.  While plankton nets physically collect plankton, the VPR takes pictures of them.  Instead of samples containing thousands of copepods, we collect tens of thousands of images.  But as Kaylyn has explained, many of these images show no organisms.  We employ computers to scan the video and capture images of particles that are in focus.  Each picture-file is associated with a time, location, depth, temperature, and salinity. The computer can also help us to automatically identify the image, but someone like me will confirm each identification and identify any images that the computer is ‘confused’ about.

Copepod

Diatom chains and marine snow

So what do we see?  Depending on the magnification used, we can view diatoms, dinoflagellates, radiolarians, marine snow, copepods, gelatinous organisms, krill, fish, and other large zooplankton.  We may see layers of the same organisms – for example, we often see krill aggregating at the bottom of our casts in the daytime and spreading out much higher in the water column at night.  Many times and often most times, the particles we see are not living, exactly.  They are called marine snow and can consist of tiny bits of dead plants and animals that stick each other, forming a tear-drop shaped particle.  We can also see larval forms of other animals such as sea anemones and starfish and swimming behavior and natural swimming position of zooplankton which we ordinarily would not be able to see from a specimen caught in a net.  This is particularly true of the gelatinous zooplankton like jellyfish and ctenophores. 

 

Jellyfish

Starfish larva

 Many times it is difficult to identify exactly what we are seeing on the video. That is why it is important to conduct net tows, like the MOCNESS to help us identify planktonic organisms as well as give us estimates of their abundance.  The acoustic instruments like the Greene Bomber and Hammarhead produce plots of back-scattering for larger areas of the water column, further helping us to see ‘the big picture’.  The ADCP gives us information on the currents which control the movements of plankton.  Using all these oceanographer’s tools helps us answer the questions of who, why, how many, and where – and to convince ourselves and others that we are closer to the truth!

Calm seas and fair winds,

Phil Alatalo, Research Associate, WHOI

Krill