The Seascapes

The Seascapes

Thursday, July 1, 2010

The vertical structure of the ocean off New Jersey

Stations where we sampled with nets today (6/30/2010) and where the CTD casts and acoustic images of the ocean referenced below were collected.  For discussion of our route planning see the earlier post today.

We use satellite, HF radar, oceanographic models and a surface conductivity, temperature and depth (CTD) sensor (the bottom left window on the computer screen) to identify surface features and position our stations in relation to those features.  To determine depths at which to fish our nets at those stations we use instruments that use sound to measure and visualize ocean structure, along with a profiling CTD.  To the left is the computer monitor on the bridge of the ship that shows real time pictures of the ocean made with sound by  acoustic instruments.  On the top is the output from an acoustic doppler current (ADCP)  profiler that emits and measures the return of high frequency sound at 600 khz.  The panel on the top left shows a vertical cross section of current speed. The middle panel at the top shows current direction and the right panel is acoustic backscatter. On the bottom right is an image of the ocean made with a longer frequency fishery hydroacoustic instrument that emits and measures the return of sound at 120 khz.  The 600 khz machine, with its shorter wavelengths of sound, will "see" smaller critters in the ocean than the 120 khz machine.

This panel shows the acoustic backscatter from the ADCP measured across the entire early morning transect we used to verify the features in the model and remotely sensed data (The map of our track is in the last post).  Inshore at mid depth there is a region of low backscatter (purple).  Offshore there are areas of high scattering (pale blue & green)  at mid-depth and deep.  The ship and pole arm on which the acoustic transducers are mounted make a lot of turbulence as they move through the water. As a result it is impossible to distinguish sound scattered by the turbulence or by particles at the surface.

We also use the CTD profiler (to the left) to see the structure of the water beneath the oceans surface .  We have added seine floats to the instrument to make it nearly neutrally bouyant and sink very slowly. As a result it makes many more measurements over the distance it sinks.  This allows us to identify small changes in water density and other characteristics over at scales as small as a few centimeters. This kind of fine scale structure in the water column is often ignored. But a typical larval fish is neutrally bouyant and less than about 15 mm, or about a half an inch long.  Because of this slight changes in density and other characteristics over distances of ranging from a few centimeters to a few meters probably make up important components of the seascape to larval fish. The scales of habitat variation should match the body size and movement scales of the animals.  So maybe larval seascapes are the "Hunt for Red October" in miniature, filled with invisible hedge rows made of structured water the larvae can hide behind and graze near.

We combine visualizations of the acoustic and CTD data to decide how deep to tow the nets.  The 5 plots below overlay the CTD profiles on top of the acoustic images of the ocean made with fishery hydroacoustics right before we fish each net. In all the plots the colored lines represent the following water column characteristics: Red=salinity, dark blue=temperature, light blue=oxygen, green=Chlorophyll-A pigment of plants, and black=turbidity, or tiny particles in the water column.

After taking this CTD cast and looking at the acoustics at this station furthest offshore we decided to fish a net from 0 to 7 meters to capture plankton at the surface.  We then used a second net to fish between 7 and 25 meters.  This is the best we can do with the equipment we have.  A 5 net electronic opening and closing net would much better but also would cost about $200k when we got through with the necessary wiring and electronics on the boat.  So the net we have will have to do for a while.  In the surface tow at this station we caught a baby seahorse and lobster.

Letting the tucker trawl out to fish.

The seahorse (closer to the quarter) and the lobster (farther from the quarter) we caught in the surface layer between 0 and 7 meters at our offshore station.  We should "look" at these organisms in our laboratory with the different frequencies of our acoustic instruments.  This might allow us to verify their acoustic signatures and one day identify lobster larvae and their locations in the water just by the return of the sound.

Acoustic image and CTD casts at station 2 (see previous post for map) Something big (the blue streak) was rising to the surface here (No it wasn't the CTD).

Acoustic image and CTD casts at station 3 (see previous post for map)

Acoustic image and CTD casts at station 2 (see previous post for map)

And below is the acoustic image and CTD casts at station 1 closest to the beach near Sandy Hook (see previous post for map).  Their often eem to be more scattering layers that coincide with steps in the density of the water along the shoreline near Sandy Hook because the flow of freshwater from the Hudson River plume pulses in with the tide. However the vertical structure of the water column in New Jersey seems to be relatively simple this year perhaps because it has been so dry (see earlier post).

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