The Seascapes

The Seascapes

Friday, July 2, 2010

Sampling off Long Island on the last day of our June cruise

A fine looking fishing vessel off the Rockaways, Long Island

Yesterday we completed the last day of our 4 day June sampling cruise. Earlier I remarked about how lucky we were to have winds shift from the south to north after we sampled each of the seascapes once. The plot on the left shows the northward and eastward components of the wind over the last few days at the “Ambrose” buoy off the mouth of the Hudson-Raritan Estuary. Our sampling periods are indicated by the hatched bars. Winds blew toward the east throughout the 4 cruises but shifted from the south to the north midway through the week. (The plots indicate the direction the wind is pushing water towards rather than the direction the wind is blowing from.  This is a convention of the physicists and all fish biologists suffer physics envy.  But why is ocean physics really interesting?  Because it matters to the fish.)

The effects of the wind shift on the oceans surface is evident in the 24 average surface currents measured with HF radar at 1300 each of the days we sampled (You can animate hourly 24 hour averaged surface currents at the RU COOL HF radar site). In these images you can see the wind drive strong offshore flow particularly along the southern flank of the Hudson Shelf valley early in the week. This flow weakened dramatically as the winds shifted southward. As a result of this change in winds the warm water we sampled relatively close to the coast of Long Island on Tuesday had moved well offshore into the mid-Atlantic bight on Thursday (7/30/10).   

The edge of the warm water is indicated by arrow in this 1030 GMT  satellite image of sea surface temperature.  On clear mornings we can count on getting one good early morning satellite image.  Once the land heats up and clouds form the images are not so useful.

This google earth map shows the NYHOPs model prediction of temperature and surface current flows for 1400 GMT which was about the time we began to sample. The beginning and ends of our transect as well the site of the front we selected from the NYHOPs model (mid_07012010) and the site of the  front from the satellite image above are marked. We towed our plankton net at 6 stations instead of usual 5 along this transect so we could sample locations of the front indicated by both the model and the satellite image.   

The acoustic image of particle backscatter along our transect to the left is from our acoustic doppler current profiler.  The rectangles show the 6 locations where we towed the plankton nets.  TT_3_1506 is the site of front in the NYHOPs model and TT_4_1558 the site of the front in the satellite image above with the arrow.  The 4 digit numbers at the end of the station designations are the Greenwich mean times (GMT) of the sampling. We use GMT time so that it is easy to match our data with data collected by satellites and other sensors available through the ocean observing system.

The image below shows in much more detail the scattering layers we towed our net through at station 2 (TT_2_1430).  We fished one net from the surface to a depth of 9 meters, and a second net from 9-23 meters.  We selected these depths based on this image and the CTD cast (not shown. But see earlier post).  There seems to be a lot going on down there.  In general the acoustic complexity of the water column on the transect to Long Island on thursday was greater than on the other 3 days of the week.  This CTD data also seemed show this but we haven't processed the data yet. We have a lot more data to processes including the plankton samples which we now need to sort.


Thursday, July 1, 2010

Who makes ECOS possible?

Today was the last day of our June ECOS cruise.  We had another of interesting day sampling on a long transect perpendicular to Long Island, New York.  Our last sample was collected in cold salty water more typical of New England and we captured some larval fish at several stations.  The day deserves description, from it's interesting oceanography to the fin whale we saw rolling toward the north off the tip of Sandy Hook. It rolled into brown water of the Hudson-Raritan river plume that ran between the bank of sand that is the New Jersey coast and the invisible riverbank on its eastern flank made by the north wind and the salty water of the Atlantic ocean.  But I'm too tired right now to process the data required to do the day real justice.

But I want to express my gratitude to Jeff Pessutti.  During all 4 days of sampling the complex integrated equipment and operations worked as smoothly as silk.  That was only possible because of the ingenuity, effort and careful work of Jeff who can take every crazy idea I can think of and make it happen using nothing but bailing wire, duct tape, and a few pieces of equipment, some of it begged for, borrowed, or ...... The ECOS research project could not happen without him. I can say the same about every other research project I've successfully completed over the past 10 years, but this project is technically the most sophisticated.

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).