Dave Grant: The Straits of Florida, March 3, 2012

NOAA Teacher at Sea
Dave Grant
Aboard NOAA Ship Ronald H. Brown
February 15 – March 5, 2012

Mission: Western Boundary Time Series
Geographical Area: Sub-Tropical Atlantic, off the Coast of the Bahamas
Date: March 3, 2012

Weather Data from the Bridge

Position:30 deg 37 min North Latitude & 79 deg 29 min West Longitude
Windspeed: 30 knots
Wind Direction: North
Air Temperature: 14.1 deg C / 57.4 deg F
Water Temperature: 25.6 deg C / 78.4 deg F
Atm Pressure: 1007.2 mb
Water Depth:740 meters / 2428 feet
Cloud Cover: 85%
Cloud Type: Cumulonimbus and Stratus

Science/Technology Log:

Entering the  Gulf Stream and Straits of Florida

“There is in the world no other such majestic flow of waters.
Its current is more rapid than the Mississippi or the Amazon.
Its waters, as far out from the Gulf as the Carolina coasts, are of an indigo blue.
They are so distinctly marked that their line of junction with the common sea-water
may be traced by the eye.”
Matthew Maury – The Physical Geography of the Sea

 While our cruise could hardly be called leisurely, most sampling has been spread out between sites, sometimes involving day-long periods on station while the CTD and moorings are recovered from great depths (5,000 meters). However, Chief Scientist Dr. Baringer regularly reminds us that west of the Bahamas in the Gulf Stream transect, our stations are in much shallower water (≤800 meters) and close together (The Florida Straits are only about 50 miles wide), so we should anticipate increased activity on deck and in the lab. In addition to the hydrology measurements, we will deploy a specialized net to sample those minute creatures that live at the surface film of the water – the neuston.

Now that we have crossed the Bahama Banks and are on-station, the routine is, as expected, very condensed, and there is little time to rest. What I did not anticipate was the great flow of the Gulf Stream and the challenge to the crew to keep the Brown on our East-West transect line as the current forces us north.  Additionally, as Wordsworth wrote, “with ships the sea was sprinkled far and wide”  and  we had to avoid many other craft, including another research ship sampling in the same area.

Ben Franklin is famous for having produced the first chart of this great Western Boundary Current, but naval officer Matthew Maury – America’s Scientist of the Sea – and author of what is recognized as the first oceanography text, best described it.  Remarkably, in The Physical Geography of the Sea, first published in 1855, he anticipates the significance of this major climate study project and summarizes it in a short and often-quoted paragraph:

“There is a river in the ocean. In the severest of droughts it never fails,
and in the mightiest floods it never overflows.
Its banks and its bottom are of cold water, while its current is of warm.
The Gulf of Mexico is its fountain, and its mouth is the Arctic seas.
It is the Gulf Stream.”

Gulf Stream water

CTD data from the Straits of Florida
1. Note that temperature (Red) decreases steadily with depth from about 26-degrees C at the surface,
to less than 10-degrees C at 700 meters. (Most of the ocean’s waters are cool where not warmed by sunlight).
2. Dissolved Oxygen (Green) varies considerably from a maximum at the surface, with a sharp decline at about 100 meters, and a more gradual decline to about 700 meters. (Phytoplankton in surface water produce excess oxygen through photosynthesis during daylight hours. At night and below about 100 meters, respiration predominates and organisms reduce the level of dissolved oxygen.)
3. Salinity (Blue) is related to atmospheric processes (Precipitation and Evaporation) and also varies according to depth, being saltiest at about 150 meters.

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“Ron Brown: Phone Home!”

At Midnight, just within sight of the beam of the Jupiter Inlet Lighthouse (And to the relief of the home-sick sailors on board – “Finally –  after  more than two weeks, we are within the range of cell phone towers!”) we began our studies of the Straits of Florida and the Gulf Stream. Nine stations in rapid order – standing-by for a CTD cast, and then turning into the current to set the neuston net for a ten-minute tow.

The purpose of the net is to sample creatures that live on or visit the interface between air and water, so the mouth of the net is only half submerged. Neuston comes from the Greek for swimming and in warm waters a variety of invertebrates and even some young mesopelagic fishes rise within a few centimeters of the surface at night to filter phytoplankton and bacteria, and feed upon other zooplankton and even drowned terrestrial insects that have been blown out to sea.

On the upper side of this water/atmosphere interface, a smaller variety of floating invertebrates, notably Physalia  and Velella (Portuguese man-of-war and By-the-wind-sailor) use gas-filled buoyancy chambers or surface tension to ride the winds and currents. This much smaller group of seafarers is further classified by oceanographers as Pleuston.

Prior to this cruise, my experience with such a sampling device was limited – Years ago, spending miserable nights sailing in choppy seas off of Sandy Hook, NJ searching  for fishes eggs and larva rising to the surface after dark; and later, much more enjoyable times studying water striders – peculiar insects that spend their lives utilizing surface tension to skate along the surface of Cape Cod ponds.

Our CTD and net casts are complicated by rising winds and chop, but some great samples were retrieved. Once the net is recovered, we rinse it down with the seawater hose, collect everything from the bottle at the cod end, rinse off and separate the great mass of weed (Sargassum) and pickle the neuston in bottles of alcohol for analysis back at the lab.

Since much of the zooplankton community rises closer to the surface at night where phytoplankton is more concentrated and the chances of being preyed upon are slimmer, there are some noticeable differences in the samples taken then and during daylight hours. Unavoidably, both samples contain great quantities of Sargassum but the weed-colored carapaces of the different crustaceans are a clue to which specimens are from the Sargassum community and which are not.

We hit the jackpot early; snaring a variety of invertebrates and fishes, including the extraordinarily well-camouflaged Sargassum fish – a piscatorial phenomenon I’ve hoped to see ever since I was a kid reading William Beebe’s classic The Arcturus Adventure. What a tenuous existence for such a vulnerable and weak swimmer, hugging the Sargassum as it is dashed about in the waves. Even with its weed-like disguise and ability to blend in with the plants, it must lead a challenging life.

A unique member of the otherwise bottom-dwelling frogfishes, the Histrio histrio has smooth skin, and spends its life hitch-hiking along in the gulf-weed forest. Like other members of the family Antennariidae, it is an ambush predator, luring other creatures to their doom by angling with its fleshy fins.

Another highlight for me is the water striders we found in several samples. These “true bugs” (Hemiptera) are remarkable for several reasons. Most varieties of these “pond-skaters” (Or Jesus Bugs if you are from Texas) are found on calm freshwater lakes and streams, but a few members of this family (Gerridae) are the only true marine insects – representing a tentative Arthropod reinvasion of the sea after their splendid foray onto land hundreds of millions of years ago.

Using surface tension to their advantage, they “skate” along by flicking their middle and hind legs, and can even “communicate” with each other by vibrating the surface of the water with the hair-like setae on their feet. On lakes their prey is other insects like mosquito larvae, confined to the surface. How they manage to find food and communicate at the surface of the raging sea is a mystery, but whatever the means, they are adept at it, and we recovered them in half of the samples.

The scientists who provided the net are generally more interested in ichthyoplankton to monitor fish eggs and larvae to predict population trends, and monitor impacts like oil spills; so this is why samples are preserved to return to the lab in Miami.

Before packing up things after our marathon sampling spree I was able to examine our catch and observed a few things:
1. I am the “High-Hook” on the cruise – catching far more fishes (albeit tiny ones) than the rest of the crew with their fishing poles. (Needlefish, sargassum fish, pipe fish, filefish and several larval species)
2. Depending on the time of day the samples were taken, there is a marked difference in the quantity and composition of organisms that have separated from the Sargassum and settled in the sample jars – (Noticeably more at night than during daylight hours).
3. There appears to be a greater variety of sea grasses present (Turtle grass, etc.) on the eastern (Bahamas side) of the Straits. We observed one seabean – drift seeds and fruits (or disseminules) from terrestrial plants.
4. Plastic bits are present in all samples – particularly plastic ties (Table 1.)

Sargassum fauna: Portunid crab – with eggs on her belly.
(Portunus was a Roman god – Protector of harbors and gates,
who supposedly also invented navigation)

Table 1.   Contents in sample jars.

With sampling completed we steer north to ride the Gulf Stream towards the Brown’s home-port,  and turn away from the bright lights of Florida …

“Where the spent lights quiver and gleam;
Where the salt weed sways in the stream;
Where the sea-beasts rang’d all around
Feed in the ooze of their pasture ground:”
Matthew Arnold

Homeward bound:

A storm battering the Midwest will impede our progress back north to Charleston and threatens to bring us the only foul weather of the cruise. Note the location of the cold front over the Florida Straits.

“Now the great winds shoreward blow;
Now the salt tides seaward flow;
Now the wild white horses play,
Champ and chafe and toss the spray.”
Matthew Arnold