ACCESS NUMBER: 30851 

STUDY TITLE: NEGOM-ADCP Backscatter Enhancement 

REPORT TITLE: Spatial and Temporal Variability of Plankton Stocks based on Acoustic 

Backscatter Intensity and Direct Measurements in the Northeastern Gulf of Mexico 

CONTRACT NUMBER: 1435-01-97-CT-30851 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREAS: Eastern Gulf of Mexico, Central Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 1998, 1999, 2000, 2001 

COMPLETION DATE OF REPORT: August 2001 

COSTS: FY 1998: $0; FY 1999: $18,879; FY 2000: $25,906; FY 2000: $9,246; 

CUMULATIVE PROJECT COST: $54,031 

PROJECT MANAGER: D.C. Biggs 

AFFILIATION: Department of Oceanography, Texas A&M University 

ADDRESS: TAMU Mail Stop 3146, College Station, Texas 77843 

PRINCIPAL INVESTIGATORS: R.L. Scott, D.C. Biggs, S.F. DiMarco 

KEY WORDS: Northeastern Gulf of Mexico, DeSoto Canyon, Acoustic Doppler Current 

Profilers (ADCPs), current meter moorings, spectral analysis, zooplankton, eddy circulation 

BACKGROUND: Because of their ability to remotely measure current velocity in the water 

column over a vertical range from a few meters to hundreds of meters, acoustic Doppler 

current profilers (ADCPs) have become increasingly important constituents of physical 

oceanographic research since their development in the mid 1980s. Shortly after ADCPs were 

put into use aboard University-National Oceanographic Laboratory System (UNOLS) research 

vessels, it was recognized that useful biological data can be extracted from the backscattered 

acoustic energy of the ADCP while simultaneously measuring current velocity. The biological 

use is based on the premise that zooplankton in the water column are the predominant 

particles that scatter sound pulses transmitted by an ADCP operating at frequencies of several 

hundred kHz. Thus, the backscatter intensity of the acoustic pulses is thought to represent a 

biological signal directly related to the size and concentration of zooplankton in the water 

column. 

As part of the DeSoto Canyon Eddy Intrusion Study (EIS), twelve 300 kHz broadband ADCPs 

were moored along the slopes of the DeSoto Canyon in the Northeastern Gulf of Mexico. The 

EIS, funded by the U.S. Department of the Interior, Minerals Management Service (OCS 

contract 1435-01-96-CT-30825) monitored the circulation of the DeSoto Canyon area for two 
ACCESS NUMBER: 30851 

years to investigate the effect of Loop Current eddies on over the continental margin of the NE 

Gulf of Mexico. The Workhorse® ADCPs used in the study were manufactured by RD 

Instruments, Inc., of San Diego, California, and deployed by Science Applications International 

Corporation (SAIC). The twenty-five month mooring deployment period ran from March 1997 

through April 1999 and consisted of six instrument deployment/re-deployment periods of 

approximately four months each. During this time the twelve moored ADCPs recorded current 

velocity as well as acoustic backscatter data. The current velocity data set has been 

summarized by SAIC (Access Number 30825), and the acoustic backscatter intensity (ABI) 

data set is the subject of this Technical Summary (Access Number 30851). 

OBJECTIVES: (1) To use a moored array of ADCPs to define spatial and temporal scales of 

variations in ABI and zooplankton biomass in the water column; and (2) To determine how 

variations in ABI and zooplankton biomass may be linked to current flow in the DeSoto Canyon 

area. 

DESCRIPTION: The twelve ADCP instruments were moored in total water depths ranging 

from 100 m to 1300 m. Most instruments were moored along cross-margin lines, with three 

ADCPs moored on each of the lines A, B, and C. Line D consisted of moorings D1 and D2. A 

narrowband ADCP was moored at a third D-line location, but ABI data recorded by this 

narrowband instrument were not evaluated for the present study. E1 was the easternmost 

mooring; this was located along the 100-m isobath off the western Florida shelf. Each of the 

ADCPs was moored in an upward-looking orientation; all hung suspended at either 80 m or 90 

m below the waters surface. Echo intensity data were recorded every 30 minutes during the 

25-month mooring deployment period, and these data were recorded in equally spaced 4-

meter depth binds. To serve as sea truth for the ABI data, zooplankton collections were made 

on three oceanographic research cruises (N2, N3, N4) conducted in the NE Gulf in support of 

the NEGOM Chemical Oceanography and Hydrography study during the 25-month ADCP 

deployment period. 

SIGNIFICANT CONCLUSIONS: Results of this combined physical and biological research 

show that ABI data recorded by ADCPs do represent a biological signal that serves as an 

approximation for zooplankton standing stocks in the NE Gulf of Mexico. Spectral analysis of 

acoustic data reveals daily variability in ABI as well as low frequency, weekly to monthly, 

variability. This low frequency variability in ABI is linked to current flow in the DeSoto Canyon 

area, and correlations between ABI and current velocity vary with the movement of warm slope 

eddies and other mesoscale features along margin and so into and out of the region of the 

moored array. Wind mixing associated with hurricane-strength storm events appears to affect 

the acoustic backscatter recorded by ADCPs, and increases in ABI during these periods are 

attributed to the downward mixing of bubbles from surface turbulence. From this research the 

interactions between physics and biology are evident, and so we recommend that ABI data 

from ADCPs which may in the future be moored in other areas of the Gulf of Mexico be studied 

as well. 

STUDY RESULTS: Regression analysis showed a positive relationship between ABI and 

zooplankton biomass, and the power of this relationship was higher for large-size organisms (> 

2 mm). Vertical analysis of ABI in the upper 90 m of the water column revealed a general 
ACCESS NUMBER: 30851 

increase in backscatter (biomass) with depth. This result was not unexpected, as plankton are 

not evenly distributed vertically, and since herbivorous (phytoplankton-eating) zooplankton are 

usually more abundant at or near the deep chlorophyll maximum, which is usually deeper than 

70 m in the deepwater Gulf of Mexico. We also found that mean ABI for the two-year record 

was highest at the shelf-break sites and lowest at the oceanic stations. Again these results 

were not unexpected, for as in other oceans, the Gulf of Mexico has higher plankton stocks in 

neritic than oceanic waters. 

In an effort to better understand the relationship between ABI and chlorophyll concentrations in 

the NE Gulf of Mexico, backscatter data were used in conjunction with SeaWiFS ocean color 

data from the study area. We found a significant positive correlation between ABI and 

remotely sensed surface chlorophyll concentrations. These results were not surprising, as 

zooplankton populations increase with enhanced food supply. However, chlorophyll 

concentrations in the DeSoto Canyon region did not follow the well-defined annual cycle of 

primary production for the Gulf of Mexico (low in spring-summer, high in fall-winter). Instead, 

remotely-sensed surface chlorophyll was annually highest in mid-summer. Analysis of 

remotely sensed altimetry data showed that this was apparently due to the consistent 

occurrence of anticyclonic eddies over the DeSoto Canyon in summertime. Data from 

NEGOM Chemical Oceanography and Hydrography cruises confirm that entrainment of low 

salinity, high chlorophyll river water by these eddies located over the study area transported 

this "green water" cross-margin to at least the 1000 m isobath in summers 1998 and 1999. 

Spectral analysis of acoustic time series data allowed us to study the variance in ABI as a 

function of frequency. Evident in all plotted spectra was a sharp peak in energy density 

centered at one cycle per day, and these peaks were indicative of the diel vertical migration of 

zooplankton in the water column. All plotted spectra were red, showing decreases in spectral 

density as frequency increased. Red variance spectra are common for oceanic long-term 

variability, and this spectral analysis revealed low frequency, weekly to monthly variability in 

the ABI record were associated with current flow in the DeSoto Canyon area. 

To further investigate the relationship between ABI and current flow, correlation analysis was 

carried out using combined backscatter and current velocity data. Significant correlations 

between ABI and offshore flow indicated the transport of neritic zooplankton off shore to 

deeper waters. Changes in ABI were also correlated with along-shelf flow, although these 

correlations varied as the direction of flow changed relative to the along-margin movement of 

mesoscale physical features. 

The effect of hurricane-strength mixing events on ABI was investigated using data from the 

time periods in which Hurricanes Georges and Earl passed close to the DeSoto Canyon study 

area. It was found that there is a backscatter response to such storm events, but that increase 

in ABI is not a biological respnse. Rather, the response is a physical consequence of the 

abrupt changes in acoustic scattering that occur with the downwared mixing of bubbles from 

surface turbulence and wave action and breaking. 

STUDY PRODUCTS: Scott, R. L., D. C. Biggs, and S. F. DiMarco. 2001. Spatial and 

Temporal Variability of Plankton Stocks based on Acoustic Backscatter Intensity and Direct 
ACCESS NUMBER: 30851 

Measurements in the Northeastern Gulf of Mexico. A final report for the U.S. Department of the 

Interior, Minerals Management Service Gulf of Mexico OCS Region, New Orleans, LA. OCS 

STUDY MMS 2001-057 Contract No. 1435-01-97-CT-30851 134 pp. 

Scott, R.L. 2001. Spatial and Temporal Variability of Plankton Stocks based on Acoustic 

Backscatter Intensity and Direct Measurements in the Northeastern Gulf of Mexico. MS thesis, 

Department of Oceanography, Texas A&M University, College Station, TX. 146 pp. 

*P.I.'s affiliation may be different than that listed for Project Manager(s)