ACCESS NUMBER: BRD 96-0006 

STUDY TITLE: Northeastern Gulf of Mexico Coastal and Marine Ecosystems Program: 

Ecosystem Monitoring, Mississippi/Alabama Shelf 

REPORT TITLE: Mississippi/Alabama Pinnacle Trend Ecosystem Monitoring, Final 

Synthesis Report 

CONTRACT NUMBER: USGS Contract 1445-CT09-96-0006. 

SPONSORING OCS REGION: Gulf of Mexico 

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

FISCAL YEARS OF PROJECT FUNDING: 1996; 1997; 1998; 1999; 2000; 2001 

COMPLETION DATE OF REPORT: November 2001 

COSTS: FY 1996: $256,179; FY 1997: $1,763,262; FY 1998: $1,065,701; FY 1999: 

$934,623; FY 2000: $429,117; FY 2001: $189,946; CUMULATIVE PROJECT COST: 

$4,638,828 

PROJECT MANAGER: D. Gettleson 

AFFILIATION: Continental Shelf Associates, Inc. 

ADDRESS: 759 Parkway Street, Jupiter, Florida 33477 

PRINCIPAL INVESTIGATORS*: William W. Sager, William W. Schroeder, Mahlon C. 

Kennicutt II, Ian D. Walsh, F.J. Kelly, Norman L. Guinasso, Jr., Dane D. Hardin, Keith D. 

Spring, Stephen T. Viada, Alan D. Hart, Bruce D. Graham, David B. Snyder, Michael B. 

Peccini, Tara J. Holmberg, Paul A. Montagna, Neal W. Phillips, David A. Gettleson 

KEY WORDS: Eastern Gulf; Central Gulf; synthesis; monitoring; geology; geophysical; 

sediment dynamics; geochemistry; physical oceanography; hydrography; pinnacles; 

mounds; benthic ecology; hard bottom; fish; microhabitat; geographic information 

system; recruitment 

BACKGROUND: The Mississippi/Alabama outer continental shelf (OCS) is an 

important multiple use area for human commerce, fisheries harvest, recreation, and 

other activities, including oil and gas exploration and development. Because of the 

petroleum industry’s interest in the area and the potential for environmental impacts, an 

understanding of hard bottom communities and environmental processes that influence 

them is critical. 

OBJECTIVES: The overall goal of this program was to characterize and monitor 

biological communities and environmental conditions at carbonate mounds along the 

Mississippi/Alabama OCS. Specific objectives were as follows: (1) to describe and 

monitor seasonal and interannual changes in community structure and zonation and 

relate these to changes in environmental conditions; and (2) to characterize the 
ACCESS NUMBER: BRD 96-0006 

geological, chemical, and physical environment of the mounds as an aid in 

understanding their origin, evolution, present-day dynamics, and long-term fate. 

DESCRIPTION: This report summarizes a 4-year program to characterize and monitor 

carbonate mounds on the Mississippi/Alabama OCS. Nine sites in the 

Mississippi/Alabama “pinnacle trend” area were selected for monitoring. The sites were 

sampled intensively during four monitoring cruises over a 2-year period. Moorings with 

sediment traps, current meters other oceanographic instruments, and settling plates 

were also deployed at four of the sites. Monitoring program components included 

geological characterization, sediment dynamics, geochemistry, physical oceanography 

and hydrography, hard bottom communities, fish communities, microhabitat studies, and 

epibiont recruitment. A synthesis chapter draws together findings from all program 

components. 

SIGNIFICANT CONCLUSIONS: Hard bottom community development was generally 

greater on higher relief features, and probably due in part to negative effects of 

resuspended sediments. A benthic nepheloid layer was present at all sites; it probably 

reaches all but the tops of the tallest mounds and may affect hard bottom community 

development. There are east-west patterns in hard bottom communities in the study 

area, but whether these reflect a direct or indirect influence of the Mississippi River or 

some other factor is not known. Substrate characteristics influence the distribution and 

abundance of hard bottom epibiota. Among sites, there was a positive relationship 

between percentage of emergent hard bottom and biological variables, including biotic 

cover and numbers of taxa. At a finer scale, relationships between microhabitat factors 

and hard bottom taxa were documented, including effects of feature morphology, smalland medium-scale roughness, slope, location on feature, and sediment veneer. 

Stochastic or unexamined processes contribute substantially to distribution patterns of 

hard bottom biota. 

There was little or no evidence of trace metal or petroleum related hydrocarbon 

contamination at any of the study sites. Relationships among community development, 

relief, and sediment flux suggest that it would be more detrimental to discharge drilling 

muds and cuttings on top of large, flat top mounds than to discharge them in low relief 

areas. Existing lease stipulations focus on avoiding physical damage to hard bottom 

features but also make it unlikely that drilling discharges could occur near large, high 

relief mounds. Data collected during this program suggest that recovery of hard bottom 

communities, following a disturbance, would be slow. 

The current meter data indicate a regional flow regime with local variations dependent 

on topography, rather than a strong east-west or onshore-offshore gradient. Current 

direction is an important influence on the orientation of filter-feeding sea fans. The 

distribution of other epifauna on hard bottom features may also be affected by current 

directions. Existing knowledge of flow over bottom features suggests that different 

mounds, and different areas within a mound or mound complex, could experience local 

flow regimes that could affect the exposure of epibiota to sedimentation, erosion, and 

food flux. 
ACCESS NUMBER: BRD 96-0006 

STUDY RESULTS: Four “megasites” (subsets of the study area) contained 

recognizable carbonate mounds varying in size, number, and morphology. The mounds 

were classified into several different forms: small, “unit” mounds; composite mounds; 

irregular mounds; smooth-top mound; and carbonate hard bottoms. Morphologic 

differences among mounds suggest differences in development. Low, wide carbonate 

hard bottoms imply slow upward growth over a large area, perhaps indicating stable sea 

level or slow sea level rise. Tall, steep-sided “pinnacle” mounds suggest rapid growth 

during faster sea level rise. Widely dispersed, shallower mounds may represent a short 

period of sea level stabilization in the middle of the deglaciation. Mound growth was 

probably most intense on the side facing the sea, which is similar to the formation of 

coral reefs and lends credence to the hypothesis that mounds were formed by biological 

action in shallow water. Sediments at the monitoring sites are mainly sand, with a small 

and variable amount of clay. Gravel content is usually highest near mounds, indicating 

them as a source or suggesting mound proximity as an important factor controlling the 

presence of organisms. While normal sedimentation is not very active, high-current 

events cause significant reworking of the sediments. 

Little or no evidence of trace metal or petroleum related hydrocarbon contamination was 

observed in sediments at the study sites. Barium, a tracer of drilling mud discharges, 

was observed to be at background levels with only a very few samples that might be 

interpreted as slightly elevated. 

Particle flux varied widely in space and time. A benthic nepheloid layer was present at 

all sites in all casts, though its intensity was variable. Water column profiles indicate a 

local origin for the nepheloid layer particles. Sediment trap results reflect the influence 

of resuspension, with fluxes increasing toward the bottom for all moorings and time 

periods. Average vertical fluxes during non-hurricane periods ranged from 1.5 to 6 g m2

 d-1 in the traps 15 meters above bottom (mab) and from 6.7 to 29.3 g m-2 d-1 in the 2.5 

mab traps. There was no consistent geographic trend in the sediment trap data set at 

any given depth level. No seasonal trends were apparent. Extremely high fluxes (4 to 

70 times the non-hurricane average) were recorded during a period when Hurricane 

Georges passed near the study area. 

Across the study area the observed flow fields were similar at 16 mab. The most 

frequent direction octant and the direction of the vector mean current were east at Sites 

1 and 9. At Site 4, located farther from shore in deeper water, there was a slight 

southwesterly bias as compared with the other sites. At all sites, the most frequent 

speed range was 5 to 10 cm/s, reflecting the normal tidal influence. Strong currents 

most frequently flowed southwest or west, particularly during Hurricane Georges 

(maximum of 97 cm/s at Site 1). The near-bottom (4 mab) flow was more site specific. 

Bottom friction and local topography influenced flow, particularly at Site 1, a large 

mound that disrupted the downstream flow. 

A total of 2,997 random photographs were analyzed for percent cover of hard bottom 

biota. The 40 taxa with the highest overall cover represented 14 taxon groups. 

Octocorals were the most diverse group (10 taxa), followed by sponges (6), 
ACCESS NUMBER: BRD 96-0006 

ahermatypic corals (4), antipatharians (4), and ectoprocts (4). Ahermatypic corals (e.g., 

Rhizopsammia manuelensis) were the most abundant group. Mean biotic cover (over 

all taxa and cruises) ranged from 13.5% to 30.6%. Most of the dominant taxa varied 

with respect to both relief category and region, as well as related environmental 

variables. The highest abundance was observed at high relief sites, with organisms 

found primarily on the sides and tops of features. Sediment veneer was also an 

important influence, especially for medium-high relief taxa. Many taxa were more 

abundant toward the east (i.e., farther from the Mississippi River), but some taxa 

increased toward the west. Stochastic or unexamined processes contributed 

substantially to distribution patterns of hard bottom biota. Fixed quadrat data reveal a 

dynamic benthic environment, with numerous instances of sediment deposition, and 

occasional instances of erosion, and organism growth and damage or mortality. 

In all, 76 fish taxa representing 33 families were seen. Numbers of fish taxa per cruise 

varied from 15 to 28. The most frequently occurring fishes were roughtongue bass, 

short bigeye, bank butterflyfish, red barbier, scorpionfish, and tattler. Streamer basses 

(e.g., roughtongue bass and red barbier) numerically dominated the mounds. There 

were no significant differences in numbers of taxa among cruises, sites, or relief 

categories. Numbers of taxa were not strongly correlated with environmental variables. 

Frequency of occurrence of 17 common fish taxa was analyzed in relation to habitat 

characteristics at three scales: large (tens of kilometers to hundreds of meters), meso 

(tens of meters to 1 m), and small (1 m to centimeters). There were no strong patterns 

at the large scale. Meso-scale observations showed that some species regularly used 

portions (tops, sides, bases) of larger features, generally reflecting their feeding 

behavior. Different fishes were associated with sedimentary vs. hard bottom habitats. 

Most small species remain near features for shelter from larger groupers, amberjacks, 

and sharks that patrol the structures. There was considerable overlap in use of the 

small-scale habitats. Crevices, ledges, and holes were especially important to some 

fish species. 

A microhabitat study focused on orientations of gorgonian colonies at several sites. At 

every site, gorgonian orientations were non-random, and the results suggest that mean 

orientations of gorgonians are strongly influenced by mean directional water flow. 

Deviations from the current meter readings probably indicate local topographic steering 

that is not captured by moorings located well away from features. Substrate 

characteristics such as morphology, location on feature, and sediment veneer were also 

significant factors in determining the abundance of gorgonian and antipatharian 

colonies. 

A recruitment study using settling plates found nine phyla: Rhizopoda, Porifera, 

Cnidaria, Ectoprocta, Entoprocta, Mollusca, Annelida, Arthropoda, and Chordata. There 

were significant differences among sites for plates located on the bottom but not for 

plate’s elevated above the bottom. The attached community changed over time. 

However, just the earliest successional stages of community development were found 

on the plates, and community structure was different from the surrounding hard bottom 
ACCESS NUMBER: BRD 96-0006 

communities. If disturbed, deepwater hard bottom communities could require decades 

to recover. 

STUDY PRODUCT(S): Continental Shelf Associates, Inc. and Texas A&M University, 

Geochemical and Environmental Research Group. 1998a. Northeastern Gulf of Mexico 

Coastal and Marine Ecosystem Program: Ecosystem Monitoring, Mississippi/Alabama 

Shelf; First Annual Interim Report. U.S. Department of the Interior, U.S. Geological 

Survey, Biological Resources Division, USGS/BRD/CR-1997-0008 and Minerals 

Management Service, Gulf of Mexico OCS Region, New Orleans, LA, OCS Study MMS 

97-0037. 133 pp. 

Continental Shelf Associates, Inc. and Texas A&M University, Geochemical and 

Environmental Research Group. 1998b. Northeastern Gulf of Mexico Coastal and 

Marine Ecosystem Program: Ecosystem Monitoring, Mississippi/Alabama Shelf; Second 

Annual Interim Report. U.S. Department of the Interior, U.S. Geological Survey, 

Biological Resources Division, USGS/BRD/CR-1998-0002 and Minerals Management 

Service, Gulf of Mexico OCS Region, New Orleans, LA, OCS Study MMS 98-0044. 198 

pp. 

Continental Shelf Associates, Inc. and Texas A&M University, Geochemical and 

Environmental Research Group. 1999. Northeastern Gulf of Mexico Coastal and 

Marine Ecosystem Program: Ecosystem Monitoring, Mississippi/Alabama Shelf; Third 

Annual Interim Report. U.S. Department of the Interior, U.S. Geological Survey, 

Biological Resources Division, USGS/BRD/CR-1999-0005 and Minerals Management 

Service, Gulf of Mexico OCS Region, New Orleans, LA, OCS Study MMS 99-0055. 211 

pp. 

Continental Shelf Associates, Inc. and Texas A&M University, Geochemical and 

Environmental Research Group. 2001. Mississippi/Alabama Pinnacle Trend 

Ecosystem Monitoring, Final Synthesis Report. U.S. Department of the Interior, 

Geological Survey, Biological Resources Division, USGS BSR 2001-0007 and Minerals 

Management Service, Gulf of Mexico OCS Region, New Orleans, LA, OCS Study MMS 

2001-080. 481 pp. 

*P.I.’s affiliation may be different than that listed for Project Managers.