AMER. ZOOL., 30:23-35 (1990)
Biotic Assemblages and Ecological Controls on Reefs and
Banks of the Northwest Gulf of Mexico1
RICHARD REZAK, STEPHEN R. GITTINGS, AND THOMAS J. BRIGHT
Department of Oceanography, Texas A&M University, College Station, TX 77843-3146
SYNOPSIS. This paper summarizes the results of investigations of the ecology of reefs
and banks in the northwestern Gulf of Mexico. Nearshore surface waters are turbid out
to approximately the 10 m isobath. A turbid nepheloid layer up to 20 m in thickness exists
beyond this depth. Seasonal temperature and salinity variability is high in nearshore waters.
At the shelf edge, the temperature to 50 m depth remains above 19°C. Topographic
prominences are common in the region. Most are surface expressions of the diapirism of
underlying salt domes derived from deeply buried Jurassic salt deposits. Biotic zonation
on the banks is primarily depth related. Zones of major reef-building include the DiploriaMontastrea-Porites Zone (15-36 m), the Madracis Zone (28-46 m), the Stephanocoenia-Millepora Zone (36-52 m), and the Algal-Sponge Zone (45-98 m). A zone of minor reefbuilding on some banks is the Millepora Zone (18—52 m). An Antipatharian Transitional
Zone exists from 56 to over 100 m on some banks. The Nepheloid Zone near the base
of the banks is a zone of no reef-building. Not all zones are present on all banks. The
community structure and depths of these zones depend on and are modified by the regional
current regime, depth of the bank crests, substrate characteristics, winter temperature
minima, river influences, and the relative depth and thickness of the nepheloid layer.
Nearshore communities are warm temperate in nature, though tropical organisms are
occasionally abundant. Progressing offshore, the benthos becomes increasingly tropical.
The northerly location and isolated nature of the Flower Gardens have lead to reduced
community diversity {e.g., only 18 of the 55 Western Atlantic hermatypic coral species
exist), but not reduced abundances or growth rates of the species present. Assemblages
on other banks exist below the tolerance limits of thriving tropical reefs due to their
frequent immersion in turbid water, excessive crest depths, or lower minimum winter
temperatures.
banks (Bright and Pequegnat, 1974).
Beginning in 1974, there was a slight
Background
change in research goals in order to proThe Department of Oceanography at vide the MMS with baseline biological and
Texas A&M University has been involved geological data to assist in establishing polin research on the reefs and banks of off- icy with regard to the need and nature of
shore Texas and Louisiana since 1961. protective regulations to be imposed on
Until 1974, the research was funded by drilling operations near the banks. FundTexas A&M University and the Flower ing by MMS continued until the middle of
Garden Ocean Research Center at the 1983. Thirty-eight banks were mapped
University of Texas Marine Biomedical using precision navigation, precision depth
Institute. The Bureau of Land Manage- recorder, and side-scan sonar. High resoment (now Minerals Management Service lution subbottom surveys were conducted
[MMS]) began funding studies on the on 20 of the banks. Submersible observaFlower Garden banks and other banks off tions and sampling were conducted at 28
Texas and Louisiana in 1974. Earlier of the banks using the Texas A&M subresearch had been aimed at developing a mersible DRV DIAPHUS operated from
conceptual model of coral reef growth on the RV GYRE. Extensive scuba diving
a terrigenous shelf (Edwards, 1971) and investigations took place in the shallower
documentation of the biota of the reefs and parts of the East and West Flower Garden
banks. The study of the Flower Garden
Banks was much more extensive than on
1
From the Special Session on Ecology of the Gulf of the other banks and as a result, the more
Mexico organized by Rezneat M. Darnell and Richard complete sequence of biotic zones present
E. Defenbaugh and presented at the Annual Meeting
there was used as a standard for compariof the American Society of Zoologists, 27-30 Decem- son with other banks.
ber 1987, at New Orleans, Louisiana.
INTRODUCTION
23
24
R. REZAK ET AL.
30'
25'
E2) Turbid bottom water
•••• Zone of low salinity turbid water
— - Intersection of winter mixed layer
with the bottom
— Generalized current pattern and eddies
20° N
95°
90°
85°
80°W
FIG. 1. Map of Gulf of Mexico summarizing circulation patterns, continental shelf bottom types, and turbid
water regions (after Rezak et at, 1985).
Regional setting
The Gulf of Mexico is a semi-enclosed
oceanic basin extending from approximately 18°N to 30°N and 81°W to 97°W on
the western side of the Atlantic Ocean.
Water enters the Gulf through the Yucatan Channel (176 km wide) and exits
through the Florida Straits (144 km wide;
Fig. 1). The loop current, which directs
this flow, is restricted to the eastern Gulf.
However, warm core rings spin off from
the loop current, move westward, and
impinge on the upper continental slope of
the northwest Gulf of Mexico even during
winter months (Kirwin et al., 1984; Wallcraft, 1985).
There are several important differences
that distinguish the northwestern Gulf of
Mexico from the rest of the Gulf. First,
though the continental shelves off Florida
and the Yucatan peninsula consist of car-
bonate sediments, the shelf in the northwestern Gulf consists of terrigenous sediments, resulting in much higher nearshore
turbidity with persistently turbid water near
the bottom. Second, the influence of winter cold fronts on nearshore surface water
temperature is most pronounced in the
northwestern Gulf of Mexico. Nearshore
surface temperatures off Louisiana may be
as low as 6°C for short periods in winter.
Third, salinity in these same waters is
strongly influenced by variability in the
Mississippi/Atchafalaya discharge system
and other rivers draining into the northwestern Gulf (Rezak et al, 1983, 1985).
Most of the Mississippi River discharge
flows west along the coast of Louisiana (Fig.
1). Thus, the hydrography of the western
Gulf of Mexico is very different from that
of the eastern Gulf, especially in the nearshore waters of the northwestern Gulf,
25
REEF AND BANKS
where terrigenous sediments and highly
variable salinity and temperature regimes
predominate.
Local environmental controls
Biological community structure on the
reefs and banks depends on the following
factors: 1) geological characteristics of the
substrates, 2) regional and local current
regimes, 3) winter temperature minima, 4)
river influences on salinity and turbidity,
5) depth of the bank crests, and 6) depth
and thickness of the nepheloid layer. Geological characteristics of the substrates are
controlled by tectonic processes, hydrology, biotic assemblages, and the presence
or absence of a nepheloid layer. The substrate may be solid bedrock or reef rock
or it may be soft sediment (either the terrigenous muds and sandy muds that are
normal on this part of the shelf or calcareous skeletal sands and gravels created by
reef and bank dwelling organisms). Salt
tectonics are responsible for elevating the
seafloor above the nepheloid layer and creating a hard substrate on which reef-building organisms can grow above the surrounding turbid water environment.
Local hydrology
Hydrology includes seawater temperatures, salinities, turbidity, and currents.
Minimum temperatures at the East Flower
Garden (27 m depth) measured by an
instrument moored on the reef are on the
order of 19.5°C (Rezak et al, 1985). Angelovic (1976) reported that the minimum
temperature in the vicinity of the Flower
Garden Banks occurs in late February. At
that time the surface mixed layer is 30 to
50 m thick, with temperatures in the range
of 19 to 20°C (Harrington, 1966). Temperatures at the bottom (125 m) are 17 to
18°C. Dodge and Lang (1983) speculated
that low salinity, turbid water caused by
the outflow of the Mississippi River during
flood stage stresses the Flower Garden
reefs. Our studies show that the low salinity, turbid water never impinges on the
reefs. It is a thin surface layer with surface
salinities rarely lower than 31.5 ppt and
salinities at 15 m depth no less than 35.5
ppt. Any suspended sediment in the low
salinity surface mixed layer will be retained
in it because of the strong density stratification in the water column. Surface currents are primarily wind driven. Bottom
currents are primarily tidal, but are complicated by inertial oscillations and shelf
waves. As the currents pass over irregularities on the bottom, turbulent flow is created in the Bottom Boundary Layer (BBL)
and fine sediment may be resuspended from
the bottom to form a nepheloid layer at
the base of the water column. Where no
fine sediment is available for resuspension,
the turbulence in the BBL may move sand
grains in a traction mode along the bottom
but no nepheloid layer can develop. The
thickness of the nepheloid layer over
muddy bottoms rarely exceeds 20 m.
DISCUSSION
Classification and characterization of
banks
After extensive surveys of the reefs and
banks of the northwest Gulf of Mexico,
UcGvzWetal. (1982) and Rezak e* a/. (1983,
1985) developed both geological and biological classifications for the banks. The
geological categorization involved three
criteria. First was the nature of the structural expression of the feature. The communities on the banks off East Texas and
Louisiana developed on diapiric structures. The banks off South Texas developed on relatively undisturbed strata and
are considered drowned coralgal reefs (i.e.,
primarily composed of coralline algae and
coral) on a relict carbonate shelf (Fig. 2;
see Berryhill et al., 1976; Bright and Rezak,
1976). The second criterion was the nature
of the structure underlying the bank. The
bank substructures are either those normally associated with salt diapirism or are
inherited from early Jurassic or Triassic
tectonic features. Finally, the substrate may
either be composed of bedded Mesozoic
and/or Cenozoic sandstone, siltstone, claystone, or basalt, or may be a carbonate cap
that conceals the original bedrock.
On the basis of these geologic criteria,
Rezak et al. (1985) identified three cate-
N
97°
96"
95
0°
TEXAS
9°
5
M
N
San Antonio Ba
28°
'
Applebaum
"9 h l
Ge'ver- E l v e r s
Phleger
Sweet
WEST FLOWER GARDEN
EAST FLOWER GARDEN
Salt Diapir Area
50
; ;o(Ieam^ Reljct
'
carbonate
25
Shelf
0
50
100
KILOMETRES
\ J Blacklist!
127°
CONTOURS IN METRES
• BANK LOCATIONS
Small Adam^ ^Mysterious
1
97°
95°
94°
93°
92°
91°
FIG. 2. Map showing locations of topographic features examined during this study.
90°
89°
REEF AND BANKS
27
gories of banks in the northwest Gulf of were classified within four more general
Mexico: mid-shelf bedrock banks, outer categories based on the degree of reefshelf bedrock banks with carbonate reef building and primary productivity:
caps, and reefs growing on a relict carbonate shelf. The first two categories are com- 1. Zones of Major Reef-Building Activity
posed of banks off East Texas and Louiand Primary Production.
siana. The third category consists of the
I. Diploria-Montastrea-Porites Zone:
banks off South Texas (Fig. 2). The midThis zone consists of living, high
shelf bedrock banks rise from depths of 80
diversity coral reefs. Hermatypic
m or less and have relief of 4 to 50 m.
corals dominate. The corals
These include 32 Fathom, Stetson, Clayinclude Diploria strigosa, Montaspile, Coffee Lump, Sonnier, and Fishnet
trea annularis, M. cavernosa, Porites
(Fig. 2). All are associated with salt domes
astreoides, P. furcata, Colpophyllia
and are outcrops of relatively bare, bedded
natans, C. amaranthus, Siderastrea
Tertiary limestones, sandstones, claysiderea, Madracis decactis, Stephanstones, and siltstones.
ocoenia asperula, Agaricia agaricites,
A. fragilis, Helioseris cucullata,
The outer shelf bedrock banks with carMussa angulosa, Scolymia cubensis,
bonate reef caps are located on complex
Paracyathus sp., and Millepora alcidiapiric structures. They rise from shelf
cornis. Coralline algae are abundepths of 80 to 300 m and crest at depths
dant. Leafy algae are limited.
as shallow as 15 m to over 100 m. Eighteen
Other species common in this zone
such banks are shown on Figure 2. These
are shown in Figure 3. Interestbanks are all east of a line drawn from Matingly, acroporid corals and shalagorda Bay to the shelf break and are
low water alcyonarians (gorgonabeyond the 30 m isobath. (Fishnet is the
ceans) are absent.
only bank beyond the 30 m isobath that is
not considered an outer shelf carbonate
II. Madracis Zone and Leafy Algae
bank; MacNeil and 29 Fathom Banks were
Zone: The Madracis Zone is domnot surveyed extensively enough for defininated by the small branching coral
itive classification.)
Madracis mirabilis, which produces
large amounts of carbonate sediThe banks off South Texas (west of the
ment. In places, large (possibly
line drawn from Matagorda Bay to the shelf
ephemeral) populations of leafy
break in Fig. 2) are not associated with salt
algae dominate the Madracis rubdiapirism. They are dead coralgal reefs that
ble substrate (Leafy Algae Zone).
were apparently living near the Late PleisIII. Stephanocoenia-Millepora Zone:
tocene to Early Holocene shoreline (based
This is a zone consisting of living,
on radio-carbon dating, approximately
low diversity coral reefs. Herma18,000 to 10,580 years bp). They rise from
typic corals dominate. The species
depths of 50 to 80 m and have relief up to
known to occur are Stephanocoenia
20 m. Surficial sediments on the banks are
michelini, Millepora alcicornis, Agarcomposed of fine-grained silt, mediumicia spp., Siderastrea siderea, Porites
grained clayey silt, coarse-grained shelly
astreoides, Colpophyllia spp., Diplomud, and very coarse-grained carbonate
ria strigosa, Montastrea annularis,
rubble on a carbonate substrate. CarbonM. cavernosa, Mussa angulosa, and
ate patch reefs 1.5 to 2 m high occur on
Scolymia sp. Coralline algae are
some banks.
abundant. Leafy algae are limited.
The biological categorization of the
Other species occurring in this
banks by Rezak et al. (1985) involved first,
zone are shown in Figure 3
the recognition of a number of distinct
(labeled "Low Diversity Coral
benthic biotic zones characteristic of these
Reef). A variation of this zone
banks, and second, the depth range of each
at 18 Fathom Bank and has
occurs
biotic zone on each bank. Seven characbeen designated the Stephanoteristic biotic zones were identified which
R. REZAK. ET AL.
Based on observations mad* from the
Texai AAM Oceanography Department
research submersible DIAPHUS
EAST FLOWER GARDEN
BANK
bbl*
CoarM carbons!* cand
Medium carbonat* u n d
Fin* Mdlm.nt
Leafy alga*
Manta blrottrit
Cmllypongla
Hotoctntni*
flfc Branching spong*
Nmcflbulmrtm
barracuda
Myef*rop*rea
Eplntpbetut
PamntMa* lureltmr
Live harmalyptc corala
Condyfact/f
Aparicild coral
Madncl*
Sponaylut emertcanuj
Uopropom*
Cintropyg*
PrfacantMdat
Matecanlhut plumltri
burrow
Sarfola dumwili
Chamlodon ••dMitarfua
Mnchryturua
<$Dl
Eupomtcentrvt
Narefaala rriponaria
Olatfm*
CfrP«att«r
Crlnold
canrp*chanu*
milanurvm
•va** Thstaiaoma
<CSa Ctoptlcut paml
pulchtltut
Pattarnad burrowa
? r Brtnaaaap
FIG. 3. Conspicuous biota and biotic zones on the southeast flank of the East Flower Garden Bank. High
diversity coral reefs = Diploria-Montastrea-Porites Zone. Low diversity coral reefs = Stephanocoenia-Millepora
Zone. This is also representative of the West Flower Garden Bank. From Rezak et al. (1983).
coenia-Montastrea-Agaricia Zone
due to the abundance of the latter
two genera and the limited abundance of the fire coral, Millepora.
IV. Algal-Sponge Zone: This is a zone
dominated by crustose coralline
algae actively producing large
quantities of carbonate substrate,
including algal nodules. The zone
extends downward, past the depth
at which algal nodules diminish in
abundance, to the greatest depth
at which coralline algal crusts are
known to cover a substantial portion of the substrate. This is the
largest of the reef-building zones
in terms of area of sea bottom. For
example, the East and West Flower
Garden Banks together contain
approximately 19.2 km2 (1,920
hectares or 4,744 acres) of AlgalSponge Zone and only 1.83 km2
(183 hectares or 454 acres) of high
diversity coral reefs. That is, the
Algal-Sponge Zones cover over 10
times the area of the high diversity
reefs. Corals reported from the
Algal-Sponge Zone include Agaricia agaricites, Helioseris cucullata,
Madracis mirabilis, M. cf. formosa, M. myriaster, Montastrea cavernosa, Millepora alcicornis, and possibly Agaricia fragilis. Leafy algae
are very abundant. Other organisms of this zone are shown in Figure 3.
2. Zone of Minor Reef-Building Activity.
V. Millepora -Sponge Zone: In this
zone, crusts of the hydrozoan coral
Millepora share the tops of silt-
29
REEF AND BANKS
SONNIER
BANK
Based on observations mod* from the
Teias A S M
research submersible DIAPHUS
^~
^*f
w
1
steeply dipping ridges
siltstone chips
patterned burrows
siltstone outcrops
rock boring pelecypods
encrusting coralline olgoe
4fi) saucer-shoped Agoriciid
^ ^
^
^QB\
«**
«/
Milleporo
(Fir* coral)
Stephonocoenio
(coral)
Spirobronchm
(feather duster worm)
Cirripolhes
(Thorny coral)
, Antipothea
(Thorny coral)
Comatulid
(crinoid)
^ Spondylus omericonus
(American thorny oyster)
4 t Diodemo
(Black urchin)
r
^
f
(sea cucumber)
Panulirui argus
(Spiny lobster)
rg, Holoconthus tricolor
^ - (Rack beauty)
s-m Holoconthus bermudensis
^ ^ (Blue angelfith)
f^T Holoconthus ciliaris
<
S-fi-(Que#n angelfish)
• g r Pomoconthus poni
^ ® (French angslfish)
_ ^ Tholossomo bifosciotum
(Bluehead)
^,r Holocentrus
^
(Squirrelfish)
^^ Chromis multilineotus
(Brown chromis)
Chfomis sp.
(chromis)
Clepticus parrai
(Cr«ol« wrattt)
Choetodon Mdentarius
(RMf butterflyfish)
Conthioottf rosfrpto
(Sharpnose puffer)
Mycteroperco
0
(grouper)
Poronthias furcifer
(Creolefish)
Lutjonus campechanus
(Red snapper)
Rhomboplites aurorubens
(Vermilion snapper)
Bodionus pulchellus
(Spotfin hogfish)
Bodionus rufus
(Spanish hogfish)
y Epinephelus odscsnsionis
(Rock hind)
log k m us (Hovering goby)
plume-likt hydroid
FIG. 4. Biota of Sonnier Bank. From Rezak et al. (1983).
stone, claystone, or sandstone outcrops with sponges and other epifauna. Isolated scleractinian coral
heads may be present, but are rare.
Coralline algae are rare. Species
occurring in this zone are shown
in Figure 4.
Transitional Zones (reef-building activity may range from minor to negligible).
VI. Antipatharian Zone: Limited
crusts of coralline algae and several species of coral exist within a
zone typified by sizeable populations of antipatharian corals
(mostly the bedspring-shaped Cirrhipathes). Other conspicuous
members of this zone are shown
in Figure 5. Banks supporting
Algal-Sponge Zones (see above
and Fig. 3) generally possess something comparable to an Antipatharian Zone as a "transition"
between the Algal-Sponge Zone
and the deeper, turbid-water,
Nepheloid Zone of the lower bank.
4. Zone of No Reef-Building Activity.
VII. Nepheloid Zone: In this zone high
turbidity, sedimentation, resuspension of sediments, and resedi-
OS
o
Based
the
an observations made
Texas A S M
Department
from
Oceanography
research
submersible
DIAPHUS
SOUTHERN BANK
JO
w
N
Partsrntd burrows
tncnistmg
coralllns algas
Limsstons
block
Lim.itc
,
\
3
V ircinia
(
X-^"
Plum.-llm bydroM
AntlpothdrlcRB
Ucyonorions
MycHroptrco
Liopropomo
5«rlola dnm.rlli
Lutjonut comptchonm
Rhombopllut ourorubwt
Chmtodon ndtntarlm
'
^
ts
•
Agorlclld
Spondylus omsrlcoi
Astsrold stortlsh
Dlodsma
Argyrothsco borrsttlono
FIG. 5. Biota of Southern Bank. From Rezak et al. (1983).
Chromli
•nchryiurm
Bodlonm pulchtllut
I * Oup-raaf fish "A*
* > FISH "B"
* ^ Burrowing trsh "C"
REEF AND BANKS
mentation dominate. Rocks and
drowned reefs here are generally
covered with veneers of fine sediment. Epifauna are depauperate
and variable; deep-water octocorals and solitary stony corals are
often conspicuous (some species
are shown in Figs. 3, 4, and 5).
This zone occurs in some form on
lower parts of all banks below the
depths of the Antipatharian or
Transitional Zones.
31
water temperature, salinity, river runoff,
turbidity, sedimentation, currents, and
seasonal variation in the last six. The distributions of biotic zones in relation to these
factors are summarized in Figure 6.
Conditions at the shelf edge near and
beyond the 80 m isobath on the broad
North Texas-Louisiana shelf west of about
91°W longitude are favorable to the development of tropical reef communities. Currents come primarily from the southwest
and are oceanic. These currents carry larvae, spores, and juveniles from the tropical
Depth ranges for each of these zones, waters of the southern Gulf of Mexico, and,
where they exist, are given for each bank possibly, the Caribbean.
in Table 1. No single bank contains all the
Coastal water masses, which are highly
zones described, though the Flower Gar- influenced by outflow from the Mississippi
den Banks contain all but the Millepora- and other rivers in Louisiana and North
Sponge Zone (see Fig. 3). Furthermore, in Texas, are held onshore and shunted west
the northwest Gulf of Mexico, only the most of the year. As a result, turbidity
Flower Garden Banks contain high diver- beyond the 80 m isobath is usually nil
sity coral reefs. (Diploria-Montastrea-Porites (except in the bottom boundary layer), and
Zone). Lower diversity reefs (Stephano- salinity averages 36 ppt.
coenia-Millepora Zone) are present at the For most of the year, near-surface water
Flower Gardens and at 18 Fathom and temperatures throughout the Gulf are
Bright Banks.
tropical to subtropical. Near shore in the
Three of the six mid-shelf bedrock banks northern Gulf, however, temperatures
peak at depths shallow enough to contain become more temperate from December
the Millepora-Sponge Zone (Claypile, Son- through March (Etter and Cochrane, 1975;
nier, and Stetson; see Fig. 4 for example). Temple et al, 1977). During the coldest
The shallowest zone on the other mid-shelf months (January and February) temperabanks is the Antipatharian Zone at approx- tures grade from below 10°C in the estuaries to 18°C on the outer shelf.
imately 52-73 m.
Onshore-offshore seasonal movements
The shallowest portions of all the South
Texas relict carbonate banks (56-70 m) of the 18 and 16°C surface isotherms probare occupied by assemblages comparable ably have a significant influence on the disto those of the Antipatharian Zone found tribution of tropical reef biota in the northat similar depths on some mid-shelf bed- western Gulf. The minimum temperature
rock banks and somewhat deeper on outer for vigorous growth of coral reefs is 18°C
shelf banks. The lower depth of this zone (see Stoddart, 1969). A temperature of
is demarked by the presence of the neph- 16°C is stressful for most reef-building coreloid layer (generally about 70 m around als. In the northwestern Gulf, surface water
the South Texas banks; see example in Fig. seaward of the 30 to 80 m depth contour
remains above 18°C year round. Surface
5).
water seaward of the 20 to 40 m depth
Environmental controls
contour remains above 16°C. The mixed
Environmental factors that can be cor- {i.e., isothermal) layer during winter is 50related with and probably control regional 75 m thick, with temperatures only 1 to
patterns of community structure, distri- 3°C lower at 100 m. Thus above 50 m depth
bution, abundance, and zonation of trop- off North Texas and Louisiana and genical epibenthos in the northwestern Gulf erally seaward of the 80 m bottom depth
of Mexico are distance from shore, sub- contour, salinities are high and temperastrate type, bottom depth, bank relief, tures range annually from approximately
32
R. REZAK ET AL.
TABLE 1.
Depth ranges (in meters) of biotic zones on the continental shelf hard banks in the northwestern Gulf of
Mexico.
Biotic Zones
Milltpora-
Banks
Claypile (MS)*
Sonnier(MS)
Stetson (MS)
Small Adam (ST)
Big Adam (ST)
North Hospital (ST)
Aransas (ST)
Baker (ST)
Blackfish (ST)
Hospital Rock (ST)
Mysterious (ST)
Southern (ST)
Dream (ST)
South Baker (ST)
32 Fathom (MS)
Coffee Lump (MS)
Fishnet (MS)
Alderdice (OS)
Ewing (OS)
Bouma (OS)
Parker (OS)
Sackett (OS)
East Flower Garden
Sponge
Madracis
Stephanocoenia
Algal-Sponge
Antipatharian
transitional Nepheloid
40-45
18-52
20-52
45 +
52 +
52 +
60?
60?
P"
P'
58-70
57-70
56-70
70 +
70+
70 +
P"
70 +
P*
70+
70 +
70 +
60?
59-70
70?
58-70
62-70
59-70
15-36
(OS)
Applebaum (OS)
Bright (OS)
West Flower Garden
(OS)
Diaphus (OS)
18 Fathom (OS)
28 Fathom (OS)
Jakkula (OS)
Rezak-Sidner (OS)
Sweet (OS)
Elvers (OS)
Geyer (OS)
Phleger (OS)
DiploriaMontastreaPorites
28-46
20-36
37-52
36-52
52?
P«
55-67
56-72
60-75
60-82
67-82"
62-68
66-73
67-82
72-80
75-84
82-?
65-85
68 +
73 +
82 +
80 +
84 +
46-82
82-86
86 +
76?
P"
P*
P'
37
52-74
74-?
36-50
46-88
45-47
45-82
52-92
59-90
55-93
75-80+
60-97
60-98
88-89
73-98
82-?
92-100
90-98
93-100
P*
97-123?
98-123?
?
P*
85 +
89 +
98 +
P'
100 +
98 +
100 +
P'
123 +
123 +
122+c
Soft bottom
50 +
60 +
62-64+
64+
66 +
68-70+
70-72+
70-74+
70-74+
70-74+
74-86+
80 +
80+
80-84+
55 +
70+
78 +
84-90+
85-100 +
90-100+
100 +
100 +
100-120 +
100-120 +
110 +
110-130 +
110-130 +
110-130 +
100-140+
120-140 +
120-150 +
130-200 +
180 +
190-210 +
200 +
* Letters in parentheses following bank names indicate geologic category. ST = south Texas relict carbonate
reefs. MS = mid-shelf bedrock banks. OS = outer shelf carbonate-capped banks.
* = zone present, but depth range uncertain.
b
Weakly represented, stressed.
c
Clear water, but biota typical of nepheloid zone.
18 to 30°C (Fig. 6). Wherever suitable hard
substrate exists above about 95 m depth in
the absence of chronically turbid water,
conditions on this part of the shelf are
favorable to the growth of tropical reef
communities dominated by corals or coralline algae, or both.
The degree of light penetration into clear
surface waters and the antagonistic effects
of turbidity in bottom nepheloid layers are
probably the factors that control depth
ranges of biological communities on the
various shelf-edge banks. High turbidity
decreases light penetration and is inimical
to the development of coral and algal reef
communities. Sedimentation associated
with high turbidity causes smothering of
encrusting epibenthos. In the northwestern Gulf, because of the enormous amount
of sediment entering from rivers, turbidity
REEF AND BANKS
Substantial Neritic Influence - Periodic
- High Turbidity and Reduced Salinity and Temperature
River
Runoff
Winter Temperatures
Often Less Than 16° C
Legend
Millepora Sponge Zone
OUJ Coral Reefs
Algal-Sponge Zone
• • Antipatharian Zone
I I Nepheloid Zone
113 Nepheloid Layer (high turbidity and
sedimentation)
Representative Banks
Claypile
Sonnier
Coffee Lump
Southern
Alderdice
East Flower Garden
Jakkula
Geyer
• Light Penetration to Approximately 1% Level
Winter Temperatures
16° to 18° C
-Oceanic WaterTemperatures Rarely
Below 18° C
Temperature Less
Than 16°C
FIG. 6. Distribution of biotic zones relative to conditions of temperature, salinity, turbidity, and light at
selected banks. From Rezak el al. (1983).
and sedimentation are major factors limiting the development of tropical reef
assemblages. It is speculated that reef
development on Sackett Bank is seriously
attenuated (the shallowest zone is the AlgalSponge Zone), due in part to increased turbidity in surface waters from admixed outflow from the nearby Mississippi River (see
Fig. 2). This influence, accompanied by
somewhat reduced salinities, diminishes
westward but may extend as far as Diaphus
Bank (91°W) during periods of particularly
high runoff.
The effects of bottom nepheloid layers
and associated sedimentation are certainly
more pronounced on the mid-shelf banks
than at the shelf-edge banks. Most or all
of the mid-shelf banks may frequently be
totally covered by the nepheloid layer. At
the Flower Garden Banks a substantial
nepheloid layer shallower than about 80 m
has not been observed and the water is
usually fairly clear. Fine, terrestrial sediments are not found on shelf-edge carbonate banks above the lower limit of their
Algal-Sponge Zones. The mid-shelf banks,
however, are generally coated with thin to
thick layers of fine sediment, presumably
derived from nepheloid layers.
Mid-shelf banks rise from surrounding
depths of 60 to 80 m. Their tops, which
support Antipatharian Zone type assemblages between 56 and 73 m, exist in a
depth range which on most shelf-edge carbonate banks is occupied by diverse, clearwater Algal-Sponge Zones. The lack of
Algal-Sponge Zones on the mid-shelf banks
and the occurrence instead of Antipatharian assemblages found typically in deeper
water at the shelf edge is probably due
largely to higher turbidity.
Relief above the surrounding bottom is
of considerable importance in alleviating
the negative impacts of bottom nepheloid
34
R. REZAK ET AL.
layers and attendant sedimentation on may, however, be fairly well isolated from
development of epibenthos. An Algal- the effects of bottom nepheloid layers due
Sponge Zone probably will not become to their relief (40 and 42 m, respectively)
established on banks that have less than above the surrounding mud bottom. The
about 15 m relief above the surrounding other mid-shelf claystone-siltstone bank,
mud. Farther offshore, at the East Flower Claypile, with a crest depth of 40 m and
Garden, the Algal-Sponge Zone extends only 10 m relief, is often covered by the
downward to 82 m, about 18 m above the nepheloid layer. Consequently, the abunsurrounding soft bottom. In even deeper dance of dominant epibenthos is least at
water, where surrounding depths are more Claypile (lowest relief) and greatest at Sonthan 180 m (Geyer and Elvers banks), the nier (highest relief).
vertical extent of the Algal-Sponge Zone
Speculation on environmental factors
is not limited by bottom nepheloid layers governing the development of the Millebecause of the high relief. Here the zone pora-Sponge Zone on the mid-shelf banks
extends down to more than 95 m depth is complicated by the fact that this zone
and is probably limited primarily by the also occurs on shelf-edge bedrock outcrops
degree of light penetration from above.
that protrude from the crest of Geyer Bank
Thus, there is a gradual increase in the between 37 and 52 m depth. The implimaximum depth of expression of coralline cation is that the development of this zone
algae-dominated communities and in the is dependent on the presence of exposed
depth of the Antipatharian Zones with bedrock outcrops; and vigorous developincreasing surrounding depth. These ment of the biota is favored by clear water
observations imply that the bottom neph- and winter minimum temperatures above
eloid layers are of great importance as eco- 16°C.
Why tropical coral reefs have not devellogical limiting factors on the lower 15 to
oped on the claystone-siltstone outcrops on
20 m of the banks.
The deeper clear-water reefal commu- Geyer Bank, which are exposed to the same
nities (Algal-Sponge Zones) are therefore oceanic conditions as the coral reefs of the
probably excluded from the mid-shelf car- Flower Garden Banks, is not known. Hypobonate banks by high turbidity, reduced thetically, the claystone, which is soft and
light, and high sedimentation and are lim- disintegrates readily on exposure to water,
ited in downward extent on some of the may be unsuitable substrate for most reefshelf-edge banks by the effects of nephe- building corals. Indeed, the epifauna that
loid layers. In their place is a less diverse inhabit the Millepora-Sponge Zone obviand less abundant Antipatharian Zone ously prefer the thin beds of rock-hard siltassemblage made up of epibenthic forms stone that protrude from the softer but
which, though basically tropical in origin, more extensive claystone on these outcrops.
are tolerant of the stress imposed.
Coral reefs would not be expected on
ACKNOWLEDGMENTS
the South Texas carbonate banks because
the depth of the crests is too great (more
Partially supported through Instituthan 52 m). Hard substrate does exist within tional Grant NA85AA-D-SG128 to Texas
suitable depth for coral reef development A&M University by the National Oceanic
on some mid-shelf claystone-siltstone banks and Atmospheric Administration's Office
rising from surrounding depths of 50 to of Sea Grant, Department of Commerce.
62 m and extending upward to 18 m (SonREFERENCES
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