Long term observations in benthic communities of tropical seas:
An interdisciplinary approach in the IAS initiative
Elva Escobar Briones
escobri@mar.icmyl.unam.mx
Unidad Académica en Sistemas Oceanográficos y Costeros
Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México
A.P. 70-305 Ciudad Universitaria, 04510 México, D.F.

Abstract.
Tropical seas provide excellent naturallaboratories for studies of the deep-sea benthic
communities
subjectedtooligotrophic
conditions that are altered by the hydrodynamic
processes, mesoscale features and basin scale
climatic changes. Since heterotrophic life in theaphotic benthic zone of the sea is based on theexport of primary production that takes place inthe euphotic pelagic zone, transport mechanismsthat couple the primary production zone withsecondary production habitats are of interestwithin
theinterdisciplinary
approach.
Information has been gathered in the Gulf ofMexico, a regional sea within the Intra Americas
Sea (IAS), for more than a decade. Results fromthese observations show that the deep sea thatwas long regarded as a homogeneousenvironment, weakly influenced by processes at
its surface has a large variability and is a
dynamic environment pulsed mainly byprocesses at the surface. Inter-disciplinary
registration of key events at selected sites
investigating the variability in time and space
are of utmost importance. Long term and largescale investigation are important and efforts areneeded to carry out a large number ofexperiments and measurements over longer
periods of time.

Introduction.
The
extremeoligotrophythatcharacterises the Gulf of Mexico, related tostratification of the euphotic layer has rated thebasin among the most oligotrophic of the world
oceans. From this standpoint the deep sea can be
regarded as a homogeneous environment,
weakly influenced by processes at its surface.
Studies in similar impoverished tropical ocean

areas have revealed significant correlation
between deep-sea infaunal abundances and thosefactors in the water column indicating foodavailability (Tselepides & Eleftheriou, 1992).
The time-varying fluxes of carbon andassociated biogenic elements in the ocean andthe related exchanges with the sea floor arelacking in the Gulf of Mexico, howevercharacteristics of the upper and intermediatewaters restrict the organic matter supply to thedeep- sea benthos. The limited export ofbiogenic carbon to deep may be expressed interms of higher benthic species richness(Escobar et al., 1999). Similar high species
richness recorded in tropical deep-sea
environments have been attributed to trophic
specialization of the benthic fauna to limitedorganic matter content in sediment (Pasternak etal., 1975).
At present, we have a broad appreciationof the nature of the types of benthic
communities we will encounter in tropical deepseas (Sanders, 1969; Pequegnat, 1983), and a
sketchy knowledge of the linkage between thearrival of organic matter to deep. We know very
little about the processes mediating theircommunity structure. We have empirical models
describing the expected species richness in the
deep-sea, but we are far from understanding themechanisms governing diversity and biomass
distribution in the tropical deep-seas. We haverecognised the importance of some key issues,
such as the role of perturbation caused by nonpredictable meteorological phenomena, such as
hurricanes, winter fronts or El Niño teleconnection effect.
In formulating a work plan for the Gulfof Mexico benthic studies major objectives wereadopted. The first objective was to investigate
the distribution of benthic invertebrates (micro-,meio-, macro and megafauna) in the southern
Gulf of Mexico. The second objective was toimprove the knowledge of the distribution ofbenthic infauna of and its relation to foodavailability. The third objective was to achieve amore quantitative understanding of processes in
the upper water column that contribute to theexport of biogenic carbon in the deep Gulf ofMexico. This abstract presents data on thevariability in time and space of the deep-sea
benthic infauna (meio- and macrofauna) of thesouthern Gulf of Mexico.

Methods.
The strategy adopted involved a seriesof cruises (R/V Justo Sierra) that wereconducted in the decade of the 1990's in thesouthern Gulf of Mexico (Fig. 1). The sediments
and benthic samples of the top 10 cm layer indifferent sectors and depth zones of the Gulf of
Mexico have been recovered with a 0.25 USNEL box corer deployed on replicate basis ateach station.
30200 m28

2000 m1000 m3000 m26

Gulf of Mexico

2422Yucatan
20

1898

96

94

92

9088Fig. 1 Area of study and distribution of the samplingstations.

Macrofauna were sieved (125 µm to 1mm) and fixed onboard, meiofauna were fixedonboard. Both groups were sorted and identified

to the lowest taxonomic level possible in thelaboratory. Biomass has been expressed, asaverage values, in units of gC.m² by station ateach cruise. Sediment and water column factors
were recorded at each sampling station. Data
were analysed in a comparative format with theexisting data from the northern Gulf of Mexico
according to the depth zones described by
Pequegnat (1983): transition zone (300 to 450m),archibenthic (460 to 950 m), upper abyssal(975 to 2250 m), mesoabyssal (2275 to 3200 m
and lower abyssal (3225 to 3850 m). Statisticalanalyses were carried out using a linear models
nested ANOVA procedure, with terms for depthzones, the season (stratified vs mixed
conditions) and year at which each cruise wascarried out on replicated measures.

Results.
Meiofauna. Average biomass values
ranged from 0.0004 to 0.096gC.m². The largest
average values were recorded in the lower
abyssal zone (0.050 gC.m², n=80). The
transition and the upper abyssal zones werecharacterised by the lowest average biomass
values (0.002 gC.m², 0.003 gC.m²; n= 20, 60
respectively). The archibenthic zone (0.005gC.m², n= 35) showed values that were twotimes higher than those recorded in the transition
and upper abyssal zones. The mesoabyssal zone
showed the second largest biomass averagevalues(0.036 gC.m², n=10).
Seasonaldifferences (Fig. 2) and between years (1997 and1999; Fig. 3) were recorded in mesoabyssal
biomass average values.
Macrofauna. Average biomass values
ranged from 0.002 to 0.37 gC.m². The largest
average values were recorded in the lower
abyssal zone (0.21 gC.m², n=106). The
mesoabyssal zone was characterised by thelowest average biomass values (0.019 gC.m², n=78). The transition (0.060 gC.m², n= 73) and thearchibenthic zones (0.078 gC.m², n=150)
showed similar values that were half as low as
those recorded in the upper abyssal zone (0.119gC.m², n=194). The Campeche Bay attained thelargest biomass values followed by theCampeche Escarpment; the lowest values havebeen recorded off the Mexican Ridges.

The average biomass values showedsignificant between year differences on theupper abyssal, mesoabyssal and lower abyssalzones during the stratified condition (Fig. 5).

Categorized Plot for Variable: BIOMASS
18
14
1062BIOMASS-2

OG16 SIG.1 SIG.2

OG16 SIG.1 SIG.2

OG16 SIG.1 SIG.2ZONE:
ARQ

ZONE:
LOA

ZONE:
MAB

b. Uppera. All18

Categorized Plot for Variable: BIOMASS

14Categorized Plot for Variable: BIOMASS
0,280,260,22100,220,186-2

OG16 SIG.1 SIG.2

OG16 SIG.1 SIG.2

ZONE:
TRA

ZONE:
UPA

±Std. Dev.
±Std. Err.
Mean

0,16BIOMASSBIOMASS

0,1420,100,060,02-0,06094

95979899±Std. Dev.
±Std. Err.
Mean-0,02c. Mesoabyssal

Fig. 2 Seasonal variability of the meiofaunal biomass(gC.m²) in the deep Gulf of Mexico.

9798±Std. Dev.
±Std. Err.
Mean99d. Lower
YEAR

YEAR

CRUISE

0,100,04-0,02

Categorized Plot for Variable: BIOMASS

Categorized Plot for Variable: BIOMASS
0,055

0,080,070,0450,060,035

Categorized Plot for Variable: BIOMASS

0,04
0,030,005

0,00

98

99

±Std. Dev.
±Std. Err.
Mean-0,005097

98

99±Std. Dev.
±Std. Err.
MeanYEAR

Fig. 5 Between years variability of the meiofaunalbiomass (gC.m²) in the deep Gulf of Mexico.

2BIOMASS

97

YEAR

897

9997

9997

ZONE:
LOA

ZONE:
ARQ99
ZONE:
MAB

20148297

99

97±Std. Dev.
±Std. Err.
Mean99ZONE:UPAZONE:
TRA

YEAR

Fig. 3 Between years variability of the meiofaunalbiomass (gC.m²) in the deep Gulf of Mexico.

Seasonal differences were recorded in the years
1993, 1995 and 1997: ANOVA F 3,15; p= 0.02
with highest values occurring throughout themonths with mixing of the water column (Fig. 4).
Categorized Plot for Variable: VAR3
0.22
0.18
0.14
0.10
0.06
0.02-0.02
Mixed

VAR30,015

0,0114-40,025

0,0220-4BIOMASSBIOMASS

0,05

Stratifi

Mixed

Stratifi

VAR2: 95

VAR2: 93
0.22
0.18
0.14
0.10
0.06
0.02

±Std. Dev.

-0.02
Mixed

±Std. Err.

Stratifi

Mean

VAR2: 97

VAR1Fig. 4 Seasonal variability of the macrofaunalbiomass (gC.m²) in the deep Gulf of Mexico.

The three abyssal zones showed similarbiomass values in the years 1997 and 1998, asignificant decrease in biomass values wasobserved for the two zones in 1999 thatremained equally low in the year 2000 (Fig. 5).
In general the biomass values recorded in 1994were higher than in 1995 and then in 1997 incontrast to 1998 and 1999.

Discussion.
The variability observed in the infaunalbiomass between years and in the spatial scale isin part attributed to be a response to food supply.
In open ocean areas the offshore deep-sea
communities rely on the export of biogenic
carbon as the major mechanisms that drives a
net removal of carbon to deep (Tselepides &Eleftheriou, 1992). In the Gulf of Mexico wehave only a vague idea of how hydrodynamic
processes contribute to the redistribution oforganic matter below the thermocline on thescale of the size of the basin (Rowe & Pariente,1992; Escobar et al., 1999). Part of this variation
is attributed to the presence of the minimum
oxygen layer in the transition, archibenthic andupper abyssal zones, generated by pulses ofsedimentation of organic matter (Pequegnat,1983). The minimum oxygen layer has been
observed following phytoplankton blooms that
may impart a seasonal signal from the euphotic
zone into the deep-sea (Thiel, 1989; Lochte,1992). Events in the scale of Niño may suggest
large inputs of fresh biogenic carbon to deep that
may lead to elevated oxygen consumption in thetransition, arquibenthic and upper abyssal zones
and have seasonal or yearly effects in themacrobenthic biomass distribution. Similareffects have been observed in Peruvian benthic
assemblages (Tarazona et al., 1988).
The transport of this phytodetritus andits state of degradation will depend highly on thesinking rates which vary widely in the tropicalseas ranging from a few meters per day up toover 500 m.d-1 (Madin, 1982). The deep Gulf of
Mexico attains higher temperatures than thetropical western Atlantic at similar depth, theremineralization of labile organic matter may behigher in the deep Gulf of Mexico (Pasternak etal., 1975). Degradation rates of algae in the deepsea have been suggested to be half as low asfrom shallow water sediments (Lochte, 1992).
Traces of phytodetritus and macroalgae(Sargassum) have been found in different box
core samples (Thiel, 1978; Tselepides &Eleftheriou, 1992). The freshness of thephytodetritus material more than the quantity ofthe organic matter may be of greater importancein the tropical seas to enhance a response of thebenthic biomass. Our results show so far that thetropical deep sea, which was long regarded as ahomogeneous environment, weakly influenced
by processes at its surface has a large variability
and is a dynamic environment pulsed mainly byprocesses at the surface. Meso-scale features
may play a relevant role, additional to otherenvironmental factors in the sediment and deepwater masses, in the distribution of the benthic
biomass in the deep Gulf of Mexico. Interdisciplinary registration of key events in thetropical seas investigating the variability in timeand space are of utmost importance in the longterm.
Acknowledgments. The invaluable assistance ofthe captain, crews, pilots and scientific participants ofcruises of the R/V Justo Sierra. Support for thisresearch was provided in different steps by DGAPA
UNAM projects IN 211200, 217298, 213197 andCONACyT grants 400356-5-050PÑ-1297, G-27777B,
0004V-T.

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