Volume 4, Number 1, Spring 1996
Epibenthic invertebrates and
fishes of the northwestern Gulf of Mexico
Micro-phytoplankton in the Equatorial
Isotopic constraints on cycling
of dissolved organic carbon in the ocean
"Bringing to light"
anchialine cave ecology
John W. Pohlman
The organisms that live in, on, or attached to the seafloor are called the benthos. The relationship linking benthic organisms into the epibenthic community is a common food source, derived directly or indirectly on the bottom. Therefore, these assemblages include demersal fishes that swim near the seabed and feed from it.
The data used in this study were collected from October 1988 through July 1990 on the continental shelf of the northwestern Gulf of Mexico. Fish and invertebrate samples were obtained during five cruises aboard the Texas A&M University oceanographic research vessel, Gyre. Station positions were determined when the trawl was deployed on the bottom of the contin-ental shelf.
The epibenthic organisms were collected with a nine-meter otter trawl during the first four cruises and a five-meter otter trawl during the fifth cruise. A total of 34 trawl samples from 34 stations were taken at speeds of 2.8 to 5.6 kilometers per hour with trawling periods lasting between 14 and 40 minutes at depths ranging from 13 to 400 meters.
This study presents a systematic, quantitative measurement of epibenthos structure in the northwestern Gulf of Mexico. In the past, the lack of studies in this part of the gulf has hindered attempts to assess changes in commun-ity structure that result from natural and artificial events. The main objective of this study is to contribute to our knowledge of the biology and commun-ity structure of the epibenthic assemblages (invertebrates and fishes) of the continental shelf of the northwestern Gulf of Mexico. This involves not only species identification but also understanding of population densities and group biomass, as well as diversity and similarity of species composition.
Most of the fish and invertebrates collected by the otter trawls were initially sorted on the ship and fixed in 10% buffered formaldehyde prior to their preservation in an ethanol solution. This prevents misidentification due to changes such as decoloration. The samples were stored in buckets for later sorting. Later, the organisms were washed in tap water to remove the formalin and blotted dry with paper towels for a wet "live" weight. All samples were eventually checked for their identifications, counted, and weighed to determine the biomass for each species. Following the second laboratory sorting and identification, the samples were transferred to 70% ethyl alcohol solution for preservation and storage.
In this study the average weight of the offshore trawl samples was not statistically higher than the average weight of the nearshore samples. Furthermore, the mean of the total biomass was almost the same for both regions. Despite the high diversities in most of the communities, small or large, the species in these communities were different. This suggests that in the northwestern gulf there are no continuous recurrent assemblages in time or space on the shelf.
I found no correlations between community structure and season, depth, or temperature within the seasonal and spatial scales of this study. In addition, the epibenthic assemblages of the continental shelf of the northwestern Gulf of Mexico do not experience hypoxic conditions. Finally, the northwestern Gulf of Mexico investigation showed a relatively high diversity compared with the northeastern gulf and the U.S. North Atlantic shelves. However, the northwestern gulf was less diverse than the Campeche Bank, the Red Sea, the Arabian Sea and the Arabian Gulf. Therefore, the diversity of the northwestern gulf lies between that of stable tropical shelves and those at temperate latitudes.
Portions of the six most dominant invertebrate species (top) and fish species (bottom) in their respective assemblages.
Return to top of page
Carbon dioxide (CO2) is the most prevalent type of man-made pollution. Ocean phytoplankton absorb CO2 during normal life processes and sink to the ocean floor when they die, where they are gradually buried under accumulating sediment. This is one of the most important ways by which oceans remove CO2 from the atmosphere. To assess the effect of this we need to know how much phytoplankton lives in the ocean.
My thesis project was designed to measure the amount of phytoplankton in the equatorial Pacific Ocean, and assess their variability and characteristics. Sizes of marine phytoplankton usually range from one micron to one millimeter. The group I studied consists of species larger than 20 microns, called micro-phytoplankton.
My samples were taken from thirteen locations along 140°W from 12°N to 12°S. When the research vessel arrived at a designated sample location, researchers dropped a water sampler into the ocean. The sampler has multiple containers which collect water at different depths. My samples were collected at eight depths from three to 200 meters below the surface, preserved, and shipped to our lab at Texas A&M University.
In the lab, we left the sample still for about one day to let the micro-phytoplankton settle to the bottom of the container. Then I used a microscope to count and measure the phytoplankton present. From the number and sizes we can estimate the biomass of the phytoplankton, often expressed as milligrams of carbon per liter of seawater.
I found three major groups of phytoplankton: dinoflagellates, diatoms, and coccolithophorids. Dinoflagellates have two flagella, and make up about 55% of the micro-phytoplankton cells in our samples. Both diatoms and cocco-lithophorids cells each contributed nearly 20%.
Most phytoplankton are found near the surface because they need light to grow. In the equatorial Pacific deeper than 100 meters the light is too low for phytoplankton. In surface water, however, there is usually a shortage of plant nutrients. There is normally a layer of maximum phytoplankton biomass, called the near-surface maximum, where phyto-plankton find optimal amounts of both light and nutrients. In this study the near-surface-maximum layer was between three and sixty meters deep. In the near-surface maximum at each station we found 1400-4500 cells per liter. Converted to biomass this equals 0.4-1.4 micrograms of carbon per liter. At 200-meter depths, the micro-phytoplankton cell numbers were as low as 92 cells per liter.
Micro-phytoplankton biomass in equatorial areas is lower than in coastal areas and higher than in each hemisphere's central gyre. The cell numbers we found during an El Niño event are about one order of magnitude lower than those during non-El Niño conditions in the same area. We concluded that this was the result of El Niño. The low phytoplankton biomass was accompanied by a high number of species. At each station we found 81 to 137 species, although two to seven dominant species contributed more than 50% of the cell numbers.
We know little about ocean phytoplankton compared to our knowledge of their terrestrial counterparts, plants. This study provided a better understanding of the phyto-plankton in the equatorial Pacific area, especially during an El Niño event.
Latitudinal distribution of micro-phytoplankton cell numbers.
Micro-phytoplankton biomass distribution. Observation points are marked with white dots.
Return to top of page
Carbon is one of the most abundant elements in the universe and is the basis for the existence of life on Earth. Dissolved organic carbon (DOC) is one of the largest organic carbon reservoirs and is a key component in the interplay between the biosphere, hydrosphere, and geosphere. Knowledge of the cycling of DOC in the ocean is important not only for the understanding of the biogeochemistry of a variety of elements, but also for the global carbon cycle and thus climate changes of human concern.
My dissertation research contains both laboratory and field experiments. Laboratory experiments were required to develop a reliable and accurate procedure for measuring low concentrations of oceanic DOC and for sampling marine colloids (including macromolecules and microparticles), or colloidal organic carbon (COC). Field studies were conducted in the Gulf of Mexico off Texas and the Middle Atlantic Bight off Cape Hatteras, as well as in estuarine waters of Galveston Bay and Chesapeake Bay. Objectives of the field studies were to investigate the abundance, distribution, and fluxes of DOC in both oceanic and estuarine environments, examine the molecular weight distribution of DOC, and gain a better understanding of the cycling of dissolved and colloidal organic carbon in the ocean using a multiple-tracer approach. In addition, I studied the proportions of thorium (Th) isotopes in dissolved, colloidal, and particulate phases, and the interaction of thorium isotopes with COC in order to better use them as tracers for the cycling of organic carbon in the ocean.
I found that a considerable portion of traditionally defined "dissolved" organic carbon is in a colloidal form (1 kilo-Dalton0.2 microns), from about 60% of the bulk DOC in estuarine waters to about 30-40% in oceanic waters. Radiocarbon measurements showed that high-molecular-weight COC contains contemporary 14C ages which are considerably younger than the apparent 14C ages of the bulk DOC.
This research provides direct evidence that bulk DOC is a mixture of different components with varying molecular weights and apparent 14C ages. High-molecular-weight COC is more reactive than low-molecular-weight DOC and thus is a active component in the marine carbon cycle and the biogeochemistry of other trace elements in the ocean. Three types of colloids with different origins were identified in both Gulf of Mexico and Middle Atlantic Bight waters: estuarine colloids, offshore surface water (pelagic) colloids, and deep water colloids. Sources of COC are distinguished by their radiocarbon signatures and concentrations of carbon and nitrogen.
Using secondary ion mass spectrometry, the first detailed profiles of 230Th and 232Th in the Gulf of Mexico were measured. Results showed that the distribution of 234Th was similar to that of organic carbon among dissolved, colloidal, and particulate phases. Residence times of macromolecular COC calculated from 234Th measurements were consistently short (1-60 days) regardless of apparent 14C ages, indicating that high-molecular-weight colloids are turning over more rapidly than the bulk DOC pool.
Values of distribution coefficients of thorium were negatively correlated with those of particle or colloidal concentrations in seawater not only for the long-lived thorium isotope but also for short-lived thorium isotope. The significance of my research lies mainly in providing an improved understanding of fluxes, turnover times, and sources of DOC in the ocean.
My research was supported by the National Science Foundation, the Department of Energy and the Texas Institute of Oceanography. My advisor, Dr. Peter Santschi, and other advisory committee members, Drs. John Morse, Luis Cifuentes, Ethan Grossman, and Bruce Herbert guided me through this project.
Radiocarbon signatures and relative concentrations of carbon and nitrogen differentiate colloids that come from estuaries, offshore surface waters, and deep waters.
Return to top of page
John W. Pohlman
One of the best kept secrets of the Yucatan Peninsula concerns the countless, clear pools called "cenotes" that speckle the tropical landscape. Cenotes are surface lesions formed by collapse of the ceilings of the labyrinthian, submerged cave systems that meander through the Pleistocene bedrock of the Yucatan. Aside from providing direct access to groundwater, cenotes serve as gateways into the world's most extensive underwater cave systems. Caves that connect to the cenotes are technically called anchialine (ank-ee-AY-leen) caves, which means they are influenced by intruding coastal seawater. My research provides the first description of ecological and biogeochemical processes that drive the anchialine ecosystem.
Locals refer to the caves as "underground rivers" because they drain fresh water from the riverless Yucatan Peninsula, but "underground estuary" more accurately describes the caves due to the mixture of fresh and sea waters found within them. Primary passages are oriented along the dramatic and aggressive boundary between fresh and salt water, called a halocline, that corroded the bedrock to form the caves. Many of the caves are lavishly decorated by stalagtites and stalagmites that formed during a low sea level stand over 18,000 years ago.
A unique group of organisms, cave-limited troglobites, is found in the caves. Troglobitic bodies emphasize economy over specialty. They have essentially reversed evolution to remove unneccessary body parts and functions. Simplified yet specialized adaptations include loss of color and eyes, the presence of long, spiny appendages, and reduced size. At present, nineteen species of Crustacea and two species of fish are known to permanently inhabit anchialine caves in the Mexican state of Quintana Roo.
As part of my research, trained cave-divers collected fauna and all possible types of organic and inorganic carbon and nitrogen from the caves for stable-isotope and quantitative analyses. The results, in conjunction with data generated from samples collected in the cenote pool and forest, allowed me to describe the biogeochemical cycling of carbon and nitrogen through the anchialine system, define the hierarchy of the food web, identify several unique ecological phenomena and present evidence for the occurrence of chemosynthesis (production of organic material using reduced chemicals rather than light for energy) by nitrifying bacteria.
Organic matter needed to support the two trophic levels of the troglobitic community appears to come from three sources, two external and one internal. External sources of organic matter are the cenote pool and forest soil, while the internal source is chemosynthetic production occurring along the halocline. An equally fascinating feature is that different species of crustaceans occupying the lower trophic level, although they inhabit the same niche, specialize in procuring organic matter from different sources. This phenomenon is known as niche partitioning and had not been previously reported for any cave community, dry or aquatic.
Cenotes had tremendous spiritual and cultural significance for the ancient Mayan civilizations and maintain their importance today as economic resources and primary water supplies for many Mexican citizens. The welfare of the people and the growing tourism industry in the Yucatan Peninsula depend upon the sanctity of this environment. With the knowledge we now have about the biogeochemical operation of anchialine systems, we can better assess how to protect these environmentally sensitive resources in the future.
Surface cenote and associated cave leading to the coastline.
Typhlatya mitchelli. One of the troglobitic crustaceans found in the Mexican anchaline caves.
Return to top of page
Comments to: firstname.lastname@example.org
Last updated February 24, 1997