Serge Thomas
Central Theme Of My Research:
A central theme of much of my research is to determine ecosystem level consequences of natural and anthropogenic stresses in aquatic ecosystems. I work primarily with algal communities in shallow hydrosystems, linking ecosystem structure to physico-chemical variation by understanding functional processes at the base of the food web. My research has combined descriptive and experimental approaches to determine causal relationships between biotic and environmental variation.
Ph.D. at the French Institut de Recherche pour le Developpement (IRD, France, the Ivory Coast):
My Ph.D. focused on primary producer dynamics in a tropical shallow reservoir in the Ivory Coast, West Africa. My research took place during a severe drought that drove the hydrosystem to switch abruptly from mesotrophy to eutrophy. During this shift, I was able to show that the hydrosystem oscillated between two stable states: a clear mesotrophic state with clear water dominated by benthic producers and a turbid eutrophic state dominated by phytoplankton. The shift could be explained by a combination of competition for changing quantities of limited resources (both light and nutrients) and also allelopathy (telemediated allochemical reducing the growth of a competing autotroph). These results, published in Freshwater Biology, confirmed that alternating stable states, described in lakes in the northern hemisphere, can also occur in tropical regions.
Research Associate at the Southeast Environmental Research Center (SERC), Periphyton Group, Florida International University, Miami, Fl, USA:
My first experience at SERC has mainly focused on periphyton metabolism and taxonomy. Periphyton is an important primary producer in the Everglades, being at the base of a diverse food web, influencing (at times controlling) soil and water quality and also interacting with plants to effect total primary production. It is functionally and taxonomically influenced by environmental change, particularly alterations in hydrology and nutrients that are at the heart of Everglades restoration plans. My research role in the periphyton laboratory at FIU has been to focus on functional linkages in periphyton mats to both hydrology and nutrients.
My research allowed me to understand how periphyton influences water quality but also can be used in phytoremediation settings to remove the excess of phosphorus from the water (Periphyton-based Stormwater Treatments Areas). In particular, I have shown that mechanically harvesting periphyton mats at appropriate intervals dramatically increases phosphorus retention (Lake and Reservoir Management, 2002). In coupling this field experiment to a smaller-scale laboratory manipulation, I found that water must be retained for 3-days subsequent to re-flooding in order to avoid a nutrient flush (Aquatic Botany, 2006). In this paper, I also demonstrated that periphyton is likely responsible for the low phosphorus levels in the short-hydroperiod Everglades and that annual drying/reflooding is required to maintain phosphorus retention in benthic periphyton mats.
In 2004, I worked with Drs. Evelyn Gaiser and Mike Ross on periphyton response to hydroperiod in Everglades marl prairie wetlands. I was coordinating a large-scale survey (over 800 sites) to determine interactions among plants and periphyton that structure habitat used by the endangered Cape Sable Seaside Sparrow. In conjunction with this project, I initiated two experiments. The first examined how fire (common in the marl prairie) affects periphyton structure and function (Wetlands, about to be submitted). The second determined the effect of plant canopy cover on recovery of desiccated periphyton after re-flooding (Hydrobiologia, in press). Another related side project I have recently begun with Dr. Miro Gantar at FIU is testing the xenobiotic and allelopathic effects on photosynthesis of periphytic algae using Pulse Amplitude Modulated fluorometry (Phyto-Pam apparatus, Walz, Germany; Aquatic Botany, submitted). These experiments should contribute to our understanding of functional linkages between plants and periphyton in the Everglades and other shallow wetlands.
Current research at the Microbial Ecology Laboratory (MEL):
Currently, I am working at MEL on the phytoplankton in Florida bay. MEL has accumulated over the past 4 years a tremendous amount of marine phytoplankton data generated by the Phyto-Pam. The Phyto-Pam is a tetra Pulse Amplitude Modulation (PAM) fluorometer, which gives, for the three main algal groups (green, blue-green and brown/dinoflagellates), the chlorophyll concentration in the sample as well as a measure of the photosynthesis efficiency at various irradiances (rapid light curves, c.f. Hall, Thomas & Gaiser, book chapter in press). This is performed in a matter of minutes!
However, the data have arbitrary units and thus are not publishable in peer reviewed journals. I am therefore working on the inter-calibration of the Phyto-Pam with other methods such as Winkler, 14C, 13C incubations (Photosynthesis-Irradiance curves) and pigment analyses (fluorimetric and HPLC).
As I write, I am isolating the dominant algae of Florida bay to record their reference spectrum. So far, I have isolated about 20 species of diatoms, as well as 5 species of green algae and 10 species of cyanophytes. I use the dinoflagellate cultures from Dr. Kelly Rein at FIU (over 50!) to fulfill my needs. Recording the reference spectra of the dominant species in the water is indeed required to deconvolute right the fluorescence signal and get a good differentiation between the 3 algal groups mentioned above.
I am also working on getting the Absorbtion cross section of the photosystem II (PSII) of the algae present in my samples to get more or less directly photosynthesis measurements by the Phyto-Pam in carbon units. For this, I use a Variant Cary-50 spectrophotometer which will be ideally equipped with an integrating sphere to capture the forward light scattering as the absorption of particles in seawater is performed on a GF/F filter.
Future research perspectives:
In the future, I hope to continue studying how transitional events in the environment influence the primary producers (phytoplankton, periphyton, macrophytes) and how these producers, in turn, contribute to ecosystem stability. In particular, I am looking forward to applying theories on alternative stable states developed from my dissertation research to various shallow hydrosystems. Over the long term, the better understanding of the influence of primary producer structure on aquatic ecosystem function will contribute to rehabilitate impacted hydrosystems to their (closest) original state.
Selected publications in peer reviewed journals:
Published or in press
1- Thomas S., Gaiser E.E., Tobias F.A. (2006) Effects of shading on calcareous
benthic periphyton in a shorthydroperiod oligotrophic alkaline wetland
(Everglades, FL, U.S.A). Hydrobiologia, xx, 113.
2- Thomas S., Gaiser E.E., Gantar M., Scinto L.J. (2006) Quantifying the
responses of calcareous periphyton crusts to rehydration: a microcosm study
(Florida Everglades). Aquatic Botany, 84, 317-323.
3- Hall R.O., Thomas S., Gaiser E.E. (in press) Measuring primary production and
respiration in freshwater ecosystems. In Principles and Standards for Measuring
Net Primary Production in Long-Term Ecological Studies. T.J. Fahey and A. K.
Knapp, editors. Oxford University Press.
4- Thomas S., Gaiser E.E., Gantar M., Pinowska A., Scinto L.J., Jones R.D.
(2002) Growth of calcareous epilithic mats in the margin of the natural and
polluted hydrosystems: phosphorus removal implications in the C-111 basin,
Florida Everglades, USA. Lake and Reservoir Management, 18, 324330.
5- Arfi R., Bouvy M., Cecchi P., Pagano M., Saint-Jean L., Thomas S. (2001)
Factors limiting phytoplankton productivity in 49 shallow reservoirs of North
Côte d'Ivoire (West Africa). Aquatic Ecosystem Health and Management, 4,
123-138.
6- Havens K.E., Hauxwell J., Tyler A.C., Thomas S., McGlathery K.J, Cebrian J.,
Valiela-I., Steinman A.D., Hwang S.J. (2001) Complex interactions between
autotrophs in shallow marine and freshwater ecosystems: implications for
community responses to nutrient stress. Environmental Pollution, 113, 95-107.
7- Aka M., Pagano M., Saint-Jean L., Arfi R., Bouvy M., Cecchi P., Corbin D.,
Thomas S. (2000) Zooplankton variability in 49 shallow tropical reservoirs of
Ivory Coast (West Africa). International Review of Hydrobiology, 85, 491504.
8- Thomas S., Cecchi P., Corbin D., Lemoalle J. (2000) The different primary
producers in a small African tropical reservoir during a drought: temporal
changes and interactions. Freshwater Biology, 45, 4356.
9- Bouvy M., Arfi R., Cecchi P., Corbin D., Pagano M., Saint Jean L., Thomas S.
(1998) Trophic coupling between bacterial and phytoplanktonic compartments in
shallow tropical reservoirs (Ivory Coast, West Africa). Aquatic Microbial
Ecology, 15, 25-37.
Submitted
1- Gantar M., Berry J.P., Thomas S., Wong M., Rein K., Gawley R.E. (submitted)
Allelopathic activity among cyanobacteria and microalgae isolated from Florida
freshwater habitats. Aquatic Boatany MS# AQBOT409
2- Thomas S. & Gaiser E.E. Effects of fire on periphyton function in Everglades
wetlands. (About to be submitted) Wetlands
3- Thomas S. & Gaiser E.E. Phosphorus effect on periphyton in South Florida. (To
be submitted by April ’06). Ecotoxicology.
In preparation
1- Thomas S., Cecchi P., Corbin D., Lemoalle J. Vertical distribution of
attached algae as a function of optical depth.
2- Thomas S., Cecchi P., Corbin D., Lemoalle J. Environmental factors and
phytoplankton community changes in a tropical reservoir (L. Brobo, West Africa).