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Hunter, J. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications

DIATOMS AS ENVIRONMENTAL INDICATORS: A CASE STUDY IN THE BIOLUMINESCENT BAYS OF VIEQUES, PUERTO RICO

JENNA M. HUNTER Beloit College Advisors: Tim Ku; Anna Martini; Carl Mendelson

INTRODUCTION

Diatoms, microscopic, unicellular, eukaryotic algae abundant in most aquatic habitats, are useful proxies for the ecological analysis of three bays on the island of Vieques, Puerto Rico. Acutely sensitive to changes in pH, salinity, temperature, hydrodynamic conditions, and nutrient concentrations, marine diatoms can be identified by their distinct assemblages and frustule shape. The ubiquitous distribution of diatoms, their high species diversity, and their siliceous frustule all enable the diatoms to function as sound environmental indicators. Samples were taken from ten of twenty-seven extruded cores within the three bays, Bahía Tapón (BT), Puerto Ferro (PF), and Puerto Mosquito (PM), and then investigated for the presence and abundance of mid- and late- Holocene marine diatoms. Puerto Mosquito (PM) is renowned for its high content of the bioluminescent dinoflagellate Pyrodinium bahamense. Core sites were chosen to represent a series of environmental conditions such as proximity to the mouth of the bay, depth, and presence of mangrove or marsh material. The open-ocean environment of the three bays, their mid-latitude location, and the limited anthropogenic impact provide favorable conditions for paleoclimate and environmental reconstructions. The classical method to infer environmental conditions involves the use of one or multiple indices, such as the Indices of Biotic Integrity (IBI) (Stevenson and Pan, 1999), Metzmeir’s Diatom Bioassessment Index, and the Practical Diatom

Index (IDP). Although slightly different in taxonomic specificity, all indices are similar in that they yield a numerical value that is constrained by both a minimum and a maximum value. The IDP, as suggested and utilized by Levêque Prygiel in 1996, provided the most straightforward guide during the analysis of diatoms in this study. The paleoecological value of the diatoms has also been well demonstrated by Koizumi (1975). Unfortunately, diatom assessment is challenging due to the developing nature of a formal taxonomy and nomenclature.

Diatoms (Bacillariophyta) are markedly distinguishable into two orders, the Centrales and the Pennales. The Centrales, or centric diatoms, have a radial symmetry and are successful as plankton in marine waters. Their frustules, or shells, can also be triangular or quadrate. The centric diatoms are mostly planktonic and non-motile (Tappan, 1980). The Pennales, or pennate diatoms, occupy and dominate the freshwater, soil, and epiphytic environments. Although they also thrive in marine habitats, their typical environmental niche is in fresh water. The Pennales have bilateral symmetry (Armstrong and Brasier, 2005).

There is a good record of the diatoms from the middle Cretaceous to the Cenozoic, with an evolutionary shift from centric to pennate diatoms in the late Miocene (Tappan, 1980). The evolutionary divergence occurred just after the Miocene, preceded by a surge in the expansion of centric diatoms. Pennate diatoms are regarded as still being in their explosive

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