MEASURING ENVIRONMENTAL CONDITIONS IN PAST AND PRESENT SEAGRASS ECOSYSTEMS THROUGH STABLE ISOTOPE ANALYSIS OF SIRENIAN SKELETAL REMAINS

Seagrasses are important primary producers in coastal areas, providing food and shelter for consumers in shallow, oligotrophic waters. The fossil record for seagrasses extends into the Late Cretaceous, but information on this important coastal ecosystem is limited by the poor preservation potential of seagrass material. Sirenians (manatees and dugongs) are the only group of mammals to consume seagrasses on a regular basis and their presence within fossil deposits is often cited as evidence of the presence of seagrasses. Skeletal remains of sirenians can provide additional insight into seagrass ecosystems through stable isotope analysis. The carbon and nitrogen isotopic composition of herbivore tissues reflects that of the plants they consume, which in turn, are affected by environmental conditions experienced during growth. Examination of the stable isotopic composition of bone from modern and fossil sirenians could provide a valuable tool for monitoring the health and productivity of seagrass ecosystems in the past and present. Here, we examine the efficacy of this approach by analyzing bone collagen from populations of two extant species of sirenians (dugong, Dugong dugon; manatee, Trichechus manatus) at three locations (Torres Strait, Australia; Red Sea, Israel; and Indian River Lagoon, FL, USA).

Bone collagen for dugongs from the Red Sea and Torres Strait showed similar δ¹⁵N values (Red Sea: 4.1 ± 0.4‰, Torres Strait: 3.9 ± 0.8‰) and δ¹³C values (Red Sea: -5.0 ± 1.1‰, Torres Strait: -6.5 ± 0.9‰), suggesting comparable growing conditions and nutrient availability for seagrass species at these sites. In contrast, bone collagen data for manatees sampled from a single site in Florida showed much greater variation. Nitrogen isotope values were higher (6.4 ± 1.1‰) and δ¹³C values lower (-9.0 ± 1.4‰) than values measured for both populations of dugongs. Though dietary flexibility may account for some of these differences, these contrasting isotopic values of seagrass consumers may also be symptomatic of major differences in environmental quality at these sites. Further work with historical and fossil remains of sirenians in these areas could provide necessary baseline environmental data to evaluate how the condition of seagrass ecosystems at these three locations has changed over time.