Historical seagrass abundance of Florida Bay, USA, based on a foraminiferal proxy

Abstract

A foraminiferal proxy for seagrass abundance was developed, tested and used to construct a record of seagrass variability for Florida Bay, USA, since its initial flooding ~ 3800 cal years BP. Of 76 species recorded in the Bay, 13 seagrass-associated foraminifera were identified based on previously observed occurrences in seagrass beds of the region. Species that occurred more in seagrass beds than in other sediments were designated seagrass-associated foraminifera (SAF). SAF from six cores taken in four areas were tested against a large, well-studied seagrass die-off that began in 1987. SAF percentages of total foraminiferal assemblages declined in 1986 (210Pb age, ± 2 years) with no recovery as of 2001 (the youngest core age), and discriminant analysis showed that SAF assemblages in 1970–1985 and 1986–2001 differed significantly. Results agree with observations of the decline in seagrass coverage, supporting SAF as a seagrass proxy for investigating earlier assemblages preserved in the cores. After anthropogenic influence began ~ 1880, the largest overall decreases in SAF percentages occurred in 1934–1940 (210Pb ages, ± 5 years), possibly from decreased salinities due to three intense hurricanes that hit South Florida, and in 1987–2002 during the seagrass die-off. SAF assemblages in the oldest core record were significantly different over the four intervals of (1) initial flooding ~ 3800 cal years BP, (2) pristine conditions until ~ 1880 when agriculture and canal construction began, (3) anthropogenic and climatic influences 1880–1986, and (4) the 1987–2002 seagrass die-off. Historically, SAF assemblages indicate that this die-off resulted in extremely low coverage (comparable to inundation and development of the benthic ecosystem > 2500 cal years BP), probably a result of cumulative anthropogenic disturbance combined with natural events. This is the first study using foraminifera as a proxy to reconstruct the seagrass history of an area. The research introduced a new approach for developing a seagrass proxy: we identified species that are more abundant in seagrass beds than in other sediments, and tested them together as a proxy against a previously documented seagrass die-off.