Biota-hydrology interactions during the Holocene in Florida

Abstract

As the concentration of atmospheric carbon dioxide (CO2) as well as global temperatures continue to increase, it is expected that sea level will rise and more extreme precipitation events will occur in the future. Moreover, warmer sea surface temperature may lead to an intensification of tropical storm development. In addition, plants respond to rising CO2 by reducing their transpiration rates, which in turn affects climate and the hydrological cycle. Particularly in densely populated low-lying coastal regions like Florida such changes can have severe economical and ecological consequences. Investigating how landscapes and the hydrological cycle was affected by past changes in climate and sea level allows for a better prediction of future changes. Florida’s estuarine and wetland environments are highly sensitive to small changes in water depth, -chemistry and hydroperiod, and their deposits present excellent archives of past hydrological variability. Multiple palynological, micropaleontological and paleobotanical proxies were applied to describe how changes in sea level and precipitation patterns have affected the landscape during the Holocene, to investigate natural variability in tropical storm activity in this region, and to determine the strength and duration of the plant response to CO2. The current Tampa Bay and Charlotte Harbor estuaries were gradually flooded around 8 ka (kilo y BP) by the rising sea level, changing former freshwater environments to initially lagoonal and later marine environments. During the late Holocene, water salinity increases and a transition from seasonally stratified to a mixed water column is inferred from algal assemblages. Superimposed on this trend, peaking runoff is inferred from increased terrestrial input and stratification indicator dinocyst abundance around 5 ka and 2.5 ka. These runoff phases possibly reflect increased precipitation, related to regionally higher sea surface temperatures (SSTs) and enhanced El Niño-Southern Oscillation activity. However, runoff could also be modified by increased water retaining capacity due to peat and soil development inland, as a consequence of the reduced hydrological gradient as sea level rises. Both trends in precipitation and sea level rise have likely led to the state-wide transition from dry oak to wetter pine dominated vegetation. Lithological, palynological and geochemical evidence from these estuaries suggests tropical storms were regionally more frequent between 6.4-5.5 ka, 5.0-4.0 ka and 3.2-1.9 ka. This variability is likely determined by both increased SSTs and shifts in the position of the Bermuda High. Shifts in vegetation and diatom assemblages during the late Holocene in an elevated central Florida wetland suggests shifts to wetter conditions around ~2.5 and 1.2 ka. These trends are possibly related to regionally warmer SSTs, whereas environmental changes during the 20th century are related to human efforts to protect the site from wildfires. Measurements on leaf fragments from Florida angiosperm, conifer and a fern species, covering the 100 ppm CO2 rise of the past 150 years, show a distinct reduction in maximal leaf conductance of on average 34%. Models predict this plant response will likely continue to beyond double current CO2, which will result in a 50% reduction of canopy transpiration, potentially altering the hydrological cycle and climate.