Meristem anoxia and sulfide intrusion: A mechanism for Thalassia testudinum short shoot mortality in Florida Bay.
Ole Pedersen, Jens Borum, and Tina M. Greve
University of Copenhagen, Hillerød, Denmark
Joseph C. Zieman , and Thomas A. Frankovich
University of Virginia, Charlottesville, VA
James W Fourqurean
Florida International University, Miami, FL
Seagrasses form dense populations in coastal waters in many parts of the world if substrate and light climate allows rooted plant growth. During the last decade, seagrasses of Northern Europe and North America have been observed to die-off during episodic events characterized by a combination of environmental stress factors including high temperature, high salinity, low light flux and/or low water column oxygen. In seagrasses, nighttime belowground aerobic respiration is supported by oxygen diffusing into the leaves from the surrounding water column and subsequently spreading via a well-developed aerenchyma to roots and rhizomes. Diel in situ measurements of meristematic oxygen pressure in Thalassia testudinum beds show that internal nighttime oxygen concentrations are closely coupled to the water column oxygen level rather than the photosynthetic history of the plant.
Hence, if the lower water column is depleted from oxygen due to increased system respiration, anoxia may develop in the roots. This can subsequently lead to sulfide intrusion into the roots where gaseous sulfide may spread throughout the aerenchyma and poison the leaf meristems because sulfide is no longer oxidized to sulfate by root-mediated oxygen diffusing into the near rhizosphere.
As primary seagrass die-off in Florida Bay typically began in the fall, experiments investigating oxygen and sulfide dynamics within Thalassia beds were conducted in November of 2000 and October 2001. The studies took place in healthy Thalassia beds near Porjoe Key and in Rabbit Key Basin, and in very dense beds experiencing primary die-off in southern Twin Key Basin north of Barnes Key. Vertical profiles of oxygen and sulfide were measured with electrodes that profiled from 10 cm in the water column to 70 cm in the sediments. Diel meristematic oxygen and sulfide dynamics were measured using microelectrodes attached to 3-axis micromanipulating micrometers. The microelectrodes were inserted into the basal meristems to within 500 um of the center of the meristem. Power was supplied to the microelectrodes on a small

boat moored immediately adjacent to the study sites. The output was fed to a lab on a houseboat moored on site and data was recorded continuously.
Vertical oxygen and sulfide profiles at Rabbit Key Basin at 21:00 revealed water column oxygen just below saturation. Oxygen declined rapidly within the sediment with hypoxic conditions down to 13 cm and anoxia below that.
Sulfide levels were near zero within the Thalassia root and rhizome layer (0 -
25 cm), but increased to 100 uM at 40-45 cm. In contrast, within the die-off affected beds at Barnes Key, the water-column became anoxic within 7 cm of the sediment surface and the sediments were completely anoxic with sulfide 3 cm below the sediment surface.
At all sites meristematic oxygen pressure showed typical diel patterns. Oxygen pressure declined with decreasing light in the late afternoon and reached minima just before dawn. At sunrise meristematic oxygen pressures rose rapidly reaching atmospheric oxygen pressures during mid to late morning.
During nighttime, basal meristems at the die-off affected sites at Barnes went anoxic for as long as 5 hours. In contrast, the basal meristems at Porjoe Key experienced nighttime hypoxia but did not go anoxic.
Where primary seagrass die-off has occurred in Florida Bay, the seagrasses are characterized by extremely high density and biomass. In addition Florida Bay
T. testudinum commonly has 85-90% of its total biomass in nonphotosynthetic belowground tissue which exerts relatively large respiratory oxygen demands. Further the dense canopy of these beds and the decomposition of the accompanying dense litter layer greatly restricts nighttime replenishment of oxygen from the overlying water column. Florida
Bay die-off events have primarily occurred in late summer and fall when solar irradiances and day lengths are declining. At the same time, water-column oxygen solubility is low and respiration rates are relatively high (both of which are in large part driven by temperature). When die-off was active,
Thalassia short-shoots exhibited rotting meristems with a foul odor. The basal meristems are the most metabolically active portions of the short-shoot, and, in theory, would be the first area damaged by insufficient oxygen. All areas of
Florida Bay that experienced the initial primary seagrass dieoff in the late
1980's and the recent instance of primary seagrass dieoff near Barnes Key showed these common characteristics. These spatial and temporal observations are consistent with a mechanism of oxygen depletion and sulfide intrusion at the basal meristem.

Environmental conditions that produce oxygen depletions (e.g., high salinity, high water temperatures, reduced circulation) may lead to meristem damage and thereby causing the observed die-offs within Thalassia beds.
Zieman, Joseph C., Dept. of Environmental Sciences, University of Virginia,
291 McCormick Rd., PO Box 400123, Charlottesville, VA 22904-4123
Phone: 434-924-0570, FAX 434-982-2137, jcz@virginia.edu
, Question 4
Seagrass