that isolated survivor Thalassia short-shoots have the ability to initiate new lateral 
growth. However, the rate of shoot initiation (average 2.5 new short-shoots) was quite 
low during the study period. The occurrence of flowering short-shoots and seedlings 
was very patchy and no flowering short-shoots or seedlings were observed at the two 
basins most affected by the die-back. Halodule wrightii (shoal grass) was present in 
52-78% of the quadrats in die-back patches in the two most severely affected basins, 
and in 28% of the quadrats in the least affected basin. These observations indicate that 
the rapid vegetative spread of Halodule may outstrip Thalassia in the initial recovery 
process. Seedling bioassays of toxicity and pathogenicity of chemical and biological 
system elements were conducted in March 1990. Seedlings growing in peat pellets 
received one of nine experimental treatments - raw or autoclaved: seawater from a 
die-back site, dieback sediments, Thalassia leaves with lesions, and dying Thalassia 
rhizomes. The last treatment was inoculation with the slime mold Labyrinthula, which 
had been isolated from lesions on Thalassia leaves from die-back sites in Florida Bay. 
No acute toxicity was noted for water, sediment, or plant material from the die-back 
site. All of the seedlings inoculated with Labyrinthula developed necrotic lesions within 
one week, however the spread of the lesions was quite slow and none of the seedlings 
died during the coarse of the experiment. Field experiments were conducted to test the 
role of environmental stressors in the die-back process. Short-term anoxia caused 
dramatic reductions in rhizome oxygen concentrations and increased carbon dioxide 
concentrations. Dead short-shoot densities were highest and live densities lowest, in 
the glucose addition treatment. Iron addition lowered porewater sulfide levels, but did 
not significantly affect short-shoot survival. Etiological studies of a die-back episode 
suggest that elevated porewater sulfide and rhizome ethanol concentrations precede the 
appearance of necrotic Labyrinthula lesions on Thalassia leaf blades by two months. A 
conceptual model of interacting causes of seagrass mortality suggests that (1) 
environmental stresses weaken Thalassia and make it vulnerable to infection, and (2) 
the proximal cause of death is probably by the pathogenic slime mold Labyrinthula. 
1989 - 1990 
Fourqurean, J. W., R. D. Jones, and J. C. Zieman (1993) Processes influencing water 
column nutrient characteristics and phosphorus limitation of phytoplankton biomass in 
Florida Bay, FL, USA: interferences from spatial distributions. Est. Coastal Shelf Sci. . 
36(3):295-3 1 4. 
The concentrations of nutrients, dissolved and particulate organic matter, salinity and 
chlorophyll-a in the water column were measured over the period of June 1989 to 
August 1990 at a network of 26 sampling locations across Florida Bay. Florida Bay was 
hypersaline during this time period, with an average salinity of 41.4 %o. Dissolved 
organic phosphorus was the dominant form of P in the water column, while soluble 
reactive P was generally less than 5% of the total P. Organic nitrogen forms dominated 
the N pool, and NH 4 + was the dominant form of dissolved inorganic nitrogen. Many of 
the measured parameters were correlated. Principal Components Analysis extracted 
three composite variables that described 90.3% of the variation in the original data 
set. PC, was highly correlated with total organic N, total N, total organic C and salinity. 
PC,| was correlated with all measures of P and chlorophyll-a. PC,,, was correlated with 
measures of inorganic N. The spatial distribution of factor scores for these principal 
components indicates three processes acting independently to control the composition of 
the water column of Florida Bay: the evaporation-driven concentration of dissolved 
material in Florida Bay, the delivery of P to the Bay through water exchange with the 
Gulf of Mexico; and the delivery of freshwater with an excess of N with respect to P to 
the Bay. The phytoplankton biomass in the water column of Florida Bay was shown to be 
P-limited. 
336 
