Notes from Seagrass Research Team Meeting held on November 16-17, 2000.

CURRENT IDEAS RE: FLORIDA BAY DIE-OFF CONCEPTUAL MODELS

1. List of things we agree on regarding die-off:

a. Die-off in Florida Bay is species specific - Thalassia.

b. Occurs in dense beds.

c. Starts in late summer-fall. (PC)

d. Occurs in continentally influenced regions (Jay Zieman (JZ)). Reduced circulation, reduced flushing.

e. PC feels sediment porosity is an important component; Die-off more prevalent in regions with high sediment porosity. May simply be a correlative factor. Fine carbonate sediments with lots of water.

f. High organic matter in sediment??? We know it is correlated with die-off areas - but it may not be a causative factor. Labile vs. refractory - not total organic matter.

g. Shorter day lengths.

h. Appears to be a threshold level of sulfide required to kill Thalassia.

NOTE: Danish die-off associated with reduced light from eutrophication. Affects Zostera. Does not occur in patches.

NOTE: Upper part of Barnes Key shoots have long internode distances. Reflects rapid growth. Could be a response to sedimentation events. Could be a response to density - self-shading. PC Thinks these shoots have less structural carbon; longer conductive pathway.

2. Testable Hypotheses:

a) Sediment

b) Density

c) Disease

JZ - Equally dense beds elsewhere do not exhibit die-off. Why? Reduced flushing in die-off areas. PC - Much lower porosity, larger grain size - holds sulfide levels down.

PC - Initial rounds of die-off - JKB, RL and RKB were all affected. They had different initial density/biomass levels. Is there a threshold level of density/biomass?

Do we need to get some marine microbiologists involved?

BIG QUESTION REMAINS - WHY FLORIDA BAY?

Unique circulation- central basins isolated rainfall and evaporation most important

Continental influence

Other areas have densities as high

Other areas have carbonate sediments

Lakes region has banks and basins but has open boundaries

3.  Epidemiology 101

1) What we think

2) What we know

3) What we can prove

Carlson & Durako Model- details of what we now know

Hurricanes

Georges main effects were to remove the litter layer and remove the older blades from SS. Also sediments were resuspended (longer internodes of Barnes Key SS suggests recent sediment deposition- meristems still relatively deep)

Halophila in JKB was dislodged and distribution reduced - following spring Halophila found in RAN, WHP, TWN and obs in RKB - storm may have increased distribution

Irene - JKB FHAP before and after - Tt cover reduce, He distribution reduced

Die-off @ Barnes raises questions regarding the role of salinity

Low salinity inhibits Laby

Tt salinity tolerance skewed rapid drop of at high (50-60ppt) salinities

Temperature records indicate higher than long-term averages ???

(Assumes Q10 of photo lower than for respiration) - high temp reduces C gain

Turkey Pt & Anclote Key research? - (Behrens and Barber, Aquat Bot))Shows Tt near upper temp limit (SG in Chesapeake have twice the thermal range - upper is similar to tropics but lower limit much lower)

Fourq & Z 1991 Model

Z (1975) Biscayne Bay T&S curves - 28 C optimal Temp response skewed

H2S a known plant toxin

[H2S] > 2-4 mM in die-off sites, cause or effect?

[H2S] < 2 mM no die-off observed

High porosity sediments correlate with high [H2S]- Low Permeability

Labyrinthula infection reduces oxygen production - may increase susceptibility to H2S

Barnes Key surface sediments very high porosity (lots of water and fine seds)

Erskine & Koch (2000) suggest tolerant to up to 6mM - short-term exposure, don't think it's a primary cause

Question - short-term vs long-term exposure to H2S

Meristems @ Barnes go anoxic in die-off sites

meristems @ RBK go hypoxic

Long SS stems @ Barnes - increases diffusion distance

Long internodes may be a response to hypoxia

ADH increases in Tt under hypoxic conditions

Barnes Key over 1200-1500 SS/m2 very high biomass (SC 300 g/m2), 1.2%/ day

Turnover and very high litter layer

RBK - 1200-1300 (SC 109 g/m2) 1.7% /day turnover

Nutrient defficiency (Boron)?? May excacerbate hypoxic stress

Infection reduces photo increases resp - Durako and Kuss (1994)

Not thought to be a primary factor at Barnes Key, comes in secondarily- leaves rotten at base, lesions absent in active patches

May be a primary factor at SC - leaves not rotten at base, lesions at active patches

May be transported by detached leaves

Seasonal pattern model (get from Barb Blakesley) Winter no die-off pathogen inactive. Spring no die-off SS active, pathogen inactive, Summer pathogen active

Older leaves have higher infection levels and are more susceptible to infection (seedling infection and field-collected SS experiments)

Need to look at phenol levels in leaves (sensu Short and Burdick)

Barnes Key SS - meristems go anoxic at night, [H2S] very high - Zieman and the Danes

RBK - SS go hypoxic, but not anoxic and bounce back earlier in the morning

Major factor in Fall - short day length high temp

Labyrinthula infection - no net photosyn, respiration increased

C reserves a resevoir - Laby negatively affects this

Can meristem hypoxia result in death when there exists a carbon reserve

What metabolic products accumulate during hypoxia/anoxia - ethanol accumulation ???

Ethanol may affect membrane characteristics

Carbonate sediments-low FE

Die-off only observed in carbonate sediments

FlaBay carbonate sediments have relatively high Fe levels for carbonate sediments (need numbers)

Spatially Fe high near mainland and high toward west. Atmospheric distribution ?? Role unknown

Addition of Fe to seds decreases flux of sulfide to Tt (Carlson bucket experiments), sig. but small increase in Tt growth

FMRI water color maps

Stumpf AVHRR- data shows an increase seasonally and

WQ monitoring data shows an increase until

Long term vs seasonal patterns

FHAP shows an increase in clarity from 97-99, not much change from 99-2000

Time lag between low light conditions and Tt loss

1984 96% of the bay bottom had SG using small-scale quads (Zieman et al 1989)

84-94 comparisons show changes

FHAP decrease in 0 seagrass cover area from 95-99

Boyer (estuaries paper) chlorophyll time line. Low during initial die-off a spike in 1991 then low then increasing 93-98 then recent declines

Periodic slimes of microalgae assoc w/early

91-93 loads low - Gradient in sp composition coralline more dominant in west high-flow areas

Highest loads in area of highest SG productivity - so epis not reducing producting

@ Barnes loads at lower 5% of Baywide averages

Fleshy epis only occur near bird islands near the keys, restricted distribution

Lynbya very common in Sf beds in the west.

Laurencia - patchy - Has it's overall abundance changed

Holmquist - Laurencia accumulation did not kill underlying Tt

Role of Grazers

Frankovich- snails (50/m2) and hermit crabs conspicuous at RBK . Baywide Bittium 3/m2 (versus IRL 20/Hw short shoot- 1000s/m2)