MARINE ECOLOGY PROGRESS SERIES
  Vol. 71: 297-299, 1991                                           Published April 30
                          Mar. Ecol. Prog. S e r .




NOTE


 Mass mortality of the tropical seagrass Thalassia testudinum
           in Florida Bay (USA)
M. B. ~ o b b l e e ' T. R. ~arber',P. R. Carlson, ~ rM. J. ,~ u r a k oJ. W. ~ o u r q u r e a nL. ,K. ~ u e h l s t e i n ~ ,
           ,                 ~        ~,           ~
               D. Porter5, L. A. yarbro2, R. T. zieman3, J. C. zieman3

            ' Everglades National Park, South Florida Research Center, Homestead, Florida 33030. USA
           Florida Marine Research Institute, 100 Eighth Avenue SE. St. Petersburg, Florida 33701. USA
          University of Virginia. Department of Environmental Sciences. Charlottesville, Virginia 22903, USA
          Caribbean Research Institute. University of the Virgin Islands. St. Thomas 00802, U.S. Virgin Islands
               S University of Georgia. Department of Botany. Athens. Georgia 30602, USA




ABSTRACT. This report documents rapid and widespread           Flonda Bay is a shallow, triangular lagoon a t the
mortality of the seagrass Thalassia testudinum Banks ex        southern tip of peninsular Florida bordered on the
Konig (turtle grass) in Florida Bay at the southern tip of the     north by the Florida mainland and on the southeast by
Flonda peninsula (USA). More than 4000 ha of seagrass beds
have been completely lost in recurring episodes of mortality      the Florida Keys; its western margin is open to the Gulf
since summer 1987 An additional 23 000 ha have been          of Mexico. Shallow carbonate mud banks divide the
affected to a lesser degree. Loss of T. testudinurn, the domi-     bay into basins, restrict circulation, and attenuate the
nant rnacrophyte species in thls highly productive system,       Gulf's lunar tidal influence. Freshwater enters the bay
may affect ecosystem function within the bay a s well as
estuarine-dependent sport and commercial fisheries. A
                                    in the northeast from Taylor Slough, a s overflow from
pathogenic protist related to the causal agent of the eelgrass     the C-l l l Canal that is part of the South Florida Water
wasting disease may be involved in the mortality and may        Management District's canal system, a n d as sheet flow
place 7. testudinunl populations outside Florida Bay at risk.     generated by local rainfall. Depending on the timing
Environmental factors and chronic hypoxia of below-ground
                                    and amounts of local rainfall, water conditions in the
T testudinum tissue may also contnbute to the die-off.




                                   FLORIDA BAY   -            Fig. 1. Distribution of
                                                       Thalassia testudinurn die-
                                                       off in Florida Bay. Contin-
                                                       uous double line is Ever-
                                                       glades National Park boun-
                                                       dary. Dashed line encloses
                                                       most known die-off areas.
                                                       Hatched areas are severely
                                                       affected, having lost up to
                                                       95 % of T. testudinum. Dot-
                                                       ted lines mark shallow car-
                                                       bonate mud banks. JKB:
                                                       Johnson Key Basin; RKB:
                                                       Rabbit Key Basin; RNK:
                                                       Rankin Lake; CRO: Cross
                                                       Bank; SUN: Sunset Cove

O Inter-Research/Printed in Germany
298                   Mar. Ecol. Frog Ser. 71: 297-299, 1991




bay oscillate between brackish and hypersaline.                                     0
Restricted circulation in the bay results in environ-                       0 PLOT l
                                                   PLOT 2
mental a n d biological gradients along a southwest to
northeast transect (Schomer & Drew 1982, Tilmant
1989). Zieman et al. (1989) have determined that sea-
grasses cover more than 80 % of the 1800 km2 area of
Florida Bay within the Everglades National Park
boundaries.
  Loss or changes in species composition of seagrass
habitat may affect many economically important fish
and shellfish species. Over 100 species of fishes a n d
over 30 crustacean species are found in Florida Bay,
including both permanent residents and temporary
residents which use seagrass habitat a s a nursery
ground, such as spotted seatrout, redfish, snook, tar-
pon, snappers, and grunts. Important shellfish species
                                         JUN AUC OCT DEC UAR NAY JUL  SW N W J W U4R
include pink shrimp (Penaeus duorarum Burkenroad)
                                          1988      l989         1990
from the Tortugas bank, blue crabs, a n d spiny lobsters
(TaSb et al. 1962, Allen et al. 1980, Klima et al. 1986,   Fig. 2. (a) Leaf blade density of Thalassia testudjnum a! 2
Thayer & Chester 1989).                    monitori.ng 'plots' in Johnson Key Basin. Data are mean blade
                               density in 8 replicate 200 cm2quadrats (f1 SD). (b) Recolonl-
  Dead a n d dying Thalassia testudinum were first
                               zation of die-off patches in a nearby plot by Halodule wnghtii
observed near Cross Bank a n d in Rankin Lake during
summer 1987 (Fig. 1). By summer 1988, die-off was      new areas in the southern half of Florida Bay. Dense
evident in ca 30 % of the dense seagrass beds of       Thalassia beds within protected basins appear to be
western Florida Bay. An extensive Thalassia bed in      most severely affected by die-off. The depth of affected
Sunset Cove, some distance from other sites but adja-    seagrass beds varies widely, but most die-off patches
cent to the Everglades National Park boat dock, died     are located in shallower portions (< 1.5 m below mean
between November 1988 a n d January 1989. At pre-      sea level) along the margins of basins.
sent, die-off continues to spread within the areas where    Die-off occurs most rapidly during fall and spring.
it was in~tiallyobserved, and has been reported from     At one 'plot' in Johnson Key Basin where divers count




                                              Fig. 3. Thalassia testudinum. Ex-
                                              pansion of die-off patches in east-
                                              ern Rabblt Key Basin estimated
                                              from SPOT satellite imagery be-
                                              tween February 1987 and J u n e
                                              1988. Dimensions of each frame
                                              are 2.2 km (E-W) x 3 . 4 km (N-S).
                                              Light areas represent sparse
                                              coverage; dark areas represent
                                              dense coverage. See text for fur-
                                                  ther description
                      Robblee et al.: Mortality of seagrass                      299




seagrass shoots in permanent quadrats, Thalassia tes-      Acknowledgement. Preliminary and ongoing work has been
tudinum completely disappeared between August and       funded in part by Cooperative Agreement OA-5280-9-8002
                                from the US Department of the Intenor, National Park Service,
October 1988, while a nearby plot was unaffected
                                Everglades National Park, in part by Project F-44 from the
(Fig. 2a). Because T. testudinum is much more abun-       Department of the Interior, US Fish and Wildlife Service,
dant in affected areas than Halodule wrightii Aschers.     Federal Aid for Sportfish Restoration, and in part by funds
or Syringodium filiforme Kutzing, the susceptibility of     provided by the Florida Department of Environmental Regula-
these latter species is not known. The rapid increase      tion, Office of Coastal Management, using funds made avail-
                                able through the National Oceanic and Atmospheric
of H. wrightii shoot density seen in the nearby plot      Administration under the Coastal Zone Management Act of
and other die-off patches suggests that it might out-      1972, a s amended.
strip T. testudinum in the initial phases of recovery
(Fig. 2b).
  The size of die-off patches varies within and among                LITERATURE CITED
basins. The development of a large (63 ha) die-off patch
in eastern Rabbit Key Basin between February 1987 a n d     Allen, D. M., Hudson, J. H., Costello, T. J. (1980). Postlarval
June 1988 is shown in Fig. 3. In these satellite (SPOT)      shrimp (Penaeus) in the Florida Keys: species, size, and
                                  seasonal abundance. Bull. mar. Sci. 30: 21-33
photographs, the white area in the upper right one-third    Cambridge, M. L., McComb, A. J. (1984). The loss of sea-
of the picture area is shallow 'bank' habitat covered by      grasses in Cockburn Sound, Western Australia. I. The time
sparse Thalassia testudinum. Two islands, the Rabbit        course and magnitude of seagrass decline in relation to
Keys, are visible along the right (east) margin of each      industrial development. Aquat. Bot. 20: 229-242
                                Kemp, W. M., Twilley, R. R., Stevenson, J. C., Boynton, W. R.,
frame. The February 1987 photograph shows fairly
                                  Means, J. C. (1983). The decline of submerged vascular
uniform, dense coverage of seagrass (represented by        plants in upper Chesapeake Bay: summary of results con-
dark areas) in the lower left two-thirds of the picture      cerning possible causes. J. mar. technol. Soc. 17: 78-85
area, although the initial nucleus of the die-off patch is   Kirkman, R. H. (1976). A review of the literature on seagrass
visible a s a lighter gray region in the center of the upper    releated to its decline in Moreton Bay, Qld. CSIRO Rep.
                                  No. 64
t h r d of the earlier photo. The June 1988 photo shows     k a , E. F., Matthews, G . A., Patella, F. J . (1986).Synopsis of
complete mortality of T. testudinum from an elliptical       the Tortugas pink shrimp fishery, 1960-1983, and the
region, ca 1.9 km long, in the center of the photograph.      impact of the Tortugas sanctuary, N. Am. J. Fish. Man. 6:
  The recurring episodes of rapid, but patchy, mortality     301-310
                                Muehlstein, L. K. (1989). Perspectives on the wasting disease
are very different from the gradual loss of seagrass due
                                  of eelgrass Zostera marina. Dis. aquat. Org. 3: 211-221
to eutrophication reported in other estuaries (Kirkman     Schomer, N. S., Drew, R. D. (1982).An ecological characteris-
1976, Kemp et al. 1983, Cambridge & McComb 1984).         tizahon of the lower Everglades. Florida Bay and the
Because die-off is occurring in areas far removed from       Florida Keys. U.S. Fish Wildl. Sew. Publ. FWS/OBS-82/58
surface-water pollution sources (Fig. l ) ,anthropogenic    Short, F. T., Muehlstein, L. K., Porter, D. (1987). Eelgrass
                                  wasting disease: cause a n d recurrence of a marine
contaminants are probably not involved. Abnormally         epidemic. Biol. Bull. mar. biol. Lab., Woods Hole 173:
high water temperature, recent reduced frequency of        557-562
hurricanes, high salinity, a pathogenic slime mold       Tabb, D. C., Dubrow, D. L., Manning, R. B. (1962). The
(Labyrinthula sp.) related to the causative agent of        ecology of northern Florida Bay and adjacent estuaries.
                                  Florida Board of Conservation Technical Series No. 39, St.
eelgrass wasting disease (Short et al. 1987, Muehlstein
                                  Petersburg, Florida
1989), and chronic hypoxia of Thalassia testudinum       Thayer, G. W., Chester, A. J. (1989). Distribution and abund-
roots and rhizomes are potential causes of the phenome-      ance of fishes among basin and channel habitats in Florida
non which are currently under investigation. Without        Bay. Bull. mar. Sci. 44: 200-219
clear demonstration of causal factors, however, w e can-    Tilmant, J. T (ed.) (1989). Proceedings of the Symposium on
                                  Florida Bay. Bull. mar. Sci. 44 (1)
not predict the rate of spread, the long-term recovery     Zieman, J. C., Fourqurean. J. W., Iverson, R. L. (1989). Dis-
potential of affected areas or the geographical limits of     tribution, abundance and productivity of seagrasses and
T, testudinum populations at risk.                 macroalgae in Florida Bay. Bull. mar. Sci. 44: 292-311

This note was presented by Dr G. W. Thayer, Beaufort, N.    Manuscript first received: November 19, 1990
CaroLina, USA                          Revised version accepted: February 22, 1991