Floating Peat Islands and Patch Formation in the Northeastern Everglades
Patrick J. Gleason
Camp Dresser & McKee Inc., West Palm Beach, FL
Peter A. Stone
SC Dept. Health and Environmental Control, Columbia SC
Floating islands are common in the waterlily-rich northeastern Everglades where
they appear important in the formation of vegetational patches in the marshes,
including the predominant type of tree-island. Floating islands of various sorts
are an interesting phenomenon found worldwide including in other peat marshes
such as the waterlily-rich “prairies” of Okefenokee Swamp, Georgia. Waterlilys
facilitate the formation of floating peat mats. These initiate by detachment of
buoyant uppermost peat and rootmat from the marsh bottom. Gas-filled
arenchyma tissue, especially in the large rhizomes, provides buoyancy that adds
to that from bubbles of decompositional gasses in the peat. Few or no floating
peat islands were observed to originate except from waterlily vegetation.
Possibly some physical characteristic of waterlily peat also accommodate, e.g.,
perhaps they more easily separate along planes to allow formation of the mats.
The floating mats immediately create emerged sites, which soon are colonized by
plants requiring shallower water than is available in the surrounding waterlily-rich
marshes (the deeper marshes) or colonized by plants that tolerate no flooding at
all at least at younger stages. Many floating islands drift slightly (mainly by
wind) and leave behind an exposed shallow depression. The eventual resunk
relicts of island mats and the depressions at their formation sites both are
lingering topographical influences on the marsh vegetation. These are involved in
both the formation and enlargement of vegetational patches, or rarely instead their
obliteration (a floating island was once observed to encompass and eliminate one
of the many small tiny patches of waterlily marsh found in the broader sawgrass
marshes). The small mounds (low peat plateaus) are strongly implicated in the
origin of the extremely numerous small bayhead tree-islands that occupy larger
peat mounds in the same region of the Everglades (mainly the A.P. Marshall
Loxahatchee National Wildlife Refuge). At least one fully established such island
has been seen seemingly to expand by means of a floating island forming in the
slough along its edge.
The northeastern Everglades now in CA1 and northern CA2A are (or were)
waterlily dominated or codominated, but also host numerous sawgrass strands
(elongated stands) and, more importantly here, vast numbers of smaller irregular
sawgrass patches and patches of other emergent marsh plants as well as thousands
of bayhead tree-islands. Most of the tree-islands occupy small peat mounds.
Floating peat islands are common and were noted before impoundment (J.H.
Davis, pers. comm.). The waterlily root mat here helps facilitate flotation and
occasionally little more than the rootmat layer floats, though more often with a
layer of peat accompanying (total 15+ cm thick). Mats are both strong and
buoyant enough to support a standing person.
Many floating islands move laterally, sometimes just a few meters offside the
emergence depression, sometimes tens of meters (distinctive size and shape of
island and emergence hole are the evidence). Wind is the main mechanism, at
least for more distant movement, with taller vegetation (e.g., sawgrass, bushes,
small trees) likely acting as “sails.” Movement means that a low peat “plateau”
forms even upon resinking or reattachment after dry season stranding, which is
promoted by drifting into shallows toward the edge of the waterlily sloughs.
New floating peat islands, with bare peat surfaces, and other recently formed
ones, with an adjacent unvegetated emergence site, were initially examined and
the flora listed, re-examined over the next year or so, examined several years
later, and a few observed again about 25 years after initially. Colonization by
marsh vegetation that differs from that immediately adjacent in the surrounding
slough is very rapid (within a year) and mostly is by seed. Where tree-islands are
common within tens or hundreds of meters, colonization by tree-seedlings is
sometime rapid also. Bushy vegetation and fruiting specimens of dahoon holly
and red bay were observed on islands still fully afloat. Tree-species colonization
is faster than that reported for Okefenokee Swamp.
After a quarter century there still was not found (in the several sites re-examined)
an example of the many small attached patches of bushes or small trees that are
seen in these same marshes and assumed to be main stages in the development of
the small peat-mound tree-islands. However, peat stratigraphy from fully
established tree-islands suggests that many went through prolonged stages as
patches of shallower marsh (sawgrass or arrowhead) within the waterlily marshes.
Many small distinct patches of sawgrass, or of other marsh plants such as
pickerelweed or arrowhead, are presently found in the waterlily sloughs and
waterlily-rich mixed wet-prairies. These plants typify shallower water than
waterlilys. One patch specifically examined lay upon a low (roughly 10-15 cm)
but steep-sided peat “plateau” that likely formed by reattachment of a floating
peat island (for what other mechanism exists?). Many of the innumerable other
small patches of shallower marsh are suspected to have the same origin.
A unique condition (for the Everglades) of hydrologic stability occurs atop a peat
island while afloat, a condition that may last for years in deeper sloughs. The
surface is always damp but never inundated. Sag areas found in some mats are
very shallowly inundated but at stable depth. This unusual stability in habitat may
in large part explain the presence of some rarer marsh species and probably
explains the almost singular occurrence of sundew (Drosera) in the Everglades
(CA1). Unfortunately, the recent re-examination of floating islands shows that
they also can facilitate growth of the troublesome weed tree, Melaleuca.
The depressions formed by the emergence of those floating islands that eventually
drift away (even slightly) are likely important in maintaining patches of the
deepest marsh. A conspicuously dense stand of white-waterlily pads (leaves)
often marks them after a few years.
Patrick Gleason, Camp Dresser & McKee Inc, 1601 Belvedere Road, Suite 211
South, West Palm Beach, FL 33406
Phone: 561-689-3336, Fax: 561-689-9713, gleasonpj@cdm.com