EPA's Vessel General Permit and Small Vessel General

2009). The largest known groups of patches are located near Sebastian Inlet and Lake Worth.
Habitat
Patches of Johnson’s seagrass have been observed to grow from the intertidal zone down to 3.3
feet water depth and in waters with variable temperatures and salinities (15 to 43 parts per
thousand) and temperatures (Dawes et al. 1989, Kenworthy 1993, Virnstein et al. 1997, Kahn
and Durako 2009). Patches near freshwater discharges have been observed (Gallegos and
Kenworthy 1996), although Torquemada et al (2005) noted that highly hypo- or hypersaline
conditions can negatively impact growth. Intertidal patches may be completely exposed at low
tides, suggesting tolerance to desiccation and wide temperature ranges (Kahn and Durako 2009).
Growth and reproduction
Only female flowers have been observed; no fruit or seeds have been found to date (Eiseman and
McMillan 1980, Heidelbaugh et al. 2000). Meiosis does occur however, meaning that if male
pollen ware even rarely present, sexual reproduction could take place (York 2005). However,
there is no evidence of male flowers, meaning Johnson’s seagrass may only reproduce by
cloning or asexual branching and fragmentation (Jewitt-Smith et al. 1997, Hammerstom and
Kenworthy 2003). Consequently, genetic diversity is low (Freshwater and York 1999).
Clonal reproduction in plants occurs when plants form new leaf-pair, root and rhizome segments
that arise from terminal buds (Posluszny and Tomlinson 1990). On average, new buds are
formed on rhizomes every two to four days and rhizomes can grow at 0.2 inch per day (Bolen
1997, Kenworthy 1997). However, these clones can expand rapidly (1 to 3 feet per month)
during periods of prolific branching (Kenworthy 1997, Greening and Holland 2003, Kenworthy
2003). As clones expand, high density “patches” are formed ranging from three to 66 feet 2 in
size (Kenworthy 1997, Virnstein et al. 1997, Kenworthy 2000, 2003, Virnstein and Morris
2007). Patches can expand rapidly (nine feet 2 per month)(Kenworthy 2003) leading to
coalescence with adjacent patches and large meadows of up to 30 acres (Kenworthy 1997).
Fragments or entire plants can be uprooted and drift extensively, providing a mechanism for
dispersal and colonization of new areas (Hall et al. 2006). Virnstein et al. (2009) recently
proposed that Johnson’s seagrass occurs in “pulsating patch,” with two to three consecutive
summers of growth followed by a rapid decline. Johnson’s seagrass populations may undergo
whole patch mortality followed by recolonization (Virnstein et al. 1997, Heidelbaugh et al. 2000,
Greening and Holland 2003, Kenworthy 2003, Virnstein and Morris 2007).
Johnson’s seagrass appears to be physiologically adapted to exploit unstable environments and
unvegetated patches, with minimal resources allocated to the holding of space (Dean and Durako
2007). This characteristic may allow for more rapid overall patch growth and the exploitation of
areas in which Johnson’s seagrass could not otherwise compete (Dean and Durako 2007). These
growth characteristics also help explain its patchy distribution (Kenworthy 1993, Virnstein et al.
1997). Although successful in unstable areas, Johnson’s seagrass may be out-competed by more
stable-selected plants in areas not subject to regular disturbance (Durako 2003). Johnson’s
seagrass thrive in unstable or newly-created unvegetated environments, but have little capacity
for holding occupied space. As a result, Johnson’s seagrass can be highly variable in its
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