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the canals determine the adjacent water table and, therefore,
the water level in the CMZ. Water enters the area as groundwater seepage and as rainfall. These changes in surficial drainage have affected surface and groundwater hydrology (Parker
et al. 1955; Parker 1974), reducing or eliminating coastal
creek function and productivity. West of the L31E levee, the
landscape is drained and compartmentalized by a network
of mosquito control ditches, wetland drainage ditches, and
flood control canals (e.g., Ruiz and Ross 2004). Although the
wetland value of this area has decreased substantially because
of increasing salinities and drainage, the ecological function
of habitat created by the invasion of salinity-tolerant coastal
mangroves remains particularly important to Biscayne Bay
(Odum and Heald 1975; Teas 1976).
The area between Convoy Point and the northern mainland extent of the park covers some 1,500 acres of potentially
high value fish and wildlife habitat that forms the longest contiguous coastal mangrove forest on the east coast of Florida.
It is comprised of coastal fringe forest (primarily red mangroves), interior transitional forest (red and white mangroves),
dwarf mangrove forest (red mangroves), and a mosaic of relict
stream channels and coastal wetlands. This area provides
food, shelter, and nursery habitat critical to sustaining the
recreational and commercial fisheries in south Florida (Teas
1976; Serafy et al. 2003). Salinity fluctuations imposed by water management operations and loss of water to coastal creeks
that once created brackish conditions in the prop root zone
have created impaired fishery habitat with impaired nursery
function.
Western Bay Zone. East of the mangroves, the WBZ is dominated by the seagrass (approximately 64% of the bottom) and
hard bottom (17%) benthic communities (NOAA 1996). Submerged aquatic vegetation (SAV) in the bay is typically a mixed
species community that may include shoal grass (Halodule
wrightii), manatee grass (Syringodium filiforme), turtle grass
(Thalassia testudinum), wigeon grass (Ruppia maritime), and
three species of an uncommon SAV Halophila. Freshwater
algae {Chara spp.) may also be found in low salinity coastal
streams, tidal creeks, ponds, and around canal and creek
outlets where fresher water predominates. The distribution
of Chara in the WBZ is limited by its intolerance of the marine conditions now common in nearshore areas of the bay.
Currently, the SAV community is dominated by turtle grass
(Christian et al. 2004), an indicator of true marine conditions
because of its low tolerance for brackish conditions.
The CERP Monitoring and Assessment Plan conceptual
models identify salinity as the primary stressor in the WBZ,
affecting biology when conditions deviate from a natural
range. Salinity in this area is controlled by tidal circulation,
which tends to increase salinities toward an offshore level (33-
35 ppt), and by canal discharge, which is a function of canal
operations and introduces freshwater (<1 ppt). To a lesser extent, salinity in this zone is affected by local rainfall (SFWMD
1995; Lirman and Cropper 2003; Faunce et al. 2003; Serafy
et al. 1997). Another stressor is poor-quality water containing nutrients and contaminants (SFWMD 1995), carried by
the six canals discharging into this zone (Black Creek (C -l),
Princeton Canal (C-102), Military Canal, and Mowry Canal
(C-103), C - l ll , Snapper Creek (C-2), and the Coral Gables
Waterway (C-3)).
Indicator Species: Benthic Community,
Endangered Species, and Important
Fishery Resources
Many of the species described in this report are considered to
be dependent on estuarine conditions. They require a brackish environment or they are reliant on estuarine salinities at
specific stages in their life cycle. The presence of these species,
both historically and at present, supports the importance of
maintaining and restoring the estuarine nature of the WBZ.
Many of these species are declining and some have presumably disappeared because of the lack of freshwater currently
reaching the bay; therefore, the salinity range of these organisms can be used to define a beneficial salinity range for Biscayne Bay that will protect fish and wildlife. Many species
including stone crab, Spanish mackerel, Crevalle jack, grey
snapper, and tarpon are either recreationally or commercially
important within the WBZ fishery. However, more historical
information and field studies with site-specific information on
Ecological and Hydrologic Targets for Western Biscayne National Park 9
preferred salinity ranges were available for the six species presented in Table 1, therefore we chose to use them as biological
indicators.
Biscayne National Park’s creel data (based on interviews
of anglers when they return from fishing to local marinas) has
been collected since about 1976. Most of the data pertain to
reef fish, since a majority of the anglers fish on the reef, and
are, therefore, less useful in this paper which focuses on the
near-shore area. The bay species that might be affected by
salinity changes near the coast (i.e., snook, spotted sea trout,
red drum) are now landed infrequently in the areas we survey
making it impossible to identify trends in population abundance from the creel data. The only species that are affected by
salinity changes and are frequently reported are snappers and
grunts, which utilize bayside seagrass beds as juvenile habitat. Also, it is impossible to distinguish the effects of salinity
changes over time from fishing effects over time. For all these
reasons, the Biscayne National Park creel dataset was used to
provide supporting evidence of the continued presence of the
biological indicator species in the bay, and was not used for
more quantitative analyses of the effects of salinity changes.
The American crocodile (Crocodylus acutus) is primarily
found in areas characterized by brackish estuarine conditions
and average salinities of 14 ppt (Mazzotti 1983; Kushlan 1988;
Kushlan and Mazzotti 1989). Hatchling crocodiles are particularly reliant on low salinities, generally 0 to 5 ppt (Mazzotti
and Cherkiss 1998). Mazzotti and Cherkiss also determined
that all size classes (hatchlings, juveniles and adults) favor
water bodies with an intermediate salinity of 20 ppt. Juvenile
crocodiles, in particular, seek out and seem to require a mesohaline (<20ppt with an optimum of 9 ppt) habitat for survival
(Mazzotti et al. 2002).
The Eastern oyster (Crassostrea virginica) has long been
recognized as an indicator of estuarine conditions (Dean
1892; Oemier 1894, Ritter 1896; Grave 1905; Pearse and
Wharton 1938; Nelson et al. 1991; Estevez 2000; Meeder et al.
2001). The life cycle of oysters occurs entirely within estuaries. Oysters are capable of surviving salt concentrations from
5 to 40 ppt, however the optimum range for oyster reef growth
is 10-20 ppt (Stenzel 1971; Meeder et al. 2000). This allows for
oyster reproduction while decreasing predation rates (Grave
1905; Gunter 1950; Wells 1961).