A 14 mo trawl survey was conducted at 8 study sites in Biscayne Bay, Florida, USA, to compare the species composition and structure of juvenile fish assemblages found near the mouths of freshwater flood control canals with those in similar areas with relatively stable salinity regimes. Water temperature, salinity, dissolved oxygen and depth measurements were recorded during fish sampling and bottom vcgctation was also quantified. The survey yielded a total of 38 134 individuals from 95 taxa. Fish species composition was similar among sites, but more species were collected from stableversus variable-salinity areas Mean fish abundance dnd the mean abundances of Euc~nostonius gula. Lagodon rhornboides, Opsanus beta and Lutjanus grlseus shared a general pattern of increase from north to s o~.~t h , with highest values occurring at one or more of the canal-influenced sites In contrast, mean specles nchness and the mean abundances of Lucanlaparva, Haenlirlon sciurw, H. plurnleri, and H. parra were signif~cantly greater at stable-salinity sltes than at variable-salinity sites Freshwdter challenge experiments \yere then condu.ctcd on each of the fishes above, as well as on 2 relatively uncommon species, Cynoscion nebulosus and Cypnnodon vadegatus. The mortality of groups exposed to a single, rapid, freshwater pulse (i.e. salinity was changed from approximately 32 ppt to 0 to 32 ppt over 2 h ) was compared with that of controls. Of the 8 fishes that dominated the nearshore habitats of Biscaync Bay, 5 exhibited no mortality and L. rhornboldrs, L. parva, and H. plurnleri exhibited 12.5, 50 and loo'% mortality rates, respectively. Mortality was 10Oo0 for the relatively uncommon C. nebulos~ls and C. variegatus. Results suggest that the differential osmoregulatory abilities of the species tested may underlie some, but not all, of the structural differences observed between fish assemblages from stable-sa1i:nity habitats versus those ddjacent to freshwater canals.

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A brackish water ecotone of coastal bays and lakes, mangrove forests, salt marshes, tidal creeks, and upland hammocks separates Florida Bay, Biscayne Bay, and the Gulf of Mexico from the freshwater Everglades. The Everglades mangrove estuaries are characterized by salinity gradients that vary spatially with topography and vary seasonally and inter-annually with rainfall, tide, and freshwater flow from the Everglades. Because of their location at the lower end of the Everglades drainage basin, Everglades mangrove estuaries have been affected by upstream water management practices that have altered the freshwater heads and flows and that affect salinity gradients. Additionally, interannual variation in precipitation patterns, particularly those caused to El Nin ˜o events, control freshwater inputs and salinity dynamics in these estuaries. Two major external drivers on this system are water management activities and global climate change. These drivers lead to two major ecosystem stressors: reduced freshwater flow volume and duration, and sea-level rise. Major ecological attributes include mangrove forest production, soil accretion, and resilience; coastal lake submerged aquatic vegetation; resident mangrove fish populations; wood stork (Mycteria americana) and roseate spoonbill (Platelea ajaja) nesting colonies; and estuarine crocodilian populations. Causal linkages between stressors and attributes include coastal transgression, hydroperiods, salinity gradients, and the ''white zone'' freshwater/estuarine interface. The functional estuary and its ecological attributes, as influenced by sea level and freshwater flow, must be viewed as spatially dynamic, with a possible near-term balancing of transgression but ultimately a long-term continuation of inland movement. Regardless of the spatio-temporal timing of this transgression, a salinity gradient supportive of ecologically functional Everglades mangrove estuaries will be required to maintain the integrity of the South Florida ecosystem.

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