The Ecology of Hydric Hammocks - USGS National Wetlands ...

CHAPTER 6. LINKAGES WITH OTHER ECOSYSTEMSHydric hammocks are interconnectedwith up1 and and downs1 ope communi ti esby flows of water and movements of animals. Because of high ground-waterlevel and, sometimes, discharge fromdeep aquifers, a hammock may be connectedto recharge areas, such as thesandy uplands of the central Floridaridge, which are located far beyondthe watershed defined by surfacedrainage. Hydric hammocks often interceptsurface and ground water beforeit enters rivers and estuaries,perhaps altering the pattern and qualityof flow. Some animals merely passthrough hydric hammocks en route topreferred habitat, but for others,this forested wetland provides criticalshelter and food during part oftheir life cycles. Movements of animals,and their transport of fruitsand seeds, 1 ink both the plant and animalcomponents of hydric hammocks toadjacent and distant communities,6.1 WITH ESTUARIESThe extensive band of hydric hammocksalong the gulf coast of Florida(Figure 1) links upland and saltmarsh/estuarine systems. The most irnpor.tantfeature of this coupling isthe modification, by passage of waterthrough hydric hammock, of the quantity,timing, and quality of freshwaterthat reaches the estuaries.Changes in one or more of these parameterscan greatly alter estuarinestructure and productivity.The flow of freshwater into estuarieshas numerous consequences(Snedaker and deSylva 1977), of whichthe major ones are the reduction ofsal ini ty; the induction of freshwaterand saltwater mixing; the establishmentof horizontal and vertical gradi -ents of salinity; and the addition ofnutrients, dissolved and particulateorganic matter, and sediments derivedfrom the watershed. The quantity andtiming of freshwater del ivery determine,in part, seasonal changes inphysico-chemical regimes that in turnare reflected in regular patterns ofestuarine use by animals. Year-roundinhabitants, mainly invertebrates, areplentiful, but the great number andvariety of part-time residents distinguishestuaries. In the southeasternUnited States, most fish and invertebratespecies of commerci a1 importanceuse estuaries and their fringing saltmarshes as nurseries, entering andleaving the shallow waters in a regularseasonal progression from 1 atefall through early summer (Sheridanand Livingston 1979; Rogers et a].1984). Temporal and spati a1 di stributionsof species within estuaries areassociated with physico-chemical andbiotic factors, many of which are influenced by freshwater inflow (e.g.,sal ini ty and food availability) . Incoastal waters, annual freshwater inputi s correlated with production ofsome invertebrate and fish species,but the mechanisms generally are notwell understood (Meeter et a7. 1979;Sinclair et a7. 1986). The effect offreshwater discharge on the abundanceof oysters is mediated by changes insalinity. The optimal salinity rangefor oyster growth in Apalachicola Bayis 15%-25%, but a more important roleof salinity is indirect in that lowsal ini ty excludes important predatorsof oysters (Menzel et a7. 1966).How forested wet1 ands influence thedelivery of freshwater to estuaries