The pre-development IRL watershed was bound to the east by the barrier island and to the west by the Atlantic Coastal Ridge, a relict shoreline from a Pleistocene high stand when sea level was higher than at present (Cook, 1945). South of Melbourne the ridge becomes discontinuous, allowing the Sebastian and St. Lucie rivers to drain inland areas into the IRL (White, 1958). In many areas the watershed has been increased by drainage canals, which divert water from the St. Johns watershed into the IRL, more than doubling the total watershed area and significantly increasing surface water inflows (Woodward and Clyde, 1994). Additionally, utilities import ground and surface water from the St. Johns watershed into the IRL watershed. In Banana River Lagoon for example, discharges from wastewater treatment facilities, the ultimate source of which is the St. Johns watershed, exceed inflows from runoff (Woodward and Clyde, 1994).

In addition to changes to the watershed, the IRL has been modified by dredge and fill, resulting in horizontal compression of the land-water interface and unnaturally deep areas where boat channels and mosquito canals have been cut. In many places these canals have lowered the benthic interface to well below the compensation depth, effectively eliminating submerged aquatic vegetation and favoring planktonic algae in the role of primary production, and probably changing the interstitial chemical characteristics of the sediments as oxygenated rhyzopheres associated with rooted vascular plants decline.

These canals act as sediment traps, allowing dissolved organic matter to flocculate. Deeper canals can tap into ground water, acting as thermal refugia, affecting the migration of the West Indian Manatee (Sette, unpublished master’s thesis). The canals also tend to be very low in dissolved oxygen and can have zooplankton densities up to two orders of magnitude less than what is found in natural areas (Kozusko, unpublished data).

The IRL is often referred to as an estuary. Although the IRL has many physicochemical processes in common with an estuary, it is morphologically distinct. These definitions are complicated by the need to somewhat arbitrarily delineate a continuum of coastal morphological features ranging from an open river mouth through embayment to a closed lagoon (Moore, 1992).

Simply put, an estuary is a place where a river meets the sea. It will have a diurnal or semi-diurnal astronomical tide, a flow primarily driven by gravity that might change direction with the tide, and a salinity gradient ranging from fresh at one end to seawater at the other end (Stickney, 1984). The connection to the sea is significant in comparison to the volume of the estuary.

The IRL has none of these aspects, and referring to it as an estuary trivializes the features that make it so unique. Its tides are meteorological and whatever flow is present is also wind-driven. The salinity is relatively constant throughout and there is little direct exchange of water between the IRL and Atlantic due to insignificant connection between the two. Approximately three quarters of the IRL is at least 5 miles from an inlet, and the five inlets that are present are artificially sustained by jetties and dredging. Historically inlets closed after 200 to 300 years and then another opened elsewhere due to hydrodynamic forces (Parkinson, pers. comm.). The remnants of relict inlets can be seen as flood tide deltas on the west side of the barrier island (Fig. 2). The Thousand Islands area in Cocoa Beach is the best example of a flood tide delta on the east coast (Parkinson, pers. comm.).

The area surrounding the St. Lucie River can be thought of as a true bar-built estuary, although its inlet is held open and its function has been dramatically altered by artificial connection of the St. Lucie River to Lake Okeechobee. During large storms massive water releases can take place through the St. Lucie, resulting in severe declines in water quality and damage to any species unable to tolerate fresh water. When water is withheld for storage to satisfy agricultural needs brackish water moves too far up the river, killing species unable to tolerate the increased salinity. 

Physical Development

The IRL developed as Holocene sea level rose to fill a topographical depression 5000 to 6000 years ago (Bader and Parkinson, 1991). The western boundary of the IRL watershed, the Atlantic Coastal Ridge, formed during the Pamlico high stand 120,000 to 140,000 years ago when sea level was approximately 9 meters higher than at present (Cook, 1945; Bader and Parkinson, 1991). Sea level fell as climate cooled, pausing to form the Merritt Island complex and continuing to fall after 110,000 years ago (Osmond, May, and Tanner, 1970).

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