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Section 2 Existing Conditions/Affected Environment

EAA Storage Reservoirs Revised Draft PIR and EIS February 2006

2-34

Within its range, Johnson’s seagrass critical habitat designations have been

designated for 10 areas: a portion of the Indian River Lagoon, north of the

Sebastian Inlet Channel; a portion of the Indian River Lagoon, south of the

Sebastian Inlet Channel; a portion of the Indian River Lagoon near the Fort

Pierce Inlet; a portion of the Indian River Lagoon, north of the St. Lucie Inlet; a

portion of Hobe Sound; a site on the south side of Jupiter Inlet; a site in central

Lake Worth Lagoon; a site in Lake Worth Lagoon, Boynton Beach; a site in Lake

Wyman, Boca Raton; and a portion of Biscayne Bay.

The essential features of habitat appear to be adequate water quality, salinity,

water clarity and stable sediments free from physical disturbance. Important

habitat characteristics include shallow intertidal as well as deeper subtidal

zones (2-5 m). Water transparency appears to be critical for Johnson’s seagrass,

limiting its distribution at depth to areas of suitable optical water quality

(Kenworthy 1997). In areas in which long-term poor water and sediment quality

have existed until recently, such as Lake Worth Lagoon, H. johnsonii appears to

occur in relatively higher abundance perhaps due to the previous inability of the

larger species to thrive. These studies support unconfirmed previous

observations that suspended solids and tannin, which reduce light penetration

and water clarity, may be important factors limiting seagrass distribution. Good

water clarity is essential for Halophila johnsonii growth in deeper waters.

Johnson’s seagrass occurs over varied depths, environmental conditions,

salinities, and water quality. In tidal channels H. johnsonii is found in coarse

sand substrates, although it has been found growing on sandy shoals, in soft

mud near canals and rivers where salinity many fluctuate widely (Virnstein et

al. 1997). Virnstein has called Johnson’s seagrass a “perennial opportunistic

species.” Within study areas in the Indian River Lagoon, H. johnsonii was found

by itself, with other seagrass species, in the intertidal, and (more commonly) at

the deep edge of some transects in water depths of up to 180 cm. H. johnsonii

was found shallowly rooted on sandy shoals, in soft mud, near the mouths of

canals, rivers and in shallow and deep water (Virnstein et al. 1997).

Additionally, recent studies have documented large patches of Johnson’s

seagrass on flood deltas just inside Sebastian Inlet, as well as far from the

influence of inlets (reported at the workshop discussed in Kenworthy, 1997).

These sites encompass a wide variety of salinities, water quality, and substrates.

Areas of concern include seagrass beds located in proximity to rivers and canal

mouths where low salinity, highly colored water is discharged. Freshwater

discharge into areas adjacent to seagrass beds may provoke physiological stress

upon the plants by reducing the salinity levels. Additionally, colored waters

released into these areas reduce the amount of sunlight available for

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Wenger 72944 Spezifikationen Seite 98