Large-scale patterns in seagrass (Thalassia testudinum) demographics in south Florida

An examination of the population age structure of 118 spatially separated subpopulations of Thalassia testudinum over the extent of the Florida Keys National Marine Sanctuary (FKNMS) during a 2-yr period revealed significant spatial variation in short shoot (SS) demographic characteristics and population dynamics. Shoot age was determined for 12.031 SS. The number of leaf scars on individual shoots was converted to shoot age by use of observed seasonally and spatially variable leaf emergence rates. The yearly mean leaf emergence rate was 0.0295 +/- 0.0128 leaves SS-1 d(-1) (+/-1 SD), and the median age of counted shoots was similar to5 yr. A significant relationship between asexual reproductive effort and gross recruitment of SS into the populations (r(2) = 0.15, P = 0.001) and between mortality of SS and gross recruitment (r(2) = 0.72, P < 0.001) existed. Thus, the greatest risk of mortality occurred in areas where gross recruitment was highest. The net population growth for T. testudinum within the boundaries of FKNMS was stable (mean = -0.006 +/- 0.089 yr(-1)). However, areas within FKNMS fluctuated between positive and negative net growth rates (-0.20-0.50 yr(-1)). The power of such large-scale observations is the ability to identify areas of management concern and to frame questions that address the controlling mechanisms that influence these regions of fluctuating population growth.