Effects of nitrate fertilization and shading on physiological and biomechanical properties of eelgrass (Zostera marina L.)
The relationship between nutrient enrichment and global declines in seagrass habitat continues to be a focus of considerable scientific effort. A recurring theme in the literature is that light limitation is responsible for much of the loss, and that nutrient enrichment is the principle mechanism (by stimulating the growth of other autotrophs). ^ The intent of these studies on the seagrass Zostera marina L., was to separate out first-order effects of nitrate enrichment from second-order shading responses, and particularly to examine fertilization effects on the mechanical properties of shoots. Of additional interest was a report in the literature attributing an acute die-off of Zostera to elevated nitrate concentrations. ^ During a nine-week incubation in outdoor tanks (with independent control of shading and nitrate loading), no acute die-off was observed due to extreme nitrate enrichment. Enrichment did, however, reduce the tensile forces that leaves could withstand. This mechanical response was significantly correlated with nitrogen and cellulose content of leaves, and the area-specific leaf mass of leaf tissue. Nitrate also increased the susceptibility of shoots to an infectious wasting disease. ^ A year-long monitoring study at two field stations with disparate nutrient regimes confirmed that natural populations of eelgrass behave consistently with most trends observed in the autecological experiment. On average, leaves from shoots at the low-nutrient site could withstand 26% more force than those from the high-nutrient site, and were stronger at both sites during the active growing season when tissue C:N was high and blades were wide and blade tissue was dense. ^ In order to determine if natural and experimentally induced differences in tensile breaking force were ecologically significant, flume measurements of hydrodynamic drag were made on shoots. These were, in turn, used to calculate coefficients of drag and predict water speeds at which shoots from the field study would have been damaged. Results indicate that considerable loss would be expected in as little as 4 ms-1 of flow, and support the hypothesis that tensile breaking force (and hence nitrate enrichment), is important in determining the distribution of eelgrass, especially at shallow depths. ^
Biology, Ecology|Biology, Oceanography|Biophysics, General
Blaine S Kopp,
"Effects of nitrate fertilization and shading on physiological and biomechanical properties of eelgrass (Zostera marina L.)"
Dissertations and Master's Theses (Campus Access).