Experimental studies of influence of hydrogen sulfide on species of eelgrass (Zostera japonica and Zostera marina) in Padilla Bay, coastal waters of southeast Alaska, from 2013-06-01 to 2013-09-30 (NCEI Accession 0137907)

Two species of eelgrass can be found in Padilla Bay, Washington (Zostera japonica and Zostera marina) and act as a bioindicators of ecosystem health. Many factors can contribute to the status of an eelgrass bed, including light, temperature, salinity, and nutrients. However, following several cases of seagrass die-off events worldwide, another factor is suspected to contribute to eelgrass health: pore-water sulfide. This study examined the relationships between Z. japonica, Z. marina, and pore-water sulfide in Padilla Bay and the effects of elevated pore-water sulfide concentrations on eelgrass. Forty sites were surveyed for eelgrass shoots and sulfide concentration profiles were measured at depths of 0 to 12 cm. A correlation was expected between eelgrass and the inventory of sulfide during August and September due to increased temperature and increased bacterial respiration as a result of higher quantities of organic matter accumulation. While the data hinted at patterns between eelgrass density and sulfide, there were no significant correlations found between Z. japonica and Z. marina and the inventory of sulfide from June 2013 through September 2013. This is perhaps due to relatively low concentrations of sulfide at the study sites and documented eelgrass tolerance to the concentration range, as well as the overall health of eelgrass in this location. To further examine the relationship between eelgrass and sulfide, Zostera japonica and Zostera marina were grown in sediment amended with sulfide in an outdoor laboratory tank to study growth response and photosynthetic yield. Eelgrass shoots were grown for four weeks under different sulfide manipulations and shoot growth was recorded weekly. Quantum efficiency of PSII in eelgrass shoots was measured by PAM fluorometry at the conclusion of the experiment. The growth rates of Z. japonica and Z. marina were significantly reduced in treatments with elevated sulfide concentrations. Manipulated concentrations of pore-water sulfide resulted in significantly lower growth rates among Z. japonica shoots treated with moderate and high levels of sulfide. The decrease in growth in both species suggests that elevated levels of pore-water sulfide have an impact on eelgrass in Padilla Bay. The average photosynthetic yield of the shoots for Z. japonica and Z. marina was lower in shoots treated with sulfide, although this difference was not statistically significant, suggesting the drop in growth was not due to chloroplast damage.