Silvera, Owen

The overall objective was to elucidate the effect of iron (Fe) on nitrogen (N) diagenesis in Lake Okeechobee. Somewhat counterintuitively, sediment ammonium (NH+4) inventories decreased during algal growth as dissolved organic nitrogen (DON) inventories increased. Whole core incubations were staged for denitrification experiments using isotopic N tracer. Core incubations showed the percentage of sediment N removal increase between summer (25 ± 21 %) and winter (39 ± 13 %). The amendment of Fe2+ enhanced this seasonal effect likely via dissimilatory nitrate reduction to ammonium (DNRA). The isotopic signature of N2 flux also suggested an additional, sedimentary, N2 source via Fe coupled anaerobic oxidation of ammonium (feammox). Sediment slurry incubations supported the occurrence of both DNRA and feammox, showing first that nitrate (NO3−) was converted to NH4+ via DNRA, which contributed 23-26% of overall NO3− reduction.
Fe amendment in slurries similarly stimulated the feammox process. However, aged Fe minerals accumulated linearly with N bound to Fe (Fe-N) in a subseasonal sediment time series, suggesting Fe-organic matter aggregation may lower the sediment NH4+ equilibrium concentration and benthic flux.

Seagrasses are essential habitats for many species and provide critical ecosystem functions. Globally seagrasses are declining at an alarming rate. Locally, seagrass loss is concerning because of the magnitude of habitat loss as well as the uncertainty surrounding the mechanisms driving seagrass decline. We exposed two prevalent seagrasses in the IRL to the terrestrial herbicide Roundup QuikPROTM (active ingredient 73% glyphosate) and quantified seagrass response over 53 days in a closed tank system. Direct application of 112 mg glyphosate (15mL of 7.5 g/L) onto Halodule wrightii and Halophila johnsonii, simulating overspray during low tide, yielded persistent concentrations of 2.58 mg/L glyphosate after 13 days. Direct application had acute effects, but survivorship after 53 days was equivalent to Control tanks. Indirect application of 125 mg/L glyphosate, with persistent concentrations of 83.4 mg/L glyphosate after 13 days, resulted in complete H. johnsonii mortality and nearly complete mortality of H. wrightii. Despite high-water solubility, glyphosate poses a measurable threat to seagrasses which warrants further study.