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Appendix C. Additional measures of dissolved oxygen. Oxygen profile in July 2005 at 4 different depths (at sunset, noon and dusk) and oxygen evolution in fertilized and control plots in July and August 2005.

We measured oxygen concentrations following two additional approaches. First, we measured oxygen concentrations at different depths in both types of plots at sunrise, noon, and sunset on 1 July 2005 using a HQ 10 Hach Portable LDO oxygen meter. Second, we obtained three-day long continuous (measurements taken every 15 min) records of oxygen concentration at ~15 cm from the sediment surface in both types of plots in July and August of 2005 using a YSI-6600 EDS with a Clark type DO probe. Our measurements of oxygen concentrations at different depths done on 1 July 2005 showed a larger diel cycle in control than in fertilized plots. This was consistent with the continuous records obtained at 15 cm from the sediment surface in July and August, where, even though maximum daily concentrations were not always higher in control plots, we also found larger diel cycles in those plots. The larger diel oxygen cycles in control plots were probably due to the presence of seagrass in those plots and its absence in fertilized plots from July to the conclusion of the experiment. Mean oxygen concentrations integrated throughout the diel cycle, however, were similar in control and fertilized plots. We never encountered anoxic (< 2 mg l-1) conditions in either type of plot.

FIG. C1. Dissolved oxygen profile (mg O2 l-1) measured at four depths (5, 10, 20, and 50 cm) in control and fertilized plots. Measurements were taken at 3 control and 4 fertilized plots on 1 July 2005. Filled circles represent control plots and gray inverted triangles fertilized plots.

FIG. C2. Dissolved oxygen concentration (mg l-1) during six consecutive days in July (above) and August (below). The YSI measured dissolved oxygen every 15 minutes with an in situ probe located at the canopy level (~15 cm) for three days in one fertilized (black dots) and three days in one control plot (gray dots). Dotted line separates fertilized from control data. Dashed line indicates hypoxia limit at 2 mg l-1.