Investigations of pond metabolism in temperate salt marshes of Massachusetts

Abstract

Salt marshes provide important ecosystem services, including carbon sequestration. Permanently inundated ponds are prominent features in the marsh landscape, encompassing up to 60% of the total marsh area, but they are rarely considered in biogeochemical assessments. I investigated two ponds in Plum Island Estuary, MA to measure and analyze their metabolism. The ponds varied in size and vegetation cover. Oxygen concentrations and pH values were recorded in 15-minute intervals during the entire study period. The ponds regularly become hypoxic or anoxic during night. This is a problem for the estimation of respiration rates which are based on nighttime measurements. To investigate this potential underestimation, several approaches to estimate respiration were used. First, additional measurements of surface water concentrations of dissolved inorganic carbon were made. A comparison of respiration estimates based on oxygen and DIC changes during tidal isolation revealed a reasonable agreement for the most time but not during periods of high productivity during the day or late at night. At this point, oxygen concentrations are so depleted that a change in concentration – the indicator of respiration – is barely detectable. However, DIC based respiration rates indicate that respiration is occurring under these hypoxic/anoxic conditions. This saturation changes during periods of tidal inundation, when a nighttime peak in oxygen concentrations indicates that the flood water is relatively enriched in oxygen compared to the pond water. On three days, it was tested whether under these conditions the oxygen-based respiration rate was higher than under hypoxic conditions (i.e., during tidal isolation). The rates were indeed higher than those under tidal isolation but still not in the range of DIC-based rates. Overall, metabolic rates differed between the two ponds in magnitude, which is likely caused by different vegetation cover, but may be influenced by size, sampling period, and duration as well.