Continuous automated sampling of tidal exchanges of nutrients by brackish marshes

Abstract

Automated instruments were used to measure tidal exchanges of materials at two brackish marshes of differing elevation. The instruments, located at the mouths of the tidal creeks draining the marshes, measured depth and tidal current velocity, and pumped water samples in volumes proportional to flow. Separate composite samples of flood and ebb water were collected weekly for about two-thirds of the weeks throughout a 2–3-year period and analysed for orthophosphate (PO 4 3−), total organic P (TOP), ammonium (NH 4 +), nitrate plus nitrite (NO 3 −), total organic N (TON), total organic C (TOC), and total suspended particles (TSP). Exchanges of these materials differed for the two marshes. At the lower elevation marsh, the largest net flux was an import of TON, but TON was exported by the high marsh. The low marsh also imported TOP, NO 3 −, and TSP, but there were no significant exchanges of these materials at the high marsh. The largest net flux at the high marsh was an export of TOC, but there was no significant net exchange of TOC at the low marsh. Both marshes exported NH 4 +. At the low marsh the tidal exchange of PO 4 3− and NH 4 + differed among years, with PO 4 3− exchange differing in direction as well as magnitude. In general, the high marsh was more prone to export materials, especially organic N and C, while the low marsh was more dominated by deposition and tended to import materials. Compared to manual techniques previously used to measure tidal exchanges, automated sampling permits lower-cost observations of many more tidal cycles, including observations during episodic storms or extreme tides. Although strong effects of such episodic events were not detected by the weekly composite sampling used here, the inclusion of such events in the sampling probably increases the accuracy of estimates of average tidal exchanges.