Monterey Bay Aquarium Research Institute
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Land/Ocean Biogeochemical Observatory in Elkhorn Slough

The Relationship Between Nitrate Concentration and the Tidal Cycle

The nitrate concentration in Moss Landing Harbor and at the L01 mooring provides an illustration of how the tidal cycle creates significant changes in water properties of the main channel of Elkhorn Slough. Changes in nitrate, salinity, and temperature (Figure 1) record the different water masses that pass the L01 mooring. Hypersaline water from the upper slough can be seen at low tide, while the incoming tide brings nitrate into the slough.

Figure 1

At least two different end-members contribute to the nitrate concentration of the water during the incoming tide. The first is via internal waves that bring deep, nitrate rich water from Monterey Bay to the near-shore surface, which is then advected into the slough with the tidal bore (Chapin et al, 2004). This water mass can be detected from the low temperature signal (e.g. Figure 1). The second is via the Old Salinas River that enters the South end of Moss Landing Harbor. The nitrate concentrations in these waters can be as high as 1500 µM. The high nitrate, low salinity waters are mixed with the incoming tide water, and exit on the following ebb tide. These waters reach the mouth of Elkhorn Slough during low tide, where they are mixed and transported up the main channel during the next flood tide (Figure 2).

Figure 2






Monterey Bay water fills much of Elkhorn Slough at high tide. Water from the upper slough (pink) and Old Salinas River (yellow) reach the mouth of the Slough during low tide, therefore water from Old Salinas River can be mixed and transported up the slough on the next flood tide.







This process can be see when comparing the peak height of nitrate in the Moss Landing Harbor and the Main channel, as they are seperated by an ebb tide-flood tide change (Figure 3).

Figure 3
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Last updated: Jan. 04, 2005