Sediment and copepoda eggs suspension by waves on a salt marsh, Bahia Blanca estuary, Argentina

PERILLO, M.M.; MOLINA, L.M.; PERILLO G.M.E.

Bariloche

Congreso; IV Congreso Latinoamericano de Sedimentología; 2006

Asociación Argentina de sedimentología

The Bahía Blanca Estuary is characterized by being in an erosive stage due to the lack of sediment input from rivers and inner shelf. Typical suspended sediment concentration (SSC) in the channels range between 100 and 200 mg/l. Most of the energy in the estuary is provided by a semidiurnal tide with tidal amplitude varying from 2 m at the mouth and 4 m at the head. However, wind is a major feature of the area, being predominantly from the NW and N for over 40% of the time. The present study was directed to estimate the effect of short, very steep waves produced by strong winds on a Spartina alterniflora marsh in comparison with the dynamics of tidal currents. Further, we have investigated the degree of resuspension of sediment and copepod eggs from the surface sediments. The marsh, at the study site, is a silty sand (very fine) with low density of plants.Measurements were made with an Acoustic Doppler Velocimeter located at 5 cm over the bed and water samples taken at four levels (2, 3.5, 10 and 20 cm) from the surface by pumping 1 l every 15 min during all the flooding period of the marsh. Maximum water depth was 1.26 m. The three components of the flow were filtered and spectrally analyzed, while SSC was estimated by filtering through 0.45 μm pore diameter filters. Recognition and counting of copepod eggs were made using a binocular magnification stereoscope.Even though waves were, on average, 2 s in period and only up to 5 cm high, the wave shear stress was up to 2 orders of magnitude than those induced by the tidal currents. As a result, SSC reached up to 8 gm/l, values that were observed in the estuary only during heavy dredging periods by formation of fluid mud by water injection. Copepod eggs require being in suspension to be fertilized, otherwise they remain in the sediment surface. There is a clear increase in the egg concentration (up to 450 eggs/l) from the initial flooding to the high tide (where wave activity was larger). This high concentration was maintained, except for a strong drop during mid ebb, following which it returned to high values by the end of the inundation period. Wave activity on the marsh was very important to insure this reproductive process since the eggs were maintained in suspension (as well as the sediment) and transported to the main channel of the estuary by the ebbing tide.