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Neoichnological Characterization of Gironde Estuary and Arcachon Bay, France: Testing the Brackish-Water Model


Neoichnological characterization of estuaries and fluvio-tidal mixing zones in delta distributaries repeatedly demonstrate predictable trends in infaunal size and trace distributions. These studies culminated recently in the development of a predictable and semi-quantitative model showing a decrease in trace diversity and density with decreasing salinity and into freshwater environments. The semi-quantitative model has not been tested extensively, and hence has not been widely adopted. Gironde Estuary, France is recognized as a “textbook” example of a tide-dominated estuary and has been heavily studied hydrodynamically and sedimentologically; it has not been characterized neoichnologically. Arcachon Bay, France is wave-dominated estuary that experiences nearly fully marine salinity throughout most of the bay, but also not been evaluated neoichnologically.

Neoichnological trends in both Gironde Estuary and Arcachon Bay follow those reported elsewhere. With decreasing salinity there is a corresponding decrease in trace size and trace diversity reflecting a decrease in the size and diversity of infauna. Gironde Estuary is polyhaline to euhaline (> 18 psu) at its mouth and salinity decreases to nearly fresh at approximately the midpoint of the tidal “funnel”. Traces are largest and most abundant at the mouth of Gironde Estuary and BI on intertidal flats can be very high. Outer Arcachon Bay is also polyhaline to euhaline, and sediments in these positions are colonized by multiple large diameter infauna; however, shifting sediments and wave reworking limit infaunal colonization such that bioturbation intensity remains low to moderate (BI 2-3; 6-60% bioturbation). A reduced number of infauna colonize central Arcachon Bay, and these infauna construct smaller burrows. Lower substrate mobility in central Arcachon Bay ensures a longer colonization window, which manifests as highly bioturbated sediments (BI 4-6: 61-100% bioturbation). In both Gironde Estuary and Arcachon Bay, where salinity approaches 1 psu (i.e., in the tidal funnel), traces are rare to absent (BI 0-1; 1-5% bioturbation), and trace sizes decrease. The freshwater reaches of rivers feeding into both systems are BI 0 and very rarely 1. This study reinforces the general tenants of the brackish-water model, but also demonstrates that trace distributions are both quantifiable and predictable as a function of salinity.