The Marine Influence Index (MII): A Tool to Assess Estuarine Intertidal Mudflat Environments for the Purpose of Foraminiferal Biomonitoring

Abstract

In this paper, we propose a marine influence index (MII), which is thought to give an integrated quantitative description of the complex of the environmental parameters controlling the foraminiferal fauna in estuarine intertidal mudflats. The MII contains three components, as follows: (1) the relative distance along the salinity gradient, (2) the emergence time relative to a reference tidal cycle, and (3) the relative importance of river outflow in the 30 days before sampling the foraminiferal fauna. Although these three parameters all have a strong relation with salinity, they also implicitly include other environmental parameters, such as the introduction of marine and continental organic matter and biota, hydrodynamic energy, or temperature. In order to show the functioning of this new index, MII is calculated for 28 stations in the Auray and Vie estuaries, for two different periods. The next step will be to compare the MII with faunal data sets. Ideally, this comparison should allow us to find strong correlations between some characteristics of the foraminiferal assemblages and the MII. If such strong correlations were indeed found, any major deviation of this relationship could then be interpreted as being due to strong anthropogenic disturbance.

Foraminiferal Distribution in Two Estuarine Intertidal Mudflats of the French Atlantic Coast: Testing the Marine Influence Index

“…At present, salt water penetrates up to 8 km landward where it is blocked by a dam. Based on the thalweg topography, Jorissen et al [40] deduced that before the construction of this dam, salt water penetrated up to the village of St. Maixent, 16.8 km from the mouth of the estuary. A 2019 environmental survey, performed in the context of the Marine Framework Directive, evaluated the environmental quality as "good", on the basis of intertidal and opportunistic macroalgae and autotrophs other than phytoplankton [43].…”

“…The Marine Influence Index proposed by Jorissen et al [40] was tested in this study. The MII combines the position of the sampling site along the estuarine gradient, the duration of the tidal emergence, as well as the riverine fresh water input during the 30 days before sampling.…”

“…In this study, these constants are set to 1, giving equal weight to the three components. See Jorissen et al [40] for further explanations. The values of MII computed for this study are presented in Supplementary Table S10.…”

“…In the Vie estuary, stations 3 to 5 had values between 0.95 and 1.5 (high to good ecological quality), whereas for all other sites, the values varied from 2.60 to 3.03 (good ecological quality). Jorissen et al [40] suggested that the Marine Influence Index (MII) should explain part of the foraminiferal distribution in estuaries. MII values are presented in Supplementary Table S10.…”

“…In an attempt to integrate the main natural factors affecting estuarine foraminifera in a single environmental index, Jorissen et al (this volume) [40] proposed the Marine Influence Index (MII). The MII, calculated for each sampling station, is based on the position of the studied station along the salinity gradient, the duration of tidal emergence and the relative importance of river runoff in the ambient water turnover.…”