Ecological requirements of benthic foraminifera in natural and anthropized transitional environments : implications for biomonitoring

Overview & Synopsis

Transitional environments are at the interface of marine and terrestrial waters. Influenced by tides and flows, these naturally variable ecosystems, rich in organic matter, constrain their organisms. Species assemblages are tolerant to this variability and characterized by a low diversity. Increasing anthropogenic pressures on these environments also impact species assemblages and create difficulties to assess the origin of these pressure. Current indicators show their limits in these environments by attributing a bad ecological quality similarly to more anthropized ones. Therefore, to implement an indicator in these environments requires a good knowledge of species and environmental drivers influencing their repartition to decipher between anthropogenic and natural stresses. To address this question, we used benthic foraminifera, an acknowledged group of bioindicator of the meiofauna, in several transitional environments of the English Channel (bay, estuary, harbour) and the Gironde estuary. We first developed a method to quickly and morphologically identified three pseudo-cryptic species of Ammonia to better understand their ecological requirements and their distribution in transitional environments. One of them, Ammonia confertitesta, is a reported non-indigenous species coming from Asia and potentially replaced the two other indigenous species/ones. We defined commercial harbours as the specific habitat of this species, reinforcing its exotic status in Europe. In the second part of this thesis, we determined that species composition was influenced by a biogeographical gradient and gradient of disturbance, in organic carbon, trace metals and grain size and established a list of indicator species according to the level of disturbance. We further identified four non-indigenous species yet unknown in the English Channel. We also compared foraminiferal assemblages of the Dunkerque harbour with those of a classically used indicator, the macrofauna. The diversity was richer than expected in the harbour, though considered as depleted ecosystem, highlighting the need to give more attention to harbour ecosystems. Although foraminifera showed a better pertinence to detect organic pollution, the related biotic indices showed some limits to detect moderately polluted states. This result suggests that more quantitative data are still needed on benthic foraminifera from these environments and that foraminiferal biotic indices may benefit from upgrades to better monitor the ecological quality when there is less or low pollution.