Site Characteristics More Than Vegetation Type Influence Food Web Structure of Intertidal Salt Marshes

Salt marshes are under increasing anthropogenic pressures that have been reported to affect the diet of fish (e.g., change in prey composition and availability), eventually resulting in alterations in their nursery function. Most studies in Europe are based on fish gut content analysis, which only reflect a small proportion of pressures to salt marshes, and do not necessarily reflect long-term disturbances. In this study, we investigated the impact of salt-marsh vegetation type on trophic network structures (i.e., fish diet and trophic position). Primary producers (particulate organic matter, microphytobenthos, and dominant terrestrial plants), potential aquatic and terrestrial prey, and fish of two dominant species (sea bass and thinlip mullet) were sampled during the summer of 2010 in four creeks from two sites from Western France (the Mont-Saint-Michel Bay and the Seine Estuary). Analysis was undertaken using C and N stable-isotope compositions. Tested response variables (diet and trophic position) suggested a dominant site effect and a weaker effect of surrounding vegetation type. Site effect was attributed to differences in anthropogenic nitrogen inputs (with a steep increase in the Mont-Saint-Michel Bay) and tidal regime between the two bays, with more marine signatures associated with a higher frequency and duration of tidal flooding events in the Seine Estuary. A second hypothesis is that invasive Elytrigia acuta, which has recently replaced typical salt-marsh vegetation in Mont-Saint-Michel Bay, negatively impacted the native salt-marshes nursery function by modifying the access to terrestrial prey on this site. The trophic position of the sea bass and the thinlip mullet was unchanged by local salt-marsh vegetation, and considered consistent with their diet. This study highlights the relevance of stable-isotopes analyses for assessing long-term and integrative effects of changes in vegetation resulting from human disturbances in salt marshes.

Does the Invasive Predator Mnemiopsis leidyi A. Agassiz, 1868 Control Copepod Abundances in the Seine Estuary?

The indigenous ctenophore Pleurobrachia pileus (O.F. Müller, 1776) was common in the coastal waters of the English Channel in the early 1990s and showed very abundant populations in the downstream part of the Seine estuary. In 2005, the non-indigenous ctenophore Mnemiopsis leidyi A. Agassiz, 1868, a species native to the Western Atlantic, was reported for the first time in Europe in Norwegian fjords and in Le Havre harbour (Seine estuary, France). More recently, in 2017, both Pleurobrachia pileus and Mnemiopsis leidyi were recorded during suprabenthos and zooplankton sampling in the Seine estuary along a downstream-upstream transect. Both species show more abundant populations in May than in September. Conversely, copepods show a spatial distribution depending on the ctenophore distribution, with low copepod abundances in the downstream part of the estuary being associated with high ctenophore abundances, while high copepod abundances are recorded where ctenophores are absent or display low abundances. We propose that the intense predation of ctenophores on copepods is related to changes in hydrological conditions over the two last decades. This may explain the dramatic decline of copepod abundance in the Seine estuary, which could have a negative effect on its nursery role.

Use of environmental management data for mass flow estimations of plastic debris in rivers: The Seine River and The Huveaune River, France.

<p>Methods to quantify plastic transport in rivers have greatly improved during the past few years. As a first approach, visual counting is currently the simplest way to assess plastic transport with minimal effort and cost. It usually results in underestimations of plastic input into the sea of about one to two order of magnitude when compared to models such as the Jambeck’s approach. The latter shows statistical weaknesses and data availability issues leading to large uncertainties, while visual counting miss the water column compartment and often has a low spatiotemporal representativeness. In order to give another ground-truth estimation of plastic transport able to challenge both models and visual counting, we developed innovative methods based on environmental management data in the Seine estuary (500 m<sup>3</sup>/s) and the Huveaune River ( 2 m<sup>3</sup>/s; Marseille, France). First, we used data from institutional cleaning in the Seine estuary that consist in litter collection on riverbanks. Their efficiency was measured based on capture-recapture design. Mass flows of plastic debris were then calculated based on the capture rate over one year, the estimation of the fraction of plastic debris which are never collected (hidden or too small) and the assumption that all plastic debris strand on riverbanks. Second, we used data from bar screens spaced of 3 cm in the Huveaune, a small urban river flowing in Marseille, South France. All the water column is screened, and captured waste are automatically collected in dumpsters. Grab sampling were performed after a dry, a wet and a flood period. The corresponding annual mass flows of plastic debris was then calculated relative to the mean fraction of time corresponding to those hydrological periods over 2017 and 2018. Annual mass flows of plastic debris were normalized to the population in both basins. Although methods were different, mass flows of plastic debris per capita are very similar with 8.5 – 13.6 g/cap/yr for the Seine River and 2.4 – 14.9 g/cap/yr for the Huveaune River. This is one to two order of magnitude lower than the Jambeck’s approach. However, when focusing on the fraction ending into the Sea, bar screens in Marseille enable to decrease the mass flow of plastic debris of about one additional order of magnitude, while cleaning of riverbanks decreases it of about 10%. This is related to the nature of the rivers that calls for different solutions, screening the whole Seine River being a tricky idea. Nevertheless, when normalized to water volume, the Huveaune River is visually much more polluted (16.4–102.2 mg/m<sup>3</sup>) than the Seine estuary (9.0–14.5 mg/m<sup>3</sup>). In conclusion, environmental management data can help to estimate mass flows of plastic debris and calls for better consideration. However, they often need an improved scientific framework.</p>