Development of the benthic macrofauna community after tidal restoration at Rammegors : Progress report 1

Abstract

The managed realignment project Rammegors aims at re-establishing estuarine nature in a formerly freshwater wetland. For this purpose, a tidal inlet was constructed in the Krabbekreekdam to generate a reduced tidal influence that should allow the development of salt marsh area, tidal flat area and low energetic shallow sub-tidal area. A central question of the Rammegors tidal restoration project is how the flora and fauna will develop in the area. To understand the biotic and abiotic processes related to the tidal restoration in Rammegors, a monitoring programme is being executed by the Centre of Expertise Delta Technology. The monitoring focuses on the main biotic and abiotic developments in the area. This report only focuses on the early colonization of macrobenthic invertebrates in Rammegors. Benthic macrofauna and sediment sampling took place in spring and autumn 2017 and 2018 at twenty-four stations along 5 transects with varying distances from the inlet. A fast colonization of the benthic macrofauna was observed in Rammegors. Twenty-two taxa were observed in the area in spring 2017. The communities significantly differed between spring and autumn 2017. In spring densities reached high values, especially for the brackish mud shrimp Manocorophium insidiosum and mosquito larvae Chironomidae. In autumn these brackish species almost disappeared from the area. Their presence in autumn 2017 and spring 2018, even in low numbers, as well as the observation of the brackish cockle Cerastoderma glaucum indicate that a year after the tidal restoration, parts of Rammegors are still under influence of brackish water. In 2018 these species declined, indicating the transition to a marine environment. A significant relation was found between elevation and the benthic macrofauna. Total density, biomass and species richness showed a significant relation along the elevation gradient with an optimum around +0.4m NAP and lower values at both lower and higher elevations. Total density significant increases with an increase in chlorophyll-a, silt content and organic matter. A significant decrease in total density and species richness was observed with increasing bulk density, indicating that more compact sediments contain less species in lower abundances. Total density, biomass and species richness increased with increasing distance from the inlet. After two years, it is unknown if the community within the Rammegors area reached stability or develops at a slow pace. Future monitoring and a comparison to a community found in a comparable natural habitat is needed to study this.