Macrofauna succession in an infilling salt marsh clay pit

The high demand of clay as soil material for dikes leads to the problem of clay excavation in salt marshes, which is a major disturbance for salt marsh ecosystems. Clay was excavated from the salt marsh Petersgroden (western Jade Bay, Germany) in 1998/99. The 10 ha dredging hole (clay pit) was connected with the tidal flat system to infill and recover more naturally, with creeks as natural structures. The succession of benthic macrofauna was studied in comparison with the adjacent tidal flats (mud flat) at three stations at both sites directly after the excavation and during the following eight years (2000 to 2007). This long-term study revealed directional changes of macrofauna communities within the clay pit. The comparison to the adjacent mud flat enabled the differentation between the successional steps and the general high variability of intertidal habitats. Overall, during the first eight years of infilling the clay pit macrofauna resembled similar species and population dynamics as the macrofauna on the mud flat, but abundances and dominant species differed in space and time. Polychaetes and oligochaetes dominated both sites. As hypothesized, differences between macrofauna communities in the clay pit and on the mud flat resulted from successional processes within the clay pit. While the mud flat community remained rather similar, the clay pit community changed from 2000 to 2007. The results give evidence for directional succession in three phases. During the first three years (2000 to 2002), the clay pit community became more similar to the mud flat. We observed early successional stages: marine early pioneers (e.g. oligochaete Paranais litoralis ) recolonized the clay pit fastest and marine mud flat species with lower recolonization potential (polychaete Manayunkia aestuarina , ostracod Cyprideis torosa ) followed. During a second phase (2003 and 2004), clay pit and mud flat communities were almost identical, the clay pit macrofauna shared early successional properties with the mud flat macrofauna. The early pioneer species P. litoralis became scarce. In a third phase (2005 to 2007), intermediate stages of succession were observed: the clay pit macrofauna started to change from a mud flat towards a salt marsh community. Major indications were the general decrease of total abundance, the loss of marine species ( C. torosa and polychaete H. filiformis ), and the increase in abundance of terrestrial enchytraeid oligochates.