Pronounced early human impact on lakeshore environments documented by aquatic invertebrate remains in waterlogged Neolithic settlement deposits

At waterlogged archaeological sites paleolimnological approaches can provide important supporting information about conditions and processes of past human life and human impact on environments around former settlements. In this study, subfossil Cladocera and Chironomidae assemblages were analysed from Neolithic lakeside sediments uncovered at Zürich-Parkhaus Opéra (OP), Switzerland. Our main objectives were to assess how periodic settlement phases altered lakeshore environments and aquatic invertebrate communities during the Neolithic. Aquatic invertebrates occurred in considerable numbers throughout the investigated sediment sections, supporting that Neolithic settlements at site OP were established above the lake surface and sedimentation occurred mostly under water. Two separate aquatic invertebrate communities were distinguished: an impacted community within cultural layers and a pre- and post-impacted community in sediments above, below and in between cultural layers. Aquatic invertebrates indicated that human impact likely resulted in surplus organic material load and nutrient input into the water during the cultural periods. This substantially increased biological oxygen demand of the sediments and overall nutrient concentrations of the near-shore water and thereby led to hypoxic conditions. Chironomids showed generally higher amplitude assemblage changes than cladocerans. This could be explained by the very local influence of humans and higher susceptibility of the less mobile chironomids to local hypoxia in and above the sediment. After settlements were abandoned invertebrate assemblages rapidly recovered to pre-impacted states, suggesting their considerable resilience to local human impact. Our results confirm that cladoceran and chironomid remains can trace localised environmental changes associated with human presence and provide important information for the interpretation of prehistoric human activities.