We present the features and architecture of the LakeBase Semantic Service. The LakeBase Semantic Service is a web service for the management of semantic annotations and semantic search of ecological research data. It was developed in the scope of the LakeBase project to support the management of long-term research data of German lake ecosystems. The basic workflow using the service consists of the following steps: (a) Suggestion of semantic annotations–represented by concept IRIs–for provided data and metadata, (b) optional adjustment of suggested annotations by the data provider, (c) storing the suggested and adjusted annotations, (d) auto completion of search queries typed by data users, (e) suggestion of semantic annotations for search queries, (f) optional selection of suggested annotations for search queries by the data users, (g) semantic search of annotated data with (optionally annotated) queries. To enable presentation and management of the annotations, the LakeBase Semantic Service further allows outputting stored annotations, to copy annotation, to delete annotation, and to provide descriptions of annotation concepts. There are several sources of concepts for the semantic annotation. We defined a default programming interface for semantic data sources that has to be implemented for each source. These implementations are provided for ontology files, SPARQL endpoints, and other services like the World Register of Marine Species (WoRMS) webservice and the GFBio Terminology Service. Due to this modular architecture, further semantic data sources can be added with only little programming effort. The LakeBase Semantic Service will be used in the IGB Freshwater Research and Environmental Database (FRED). Further, the source code will be freely available to also allow the usage beyond the LakeBase project. KEYWORDS: Semantic Annotation, Semantic Search, Ontology Based Applications ACKNOWLEDGMENTS: This work was funded by DFG in the scope of the LakeBase project within the Scientific Library Services and Information Systems (LIS) program.