The complex nature of sea pollution in urban areas due to antropic activities prevents traditional monitoring techniques from obtaining measurements representative of true human exposure and the real health status of an urban context. 'Citizen Observatories', at urban, regional or global level, used together with innovative technologies can strengthen environmental monitoring capabilities and reduce running costs of institutional protocols. Citizens Observatories (CO) by now are viewed as a valuable approach to better observe, understand, protect and enhance the environment. The increasing number of mobile phone and low cost devices enable citizens to provide timely geospatial information to participate with scientists, stakeholders and policy maker to manage both emergencies and the day-to-day fundamental resources. This methodology in which citizens are active participants in the generation of an innovative and non-formal scientific knowledge is named Citizen Science. As such, CO contribute to transparency in governance, involvement of citizens in co-design of envirnmental applications and decision-making. We are currently translating into practice a model of citizens' observatory, named Osservatorio del Mare a Molfetta, to monitor a toxic algal bloom and the status of a posidonia medow along the coastal area of Molfetta, a city on the South Adriatic. We produced spatially distributed datasets of concentration of Ostrepsis ovata for the dual purpose of facilitating scientific knowledge building and providing a resource for citizens to inform themselves of the risks associated with recreational use of the coastal area. We also perform population genetic studies on the declining Posidonia oceanica meadow along the city coast with the aim of providing new insights into the knowledge of the sea grass necessary to promote good conservation practices before the meadow regresses irreversibly.