Abstract
Offshore wind farms are part of the transition to renewable sources of energy, and both the total numbers and sizes of wind turbines are rapidly increasing. While the impact of underwater sound related to construction work has been in the focus of research and regulation, few data exist on the potential impact of underwater sound from operational offshore wind farms (OWF). Here, the knowledge gained on operational noise from OWF and its potential effects on marine mammal behavior is synthesized. In a case study, effects of a 10- and 20-MW direct drive OWF turbine on the hearing of marine mammals were assessed using a simple model spreadsheet based on geometrical spreading loss and hearing types as defined by NMFS/NOAA. Effects over 24 h exposure from a single turbine ranged between a few meters and a little over 700 m depending on the effect (permanent threshold shift (PTS)/temporary threshold shift (TTS)) and hearing group investigated. In general, impact ranges for PTS were small (i.e., up to 50 m from the sound source) and are thus likely negligible. The same was true for TTS impact ranges for 10 MW. However, impact ranges for TTS from a 20 MW turbine could reach a little over 700 m for low-frequency cetaceans which could bridge distances between wind turbines. Thus, impact areas from single turbines might overlap and the whole wind farm can be considered an impact area. It is concluded that for larger size wind turbines, operational noise needs to be considered in sufficient detail as a part of the environmental impact assessment in the wind farm planning phase. In addition, further observations and modeling efforts are necessary to increase the accuracy of the estimates.