Ocean models as shallow sea oxygen deficiency assessment tools: from research to practical application.

Oxygen is a key indicator of ecosystem health and part of environmental assessments used as a tool to achieve a healthy ocean. Oxygen assessments are mostly based on monitoring data that are spatially and temporally limited, although monitoring efforts have increased. This leads to an incomplete understanding of the current state and ongoing trends of the oxygen situation in the oceans. Ocean models can be used to overcome spatial and temporal limitations and provide high-resolution 3D oxygen data but are rarely used for policy-relevant assessments. In the Baltic Sea where environmental assessments have a long history and which is known for the world's largest permanent hypoxic areas, ocean models are not routinely used for oxygen assessments. Especially for the increasingly observed seasonal oxygen deficiency in its shallower parts, current approaches cannot adequately reflect the high spatio-temporal dynamics. To develop a suitable shallow water oxygen deficiency assessment method for the western Baltic Sea, we evaluated first the benefits of a refined model resolution. Secondly, we integrated model results and observations by a retrospective fitting of the model data to the measured data using several correction functions as well as a correction factor. Despite its capability to reduce the model error, none of the retrospective correction functions applied led to consistent improvements. One reason is probably the heterogeneity of the used measurement data, which are not consistent in their temporal and vertical resolution. Using the Arkona Basin as an example, we show a potential future approach where only high temporal and/or vertical resolution station data is integrated with model data to provide a reliable and ecologically relevant assessment of oxygen depletion with a high degree of confidence and transparency. By doing so we further aim to demonstrate strengths and limitations of ocean models and to assess their applicability for policy-relevant environmental assessments. [ABSTRACT FROM AUTHOR]