Background

The agency for Green Transition and Aquatic Environment (Styrelsen for Grøn Arealomlægning og Vandmiljø, formerly Miljøstyrelsen/Danish EPA) is a governmental body responsible for environmental management and monitoring. As part of their national environmental monitoring program, NOVANA, the agency conducts periodic measurements of water quality parameters in the Danish seas.

Historically, the agency has used ship-based profile measurements to monitor water quality parameters such as temperature, salinity and dissolved oxygen. While this method provides good accuracy and information about the full water column, it is costly and therefore only conducted at each location every few weeks. This sampling frequency is too low to capture the large temporal variations typically seen for parameters like dissolved oxygen concentration. This means that important events such as short periods of hypoxia may go undetected. .

Challenge

Water quality sensors mounted in fixed locations can provide the high temporal resolution needed to capture the dynamic behaviour of the ocean. However, the high cost of the required equipment has traditionally prevented the widespread use of continuous online measurements. The agency sought a cost-effective and scalable solution to complement their profile measurements, and enable a more comprehensive registration of oxygen concentrations in the Danish waters.

Solution: Deploying Havsans Marine Buoys

As part of the development project “Integreret Marin Miljøovervågning” the agency deployed six Havsans Marine Buoys at strategic locations around the island of Funen, Denmark. These sites were selected to investigate and demonstrate the extent to which water quality parameters vary across different locations within the inner Danish Waters.

Each buoy was connected to sensors measuring dissolved oxygen, temperature, and salinity at a fixed position just above seabed. Measurements were taken every 30 minutes and data was immediately transmitted and made accessible online via the Havsans data platform. This enabled the agency to track how the environmental conditions in the sea were changing on a day-to-day basis. In some shallow locations it was possible to see how the daily photosynthesis in eelgrass and other vegetation sent oxygen soaring, while in other deeper locations, respiration and lack of water exchange led to gradual oxygen depletion and eventually hypoxia.

Since the agency was already conducting monitoring under the NOVANA program, the Havsans Marine Buoys could be added as another component to this monitoring with very little extra cost and effort. Buoys were simply deployed or recovered as part of the agency’s regular monitoring activities. Because the buoys and associated bottom rigs are lightweight, no additional equipment was needed for deployment, allowing the agency to use their existing vessels.

Results and Impact

By employing Havsans Marine Buoys, the agency is now able to perform more comprehensive monitoring of a number of environmental parameters in the Danish Waters. The addition of online in-situ measurements to the existing profile measurement program means that the combined monitoring efforts now provide a good resolution of how each parameter varies both in vertical space and in time.

While this capability is valuable for all measured parameters, it is particularly important for the monitoring of dissolved oxygen. Excessive nutrient input and resulting hypoxia is one of the main factors contributing to the poor environmental state of most of the Inner Danish Waters. As demonstrated by the recent monitoring data, hypoxia events are often shorter than 2 weeks making them difficult to detect using periodic profile measurements only. With the Havsans Marine Buoys these events can now be detected in real-time.

Examples of dissolved oxygen concentrations obtained from profile measurements (red crosses) and Havsans Marine Buoys (blue lines). The top graph shows data from a very shallow location (Odense indre fjord) where photosynthesis causes very high oxygen concentrations in the middle of the day. The bottom graph shows data from 20 m depth (Ballen) where respiration typically causes the oxygen concentrations to decline steadily towards zero. Sometimes however, the arrival of more oxygen rich water causes a sudden increase in dissolved oxygen levels. Notice these dynamics are not visible when only considering data from profile measurements (red crosses).

The availability of more data and new types of data also opens up new possibilities for how the data can be used. The agency has a close collaboration with DHI, who specializes in numerical modelling of ocean dynamics and water quality. In a new research project, the agency and DHI are working on improving the accuracy of such numerical models by using data assimilation techniques. They found that incorporating continuous measurements obtained from the Havsans Marine Buoys significantly improved the accuracy of the numerical models. The monitoring campaign has demonstrated some of the multiple ways in which continuous measurements from Havsans Marine Buoys can improve our understanding of the dynamics and environmental state of the sea. At Havsans, we are excited to continue our collaboration with the agency and DHI to further document and improve the state of the marine environment.