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We assessed the importance of boulder reefs to the oxygen dynamics of a shallow estuary during two growing seasons in 2017 and 2018. Using open-system diel oxygen measurements and benthic and pelagic incubations, we evaluated the relative contribution of pelagic and benthic habitats to the ecosystem metabolism along a depth gradient in two areas, with (Reef) and without (Bare) boulder reefs in the Limfjorden, Denmark. System integrated areal rates of gross primary production (GPP) and ecosystem respiration (ER) both increased with depth in both areas. Benthic contribution to system GPP and ER was highest at shallow depth where it represented 47 and 53% respectively. However, with increasing depth pelagic processes dominated GPP and ER (98 and 94%) even in the Reef area. Although the Reef area had higher biomass of auto- and heterotrophic organisms (macroalgae and macrofauna), benthic GPP was at similar level in both areas, due to a significant contribution from micro-phytobenthic organisms. The Reef area had lower sediment pools of organic matter, nitrogen and phosphorous and was slightly more oxygenated compared to the nearby Bare area. Extreme temperatures and higher levels of nutrients in 2018 caused a marked increase in benthic ER rates resulting in net heterotrophy (NEM = GPP − ER < 0) in 2018 compared to net autotrophy (NEM > 0) in 2017. Under current eutrophic conditions, boulder reefs do not contribute positively to the oxygen dynamics in the estuary. Reoccurring blooms of phytoplankton with high organic matter decomposition combined with high temperatures and dominance of fauna stimulate depletion of oxygen around the reefs. Significant improvements in water clarity are needed to regrow perennial macroalgae and induce net autotrophy. Under current turbid conditions, it is only recommended to establish boulder reefs in shallow (We assessed the importance of boulder reefs to the oxygen dynamics of a shallow estuary during two growing seasons in 2017 and 2018. Using open-system diel oxygen measurements and benthic and pelagic incubations, we evaluated the relative contribution of pelagic and benthic habitats to the ecosystem metabolism along a depth gradient in two areas, with (Reef) and without (Bare) boulder reefs in the Limfjorden, Denmark. System integrated areal rates of gross primary production (GPP) and ecosystem respiration (ER) both increased with depth in both areas. Benthic contribution to system GPP and ER was highest at shallow depth where it represented 47 and 53% respectively. However, with increasing depth pelagic processes dominated GPP and ER (98 and 94%) even in the Reef area. Although the Reef area had higher biomass of auto- and heterotrophic organisms (macroalgae and macrofauna), benthic GPP was at similar level in both areas, due to a significant contribution from micro-phytobenthic organisms. The Reef area had lower sediment pools of organic matter, nitrogen and phosphorous and was slightly more oxygenated compared to the nearby Bare area. Extreme temperatures and higher levels of nutrients in 2018 caused a marked increase in benthic ER rates resulting in net heterotrophy (NEM = GPP − ER < 0) in 2018 compared to net autotrophy (NEM > 0) in 2017. Under current eutrophic conditions, boulder reefs do not contribute positively to the oxygen dynamics in the estuary. Reoccurring blooms of phytoplankton with high organic matter decomposition combined with high temperatures and dominance of fauna stimulate depletion of oxygen around the reefs. Significant improvements in water clarity are needed to regrow perennial macroalgae and induce net autotrophy. Under current turbid conditions, it is only recommended to establish boulder reefs in shallow (

Rapporten analyserer betydningen af klimaændringer siden år 1900 for det estimerede behov for reduktion i næringsstoftilførsler, som skal sikre mindst god økologisk tilstand i kystvande. Resultaterne viser, at miljømålet for klorofyl ville have været højere i et år 1900 klima sammenlignet med det nuværende klima og indsatsbehovet ville være ca 1500 Ton N/år lavere. Endvidere viser resultaterne, at statusværdien for ålegræs indikatoren i de fleste vandområder ville være lavere i et år 1900 klima, hvilket resulterer i et indsatsbehov som er ca 600 Ton N/år højere, såfremt statusværdien var fastlagt i et år 1900 klima og ikke i det nuværende klima.

Cyanobacterial blooms continue to be a serious nuisance to water quality managers, and remedial measures that are ecologically sound, economically feasible, and efficient in responding to the scale and intensity of the problem are required for their control. Here, we evaluated the potential of a new technique to adequately mix the water column and control cyanobacterial blooms, employing a surface-mounted solar-powered water mixer (SWM). Flow dynamics, thermal structure of the water column and phytoplankton photo-physiology were compared before and during operation of the SWM in Falling Creek Reservoir to determine whether the device helps weaken thermal stratification, deepen surface mixed layer, and entrain surface cyanobacteria into deeper water to light-limit their growth. Water circulation generated by the SWM resulted in weak velocity fields to a lateral distance of 5 m from the device, to a depth of 1.5 m from the surface. Moreover, the mixing minimized the surface water temperatures by approximately 2 °C and deepened the surface mixed layer by 1 m up to a lateral distance of 3 m from the SWM, but little change occurred beyond this distance even after continuous operation of the device for more than a week. Photo-physiological observations also suggested light-limitation in cyanobacterial growth up to a lateral distance of 5 m from SWM during intense sunlight conditions. The results showed that the SWM has limited zone of influence and is unable to adequately mix lakes and reservoirs to effectively control cyanobacterial blooms.