Aarhus University Seal / Aarhus Universitets segl

No. 394: Rock reef as a possible nitrogen agent

Stæhr, P.A., Markager, S., Høgslund, S., Hansen, J.W., Tonetta, D., Upadhyay, S. & Nielsen, M.M. 2020. Stenrev som muligt kvælstofvirkemiddel. Vækstbetingelser for bentiske alger og deres betydning for ilt- og næringsstofdynamikken i Limfjorden. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 108 s. - Videnskabelig rapport nr. 394. http://dce2.au.dk/pub/SR394.pdf

Summary

Stone reefs are very productive habitats which under favorable environmental conditions can establish a high coverage of macroalgae, extending from shallow to deep waters. This report provides a scientific basis for assessing whether the reestablishment of a stone reef in The Limfjord may, in a long term, contribute to reducing internal nutrients loads, and at the same time improve both the ecological and the conservation state of The Limfjord. The investigation included a comprehensive series of experimental and field-based studies of light requirements, physiological stress in macroalgae, light conditions at the bottom waters, water quality and different processes for oxygen and nutrients dynamics in The Limfjord.

Our results demonstrated that the stone reef in The Limfjord under the current poor light conditions, had no positive effect on either oxygen or retention and removal of nitrogen or phosphorus. Rather, we found that dominance of oxygen-consuming organisms on the larger stones contributed to an elevated benthic oxygen consumption which increased with depth. Under current conditions, stone reefs pose a threat to oxygen conditions, especially if established below the pycnocline. This was particularly evident in 2018, where extreme high temperatures combined with an elevated oxygen demand by heterotrophic organisms caused oxygen depletion even in shallow waters. Furthermore, as new stone reefs are to be placed on a consolidated base with a high carrying capacity, new stone reefs are also expected to replace a mixed hard bottom with a powerful film of benthic microalgae that is effective in producing oxygen and binding nitrogen and phosphorus, at least temporarily.

An important reason for the non-effects of macroalgae was the low coverage of large perennial macroalgae, and the relatively small area of the stone reef. Also the reefs were dominated by oxygen demanding sessile animals. Results from a series of laboratory experiments showed that, under realistic conditions, the minimum light requirements of macroalgae was up to 6 times higher than previously assumed. In addition, recurrent periods of oxygen depletion and high water temperatures were found to have a markedly negative effect on the macroalgal performance. Therefore, there is a low probability that macroalgae will establish viable populations in the stone reef at depths where low oxygen concentrations occurs in late summer.

By combining results from macroalgal light requirements, with data of light availability, a reduction in light attenuation up to 42% was needed for macroalgae to contribute positively to oxygen production at greater depths in the central parts of The Limfjord such as Løgstør Bredning. Such a reduction, requires a significant reduction in the external nitrogen and phosphorus supplies, which will primarily reduce the light attenuation from phytoplankton. A significant improvement in the overall light attenuation is expected to take many years, because it requires a reduction in the organic matter concentration, which currently dominates the light attenuation. Under current turbid conditions, it is not recommended to establish stone reefs in deeper parts of the fjord as they will become dominated by oxygen consuming fauna. Given that a positive effect of stone reefs on benthic oxygen conditions requires large nutrient reductions, establishment of reefs cannot be recommended as an N-mitigation tool.