Kathrine Jul Hammer:
Aquatic phototroph performance under global human impacts

Date: 03-09-2019    Supervisor: Jens Borum & Kaj Sand-Jensen

Human activities have a growing effect on coastal and lake environments. Agriculture, direct physical activities, and climatic changes have caused increased eutrophication, more suspended particles, higher temperatures, and alterations in the concentration and composition of dissolved inorganic carbon (DIC). I have addressed these anthropogenic impacts on freshwater phytoplankton and marine seagrasses. Phytoplankton and seagrasses are among the most productive primary producers in lakes and coastal waters with large roles in forming their specific environment. In this thesis, I present new aspects of how phytoplankton and seagrasses are affected by major human impacts on their environments.

Using a combination of traditional growth measurements and novel techniques for analysing internal oxygen and metabolic composition, I show (chapter 1), that high temperature can have negative impacts on Zostera marina growth and survival due to altered nitrogen metabolism, despite high oxygen concentrations in the tissues. This indicated direct temperature damage to the plants.

Chapter 2 reveals that increasing DIC and CO2 concentrations had a positive effect on Zostera marina photosynthesis in short-term laboratory experiments under saturating light conditions. The positive effect of CO2 was highest in DIC-poor water because less bicarbonate was available to support photosynthesis. This result calls for more careful and local predictions of future effects of increasing CO2 levels in coastal waters of different DIC concentrations. The impact of DIC was also profound in mesocosms with freshwater phytoplankton under nutrientreplete conditions (chapter 3). DIC supply stimulated photosynthesis, growth and biomass buildup of phytoplankton on both daily and long-term scales. Alkalinity and pH in the water played an important role in determining CO2 uptake from the atmosphere.

In chapter 4, we demonstrate that suspended particles in the water column, which settled on leaves of Zostera muelleri, hampered internal aeration and reduced photosynthesis under illumination and promoted toxic sulphide intrusion in darkness in the field.

I conclude that high temperatures can have a direct negative effect on seagrass health, and I show that direct temperature stress may be caused by an altered nitrogen metabolism and cell damage. Likewise, seagrass may be negatively affected by suspended particles typical of physically disturbed coastal waters. I conclude, that DIC, under some conditions, may play a much larger role in controlling aquatic primary production than previously presumed, and that high inorganic carbon supply may have a positive impact on both seagrass and phytoplankton growth.