Reflections regarding our biogeochemical studies in lakes and marine environments

My fascination with the biogeosciences started with the investigation of nitrogen (N) and phosphorus (P) enrichment of lakes stimulating the growth of diatoms leading to increased sedimentation and eventual depletion of dissolved silicate from the water column. At that time most research on the global Si cycle was focused on weathering and had not explored the complexity of the terrestrial biogeochemical cycle. Our research demonstrated that diatoms and phytoliths, e.g. biogenic silica that accumulates in the living tissues of growing plants, are transported from lakes and rivers on the continents into the oceans. The emerging understanding is that the flux and isotopic composition of dissolved silicate delivered to the ocean has likely varied over time mediated by a fluctuating continental pool.

 

An important question we addressed was if reductions of P and N could reduce eutrophication and degradation of freshwater and marine ecosystems. Our analysis explored the rationale for only P or only N reductions and concluded that dual–nutrient reduction strategies were needed for aquatic ecosystems. A focus on only P or only N reduction should not be considered unless there is clear evidence or strong reasoning that reducing the inputs of only one nutrient is justified in that ecosystem and will not harm downstream ecosystems.

 

The depletion of dissolved oxygen in bottom waters is one of the common responses of aquatic ecosystems to eutrophication. A classic example is the semi-enclosed brackish Baltic Sea. Our research has shown that the Baltic Sea is the largest anthropogenically induced hypoxic area in the world, which has increased 10-fold during the last century due to increased nutrient inputs. Concurrently, the coastal zone has experienced increasing hypoxia during the same period with the Baltic Sea coastal zone containing over 20% of all known sites suffering from hypoxia worldwide. Our research has highlighted the continuously growing problems of hypoxia and anoxia with eutrophication.

 

Altered global biogeochemical cycles is not only a feature of the Anthropocene but ongoing geological processes. Our recent research has focused on the use of silicon isotope signatures of unaltered sponges and radiolarians to estimate dissolved silicate drawdown as a proxy for the changes in the productivity of diatom communities in the Mesozoic oceans and how the ocean chemistry changed with the evolution of diatoms. Our major results to date suggest that dissolved silicate has been low in the oceans for at least the last 100 million years because of the extreme efficiency of dissolved silicate uptake by diatoms reducing ocean concentrations.

 

My continued enchantment with biogeochemical processes and collaboration with other creative scientists has lead to uncovering new biogeochemical pathways which stimulates the drive to learn more about how ecosystems operate.