Description: Shallow, semi-enclosed coastal systems are particularly prone to eutrophication. Depending on local site conditions and historical nutrient legacies, sea-based measures might be necessary in addition to land-based nutrient removal. In this study, C:N:P ratios were combined with open-source bathymetric information and linked with the prevailing geomorphological and sedimentological regimes to gain insights into nutrient hotspots and understand their sources and fate in coastal waters. Land-based sediment samples were taken behind outlets at three sites in Eckernförde Bay (Baltic Sea), and complemented with ship-based sampling at locations approximately 8 m and 12 m water depth. The total carbon, nitrogen and phosphorus concentrations in surface sediments increased at deeper sites. This suggests that an increased downslope particle transport and deposition regime, based on local geomorphology, might influence nutrient hotspots to a larger extent than proximity to sources (e.g., outlets). Overall, the recorded C:N ratios (mean = 28.12) were closer to the ratio of terrestrial plants than those of marine phytoplankton, indicating allochthonous sources of organic matter.

Description: Domoic acid (DA) is primarily produced by the ubiquitous marine diatom genus Pseudo-nitzschia. DA is primarily known for its neurotoxic effect on higher trophic level organisms and particulate DA (pDA) has thus been well studied. Dissolved DA (dDA) can, however, substantially contribute to the total DA of toxic blooms since it can be released into the water in high amounts. So far, dDA has not been as extensively studied, leaving knowledge gaps about its large-scale distribution, its carbon contribution to marine dissolved organic matter (DOM) and its ecological function. The motivation of this thesis was to fill in these gaps by elucidating the ecological and biogeochemical role of dDA in the Atlantic Ocean with a spatial focus on the Arctic and Antarctic sectors. I aimed at (i) developing a sensitive quantification method for dDA in seawater, (ii) investigating its broad-scale distribution and contribution to DOM, and (iii) improve our understanding about the ecological role of dDA. The highly sensitive quantification method allowed the determination of dDA concentrations in the East Atlantic Ocean, where it was ubiquitous and decreased with water depth. Together with the radiocarbon dates of dissolved organic carbon this points to a fairly high persistence of dDA in the ocean. In a standard DOM solid-phase extraction method DA showed high recovery rates, allowing a chemical identification within DOM. In the Southern Ocean, a high-nutrient, low-chlorophyll (HNLC) area due to low iron bioavailability, dDA also occurs. I thus tested its suggested ligand function for the Antarctic species P. subcurvata. Although dDA availability did not increase iron uptake, intracellular copper levels of iron-depleted treatments increased, potentially due to dDA. I also studied dDA’s occurrence in three iron-replete Arctic fjords, differing in their glaciation state. Pseudo-nitzschia counts and pDA correlated with dDA, which was influenced by macronutrient availability. This thesis contains the first verification of the in situ presence of DA biosynthesis genes, which points to an active DA production. Overall, this thesis provides new insights into the distribution of dDA in the East Atlantic Ocean, its contribution to DOM, its connection to environmental parameters, and its potential ligand function.