Marine carbon dioxide removal (mCDR) and geological carbon storage in the marine environment (mCS) promise to contribute to the mitigation of global climate change in combination with drastic emission reductions. However, the implementable potential of mCDR and mCS depends, apart from technology readiness, also on site-specific conditions. In this paper, we explore different options for mCDR and mCS, using the German context as a case study. We challenge each option to remove 10 Mt CO2 yr-1, which accounts for 8-22% of projected hard-to-abate and residual emissions of Germany in 2045. We focus on the environmental, resource, and infrastructure requirements of individual mCDR and mCS options at a specific site, within the German jurisdiction when possible. Furthermore, we discuss main uncertainty factors and research needs, and, where possible, cost estimates, expected environmental effects, and monitoring approaches. In total, we describe ten mCDR and mCS options; four aim at enhancing the chemical carbon uptake of the ocean through alkalinity enhancement, four aim at enhancing blue carbon ecosystems’ sink capacity, and two employ geological off-shore storage. Our results indicate that five out of ten options would potentially be implementable within German jurisdiction, and three of them could potentially rise to the challenge. This exercise provides a basis for further studies to assess the socio-economic, ethical, political, and legal aspects for such implementations.

In light of ongoing climate change, it is increasingly important to know how nutritional requirements of ectotherms are affected by changing temperatures. Here, we analyse the wide thermal response of phosphorus (P) requirements via elemental gross growth efficiencies of Carbon (C) and P, and the Threshold Elemental Ratios in different aquatic invertebrate ectotherms such as the freshwater model species Daphnia magna, the marine copepod Acartia tonsa, the marine heterotrophic dinoflagellate Oxyrrhis marina, and larvae of two populations of the marine crab Carcinus maenas. We show that they all share a non-linear cubic thermal response of nutrient requirements. Phosphorus requirements decrease from low to intermediate temperatures, increase at higher temperatures, and decrease again when temperature is excessive. This universality in the thermal response of nutrient requirements is of great importance if we aim to understand or even predict how ectotherm communities will react to global warming and nutrient-driven eutrophication.

Foraging based on prey elemental content could be more pronounced if consumers leveraged differences in physical traits among stoichiometrically distinct prey to ingest more favorable food, allowing consumers to meet their threshold elemental ratios (TER). We offered the marine copepod, Acartia tonsa, different combinations of large and small diatom congeners, grown to be either stoichiometrically replete or nitrogen-deficient. When offered each diatom alone, A. tonsa exhibited compensatory grazing on large diatoms, and ingested more nitrogen-deficient cells. When offered diatoms that differed in both size and stoichiometry, copepods mixed their diets to achieve a C:N close to their independently estimated optimal TER. When offered pairs of different-sized diatoms of similar stoichiometry, A. tonsa preferred larger cells, suggesting evolutionary constraints drive choice when stoichiometric differences are minimal. Experiments describing selectivity on single traits may seriously underestimate how precisely copepods can optimize their dietary stoichiometry when feeding on a diverse natural prey assemblage.