Nutrient flows and trophic interactions in natural microcosms: typologies, boundaries and complexities

Natural microcosms are small ecosystems with a range of nutrient sources and sinks. Natural microcosms vary in longevity and predictability, and have complex nutrient networks tractable to profiling. However, the boundaries of microcosms are sometimes difficult to demarcate, making measurement of nutrient inflows (subsidies) challenging. Principal dichotomies in microcosm classification are whether they are open or closed to chance nutrient inputs, and whether they are biological or non-biological in origin. These dichotomies affect microcosm boundary demarcation and nutrient flows. We argue that there is strong yet unrealized potential for natural microcosms to advance theory in biogeochemistry and ecological stoichiometry owing to their amenability to experimentation and measurement of elemental sources and sinks for entire microcosms. Correspondingly, the understanding of natural microcosms would benefit from biogeochemical and ecological stoichiometry, which includes the characterization of the elemental phenotypes of organisms and their proximate or ultimate drivers. Considering widespread stoichiometric flexibility in organisms and mixotrophy in ecosystems, new nutrient network models are required to incorporate this trophic complexity. Microcosms harbour conglomerates of nutrients and organisms within which accurate nutrient book-keeping can be achieved. This Opinion piece provides ideas, outlines problems that need solutions and re-emphasizes the advantages of natural microcosms as model systems. This article is part of the theme issue 'Life in natural microcosms'.