Ecosystem Function Measurement, Aquatic and Marine Communities John T Lehman, University of Michigan, Ann Arbor, MI, USA r 2013 Elsevier Inc All rights reserved Glossary Aerobic In the presence of oxygen Allochthonous Imported from outside the ecosystem Chemoautotrophy Use of energy-yielding chemical reactions as an energy source for synthesis of organic matter from inorganic precursors Compensation depth Depth where photosynthesis and respiration are in balance Euphotic zone Water depth with sufficient light for photosynthesis Photoautotrophy Use of light as an energy source for synthesis of organic matter from inorganic precursors Conceptual Framework Biological mass flux and energy transfer obey the first law of thermodynamics Investigations that follow this theme require basic accounting for inputs and outputs balanced on carbon mass, nitrogen mass, or the chemically bound potential energy present in organic matter The different bases for accounting are interlinked by the common stoichiometry of all protoplasm As Alfred Redfield and his colleagues began to point out in the 1930s, there are consistent ratios among carbon, nitrogen, and phosphorus in living matter drawn from lakes and oceans The ratios are not as rigid as the elemental composition of a crystalline mineral, but they are reliable within limits They stem from the fact that organisms exist as biochemical aggregates of proteins, lipids, carbohydrates, and nucleic acids The basic biochemical building blocks are universal, so the scope for differences among species is limited If a class of organisms dominates the organic nitrogen pool within an ecosystem, it likely dominates in terms of organic carbon or calories as well Thus the different measurement bases converge to common results The energy and mass-based approaches offer consistency of accounting, but they are inherently abiotic They are indifferent to the forces that govern self-propagating organisms with heritable traits, for which the law of natural selection is as important as the laws of thermodynamics Survival, persistence, and production of viable offspring are properties that define successful species, whether or not they dominate ecosystem biomass and material flux Another theme in measuring ecosystem function focuses on the properties of genetic entities rather than on the properties of conservation laws This second theme focuses on processes that add or subtract individuals or genomes within an ecosystem, processes such as birth, death, or migration, as well as the explicit match between individual age and the passage of time Viewed through a lens that traces heritable lines of descent, mass and energy transformations are incidental consequences Transformations of energy and material are the byproducts of life struggles whose object is indefinite persistence 64 Production Newly formed biomass of a population or trophic level, including the organic matter eliminated during the period of observation Trophic level Position in a food chain, defined by the number of energy transfer steps to that level Trophogenic region Region where net production of organic matter occurs by photoautotrophy or chemoautotrophy Tropholytic region Region where respiration and decomposition of organic matter proceed in the absence of primary production In 1961, Hutchinson coined the phrase ‘‘paradox of the plankton’’ to define the challenge to explanation presented by the species diversity of plankton communities Nets towed from several hundred meters depth to the surface in the Pacific Ocean north of Hawaii, for example, routinely collect more than 300 species of zooplankton, and the richness of phytoplankton species in the surface waters is equally great The oceanic pelagic region is the oldest continual habitat on the planet and its denizens are the product of ceaseless natural experiments Near coastal regions and in lakes and rivers, the permanent plankton are joined by the larvae of benthic organisms To Hutchinson, the contemporaneous existence of so many distinct genetic lines in habitats that lack obvious structural complexity begged important questions about coexistence, interactions, competition, and resource use Faced with a bewildering array of potential species interactions, investigations of the marine pelagic ecosystems historically have emphasized mass and energy flow rather than individual species dynamics Compounding the problem of overwhelming diversity is the fact that marine plankton are notoriously difficult to census accurately, owing to the physical movements of water masses, to such an extent that some workers believe that time series data that are essential to population studies are nearly impossible to gather In an effort to span the breach between measurements of organic composition in units of carbon or nitrogen and the realities of the ways matter is packaged as individual organisms, there has been great interest in finding rules of general validity and broad predictive strength that blend the approaches Special attention has been paid to relationships based on individual body size or biomass These size-based or allometric models of ecosystem processes offer the attraction of predicting metabolic rates and trophic interactions from simple measurements Extensive tabulations have become available for physiological processes such as respiration, motility, body growth, and feeding rates versus individual size This activity has spurred the drive toward models of ecosystem dynamics that treat the size structure of organisms present in a system more so than the phyletic composition of the communities Encyclopedia of Biodiversity, Volume http://dx.doi.org/10.1016/B978-0-12-384719-5.00154-4