Species-specific biomass estimation from gene copy number in metazoan plankton

Despite the importance of early life-history stages to ecosystem processes, challenges in the identification of the larval stages of species continue to slow progress in understanding the dynamics of marine ecosystems. A quantitative polymerase chain reaction-based method was developed and tested for estimating the species-specific biomass of juvenile copepods through targeted measurements of mitochondrial cytochrome c oxidase subunit I (mtCOI) gene copies in size-fractionated, mixed-species plankton samples. The assays are species-specific, amplifying a single product in the presence of focal-species DNA at levels as low as 2 pg total DNA, and in mixed-species samples where the focal species represented <1% of total DNA. Tests comparing mtCOI copy number to measurements of biomass across developmental stages found a robust linear relationship across a five order of magnitude range in animal biomass and DNA copy number (r(2) = 0.998). These results demonstrate that quantification of DNA copy number can be used to obtain estimates of species-specific biomass for metazoan plankton, which could transform population and community dynamics studies that include early life history stages.