Evaluating Species-Specific Naupliar Recruitment During the Winter-to-Spring Transition in the Northern Gulf of Alaska Using Molecular Tools

Abstract

The Gulf of Alaska is a highly seasonal environment that is characterized by an order-of-magnitude increase in copepod biomass in the photic zone between winter and spring. Copepod recruitment processes, including the location and timing of naupliar production, responsible for the transition from low-biomass winter conditions to the highly productive spring are not well characterized. The recruitment patterns of copepod nauplii were examined in Resurrection Bay, Alaska using biweekly sampling between January and March with zooplankton collected from three depth strata. Nauplii were identified using DNA metabarcoding and species-specific naupliar phenologies were contextualized with environmental data and copepodite and adult copepod population data. This study revealed that nauplii were abundant throughout the winter and were comprised of a diverse assemblage of species. The community composition changed over the course of the season, with different copepod species exhibiting three distinct naupliar phenologies. These include species with nauplii that were 1) present during the winter and absent during the spring, 2) absent during the winter and present during the spring, and 3) present during both winter and spring. Several closely related species were split across groups, revealing temporal niche partitioning of reproduction and naupliar phenologies. For most species in the third group, the presence of nauplii during the winter occurred despite the absence of ovigerous females. While ovigerous females may have been missed or the nauplii could have been sourced from reproductive populations outside of Resurrection Bay, it is also possible that some copepods overwinter as nauplii. Prior to the spring phytoplankton bloom, a moderate increase in chlorophyll α concentrations occurred during March, coinciding with a period of female maturation, an increase in naupliar abundances, and the appearance of later developmental stages. These observations suggest smaller increases in chlorophyll prior to the large spring bloom may be critically important to recruitment of copepod nauplii, their survival, and their growth.