not-yet-known not-yet-known not-yet-known unknown Invasive species are a major challenge to managers worldwide due to their ability to rapidly change their niche space and behavior in order to reproduce and survive in a novel environment. One globally distributed invasive species, the Pacific oyster, Crassostrea (Magallana) gigas, first arrived in Sweden in 2006. To date, they have not colonized the low salinity waters of the Baltic Sea, and it has generally been assumed that the inability of Pacific oysters to reproduce in low salinities has acted as a barrier. We have studied differences in fertilization rates among oysters along the invasion range, and examined heritability of salinity tolerance through classic mating designs across five different salinity levels. We found that fertilization rates in low salinities increased with proximity to the range front, and that there was a strong heritable component to these differences. We then used whole genome sequence data to identify genomic regions that diverged significantly from expected Mendelian inheritance in larval full-sib families that survived in low salinities. Our results show that a chromosomal region containing coding sequences for all histones forming the nucleosome, as well as a region containing LSU and SSU subunit ribosomal DNA in the sperm, are involved in low-salinity tolerance at fertilization and early development. We also found no evidence of recent bottlenecks or reductions in genetic diversity along the invasion front compared to more established populations. We therefore conclude that the Pacific oyster has the potential to adapt further to low-salinity conditions and may invade the Baltic Sea.

Ocean phytoplankton play a critical role in the global carbon cycle, contributing ~50% of global photosynthesis. As planktonic organisms, phytoplankton encounter significant environmental variability as they are advected throughout the ocean. How this variability impacts phytoplankton growth rates and population dynamics remains unclear. Here, we systematically investigated the impact of different rates and magnitudes of sea surface temperature (SST) variability on phytoplankton community growth rates using surface drifter observations from the Southern Ocean (> 30oS) and a phenotype-based ecosystem model. Short-term SST variability (<7 days) had a minimal impact on phytoplankton community growth rates. Moderate SST changes of 3-5oC over 7-21 days produced a large time lag between the temperature change and the biological response. The impact of SST variability on community growth rates was nonlinear and a function of the rate and magnitude of change. Additionally, the nature of variability generated in a Lagrangian reference frame (following trajectories of surface water parcels) was larger than that within an Eulerian reference frame (fixed point), which initiated different phytoplankton responses between the two reference frames. Finally, we found that these dynamics were not captured by the Eppley growth model commonly used in global biogeochemical models and resulted in an overestimation of community growth rates, particularly in dynamic, strong frontal regions of the Southern Ocean. This work demonstrates that the timescale for environmental selection (community replacement) is a critical factor in determining community composition and takes a first step towards including the impact of variability and biological response times into biogeochemical models.