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Small crustaceans, such as the mysid Neomysis americana (S.I. Smith 1873), are a central component of coastal food webs and, while generally tolerant of a wide-range of environmental conditions, can be negatively affected by poor water quality. In this study, daily growth rates (GRD) and clutch size metrics of N. americana collected during the early and late summer of 2018–2019 were evaluated for the Choptank and Patuxent rivers, major tributaries of Chesapeake Bay known to exhibit different oxygenation regimes. Genetic variation in the mitochondrial CO1 locus was assessed to evaluate the potential intraspecific genetic structure within Chesapeake Bay. CO1 haplotype network analysis, phylogenetic analysis, and analysis of molecular variance revealed no genetic differences between Choptank and Patuxent river populations, with all Chesapeake Bay individuals belonging to a single genetic lineage (lineage C), of the N. americana cryptic species complex. Total and size-specific clutch size were approximately 18% and 53% higher, respectively, in the normoxic Choptank River during the early summer. Embryos within the marsupium, corrected for clutch size and female length, were consistently larger in the Choptank River during later larval development stages. Size-specific clutch size showed correlations with bottom water dissolved oxygen concentration (positive) and water temperature (negative). GRD did not differ between rivers or seasonally but juveniles grew twice as fast as adults. Given that all individuals genotyped from both rivers belonged to lineage C of the N. americana cryptic species complex, it is hypothesized that bottom water hypoxia (rather than genetic differentiation) is responsible for reduced clutch size in the Patuxent River. Our findings build on other recent work by providing evidence of a direct, negative relationship between hypoxia and local population dynamics of N. americana, a key ecological component of Chesapeake Bay’s food web. 

The expected reduction of ice algae with declining sea ice may prove to be detrimental to the Pacific Arctic ecosystem. Benthic organisms that rely on sea ice organic carbon (iPOC) sustain benthic predators such as the Pacific walrus ( Odobenus rosmarus divergens ). The ability to track the trophic transfer of iPOC is critical to understanding its value in the food web, but prior methods have lacked the required source specificity. We analyzed the H-Print index, based on biomarkers of ice algae versus phytoplankton contributions to organic carbon in marine predators, in Pacific walrus livers collected in 2012, 2014 and 2016 from the Northern Bering Sea (NBS) and Chukchi Sea. We paired these measurements with stable nitrogen isotopes ( δ 15 N) to estimate trophic position. We observed differences in the contribution of iPOC in Pacific walrus diet between regions, sexes, and age classes. Specifically, the contribution of iPOC to the diet of Pacific walruses was higher in the Chukchi Sea (52%) compared to the NBS (30%). This regional difference is consistent with longer annual sea ice persistence in the Chukchi Sea. Within the NBS, the contribution of iPOC to walrus spring diet was higher in females (~45%) compared to males (~30%) for each year (p < 0.001), likely due to specific foraging behavior of females to support energetic demands associated with pregnancy and lactation. Within the Chukchi Sea, the iPOC contribution was similar between males and females, yet higher in juveniles than in adults. Despite differences in the origin of organic carbon fueling the system (sea ice versus pelagic derived carbon), the trophic position of adult female Pacific walruses was similar between the NBS and Chukchi Sea (3.2 and 3.5, respectively), supporting similar diets (i.e. clams). Given the higher quality of organic carbon from ice algae, the retreat of seasonal sea ice in recent decades may create an additional vulnerability for female and juvenile Pacific walruses and should be considered in management of the species. 

Abstract The mysid Neomysis americana (Smith, 1873) is native to shallow shelf waters and estuaries of the western Atlantic coast of North America. Despite the important role mysids such as N. americana play in estuarine ecosystems as both consumers and as prey for higher trophic levels, there is limited information on how metabolism influences their spatial ecology and habitat requirements. In tributaries of Chesapeake Bay, MD, USA, previous research has shown that summer water temperatures can approach the lethal upper tolerance limit for N. americana. We measured the per capita metabolic rate (µgO2 min–1) of N. americana from the upper Patuxent River near Benedict, MD, a tributary of Chesapeake Bay in the laboratory to evaluate the metabolic response to salinity and temperature conditions that mysids experience in natural habitats. Sex-specific and diel patterns in metabolic rate were quantified. Metabolic rates did not differ between night and day and there was no significant difference in metabolic rate between males and females, exclusive of gravid females. Metabolic rates were lowest in salinity treatments of 2 and 8 at 29 °C, and highest in the salinity 2 treatment at 22 °C. Only temperature had a statistically significant, albeit unexpected, effect. This study shows that the metabolic response of N. americana to temperature and salinity conditions is complex and plastic, and that metabolic rates can vary 3–4 fold within realistic summer temperature and salinity conditions. As environmental conditions continue to change, understanding metabolic response of mysids to realistic salinity and temperature conditions is necessary for understanding their distributions in temperate estuaries.