One pathway of the biological pump that remains largely unquantified in many export models is the active transport of carbon from the surface ocean to the mesopelagic by zooplankton diel vertical migration (DVM). Here, we develop a simple representation of zooplankton DVM and implement it in a global export model as a thought experiment to illustrate the effects of DVM on carbon export and mesopelagic biogeochemistry. The model is driven by diagnostic satellite measurements of net primary production, algal biomass, and phytoplankton size structure. Due to constraints on available satellite data, the results are restricted to the latitude range from 60°N to 60°S. The modeled global export flux from the base of the euphotic zone was 6.5 PgC/year, which represents a 14% increase over the export flux in model runs without DVM. The mean (± standard deviation, SD) proportional contribution of the DVM‐mediated export flux to total carbon export, averaged over the global domain and the climatological seasonal cycle, was 0.16 ± 0.04 and the proportional contribution of DVM activity to total respiration within the twilight zone was 0.16 ± 0.06. Adding DVM activity to the model also resulted in a deep local maximum in the oxygen utilization profile. The model results were most sensitive to the assumptions for the fraction of individuals participating in DVM, the fraction of fecal pellets produced in the euphotic zone, and the fraction of grazed carbon that is metabolized.

Rising ocean temperatures affect marine microbial ecosystems directly, since metabolic rates (e.g., photosynthesis, respiration) are temperature‐dependent, but temperature also has indirect effects mediated through changes to the physical environment. Empirical observations of the long‐term trends in biomass and productivity measure the integrated response of these two kinds of effects, making the independent components difficult to disentangle. We used a combination of modeling approaches to isolate the direct effects of rising temperatures on microbial metabolism and explored the consequences for food web dynamics and global biogeochemistry. We evaluated the effects of temperature sensitivity in two cases: first, assuming that all metabolic processes have the same temperature sensitivity, or, alternatively, that heterotrophic processes have higher temperature sensitivity than autotrophic processes. Microbial ecosystems at higher temperatures are characterized by increased productivity but decreased biomass stocks as a result of transient, high export events that reduce nutrient availability in the surface ocean. Trophic dynamics also mediate community structure shifts resulting in increased heterotroph to autotroph ratios at higher temperatures. These ecosystem thermal responses are magnified when the temperature sensitivity of heterotrophs is higher than that of autotrophs. These results provide important context for understanding the combined food web response to direct and indirect temperature effects and inform the construction and interpretation of Earth systems models used in climate projections.

Mixotrophic protists combine photosynthesis with the ingestion of prey to thrive in resource-limited conditions in the ocean. Yet, how they fine-tune resource investments between their two different metabolic strategies remains unclear. Here, we present a modeling framework (Mixotroph Optimal Contributions to Heterotrophy and Autotrophy) that predicts the optimal (growth-maximizing) investments of carbon and nitrogen as a function of environmental conditions. Our model captures a full spectrum of trophic modes, in which the optimal investments reflect zero-waste solutions (i.e., growth is colimited by carbon and nitrogen) and accurately reproduces experimental results. By fitting the model to data for Ochromonas , we were able to predict metabolic strategies at a global scale. We find that high phagotrophic investment is the dominant strategy across different oceanic biomes, used primarily for nitrogen acquisition. Our results therefore support empirical observations of the importance of mixotrophic grazers to upper ocean bacterivory.

Most landbirds migrate at night and typically make landfall in suitable stopover habitat before dawn. However, when birds find themselves over large water bodies at dawn, they must continue flying into the day and either finish crossing the water body and land on the far shore or backtrack to the near (i.e. first encountered) shore to land. Their collective decisions will influence how migrants are distributed among shoreline stopover habitats on either side of the water crossing. We studied birds during 4 spring migration seasons from 2010 to 2013 in the Great Lakes region, USA. We used 3 weather surveillance radars to observe migrating landbirds' behavior at dawn and subsequent terrestrial distributions during stopover. Mean flight heights over land and water were higher and mean flight directions were more oriented toward the closest shore at dawn when compared to peak migration earlier in the night. The wider the lake crossing, the higher that birds along the lakeshore flew at dawn. Seasonal mean stopover densities of migrants on land within 3 km of shorelines were 48% higher on the near shores of lakes (based on the seasonal mean direction of migration) than on the far shores. There was a moderate positive correlation (r = 0.584, P < 0.001, n = 358) between the seasonal mean density of birds aloft over water at dawn and the stopover density of birds in adjacent shorelines. Thus, birds over the water at dawn may tend to return to the near shore of the Great Lakes for stopover rather than continuing across the water. As a result, shoreline habitats on the near shores of lakes harbor greater densities of migrants and are thus critical stopover sites for migrating landbirds in the Great Lakes region. La mayoría de las aves terrestres migran durante la noche y típicamente paran en hábitats adecuados antes del amanecer. Sin embargo, cuando las aves se hallan sobre grandes cuerpos de agua al amanecer, deben seguir volando entrado el día y terminar de cruzar el cuerpo de agua y aterrizar en la orilla del lado opuesto o dar marcha atrás hasta la orilla que encontraron en primera instancia para aterrizar. Sus decisiones colectivas van a influenciar el modo en que las aves migratorias están distribuidas entre los ambientes de parada en las orillas a cada lado de las aguas atravesadas. Estudiamos aves durante cuatro estaciones migratorias de primavera desde 2010 a 2013 en la región de los Grandes Lagos. Usamos tres radares de vigilancia climática para observar el comportamiento de las aves terrestres migratorias al amanecer y las subsecuentes distribuciones terrestres cerca de la orilla durante la parada. Las alturas promedio del vuelo sobre la tierra y el agua fueron más elevadas y las direcciones promedio de vuelo estuvieron más orientadas hacia la primera orilla encontrada al momento del amanecer en comparación con el pico de migración que ocurre más temprano en la noche. Cuanto más ancho fue el lago que cruzaron, más alto volaron las aves a lo largo de la orilla del lago al amanecer. Las densidades estacionales promedio en los sitios de parada a menos de 3 km de la orilla fueron 48% más altas en la orilla primeramente encontrada de los lagos usando la dirección estacional promedio de los migrantes en comparación con el lado opuesto de los lagos. Encontramos una correlación positiva moderada (r=0.584, n=358, p<0.001) entre la densidad estacional promedio de las aves sobre el agua al amanecer y la densidad de aves en los sitios de parada en las orillas adyacentes. Por ende, las aves sobre el agua al amanecer tienden a regresar a las orillas encontradas en primera instancia de los Grandes Lagos para parar, más que seguir a través del agua. Como resultado, los hábitats de las orillas encontradas en primera instancia de los lagos albergan densidades mayores de migrantes y son por ende sitios críticos de parada para las aves migratorias terrestres en la región de los Grandes Lagos. Palabras clave: comportamiento de vuelo, Grandes Lagos, migración, orilla, radar, sitio de parada

The Mid-Atlantic Bight (MAB) hosts a large and productive marine ecosystem supported by high phytoplankton concentrations. Enhanced surface chlorophyll concentrations at the MAB shelf-break front have been detected in synoptic measurements, yet this feature is not present in seasonal means. To understand why, we assess the conditions associated with enhanced surface chlorophyll at the shelf break. We employ in-situ and remote sensing data, and a 2-dimensional model to show that Ekman restratification driven by upfront winds drives ephemerally enhanced chlorophyll concentrations at the shelf-break front in spring. Using 8-day composite satellite-measured surface chlorophyll concentration data from 2003-2020, we constructed a daily running mean (DRM) climatology of the cross-shelf chlorophyll distribution for the northern MAB region. While the frontal enhancement of chlorophyll is apparent in the DRM climatology, it is not captured in the seasonal climatology due to its short duration of less than a week. In-situ measurements of the frontal chlorophyll enhancement reveal that chlorophyll is highest in spring when the shelf-break front slumps offshore from its steep wintertime position causing restratification in the upper part of the water column. Several restratification mechanisms are possible, but the first day of enhanced chlorophyll at the shelf break corresponds to increasing upfront winds, suggesting that the frontal restratification is driven by offshore Ekman transport of the shelf water over the denser slope water. The 2-dimensional model shows that upfront winds can indeed drive Ekman restratification and alleviate light limitation of phytoplankton growth at the shelf-break front.

In food web models that include more than one prey type for a single predator, it is common for the predator's functional response to include some form of switching-preferential consumption of more abundant prey types. Predator switching promotes coexistence among competing prey types and increases diversity in the prey community. Here, we show how the dynamics of a diamond-shaped food web model of a marine plankton community are sensitive to a parameter that sets the strength of predator switching. Stronger switching destabilizes the model's coexistence equilibrium and leads to the appearance of limit cycles. Stronger switching also increases the evenness of the asymptotic prey community and promotes synchrony in the dynamics of disparate prey types. Given the dependence of model behavior on the strength of predator switching, it is important that modelers carefully consider the parameterization of functional responses that include switching.