Phytoplankton balance in the oceanic subarctic Pacific: Grazing impact of Metridia pacifica

Ingestion and respiration by Metridia pacifica, the dominant large copepod during autumn and winter in the subarctic Pacific, were investigated by shipboard and laboratory experiments. Diel variation in the rate of grazing on phytoplankton by M. pacifica was determined from measurements of gut pigment content and gastric evacuation rate. Both adult females and C₅ copepodites exhibited marked diel variation in gut contents, and thus feeding intensity. Night gut pigment values were 10 times greater than daytime values. Ingestion rates during May 1984 were 51.7 and 9.8 ng chl a copepod⁻¹ d⁻¹ for adult females and C₅ copepodites, respectively. Estimated filtering rates were 76 ml female⁻¹ d⁻¹ and 15 ml C₅⁻¹ d⁻¹. Feeding rates at low food concentrations in incubation bottles were similar to estimates obtained from in situ studies. Adult females consumed approximately 7.5 % of body carbon d⁻¹, and C₅ copepodites only 2.5 % d⁻¹. Respiration was 4 to 10 % of body carbon d⁻¹ for both C₅ and adults, indicating approximate energy balance for females but higher daily energy expenditure than gain for C₅. Based on the respiration measurements, a model was developed to evaluate the seasonal grazing impact of M. pacifica on the phytoplankton standing crop in the subarctic Pacific. During peak phytoplankton production in summer, the low densities of M. pacifica require less than 10 % of daily primary production to satisfy metabolic, growth and reproduction requirements. In contrast, during autumn and winter, the M. pacifica population requires 36 to 57 % and exceptionally 175 % of daily primary production to satisfy energy requirements. M. pacifica contributes significantly to the total grazing potential responsible for maintaining low stocks of phytoplankton during the unproductive, fall-winter season in the eastern subarctic Pacific

The stability of an NPZ model subject to realistic levels of vertical mixing

The linear stability of a vertically-distributed, Nutrient-Phytoplankton-Zooplankton (NPZ) ocean ecosystem model is analyzed to understand how vertical mixing influences biological dynamics. In the absence of vertical diffusion, the model generally exhibits both stable fixed point and limit cycle behavior, depending on the depth and choice of parameters. Diffusion couples the dynamics of nearby levels and can induce stable profiles as well as oscillatory dynamical trajectories that become vertically phase-locked for large mixing levels. Calculations of the Lyapunov exponent reveal that vertical diffusion can drive this model into a chaotic state, though this occurs only for levels of diffusion well below those found in nature. The dynamics of the model, assuming macrozooplankton are the dominant grazers in the ecosystem, are compared to those in which microzooplankton dominate, with a faster grazing rate and poor assimilation efficiency. While the coupled physical-macrozooplanton system has a stable profile, the coupled microzooplankton profile remains unstable, even at large mixing levels. Fluctuations occur on time scales varying between a few days and a few months, depending on the parameters and magnitude of diffusion

Modeling larval connectivity of coral reef organisms in the Kenya-Tanzania region

Most coral reef organisms have a bipartite life-cycle; they are site attached to reefs as adults but have pelagic larval stages that allow them to disperse to other reefs. Connectivity among coral reef patches is critical to the survival of local populations of reef organisms, and requires movement across gaps that are not suitable habitat for recruitment. Knowledge of population connectivity among individual reef habitats within a broader geographic region of coral reefs has been identified as key to developing efficient spatial management strategies to protect marine ecosystems. The study of larval connectivity of marine organisms is a complex multidisciplinary challenge that is difficult to address by direct observation alone. An approach that couples ocean circulation models with individual based models (IBMs) of larvae with different degrees of life-history complexity has been previously used to assess connectivity patterns in several coral reef regions (e.g., the Great Barrier Reef (GBR) and the Caribbean). We applied the IBM particle tracking approach to the Kenya-Tanzania region, which exhibits strong seasonality in the alongshore currents due to the influence of the monsoon. A 3-dimensional (3D) ocean circulation model with 2 km horizontal resolution was coupled to IBMs that track virtual larvae released from each of 661 reef habitats, associated with 15 distinct regions. Given that reefs provide homes to numerous species, each with distinctive, and in aggregate very diverse life-histories, several life-history scenarios were modeled to examine the variety of dispersal and connectivity patterns possible. We characterize virtual larvae of Acropora corals and Acanthurus surgeonfish, two coral reef inhabitants with greatly differing pelagic life-histories, to examine the effects of short (50 days) pelagic larval durations (PLD), differences in swimming abilities (implemented as reef perception distances), and active depth keeping in reef connectivity. Acropora virtual larvae were modeled as 3D passive particles with a precompetency period of 4 days, a total PLD of 12 days and a perception distance of 10 m. Acanthurus virtual larvae were characterized by 50 days precompetency period, a total PLD of 72 days and a perception distance of 4 km. Acanthurus virtual larvae were modeled in two ways — as 3D passive particles and including an idealized ontogenetic vertical migration behavior. A range of distances within which larvae were able to perceive reefs and directionally swim to settle on them during the competency period were evaluated. The influence of interannual environmental variations was assessed for two years (2000, 2005) of contrasting physics. The spatial scale of connectivity is much smaller for the short PLD coral, with successful connections restricted to a 1° radius (~100 km) around source reefs. In contrast, long distance connections from the southern to the northernmost reefs (~950 km) are common for virtual Acanthurids. Successful settlement for virtual Acropora larvae was 20% overall, with cross-region recruitment much increased compared to the coral larvae. Approximately 8% of Acropora larvae that successfully settled, recruited to their source reef (self-recruitment), an important proportion compared to only 1-2 % self-recruitment for Acanthurus. These rates and dispersal distances are similar to previous modelling studies of similar species in other coral reef regions and agree well with the few observational studies within the Kenya-Tanzania region

Late spring and summer patterns of euphausiid reproduction in Southeast Alaska fjord waters

Abundance, size and development stage data for furcilia and juvenile euphausiids and data on timing and prevalence of attached spermatophores on adult females are used to infer spawning times by four euphausiid species in Frederick Sound and lower Stephens Passage, Southeast Alaska. Results from net tows conducted between late May and September 2008 and a single, opportunistic dip-net sample on 21 April indicate that Thysanoessa raschii and T. longipes spawned in association with the spring phytoplankton bloom and continued spawning until June, with juveniles first appearing in mid-late June. Presence of female T. spinifera carrying spermatophores in mid-April indicate that T. spinifera spawns in association with the spring bloom as well; however, absence of larval T. spinifera suggest that spawning in the inshore waters is comparatively rare. In contrast, observations of female Euphausia pacifica carrying spermatophores from late May-August and the first appearance of early furcilia in August indicate that spawning occurs, at least to some extent, after the primary bloom. However, the appearance of juvenile E. pacifica in late June suggests that spawning occurred earlier as well and in discrete bouts. We argue that the absence of E. pacifica furcilia that were likely to have originated from an early spawning event may indicate that E. pacifica juveniles observed in late June were advected into the study region from the Gulf of Alaska. Overall, phenology of seasonal reproduction in this Alaskan fjord is similar to that observed in coastal waters in arctic and temperate ecosystems.Keywords: Spawning timing, Euphausiids, Southeast Alaska, Juvenile, Larva, Fjor

William (Bill) Peterson's contributions to ocean science, management, and policy

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Schwing, F. B., Sissenwine, M. J., Batchelder, H., Dam, H. G., Gomez-Gutierrez, J., Keister, J. E., Liu, H., & Peterson, J. O. William (Bill) Peterson's contributions to ocean science, management, and policy. Progress in Oceanography, 182, (2020): 102241, doi:10.1016/j.pocean.2019.102241.In addition to being an esteemed marine ecologist and oceanographer, William T. (Bill) Peterson was a dedicated public servant, a leader in the ocean science community, and a mentor to a generation of scientists. Bill recognized the importance of applied science and the need for integrated “big science” programs to advance our understanding of ecosystems and to guide their management. As the first US GLOBEC program manager, he was pivotal in transitioning the concept of understanding how climate change impacts marine ecosystems to an operational national research program. The scientific insight and knowledge generated by US GLOBEC informed and advanced the ecosystem-based management approaches now being implemented for fishery management in the US. Bill held significant leadership roles in numerous international efforts to understand global and regional ecological processes, and organized and chaired a number of influential scientific conferences and their proceedings. He was passionate about working with and training young researchers. Bill’s academic affiliations, notably at Stony Brook and Oregon State Universities, enabled him to advise, train, and mentor a host of students, post-doctoral researchers, and laboratory technicians. Under his collegial guidance they became critical independent thinkers and diligent investigators. His former students and colleagues carry on Bill Peterson’s legacy of research that helps us understand marine ecosystems and informs more effective resource stewardship and conservation

US GLOBEC: Program Goals, Approaches, and Advances

This special issue summarizes the major achievements of the US Global Ocean Ecosystem Dynamics (GLOBEC) program and celebrates its accomplishments. The articles grew out of a final symposium held in October 2009 under the auspices of the National Academy of Sciences Ocean Studies Board (http://usglobec.org/Symposium). This special issue updates the US GLOBEC "mid-life" Oceanography issue (Vol. 15, No. 2, 2002, http://tos.org/oceanography/archive/15-2.html), which put forward many of the goals and activities of the program, but was published while field work was still being conducted and results had yet to be synthesized across regional programs. The present special issue highlights the advances in understanding achieved through the synthesis of regional studies and pan-regional comparisons

Advances in Marine Ecosystem Dynamics from US GLOBEC: The Horizontal-Advection Bottom-up Forcing Paradigm

A primary focus of the US Global Ocean Ecosystem Dynamics (GLOBEC) program was to identify the mechanisms of ecosystem response to large-scale climate forcing under the assumption that bottom-up forcing controls a large fraction of marine ecosystem variability. At the beginning of GLOBEC, the prevailing bottom-up forcing hypothesis was that climate-induced changes in vertical transport modulated nutrient supply and surface primary productivity, which in turn affected the lower trophic levels (e.g., zooplankton) and higher trophic levels (e.g., fish) through the trophic cascade. Although upwelling dynamics were confirmed to be an important driver of ecosystem variability in GLOBEC studies, the use of eddy-resolving regional-scale ocean circulation models combined with field observations revealed that horizontal advection is an equally important driver of marine ecosystem variability. Through a synthesis of studies from the four US GLOBEC regions (Gulf of Alaska, Northern California Current, Northwest Atlantic, and Southern Ocean), a new horizontal-advection bottom-up forcing paradigm emerges in which large-scale climate forcing drives regional changes in alongshore and cross-shelf ocean transport that directly impact ecosystem functions (e.g., productivity, species composition, spatial connectivity). The horizontal advection bottom-up forcing paradigm expands the mechanistic pathways through which climate variability and climate change impact the marine ecosystem. In particular, these results highlight the need for future studies to resolve and understand the role of mesoscale and submesoscale transport processes and their relationship to climate

Climate Impacts on Zooplankton Population Dynamics in Coastal Marine Ecosystems

The 20-year US GLOBEC (Global Ocean Ecosystem Dynamics) program examined zooplankton populations and their predators in four coastal marine ecosystems. Program scientists learned that environmental controls on zooplankton vital rates, especially the timing and magnitude of reproduction, growth, life-cycle progression, and mortality, determine species population dynamics, seasonal and spatial distributions, and abundances. Improved knowledge of spatial-temporal abundance and distribution of individual zooplankton taxa coupled with new information linking higher trophic level predators (salmon, cod, haddock, penguins, seals) to their prey yielded mechanistic descriptions of how climate variation impacts regionally important marine resources. Coupled ecological models driven by improved regional-scale climate scenario models developed during GLOBEC enable forecasts of plausible future conditions in coastal ecosystems, and will aid and inform decision makers and communities as they assess, respond, and adapt to the effects of environmental change. Multi-region synthesis revealed that conditions in winter, before upwelling, or seasonal stratification, or ice melt (depending on region) had significant and important effects that primed the systems for greater zooplankton population abundance and productivity the following spring-summer, with effects that propagated to higher trophic levels