Components of the zooplankton production cycle in the temperate ocean

Components of the zooplankton production cycle (assimilation, respiration, and predation) on Georges Bank were analyzed using three models of increasing complexity, i.e., total biomass (Riley, 1947), size-structure (Huntley and Boyd, 1984) and species population (Davis, 1984a). Other temperate marine areas with seasonal cycles in temperature similar to Georges Bank were identified and available data on zooplankton biomass and species cycles were found. For Georges Bank, the species level model, based on well-defined empirical relationships between temperature, development rate, and fecundity, revealed that highest production occurs by small animals during warmest months (e.g., September), when biomass is low. By contrast, the total biomass model calculated maximal production during June, coinciding with peak herbivore mass, but could not account for high fall production because size-dependent effects were not considered. The size-structured model computed high fall production rates but greatly overestimated June production by failing to consider life-history characteristics of component species (i.e., diapause in Calanus). In general, it was found that the temperature cycle has a much greater influence on production rates than does food concentration. Zooplankton cycles from the North Sea, Japan Sea, and Argentinian Shelf were similar to Georges Bank, suggesting that the modeling results may be generally true for temperate areas having large annual ranges in temperature (\u3e 10°C). The traditional view that temperate zooplankton production is primarily food-limited and occurs mainly during spring/summer coinciding with peak herbivore stock can no longer be considered valid. Instead the cycle appears largely driven by temperature so that production is highest during summer/fall when biomass is low and small warm-water species dominate. The misconception of the production cycle has resulted from use of oversimplified models

Interaction of a copepod population with the mean circulation on Georges Bank

The population structure of the copepod, Pseudocalanus sp., on Georges Bank in February 1975 had distinct shifts from younger to older developmental stages in a clockwise pattern around the bank. It is hypothesized that this spatial pattern resulted from the interaction of copepod population dynamics and the mean circulation of sea water around Georges Bank. To test this idea, a combined physical/biological model is developed which simulates copepod population dynamics at each of a series of fixed spatial points around a hypothetical gyre. This simple physical scheme incorporates both advection and diffusion of copepods around the gyre. Rates of advection and diffusion are taken from the literature. Environmental influence on life history parameters (development rate, fecundity, and mortality) for Pseudocalanus sp. are found from laboratory experiments and from the literature. Spatial patterns generated by the model agree well with those observed from field samples. The results show that the hypothesis is possible thus providing a reasonable explanation for mechanisms generating these patterns. It is suggested that such mechanisms may contribute to much of the coarse and meso-scale (50 – 1000 km) heterogeneity in the distribution of marine zooplankton

Biological effects of Gulf Stream meandering

A modeling study was conducted to examine the effects of time-dependent mesoscale meandering of a jet on nutrient—phytoplankton—zooplankton (NPZ) dynamics. The jet was represented as a quasi-geostrophic flow using the method of contour dynamics. Two cases for biology were examined: 1) plankton in a mixed layer of fixed depth and 2) plankton at the base of a mixed layer (i.e., pycnocline) of variable depth. When the mixed layer depth is fixed, nutrient upwelling and dilution of the phytoplankton and zooplankton populations occur along the northward branch of the meander. The additional nutrients and reduced grazing pressure leads to significant enhancement (10–20%) of the phytoplankton production and biomass, while the zooplankton biomass decreases similarly. For plankton on a material surface of variable depth, phytoplankton growth in the pycnocline is increased by the higher light levels encountered during along-isopycnal upwelling. The nutrients decrease and the zooplankton mass in the pycnocline increases by a small amount downstream of the phytoplankton peak. Although the biological enhancements found are not large, the results suggest that vertical motions resulting from mesoscale oceanographic features such as jet meanders and mid-ocean eddies can be an important source of new nutrients for oceanic plankton production

Focus detection from digital in-line holograms based on spectral l1 norms

Author Posting. © Optical Society of America, 2007. This is the author's version of the work. It is posted here by permission of Optical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Optical Society of America A, 24 (2007): 3054-3062, doi:10.1364/JOSAA.24.003054.In this paper a rapid focus detection technique is developed for objects imaged using digital in-line holograms. It differs from previous approaches in that it is based directly on the spectral content of the object images and does not need a full reconstruction of the actual images. It is based on new focus metrics defined as the l1 norms of the object spectral components associated with the real and imaginary parts of the reconstruction kernel. Furthermore, these l1 norms can be computed efficiently in the frequency domain using a polar coordinate system, yielding a drastic speedup of about two orders of magnitude compared with image-based focus detection. The subsequent reconstruction, when done selectively over these detected focus distances, leads to significant computational savings. Focus detection results from holograms of plankton are demonstrated showing that the technique is both accurate and robust

Habitat usage by the cryptic copepods Pseudocalanus moultoni and P. newmani on Georges Bank (Northwest Atlantic)

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in Continental Shelf Research 111 (2015): 83-94, doi:10.1016/j.csr.2015.11.001.The cryptic copepod species, Pseudocalanus moultoni and P. newmani, co-occur on Georges Bank and in the Gulf of Maine (Northwest Atlantic); even recent studies have reported results and conclusions based on examination of the combined species. Species-specific PCR (SS-PCR) based on mitochondrial cytochrome oxidase I (COI) sequence divergence was used in this study to discriminate the species. Species-specific descriptions of habitat usage and predicted patterns of transport and retention on Georges Bank were made by mapping distributions and calculating abundances of each species from January to June, 1999 for four vertical strata (0-15 m, 15-40 m, 40-100 m, and 0-100 m) and five regions (Northern Flank, Bank Crest, Northeast Peak, Southern Flank, and Slope Water) identified on the basis of bathymetry and circulation. Patterns of distribution and abundance for the two species during January to June, 1999 were largely consistent with those described based on vertically integrating mapping and analysis for the same period in 1997 by McGillicuddy and Bucklin (2002). The region-specific and depth-stratified analyses allowed further discrimination in habitat usage by the species and confirmed the distinctive patterns for the two species. The observed differences between the species in abundances among the five regions and three depth strata over Georges Bank impact their transport trajectories. The concentration of P. moultoni in deep layers likely explains the higher rates of retention and lower rates of advective loss of this species from the Bank, compared to P. newmani, which may be more subject to wind-driven transport in the surface layer. Accurate identification and discrimination of even closely-related and cryptic species is needed to ensure full understanding and realistic predictions of changes in diversity of zooplankton and the functioning of pelagic ecosystems.Funding was provided by the National Science Foundation as part of the U.S. GLOBEC Program (Award Nos. OCE-9529100 and OCE-9632840 to Ann Bucklin; Award No. OCE-0815047 to Dennis McGillicuddy)

Micropatchiness, turbulence and recruitment in plankton

A series of models are presented which examine the relative importance of microscale patchiness and turbulence to growth and recruitment in planktonic consumers. The analyses apply over scales from centimeters to meters (e.g. from copepods to fish larvae), and we assume food-limited conditions, since, otherwise, patchiness would not affect growth. A model of individual growth response to fluctuating food is developed which shows that growth is approximately exponential and is linearly related to food concentration. A random walk model reveals that the swimming process can be approximated as a simple diffusion term which, when included in the exponential growth model, leads to accumulation of consumers in high growth (=prey) areas. This diffusive migration of consumers up the prey gradient is rapid; for example, half- maximum growth is reached in \u3c2 hours for fish larvae swimming in a 10 m patch of copepod nauplii. Enhancement of the net growth by this process is substantial; larval fish growth rates increase by 25% when 10 m prey patches appear at 5 hour intervals and by \u3e100% for steady patches. Physical turbulence, at intermediate levels, causes patch dissipation and reduced growth, whereas, at higher levels, it causes growth to be restored to original, low-turbulence, values due to increased encounter velocities. Variations in population growth rate due to turbulence and micropatchiness, even when small (\u3c10%), can cause large fluctuations in recruitment by affecting duration of pre-recruit life

Remote silicate supply regulates spring phytoplankton bloom magnitude in the Gulf of Maine

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Zang, Z., Ji, R., Liu, Y., Chen, C., Li, Y., Li, S., & Davis, C. S. Remote silicate supply regulates spring phytoplankton bloom magnitude in the Gulf of Maine. Limnology and Oceanography Letters, 7, (2022): 277-285, https://doi.org/10.1002/lol2.10245.Spring phytoplankton blooms in the Gulf of Maine (GoM) are sensitive to climate-related local and remote forcing. Nutrient supply through the slope water intrusion has been viewed as critical in regulating the GoM spring blooms, with an assumption that nitrogen is the primary limiting nutrient. In recent years, this paradigm has been challenged, with silicate being recognized as another potential limiting nutrient, but the source of silicate and its associated water mass remain difficult to be determined. In this study, a time series of spring bloom magnitude was constructed using a self-organizing map algorithm, and then correlated with the fluctuation of water composition in the deep Northeast Channel. The results reveal the importance of silicate supply from previously less-recognized deep Scotian Shelf Water inflow. This study offers a new hypothesis for spring bloom regulation, providing a better understanding of mechanisms controlling the spring bloom magnitude in the GoM.This study was supported by NOAA Coastal and Ocean Climate Application (COCA) Program (NA17OAR4310273) and NSF Northeast US Shelf-Long-Term Ecological Research (NES-LTER) Program (OCE-1655686)

Wind-induced interannual variability of sea level slope, along-shelf flow, and surface salinity on the Northwest Atlantic shelf

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 2462–2479, doi:10.1002/2013JC009385.In this study, we examine the importance of regional wind forcing in modulating advective processes and hydrographic properties along the Northwest Atlantic shelf, with a focus on the Nova Scotian Shelf (NSS)-Gulf of Maine (GoM) region. Long-term observational data of alongshore wind stress, sea level slope, and along-shelf flow are analyzed to quantify the relationship between wind forcing and hydrodynamic responses on interannual time scales. Additionally, a simplified momentum balance model is used to examine the underlying mechanisms. Our results show significant correlation among the observed interannual variability of sea level slope, along-shelf flow, and alongshore wind stress in the NSS-GoM region. A mechanism is suggested to elucidate the role of wind in modulating the sea level slope and along-shelf flow: stronger southwesterly (northeastward) winds tend to weaken the prevailing southwestward flow over the shelf, building sea level in the upstream Newfoundland Shelf region, whereas weaker southwesterly winds allow stronger southwestward flow to develop, raising sea level in the GoM region. The wind-induced flow variability can influence the transport of low-salinity water from the Gulf of St. Lawrence to the GoM, explaining interannual variations in surface salinity distributions within the region. Hence, our results offer a viable mechanism, besides the freshening of remote upstream sources, to explain interannual patterns of freshening in the GoM.This work was supported by NOAA’s Fisheries and the Environment Program, Grant #12-03 and through NOAA Cooperative Agreement NA09OAR4320129.2014-10-1

Influence of ocean freshening on shelf phytoplankton dynamics

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 34 (2007): L24607, doi:10.1029/2007GL032010.Climate change-induced freshening of the ocean can enhance vertical stratification and alter circulation patterns in ways that influence phytoplankton dynamics. We examined the timing of spring phytoplankton blooms and the magnitude of net primary productivity in the Nova Scotian Shelf (NSS) - Gulf of Maine (GoM) region with respect to seasonal and interannual changes in surface water freshening from 1998 to 2006. The general pattern of temporal westward progression of the phytoplankton bloom corresponds with the gradient of increasing sea surface salinity from the NSS in the east to the western GoM. Increased freshening enhances the spatial gradients in bloom timing by stimulating earlier blooms upstream (NSS), but it has less impact downstream (the western GoM). Strong spatial gradients (increasing westward) of mean chlorophyll concentration and net primary productivity during post-bloom months (May–June) indicate that lower sea surface salinity upstream can likely impede nutrient fluxes from deep water and therefore affect overall productivity.We thank NSF grant OCE-0727033 and NOAA grant NA17RJ1223 to RJ, CSD and RCB, NSF grants OCE- 0606612 and OCE-0726577 to DWT, and NSF grants OCE-0606928 and OCE-0726851 to CC

Does predation control the diapausing stock of Calanus finmarchicus in the Gulf of Maine?

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Wiebe, P., Baumgartner, M., Copley, N., Lawson, G., Davis, C., Ji, R., & Greene, C. Does predation control the diapausing stock of Calanus finmarchicus in the Gulf of Maine? Progress In Oceanography, 206, (2022): 102861, https://doi.org/10.1016/j.pocean.2022.102861.The variability of zooplankton populations is controlled by external and internal forcing, with the former being principally large-scale changes in circulation, and the latter being driven by in situ growth, competition, and predation. Assessing the relative importance of these forcings is challenging and requires analyses of multifaceted observational data. As part of the U.S. GLOBEC Georges Bank program, a series of cruises were conducted in fall 1997, 1998, and 1999 to survey diapausing populations of Calanus finmarchicus and their predators in Wilkinson, Jordan, and Georges Basins of the Gulf of Maine. Station and underway sampling were conducted using net (1 m2 MOCNESS) and bioacoustic (BIOMAPER-II) systems, respectively, to acquire vertically stratified data for zooplankton biomass, taxonomic, size, and life-stage composition, together with associated environmental data. The results show that the autumn diapausing C. finmarchicus abundance was much lower in 1998 than in 1997 or 1999, even though the overall zooplankton biomass levels were comparable between the three years. The size frequency distribution of the diapausing individuals had a bi-modal pattern in 1997 and 1999, but a single mode in 1998, indicating the demise of an early cohort of the diapausing stock. The relative biomass and computed energy demand of potential invertebrate predators (euphausiids, decapods, medusae, and siphonophores) was found to be higher in 1998 and could account for the missing C. finmarchicus cohort. Evidence collected from this study supports the hypothesis that local predation has the potential to control the diapausing stock of C. finmarchicus in the Gulf of Maine.RJ received support from the Northeast US Shelf Long Term Ecological Research (NES-LTER) project (NSF OCE-1655686) and the US MBON Gulf of Maine project to NERACOOS (NOPP award NA19NOS0120197 and BOEMUMaine Cooperative Agreement M19AC00022) for analyzing the size data and working on the manuscript. Research support was provided by the US GLOBEC Georges Bank Program through the CILER Cooperative Agreement NA-67RJO148 (NOAA Coastal Ocean Program)