Stronger density-dependent growth of Japanese sardine with lower food availability: Comparison of growth and zooplankton biomass between a historical and current stock-increase period in the western North Pacific

Density dependence is a fundamental concept for fish population dynamics. Although density-dependent growth and maturity among older juveniles and adults is important for regulating fish population size and for fisheries management, the mechanism of density dependence for marine fishes remains unclear. Here, we examined changes in Japanese sardine growth with increasing abundance beginning in the 2010s and how the current pattern of density-dependent growth differs from that of a previous stock-increase period from the 1970s to early 1980s. During the current period of increasing abundance, mean standard length has already dropped to the lowest level yet observed and growth has declined more sharply with increased abundance than in the 1970s and 1980s. Mesozooplankton biomass in July in the summer feeding grounds was also lower during the current period. Therefore, our results suggest that summer food availability in the western North Pacific controls the strength of density-dependent growth. A lower carrying capacity for Japanese sardine could account for the stronger density dependence of growth observed in the 2010s; this indicates that future Japanese sardine abundance might not increase as much as in the 1980s unless food availability improves.

Propagation of the 2014–2016 Northeast Pacific Marine Heatwave Through the Salish Sea

Effects and impacts of the Northeast Pacific marine heatwave of 2014–2016 on the inner coastal estuarine waters of the Salish Sea were examined using a combination of monitoring data and an established three-dimensional hydrodynamic and biogeochemical model of the region. The anomalous high temperatures reached the U.S. Pacific Northwest continental shelf toward the end of 2014 and primarily entered the Salish Sea waters through an existing strong estuarine exchange. Elevated temperatures up to + 2.3°C were observed at the monitoring stations throughout 2015 and 2016 relative to 2013 before dissipating in 2017. The hydrodynamic and biogeochemical responses to this circulating high-temperature event were examined using the Salish Sea Model over a 5-year window from 2013 to 2017. Responses of conventional water-quality indicator variables, such as temperature and salinity, nutrients and phytoplankton, zooplankton, dissolved oxygen, and pH, were evaluated relative to a baseline without the marine heatwave forcing. The simulation results relative to 2014 show an increase in biological activity (+14%, and 6% Δ phytoplankton biomass, respectively) during the peak heatwave year 2015 and 2016 propagating toward higher zooplankton biomass (+14%, +18% Δ mesozooplankton biomass). However, sensitivity tests show that this increase was a direct result of higher freshwater and associated nutrient loads accompanied by stronger estuarine exchange with the Pacific Ocean rather than warming due to the heatwave. Strong vertical circulation and mixing provided mitigation with only ≈+0.6°C domain-wide annual average temperature increase within Salish Sea, and served as a physical buffer to keep waters cooler relative to the continental shelf during the marine heatwave.

Should we account for mesozooplankton reproduction and ontogenetic growth in biogeochemical modeling?

Mesozooplankton play a key role in marine ecosystems as they modulate the transfer of energy from phytoplankton to large marine organisms. In addition, they directly influence the oceanic cycles of carbon and nutrients through vertical migrations, fecal pellet production, respiration, and excretion. Mesozooplankton are mainly made up of metazoans, which undergo important size changes during their life cycle, resulting in significant variations in metabolic rates. However, most marine biogeochemical models represent mesozooplankton as protists-like organisms. Here, we study the potential caveats of this simplistic representation by using a chemostat-like zero-dimensional model with four different Nutrient-Phytoplankton-Zooplankton configurations in which the description of mesozooplankton ranges from protist-type organisms to using a size-based formulation including explicit reproduction and ontogenetic growth. We show that the size-based formulation strongly impacts mesozooplankton. First, it generates a delay of a few months in the response to an increase in food availability. Second, the increase in mesozooplankton biomass displays much larger temporal variations, in the form of successive cohorts, because of the dependency of the ingestion rate to body size. However, the size-based formulation does not affect smaller plankton or nutrient concentrations. A proper assessment of these top-down effects would require implementing our size-resolved approach in a 3-dimensional biogeochemical model. Furthermore, the bottom-up effects on higher trophic levels resulting from the significant changes in the temporal dynamics of mesozooplankton could be estimated in an end-to-end model coupling low and high trophic levels.

Mesozooplankton biomass, grazing and trophic structure in the bluefin tuna spawning area of the oceanic Gulf of Mexico

We investigated size-fractioned biomass, isotopes and grazing of mesozooplankton communities in the larval habitat of Atlantic bluefin tuna (ABT) in the oceanic Gulf of Mexico (GoM) during the peak spawning month of May. Euphotic-zone biomass ranged from 101 to 513 mg C m−2 during the day and 216 to 798 mg C m−2 at night. Grazing varied from 0.1 to 1.0 mg Chla m−2 d−1, averaging 1–3% of phytoplankton Chla consumed d−1. Carnivorous taxa dominated the biomass of > 1-mm zooplankton (78% day; 60% night), while only 13% of smaller zooplankton were carnivores. δ15N enrichment between small and large sizes indicates a 0.5–0.6 trophic-step difference. Although characteristics of GoM zooplankton are generally similar to those of remote oligotrophic subtropical regions, zooplankton stocks in the ABT larval habitat are disproportionately high relative to primary production, compared with HOT and BATS averages. Growth-grazing balances for phytoplankton were resolved with a statistically insignificant residual, and trophic fluxes from local productivity were sufficient to satisfy C demand of suspension feeding mesozooplankton. While carnivore C demand was met by local processes in the central GoM, experiments closer to the coastal margin suggest the need for a lateral subsidy of zooplankton biomass to the oceanic region.

Mesozooplankton biomass and temperature-enhanced grazing along a 110°E transect in the eastern Indian Ocean

Low-latitude waters of the Indian Ocean are warming faster than other major oceans. Most models predict a zooplankton decline due to lower productivity, enhanced metabolism and phytoplankton size shifts that reduce trophic transfer efficiency. In May-June 2019, we investigated mesozooplankton biomass and grazing along the historic 110°E transect line from the International Indian Ocean Expedition (IIOE) of the 1960s. Twenty sampling stations from 39.5 to 11.5°S spanned latitudinal variability from temperate to tropical waters and a pronounced 14°C gradient in mean euphotic zone temperature. Although mesozooplankton size structure was similar along the transect, with smaller (<2 mm) size classes dominant, total biomass increased 3-fold (400 to 1500 mg dry weight m-2) from high to low latitude. More dramatically, gut-fluorescence estimates of grazing (total ingestion or % euphotic zone chl a consumed d-1) were 14- and 20-fold higher, respectively, in the low-latitude warmer waters. Biomass-normalized grazing rates varied more than 6-fold over the transect, showing a strong temperature relationship (r2 = 0.85) that exceeded the temperature effects on gut turnover and metabolic rates. Herbivory contributed more to satisfying zooplankton energetic requirements in low-chl a tropical waters than chl a-rich waters at higher latitude. Our unexpected results are inconsistent with trophic amplification of warming effects on phytoplankton to zooplankton, but might be explained by enhanced coupling efficiency via mixotrophy. Additional implications for selective herbivory and top-down grazing control underscore the need for rigorous field studies to understand relationships and validate assumptions about climate change effects on the food webs of tropical oceans.

Quantifying spatiotemporal variability in zooplankton dynamics in the Gulf of Mexico with a physical–biogeochemical model

Zooplankton play an important role in global biogeochemistry, and their secondary production supports valuable fisheries of the world's oceans. Currently, zooplankton standing stocks cannot be estimated using remote sensing techniques. Hence, coupled physical–biogeochemical models (PBMs) provide an important tool for studying zooplankton on regional and global scales. However, evaluating the accuracy of zooplankton biomass estimates from PBMs has been a major challenge due to sparse observations. In this study, we configure a PBM for the Gulf of Mexico (GoM) from 1993 to 2012 and validate the model against an extensive combination of biomass and rate measurements. Spatial variability in a multidecadal database of mesozooplankton biomass for the northern GoM is well resolved by the model with a statistically significant (p < 0.01) correlation of 0.90. Mesozooplankton secondary production for the region averaged 66±8×109 kg C yr−1, equivalent to ∼10 % of net primary production (NPP), and ranged from 51 to 82×109 kg C yr−1, with higher secondary production inside cyclonic eddies and substantially reduced secondary production in anticyclonic eddies. Model results from the shelf regions suggest that herbivory is the dominant feeding mode for small mesozooplankton (< 1 mm), whereas larger mesozooplankton are primarily carnivorous. In open-ocean oligotrophic waters, however, both mesozooplankton groups show proportionally greater reliance on heterotrophic protists as a food source. This highlights an important role of microbial and protistan food webs in sustaining mesozooplankton biomass in the GoM, which serves as the primary food source for early life stages of many commercially important fish species, including tuna.

Response of size and trophic structure of zooplankton community to marine environmental conditions in the northern South China Sea in winter

The South China Sea (SCS) is a semi-enclosed marginal sea. And in the northern SCS (NSCS), the Pearl River plume, mesoscale eddies and Kuroshio intrusion may influence the structure of pelagic ecosystems. Here, based on mesozooplankton samples collected in the NSCS from December 2014 to January 2015, spatial variations of mesozooplankton biomass, abundance, normalized biomass size spectra (NBSS), size structure and trophic structure were analyzed to study the response of mesozooplankton community to the influence of highly variable oceanographic environment. High biomass, abundance and intercept of NBSS for the mesozooplankton community were observed in highly productive regions (net primary production &gt; 400 mg C m−2 day−1), such as river plume and cold eddy. No consistent spatial variation was observed for the slope of NBSS, probably because of highly dynamic marine environment in the NSCS. For the trophic structure of mesozooplankton community, the ratio of total chaetognaths biovolume to herbivores/omnivores biovolume (RTCH) was significantly high in river plume and Kuroshio intrusion. The high RTCH indicated unstable trophic structures, which may be due to extreme high proportion of smaller size individuals supported by discharge of river and transport of warm water species from the tropical western Pacific by Kuroshio intrusion, respectively.

Quantifying spatiotemporal variability in zooplankton dynamics in the Gulf of Mexico with a physical-biogeochemical model

Zooplankton play an important role in global biogeochemistry and their secondary production supports valuable fisheries of the world's oceans. Currently, zooplankton abundances cannot be estimated using remote sensing techniques. Hence, coupled physical-biogeochemical models (PBMs) provide an important tool for studying zooplankton on regional and global scales. However, evaluating the accuracy of zooplankton abundance estimates from PBMs has been a major challenge as a result of sparse observations. In this study, we configure a PBM for the Gulf of Mexico (GoM) from 1993–2012 and validate the model against an extensive combination of in situ biomass and rate measurements including total mesozooplankton biomass, size-fractionated mesozooplankton biomass and grazing rates, microzooplankton specific grazing rates, surface chlorophyll, deep chlorophyll maximum depth, phytoplankton specific growth rates, and net primary production. Spatial variability in mesozooplankton biomass climatology observed in a multi-decadal database for the northern GoM is well resolved by the model with a statistically significant (p