Sinking organic particles transfer ∼10 gigatonnes of carbon into the deep ocean each year, keeping the atmospheric CO2 concentration significantly lower than would otherwise be the case. The exact ...size of this effect is strongly influenced by biological activity in the ocean's twilight zone (∼50–1,000 m beneath the surface). Recent work suggests that the resident zooplankton fragment, rather than ingest, the majority of encountered organic particles, thereby stimulating bacterial proliferation and the deep‐ocean microbial food web. Here we speculate that this apparently counterintuitive behaviour is an example of ‘microbial gardening’, a strategy that exploits the enzymatic and biosynthetic capabilities of microorganisms to facilitate the ‘gardener's’ access to a suite of otherwise unavailable compounds that are essential for metazoan life. We demonstrate the potential gains that zooplankton stand to make from microbial gardening using a simple steady state model, and we suggest avenues for future research.
Zooplankton in the ocean's twilight zone typically consume nutritionally poor detritus. We speculate that these animals fragment large detrital particles to stimulate the growth of microorganisms. This ‘gardening’ of microbes exploits their enzymatic and biosynthetic pathways to produce biomass that can be harvested as an energy‐rich and nutritious food source.
Zooplankton are fundamental to aquatic ecosystem services such as carbon and nutrient cycling. Therefore, a robust evidence base of how zooplankton respond to changes in anthropogenic pressures, such ...as climate change and nutrient loading, is key to implementing effective policy-making and management measures. Currently, the data on which to base this evidence, such as long time-series and large-scale datasets of zooplankton distribution and community composition, are too sparse owing to practical limitations in traditional collection and analysis methods. The advance of
in situ
imaging technologies that can be deployed at large scales on autonomous platforms, coupled with artificial intelligence and machine learning (AI/ML) for image analysis, promises a solution. However, whether imaging could reasonably replace physical samples, and whether AI/ML can achieve a taxonomic resolution that scientists trust, is currently unclear. We here develop a roadmap for imaging and AI/ML for future zooplankton monitoring and research based on community consensus. To do so, we determined current perceptions of the zooplankton community with a focus on their experience and trust in the new technologies. Our survey revealed a clear consensus that traditional net sampling and taxonomy must be retained, yet imaging will play an important part in the future of zooplankton monitoring and research. A period of overlapping use of imaging and physical sampling systems is needed before imaging can reasonably replace physical sampling for widespread time-series zooplankton monitoring. In addition, comprehensive improvements in AI/ML and close collaboration between zooplankton researchers and AI developers are needed for AI-based taxonomy to be trusted and fully adopted. Encouragingly, the adoption of cutting-edge technologies for zooplankton research may provide a solution to maintaining the critical taxonomic and ecological knowledge needed for future zooplankton monitoring and robust evidence-based policy decision-making.
Low dissolved iron (DFe) concentrations limit primary production in most high‐nutrient low‐chlorophyll (HNLC) regions. Increased recycling of iron (Fe) relative to nitrogen (N) by zooplankton may ...help to sustain phytoplankton production in these conditions. We concurrently determined rates of DFe and ammonium (NH4+) recycling by natural mesozooplankton communities in HNLC conditions of the Northeast Atlantic. NH4+ excretion remained constant and ranged between 14.2–54.1 nmol NH4+ mg dry weight−1 h−1. Fe recycling ranged between 6–138 pmol DFe mg dry weight−1 h−1during the first hour and decreased thereafter, reflecting the transition from the loss of phytoplankton‐derived Fe to basal DFe excretion. Mesozooplankton‐driven nutrient recycling was estimated to support 6–59% and <1–13% of the respective phytoplankton requirements for DFe and N; DFe:N regeneration ratios were 5–26 times larger than those required by phytoplankton. Our data suggest that Fe recycling by grazing organisms has the potential to reduce the intensity of HNLC conditions.
Key Points
Mesozooplankton regenerate high amounts of dissolved iron during grazing
DFe:N recycling ratios by mesozooplankton exceed requirements by phytoplankton
Mesozooplankton have the potential to reduce the intensity of HNLC conditions
The Agulhas Bank is a moderately productive and dynamic shelf ecosystem. In this region, underwater visibility can, at times, be poor owing to high turbidity near the seabed. However, it is currently ...unclear what causes high-turbidity events and how far they extend. Using an analysis of optical particle data (backscatter and fluorescence), we show a strong cross-shelf gradient of near-bottom turbidity and distinct particle dynamics across the study region. The region near Port Alfred was characterized by high levels of new production and vertical transport of organic matter to depth via sinking particles. On the Central Agulhas Bank, our observations were consistent with the suggestion of particle retention owing to a larger cyclonic recirculation pattern. Close to the coast we observed four sites with benthic nepheloid layers (BNL) that were likely formed because of resuspension of sediments. Our data indicate that there was no instantaneous link between BNL and sinking phytodetritus, though a time lag between export events and benthic remineralization could have obscured a direct link between surface productivity and BNL formation.
Phytoplankton play a central role in the regulation of global carbon and nutrient cycles, forming the basis of the marine food webs. A group of biogeochemically important phytoplankton, the ...coccolithophores, produce calcium carbonate scales that have been hypothesized to deter or reduce grazing by microzooplankton. Here, a meta-analysis of mesocosm-based experiments demonstrates that calcification of the cosmopolitan coccolithophore,
Emiliania huxleyi
, fails to deter microzooplankton grazing. The median grazing to growth ratio for
E. huxleyi
(0.56 ± 0.40) was not significantly different among non-calcified nano- or picoeukaryotes (0.71 ± 0.31 and 0.55 ± 0.34, respectively). Additionally, the environmental concentration of
E. huxleyi
did not drive preferential grazing of non-calcified groups. These results strongly suggest that the possession of coccoliths does not provide
E. huxleyi
effective protection from microzooplankton grazing. Such indiscriminate consumption has implications for the dissolution and fate of CaCO
3
in the ocean, and the evolution of coccoliths.
As the major term in downward organic carbon flux attenuation, determining prokaryotic metabolism over depth in the mesopelagic ocean is crucial for constraining the efficiency of the gravitational ...biological carbon pump (BCP). We hypothesize that the enhancement of particulate organic carbon (POC) concentrations in the mesopelagic twilight zone during export events leads to a temporally dynamic prokaryotic metabolic response, which likely has consequences for the efficiency of the BCP. We tested this hypothesis by making repeated measurements of leucine assimilation and leucine respiration at in situ concentrations over six depths throughout the upper 500 m of the water column during the collapse of a large-scale Southern Ocean spring diatom bloom. Rates of prokaryotic leucine assimilation were used to indicate levels of prokaryotic heterotrophic production, and leucine assimilation efficiency (LAE; the proportion of leucine used for growth versus respiration) was taken as an indicator of prokaryotic growth efficiency. Thus, relative shifts in LAE are indicative of shifts in rates of prokaryotic production relative to respiration. The flux of POC through the oceans’ interior led to a dynamic prokaryotic response, characterized by a temporary elevation in mesopelagic prokaryote leucine assimilation rates, LAE and prokaryotic abundance. By the final measurement these changes had already begun to revert, despite POC concentrations still being enriched. As hypothesized, our data revealed distinctions in the phases of the mesopelagic system, likely due to an evolution in bulk prokaryotic metabolic status and the amount and composition of organic matter available. This indicates that estimating ocean carbon sequestration during export events necessitates a time course of measurements throughout the period of POC downward flux. Our findings also revealed distinctions in the ecophysiological prokaryotic responses to substrate regimes between the surface mixed layer and the mesopelagic. Specifically, in the latter in situ leucine concentrations appeared more significant in controlling prokaryote metabolism than POC concentration, and were more closely related to per cell leucine assimilation, than respiration. Whereas, in the mixed layer, the concentration of in situ leucine did not seem to drive rates of its assimilation, rather POC concentration was a strong negative driver of cell specific leucine respiration. These findings are suggestive of stronger levels of energy limitation in the deeper ocean. We surmised that ocean regions with sporadic substrate supply to the mesopelagic are likely to experience stronger energy limitation which favors prokaryotic respiration over production.
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Zooplankton on continental shelves represent an important intermediary in the transfer of energy and matter from phytoplankton to the wider ecosystem. Their taxonomic composition and ...trophic interactions with phytoplankton vary in space and time, and interpreting the implications of this constantly evolving landscape remains a major challenge. Here we combine plankton taxonomic data with the analysis of biovolume spectra and stable isotopes to provide insights into the trophic interactions that occur in a shelf sea ecosystem (Celtic Sea) across the spring-summer-autumn transition. Biovolume spectra captured the seasonal development of the zooplankton community well, both in terms of total biomass and trophic positioning, and matched trophic positions estimated by stable isotope analysis. In early April, large microplankton (63–200 µm) occupied higher trophic positions than mesozooplankton (>200 µm), likely reflecting the predominance of nanoplankton (2–20 µm) that were not readily available to mesozooplankton grazers. Biomass and number of trophic levels increased during the spring bloom as elevated primary production allowed for a higher abundance of predatory species. During July, the plankton assemblage occupied relatively high trophic positions, indicating important links to the microbial loop and the recycling of organic matter. The strong correlation between biomass and community trophic level across the study suggests that the Celtic Sea is a relatively enclosed and predominantly energy-limited ecosystem. The progression of the zooplankton biomass and community structure within the central shelf region was different to that at the shelf-break, potentially reflecting increased predatory control of copepods by macrozooplankton and pelagic fishes at the shelf break. We suggest that the combination of size spectra and stable isotope techniques are highly complementary and useful for interpreting the seasonal progression of trophic interactions in the plankton.
The fraction of net primary production that is exported from the euphotic zone as sinking particulate organic carbon (POC) varies notably through time and from region to region. Phytoplankton ...containing biominerals, such as silicified diatoms have long been associated with high export fluxes. However, recent reviews point out that the magnitude of export is not controlled by diatoms alone, but determined by the whole plankton community structure. The combined effect of phytoplankton community composition and zooplankton abundance on export flux dynamics, were explored using a set of 12 large outdoor mesocosms. All mesocosms received a daily addition of minor amounts of nitrate and phosphate, while only 6 mesocosms received silicic acid (dSi). This resulted in a dominance of diatoms and dinoflagellate in the +Si mesocosms and a dominance of dinoflagellate in the –Si mesocosms. Simultaneously, half of the mesocosms had decreased mesozooplankton populations whereas the other half were supplemented with additional zooplankton. In all mesocosms, POC fluxes were positively correlated to Si/C ratios measured in the surface community and additions of dSi globally increased the export fluxes in all treatments highlighting the role of diatoms in C export. The presence of additional copepods resulted in higher standing stocks of POC, most probably through trophic cascades. However it only resulted in higher export fluxes for the –Si mesocosms. In the +Si with copepod addition (+Si +Cops) export was dominated by large diatoms with higher Si/C ratios in sinking material than in standing stocks. During non-bloom situations, the grazing activity of copepods decrease the export efficiency in diatom dominated systems by changing the structure of the phytoplankton community and/or preventing their aggregation. However, in flagellate-dominated system, the copepods increased phytoplankton growth, aggregation and fecal pellet production, with overall higher net export not always visible in term of export efficiency.
Often considered detrimental to the environment and human activities, jellyfish blooms are increasing in several coastal regions worldwide. Yet, the overall effect of these outbreaks on ecosystem ...productivity and structure are not fully understood. Here we provide evidence for a so far unanticipated role of jellyfish in marine nitrogen cycling. Pelagic jellyfish release nitrogen as a metabolic waste product in form of ammonium. Yet, we observed high rates of nitrification (NH4+ → NO3−, 5.7–40.8 nM gWW−1 wet weight h−1) associated with the scyphomedusae Aurelia aurita, Chrysaora hysoscella, and Chrysaora pacifica and low rates of incomplete nitrification (NH4+ → NO2−, 1.0–2.8 nM gWW−1 h−1) associated with Chrysaora fulgida, C. hysoscella, and C. pacifica. These observations indicate that microbes living in association with these jellyfish thrive by oxidizing the readily available ammonia to nitrite and nitrate. The four studied species have a large geographic distribution and exhibit frequent population outbreaks. We show that, during such outbreaks, jellyfish‐associated release of nitrogen can provide more than 100% of the nitrogen required for primary production. These findings reveal a so far overlooked pathway when assessing pelagic nitrification rates that might be of particular relevance in nitrogen depleted surface waters and at high jellyfish population densities.
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