A critical examination of the role of marine snow and zooplankton faecal pellets in removing ocean surface microplastic

Numerical simulations and emissions estimates of plastic in and to the ocean consistently over-predict the surface inventory, particularly in the case of microplastic (MP), i.e. fragments less than 5 mm in length. Sequestration in the sediments has been both predicted and, to a limited extent, observed. It has been hypothesized that biology may be exporting a significant fraction of surface MP by way of marine snow aggregation and zooplankton faecal pellets. We apply previously published data on MP concentrations in the surface ocean to an earth system model of intermediate complexity to produce a first estimate of the potential global sequestration of MP by marine aggregates, including faecal pellets. We find a MP seafloor export potential of between 7.3E3-4.2E5 metric tons per year, or about 0.06-8.8% of estimated total annual plastic ocean pollution rates. We find that presently, aggregates alone would have the potential to remove most existing surface ocean MP to the seafloor within less than 2 years if pollution ceases. However, the observed accumulation of MP in the surface ocean, despite this high potential rate of removal, suggests that detrital export is an ineffective pathway for permanent MP removal. We theorize a prominent role of MP biological fouling and de-fouling in the rapid recycling of aggregate-associated MP in the upper ocean. We also present an estimate of how the potential detrital MP sink might change into the future, as climate change (and projected increasing MP pollution) alters the marine habitat. The polar regions, and the Arctic in particular, are projected to experience increasing removal rates as export production increases faster than MP pollution. Northern hemisphere subtropical gyres are projected to experience slowing removal rates as stratification and warming decrease export production, and MP pollution increases. However, significant uncertainty accompanies these results

The role of microzooplankton trophic interactions in modelling a suite of mesocosm ecosystems

Highlights: • Optimality-based modelling of microzooplankton trophic interactions in mesocsosm ecosystems. • Intraguild predation is important for modelling microzooplankton feeding interactions. • Trophic interactions structured solely by size may fail to capture feeding diversity. • Adequate representation of feeding interaction is needed for modelling ecosystem dynamics. Abstract: The zooplankton components in biogeochemical models drive top-down control of primary production and remineralisation, and thereby exert a strong impact on model performance. Who eats whom in oceanic plankton ecosystem models is often largely determined by body size. However, zooplankton of similar size can have different prey-size spectra. Thus, models with solely size-structured trophic interactions may not capture the full diversity of feeding interactions and miss important parts of zooplankton behavior. We apply an optimality-based plankton ecosystem model to analyse trophic interactions in a suite of mesocosm experiments in the Peruvian upwelling region. Sensitivity analyses reveal a dominant role of trophic structure for model performance, which cannot be compensated by parameter optimisation. The single most important aspect governing model performance is the trophic linking between dinoflagellates and ciliates. Only with a bidirectional link, i.e., both groups can prey on each other, is the model able to reproduce the differential development of the microzooplankton communities in the mesocosms. Thus, we conclude that a solely size-based trophic structure may not be appropriate to represent the most important trophic interactions in plankton ecosystems. The diversity of feeding interactions needs to be adequately represented to capture community dynamics

Mixed layer depth dominates over upwelling in regulating the seasonality of ecosystem functioning in the Peruvian Upwelling System

The Peruvian Upwelling System hosts an extremely high productive marine ecosystem. Observations show that the Peruvian Upwelling System is the only Eastern Boundary Upwelling Systems (EBUS) with an out-of-phase relationship of seasonal surface chlorophyll concentrations and upwelling intensity. This "seasonal paradox" triggers the questions: (1) what is the uniqueness of the Peruvian Upwelling System compared with other EBUS that leads to the out of phase relationship; (2) how does this uniqueness lead to low phytoplankton biomass in austral winter despite strong upwelling and ample nutrients? Using observational climatologies for four EBUS we diagnose that the Peruvian Upwelling System is unique in that intense upwelling coincides with deep mixed layers. We then apply a coupled regional ocean circulation-biogeochemical model (CROCO-BioEBUS) to assess how the interplay between mixed layer and upwelling is regulating the seasonality of surface chlorophyll in the Peruvian Upwelling System. The model recreates the "seasonal paradox" within 200 km off the Peruvian coast. We confirm previous findings that deep mixed layers, which cause vertical dilution and stronger light limitation, mostly drive the diametrical seasonality of chlorophyll relative to upwelling. In contrast to previous studies, reduced phytoplankton growth due to enhanced upwelling of cold waters and lateral advection are second-order drivers of low surface chlorophyll concentrations. This impact of deep mixed layers and upwelling propagates up the ecosystem, from primary production to export efficiency. Our findings emphasize the crucial role of the interplay of the mixed layer and upwelling and suggest that surface chlorophyll may increase along with a weakened seasonal paradox in response to shoaling mixed layers under climate change

The global biological microplastic particle sink

Every year, about four percent of the plastic waste generated worldwide ends up in the ocean. What happens to the plastic there is poorly understood, though a growing body of evidence suggests it is rapidly spreading throughout the global ocean. The mechanisms of this spread are straightforward for buoyant larger plastics that can be accurately modelled using Lagrangian particle models. But the fate of the smallest size fractions (the microplastics) are less straightforward, in part because they can aggregate in sinking marine snow and faecal pellets. This biologically-mediated pathway is suspected to be a primary surface microplastic removal mechanism, but exactly how it might work in the real ocean is unknown. We search the parameter space of a new microplastic model embedded in an earth system model to show that biological uptake can significantly shape global microplastic inventory and distributions and even account for the budgetary “missing” fraction of surface microplastic, despite being an inefficient removal mechanism. While a lack of observational data hampers our ability to choose a set of “best” model parameters, our effort represents a first tool for quantitatively assessing hypotheses for microplastic interaction with ocean biology at the global scale

Zooplankton grazing of microplastic can accelerate global loss of ocean oxygen

Global warming has driven a loss of dissolved oxygen in the ocean in recent decades. We demonstrate the potential for an additional anthropogenic driver of deoxygenation, in which zooplankton consumption of microplastic reduces the grazing on primary producers. In regions where primary production is not limited by macronutrient availability, the reduction of grazing pressure on primary producers causes export production to increase. Consequently, organic particle remineralisation in these regions increases. Employing a comprehensive Earth system model of intermediate complexity, we estimate this additional remineralisation could decrease water column oxygen inventory by as much as 10% in the North Pacific and accelerate global oxygen inventory loss by an extra 0.2–0.5% relative to 1960 values by the year 2020. Although significant uncertainty accompanies these estimates, the potential for physical pollution to have a globally significant biogeochemical signal that exacerbates the consequences of climate warming is a novel feedback not yet considered in climate research

Food web structure and intraguild predation affect ecosystem functioning in an established plankton model

Understanding how marine microbial food webs and their ecosystem functions are changing is crucial for projections of the future ocean. Often, simplified food web models are employed and their solutions are only evaluated against available observations of plankton biomass. With such an approach, it remains unclear how different underlying trophic interactions affect interpretations of plankton dynamics and functioning. Here, we quantitatively compare four hypothetical food webs to data from an existing mesocosm experiment using a refined version of the Minimum Microbial Food Web model. Food web representations range from separated food chains to complex food webs featuring additional trophic links including intraguild predation (IGP). Optimization against observations and taking into account model complexity ensures a fair comparison of the different food webs. Although the different optimized model food webs capture the observations similarly well, projected ecosystem functions differ depending on the underlying food web structure and the presence or absence of IGP. Mesh-like food webs dominated by the microbial loop yield higher recycling and net primary production (NPP) than models dominated by the classical diatom-copepod food chain. A high degree of microzooplankton IGP increases NPP and organic matter recycling, but decreases trophic transfer efficiency (TTE) to copepods. Copepod production, the trophic role of copepods, and TTE are more sensitive to initial biomass changes in chain-like than in complex food webs. Measurements resolving trophic interactions, in particular those quantifying IGP, remain essential to reduce model uncertainty and allow sound conclusions for ecosystem functioning in plankton ecosystems

Biogeography of zooplankton feeding strategy

The trait‐based approach is increasingly used in plankton ecology to understand diversity, community dynamics, and biogeography. While on the global scale phytoplankton traits are fairly well established, zooplankton traits are only beginning to be understood. One taxa‐transcending aspect of zooplankton diversity is the distinction between ambush and active feeding strategies. We present a global‐scale empirical estimate of feeding strategy derived from copepod abundance observations, which for the first time suggests a distinct trait biogeography with ambush feeding as the dominant feeding strategy at higher, but not at lower latitudes. To explain this trait biogeography, we develop a minimalist trade‐off based model of feeding strategies based on encounter rates between zooplankton predators and their phyto‐ and zooplankton prey. Encounter rates are governed by the two traits, size and motility, that trade off against predation risk. Coupled to a three‐dimensional dynamic green ocean model, our idealized encounter model captures the observed feeding strategy biogeography. In the model, this pattern arises from competing dominant food chains within the food web and is shaped by a trophic trait cascade of active vs. passive feeding in adjacent trophic levels. The dominant feeding strategy structures the pathways and efficiency of energy and biomass transfer through the model food web, with consequences for primary production, export and higher trophic levels. Understanding feeding strategies is therefore important for fisheries, biogeochemical cycling, and long‐term predictions of ecosystem dynamics and functioning by global dynamic green ocean models

Can top-down controls expand the ecological niche of marine N2 fixers?

The ability of marine diazotrophs to fix dinitrogen gas (N₂) is one of the most influential yet enigmatic processes in the ocean. With their activity diazotrophs support biological production by fixing about 100-200 Tg N/yr of bioavailable nitrogen (N), an essential limiting nutrient. Despite their important role, the factors that control the distribution of diazotrophs and their ability to fix N₂ are not fully elucidated. We discuss insights that can be gained from the emerging picture of a wide geographical distribution of marine diazotrophs and provide a critical assessment of environmental (bottom-up) versus trophic (top-down) controls. We present a simplified theoretical framework to understand how top-down control affects competition for resources that determine ecological niches. Selective grazing on non-fixing phytoplankton is identified as a critical process that can broaden the ability of diazotrophs to compete for resources in top-down controlled systems and explain an expanded ecological niche for diazotrophy. Our simplified analysis predicts a larger importance of top-down control in nutrient-rich systems where grazing controls the faster growing phytoplankton, allowing the slower growing diazotrophs to become established. However, these predictions require corroboration by experimental and field data, together with the identification of specific traits of organisms and associated trade-offs related to selective top-down control. Elucidation of these factors could greatly improve our predictive capability for marine N2 fixation. The susceptibility of this key biogeochemical process to future changes may not only be determined by changes in environmental conditions but also via changes in the ecological interactions

Enhanced ocean carbon storage from anaerobic alkalinity generation in coastal sediments

The coastal ocean is a crucial link between land, the open ocean and the atmosphere. The shallowness of the water column permits close interactions between the sedimentary, aquatic and atmospheric compartments, which otherwise are decoupled at long time scales (≅ 1000 yr) in the open oceans. Despite the prominent role of the coastal oceans in absorbing atmospheric CO2 and transferring it into the deep oceans via the continental shelf pump, the underlying mechanisms remain only partly understood. Evaluating observations from the North Sea, a NW European shelf sea, we provide evidence that anaerobic degradation of organic matter, fuelled from land and ocean, generates total alkalinity (AT) and increases the CO2 buffer capacity of seawater. At both the basin wide and annual scales anaerobic AT generation in the North Sea's tidal mud flat area irreversibly facilitates 7–10%, or taking into consideration benthic denitrification in the North Sea, 20–25% of the North Sea's overall CO2 uptake. At the global scale, anaerobic AT generation could be accountable for as much as 60% of the uptake of CO2 in shelf and marginal seas, making this process, the anaerobic pump, a key player in the biological carbon pump. Under future high CO2 conditions oceanic CO2 storage via the anaerobic pump may even gain further relevance because of stimulated ocean productivity

Rapid decline of the CO2 buffering capacity in the North Sea and implications for the North Atlantic Ocean

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 Global Biogeochemical Cycles 21 (2007): GB4001, doi:10.1029/2006GB002825.New observations from the North Sea, a NW European shelf sea, show that between 2001 and 2005 the CO2 partial pressure (pCO2) in surface waters rose by 22 μatm, thus faster than atmospheric pCO2, which in the same period rose approximately 11 μatm. The surprisingly rapid decline in air-sea partial pressure difference (ΔpCO2) is primarily a response to an elevated water column inventory of dissolved inorganic carbon (DIC), which, in turn, reflects mostly anthropogenic CO2 input rather than natural interannual variability. The resulting decline in the buffering capacity of the inorganic carbonate system (increasing Revelle factor) sets up a theoretically predicted feedback loop whereby the invasion of anthropogenic CO2 reduces the ocean's ability to uptake additional CO2. Model simulations for the North Atlantic Ocean and thermodynamic principles reveal that this feedback should be stronger, at present, in colder midlatitude and subpolar waters because of the lower present-day buffer capacity and elevated DIC levels driven either by northward advected surface water and/or excess local air-sea CO2 uptake. This buffer capacity feedback mechanism helps to explain at least part of the observed trend of decreasing air-sea ΔpCO2 over time as reported in several other recent North Atlantic studies.S. Doney and I. Lima were supported by NSF/ONR NOPP (N000140210370) and NASA (NNG05GG30G)