Natural Ecosystems

Testing the skill of a species distribution model using a 21st century virtual ecosystem

Authors' Summary: Marine plankton communities play a central role within Earth's climate system, with important processes often divided among different “functional groups.” Changes in the relative abundance of these groups can therefore impact on ecosystem function. However, the oceans are vast, and samples are sparse, so global distributions are not well known. Statistical species distribution models (SDM's) have been developed that predict global distributions based on their relationships with observed environmental variables. They appear to perform well at summarizing present day distributions, and are increasingly being used to predict ecosystem changes throughout the 21st century. But it is not guaranteed that such models remain valid over time.

Rather than wait 100 years to find out, we applied a statistical SDM to a complex virtual ocean, and trained it using virtual observations that match real-world ocean samples. This allows us to jump forward to the end-of-century to test the accuracy of our predictions. The SDM performed well at qualitatively predicting “present day” plankton distributions but yielded poor end-of-century predictions. Our case study emphasizes both the importance of environmental variable selection, and of changes in the underlying relationships between environmental variables and plankton distributions, in terms of model validity over time.

Bottom-heavy trophic pyramids impair methylmercury biomagnification in the marine plankton ecosystems

Abstract: Methylmercury (CH3Hg+, MMHg) in the phytoplankton and zooplankton, which form the bottom of marine food webs, is a good predictor of MMHg in top predators, including humans. Therefore, evaluating the potential exposure of MMHg to higher trophic levels (TLs) requires a better understanding of relationships between MMHg biomagnification and plankton dynamics.

In this study, a coupled ecological/physical model with 366 plankton types of different sizes, biogeochemical functions, and temperature tolerance is used to simulate the relationships between MMHg biomagnification and the ecosystem structure. The study shows that the MMHg biomagnification becomes more significant with increasing TLs. Trophic magnification factors (TMFs) in the lowest two TLs show the opposite spatial pattern to TMFs in higher TLs. The low TMFs are usually associated with a short food-chain length. The less bottom-heavy trophic pyramids in the oligotrophic oceans enhance the MMHg trophic transfer. The global average TMF is increased from 2.3 to 2.8 in the warmer future with a medium climate sensitivity of 2.5 °C.

Our study suggests that if there are no mitigation measures for Hg emission, MMHg in the high-trophic-level plankton is increased more dramatically in the warming future, indicating greater MMHg exposure for top predators such as humans.

Marine phytoplankton functional types exhibit diverse responses to thermal change

Abstract: Marine phytoplankton generate half of global primary production, making them essential to ecosystem functioning and biogeochemical cycling. Though phytoplankton are phylogenetically diverse, studies rarely designate unique thermal traits to different taxa, resulting in coarse representations of phytoplankton thermal responses.

Here we assessed phytoplankton functional responses to temperature using empirically derived thermal growth rates from four principal contributors to marine productivity: diatoms, dinoflagellates, cyanobacteria, and coccolithophores. Using modeled sea surface temperatures for 1950–1970 and 2080–2100, we explored potential alterations to each group’s growth rates and geographical distribution under a future climate change scenario.

Contrary to the commonly applied Eppley formulation, our data suggest phytoplankton functional types may be characterized by different temperature coefficients (Q10), growth maxima thermal dependencies, and thermal ranges which would drive dissimilar responses to each degree of temperature change. These differences, when applied in response to global simulations of future temperature, result in taxon-specific projections of growth and geographic distribution, with low-latitude coccolithophores facing considerable decreases and cyanobacteria substantial increases in growth rates. These results suggest that the singular effect of changing temperature may alter phytoplankton global community structure, owing to the significant variability in thermal response between phytoplankton functional types.

This project will apply the Darwin ocean ecology model and ECCO-MITgcm estimates of ocean circulation to study the relationship between upper ocean community production, particulate organic carbon export fluxes, interior ocean remineralization, and biological carbon stores, and compare observations with biogeochemical budget computations. 

Using a data-constrained global-ocean ecology and biogeochemistry model to study the role of ocean circulation and the biological pump in driving ocean carbon cycle variability

The preeminent conference for the advancement of Earth and space sciences, the AGU (American Geophysical Union) Fall Meeting draws more than 25,000 attendees from over 100 countries each year to share research findings and identify innovative solutions to complex problems. Organized around the theme “Science is Society,” this year’s AGU Fall Meeting will take place in New Orleans and online on December 13 - 17.

AGU Fall Meeting goes hybrid for 2021
Abrupt shifts in 21st-century plankton communities

Abstract: Marine microbial communities sustain ocean food webs and mediate global elemental cycles. These communities will change with climate; such changes can be gradual or foreseeable, but likely have much more substantial consequences when sudden and unpredictable.

In a complex virtual marine microbial ecosystem, we find climate-change-driven shifts over the 21st century are often abrupt, large in amplitude and extent, and unpredictable using standard early warning signals. Phytoplankton with unique resource needs, especially fast-growing species such as diatoms, are more prone to abrupt shifts.

Abrupt shifts in biomass, productivity, and community structure are concentrated in Atlantic and Pacific subtropics. Abrupt changes in environmental variables such as temperature and nutrients rarely precede these ecosystem shifts, indicating that rapid community restructuring can occur in response to gradual environmental changes, particularly in nutrient supply rate ratios.

How predictable is plankton biogeography using statistical learning methods?

Abstract: Plankton play an important role in marine food webs, in biogeochemical cycling, and in moderating Earth's climate. Their possible responses to climate change are of broad scientific and social interest; yet observations are sparse, and mechanistic and statistical methods yield diverging predictions.

Here, we evaluate a statistical learning method using output from a 21st Century marine ecosystem model as a 'ground truth'. The model is sampled to mimic historical ocean observations, and Generalised Additive Models (GAMs) are used to predict the simulated plankton biogeography in space and time. Predictive skill varies across test cases, and between functional groups, and errors are more attributable to spatiotemporal sampling bias than to sample size.

Overall, the GAMs yield poor end-of-century predictions. Given that statistical methods are unable to capture changes in relationships between variables over time, we advise caution in their application and interpretation, particularly when modelling complex, dynamic systems.

Future phytoplankton diversity in a changing climate

Abstract: The future response of marine ecosystem diversity to continued anthropogenic forcing is poorly constrained.  An extremely diverse set of organisms form the base of the marine ecosystem: phytoplankton. Currently, ocean biogeochemistry and ecosystem models used for climate change projections typically include only 2-3 phytoplankton types, and are thus too simple to adequately assess the potential for changes in plankton community structure.

Here we analyze a complex ecosystem model with 35 phytoplankton types to evaluate the changes in phytoplankton community composition, turnover and size structure over the 21st century. We find that the rate of turnover in the phytoplankton community becomes faster during this century, i.e. the community structure becomes increasingly unstable in response to climate change. 

Combined with alterations to phytoplankton diversity, our results imply a loss of ecological resilience, with likely knock-on effects on the productivity and functioning of the marine environment. 

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