Biology subjects

Visser, A. W.

Publications and source records attributed to Visser, A. W..

6 recordsLinked to original sources

bioRxiv2025

Diatom -lipid - copepod nexus under threat by global change

The central role of lipids in the ecology of high latitude seas is under threat by global change. This hinges on tight ecological links between diatoms and copepods of the genus Calanus, that produce and accumulate energy rich lipids. Strongly coupled through seasonal water column stability, nutrient supply, spring bloom succession and overwintering life strategies, this nexus is foundational to the highly productive food webs of Arctic/sub-Arctic North Atlantic. We report that the ability of Calanus finmarchicus to accumulate lipids is seriously impaired by changing plankton community and by rising temperatures, being 3 times higher on diatoms than flagellate based diets while suffering around a 60% reduction associated with a 2oC temperature change. This together with higher respiration rates, smaller size at maturity and reduced diatom productivity suggests their ability to overwinter will be compromised, threatening not only their contribution to regional productivity but also in maintaining a significant pool of sequestered carbon via the lipid pump. Similar lipid mediated couplings are prevalent throughout high latitude seas, bringing uncertainty to the ecosystem services they provide.

ecology↗

bioRxiv2024

Residence Times and Legacy of Biogenic Carbon in Ocean Reservoirs

Quantifying the sequestration potential of biologically driven carbon fluxes in the ocean depends critically on residence times - how long carbon remains stored in reservoirs before being re-exposed to the atmosphere. Simple mass balance provides estimates for many of the major ocean biogenic carbon reservoirs. For vegetated coastal ecosystems (mangroves, sea grass meadows, salt marshes) that globally store 20 to 40 PgC, this is 200 to 500 years, while for the biological carbon pump, a reservoir of about 2000 PgC, it is between 200 to 800 years. Over these time scales respective reservoirs reach equilibrium if left undisturbed. Importantly, near equilibrium of ocean reservoirs during the Holocene can be inferred from the near steady atmospheric concentrations during this period. The degradation of habitats and the over-exploitation of living marine resources particularly in the last 75 years have tipped these natural processes out of balance, to the extent where many are now net emitters of legacy carbon back to the atmosphere. The analysis exposes a conflict between how sequestration is reported in oceanographic literature and how it is understood with regards durable carbon capture and storage. Nature-based solutions can be sought to address parts of the climate crisis, by improving ecosystem health and biodiversity, but are unlikely to provide solutions to carbon management on a scale commensurate with anthropogenic emissions. The best we can do is to limit net emissions by restoring what we can, and to ensure that future practices do not further tip ocean carbon reservoirs out of balance. Significance StatementMarine animals and plants maintain large pools of carbon in the ocean and coastal areas that have been laid down by generations past. This legacy carbon is continuously being recycled on time scales of 100s of years. Left undisturbed, as they were for most of the last 10000 years, these carbon pools tend to equilibrium; flux in equals flux out. Human activities such as over fishing and coastal construction, particularly in the past 75 years, have tipped these natural cycles out of balance to the extent where many pools are now net emitters of carbon. Conservation and restoration of marine habitats can bring these cycles back into balance but cannot be counted as offsetting fossil fuel emissions.

ecology↗

bioRxiv2022

From first principles to structure and function of unicellular plankton communities

Here we review, synthesize, and analyse the size-based approach to model unicellular plankton cells and communities. We first review how cell size influences processes of the individual the cell: uptake of dissolved nutrients and dissolved organic carbon, phototrophy, phagotrophy, and metabolism. We parameterise processes primarily from first principles, using a synthesis of existing data only when needed, and show how these processes determine minimum and maximum cell size and limiting resource concentrations. The cell level processes scale directly up to the structure and function of the entire unicellular plankton ecosystem, from heterotrophic bacteria to zooplankton. The structure is described by the Sheldon size spectrum and by the emergent trophic strategies. We develop an analytical approximate solution of the biomass size spectrum and show how the trophic strategies of osmotrophy, light- and nutrient-limited phototrophy, mixotrophy, phagotrophy depend on the resource environment. We further develop expressions to quantify the functions of the plankton community: production, respiration and losses, and carbon available to production of higher trophic levels, and show how the plankton community responds to changes in temperature and grazing from higher trophic levels. We finally discuss strengths and limitations of size-based representations and models of plankton communities and which additional trait axes will improve the representation of plankton functional diversity.

ecology↗

bioRxiv2021

Metazoans, migrations, and the ocean's biological carbon pump

The daily vertical migrations of fish and other metazoans actively transport organic carbon from the ocean surface to depth, contributing to the biological carbon pump. We use an oxygen-constrained, game-theoretic food-web model to simulate diel vertical migrations and estimate global carbon fluxes and sequestration by fish and zooplankton due to respiration, fecal pellets, and deadfalls. Our model provides estimates of the carbon export and sequestration potential for a range of pelagic functional groups, despite uncertain biomass estimates of some functional groups. While the export production of metazoans and fish is modest (~20% of global total), we estimate that their contribution to carbon sequestered by the biological pump (~ 800 PgC) is conservatively more than 50% of the estimated global total (~1300 PgC) and have a significantly longer sequestration time scale (~250 years) than previously reported for other components of the biological pump. Fish and multicellular zooplankton contribute about equally to this sequestered carbon pool. This essential ecosystem service could be at risk from both unregulated fishing on the high seas and ocean deoxygenation due to climate change.

ecology↗

bioRxiv2021

Zooplankton trophic dynamics drive carbon export efficiency

The magnitude and efficiency of particulate carbon export from the ocean surface depends not only on net primary production (NPP) but also on how carbon is consumed, respired, and repackaged by organisms. We contend that several of these processes can be captured by the size spectrum of the plankton community. However, most global models have relatively simple food-web structures that are unable to generate plankton sizespectra. Moreover, the life-cycles of multicellular zooplankton are typically not resolved, restricting the ability of models to represent time-lags that are known to impact carbon export and its efficiency (pe-ratio). Here, we use a global mechanistic size-spectrum model of the marine plankton community to investigate how particulate export and pe-ratio relate to the community size spectrum, community composition, and time-lags between predators and prey. The model generates emergent food-webs with associated size distributions for organisms and detrital particles. To resolve time-lags between phytoplankton and zooplankton, we implement the life-cycle of multicellular zooplankton (here represented by copepods). The simulation successfully captures observed patterns in biomass and energy fluxes across regions. We find that carbon export correlates best with copepod biomass and trophic level, whereas the pe-ratio correlates best with the exponent of the size spectrum and sea surface temperature (SST). Community metrics performed better than NPP or SST for both deep export and pe-ratio. Time-lags between phytoplankton and copepods did not strongly affect export or pe-ratio. We conclude by discussing how can we reconcile size-spectrum theory with field sampling. Plain Language SummaryPlankton are tiny but extremely abundant aquatic organisms. Plankton lock CO2 away from the atmosphere as they sink to the deep ocean, where carbon can be stored for hundreds of years. However, how much carbon is locked away and for how long depends on how organisms eat, defecate, and respire. We argue that these processes are reflected in the size composition of the plankton community. The size composition shows a clear relationship between the number of organisms and their body-size. The steepness of this "size-abundance relationship" describes the balance between small vs. large organisms, and has been argued to reflect how energy is transferred from small to large organisms. Since large organisms create fast-sinking particles, the size-abundance relationship could be used to estimate how much carbon is being stored in the deep ocean. Here we use a computer simulation of the global plankton community to investigate how the removal of carbon relates to the plankton community and the steepness of the sizeabundance relationship. The model successfully captures patterns observed in nature. We found that the size-abundance relationship, together with the quantity of large zooplankton better explained carbon export than other measures typically used, such as photosynthesis and temperature. Key PointsO_LIWe use a global mechanistic size-spectrum model to investigate the relation between particulate export and plankton community metrics. C_LIO_LIWe find a good correlation between export efficiency and the exponent of the size spectrum. C_LIO_LITotal carbon export correlated well with copepod biomass and trophic level of active copepods in the model. C_LI

ecology↗

bioRxiv2020

Optimal seasonal routines across animals and latitudes

Most animals face seasonal fluctuations in food availability and need to develop an annual routine that maximizes their lifetime reproductive success. Two particularly common strategies are reducing energy expenditure and building storage to sustain the animal in meager periods (winters). Here, we pose a simple and generic model for an animal that can decide, at each time during the season, on its level of foraging effort and on building energy stores. Using dynamic optimization, we identify the optimal annual routines that maximize the trade-off between energy and mortality over a life-long horizon. We investigate how the optimal strategies depend on the body size and longevity of the animal, and upon the seasonal variability in the environment. We find that with large fluctuations, the optimal annual routine for small animals is to develop a surviving egg/spore stage rather than to attempt to survive the winter. Medium sized animals invest heavily in reserves to allow long hibernation, while larger animals only need smaller reserves and a shorter hibernation period. In environments with smaller fluctuations, organisms do not need energy stores or hibernation but reduce foraging activities during spring and summer where their fitness is highest. Our optimization model can be used as a null hypothesis to explain the annual routines of animals of all body sizes across the globe.

ecology↗