Towards end-to-end (E2E) modelling in a consistent NPZD-F modelling framework (ECOSMO E2E_v1.0): application to the North Sea and Baltic Sea

Publisher's version (útgefin grein)Coupled physical-biological models usually resolve only parts of the trophic food chain; hence, they run the risk of neglecting relevant ecosystem processes. Additionally, this imposes a closure term problem at the respective "ends" of the trophic levels considered. In this study, we aim to understand how the implementation of higher trophic levels in a nutrient-phytoplankton-zooplankton-detritus (NPZD) model affects the simulated response of the ecosystem using a consistent NPZD-fish modelling approach (ECOSMO E2E) in the combined North Sea-Baltic Sea system. Utilising this approach, we addressed the above-mentioned closure term problem in lower trophic ecosystem modelling at a very low computational cost; thus, we provide an efficient method that requires very little data to obtain spatially and temporally dynamic zooplankton mortality. On the basis of the ECOSMO II coupled ecosystem model we implemented one functional group that represented fish and one group that represented macrobenthos in the 3-D model formulation. Both groups were linked to the lower trophic levels and to each other via predator-prey relationships, which allowed for the investigation of both bottom-up processes and top-down mechanisms in the trophic chain of the North Sea-Baltic Sea ecosystem. Model results for a 10-year-long simulation period (1980-1989) were analysed and discussed with respect to the observed patterns. To understand the impact of the newly implemented functional groups for the simulated ecosystem response, we compared the performance of the ECOSMO E2E to that of a respective truncated NPZD model (ECOSMO II) applied to the same time period. Additionally, we performed scenario tests to analyse the new role of the zooplankton mortality closure term in the truncated NPZD and the fish mortality term in the end-to-end model, which summarises the pressure imposed on the system by fisheries and mortality imposed by apex predators. We found that the model-simulated macrobenthos and fish spatial and seasonal patterns agree well with current system understanding. Considering a dynamic fish component in the ecosystem model resulted in slightly improved model performance with respect to the representation of spatial and temporal variations in nutrients, changes in modelled plankton seasonality, and nutrient profiles. Model sensitivity scenarios showed that changes in the zooplankton mortality parameter are transferred up and down the trophic chain with little attenuation of the signal, whereas major changes in fish mortality and fish biomass cascade down the food chain.This work is a contribution to the FP7 SEAS-ERA SEAMAN collaborative project financed by the Norwegian Research Council (grant no. NRC-227779/E40). We would like to thank Marie Maar for her constructive comments on an earlier version of the paper. Furthermore, we are grateful to an anonymous reviewer and Hagen Radtke, whose thoughtful comments helped to improve the paper.Peer Reviewe

Environmental Change at Deep-Sea Sponge Habitats Over the Last Half Century: A Model Hindcast Study for the Age of Anthropogenic Climate Change

Deep-sea sponges inhabit multiple areas of the deep North Atlantic at depths below 250 m. Living in the deep ocean, where environmental properties below the permanent thermocline generally change slowly, they may not easily acclimatize to abrupt changes in the environment. Until now consistent monitoring timeseries of the environment at deep sea sponge habitats are missing. Therefore, long-term simulation with coupled bio-physical models can shed light on the changes in environmental conditions sponges are exposed to. To investigate the variability of North Atlantic sponge habitats for the past half century, the deep-sea conditions have been simulated with a 67-year model hindcast from 1948 to 2014. The hindcast was generated using the ocean general circulation model HYCOM, coupled to the biogeochemical model ECOSMO. The model was validated at known sponge habitats with available observations of hydrography and nutrients from the deep ocean to evaluate the biases, errors, and drift in the model. Knowing the biases and uncertainties we proceed to study the longer-term (monthly to multi-decadal) environmental variability at selected sponge habitats in the North Atlantic and Arctic Ocean. On these timescales, these deep sponge habitats generally exhibit small variability in the water-mass properties. Three of the sponge habitats, the Flemish Cap, East Greenland Shelf and North Norwegian Shelf, had fluctuations of temperature and salinity in 4–6 year periods that indicate the dominance of different water masses during these periods. The fourth sponge habitat, the Reykjanes Ridge, showed a gradual warming of about 0.4°C over the simulation period. The flux of organic matter to the sea floor had a large interannual variability, that, compared to the 67-year mean, was larger than the variability of primary production in the surface waters. Lateral circulation is therefore likely an important control mechanism for the influx of organic material to the sponge habitats. Simulated oxygen varies interannually by less than 1.5 ml/l and none of the sponge habitats studied had oxygen concentrations below hypoxic levels. The present study establishes a baseline for the recent past deep conditions that future changes in deep sea conditions from observations and climate models can be evaluated against.publishedVersio

How is climate change affecting marine life in the Arctic?

Rising temperature is melting the ice that covers the Arctic Ocean, allowing sunlight into waters that have been dark for thousands of years. Previously barren ice-covered regions are being transformed into productive seas. Here we explain how computer modelling can be used to predict how this transformation will affect the food web that connects plankton to fish and top-predators like whales and polar bears. Images of starving polar bears have become symbolic of the effects of warming climate. Melting of the sea-ice is expected to reduce the bears’ ability to hunt for seals. However, at the same time, the food web upon which they depend is becoming more productive, so it is not completely clear what the eventual outcome will be for the bears. Computer models help us to understand these systems and inform policy decisions on the management of newly available Arctic resources

Ecosystem approach to harvesting in the Arctic : walking the tightrope between exploitation and conservation in the Barents Sea

Funidng: This study was supported by the Changing Arctic Ocean project MiMeMo (NE/R012679/1) jointly funded by the UKRI Natural Environment Research Council (NERC) and the German Federal Ministry of Education and Research (BMBF/03F0801A). Brierley was also supported by ArcticPRIZE (NE/P005721/1).Projecting the consequences of warming and sea-ice loss for Arctic marine food web and fisheries is challenging due to the intricate relationships between biology and ice. We used StrathE2EPolar, an end-to-end (microbes-to-megafauna) food web model incorporating ice-dependencies to simulate climate-fisheries interactions in the Barents Sea. The model was driven by output from the NEMO-MEDUSA earth system model, assuming RCP 8.5 atmospheric forcing. The Barents Sea was projected to be > 95% ice-free all year-round by the 2040s compared to > 50% in the 2010s, and approximately 2 °C warmer. Fisheries management reference points (FMSY and BMSY) for demersal fish (cod, haddock) were projected to increase by around 6%, indicating higher productivity. However, planktivorous fish (capelin, herring) reference points were projected to decrease by 15%, and upper trophic levels (birds, mammals) were strongly sensitive to planktivorous fish harvesting. The results indicate difficult trade-offs ahead, between harvesting and conservation of ecosystem structure and function.Publisher PDFPeer reviewe

Can environmental conditions at North Atlantic deep-sea habitats be predicted several years ahead? - Taking sponge habitats as an example

Predicting the ambient environmental conditions in the coming several years to one decade is of key relevance for elucidating how deep-sea habitats, like for example sponge habitats, in the North Atlantic will evolve under near-future climate change. However, it is still not well known to what extent the deep-sea environmental properties can be predicted in advance. A regional downscaling prediction system is developed to assess the potential predictability of the North Atlantic deep-sea environmental factors. The large-scale climate variability predicted with the coupled Max Planck Institute Earth System Model with low-resolution configuration (MPI-ESM-LR) is dynamically downscaled to the North Atlantic by providing surface and lateral boundary conditions to the regional coupled physical-ecosystem model HYCOM-ECOSMO. Model results of two physical fields (temperature and salinity) and two biogeochemical fields (concentrations of silicate and oxygen) over 21 sponge habitats are taken as an example to assess the ability of the downscaling system to predict the interannual to decadal variations of the environmental properties based on ensembles of retrospective predictions over the period from 1985 to 2014. The ensemble simulations reveal skillful predictions of the environmental conditions several years in advance with distinct regional differences. In areas closely tied to large-scale climate variability and ice dynamics, both the physical and biogeochemical fields can be skillfully predicted more than 4 years ahead, while in areas under strong influence of upper oceans or open boundaries, the predictive skill for both fields is limited to a maximum of 2 years. The simulations suggest higher predictability for the biogeochemical fields than for the physical fields, which can be partly attributed to the longer persistence of the former fields. Predictability is improved by initialization in areas away from the influence of Mediterranean outflow and areas with weak coupling between the upper and deep oceans. Our study highlights the ability of the downscaling regional system to predict the environmental variations at deep-sea benthic habitats on time scales of management relevance. The downscaling system therefore will be an important part of an integrated approach towards the preservation and sustainable exploitation of the North Atlantic benthic habitats

Modelling the effects of changes in sea-ice extent on Arctic marine food webs

Diminishing extent of sea-ice cover in the Arctic over recent decades is well documented, and linked to global warming. The ecological effects have been profound especially in areas which have transformed from extensive seasonal ice-cover, to marginal sea-ice or year-round open water. The effects include an increase in Arctic primary production and changes in habitat and food availability for iconic marine mammals. Fishing nations anticipate increased harvesting opportunities in the Arctic as ice cover retreats further, but in November 2017 an international agreement was reached to prevent fisheries development in the Central Arctic Ocean for at least the next 16 years, to give time for development of scientific understanding. The scope for changes primary production due to diminishing sea-ice to propagate through the food web and affect higher trophic levels and charismatic megafauna such as whales, seals and polar bears, is extremely uncertain and hard to predict. The classical hypothesis would be that warming climate will result in a bottom-up trophic cascade from a) increased primary production, to b) increased zooplankton production, to c) increased fish production and harvesting potential, through to d) increased populations of charismatic marine megafauna. However, this assumes that primary production is retained in the upper layers of the water column – the outcome could be quite different if changes in vertical mixing and animal behavior associated with loss of ice cover lead instead to a greater proportion of primary production being directed to the benthos. Here we report on results from a configuration of the StrathE2E marine food web model to represent the Barents Sea. First, we show a baseline model representing sea-ice and temperature conditions during the 1980s-1990s, and then compare this with results from simulation of a warmer, year-round ice-free scenario. The results show that the increase in primary production in the ice-free scenario is amplified as it cascades up the food web. The effects preferentially benefit benthos and demersal fish, but this result is sensitive assumptions about prey preferences and vertical mixing. We also show how the food web responds to harvesting of fish, under both contemporary ice-cover and future ice-free situations. The results presented here are a starting point for a much more extensive new project under the NERC Charging Arctic Ocean Programme (Microbes to Megafauna Modelling of Arctic Seas (MiMeMo)) which we briefly introduce

On the role of biogeochemical coupling between sympagic and pelagic ecosystem compartments for primary and secondary production in the Barents Sea

Primary production in the Arctic marine system is principally due to pelagic phytoplankton. In addition, sea-ice algae also make a contribution and play an important role in food web dynamics. A proper representation of sea-ice algae phenology and the linkage with the pelagic and benthic systems is needed, so as to better understand the ecosystem response to warming and shrinking ice cover. Here we describe the extension of the biogeochemical model ECOSMO II to include a sympagic system in the model formulation, illustrated by implementation in the Barents Sea. The new sympagic system formulation includes four nutrients (NO3, NH4, PO4, and SiO2), one functional group for sea-ice algae and one detritus pool, and exchanges with the surface ocean layer. We investigated the effects of linkage between the three systems (sympagic, pelagic, and benthic) on the ecosystem dynamic; the contribution of the ice algae to total primary production; and how the changes in ice coverage will affect the lower trophic level Arctic food-web dynamics. To solve the scientific and technical challenges related to the coupling, the model was implemented in a 1D application of the General Ocean Turbulence Model (GOTM). Results showed that the model simulated the seasonal pattern of the sympagic components realistically when compared to the current knowledge of the Barents Sea. Our results show that the sympagic system influences the timing and the amplitude of the pelagic primary and secondary production in the water column. We also demonstrated that sea-ice algae production leads to seeding of pelagic diatoms and an enhancement of the zooplankton production. Finally, we used the model to explain how the interaction between zooplankton and ice algae can control the pelagic primary production in the Barents Se

Understanding the role of organic matter cycling for the spatio-temporal structure of PCBs in the North Sea

Using the North Sea as a case scenario, a combined three-dimensional hydrodynamic-biogeochemical-pollutant model was applied for simulating the seasonal variability of the distribution of hydrophobic chemical pollutants in a marine water body. The model was designed in a nested framework including a hydrodynamic block (Hamburg Shelf Ocean Model (HAMSOM)), a biogeochemical block (Oxygen Depletion Model (OxyDep)), and a pollutant-partitioning block (PolPar). Pollutants can be (1) transported via advection and turbulent diffusion, (2) get absorbed and released by a dynamic pool of particulate and dissolved organic matter, and (3) get degraded. Our model results indicate that the seasonality of biogeochemical processes, including production, sinking, and decay, favors the development of hot spots with particular high pollutant concentrations in intermediate waters of biologically highly active regions and seasons, and it potentially increases the exposure of feeding fish to these pollutants. In winter, however, thermal convection homogenizes the water column and destroys the vertical stratification of the pollutant. A significant fraction of the previously exported pollutants is then returned to the water surface and becomes available for exchange with the atmosphere, potentially turning the ocean into a secondary source for pollutants. Moreover, we could show that desorption from aging organic material in the upper aphotic zone is expected to retard pollutants transfer and burial into sediments; thus, it is considerably limiting the effectiveness of the biological pump for pollutant exports

Understanding the role of organic matter cycling for the spatio-temporal structure of PCBs in the North Sea

Using the North Sea as a case scenario, a combined three-dimensional hydrodynamic-biogeochemical-pollutant model was applied for simulating the seasonal variability of the distribution of hydrophobic chemical pollutants in a marine water body. The model was designed in a nested framework including a hydrodynamic block (Hamburg Shelf Ocean Model (HAMSOM)), a biogeochemical block (Oxygen Depletion Model (OxyDep)), and a pollutant-partitioning block (PolPar). Pollutants can be (1) transported via advection and turbulent diffusion, (2) get absorbed and released by a dynamic pool of particulate and dissolved organic matter, and (3) get degraded. Our model results indicate that the seasonality of biogeochemical processes, including production, sinking, and decay, favors the development of hot spots with particular high pollutant concentrations in intermediate waters of biologically highly active regions and seasons, and it potentially increases the exposure of feeding fish to these pollutants. In winter, however, thermal convection homogenizes the water column and destroys the vertical stratification of the pollutant. A significant fraction of the previously exported pollutants is then returned to the water surface and becomes available for exchange with the atmosphere, potentially turning the ocean into a secondary source for pollutants. Moreover, we could show that desorption from aging organic material in the upper aphotic zone is expected to retard pollutants transfer and burial into sediments; thus, it is considerably limiting the effectiveness of the biological pump for pollutant exports

Mutual dependency between benthic fauna and early diagenesis in a shelf sea

Field data collected for the North Sea indicate a prominent seasonal variation in the vertical distribution of total organic carbon (TOC) and macrobenthic biomass in sediments. The vertical TOC profiles classify into three modes, with maximum at surface, middle and deep part of sediments, respectively. We here present a mechanistic model to quantify, for the first time, the dynamic interaction between sedimentary TOC and benthic fauna. The major model principles include that (i) the vertical distribution of macrobenthic biomass is a trade-off between nutritional benefit (quantity and quality of TOC) and the costs of burial (respiration) and mortality, and (ii) the vertical transport of TOC is in turn modulated by macrobenthos through bioturbation. A novelty of our model is that bioturbation is resolved dynamically depending on variation of local food resources and macrobenthic biomass. This allows capturing of the benthic response to both depositional and erosional conditions and improving estimates of the material exchange flux at the sediment-water interface. The coupling of the TOC-benthos model with 3D hydrodynamic-ecological simulations reveals that the three profile modes of sedimentary TOC (in both quantify and quality) can be explained as a combined response to pelagic conditions (shear stress and primary production) and the synergy between bioturbation, vertical redistribution of higher quality TOC and vertical positioning of benthic organisms. A model reconstruction of the benthic status in the North Sea from 1950s to 2010s indicates that despite a relatively stable pattern at decadal and regional scales, significant variations exist at smaller scales characterized by seasons and local areas. In addition, inter-annual and multi-year cycle-like variations are also prominent especially in coastal areas