Posts Tagged 'primary production'

Impact of dust addition on the metabolism of Mediterranean plankton communities and carbon export under present and future conditions of pH and temperature

Although atmospheric dust fluxes from arid as well as human-impacted areas represent a significant source of nutrients to surface waters of the Mediterranean Sea, studies focusing on the evolution of the metabolic balance of the plankton community following a dust deposition event are scarce and none were conducted in the context of projected future levels of temperature and pH. Moreover, most of the experiments took place in coastal areas. In the framework of the PEACETIME project, three dust-addition perturbation experiments were conducted in 300-L tanks filled with surface seawater collected in the Tyrrhenian Sea (TYR), Ionian Sea (ION) and in the Algerian basin (FAST) onboard the R/V “Pourquoi Pas?” in late spring 2017. For each experiment, six tanks were used to follow the evolution of chemical and biological stocks, biological activity and particle export. The impacts of a dust deposition event simulated at their surface were followed under present environmental conditions and under a realistic climate change scenario for 2100 (ca. +3 °C and −0.3 pH units). The tested waters were all typical of stratified oligotrophic conditions encountered in the open Mediterranean Sea at this period of the year, with low rates of primary production and a metabolic balance towards net heterotrophy. The release of nutrients after dust seeding had very contrasting impacts on the metabolism of the communities, depending on the station investigated. At TYR, the release of new nutrients was followed by a negative impact on both particulate and dissolved 14C-based production rates, while heterotrophic bacterial production strongly increased, driving the community to an even more heterotrophic state. At ION and FAST, the efficiency of organic matter export due to mineral/organic aggregation processes was lower than at TYR likely related to a lower quantity/age of dissolved organic matter present at the time of the seeding. At these stations, both the autotrophic and heterotrophic community benefited from dust addition, with a stronger relative increase in autotrophic processes observed at FAST. Our study showed that the potential positive impact of dust deposition on primary production depends on the initial composition and metabolic state of the investigated community. This potential is constrained by the quantity of nutrients added in order to sustain both the fast response of heterotrophic prokaryotes and the delayed one of primary producers. Finally, under future environmental conditions, heterotrophic metabolism was overall more impacted than primary production, with the consequence that all integrated net community production rates decreased with no detectable impact on carbon export, therefore reducing the capacity of surface waters to sequester anthropogenic CO2.

Continue reading ‘Impact of dust addition on the metabolism of Mediterranean plankton communities and carbon export under present and future conditions of pH and temperature’

Risk assessment for key socio-economic and ecological species in a sub-arctic marine ecosystem under combined ocean acidification and warming

The Arctic may be particularly vulnerable to the consequences of both ocean acidification (OA) and global warming, given the faster pace of warming and acidification. Here, we use the Atlantis ecosystem model to assess how the trophic network of marine fishes and invertebrates in the Icelandic waters is responding to the combined pressures of OA and warming. We develop an approach which allows us to focus on species of economic (catch-value), social (number of participants in fisheries), or ecological (keystone species) importance. We parameterize the model with literature-determined ranges of sensitivity to OA and warming for different species and functional groups in the Icelandic waters. We found divergent species responses to warming and acidification levels; (mainly) planktonic groups and forage fish benefited while (mainly) benthic groups and predatory fish decreased under warming and acidification scenarios. Assuming conservative harvest rates for the largest catch-value species, Atlantic cod, we see that the population is projected to remain stable under even the harshest acidification and warming scenario. Further, for the scenarios where the model projects reductions in biomass of Atlantic cod, other species in the ecosystem increase, likely due to a reduction in competition and predation. These results highlight the interdependencies of multiple global change drivers and their cascading effects on trophic organization, and the supply of an important species from a socio-economic perspective in the Icelandic fisheries.

Continue reading ‘Risk assessment for key socio-economic and ecological species in a sub-arctic marine ecosystem under combined ocean acidification and warming’

Will community calcification reflect reef accretion on future, degraded coral reefs?

Coral bleaching events continue to drive the degradation of coral reefs worldwide, causing a shift in the benthic community from coral to algae dominated ecosystems. Critically, this shift may decrease the capacity of degraded coral reef communities to maintain net positive accretion during warming-driven stress events (e.g., reef-wide coral bleaching). Here we measured rates of net ecosystem calcification (NEC) and net ecosystem production (NEP) on a degraded coral reef lagoon community (coral cover < 10 % and algae cover > 20 %) during a reef-wide bleaching event in February of 2020 at Heron Island on the Great Barrier Reef. We found that during this bleaching event, rates of community NEP and NEC across replicate transects remained positive and did not change in response to bleaching. Repeated benthic surveys over a period of 20 d indicated an increase in the percent area of bleached coral tissue, corroborated by relatively low Symbiodiniaceae densities (~0.6 × 106 cm−2) and dark-adapted photosynthetic yields in photosystem II of corals (~0.5) sampled along each transect over this period. Given that a clear decline in coral health was not reflected in the overall community NEC estimates, it is possible that elevated temperatures in the water column that compromise coral health enhanced the thermodynamic favourability for calcification in other, ahermatypic benthic calcifiers. These data suggest that positive NEC on degraded reefs may not equate to the net positive accretion of reef structure in a future, warmer ocean. Critically, our study highlights that if coral cover continues to decline as predicted, NEC may no longer be an appropriate proxy for reef growth as the proportion of the community NEC signal owed to ahermatypic calcification increases and coral dominance on the reef decreases.

Continue reading ‘Will community calcification reflect reef accretion on future, degraded coral reefs?’

Effect of ocean acidification on bacterial metabolic activity and community composition in oligotrophic oceans, inferred from short-term bioassays

Increasing anthropogenic CO2 emissions in recent decades cause ocean acidification (OA), affecting carbon cycling in oceans by regulating eco-physiological processes of plankton. Heterotrophic bacteria play an important role in carbon cycling in oceans. However, the effect of OA on bacteria in oceans, especially in oligotrophic regions, was not well understood. In our study, the response of bacterial metabolic activity and community composition to OA was assessed by determining bacterial production, respiration, and community composition at the low-pCO2 (400 ppm) and high-pCO2 (800 ppm) treatments over the short term at two oligotrophic stations in the northern South China Sea. Bacterial production decreased significantly by 17.1–37.1 % in response to OA, since bacteria with high nucleic acid content preferentially were repressed by OA, which was less abundant under high-pCO2 treatment. Correspondingly, shifts in bacterial community composition occurred in response to OA, with a high fraction of the small-sized bacteria and high bacterial species diversity in a high-pCO2 scenario at K11. Bacterial respiration responded to OA differently at both stations, most likely attributed to different physiological responses of the bacterial community to OA. OA mitigated bacterial growth efficiency, and consequently, a larger fraction of DOC entering microbial loops was transferred to CO2.

Continue reading ‘Effect of ocean acidification on bacterial metabolic activity and community composition in oligotrophic oceans, inferred from short-term bioassays’

Extreme levels of ocean acidification restructure the plankton community and biogeochemistry of a temperate coastal ecosystem: a mesocosm study

The oceans’ uptake of anthropogenic carbon dioxide (CO2) decreases seawater pH and alters the inorganic carbon speciation – summarized in the term ocean acidification (OA). Already today, coastal regions experience episodic pH events during which surface layer pH drops below values projected for the surface ocean at the end of the century. Future OA is expected to further enhance the intensity of these coastal extreme pH events. To evaluate the influence of such episodic OA events in coastal regions, we deployed eight pelagic mesocosms for 53 days in Raunefjord, Norway, and enclosed 56–61 m3 of local seawater containing a natural plankton community under nutrient limited post-bloom conditions. Four mesocosms were enriched with CO2 to simulate extreme pCO2 levels of 1978 – 2069 μatm while the other four served as untreated controls. Here, we present results from multivariate analyses on OA-induced changes in the phyto-, micro-, and mesozooplankton community structure. Pronounced differences in the plankton community emerged early in the experiment, and were amplified by enhanced top-down control throughout the study period. The plankton groups responding most profoundly to high CO2 conditions were cyanobacteria (negative), chlorophyceae (negative), auto- and heterotrophic microzooplankton (negative), and a variety of mesozooplanktonic taxa, including copepoda (mixed), appendicularia (positive), hydrozoa (positive), fish larvae (positive), and gastropoda (negative). The restructuring of the community coincided with significant changes in the concentration and elemental stoichiometry of particulate organic matter. Results imply that extreme CO2 events can lead to a substantial reorganization of the planktonic food web, affecting multiple trophic levels from phytoplankton to primary and secondary consumers.

Continue reading ‘Extreme levels of ocean acidification restructure the plankton community and biogeochemistry of a temperate coastal ecosystem: a mesocosm study’

Impacts of climate change on methylmercury formation and bioaccumulation in the 21st century ocean

Highlights

  • Seawater MeHg may increase in the polar oceans and decrease in the North Atlantic in 2100
  • Plankton MeHg may increase at high latitudes and decrease at mid to low latitudes
  • Ocean acidification leads to different spatial patterns compared with physical factors

Summary

Climate change-driven alterations to marine biogeochemistry will impact the formation and trophic transfer of the bioaccumulative neurotoxin methylmercury (MeHg) in the global ocean. We use a 3D model to examine how MeHg might respond to changes in primary production and plankton community driven by ocean acidification and alterations in physical factors (e.g., ocean temperature, circulation). Productivity changes lead to significant increases in seawater MeHg in the polar oceans and a decrease in the North Atlantic Ocean. Phytoplankton MeHg may increase at high latitudes and decrease in lower latitudes due to shifts in community structure. Ocean acidification might enhance phytoplankton MeHg uptake by promoting the growth of a small species that efficiently accumulate MeHg. Non-linearities in the food web structure lead to differing magnitudes of zooplankton MeHg changes relative to those for phytoplankton. Climate-driven shifts in marine biogeochemistry thus need to be considered when evaluating future trajectories in biological MeHg concentrations.

Continue reading ‘Impacts of climate change on methylmercury formation and bioaccumulation in the 21st century ocean’

Late afternoon seasonal transition to dissolution in a coral reef: an early warning of a net dissolving ecosystem?

There are concerns that reefs will transition from net calcifying to net dissolving in the near future due to decreasing calcification and increasing dissolution rates. Here we present in situ rates of net ecosystem calcification (NEC) and net ecosystem production (NEP) on a coral reef flat using a slack‐water approach. Up until dusk, the reef was net calcifying in most months but shifted to net dissolution in austral summer, coinciding with high respiration rates and a lower aragonite saturation state (Ωarag). The estimated sediment contribution to NEC ranged from 8 – 21 % during the day and 45 – 78 % at night, indicating that high rates of sediment dissolution may cause the transition to reef dissolution. This late afternoon seasonal transition to negative NEC may be an early warning sign of the reef shifting to a net dissolving state and may be occurring on other reefs.

Continue reading ‘Late afternoon seasonal transition to dissolution in a coral reef: an early warning of a net dissolving ecosystem?’

Projections of algae, eelgrass, and zooplankton ecological interactions in the inner Salish Sea – for future climate, and altered oceanic states

Highlights

  • Harmonized simulation of DO, pH, and Y2095 climate change impacts in the Salish Sea
  • A 52-fold increase in exposure and near-bed pelagic species to hypoxic waters in Y2095
  • Ocean acidification projections for Y2095 indicate ≈ 20 −114% increase in water column (ΩA) <1)
  • Primary productivity propagation to zooplankton projected for Y2095 with ≈ 13%−25% increases.
  • Eelgrass sensitive to stressors and potential for loss of eelgrass biomass in the future.

Abstract

Future projections based on the IPCC high emissions scenario RCP8.5 have previously shown that the Pacific Northwest coastal waters will be subjected to altered ocean states in the upwelled shelf waters, resulting in higher primary productivity and increased regions of hypoxia and acidification in the inner estuarine waters such as the Salish Sea. However, corresponding effects on the lower trophic levels and submerged aquatic vegetation have not yet been quantified. Supported by new synoptic field data, explicit coupled simulation of algae, zooplankton, and eelgrass biomass was accomplished for the first time in the Salish Sea. We re-applied the improved model to evaluate future ecological response and examined potential algal species shift, but with the effects of zooplankton production, metabolism, and predation-prey interactions included. We also evaluated the role of eelgrass with respect to potential for improvements to dissolved oxygen and pH levels and as a mitigation measure against hypoxia and ocean acidification. The results re-confirm the possibility that there could be a substantial area-days increase (≈52-fold) in exposure of benthic and near-bed pelagic species to hypoxic waters in 2095. The projections for ocean acidification similarly indicate ≈ 20 -114% increase in exposure to lower pH corrosive waters with aragonite saturation state ΩA <1. Importantly, projected increase in primary productivity was shown to propagate to higher trophic levels, with ≈ 13% and 25% increases in micro and mesozooplankton biomass levels. However, the preliminary results also point to sensitivity of the eelgrass model to environmental stressor and potential loss eelgrass biomass in the future.

Continue reading ‘Projections of algae, eelgrass, and zooplankton ecological interactions in the inner Salish Sea – for future climate, and altered oceanic states’

Ocean acidification and short‐term organic matter enrichment alter coral reef sediment metabolism through different pathways

Ocean acidification (OA) and organic matter (OM) enrichment (due to coastal eutrophication) could act in concert to shift coral reef carbonate sediments from a present state of net calcification to a future state of net dissolution, but no studies have examined the combined effect of these stressors on sediment metabolism and dissolution. This study used 22‐hour incubations in flume aquaria with captive sediment communities to measure the combined effect of elevated pCO2 (representing Ocean Acidification) and particulate organic carbon (representing coastal eutrophication) on coral reef sediment gross primary productivity (GPP), respiration (R), and net calcification (Gnet). Relative to control sediment communities, both OA (pCO2 ~ 1000 μatm) and OM enrichment (~ + 40 μmol C L‐1) significantly decreased rates of sediment Gnet by 1.16 and 0.18 mmol CaCO3 m‐2 h‐1, respectively, but the mechanism behind this decrease differed. The OA‐mediated transition to net dissolution was physiochemical, as rates of GPP and R remained unaffected and dissolution was solely enhanced by a decline in the aragonite saturation state (Ωarg) of the overlying water column and the physical factors governing the porewater exchange rate with this overlying water column. In contrast, the OM‐mediated decline in Gnet was due to a decline in the overlying seawater Ωarg due to the increased respiratory addition of CO2. The decrease in Gnet in response to a combination of both stressors was additive (‐ 0.09 mmol CaCO3 m‐2 h‐1 relative to OA alone) but this decrease did not significantly differ from the individual effect of either stressor. In this study OA was the primary driver of future carbonate sediment dissolution, but longer‐term experiments with chronic organic matter enrichment are required.

Continue reading ‘Ocean acidification and short‐term organic matter enrichment alter coral reef sediment metabolism through different pathways’

Phytoplankton dynamics in a changing Arctic Ocean

Changes in the Arctic atmosphere, cryosphere and Ocean are drastically altering the dynamics of phytoplankton, the base of marine ecosystems. This Review addresses four major complementary questions of ongoing Arctic Ocean changes and associated impacts on phytoplankton productivity, phenology and assemblage composition. We highlight trends in primary production over the last two decades while considering how multiple environmental drivers shape Arctic biogeography. Further, we consider changes to Arctic phenology by borealization and hidden under-ice blooms, and how the diversity of phytoplankton assemblages might evolve in a novel Arctic ‘biogeochemical landscape’. It is critical to understand these aspects of changing Arctic phytoplankton dynamics as they exert pressure on marine Arctic ecosystems in addition to direct effects from rapid environmental changes.

Continue reading ‘Phytoplankton dynamics in a changing Arctic Ocean’


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Ocean acidification in the IPCC AR5 WG II

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