Posts Tagged 'biogeochemistry'

Time series pCO2 at a coastal mooring: internal consistency, seasonal cycles, and interannual variability

Marine carbonate system monitoring programs often consist of multiple observational methods that include underway cruise data, moored autonomous time series, and discrete water bottle samples. Monitored parameters include all, or some of the following: partial pressure of CO2 of the water (pCO2w) and air, dissolved inorganic carbon (DIC), total alkalinity (TA), and pH. Any combination of at least two of the aforementioned parameters can be used to calculate the others. In this study at the Gray’s Reef (GR) mooring in the South Atlantic Bight (SAB) we: examine the internal consistency of pCO2wfrom underway cruise, moored autonomous time series, and calculated from bottle samples (DIC-TA pairing); describe the seasonal to interannual pCO2w time series variability and air-sea flux (FCO2), as well as describe the potential sources of pCO2wvariability; and determine the source/sink for atmospheric pCO2. Over the ~8.5 years of GR mooring time series, mooring-underway and mooring-bottle calculated-pCO2wstrongly correlate with r-values > 0.90. pCO2w and FCO2 time series follow seasonal thermal patterns; however, seasonal non-thermal processes, such as terrestrial export, net biological production, and air-sea exchange also influence variability. The linear slope of time series pCO2w increases by 5.2±1.4 µatm y−1 with FCO2 increasing 51 to 70 mmol m−2 y−1. The net FCO2 sign can switch interannually with the magnitude varying greatly. Non-thermal pCO2w is also increasing over the time series, likely indicating that terrestrial export and net biological processes drive the long term pCO2w increase.

Continue reading ‘Time series pCO2 at a coastal mooring: internal consistency, seasonal cycles, and interannual variability’

Organic matter export to the seafloor in the Baltic Sea: Drivers of change and future projections

The impact of environmental change and anthropogenic stressors on coastal marine systems will strongly depend on changes in the magnitude and composition of organic matter exported from the water column to the seafloor. Knowledge of vertical export in the Baltic Sea is synthesised to illustrate how organic matter deposition will respond to climate warming, climate-related changes in freshwater runoff, and ocean acidification. Pelagic heterotrophic processes are suggested to become more important in a future warmer climate, with negative feedbacks to organic matter deposition to the seafloor. This is an important step towards improved oxygen conditions in the near-bottom layer that will reduce the release of inorganic nutrients from the sediment and hence counteract further eutrophication. The evaluation of these processes in ecosystem models, validated by field observations, will significantly advance the understanding of the system’s response to environmental change and will improve the use of such models in management of coastal areas.

Continue reading ‘Organic matter export to the seafloor in the Baltic Sea: Drivers of change and future projections’

The role of biological rates in the simulated warming effect on oceanic CO2 uptakel

Marine biology plays an important role in the ocean carbon cycle. However, the effect of warming-induced changes in biological rates on oceanic CO2 uptake has been largely overlooked. We use an Earth system model of intermediate complexity to investigate the effect of temperature-induced changes in biological rates on oceanic uptake of atmospheric CO2 and compare it with the effects from warming-induced changes in CO2 solubility and ocean mixing and circulation. Under the representative CO2 concentration pathway RCP 8.5 and its extension, by year 2500, relative to the simulation without warming effect on the ocean carbon cycle, CO2-induced warming reduces cumulative oceanic CO2 uptake by 469 Pg C, of which about 20% is associated with the warming-induced change in marine biological rates. In our simulations, the bulk effect of biological-mediated changes on CO2 uptake is smaller than that mediated by changes in CO2 solubility and ocean mixing and circulation. However, warming-induced changes in individual biological rates, including phytoplankton growth, phytoplankton mortality, and detritus remineralization, are found to affect oceanic CO2 uptake by an amount greater than or comparable to that caused by changes in CO2 solubility and ocean physics. Our simulations, which include only a few temperature-dependent biological processes, demonstrate the important role of biological rates in the oceanic CO2 uptake. In reality, many more complicated biological processes are sensitive to temperature change, and their responses to warming could substantially affect oceanic uptake of atmospheric CO2.

Continue reading ‘The role of biological rates in the simulated warming effect on oceanic CO2 uptakel’

Scales and drivers of seasonal pCO2 dynamics and net ecosystem exchange along the coastal waters of southeastern Arabian Sea

The impact of seasonal coastal upwelling on the dynamics of dissolved inorganic carbon (DIC) and sea-air fluxes of CO2 along the coastal waters of Kochi was investigated during 2015, as a part of Ecosystem Modelling Project. The surface water pCO2 varied from 396 to 630 μatm during the study period. Significant inter-seasonal variations were found in the distribution of physico-chemical variables and surface pCO2. An increase of 102.1 μatm of pCO2 was noticed over a two-decade period with a rate of 5.3 μatm y− 1. There was an agreement between the fluxes of CO2 and net ecosystem production (NEP) with respect to the trophic status while NEP was higher than CO2 fluxes by a factor of 3.9. The annual net ecosystem exchange (NEE) was estimated to be 15.02 mmol C m− 2 d− 1 indicating that the coastal waters of Kochi are highly heterotrophic in nature.

Continue reading ‘Scales and drivers of seasonal pCO2 dynamics and net ecosystem exchange along the coastal waters of southeastern Arabian Sea’

Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: a baseline for ocean acidification impact assessment

The Southern Ocean provides a vital service by absorbing about one sixth of humankind’s annual emissions of CO2. This comes with a cost – an increase in ocean acidity that is expected to have negative impacts on ocean ecosystems. The reduced ability of phytoplankton and zooplankton to precipitate carbonate shells is a clearly identified risk. The impact depends on the significance of these organisms in Southern Ocean ecosystems, but there is very little information on their abundance or distribution. To quantify their presence, we used coulometric measurement of particulate inorganic carbonate (PIC) on particles filtered from surface seawater into two size fractions: 50–1000 μm to capture foraminifera (the most important biogenic carbonate forming zooplankton) and 1–50 μm to capture coccolithophores (the most important biogenic carbonate forming phytoplankton). Ancillary measurements of biogenic silica (BSi) and particulate organic carbon (POC) provided context, as estimates of the abundance of diatoms (the most abundant phytoplankton in polar waters), and total microbial biomass, respectively. Results for 9 transects from Australia to Antarctica in 2008–2015 showed low levels of PIC compared to northern hemisphere polar waters. Coccolithophores slightly exceeded the biomass of diatoms in Subantarctic waters, but their abundance decreased more than 30-fold poleward, while diatom abundances increased, so that on a molar basis PIC was only 1 % of BSi in Antarctic waters. This limited importance of coccolithophores in the Southern Ocean is further emphasized in terms of their associated POC, representing less than 1 % of total POC in Antarctic waters and less than 10 % in Subantarctic waters. NASA satellite ocean colour based PIC estimates were in reasonable agreement with (though somewhat higher than) the shipboard results in Subantarctic waters, but greatly over-estimated PIC in Antarctic waters. Contrastingly, the NASA Ocean Biogeochemical Model (NOBM) shows coccolithophores as overly restricted to Subtropical and northern Subantarctic waters. The cause of the strong southward decrease in PIC abundance in the Southern Ocean is not yet clear. Poleward decrease in pH is small and while calcite saturation decreases strongly southward it remains well above saturation (> 2). Nitrate and phosphate variations would predict a poleward increase. Temperature and competition with diatoms for limiting iron appear likely to be important. While the future trajectory of coccolithophore distributions remains uncertain, their current low abundances suggest small impacts on overall Southern Ocean pelagic ecology.

Continue reading ‘Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: a baseline for ocean acidification impact assessment’

Net community metabolism and seawater carbonate chemistry scale non-intuitively with coral cover

Coral cover and reef health have been declining globally as reefs face local and global stressors including higher temperature and ocean acidification (OA). Ocean warming and acidification will alter rates of benthic reef metabolism (i.e., primary production, respiration, calcification, and CaCO3 dissolution), but our understanding of community and ecosystem level responses is limited in terms of functional, spatial, and temporal scales. Furthermore, dramatic changes in coral cover and benthic metabolism could alter seawater carbonate chemistry on coral reefs, locally alleviating or exacerbating OA. This study examines how benthic metabolic rates scale with changing coral cover (0-100%), and the subsequent influence of these coral communities on seawater carbonate chemistry based on mesocosm experiments in Bermuda and Hawaii. In Bermuda, no significant differences in benthic metabolism or seawater carbonate chemistry were observed for low (40%) and high (80%) coral cover due to large variability within treatments. In contrast, significant differences were detected between treatments in Hawaii with benthic metabolic rates increasing with increasing coral cover. Observed increases in daily net community calcification and nighttime net respiration scaled proportionally with coral cover. This was not true for daytime net community organic carbon production rates, which increased the most between 0 to 20% coral cover and then less so between 20% to 100%. These differences in scaling resulted in larger diel variability in seawater carbonate chemistry as coral cover increased. To place the results of the mesocosm experiments into a broader context, in situ seawater carbon dioxide (CO2) at three reef sites in Bermuda and Hawaii were also evaluated; reefs with higher coral cover experienced a greater range of diel CO2 levels, complementing the mesocosm results. The results from this study highlight the need to consider the natural complexity of reefs and additional biological and physical factors that influence seawater carbonate chemistry on larger spatial and longer temporal scales. Coordinated efforts combining various research approaches (e.g. experiments, field studies, and models) will be required to better understand how benthic metabolism integrates across functional, spatial, and temporal scales, and for making predictions on how coral reefs will respond to climate change.

Continue reading ‘Net community metabolism and seawater carbonate chemistry scale non-intuitively with coral cover’

Impact of ocean acidification on Arctic phytoplankton blooms and dimethyl sulfide concentration under simulated ice-free and under-ice conditions (update)

In an experimental assessment of the potential impact of Arctic Ocean acidification on seasonal phytoplankton blooms and associated dimethyl sulfide (DMS) dynamics, we incubated water from Baffin Bay under conditions representing an acidified Arctic Ocean. Using two light regimes simulating under-ice or subsurface chlorophyll maxima (low light; low PAR and no UVB) and ice-free (high light; high PAR + UVA + UVB) conditions, water collected at 38 m was exposed over 9 days to 6 levels of decreasing pH from 8.1 to 7.2. A phytoplankton bloom dominated by the centric diatoms Chaetoceros spp. reaching up to 7.5 µg chlorophyll a L−1 took place in all experimental bags. Total dimethylsulfoniopropionate (DMSPT) and DMS concentrations reached 155 and 19 nmol L−1, respectively. The sharp increase in DMSPT and DMS concentrations coincided with the exhaustion of NO3− in most microcosms, suggesting that nutrient stress stimulated DMS(P) synthesis by the diatom community. Under both light regimes, chlorophyll a and DMS concentrations decreased linearly with increasing proton concentration at all pH levels tested. Concentrations of DMSPT also decreased but only under high light and over a smaller pH range (from 8.1 to 7.6). In contrast to nano-phytoplankton (2–20 µm), pico-phytoplankton ( ≤  2 µm) was stimulated by the decreasing pH. We furthermore observed no significant difference between the two light regimes tested in term of chlorophyll a, phytoplankton abundance and taxonomy, and DMSP and DMS net concentrations. These results show that ocean acidification could significantly decrease the algal biomass and inhibit DMS production during the seasonal phytoplankton bloom in the Arctic, with possible consequences for the regional climate.

Continue reading ‘Impact of ocean acidification on Arctic phytoplankton blooms and dimethyl sulfide concentration under simulated ice-free and under-ice conditions (update)’

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Ocean acidification in the IPCC AR5 WG II

OUP book