Measuring NEP

Some under- and post-grad students recently asked me to explain how to measure NEP in cryoconite holes, and this post represents a brief overview on their behalf – apologies to other readers who may find this a bit “niche” – something more accessible next time!

What is NEP?

NEP stands for Net Ecosystem Productivity and is a measure of the balance between primary production (PP) and respiration (R) occurring in all the organisms within a microbial community. Primary production is the conversion of atmospheric inorganic carbon (IC) into organic carbon (OC), primarily using energy from sunlight (photosynthesis). This is opposed by respiration, which is the process of metabolising OC back into IC for the purposes of energy harvesting. PP uses CO2 and releases O2, R uses O2 and releases CO2.

Proteobacteria are one of many microbial species that are commonly found on ice surfaces (ph. wikimedia)
Proteobacteria are common heterotrophs that influence NEP in cryoconite (ph. wikimedia)

How can it be measured?

Since NEP involves the usage and production of  O2, dissolved IC (DIC) and dissolved OC (DOC), NEP can be measured using changes in the concentrations of these nutrients after a period of activity. Most analyses have used closed-bottle incubations and measured changes in these nutrients over time periods of hours to days. Some incubations are undertaken under normal light conditions (to measure NEP) and some are undertaken wrapped in tin foil to eliminate irradiance (to measure R). These measurements are based upon several fundamental assumptions: firstly, that primary production ceases in the dark; second, that confining the community within a bottle does not significantly alter nutrient availability or hydrochemistry during the incubation; third that the temperature is not significantly lower in tin-foil wrapped incubations; fourth that the glass walls of the bottles do not attenuate harmful UV-B radiation such that photosynthesis is artificially enhanced; fifth that respiration rates are constant for both light and dark incubations; and finally that we can accurately correct for sulphide oxidation (in oxygen-based studies) and carbonate dissolution (in carbon-based studies). Whether these assumptions are justifiable is still somewhat uncertain.

Incubated cryoconite suspended in a cryoconite hole to simulate 'in situ' conditions on the Greenlanbd ice sheet
Incubated cryoconite suspended in a cryoconite hole to simulate ‘in situ’ conditions on the Greenlanbd ice sheet

Methods:

Accepting the assumptions listed above, there are three primary ways to measure NEP. One of these is to measure changes in DO2, and there are two ways to measure changes in C.

Changes in dissolved O2 concentration in the water overlying cryoconite in incubations can easily be measured by swirling a DO2 meter in the solution. A decrease in DO2 between pre- and post-incubation measurements indicates net respiration; an increase indicates net primary production. Although simple, oxygen probes can be large compared to the diameter of incubation bottles, and they require agitation to simulate flow >15cm/min, which introduces the possibility of both degassing of oxygen into the atmosphere and spillage of the solution. There is also simply a longer exposure of the solution to the atmosphere using DO2 meters than the other methods, increasing the time in which degassing can occur. DO2 meters sometimes require frequent calibration using very sensitive manual controls, which can be awkward and time consuming – especially in the cold in the field!

Changes in total dissolved inorganic carbon (TDIC) concentration has become the favoured technique for measuring NEP. This involves acidifying the incubated solution with HCl to force DIC to degas as CO2 into a headspace full of “scrubbed” air. The CO2 concentration of this air can then be measured using an infra-red gas analyser (IRGA). This is a slightly more convoluted procedure than using a DO2 meter; however it has been proven to be robust, even in the field. There is less opportunity for degassing since the solution hardly ever becomes open to atmospheric exchanges.

The final technique is radiolabel incorporation. Here, radio-isotopes  of carbon (14C) are added to incubated cryoconite. After incubation the original 14C concentration and the remaining 14C concentration in the solution are used to calculate a rate of 14C incorporation. This requires only very short incubation times (minimising bottle effects) and is very sensitive; however it is possible for radioactive 14C to be incorporated into biomass and then respired by heterotrophs, releasing it back into the solution – there is no way to distinguish this from non-incorporation. Therefore, radiolabel incorporation is only really useful for approximating net PP. Furthermore, this technique cannot be used in net heterotrophic systems since the incubations become flooded with DIC which dilutes the 14C.

Telling et al (2010) identified TDIC as the optimum method for calculating NEP in cryoconite incubations for the reasons outlined above. Standard procedures for carrying out NEP measurements using TDIC were developed by Hodson et al (2010) and Telling et al (2010; 2012). They suggested that, since sediment arrangement significantly impacts NEP (Cook et al, 2010; Telling et al, 2012), measurements should be normalised for sediment mass and incubations should last for entire days.

Measuring NEP is important because it illustrates whether a habitat is a net source or sink of carbon. Cryoconite holes could be particularly active sites of microbial activity, and understanding their NEP tells us about their influence on carbon cycling. The video linked below (shared from Youtuber “Cryoconite314”) shows a fascinating time lapse of cryoconite hole dynamics on Qaanaaq Glacier in 2012.

 

References:

Cook, J.; Hodson, A.; Telling, J.; Anesio, A.; Irvine-Fynn, T.; Bellas, C. 2010. The mass-area relationship within cryoconite holes and its implications for primary production. Annals of Glaciology, 51 (56): 106-110.

Hodson, A. and 6 others. 2010b. The structure, biological activity and biogeochemistry of cryoconite aggregates upon an Arctic valley glacier: Longyearbreen, Svalbard. J. Glaciol., 56(196), 349–362.

Telling, J, Anesio, A, Hawkings, J, Tranter, M, Wadham, J, Hodson, A, Irvine-Fynn, T & Yallop, M 2010, ‘Measuring rates of gross photosynthesis and net community production in cryoconite holes: a comparison of field methods’. Annals of Glaciology, vol 51(56)., pp. 153 – 162

Telling, J., Anesio, A.M., Tranter, M., Stibal, M., Hawkings, J., Irvine-Fynn, T., Hodson, A.J., Butler, C., Yallop, M.,Wadham, J. 2012. Controls on the autochthonous production of organic matter in cryoconite holes on high Arctic glaciers. Journal of Geophysical Research: Biogeosciences, 117 (G1)

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