Yes, almost any organism, that respires aerobically, will simultaneously release CO2 as it consumes the O2 it uses to reduce to reduce C. Don't forget about the heterotrophic bacteria. However, at night, when photosynthesis stops, the consumption of O2 and release of CO2, increases dramatically in eutrophic systems. It can turn the water hypoxic. I hope this helped.

Can the net emission flux (day time) of both O2 and CO2 at the surface be positive?

It's possible. CO2 has a higher affinity for water than O2. In an alkaline solution CO2 is HCO3-. The excreted CO2 may stay in the water as bicarbonate. O2 is released into the atmosphere more readily, especially when the water is saturated with O2 (during the day). To have a net postive, flux for CO2 and O2 the system would need more aerobic activity than photosynthesis, and a lot of photsynthesis to saturate the water with O2. So It's possible. But when there is this much activity, water will become hypoxic at night. I wish there was a simple answer.

CO2 will not be released to the surface directly. As CO2 can be dissolved in the water as well as to react with water, it is not easy to link the CO2 at the surface to plant photosyntheses. Similarly, O2 can be dissolved in the water.

Yes it is possible to have water-to-air fluxes of CO2 and O2 at the same time. There are processes other than productivity and respiration that drive both O2 and CO2 partial pressures. For example, if the system you are studying receives groundwater inputs (as most rivers and estuaries do) than this groundwater normally has extremely high pCO2 (due to organic matter respiration in the soils),, and this can lead to a flux of CO2 to the atmosphere that is decoupled from respiration (e.g. see https://www.researchgate.net/publication/236234856_Carbon_dioxide_dynamics_driven_by_groundwater_discharge_in_a_coastal_floodplain_creek?ev=prf_pub ). If your system receives acidic surface or groundwater water inputs (e.g. surrounded by acid sulphate soils), the lowering of the pH can drive a shift in carbonate equilibrium, leading to high pCO2 which is also decoupled from respiration. Also if your system is a stream, river or estuary, you have water inputs from upstream that may have high O2 or high pCO2 that may be driving the observed fluxes you see at your site. Coupled to these abotic controls is the solubility difference between CO2 and O2.

Article Carbon dioxide dynamics driven by groundwater discharge in a...

We have been puzzling over a similar question in coastal marine systems where ocean pH has been dropping fast, all available evidence supports CO2/bicarb concentrations as the driver, but DO is not depleted as would be expected if any respiration-based processes were the CO2 source. We have been been contemplating the idea of elevated CO2 and/or organic C input from land-based photosynthesis, perhaps linked to elevated atmCO2, which would be consistent with the prior two answers, but we have a hard time understanding why these inputs would not also be highly DO-depleted too as the respiration stoichiometry requires. How does the groundwater get recharged with DO without losing its dissolved CO2 at the same time, such that the CO2:DO stoichiometry at the end looks distinctly different than that from aquatic respiration itself? Is the idea that the residence time in the groundwater is on just the right scale that the more rapid flux of O2 into/out of water recharges the DO but the CO2 doesn't have time to leave for the most part? I'm not so sure about supersaturation per se as an explanation, because the stoichiometry would also seem to require massive reduction in dissolved CO2/bicarb at the same time.

....An additional wrinkle would be that the air spaces in the soil have to be themselves be readily recharged with oxygen to facilitate diffusion into the ground water. Where might this come from? Diffusion out of roots or simply that subsurface soil air has a surprisingly (to me) high exchange rate?

Hi John

It is possible that terrestrial derived CO2, delivered by groundwater could still lead to what you are observing, even if the groundwater is anoxic. Lets take a very simplified scaled down mass balance approach to explain how this might happen. Lets assume your surface water starts out with DO and CO2 in equilibrium with the atmosphere , and has a volume of 100 cubic units and is 25 degreesC. For simplicity, lets assume your surface water is freshwater with a pH of 7 (the calculations get a bit more complicated with carbonate buffering in seawater). This will give concentrations DO = ~ 258 uM and DIC = ~70 uM. Now, lets assume 10 cubic units of groundwater enters your surface water volume. Again for simplicity lets say the groundwater is also freshwater, with a pH of 7, has no oxygen but has a DIC concentration of 3000 uM due to terrestrial respiration. The end result is 110 cubic liters of surface water with a DO concentration of 234uM and DIC concentration of 336uM. Now lets say the groundwater also contained nutrients which stimulated primary production, If you had anymore than 24 uM of oxygen produced (i.e. to take it back over saturation) then you would have a flux of oxygen to the atmosphere. If you lost 24 uM of DIC through primary production you still have a concentration of 312 uM which will give the water a pCO2 of ~1685 uatm (i.e. there will still be a net flux of CO2 to the atmosphere). Obviously this is a very simply example and does not take into account export losses (advection and atmospheric flux), but hopefully it helps. The main thing to consider is that the groundwater can increase in DIC concentration considerably more than the DO can decrease (DO can get below 0 uM,).

sorry meant DO can not get below 0 uM obviously

It's also possible if there's a wholesale shift in gas solubility due to temperature fluctuations

It is not possible. I cite two extreme cases as argument.1. in eutrophicated conditions there will be much photosynthesis and more requirement of CO2 and more evolution of oxygen. There will be only net release of O2 and absorption of CO2 from air. 2. In waters with organic load or in any water body at night, there will be high respiration and net demand of O2. So there will be absorption of O2 and possibly evolution of CO2 if the concentration exceeds the solubility limit. Since there is a high solubility of CO2 in water, water bodies are generally the CO2 sinks. On the other hand O2 has a low solubility in water resulting in net release of O2. Therefore in all photosynthetically active water bodies there is evolution of O2 and absorption of CO2 during day time and in night the reverse is true if the concentration of these gases in water exceeds their solubility.