Water column respiration

In Amazon floodplain lakes, daily water column respiration usualiy exceeds planktonic gross photosynthesis, and dissolved oxygen is usually undersaturated (Melack and Fisher 1983). Wissmar et al. (1981) and Richey et al. (1988) found Amazon flood-plain lakes to be consistently undersaturated in O2 and supersaturated in CO2. The deficit of oxygen in these systems is a consequence of high respiration to production ratios. 

Oceanic surface waters are efficiently stripped of nutrients by phytoplankton. If phytoplankton biomass was not reconverted into simple dissolved nutrients, the entire marine water column would be depleted in nutrients and growth would stop. But as we saw from the carbon balance presented earlier, more than 90% of the primary productivity is released back to the water column as a reverse RKR equation. This reverse reaction is called remineralization and is due to respiration. An important point is that while production via photosynthesis can only occur in surface waters, the remineralization by heterotrophic organisms can occur over the entire water column and in the underlying sediments. 

Redfield (1934), who analyzed the major elemental content of many samples of mixed plankton (phytoplankton and zooplankton) caught in nets towed through the surface ocean. They compared the carbon, nitrogen, and phosphorus composition of these collections to concentration profiles of dissolved inorganic carbon (DIC), NOs, and P04 throughout the water column. This pioneering research demonstrated that these three elements are continually redistributed in the ocean by selective removal into plankton cells and their remains (i.e., fecal pellets), which are then efficiently respired as they sink through the marine water column. 

The balance between relative rates of aerobic respiration and water movement were considered in Section 4.3.4. We saw that a subsurfece concentration minimum, the oxygen minimum zone (OMZ), is a common characteristic of vertical profiles of dissolved oxygen and is produced by in situ respiration. Waters with O2 concentrations less than 2.0 ppm are termed hypoxic The term anoxic is applied to conditions when O2 is absent. (Some oceanographers use the term suboxic to refer to conditions where O2 concentrations fall below 0.2 ppm but are still detectable.) As illustrated by Figure 4.21b, this water column is hypoxic in the OMZ. The dissolved oxygen concentrations are presented as % saturations in Figure 4.21c. With the exception of the mixed layer, the water column is undersaturated with respect to dissolved oxygen with the most intense undersaturations present in mid-depths. Surface supersaturations are the result of O2 input from photosynthesis and bubble injection. 

During denitrification, nitrate is reduced to N2(g) by heterotrophic bacteria. The stoichiometry for this process was presented in Eq. 8.11 and Table 12.1 for the water column and the sediments, respectively. Denitrification is thought to be performed by fecultative aerobes that switch to nitrate respiration when O2 concentrations are less than 5 p-M (0.1 ml/L). 

Sulfur fulfills many diverse roles in lakes. As the sixth most abundant element in biomass, it is required as a major nutrient by all organisms. For most algae, S is abundant in the form of sulfate in the water column however, in dilute glacial lakes in Alaska (I) and in some central African lakes (2) low concentrations of sulfate may limit primary production. Sulfur also serves the dual role of electron acceptor for respiration and, in reduced forms, source of energy for chemolithotrophic secondary production. Net sulfate reduction can account for 10-80% of anaerobic carbon oxidation in lakes (3-5), and hence this process is important in carbon and energy flow. Sulfate reduction, whether associated with uptake of sulfate and incorpo-... 

POPs can be absorbed actively or passively by zooplankton and phytoplankton species, or by higher organisms, such as fish, via ingestion, respiration and diffuse absorption. Some compounds may accumulate in body tissue, while others are transformed into other compounds by the metabolism of the organism or excreted to the water column in its original form [75-77]. As consequence of particle deposition, POPs will eventually end up on the bottom of the lake and be incorporated into the sediment were they are generally well preserved due to the low biological activities in these lakes [50, 51]. 

Seventeen genera of facultative anaerobic bacteria (e.g., Pseudomonas and Bacillus) can perform denitrification under anaerobic or low-oxygen conditions, where they use NO3- as an electron acceptor during anaerobic respiration (Jaffe, 2000). In fact, in many estuaries, denitrification is limited by the availability of NC>3 (Koike and Sprensen, 1988 Cornwell et al., 1999). Sources of NC>3 and NC>2 for denitrification are from diffusive inputs from the overlying water column and nitrification in the sediments (Jenkins and Kemp, 1984). The activity of other bacterial processes under anoxic conditions has been shown to affect the activity of denitrifying bacteria. For example, SO42- reduction occurs in anoxic sediments whereby SC>42 is reduced to sulfide (Morse et al., 1992)—more... 

Naqvi, S. W. A., Shailaja, M. S., Kumar, M. D., and Sen Gupta, R. (1996). Respiration rates in subsurface waters of the northern Indian Ocean Evidence for low decomposition rates of organic matter within the water column in the Bay of Bengal. Deep-Sea Res. II43, 73—81. 

Since nearly aU subsurface respiration on the global scale is supported by oxygen, there is no clear a priori reason that water column denitrification should have been similarly important at aU times in the past increases in the rate of oxygen supply relative to consumption could have nearly eliminated it. However, the strong sensitivities of water column denitrification, while rendering this process susceptible to climate-driven variability, probably also make it more responsive to stabilizing negative feedbacks (see Section 2.2.3). 

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