Spatial variations in time‐integrated plankton metabolic rates in Sagami Bay using triple oxygen isotopes and O2:Ar ratios

Abstract

Plankton metabolic rates, such as rates of gross, net community production and community respiration, were measured in the surface waters using triple oxygen isotopes and O 2 : Ar ratios. The 17 D anomaly showed clear coastal and offshore gradients that were consistent with the distribution of chlorophyll a and oxygen saturation. Gross primary production (GPP) rates in coastal regions were several times higher than offshore regions, and the net-to-gross production ratio (N : G) indicated that coastal regions were net autotrophic, whereas offshore regions were net heterotrophic. On a seasonal scale, about 73% of the phytoplankton-produced carbon was respired. Based on floating-sediment-trap data, export production to the aphotic zone was about 12-20% of the GPP; the rest accumulated as dissolved organic carbon (DOC) or was respired.Marine primary production accounts for about 50% of total carbon fixation in the biosphere (Field et al. 1998). About 60-90% of primary production is respired by heterotrophs in the upper few meters of the ocean (Laws et al. 2000). The ocean ecosystem is sustained by excess of gross primary production (GPP) over community respiration (R), which is termed net community production (NCP); the balance between GPP and R (the net metabolic status) dictates whether oceans are net sources or sinks of CO 2 to the atmosphere. Several studies have suggested that metabolic processes are substantially out of balance in the open-ocean regions, i.e., respiration is higher than production (del Giorgio and Duarte 2002;Williams et al. 2004), while coastal oceans are net autotrophic (Ducklow and McAllister 2005). Williams (1997) compared depth-integrated GPP and R and found that they are substantially in balance. However, both opinions are hotly debated. For instance, dissolved O 2 incubation experiments in the oligotrophic subtropical ocean (Hawaii Ocean Time Series Station; HOTS) suggested that heterotrophy is dominant in the photic zone (Williams et al. 2004). If this is so, it raises the question: what is the carbon source that supports heterotrophy in excess of production? Karl et al. (2003) observed that incubation experiments miss relatively short and infrequent occurrences of episodic high-productivity events, which may support net heterotrophy in oligotrophic regions. Clearly, the importance of such events cannot be assessed accurately on the basis of any single sampling because of the short time period covered by incubations. Therefore, a lack of data for episodic high-productive events and the inherent limitations of incubation experiments (Williams et al. 2004) prevent us from understanding whether metabolic rates are in balance or not.Most of our knowledge of primary production and production-to-community respiration ratios are based on incubation experiments using tracers, such as 14 C (Steeman Nielsen 1952), and 18 O (Grande et al. 1989), and concentration changes of dissolved oxygen or dissolved inorganic carbon in light and dark bottles. Each one of these techniques has its stren...

Ecosystem metabolism in salt marsh tidal creeks and ponds : applying triple oxygen isotopes and other gas tracers to novel environments

“…However, there are no published measurements of triple oxygen isotope fractionations of dissolved oxygen in sediment porewaters, or in the oxygen pool during common sedimentary processes such as sulfide oxidation. It is unclear how large an error this might introduce; study of oxygen isotope systematics in porewaters may be a particularly relevant problem moving forward as the triple oxygen isotope tracer is used to study productivity in shallow and nearshore environments (Sarma et al 2008, Munro et al 2013 where sediment-water interactions may play a significant role.…”

Technical note: Consistent calculation of aquatic gross production from oxygen triple isotope measurements

“…(4) with κ = 0.521 was used initially (Luz and Barkan, 2000;Luz et al, 1999), but following publication of the paper by Angert et al (2003), Eq. (7) was adopted with λ = 0.516 (Hendricks et al, 2004(Hendricks et al, , 2005Reuer et al, 2007;Juranek and Quay, 2005) or λ = 0.518 (Sarma et al, 2005(Sarma et al, , 2006a(Sarma et al, , 2008Stanley et al, 2010;Barkan, 2005, 2009;Quay et al, 2010;Juranek and Quay, 2010).…”

“…Equation (1) has been used in numerous studies to calculate gross O 2 production rates (e.g. Sarma et al, 2005Sarma et al, , 2006aSarma et al, , 2008Juranek and Quay, 2005;Stanley et al, 2010;Barkan, 2005, 2009). Usually, statistical uncertainties in the measurement of 17 and in the calculation of k from wind speeds are cited as main contributors to the overall uncertainty in P , which may be between 15 % (Stanley et al, 2010) and 40 % Reuer et al, 2007).…”

Net and gross oxygen production from O2/Ar, 17O/16O and 18O/16O ratios

“…Together, these 2 parameters can be used to assess net and gross primary productivity as well as community respiration, the latter being the difference between them. Our 17 Δ method has been adopted by other laboratories and used extensively for estimating gross O 2 production (GOP) in different parts of the world ocean (Hendricks et al 2004, Juranek & Quay 2005, Sarma et al 2005, 2008, Reuer et al 2007.…”