PRIMARY PRODUCTION METHODS 2277 Area of Research

PRIMARY PRODUCTION METHODS 2277 area of research. The Rrst computation of global Falkowski PG and Woodhead AD (eds) (1992) Primary oceanic primary production using the remote- Productivity and Biogeochemical Cycles in the Sea. sensing approach appeared in the literature in 1995 New York: Plenum Press. (Figure 1). Other, similar computations have Geider RJ and Osborne BA (1992). Algal Photosynthesis. since appeared in the literature. It is a method that New York: Chapman & Hall. will continue to improve, with improvements in Li WKW and Maestrini SY (eds) (1993) Measurement of Primary Production from the Molecular to the Global satellite technology as well as in the techniques for Scale, ICES Marine Science Symposia, vol. 197. extrapolation of local biological measurements to Copenhagen: International Council for the Exploration large scales. of the Sea. Longhurst A (1998) Ecological Geography of the Sea. San Diego: Academic Press. See also Longhurst A, Sathyendranath S, Platt T and Caverhill C (1995) An estimate of global primary production in Microbial Loops. Network Analysis of Food Webs. the ocean from satellite radiometer data. Journal of Ocean Gyre Ecosystems. Pelagic Biogeography. Plankton Research 17: 1245}1271. Primary Production Processes. Primary Produc- Mann KH and Lazier JRN (1991) Dynamics of Marine tion Methods. Ecosystems. Biological}Physical Interactions in the Oceans. Cambridge, USA: Blackwell Science. Platt T and Sathyendranath S (1993) Estimators of pri- Further Reading mary production for interpretation of remotely sensed data on ocean color. Journal of Geophysical Research Chisholm SW and Morel FMM (eds) (1991) What Con- 98: 14561}14576. trols Phytoplankton Production in Nutrient-Rich Areas Platt T, Harrison WG, Lewis MR et al. (1989) Biological of the Open Sea? vol. 36. Lawrence, KS: American production of the oceans: the case for a consensus. Society of Limnology and Oceanography. Marine Ecology Progress Series 52: 77}88. PRIMARY PRODUCTION METHODS J. J. Cullen, Department of Oceanography, measurement programs. However, details of these Halifax, Canada patterns can depend on methodology, so it is Copyright ^ 2001 Academic Press important to appreciate the uncertainties and built- in biases associated with different methods for doi:10.1006/rwos.2001.0203 measuring primary production. Introduction De\nitions Primary production is the synthesis of organic ma- Primary production is centrally important to eco- terial from inorganic compounds, such as CO2 and logical processes and biogeochemical cycling in water. The synthesis of organic carbon from CO2 is marine systems. It is thus surprising, if not discon- R R commonly called carbon xation: CO2 is xed by certing, that (as discussed by Williams in 1993), both photosynthesis and chemosynthesis. By far, there is no consensus on a deRnition of planktonic photosynthesis by phytoplankton accounts for most primary productivity, or its major components, net marine primary production. Carbon Rxation by and gross primary production. One major reason macroalgae, microphytobenthos, chemosynthetic for the problem is that descriptions of ecosystems microbes, and symbiotic associations can be locally require clear conceptual deRnitions for processes important. (e.g., net daily production of organic material by Only the measurement of marine planktonic phytoplankton), whereas the interpretation of primary production will be discussed here. These measurements requires precise operational deRni- measurements have been made for many decades tions, for example, net accumulation of radiolabeled using a variety of approaches. It has long been CO2 in particulate matter during a 24 h incubation. recognized that different methods yield different Conceptual and operational deRnitions can be rec- results, yet it is equally clear that the variability of onciled for particular approaches, but no one set of primary productivity, with depth, time of day, deRnitions is sufRciently general, yet detailed, to season, and region, has been well described by most serve as a framework both for measuring planktonic 2278 PRIMARY PRODUCTION METHODS primary production with a broad variety of methods ed as CH2O. Carbon dioxide in sea water is found and for interpreting the measurements in a range of in several chemical forms which exchange quickly R R scienti c contexts. It is nonetheless useful to de ne enough to be considered in aggregate as total CO2 three components of primary production that can be (TCO2). In principle, photosynthesis can be quanti- estimated from measurements in closed systems: Red by measuring any of three light-dependent processes: (1) the increase in organic carbon; (2) the E Gross primary production (Pg ) is the rate of decrease of TCO2 ; or (3) the increase of O2. How- photosynthesis, not reduced for losses to excre- ever, growth of phytoplankton is not so simple: tion or to respiration in its various forms since phytoplankton are composed of proteins, E Net primary production (Pn) is gross primary lipids, nucleic acids, and other compounds production less losses to respiration by phyto- besides carbohydrate, both photosynthesis and plankton the assimilation of nutrients are required. Conse- E Net community production (Pnc) is net primary quently, many chemical transformations are asso- production less losses to respiration by hetero- ciated with primary production, and eqn [1] does trophic microorganisms and metazoans. not accurately describe the process of light-dependent growth. Other components of primary production, such as It is therefore useful to describe the growth of new production, regenerated production, and export phytoplankton (i.e., net primary production) with production, must be characterized to describe food- a more general reaction that describes how trans- web dynamics and biogeochemical cycling. As formations of carbon and oxygen depend on the pointed out by Platt and Sathyendranath in 1993, in source of nutrients (particularly nitrogen) and on any such analysis, great care must be taken to the chemical composition of phytoplankton. For reconcile the temporal and spatial scales of both growth on nitrate: the measurements and the processes they describe. \# # Marine primary production is commonly ex- 1.0NO3 5.7CO2 5.4H2O pressed as grams or moles of carbon Rxed per unit P(C H O N)#8.25O #1.0OH\ [2] volume, or pet unit area, of sea water per unit time. 5.7 9.8 2.3 2 The timescale of interest is generally 1 day or The idealized organic product, C H O N, rep- 1 year. Rates are characterized for the euphotic 5.7 9.8 2.3 resents the elemental composition of phytoplankton. zone, commonly deRned as extending to the depth Ammonium is more reduced than nitrate, so less of 1% of the surface level of photosynthetically water is required to satisfy the demand for reduc- active radiation (PAR: 400}700nm). This conve- tant: nient deRnition of euphotic depth (sometimes sim- pliRed further to three times the depth at which 1.0NH>#5.7CO #3.4H O a Secchi disk disappears) is a crude and often inac- 4 2 2 P # # > curate approximation of where gross primary pro- (C5.7H9.8 O2.3N) 6.25O2 1.0H [3] duction over 24 h matches losses to respiration and excretion by phytoplankton. Regardless, rates of The photosynthetic quotient (PQ; mol mol\1) is the R photosynthesis are generally insigni cant below the ratio of O2 evolved to inorganic C assimilated. It depth of 0.1% surface PAR. must be speciRed to convert increases of oxygen to the synthesis of organic carbon. For growth on ni- \1 Photosynthesis and Growth of trate as described by eqn [2], PQ is 1.45 mol mol ; with ammonium as the source of N, PQ is 1.10. The Phytoplankton photosynthetic quotient also reSects the end prod- Primary production is generally measured by quan- ucts of photosynthesis, the mixture of which varies tifying light-dependent synthesis of organic carbon according to environmental conditions and the spe- from CO2 or evolution of O2 consistent with the cies composition of phytoplankton. For example, if simpliRed description of photosynthesis as the the synthesis of carbohydrate is favored, as can reaction: occur in high light or low nutrient conditions, PQ is lower because the reaction described in eqn [1] be- &8hJ comes more important. Uncertainty in PQ is often CO #2H O &P (CH O)#H O#O [1] 2 2 2 2 2 ignored. This can be justiRed when the synthesis of organic carbon is measured directly, but large errors Absorbed photons are signiRed by hl and the carbo- can be introduced when attempts are made to hydrates generated by photosynthesis are represent- infer carbon Rxation from the dynamics of oxygen. PRIMARY PRODUCTION METHODS 2279 Excretion of organic material would have a small sult from that for the light bottle. It is thus assumed inSuence on PQ and is not considered here. that respiration in the light equals that in the dark. As documented by Geider and Osborne in their Approaches 1992 monograph, this assumption does not generally hold, so errors in estimation of the respiratory Primary production can be estimated from chloro- component of Pg must be tolerated unless isotopi- phyll (from satellite color or in situ Suorescence) if cally labelled oxygen is used (see below). carbon uptake per unit of chlorophyll is known. Methods based on the direct measurement of Therefore, `global' estimates of primary production oxygen are less sensitive than techniques using the depend on direct measurements by incubation. The isotopic tracer 14C. However, careful implementa- technical objectives are to obtain a representative tion of procedures using automated titration or sample of sea water, contain it so that no signiRcant pulsed oxygen electrodes can yield useful and exchange of materials occurs, and to measure light- reliable data, even from oligotrophic waters of the dependent changes in carbon or oxygen during incu- open ocean. Interpretation of results is complicated bations that simulate the natural environment. by containment effects common to all methods for Methods vary widely, and each approach involves direct measurement of primary production (see be- compromises between needs for logistical conveni- low).

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