WHY WE WILL CONTINUE TO MEASURE PHYTOPLANKTON PHOTOSYNTHESIS DURING THE NEXT 50 YEARS

Banse’s letter in the ASLO Bulletin is correct as far as it goes. However, it does not include the value of C measurements for measuring P, the ‘assimilation number’ or ‘index.’ This is a measure of the rate of photosynthesis, not the amount. The units for this value are derived from dividing the C measurement by the standing stock of chlorophyll and Pmax values in units of mg C (mg Chl a hr) generally range from < 1.0 to ca. 10 (Parsons et al. 1990). Providing the chlorophyll a is measured on the same sample as the C uptake, the value is independent of grazing. This is a valid measure of the physiological state of the phytoplankton and will reflect light and nutrient conditions. For modeling purposes, it can be used in conjunction with assumed values of chlorophyll a to create scenarios of geographical differences in primary production.


ON THE USE OF 14 C MEASUREMENTS OF PRIMARY PRODUCTION
Banse's letter in the ASLO Bulletin is correct as far as it goes. However, it does not include the value of 14 C measurements for measuring P, the 'assimilation number' or 'index.' This is a measure of the rate of photosynthesis, not the amount. The units for this value are derived from dividing the 14 C measurement by the standing stock of chlorophyll and Pmax values in units of mg C (mg Chl a hr -1 ) -1 generally range from < 1.0 to ca. 10 (Parsons et al. 1990). Providing the chlorophyll a is measured on the same sample as the 14 C uptake, the value is independent of grazing. This is a valid measure of the physiological state of the phytoplankton and will reflect light and nutrient conditions. For modeling purposes, it can be used in conjunction with assumed values of chlorophyll a to create scenarios of geographical differences in primary production. Karl Banse challenged the oceanographic community to justify continued estimation of regional rates of phytoplankton productivity, specifically via rates of 14 C-uptake by phytoplankton in a profile of the euphotic zone. His argument is that measuring the rate of photosynthesis during 24-hour incubations yields data with low predictive value, mainly because loss terms (grazing, sinking) are not constrained at the same time. (To these loss terms we could add total system respiration, but that's a topic for future discussion.) We only partially agree with Banse's assessment. He is correct that studies of phytoplankton mortality processes (and respiratory losses) have greatly lagged studies of the rate of phytoplankton production in the world ocean. But, there are other issues beyond day-to-day predictability of phytoplankton production. We are confident that over the coming decades oceanographers will continue to measure rates of photosynthesis in order to estimate marine productivity. Here's why: 1) In 50 years, it won't be the same ocean. There is high probability of major shifts in ocean systems due to global climate change in this century. Ocean currents, particularly in the North Atlantic, are poised to shift dramatically over a short time scale if the Arctic ice cap and Greenland glaciers continue to melt. Warmer temperatures in the sea surface, more frequent El Niño events, and expansion of hypoxic zones in coastal regions are also predicted. Time series measurements will provide a continuity of data from the pre-warming state of the ocean into the period when global change effects become increasingly apparent. We will want to compare the patterns of phytoplankton photosynthesis in the world ocean in the 1980's with the patterns of photosynthesis in the 2030's. Using the same methodology ( 14 C uptake) would facilitate direct comparison.

REFERENCES
2) The ocean remains woefully under-sampled (yes, even for photosynthesis! See Lament, above). Biological oceanographers are just beginning to grapple with the spatial and temporal variability in phytoplankton biomass and rates of production in the world ocean. The results of the BATS and HOTS time series projects show clearly that there is intra-annual, inter-annual, even inter-decadal variability in phytoplankton stocks, in the composition of the phytoplankton community, in rates of production, in organic matter stocks, and in sinking fluxes (e.g. DuRand et al. 2001, Karl et al. 2001. And, these data sets are from the boringly uniform, warm oligotrophic regions of the world ocean! What about the other half of oceanic productivity, upwelling systems and shelf regions? There are currently no decadal data sets, analogous to BATS and HOTS, for these very important components of ocean production. The GLOBEC Long-Term-Observation Program (if six years can be considered long term!) in the California Current system has recently documented a dramatic year-to-year increase in the concentration of inorganic nutrients in upwelled water, which fueled very dense phytoplankton blooms (Wheeler 2002). Sinking of these blooms in turn resulted in the first-ever documented subsurface hypoxia event and massive die-off of benthic fauna, including rockfish, for the Pacific Northwest coast (Grantham et al. 2002). It is clear that time-series studies of mesotrophic and eutrophic ocean systems are needed to properly evaluate their intrinsic variability, and their response to global change processes. Such monitoring should include measurements of phytoplankton productivity.
3) We are still discovering major pieces of the oceanic productivity puzzle. Major groups of light-harvesting organisms (e.g., prochlorophytes, pico-phytoflagellates, and photoheterotrophic bacteria) are fairly new to us. We are also still in the early stages of elucidating the importance of atmospheric nitrogen inputs and nitrogen fixation as sources of new nitrogen to open ocean phytoplankton. Research is needed to evaluate the quantitative importance of these findings (and undoubtedly others as yet unknown) in the context of oceanic productivity.
Phytoplankton production is a basic process in marine ecosystems. We have far to go to understand the underlying physical, chemical, and biological factors that determine the rates and fates of primary production in various regions of the world ocean. New methodology (e.g., fast repetition rate (FFR) fluorescence instruments) may allow greater temporal and spatial resolution of phytoplankton photosynthetic activity, but bottle incubations with isotopically-labeled ( 13 C or 14 C, 15 N) substrates will continue to be a standard protocol for rate quantification.
We answer Banse's challenge by stating: more research is needed, both on marine photosynthesis and on phytoplankton loss processes. The next 50 years should be as interesting and important in this effort as were the previous 50. The comments by my colleagues are correct as far as they go. However, perhaps misled by the intentionally provocative headline, T.R. Parsons and E.B. and B.F. Sherr seem to have overlooked my premise stated in the very first sentence: "A principal goal of ecology and, hence, biological oceanography and limnology, is to understand and be able to predict the abundance of organisms and the rate of temporal change." Photosynthetic rate measurements were not the actual subject (see also the end of my first paragraph).
I'd also like everyone to note that there was a typo in the 6th line of the right-hand column of the first page; it was to be (Banse 1992), not 1994. Also, Hydrobiologia 480: 15-28 is the complete reference to (Banse 2002

Ecological Stoichiometry
The Biology of Elements from Molecules to the Biosphere

Robert W. Sterner and James J. Elser
All life is chemical. That fact underpins the developing field of ecological stoichiometry, the study of the balance of chemical elements in ecological interactions. This long-awaited book brings this field into its own.
"Ecological Stoichiometry is a monumental undertaking without ecological precedent. Sterner and Elser offer a majestic and novel synthesis of a broad and diverse field of study."