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Ocean Acidification: Documenting Its Impact on Calcifying Phytoplankton at Basin Scales

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Ocean Acidification: Documenting Its Impact on Calcifying Phytoplankton at Basin Scales Vol. 373: 239–247, 2008 MARINE ECOLOGY PROGRESS SERIES Published December 23 doi: 10.3354/meps07801 Mar Ecol Prog Ser Contribution to the Theme Section ‘Effects of ocean acidification on marine ecosystems’ OPENPEN ACCESSCCESS Ocean acidification: documenting its impact on calcifying phytoplankton at basin scales William M. Balch1,*, Victoria J. Fabry2 1Bigelow Laboratory for Ocean Sciences, PO Box 475, W. Boothbay Harbor, Maine 04575, USA 2Department of Biological Sciences, California State University San Marcos, San Marcos, California 92096-0001, USA ABSTRACT: In this paper, we evaluate several approaches to discern the impact of ocean acidifica- tion on calcifying plankton, over basin scales. We focus on estimates of the standing stock of particu- late inorganic carbon (PIC) associated with calcifying plankton since it is thought that these organ- isms will be the most sensitive to ocean acidification. Chemical techniques provide the greatest accuracy and precision for measuring the concentration of PIC in seawater, but basin-scale chemical surveys are formidably expensive due to the high costs of ship time and analytical instrumentation. Optical techniques, while not yet as precise as chemical methods, provide the opportunity to rapidly sample over much greater spatial scales, with large numbers of samples contributing to each PIC determination (which reduces the SE of each mean determination). Optical measurements from autonomous platforms (buoys and gliders) will provide important depth resolution of PIC, which is otherwise not accessible to ocean color satellites. We propose a strategy for future PIC measurements that employs both optical and chemical measurements on the same water samples. This will ensure adequate knowledge of the PIC backscattering cross-section, critical for satellite PIC determinations at basin scales. KEY WORDS: Coccolithophores · Ocean acidification · Calcium carbonate · Calcite · Coccolith · Ocean backscattering Resale or republication not permitted without written consent of the publisher TIME AND SPACE CONSIDERATIONS OF marine productivity. Seasonal aliasing also could be OCEAN ACIDIFICATION significant, however (e.g. due to temperature effects on CO2 equilibrium as well as seasonal variability in Ocean acidification resulting from anthropogenic phytoplankton primary production and community CO2 will likely take place over basin scales since respiration). We could resolve seasonal variability with –1 anthropogenic CO2 is globally dispersed throughout a sampling frequency of 8 times yr , (i.e. the Nyquist the atmosphere but with notable spatial bias (Conway frequency, the minimal frequency with which one et al. 1994). Moreover, the impact of ocean acidifica- could unambiguously represent the data without alias- 2– tion will vary by latitude due to the low CO3 concen- ing; Nyquist 1928). trations in cold, polar waters as well as increased con- By far, one of the biggest predicted biogeochemical centrations of CO2 associated with recently upwelled effects of ocean acidification will be on the global water, also prevalent at high latitudes (Orr et al. 2005). ocean carbonate cycle. The global standing stock of Ideally, methods to study the impact of ocean acidifica- particulate inorganic carbon (PIC) depends on calcium tion should match the global scale of its influence. carbonate production and dissolution, both of which Sampling frequencies for detecting the impact of are expected to be affected by ocean acidification. In ocean acidification should be at least annual, in order the pelagic ocean, PIC is contributed by a host of to discern the complicating effects of other well-known marine organisms, including: coccolithophores, calci- climate phenomena such as El Niño that cause strong fying dinoflagellates (e.g. Thoracosphaera sp.), fora- interannual variability in global weather patterns and minifera, pteropods, and diverse larval species of calci- *Email: [email protected] © Inter-Research 2008 · www.int-res.com 240 Mar Ecol Prog Ser 373: 239–247, 2008 fying invertebrates. The relative contribution of these (Sherrell et al. 1998), suggesting removal of PIC parti- groups varies in space and time, but on a global basis, cles (see also Bishop et al. 1980, 1986). Such chemical coccolithophores are one of the most important pro- observations, along with a plethora of other evidence, ducers of biogenic CaCO3. have led to the speculation that calcium carbonate par- The effect of climate change and ocean acidifi- ticles are dissolving above the lysocline (Milliman et al. cation on coccolithophores will be difficult to predict 1999). unequivocally. On the one hand, a lower pH and car- Chemical techniques such as ICPAA, ICPMS, and bonate ion concentration might reduce calcification in even X-ray fluorescence are highly accurate and pre- the bloom-forming coccolithophores Emiliania huxleyi cise for measuring particulate calcium, yet seasonal, and Gephyrocapsa oceanica (Riebesell et al. 2000, basin-scale ocean surveys, such as WOCE (World Zondervan et al. 2002), but the response does not Ocean Circulation Experiment)- or GEOSECS (Geo- appear to be uniform across all coccolithophorid spe- chemical Ocean Section Study)-style decadal surveys, cies(Langer et al. 2006). On the other hand, changes in would be prohibitively expensive using such tech- climate (warming and precipitation) could enhance niques. It should be noted that, unless a large volume vertical density stratification, which is known to of water is sampled for particulate calcium, relatively encourage E. huxleyi growth over other phytoplankton large, rare PIC particles from foraminiferal tests, ptero- species such as diatoms (Tyrrell & Taylor 1996). The pod shells, and calcareous invertebrate larvae may be purpose of this essay is to discuss techniques for large- undersampled. This highlights an advantage of high- scale assessment of the impact of ocean acidification volume, in situ filtration units (Bishop et al. 1985, on the standing stock of suspended PIC, which is a Thomalla et al. 2008). Nevertheless, distinct advan- good overall proxy of the ocean carbonate cycle. The tages of the chemical techniques are the accuracy and chemical and optical methods we will discuss have precision associated with the laboratory analyses and been used primarily to assess small PIC particles the high resolution of vertical profiles of PIC. It should such as coccolithophores and calcifying dinoflagel- be noted, however, that compared to particulate lates. We will discuss the various methods and end by organic carbon, relatively few data exist on the vertical outlining an overall strategy for detecting basin-scale distribution of PIC (Bishop et al. 1980, Balch & Kil- changes in PIC. patrick 1996, Sherrell et al. 1998, Balch et al. 2000, Poulton et al. 2006) and chemical techniques remain the most accurate way to quantify PIC. MEASURING THE STANDING STOCK OF PIC Chemical techniques Optical measurements on small volumes The standing stock of PIC can be estimated over Optical approaches for estimating PIC are based fun- basin scales using chemical analyses of particulate cal- damentally on the strong refractive index of calcium cium measured from ships. A number of techniques carbonate relative to water (1.19) (Broerse et al. 2003), are available to estimate particulate calcium including which is significantly greater than relative refractive in- X-ray fluorescence (Hurd & Spencer 1991, Twining et dex of pure biogenic silica (1.06) (Costello et al. 1995) or al. 2004), and atomic absorption (AA) (including flame non-minerogenic phytoplankton (1.05 to 1.06) (Ackle- AA, graphite furnace AA, and inductively coupled son & Spinrad 1988). The high relative refractive index plasma AA [ICPAA]) (Cheng et al. 2004) or inductively means that calcium carbonate is an intense scatterer of coupled plasma mass spectrometry (ICPMS) (Platzner light. Calcium carbonate also is highly birefringent (it et al. 2008). Background seawater concentrations of rotates the plane of polarized light by 90°), a property calcium (~0.1 mM) present a challenge to measuring used by micropaleontologists for decades to enumerate particulate calcium, since dissolved Ca++ in seawater biogenic and lithogenic mineral particles (Fig. 1A). must be rinsed away completely in order to accurately Recently, a technique to document particle birefrin- discern nanogram quantities of particulate calcium. gence has been adapted to estimate PIC by adding This is usually accomplished by carefully rinsing filters polarizing filters to a transmissometer (Guay & Bishop with buffered potassium tetraborate tetrahydrate (Fer- 2002). It is calibrated using purified mineral sus- nández et al. 1993). Moreover, with AA, corrections for pensions of diatomaceous earth and calcareous sedi- background seawater can be made by simultaneous ments. The technique shows promise. One possible measurement of sodium. Profiles of particulate calcium limitation is that other organic molecules also can be made using AA have shown excellent precision and highly birefringent. For example, in observations of consistent decreases in particulate calcium concen- thousands of samples with polarization microscopy, we trations with increasing depth over the top 500 m have observed that zooplankton carapaces, certain Balch & Fabry: Documenting ocean acidification at basin scales 241 and organic detritus), such errors should be easily quantifiable. This technology has been adapted for use on autonomous drifters (Bishop et al. 2004), which pro- vides
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