Turbulence does not prevent nitrogen fixation by plankton in estuaries and coastal seas (reply to the comment by Paerl et al.)

Transcription

1 Comment 639 Limnol. Oceanogr., 40(3), 1995, , by the American Society of Limnology and Oceanography, Inc. Turbulence does not prevent nitrogen fixation by plankton in estuaries and coastal seas (reply to the comment by Paerl et al.) Heterocystic, N,-fixing cyanobacteria (blue-green algae) are rare or absent from the plankton of most estuaries and coastal marine ecosystems (Home 1977; Howarth et al. 1988a), and rates of N2 fixation are correspondingly low (Howarth et al. 1988b). In contrast to many freshwater lakes where N, fixation by these organisms can be a major factor leading to P limitation (Schindler 1977) the lack of N2 fixation in many coastal marine waters contributes to N as the factor limiting net annual primary production (Howarth 1988; Vitousek and Howarth 199 1). Many hypotheses have been put forward to explain the relative scarcity of planktonic, N,-fixing cyanobacteria in estuaries and coastal marine waters (as well as in many open-ocean waters). Several of these invoke the higher turbulence in the marine and estuarine environment as deleterious to planktonic cyanobacteria (Doremus 1982; Fogg 1987; Valiela 199 1). Turbulence can affect phytoplankton through a variety of mechanisms (Mann and Lazier 199 1; Howarth et al. 1993; Reynolds 1994). Paerl (1985) proposed the hypothesis that higher turbulence and lower dissolved organic matter in marine ecosystems as compared to lakes reduces the likelihood of anoxic microzones on the surfaces of cyanobacterial cells, making poisoning of nitrogenase by 0, more likely. We used a mesocosm experiment to test specifically whether this hypothesis could explain the relative absence of planktonic N2 fixation at turbulence levels typical in estuaries. Turbulence was generated in mesocosms containing water and ambient plankton from a lake known to have blooms of planktonic cyanobacteria (Anabaena and Aphanizomenon sp.; Howarth et al. 1993). We found high rates of N, fixation by these cyanobacteria even at very high levels of turbulence, indicating that a direct effect of turbulence is unlikely to explain the relative absence of planktonic N2 fixation in estuaries and coastal seas. Paerl et al. (1995) raise four general criticisms of our paper: they believe we inappropriately used Anabaena as a test organism since anoxic microzones are less likely to affect this heterocystic organism than would be the case for nonheterocystic N, fixers; they state that heterocystic cyanobacteria are more common in estuarine and marine ecosystems than we had implied; they contend that our experiment was poorly designed and that our interpretation of N, fixation data was flawed; and they conclude that we did not accurately measure turbulence in our mesocosms. We disagree with each of these criticisms. Our detailed responses follow below. In their comment, Paerl et al. (1995) mix discussion of cyanobacteria in estuaries and coastal seas with cyanobacteria in open-ocean systems and of benthic cyanobacteria with planktonic cyanobacteria. Our work to date has addressed why N2 fixation by planktonic cyanobac- teria is low in most estuaries and coastal seas, and we have discussed extensively how controls in benthic mats and in open oceans may differ (Howarth and Cole 1985; Howarth et al. 1988a; Marino et al. 1990; Vitousek and Howarth 199 1). The mesocosm experiment criticized by Paerl et al. (199 5) was designed only to address whether turbulence is sufficient to explain the difference between lakes and estuaries with regard to planktonic N, fixation by heterocystic cyanobacteria, as was explicitly stated by Howarth et al. (1993). The use of heterocystic cyanobacteria to test the turbulence hypothesis -We agree with Paerl et al. (1995) that turbulence may affect heterocystic N,-fixing organisms differently from nonheterocystic species which have evolved other mechanisms for dealing with the 02 problem. Our experiment showed that high levels of turbulence do not keep heterocystic species such as Anabaena from blooming in the plankton and fixing N2, but we were careful not to suggest that our results applied to nonheterocystic species of N2 fixers such as Trichodesmium (Howarth et al. 1993). These organisms live in oceanic waters which are far less turbulent than many estuaries (Table 1), perhaps allowing the evolution of species which are not particularly tolerant of high turbulence. They may be much more sensitive to turbulence. We strongly disagree that our experiment was an inappropriate test of the hypothesis originally stated by Paerl(1985) simply because Anabaena dominated: Paerl (1985) himself in his original experiments on the role of turbulence and microzones on N, fixation used a heterocystic, freshwater Anabaena species as one of only two test organisms. In marked contrast to the comment of Paerl et al (1995) Paerl (1985) stated that the Anabaena he used was to some extent sensitive to 0, and fixed the largest amount of N2 under low turbulence conditions (see figure 1 of Paerl 1985) a fact he attributed to the creation of anoxic microzones by bacterial assemblages. Thus, the current criticism that Anabaena is an inappropriate test organism because the presence or absence of anoxic microzones has little bearing on the ability of Anabaena to fix N2 (p. 634) is inconsistent with the original experiments and conclusions of Paerl (1985). Paerl s (1985) hypothesis was posed in the context of why there is so little N, fixation in N-limited portions of the world s oceans. He stated that (Paerl 1985, p ) more extensive and long-lasting episodes of N2 fixation are found in freshwaters than in marine environments because calm conditions and dissolved organic matter enrichment are more often associated with freshwaters, making microzones conducive to N2 fixation more likely. N2 fixation in the plankton of freshwater lakes is domi-

2 640 Comment Table 1. Rates of turbulent energy dissipation, size of smallest eddies, and strain rates for natural and experimental systems. Data for turbulent energy dissipation in lakes, open oceans, and the Sever-n estuary are from Reynolds (1994), in tidally mixed waters from MacKenzie and Leggett (199 l), in Narragansett Bay from Nixon et al. (1980) and in mesocosms from Howarth et al. (1993); re-estimated rates of Paerl et al. (1995) shown for comparison. Also shown are data from rotating cylinders of Thomas and Gibson (1990) for study of turbulence on phytoplankton growth. Smallest eddies calculated as by Reynolds (1994). Strain rates calculated as by Thomas and Gibson (1990). Natural systems Lakes Open ocean Narragansett Bay and other tidally mixed water columns Severn estuary Experimental systems Lower turbulence mesocosms Range of Howarth et al. Mean of Howarth et al. Re-estimate of Paerl et al. Higher turbulence mesocosms Range of Howarth et al. Mean of Howarth et al. Re-estimate of Paerl et al. Rotating cylinders Turbulent energy dissipation rate (lop6 W kgg ) CO l, l 2,000 1, l 6,400 Smallest eddy (mm> Strain rate (s >1.3 ~ U l l 32 nated by heterocystic cyanobacteria, which often make up the majority of the phytoplankton biomass; other N2- fixing organisms are seldom important as sources of new N to lakes (Howarth et al. 1988a,b). So ifwe are interested in the ecosystem-scale question of why N, fixation is so much less effective in alleviating N deficits in estuaries and coastal seas than in lakes (Howarth and Cole 1985; Howarth 1988; Vitousek and Howarth 199 I), it is important to address why heterocystic cyanobacteria such as Anabaena seem to be so rare among the plankton of these marine ecosystems. We urge the interested reader to refer to the original paper by Paerl (1985). The occurrence of heterocystic cyanobacteria in marine environments-in their comment, Paerl et al. (1995, p. 63 5) state that planktonic heterocystous cyanobacteria are more common in estuarine, coastal, and ocean waters than implied by Howarth et al. However, their discussion ignores our emphasis on planktonic cyanobacteria in estuarine and coastal ecosystems as laid out in our introductory paragraph (Howarth et al. 1993, p. 1696): L... planktonic species of N,-fixing, heterocystic cyanobacteria are absent or extremely rare in most such ecosystems.... Notable exceptions are the Baltic Sea and the Harvey-Peel estuary.... (references deleted here for ease of reading). Paerl et al. (1995) provide no examples of estuaries or coastal seas where planktonic, heterocystic cyanobacteria occur other than these same systems. In his earlier work, Paerl(1985) noted the scarcity of planktonic N, fixers in marine habitats, stating that (p. 1246) L... N,-fixing cyanobacteria are more common and more diverse in freshwater habitats.... All of the references now cited by Paerl et al. (1995) on the purported abundance of heterocystic cyanobacteria in marine systems pre-date Paerl (1985) except for the Harvey-Peel work (Huber 1986). We remain impressed by the relative absence of N,-fixing cyanobacteria from estuaries and coastal seas and intrigued by the mechanisms which may make the Baltic and Harvey-Peel such notable exceptions (see Howarth et al. 1988a). Paerl et al. (1995) are correct in stating that N2 fixation by benthic cyanobacteria is common in shallow marine ecosystems, a point we also have repeatedly made (Howarth and Cole 1985; Howarth et al. 1988a,b; Howarth 1988; Vitousek and Howarth 199 1). However, the major difference in ecosystem N2 fixation between lakes and estuaries and coastal seas is related to the lack of heterocystic cyanobacteria among the plankton of these saltwater environments (Howarth et al. 1988a,b, 1993). Paerl et al. (1995) use the common occurrence of benthic cyanobacteria in shallow marine ecosystems to conclude that (p. 635) it is difficult to implicate specific chemical characteristics (salinity, or unusual ionic or nutrient inputs and ratios) as barriers to heterocystous taxa in ma-

3 Comment 641 rine ecosystems. We agree with regard to salinity, but ecological and biogeochemical controls on N2 fixation are likely to be dramatically different in benthic mats than in planktonic systems. For example, the availability of trace metals required for N, fixation (Fe and MO) are likely to be much greater in benthic mats with anoxic microzones and underlying anoxic sediments. These differences are discussed elsewhere (Howarth and Cole 1985; Howarth et al. 1988a; Marino et al. 1990; Vitousek and Howarth 199 1). Experimental design and interpretation of nitrogen fiation data-our mesocosm experiment tested two levels of turbulence: high and very high (Howarth et al. 1993). Paerl et al. (1995) object to this design as lacking an appropriate no-turbulence control. As stated by Howarth et al. (1993, p. 1698), we believe such controls are inappropriate because they would have stratified rapidly, which would have greatly changed light and nutrient availabilities and might have made the tanks anoxic. We acknowledged that this makes it impossible to fully explore the effects of turbulence over the range of levels found in nature. Thus, we cannot rule out the suggestion of Paerl et al. (1995, p. 635) that turbulence may have actually decreased N2 fixation relative to nonturbulent conditions. However, high rates of N, fixation occurred at extremely high levels of turbulence relative to natural systems, and there was no significant effect of the two experimental turbulence levels (Howarth et al. 1993). It is unlikely that N, fixation rates would have been much higher (if higher at all) in nonturbulent conditions; the rates we found are among the highest reported for natural systems (Howarth et al. 1988b), and rates of fixation per heterocyst (a test for a physiological effect of turbulence) were typical of those found in lakes (Howarth et al. 1993). Paerl et al. (1995) replot our data on rates of N2 fixation per heterocyst (their figure 1) and conclude that (p. 636) the data exhibit a better fit to a hyperbolic tangent curve than to the linear fit we showed. However, they present no statistical information in support of a hyperbolic function as a better fit, and the data do not allow a significant distinction between linear and hyperbolic functions; both are possible. We chose the conservative approach of presenting the simpler, linear function. Accepting a hyperbolic function instead in no way affects our major conclusions: rates of N2 fixation per heterocyst are not affected by turbulence and are comparable to real-world values. Paerl et al. (1995) go on to suggest that a hyperbolic function might imply self-shading by cyanobacteria in our experiment. This was not the case, as our N2 fixation measurements were made in an incubator under light-saturating conditions (Howarth et al. 1993). Shading in situ may well have occurred, but would not have been a factor in our acetylene reduction measurements. Paerl et al. (1995, p. 636) also state that the data should be normalized to cyanobacterial biomass (e.g. chlorophyll a) to allow a more meaningful comparison.... There was no significant effect of turbulence on chlorophyll (see figure 6 of Howarth et al. 1993), and normalizing the het- erocyst abundance to chlorophyll still shows no effect of turbulence. The measurement of turbulence and efect on cyanobacteria -Turbulence is notoriously difficult to measure (Nelkin 1992; Reynolds 1994). Partly for this reason, we used two different methods: estimation from electromagnetic current meter velocity measurements and integration of the turbulence spectrum obtained with a hot-film probe. The assumptions behind these approaches are independent, yet the resulting estimates of turbulent energy dissipation in our mesocosms are comparable (Howarth et al. 1993). Ours is one of the few attempts to estimate turbulence associated with growth of phytoplankton in mesocosms, microcosms, or cultures (Thomas and Gibson 1990), and as far as we know is the only attempt to date to do so with filamentous cyanobacteria. We believe our published estimates are robust. Paerl et al. (1995, p. 636) state that our hot-film probe is inadequate for determining turbulence in the mesocosms because we did not resolve the wave numbers down to the Kolmogorov cutoff. They are correct that the hot-film probe cannot measure down to the Kolmogorov scale (as is true of virtually all measurement techniques), but they are wrong in asserting that this makes the turbulence measurement inadequate. In general, the contribution of the high wave numbers is negligible in the integration procedure we used and can be safely ignored (Sirivat and Warhaft 1983; Chu 1993). To the ex- tent this introduces error in our estimates, actual turbulent energy dissipation rates would be even higher than we stated (Howarth et al. 1993). This would not change our conclusion that even very high rates of turbulence had no obvious deleterious effect on N2 fixation by heterocystic cyanobacteria. Paerl et al. (1995) re-estimate turbulence in our mesocosms by applying a formula that relates the dissipation rate to spectral intensity in the inertial subrange. They obtain values which are fold lower than the volume-weighted mean estimates of turbulence published by Howarth et al. (1993). However, the procedure used by Paerl et al. (1995) is subject to the same criticism they level at us: it does not include turbulence at the smallest scales and thus is likely to underestimate turbulence. If this bias is greater in their analysis than in ours, it could explain why their estimates of turbulence are lower. In any event, even the estimates they present show our mesocosms were quite turbulent. Their estimate of turbu- lence in our higher turbulence mesocosms, 9.6 x 10e5 W kg- is much higher than is typical of lakes (Reynolds 1994) and is comparable to the rate of 2.5 x 1 Oe5 W kg-l reported for Narragansett Bay (Nixon et al. 1980) and other tidally mixed water columns (MacKenzie and Leggett 199 1; see Table I and Howarth et al. 1993). Paerl et al. (1995, p. 636) state that it is their opinion that a correlation between the effects of turbulence and nitrogen fixation should focus on the small-scale shearing forces to which microorganisms would be subjected. This is a reasonable approach since shearing and strain forces can be related to diffusive fluxes (Thomas and Gibson

4 642 Comment 1990; Mann and Lazier 199 1; Reynolds 1994) and so should be expected to influence the likelihood of anoxic microzones. However, such shear forces are directly related to the rate of turbulent energy dissipation (Thomas and Gibson 1990; Mann and Lazier 199 I), so our use of energy dissipation as the measure by which to evaluate the effect of turbulence on N2 fixation was appropriate (Howarth et al. 1993). If one prefers to think in terms of shear forces, these can be readily calculated from the rates of turbulent energy dissipation we presented. The rate of strain (s- ) is given by the formula s = (&lv).5 where c is the turbulent energy dissipation rate (W kg- l, or m2 se3) and v is the kinematic viscosity of water (w 1 Oe6 m2 s-l) (Thomas and Gibson 1990). Values of strain rates, turbulent energy dissipation rates, and smallest eddies of turbulence for our mesocosms and for various natural systems are presented in Table 1. The strain rates in our mesocosms were much higher than in lakes or open ocean waters and were high even in comparison to tidally turbulent estuaries (Table 1). Consequently, diffusive fluxes to and from phytoplankton cells in our mesocosms must have been quite high in comparison to most natural ecosystems, and the likelihood of anoxic microzones existing is correspondingly low. Paerl et al. (1995, p. 636) also state that phytoplankton are generally much smaller than the length scale of the smallest eddies of turbulence. While this is true for most phytoplankton (Reynolds 1994), it is not necessarily true for heterocystic, N,-fixing cyanobacteria, whose filaments can exceed 2 mm in length (Reynolds 1994); the smallest eddies of turbulence were smaller than this in our mesocosms, as is frequently true for tidally mixed ecosystems (Table 1). In addition turbulence can affect diffusion at scales smaller than those of the smallest eddies, and this effect can be related to turbulent energy dissipation (Mann and Lazier 199 1; Reynolds 1994). Conclusions and comments on the importance of experimental scale- Paerl and colleagues have published several studies asserting that turbulence is at least in part the cause of the difference in N, fixation between lakes and estuaries and coastal marine ecosystems (Paerl 1985; Paerl et al. 1987; Paulsen et al ). Their evidence consists of small-scale bioassays with turbulence generated in flasks on shaker tables and additions of high concentrations of simple sugars to create anoxic zones. While bioassays are useful to test responses of ambient plankton assemblages to short-term stimuli and one can gain important insights from such work, great care must be taken in the scaling of experimental variables. For instance, when studying the effects of turbulence by shaking flasks on a shaker table (Paerl 1985), how fast should the flasks be shaken, and how does this relate to real-world turbulence? Although it is possible to assess turbulent energy dissipation in shaken flasks in at least a semiquantitative manner (Thomas and Gibson 1990; Howarth et al. 1993) this is a difficult question and one which was not addressed in the experiments of Paerl (1985). Our experiment was the first test of the turbulence hypothesis under more environmentally realistic conditions where turbulence could be scaled to real-world values. The dominant cyanobacteria, Anabaena, were good test organisms in that they frequently dominate in lakes yet are rare among the plankton of estuaries and were also used by Paerl (1985) in his bioassays testing the turbulence hypothesis. In addition to making it easier to quantify turbulence, an advantage of the mesocosms is that they allowed for longer time scales and larger spatial scales where ecological factors such as recruitment, competition, and grazing could potentially manifest themselves. An inescapable conclusion from our data is that turbulence alone cannot cause the relative absence of N2 fixation by planktonic, heterocystic cyanobacteria in estuaries and coastal seas. This is not to say that turbulence is unimportant as a factor affecting N2 fixation by cyanobacteria in aquatic systems. Turbulence can lead to light limitation of N2 fixation in some systems, both marine and freshwater, through deepening of mixed layers; and the possibility remains that turbulence can interact with low availability of trace metals or P to adversely affect cyanobacterial growth in estuaries and coastal seas (Howarth et al. 1993). Section of Ecology & Systematics Division of Biological Sciences Corson Hall Cornell University Ithaca, New York Department of Civil Engineering National Central University Chung-Li, Taiwan References R. W. Howarth D. Swaney R. Marino T. Butler C. R. Chu CHU, C. R Experiments on gas transfer and turbulence structure in free surface flows with combined wind/bottom shear. Ph.D. thesis, Cornell University. DOREMUS, C Geochemical control of dinitrogen fixation in the open ocean. Biol. Oceanogr. 1: FOGG, G. E Marine planktonic cyanobacteria, p In P. Fay and C. V. Baalen [eds.], The cyanobacteria. Elsevier. HORNE, A. J Nitrogen fixation-a review of this phenomenon as a polluting process. Prog. Water Technol. 8: HOWARTH, R. W Nutrient limitation of net primary production in marine ecosystems. Annu. Rev. Ecol. Syst. 19: 89-l 10. -, T. BUTLER, K. LUNDE, D. SWANEY, AND C. R. CHU Turbulence and planktonic nitrogen fixation: A mesocosm experiment. Limnol. Oceanogr. 38: 1696-l 7 11.

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