Conceptual models for the biogeochemical role of the photic zone microbial food web, with particular reference to the Mediterranean Sea

Progress in Oceanography 44 (1999) 271–286

Conceptual models for the biogeochemical role of the photic zone microbial food web, with particular reference to the Mediterranean Sea T. Frede Thingstad a

a,*

, Fereidoun Rassoulzadegan

b

Department of Microbiology, University of Bergen, Jahnebakken 5, N-5020 Bergen, Norway b Station Zoologique, BP 28, F-06230 Villefranche-sur-Mer, France

Abstract Observations are reviewed which indicate that not only phytoplankton, but also heterotrophic bacteria are P-limited during those seasons when there is stratification in the Mediterranean. It is discussed how these observations fit into a general concept of a size-structured food chain where the structure is a result of combined top-down control from size-selective predators and size-dependent competition for phosphate. It is argued that, conceptually, labile DOC and silicate have symmetrical roles in potentially controlling the flux of P through the ‘microbial’ and ‘classical’ sides, respectively, of this food web. The resulting concept provides a model linking C, P, and Si fluxes to the size spectrum of biogenic particles in the photic zone.  1999 Elsevier Science Ltd. All rights reserved.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

2.

Biomass and growth rate control of bacteria and phytoplankton . . . . . . . . . . . 275

3.

Size spectrum of osmotrophs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

* Corresponding author. E-mail address: [email protected] (T..F. Thingstad) 0079-6611/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 9 9 ) 0 0 0 2 9 - 4

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

1. Introduction In the Mediterranean Sea a combination of factors present a fascinating, although perhaps somewhat overlooked, combination of challenges and opportunities to modern marine microbiology. Not only is the Mediterranean an area with an interesting hydrography and biogeochemistry, which is believed to strongly influence the conditions of marine life, but it is also a marine area associated with strong economical, recreational and conservational interests that is in need of a management plan based on sound scientific principles. The general circulation pattern, which is dominated by the negative thermo-haline circulation drawing nutrient-poor surface waters in through the Strait of Gibraltar, is supposedly the main mechanism determining the generally oligotrophic status of the Mediterranean (Redfield, Ketchum & Richards, 1963). this oligotrophic trend increases towards the eastern region. In some, so-far poorly understood manner, this circulation is coupled to an unusual peculiar biogeochemistry which leads to N:P ratios becoming increasingly skewed towards P-deficiency, particularly in the eastern parts (Krom, Kress, Brenner & Gordon, 1991). With a heavily populated coastal zone, the stage is also set for strong local gradients to develop where waste water inputs from human activities meet nutrient poor off-shore waters (Bianchi et al., 1993; Justic, Rabalais & Turner, 1995; Elbazpoulichet, Garnier, Gaun, Martin & Thomas, 1996). The Mediterranean is also an area where deep water is formed in winter (Wu¨st, 1961; MEDOC Group, 1970; Lacombe, Gascard, Gonella & Bethoux, 1981) creating a situation where the linkages between biological activity and carbon transport by downwelling occurs under light and temperature conditions that are very different from those prevailing in areas of deep water formation in, for example, the North Atlantic. The Mediterranean thus offers a unique set of conditions as a natural laboratory, for studying the interplay between marine biological, biogeochemical, and hydrographical processes. At the basis of a food chain ending in the more conspicuous forms such as economically important fish stocks and marine mammals is a microbial food web consisting of small unicellular phytoplankton, bacteria, protozoa, and viruses, invisible to the unaided human eye. These tiny life forms are connected by trophic interactions which together form a complex braid of closely linked processes. Powered by energy originating (in most cases) from sunlight, the microbial food web drives constant processes transforming matter between organic and inorganic, dissolved and particulate forms. The function of this food web in terms of material cycling is intimately linked to the food web structure, i.e., to which functional groups of organisms are present, the extent to which they interact, and how this structure is a result of external driving forces such as light, nutrient content, advection etc. Understanding this system is a challenge very much in its own right, but equally challenging is confronting the wide range of issues where fundamental knowledge

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of the functioning of the photic zone food web is central to a sound, scientifically based, management of the Mediterranean Sea (Table 1). With an almost infinite variation in trophic strategies involved in shaping the pelagic food web, extraction of general principles requires some kind of idealisation of the system. With the obvious opposite danger of ignoring important detail, we want to focus here on how generalised trophic interactions such as competition and predation, combined with inevitable physical constraints such as diffusion and sinking, can form a conceptual picture which allows us to understand some of the important properties of the pelagic Mediterranean ecosystem. Fig. 1 illustrates one way of summarising the carbon flows through the lower parts of the pelagic food web. The ‘ladder’ like structure in the figure contains (to the right) the linear food web from relatively large-celled phytoplankton species via copepods to fish, classically thought to dominate the pelagic food web (e.g., Steele, 1974). As one progresses from this ‘classical’ side towards the ‘microbial’ side (on the left), the cell size of the organisms progressively decreases. The basic principle of this conceptual structure does not seem to have been critically challenged since its original description (Johannes, 1965; Sheldon, Prakash & Sutcliffe, 1972; Pomeroy, 1974; Williams, 1981; Azam, Fenchel, Field, Gray, Meyer-Reil & Thingstad, 1983), and variations of this theme form the basis of most present conceptualisations in the field. Fig. 1 emphasises two classes of trophic strategies, osmotrophy, i.e. feeding by organisms taking up dissolved nutrients through the membrane, and phagotrophy, here used for organisms feeding by eating particulate matter. Osmotrophs include both the heterotrophic bacteria and the autotrophic phytoplankton, while the predatory food chain includes both the phagotrophic protozoa, mesozooplankton, and higher predators, all heterotrophs. Carbon fixed in photosynthesis is, through predatory processes, channelled ‘upwards’ in the food web towards larger organisms, but is also lost through a multitude of processes to the pool of dissolved organic-C (DOC) which can then, to a smaller or larger extent, be re-incorporated in the particulate food web. This cycle via DOC of Table 1 Some of the areas where a correct understanding of the mechanisms regulating the pelagic microbial food web is suggested to be of importance to a scientifically based management of marine areas Stoichiometric coupling of biogeochemical cycles of C, N, P Relevant to: Carbon sequestration from the atmosphere/climate Food production for commercially important fish stocks Fate and effect of organic and inorganic waste disposal Size spectrum and pigmentation of biogenic particles Relevant to: Adsorption area for surface active pollutants (heavy metals, ship paints, hydrocarbons etc.) Sinking flux of particulate material and vertical transport of pollutants Optical and aesthetic qualities of coastal waters Interpretation of remote sensing data Diversity controlling mechanisms Relevant to: Biodiversity, ecosystem resilience Ecosystem conservation Survival of pathogens in recreational areas

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Fig. 1. Idealised view of carbon flow through the pelagic photic zone food web (modified from Fenchel, 1987).

organic material, otherwise ‘lost’ from the particulate food web is often referred to as the ‘microbial loop’ (Azam, Fenchel, Field, Gray, Meyer-Reil & Thingstad, 1983). Organic matter produced in the photic zone may be exported not only via the ‘classical’ side, by the production of particles large enough to sink at a significant rate (Miquel, Fowler, Larosa & Buatmenard, 1994), but also as DOC, which if present at a sufficiently high concentration may be expected by diffusion or transported downwards during downwelling events and thus result in a quantitatively important

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flux (e.g. Copin-Montegut & Avril, 1993; Miquel, Fowler, Larosa & Buatmenard, 1994). 2. Biomass and growth rate control of bacteria and phytoplankton Views on population regulation in natural systems are generally dominated by two alternative schools, leaving the impression that there is either a so-called ‘top-down’ control by predators, or a ‘bottom-up’ control through resource availability. In ladder-like structures of the type shown in Fig. 1, however, this dichotomy in analysis may rapidly become confusing. It seems fruitful to recognise the fact that bacterial production is the product of growth rate and biomass (e.g., Thingstad, Hagstrom & Rassoulzadegan, 1997a). This allows the analysis to be separated into those factors affecting biomass (top-down), growth rate (bottom-up), and also diversity (Thingstad & Lignell, 1997b). For the heterotrophic bacteria in the lower left corner of Fig. 1, the main controlling mechanisms usually considered are: the availability of organic substrates (carbon and energy), available forms of nitrogen (N) or phosphorus (P), predation by (mainly) protozoans, and lysis by viruses (Fig. 2A). From simple steady state models one would expect protozoan grazing to control the bacterial biomass, and viruses mainly to affect diversity, while growth rates may be limited either by organic carbon, by mineral forms of N or P, or by a combination of inorganic and organic forms of N and P (Thingstad & Lignell, 1997b). Bacterial predation by protozoa has been a well studied phenomenon in the Mediterranean for many years (Rivier, Brownlee, Sheldon & Rassoulzadegan, 1985; Rassoulzadegan & Sheldon, 1986; Rassoulzadegan, Laval-Peuto & Sheldon, 1988; Sherr, Rassoulzadegan & Sherr, 1989; Bernard & Rassoulzadegan, 1990; Wikner, Razzoulzadegan & Hagstro¨m, 1990; Rassoulzadegan, 1993) and is believed to be the mechanism that keeps bacterial numbers relatively constant. If heterotrophic flagellates are assumed to be non-selective (any bacterium is a good bacterium) and to be the only predators on bacteria, then the steady state requirement that loss of heterotrophic flagellate biomass (H) equals growth gives the relationship: YHaHBH⫽dHH, where aH is the clearance rate of heterotrophic flagellates for bacteria (B) and dH is the specific loss rate. Solving for bacterial abundance B, this gives: B⫽

dH . YHaH

If we assume a turnover of the flagellate population once per day, dH=1.0 d ⫺1 (0.04 h⫺1), a ‘typical’ flagellate that needs in the order of 200 bacteria to divide (YH=0.005), and have a maximum clearance rate of 10⫺5 ml h⫺1 (Fenchel, 1982), insertion gives an estimated steady state population of bacteria of ⬇8 105 ml⫺1 ), which is a typical value for the Mediterranean photic zone. On such a general level, this theory thus seems consistent with observations.

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Fig. 2. Illustration of how (A) heterotrophic bacteria may be controlled ‘from below’ by the lack of either labile organic-C (L-DOC) or by available forms of nitrogen (N) or phosphorus (P). If, for simplicity, protozoan predators are assumed to be non-selective among the bacterial prey, total bacterial abundance at steady state is controlled ‘from above’ by the protozoa. If viruses are host-specific, the main effect of viruses at steady state may be on diversity inside the bacterial community (Thingstad & Lignell, 1997b). If both bacterial and phytoplankton growth rate is phosphate limited in the Mediterranean, the simple model in (B) suggests that that there is no stable algal-bacterial coexistence. Introducing selective predation as in (C), however, solves this theoretical problem.

Bacterial growth rate is often assumed to be C-limited (e.g. Tusseau, Lancelot, Martin & Tassin, 1997). If this should be the case, it would greatly complicate further analyses since bacterial production would be regulated by a set of many complex and poorly quantified processes such as viral lysis (Middelboe, Jorgensen & Kroer, 1996), sloppy feeding (Jumars, Penry, Baross, Perry & Frost, 1989), and phytoplankton excretion (Myklestad, Holm-Hansen, Vorum & Volcani, 1989). If, however, bacterial growth rate is limited by the same mineral nutrients as phytoplankton growth rate, then it is the competition with phytoplankton for mineral nutrient that deter-

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mines bacterial growth rate. Not only the biogeochemical evidence of skewed nitrate:phosphate ratios, but also direct evidence from flow cytometric analysis (Vaulot, Lebot, Marie & Fukai, 1996), and bioassays (Fiala, Cahet, Jacques, Neveau & Panouse, 1976; Justic, Rabalais & Turner, 1995) have suggested P as the most important limiting element for phytoplankton growth in Mediterranean stratified summer situations. Indications from both the north-western (Thingstad, Zweifel & Rassoulzadegan, 1998) and the eastern Mediterranean (Zohary & Robarts, 1998) indicate strongly that the growth rate of not only phytoplankton, but also of heterotrophic bacteria is Plimited during stratification. P-limited bacteria, and a resulting competition between bacteria and photosynthetic organisms, has also been suggested in Mediterranean benthic seagrass communities (Lo´pez, Duarte, Vallespino´s, Romero & Alcoverro, 1995). Interestingly, P-limited bacterial growth rates have recently been reported also in other truly marine pelagic environments (Pomeroy, Sheldon, Sheldon & Peters, 1995; Cotner, Ammerman, Peele & Bentzen, 1997), as well as in fresh water influenced coastal areas (Thingstad, Skjoldal & Bohne, 1993; Zweifel, Norrman & Hagstrom, 1993), suggesting that P-limited growth of marine bacteria is probably not a phenomenon exclusive to the Mediterranean. If both bacteria and phytoplankton growth rates are P-limited (Fig. 2B), this leads us into a theoretical problem which actually can be seen as extending Hutchinson’s classical ‘paradox of the plankton’ (Hutchinson, 1961) also to include heterotrophic bacteria. Since bacteria with their superior surface:volume ratio are usually assumed to be the best competitors at low orthophosphate concentrations, the problem can be illustrated by the following variation of the theme of such an extended Hutchinson’s paradox: Why do not the P-limited bacteria outcompete phytoplankton until phytoplankton biomass is reduced to a level where the systems’ production of organic-C is so low that bacteria become C-limited? Probably the simplest theoretical solution to this paradox is to include predation in the models. Competing species can coexist stably on one common resource if there is a mechanism such as predation, that selectively kills the winner (Fig. 2C). A higher predation pressure on the superior competitor thus solves the theoretical problem (Thingstad, Hagstrom & Rassoulzadegan, 1997a) without the need to infer variations in time and space to explain the coexistence. A consequence of such a model is that it allows for a system where heterotrophic bacteria, being ‘sandwiched’ between predation and competition, do not respond to increased concentrations of free organic carbon, either in their growth rate or in thir biomass. The theoretical consequence is that labile DOC may accumulate and thus potentially be subject to (photo)chemical processes changing its availability as bacterial substrate (Benner & Biddanda, 1998). Although not altering the general conclusion of the need for a mechanism selectively killing the winner, some experimental evidence may, however, shed doubt on the traditional assumption that heterotrophic bacteria invariably are such superior competitors. The ability of an osmotrophic organism to compete at permanently low

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substrate concentrations is given by its maximum specific affinity (a). This is the slope of the uptake-versus-substrate curve as substrate concentration approaches zero. The interpretation of specific affinity is the volume of water cleared for substrate per unit biomass per unit time, and specific affinity is thus analogous to the clearance rate of a phagotrophic organism. Few experimental data on bacterial affinity seem to be available, but recently published investigations on cultured isolates led Vadstein (1997) to suggest that the difference between phytoplankton and bacterial affinity may not be as clear as hitherto assumed. We can estimate the specific affinity in natural populations from experimental data following the procedure suggested in Table 2. Using typical values from Villefranche Bay (north-west Mediterranean) for bacterial counts and chlorophyll as biomass measures, the resulting estimates for bacterial and phytoplankton specific affinities are comparable to those obtained by Vadstein (1997) for cultured bacteria. Values for phytoplankton and heterotrophic bacteria are comparable (Table 2), supporting the suggestion that bacterial superiority should be viewed with some caution. Estimates such as these are, however, subject to large uncertainties. A major uncertainty stems from the recent proposition that only a minor portion of the bacteria seen in the epifluorescence microscope are active (Zweifel & Hagstro¨m, 1995). Any reduction in the biomass of active bacteria used in the calculations would increase the a-estimate for bacteria correspondingly. The numerical value of the affinity estimates in Table 2 deserves another comment. Based on the assumption that the cell is diffusion limited, i.e. that the cell’s uptake system is so efficient (and the bulk nutrient concentration so low) that all substrate molecules hitting the cell surface are captured, it is possible to derive a theoretical expression for maximum affinity for a spherical cell of radius r (Thingstad & Lignell, 1997b): a⫽

3D , sr2

(1)

where D is the diffusion constant for the substrate molecules, and s is the volume specific content of the element in question. Inserting D⬇10⫺5 cm2 s⫺1 for a small molecule such as orthophosphate, and assuming a phytoplankton cell to have a density of 1.2 g cm⫺3, 50% of wet weight to be dry weight, 50% of carbon to be dry weight, and a molar C:P-ratio of 106 in phytoplankton biomass, we get s⬇0.24 µmol-P cm⫺3. The estimated phytoplankton affinity of 0.016 l nmol-P⫺1 h⫺1 in Table 2 then corresponds to a diffusion limited cell of radius of about 1.7 µm. For a cell of radius 1 µm, corresponding approximately to that of unicellular cyanobacteria in the Mediterranean, we get an estimated a1µm⬇0.046 l nmol-P⫺1 h⫺1. Although imprecise, these rough estimates suggest that phytoplankton cells larger than about 2–3 µm may be diffusion limited in Villefranche Bay summer surface waters. Chemical determination of bioavailable orthophosphate Sn in P-deficient waters is difficult, both because values may be close to the detection limit, and because of uncertainties as to whether the measured values of SRP (Soluble Reactive Phosphorus), when obtained, really represent Sn. To overcome this problem, we have used a somewhat intricate way of estimating Sn by combining measured loss rate of P from the bacterial size-fraction, turnover time measured for radioactively labelled

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Table 2 Procedure suggested for estimating affinity constants from experimental data Uptake is usually described as a hyperbolic function of substrate concentration. When substrate concentration is sufficiently low, the initial part of the curve can be approximated by a linear relationship, e.g. where uptake is proportional to substrate concentration. In the case of balanced, substrate limited growth, bacterial uptake must equal growth so that mB=aBSn where mB is specific growth rate, aB is the bacterial affinity, and Sn is the natural concentration of substrate in the water. Multiplying both sides with the biomass B and solving for aB we get aB=[(mB/Sn)/B]. If B is in units of bacterial biomass-P, mB is the bacterial incorporation rate of P. If f is the fraction of total incorporation going into bacteria, mB/f is the total incorporation rate of phosphate, and mBB/fSn=1/Tt where Tt is the turnover-time for orthophosphate in the natural environment. We thus get: f aB = TtB Since f and Tt can be estimated from size-fractionated uptake of 33P or 32P labelled orthophosphate (Dolan et al., 1995), and B can be estimated from microscopic determination of bacterial biomass or size-fractionation of particulate-P, this allows estimation of an ‘apparent’ aB. If substrate concentration is beyond the range of linear uptake, this ‘apparent’ aB should be smaller than the ‘true’ value. ‘Apparent’ affinities estimated through this procedure should therefore provide lower limits for the true values. Similarly: (1−f) aA= TtA where aA is the affinity of phytoplankton, A is P in algal biomass, and only phytoplankton and bacteria have been assumed to incorporate P from orthophosphate. If an estimate for A is available from e.g. a conversion from chlorophyll, also aA can be obtained. Although f and Tt in our experience can vary widely, ‘typical’ value for Villefranche Bay (northwest Mediterranean) could be 0.5 and 1.6 h, respectively (Dolan et al., 1995). A ‘typical’ bacterial abundance of 0.8 106 bacteria ml⫺1 corresponds to B⬇27 nmol-P l⫺1 if a carbon content of 20 fg C cell⫺1 and a molar C:P ratio of 50:1 is assumed. This fits well with particulate-P measured in the size fraction 1–0.2 µm (Dolan et al., 1995). The corresponding ‘apparent’ aB is thus in the order of aB⬇0.012 l nmol-P⫺1h⫺1. Using 0.5 µg Chl l⫺1 as a ‘typical’ chlorophyll concentration (Dolan et al., 1995). A Chl:C ratio of 50 and a molar Redfield C:P ratio of 106, gives an estimated A⬇20 nmol-P l⫺1. This is about 50% of the particulate-P ⬎1 µm reported by Dolan et al. (1995), which again gives a reasonable agreement since parts of the measured particulate-P in this size fraction would be expected to be in protozoa. The corresponding aA would be in the order of aA⬇0.016 l nmol-P⫺1 h⫺1.

orthophosphate, and the fraction of this label estimated to go into heterotrophic bacteria. In Villefranche Bay (Thingstad, Dolan & Fuhrman, 1996) as well as in the brackish layer of a Norwegian fjord (Thingstad, Skjoldal & Bohne, 1993), this procedure led to estimates of Sn⬇0.8 nmol-P l⫺1. To put this number into perspective, a water volume 1000 times that of a bacterium would, at this concentration, contain in the order of 30 free phosphate molecules. If we combine this concentration with the estimated affinity a1µm⬇0.046 l nmol-P⫺1 h⫺1, the corresponding growth rate becomes mA=aASn⬇0.88 d ⫺1. This is close to the growth rate of 0.95 d ⫺1 determined

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from flow cytometric analysis for the native Synechococcus population of Villefranche Bay (Jacquet, Lennon, Marie & Vaulot, 1999). As a conclusion from the present state of knowledge, it seems that the suggestion of diffusion limited phytoplankton growth, the low estimates of bioavailable orthophosphate concentration, and the relatively short measured generation times for the small phytoplankton species, together form a consistent picture. Iron-containing dust from Sahara has been suggested to scavenge phosphate from the Mediterranean pelagic system (Krom, Kress, Brenner & Gordon, 1991). Using electron microscope X-ray analysis Mostajir, Fagerbakke, Heldal, Thingstad and Rassoulzadegan (1998) found, however, little P in inorganic particles from the NW Mediterranean. Possibly, the effect of Saharan dust may be slightly more complicated than suggested so far. The phosphate it originally carries may be exchanged with the high-affinity microbes in the surface waters, locally stimulating biological activity in the surface layer. This possibility remains, however, that the dust particles may scavenge phosphate when sinking deeper into the aphotic zone, and thus in effect move the nutricline downwards.

3. Size spectrum of osmotrophs The phosphate-bacteria-algae-protozoa system of Fig. 2(C) can be generalised a step further by adding more size classes of phytoplankton and their corresponding predators (Fig. 3). Comparison of Figs. 1 and 3 should reveal the emerging similarity of our theoretical construction to the more generally accepted ideas of carbon flow in food webs. The steady states of structures such as that in Fig. 3 can be analysed as functions of the nutrient richness of the system, i.e. of the total amount ST of the limiting element available for sharing between all particulate and dissolved pools in the food web. It then turns out that the concentration of free limiting nutrients Sn, as a general trend, will increase with increasing ST (Thingstad & Sakshaug, 1990).

Fig. 3. Generalisation of the food web in Fig. 2(C), to include mesozooplankton top-predators and two size classes of phytoplankton and of protozoa. Solid arrows represents P-flows (recycling pathways omitted for simplicity). The dotted arrows illustrate the needed input of labile organic matter and silicate, and thus their symmetric role in potentially limiting the P-flow through the left ‘microbial’, and right ‘classical’ sides of the food web, respectively.

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With higher substrate concentrations Sn, new, larger phytoplankton species, although having lower a than those already present, may potentially become established towards the ‘classical’ side of the food web. In apparent agreement with such a view, the distribution of 32PO4-uptake within the phytoplankton size-range (⬎1 mm) was found to shift towards larger organisms in parallel with the destabilisation of the water column occurring during autumn cooling of the top layer in Villefranche Bay (Dolan, Thingstad & Rassoulzadegan, 1995). The presently available evidence thus suggest that the Mediterranean system fits into the series of observations supporting the theory that nutrient enrichment stimulates the ‘classical’ right side of Fig. 3 (e.g. Kiørboe, 1993). The present impression from experimental observations is that, in oligotrophic areas, biomass of heterotrophic bacteria becomes increasingly dominant relative to phytoplankton (Fuhrman, Sleeter, Carlson & Proctor, 1989; Cho & Azam, 1990). A steady state analysis of the idealised food web in Fig. 2(C) gives such a decrease in the bacteria:phytoplankton biomass ratio with decreasing total nutrient content ST, as long as bacterial growth rate is mineral nutrient limited (Thingstad, Hagstrom & Rassoulzadegan, 1997a). In the Mediterranean, the expected pattern emerging from such a theory would be an increasing relative dominance of heterotrophic bacteria in the eastern parts. In accordance with such expectations, Van Wambeke, Christaki, Bianchi, Psarra and Tselepides (2000) reports such dominance of heterotrophic bacterial biomass, with the exception of a coastal station in spring. Robarts, Zohary, Waiser and Yacobi (1996), however, found the effect to be less pronounced with bacterial biomass about 50% of phytoplankton biomass. The establishment of new, larger species on the ‘classical’ side of the food web, depends upon the shift in balance between loss and growth processes. The positive effect of reduced loss resulting from reduced predation must overcome the negative effects of reduced a and of increased sinking loss for the large cells. For a nonswimming osmotrophic cell with density higher than seawater, an expression for the minimum concentration SC of limiting nutrient at which it can maintain a stable population, can be derived from the requirement that diffusion limited growth must balance losses by sinking. Since Stoke’s law gives sinking rate as the second power of cell radius, combination of Stoke’s law with the expression in Eq. (1) above, gives an expression for this minimum concentration which goes as the fourth power of cell radius (Thingstad, 1998): 2g(r−r)s 4 S C⫽ r. 27hhD

(2)

The proportionality factor depends on the gravitational constant (g), the viscosity of water (h), the depth of the mixed layer (h) and the diffusion constant for the substrate (D), the net buoyancy (r⫺rW), and the volume-specific content of the limiting element. Since Eq. (2) ignores the effect of increased nutrient supply on large sinking cells, the fourth power relationship is in effect a kind of ‘worst case’ estimate. From such considerations, however, it is not surprising that oligotrophic areas like the Mediterranean have phytoplankton populations that are dominated by small phytoplankton species. The more interesting question is how larger species such as diatoms

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ever become established. In the Mediterranean, a typical habitat for diatoms when they are present, is in the deep chlorophyll maximum situated on the boundary between where there is sufficient available light in the upper part of the water column, and the upper boundary of available free nutrients in the deeper, aphotic zone (Estrada, 1985). Here they presumably benefit from diffusion and from intermittent hydrographic phenomena that inject nutrient rich deep water up into the base of the photic zone (Estrada, 1996). With the strong penalty for being large suggested above, one may speculate whether nutrient enrichment alone is a sufficient mechanism to explain the occurrence of these larger phytoplankton species. If a 1 µm cell needs a concentration around 1 nmol-P l⫺1, a 10 µm cell would, in the extreme case, need a concentration of 10 µmol l⫺1 which is much higher than observed in Mediterranean deep waters (e.g., Redfield, Ketchum & Richards, 1963). It is, therefore, tempting to speculate that larger phytoplankton species must have special strategies to counteract the proposed size-penalty. In the proportionality factor of Eq. (2), the only two factors readily available for phytoplankton adaptation strategies are their buoyancy (r), and their volume-specific nutrient content (s). A particularly important feature of diatoms in this context is their vacuole (Egge, 1998), which means that the cytoplasm is concentrated along their silicate wall. This is probably the reason for diatoms having a lower volume-specific carbon content than flagellates, and a carbon content that is correlated better with plasma volume than with total cell volume (Smayda, 1978). Assuming P only to be in the cytoplasm, and a constant cytoplasm thickness of ⬇1 µm, the relative advantage of diatoms over flagellates of the same size would, theoretically, increase with cell radius (Fig. 4). Adding the possibility that the vacuole may play a role in buoyancy regulation and perhaps also in storage of P during rapid luxury uptake, it would seem that diatoms may have a means to ‘look large’ to predators while still being efficient in terms of nutrient uptake, combined with a means to reduce sinking loss rate. Thus it may be speculated that diatoms may have evolved a strategy particularly well suited for rapid exploitation of ephemeral situations where nutrient enrichment opens for the possibility for larger phytoplankton species to become established. The price for this seems to be the need for silicate. However, in an environment such as the north-west Mediterranean, where the nutricline for silicate does not seem to be situated deeper than that for phosphate (e.g., Estrada, 1985), natural P-enrichment episodes would generally be expected also to supply silicate. Mesocosm experiments in Norwegian waters have suggested that Pdeficiency may reduce the competitive abilities of diatoms, relative to that in Ndeficient systems (Egge, 1998). The observed occurrence of diatoms (Estrada, 1985), however, does not suggest that such an effect prevents diatom establishment in Mediterranean deep chlorophyll maxima. Based on pigment signatures Claustre (1994) found both diatom and dinoflagellate pigments to increase linearly with chlorophyll, whereas other pigments did not correlate with chlorophyll. Within our theoretical framework, autotrophic dinoflagellates would be able to reduce the fourth power size penalty by swimming. For large cells, swimming can not only compensate for sinking, but will also increase nutrient flux to the cell (Lazier & Mann, 1989). Dinoflagellates are, however, usually (though not always (Chang & Carpenter, 1994))

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Fig. 4. Theoretical reduction in maximum specific affinity with cell size when the difference between diatoms and flagellates is the assumption of diatoms having a vacuole free of P, and a fixed cytoplasm thickness of 1 µm. Since the y-axis is logarithmic, the distance between the curves gives the ratio between the two affinities.

assumed to have low maximum growth rates, and may need time to fill niches temporarily opened to the ‘classical’ side of the food web. Conceptually, these arguments lead us to a view of the pelagic food web where L-DOC and Si have somewhat symmetric roles in regulation of the P-flow through the food web. For the system in Fig. 3, there is one steady state of the P-flow through the system when both L-DOC and Si are in excess of the need of heterotrophic bacteria and diatoms, respectively. If L-DOC supply sinks below this, P-flow through the left, ‘microbial’ side is restricted, if Si-supply is insufficient, P-flow through the right ‘classical’ side is restrained (at least until this niche can be filled by dinoflagellates). The size selective grazing central to this concept, resembles that of Sheldon’s theory for the particle size-spectrum (Sheldon, Prakash & Sutcliffe, 1972). In Sheldon’s analysis, growth rate is, however a property determined by cell size. In the concept suggested here, the size spectrum is much less fixed because growth rate is a function of food availability and its potential modification by L-DOC and silicate. The size spectrum of mineral nutrient uptake should, from theoretical con-

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siderations, greatly influence the amount of carbon reaching trophic levels of potential importance as food for fish (Suttle, Fuhrman & Capone, 1990). Since the mechanisms for production, import, and loss for L-DOC and Si are very different, this picture allows for large variations in the structure of the photic zone food web and its coupling to higher trophic levels, as Si and phosphate is imported and lost through the seasons and with upwelling events.

Acknowledgements This work was financed by the EU MAST-III programme through contract MAS3CT95-0016 ‘MEDEA’.

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