Nitrogen supply effects on productivity and potential leaf litter decay of Carex species from peatlands differing in nutrient limitation

We investigated the effect of increased N-supply on productivity and potential litter decay rates of Carex species, which are the dominant vascular plant species in peatlands in the Netherlands. We hypothesized that: (1) under conditions of N-limited plant growth, increased N-supply will lead to increased productivity but will not affect C:N ratios of plant litter and potential decay rates of that litter; and (2) under conditions of P-limited plant growth, increased N-supply will not affect productivity but it will lead to lower C:N ratios in plant litter and thereby to a higher potential decay rate of that litter. These hypotheses were tested by fertilization experiments (addition of 10 g N m^-2 year^-1) in peatlands in which plant growth was N-limited and P-limited, respectively. We investigated the effects of fertilization on net C-fixation by plant biomass, N uptake, leaf litter chemistry and potential leaf litter decay. In a P-limited peatland, dominated by Carex lasiocarpa, there was no significant increase of net C-fixation by plant biomass upon enhanced N-supply, although N-uptake had increased significantly compared with the unfertilized control. Due to the N-fertilization the C:N ratio in the plant biomass decreased significantly. Similarly, the C:N ratio of leaf litter produced at the end of the experiment showed a significant decrease upon enhanced N-supply. The potential decay rate of that litter, measured as CO₂-evolution from the litter under aerobic conditions, was significantly increase upon enhanced N-supply. In a N-limited peatland, dominated by C. acutiformis, the net C-fixation by plant biomass increased with increasing N-supply, whereas the increase in N-uptake was not significant. The C:N ratio of both living plant material and of dead leaves did not change in response to N-fertilization. The potential decay rate of the leaf litter was not affected by N-supply. The results agree with our hypotheses. This implies that atmospheric N-deposition may affect the CO₂-sink function of peatlands, but the effect is dependent on the nature of nutrient limitation. In peatlands where plant growth is N-limited, increased N-supply leads to an increase in the net accumulation of C. Under conditions of P-limited plant growth, however, the net C-accumulation will decrease, because productivity is not further increased, whereas the amount of C lost through decomposition of dead organic matter is increased. As plant growth in most terrestrial ecosystems is N-limited, increased N-supply will in most peatlands lead to an increase of net C-accumulation.     

Density-dependent outcomes in a digestive mutualism between carnivorous <Emphasis Type="Italic">Roridula</Emphasis> plants and their associated hemipterans

Recent studies have shown that mutualisms often have variable outcomes in space and time. In particular, the outcomes may be dependent on the density of the partners with unimodal or saturating outcomes providing stability to the mutualism. We examine density-dependent outcomes of an obligate, species-specific mutualism between a South African carnivorous plant (Roridula dentata) and a hemipteran (Pameridea) that facilitates prey digestion, but also sucks plant sap. Plants occur in sandy, leached, nitrogen-poor soils and have no digestive enzymes to digest prey. Instead they rely on obligately dependent hemipterans to supply nitrogen by digesting prey for them and defecating on their leaves. We documented the densities of Pameridea on Roridula in the field. In the greenhouse, we manipulated the hemipteran densities on Roridula and measured the mean relative growth rates of plants with differing hemipteran densities. Plants exhibited a unimodal response to the density of their mutualist partners. Those with no hemipterans had negative growth rates, suggesting that hemipterans are important in facilitating nitrogen absorption. Plants with intermediate hemipteran densities had positive growth rates but growth rates were negative under very high hemipteran densities. Our research provides support for variable and unimodal outcomes in mutualism. Unimodal outcomes may be particularly important in obligate mutualisms and this is one of the few studied outside of pollinating seed parasite mutualisms. In this system, extrinsic factors such as other predators may affect the mutualism by altering the numbers of hemipterans.     

Nutrient Addition Differentially Affects Ecological Processes of <Emphasis Type="Italic">Avicennia germinans</Emphasis> in Nitrogen versus Phosphorus Limited Mangrove Ecosystems

Nutrient over-enrichment is a major threat to marine environments, but system-specific attributes of coastal ecosystems may result in differences in their sensitivity and susceptibility to eutrophication. We used fertilization experiments in nitrogen (N)- and phosphorus (P)-limited mangrove forests to test the hypothesis that alleviating different kinds of nutrient limitation may have different effects on ecosystem structure and function in natural systems. We compared a broad range of ecological processes to determine if these systems have different thresholds where shifts might occur in nutrient limitation. Growth responses indicated N limitation in Avicennia germinans (black mangrove) forests in the Indian River Lagoon (IRL), Florida, and P limitation at Twin Cays, Belize. When nutrient deficiency was relieved, A. germinans grew out of its stunted form by increasing wood relative to leaf biomass and shoot length relative to lateral growth. At the P-limited site, P enrichment (+P) increased specific leaf area, N resorption, and P uptake, but had no effect on P resorption. At the N-limited site, +N increased both N and P resorption, but did not alter biomass allocation. Herbivory was greater at the P-limited site and was unaffected by +P, whereas +N led to increased herbivory at the N-limited site. The responses to nutrient enrichment depended on the ecological process and limiting nutrient and suggested that N- versus P-limited mangroves do have different thresholds. +P had a greater effect on more ecological processes at Twin Cays than did +N at the IRL, which indicated that the P-limited site was more sensitive to nutrient loading. Because of this sensitivity, eutrophication is more likely to cause a shift in nutrient limitation at P-limited Twin Cays than N-limited IRL.     

Prey Food Quality Affects Flagellate Ingestion Rates

Flagellate feeding efficiency appears to depend on morphological characteristics of prey such as cell size and motility, as well as on other characteristics such as digestibility and cell surface characteristics. Bacteria of varying morphological characteristics (cell size) and mineral nutrient characteristics or food quality (as determined by the C:N:P ratio) were obtained by growing Pseudomonas fluorescens in chemostats at four dilution rates (0.03, 0.06, 0.10, and 0.13 h⁻¹) and three temperatures (14°C, 20°C, and 28°C). Cells of a given food quality were heat-killed and used to grow the flagellate Ochromonas danica. Ingestion and digestion rates were determined by using fluorescently labeled bacteria of the same food quality as the bacteria supporting growth. Ingestion rates were affected by both food quality and cell size. Cells of high food quality (low carbon:element ratio) were ingested at higher rates than cells of low food quality. Multiple regression analysis indicated that cell size also influenced ingestion rate but to a much lesser extent than did food quality. Digestion rates were not correlated with either food quality or cell size. Results suggest that flagellates may adjust feeding efficiency based on the quality of food items available.     

Co-utilization of polymerized carbon sources by Bacteroides ovatus grown in a two-stage continuous culture system

Bacteroides ovatus NCTC 11153 was grown in a two-stage continuous culture system at various growth rates (vessel 1, D = 0.06 to 0.19 h-1; vessel 2, D = 0.03 to 0.09 h-1) on media containing mixtures of starch and arabinogalactan as carbon sources. The cell-associated enzyme activities needed to hydrolyze both substrates (amylase, arabinogalactanase, alpha-glucosidase, beta-galactosidase, and alpha-arabinofuranosidase) were variously influenced by growth rate and polysaccharide availability but were detected under all growth conditions tested. Measurements of residual carbohydrate in spent culture media showed that both polysaccharides were co-utilized during growth under putative C-limited conditions. The arabinogalactan was partly depolymerized in N-limited chemostats, and significant amounts of arabinose- and galactose-containing oligosaccharides accumulated in the cultures, indicating that starch was being preferentially utilized. Acetate, propionate, and succinate were the major fermentation products formed by C-limited bacteria, but under N limitation, lactate was also produced. Molar ratios of succinate increased concomitantly with the dilution rate in C-limited chemostats, whereas molar ratios of propionate decreased. During N-limited growth, however, decarboxylation of succinate to propionate was relatively independent of growth rate. Cell viability was higher in C-limited cultures compared with those grown under N limitation and was greatest at high dilution rates, irrespective of nutrient limitation.     

Co-utilization of polymerized carbon sources by Bacteroides ovatus grown in a two-stage continuous culture system.

Bacteroides ovatus NCTC 11153 was grown in a two-stage continuous culture system at various growth rates (vessel 1, D = 0.06 to 0.19 h-1; vessel 2, D = 0.03 to 0.09 h-1) on media containing mixtures of starch and arabinogalactan as carbon sources. The cell-associated enzyme activities needed to hydrolyze both substrates (amylase, arabinogalactanase, alpha-glucosidase, beta-galactosidase, and alpha-arabinofuranosidase) were variously influenced by growth rate and polysaccharide availability but were detected under all growth conditions tested. Measurements of residual carbohydrate in spent culture media showed that both polysaccharides were co-utilized during growth under putative C-limited conditions. The arabinogalactan was partly depolymerized in N-limited chemostats, and significant amounts of arabinose- and galactose-containing oligosaccharides accumulated in the cultures, indicating that starch was being preferentially utilized. Acetate, propionate, and succinate were the major fermentation products formed by C-limited bacteria, but under N limitation, lactate was also produced. Molar ratios of succinate increased concomitantly with the dilution rate in C-limited chemostats, whereas molar ratios of propionate decreased. During N-limited growth, however, decarboxylation of succinate to propionate was relatively independent of growth rate. Cell viability was higher in C-limited cultures compared with those grown under N limitation and was greatest at high dilution rates, irrespective of nutrient limitation.     

Environmental control of protein export of Bacillus licheniformis NCIB 6346 reveals two types of extracellular proteins

Teichuronic acid was the major anionic polymer of Bacillus licheniformis NCIB 6346 durign phosphate-limited (P-limited) growth in the chemostat. This polymer was also present in significant quantities when B. licheniformis was grown under carbon-limited (C-limited) or magnesium-limited (Mg-limited) conditions where teichoic acid predominated in the cell wall. However, the cell wall composition was not of significance in protein export and the parameters for the excretion process were found to be environmental. In particular, two types of extracellular proteins were identified: the first type of enzyme, penicillinase, was only weakly catabolite repressed; was maximally synthesized and secreted during P-limited growth; was unaffected by growth in high Na⁺ media but its production was inhibited by gramicidin. The second type of enzyme, -amylase, was strongly catabolite repressed and its export was markedly inhibited during P-limited growth or in the presence of Na⁺ or gramicidin. It is noteworthy that the penicillinase carries a glyceride-cysteine modification at its N-terminus whilst the -amylase does not.     

Polyphenols in litter from tropical montane forests across a wide range in soil fertility

In nutrient-poor ecosystems high polyphenol concentrations in plant litter have been proposed to influence soil nutrient availability in benefit of the plants. We addressed the question whether litter polyphenol concentrations vary across a soil chronosequence of almost identical geology, climate and plant species composition, but of a wide range in nitrogen (N) and phosphorus (P) availability in the Hawaiian Islands. Concentrations of total phenolics (TPh) and proanthocyanidins (PA) in leaf litter of the dominant tree species Metrosideros polymorpha were higher at the oldest, P-limited site compared to the youngest, N-limited site, with intermediate values at the two relatively fertile sites co-limited by N and P. Polyphenol concentrations in fine root litter differed considerably from those observed in leaf litter and varied differently across the soil age gradient. Long-term fertilization did not significantly alter polyphenol concentrations in Metrosideros litter at either site. Moreover, green leaves and leaf litter of Metrosideros showed similar relative differences among sites when compared between natural populations and plants from the same populations but grown in a common garden. These results suggest that polyphenol concentrations inherently vary among populations of the dominant tree species in Hawaiian montane forests possibly indicating an adaptation to ecosystem properties such as substrate age related differences in soil fertility. The combined above- and below-ground input rate of TPh ranged from 62.4 to 170.8 g/m²/yr and was significantly higher at the P-limited than at the N-limited site. Root-derived polyphenols contributed a much higher absolute and relative amount of phenolic input at the N-limited than at the P-limited site. The differences in amount, quality, and pathways of input might suggest specific interactions with soil processes and nutrient cycling among the Hawaiian rainforests studied here.     

Fertilization with nitrogen and phosphorus increases abundance of non-native species in Hawaiian montane forests

Weexamined the effects of fertilization on the diversity, abundance, and cover ofthe understory plant community of two montane wet forests in Hawaii. One siteoccupies a young substrate, where aboveground tree growth is limited bynitrogen(N), while the other site is on an older substrate, where aboveground treegrowth is limited by phosphorus (P). Both sites contained an on-going,long-termfactorial fertilization experiment in which plots were fertilized semi-annuallywith N, P, or N and P in combination. In each fertilization treatment, wemeasured density of species 0.5 m tall and percent cover ofspecies <0.5 m tall. Fertilization with N reducedspeciesrichness at the young, N-limited site, but none of the nutrient additionsaltered species richness at the older, P-limited site. Species diversity andevenness were not affected by fertilization at either site. At the site withlowN availability, plots fertilized with NP had higher densities of the non-nativeginger Hedychium gardnerianum, and at the site with lowP-availability, densities of the exotic shrub Rubusargutuswere higher in P- and NP-fertilized plots. Other effects included declines inmoss cover with fertilization at both sites, and reduced abundance of nativeseedlings in response to N and NP addition at the N-limited site. Continuedlong-term fertilization could lead to greater dominance of non-native speciesbyencouraging their growth at the expense of native species, which may sufferdecreased recruitment as fertilization and increased abundance of thenon-nativespecies may reduce suitable substrates for seedling establishment.     

Element Stoichiometry of a Mixotrophic Protist Grown Under Varying Resource Conditions

ABSTRACT. The balance of essential elements (e.g. carbon [C], nitrogen [N], and phosphorus [P]) between consumers and their resources influences not only the growth and reproduction of the consumers but also the nutrients they regenerate. Flagellate protists are significant predators of aquatic bacteria and directly influence nutrient flow to higher trophic levels and, through excretion, influence the mineral element composition of dissolved nutrients. Because the element stoichiometry of protists is poorly characterized, we varied the resource composition of the bacterium Pseudomonas fluorescens and used it to grow the mixotrophic bacterivorous flagellate Ochromonas danica. Using a mass balance approach, the element composition of O. danica was found to vary depending upon the nutrient composition of the prey and ranged between 482:36:1 and 80:12:1 (C:N:P molar). Homeostasis plots suggested that flagellate protists weakly regulate their element composition and are likely to regenerate different elements depending upon the nature of the element limiting growth of their prey.     

Bioassays using Stigeoclonium tenue Kütz. and Scenedesmus quadricauda (Turp.) Bréb. as testorganisms; a comparative study

Three different bioassays, two culture tube test methods with respectively Stigeoclonium tenue or Scenedesmus quadricauda and one bottle test with S. quadricauda, were compared. The yields obtained in the various tests were linearily correlated (r = 0.86, P < 0.001). The same primary limiting nutrient was indicated by the bioassays in most cases. However the algal growth in the tube test using Stigeoclonium was more often P-limited. In the case of S. quadricauda both test methods (tube and bottle) were nearly equally effective. The yields of N-limited samples were significantly correlated with the inorganic-N as well as total-N concentration of the water sample. A significant correlation of the ortho-P as well as total-P concentration with the yield of the P-limited assays was only found for Stigeoclonium tenue. The critical total N/P ratio (by weight) for N or P limitation was approximately 17: 1 for Stigeoclonium tenue and 22: 1 for Scenedesmus quadricauda.     

Effects of food deprivation and type of prey on predation by Notiophilus biguttatus F. (Carabidae) on springtails (Collembola)

Summary With diurnally active predators like Notiophilus biquttatus F. food deprivation is involved in predation in two ways: as a consequence of food shortage (i.e., low prey density), and as a consequence of the night period. The pattern of food intake after deprivation at night has been studied with respect to two prey species and differently deprived predators. They prey species represent a locomotory active one (Orchesella cincta) and a locomotory inactive one (Tomocerus minor). It appears that the rate of predation after deprivation shows a distinct pattern, initially high and then slowing to a more or less constant value. This pattern is influenced by food deprivation and type of prey. The beetles preying on O. cincta consumed more prey than those preying on T. minor. The more deprived predators compensated for deprivation by a higher daily predation when O. cincta was the prey, but not when T. minor was. Consequences of these findings are discussed with respect to diet composition and functional response.     

Consumer identity,abundance and nutrient concentration affect epiphyte diversity in an experimental eelgrass system

Conceptual models predict a unimodal effect of consumer abundance on prey diversity with the highest diversity at intermediate consumer abundance (intermediate disturbance hypothesis). Consumer selectivity and prey productivity are assumed to be further important determinants. Preferential grazing on dominant prey species favoured by high nutrient supply is supposed to increase prey diversity, whereas the effect of consumers on prey diversity may be negative under low nutrient conditions (grazer reversal hypothesis). We tested the effect of four common consumers the isopod Idotea baltica, the amphipod Gammarus oceanicus, and the gastropods Littorina littorea and Rissoa membranacea on diversity and composition of epiphytes growing on eelgrass Zostera marina. Consumer density was manipulated (four levels: grazer free control, low, medium, high) based on abundances observed in eelgrass systems. Additionally, we manipulated nutrient supply (three levels) and the presence of Idotea in a factorial experiment. The impact of consumer abundance on epiphyte diversity varied depending on consumer identity and epiphyte evenness was affected rather than species number in this short‐term experiment. Idotea reduced epiphyte diversity (Shannon‐Wiener index H') and Gammarus increased epiphyte diversity. Littorina had no effect at low and medium abundance, but a negative effect in the high density treatment. Only Rissoa supported the conceptual models as it caused the proposed unimodal pattern in epiphyte diversity. The varying species‐specific selectivity of the studied consumers is likely to explain their diverse impact on epiphyte diversity. Nutrients enhanced epiphyte diversity at medium enrichment, whereas higher nutrient supply reduced epiphyte diversity. The effect of Idotea changed from negative at low nutrient concentration to positive at higher nutrient supply, supporting the grazer reversal hypothesis. This study implies that consumer species identity and nutrient concentrations are important in controlling prey diversity and composition. Different consumer selectivity and changes in selectivity with growing consumer abundance and nutrient concentration are the causal factors for this effect.     

The effect of growth in continuous culture at various input C:N ratios on the expression of proteins involved in the transport and metabolism of methanol and short-chain amides by Methylophilus methylotrophus

Physiological regulation of proteins involved in the transport and metabolism of methanol and short-chain amides by Methylophilus methylotrophus was investigated following growth in continuous culture at various input C:N ratios. The concentrations of the methanol porin and methanol dehydrogenase were highest during C-limited growth (C:N<4.6), but declined gradually as a function of the increasing C:N ratio and were lowest during N-limited growth (C:N>16.3). In contrast, the concentrations of the amide-urea porin, the amide-urea binding protein, formamidase and acetamidase (together with formamidase and acetamidase activities) were lowest during C-limited growth, but increased sharply as a function of the C:N ratio and were highest during dual CN-limited and N-limited growth (C:N 4.6–16.3). The results are discussed in terms of the physiological and biochemical requirements of growth at varying C:N ratios.     

Inducible Mixotrophy in the Dinoflagellate Prorocentrum minimum

Prorocentrum minimum is a neritic dinoflagellate that forms seasonal blooms and red tides in estuarine ecosystems. While known to be mixotrophic, previous attempts to document feeding on algal prey have yielded low grazing rates. In this study, growth and ingestion rates of P. minimum were measured as a function of nitrogen (‐N) and phosphorous (‐P) starvation. A P. minimum isolate from Chesapeake Bay was found to ingest cryptophyte prey when in stationary phase and when starved of N or P. Prorocentrum minimum ingested two strains of Teleaulax amphioxeia at higher rates than six other cryptophyte species. In all cases ‐P treatments resulted in the highest grazing. Ingestion rates of ‐P cells on T. amphioxeia saturated at ~5 prey per predator per day, while ingestion by ‐N cells saturated at 1 prey per predator per day. In the presence of prey, ‐P treated cells reached a maximum mixotrophic growth rate (μ_max) of 0.5 d^?1, while ‐N cells had a μ_max of 0.18 d^?1. Calculations of ingested C, N, and P due to feeding on T. amphioxeia revealed that phagotrophy can be an important source of all three elements. While P. minimum is a proficient phototroph, inducible phagotrophy is an important nutritional source for this dinoflagellate.     

Bacterioplankton Growth and Nutrient Use Efficiencies Under Variable Organic Carbon and Inorganic Phosphorus Ratios

We carried out enclosure experiments in an unproductive lake in northern Sweden and studied the effects of enrichment with different dissolved organic carbon (glucose)/inorganic phosphorous (DOC/Pi) ratios on bacterioplankton production (BP), growth efficiency (BGE), nutrient use efficiency (BNUE), growth rate, and specific respiration. We found considerable variation in BP, BGE, and BNUE along the tested DOC/Pi gradient. BGE varied between 0.87 and 0.24, with the highest values at low DOC/Pi ratios. BNUE varied between 40 and 9 g C g P⁻¹, with high values at high DOC/Pi ratios. More DOC was thus allocated to growth when bacteria tended to be C-limited, and to respiration when bacteria were P-limited. Specific respiration was positively correlated with bacterial growth rate throughout the gradient. It is therefore possible that respiration was used to support growth in P-limited bacteria. The results indicated that BP can be limited by Pi when BNUE is at its maximum, by organic C when BGE is at its maximum, and by dual organic C and Pi limitation when BNUE and BGE have suboptimal values.     

NUTRIENT-LIMITED GROWTH OF AUREOUMBRA LAGUNENSIS (PELAGOPHYCEAE), WITH IMPLICATIONS FOR ITS CAPABILITY TO OUTGROW OTHER PHYTOPLANKTON SPECIES IN PHOSPHATE-LIMITED ENVIRONMENTS

Growth and chemical compositional characteristics of the brown tide-forming alga, Aureoumbra lagunensis Stockwell, DeYoe, Hargraves et Johnson, were studied through a series of nitrogen-limited and phosphate-limited continuous cultures over a range of growth rates. The specific growth rate of A. lagunensis was hyperbolically related to the cell quota of the limiting nutrient in ammonium-limited cultures. In phosphate-limited cultures, the relationship was best described by a straight line. The N cell quota of A. lagunensis ranges from about 20 fmol at zero growth rate under N-limited conditions to a high of roughly 85 fmol under N-replete conditions. Similarly, the P cell quota of A. lagunensis ranges from about 0.15 fmol at zero growth rate under P-limited conditions to a high of 2 fmol under P-replete conditions. Aureoumbra lagunensis has a very high N:P critical ratio (>100). The high N:P critical ratio, as well as the organism's apparent ability to use forms of phosphorus other than phosphate under severe phosphate deficiency, may partially explain its success in P-limited environments, such as the Laguna Madre. In addition, a uniqe quadratic relationship between the productivity index (PI) and growth rate was discovered. Such a relationship supports an earlier argument that PI may not be a good indicator of nutritional status.     

Differences in plant function in phosphorus- and nitrogen-limited mangrove ecosystems

Mangrove ecosystems can be either nitrogen (N) or phosphorus (P) limited and are therefore vulnerable to nutrient pollution. Nutrient enrichment with either N or P may have differing effects on ecosystems because of underlying differences in plant physiological responses to these nutrients in either N- or P-limited settings. Using a common mangrove species, Avicennia germinans, in sites where growth was either N or P limited, we investigated differing physiological responses to N and P limitation and fertilization. We tested the hypothesis that water uptake and transport, and hydraulic architecture, were the main processes limiting productivity at the P-limited site, but that this was not the case at the N-limited site. We found that plants at the P-deficient site had lower leaf water potential, stomatal conductance and photosynthetic carbon-assimilation rates, and less conductive xylem, than those at the N-limited site. These differences were greatly reduced with P fertilization at the P-limited site. By contrast, fertilization with N at the N-limited site had little effect on either photosynthetic or hydraulic traits. We conclude that growth in N- and P-limited sites differentially affect the hydraulic pathways of mangroves. Plants experiencing P limitation appear to be water deficient and undergo more pronounced changes in structure and function with relief of nutrient deficiency than those in N-limited ecosystems.     

Behavioural and developmental responses of predatory coral reef fish to variation in the abundance of prey

Ecological theory suggests that the behaviour, growth and abundance of predators will be strongly influenced by the abundance of prey. Predators may in turn play an important role in structuring prey populations and communities. Responses of predators to variation in prey abundance have most commonly been demonstrated in low-diversity communities where food webs are relatively simple. How predators respond in highly diverse assemblages such as in coral reef habitats is largely unknown. This study describes an experiment that examined how the movement, diet and growth of the coral reef piscivore, Cephalopholis boenak (Serranidae) responded to variation in the abundance of its prey. Predator densities were standardised on small patch reefs made from the lagoonal reef-building coral, Porites cylindrica. These patch reefs exhibited natural variation in the abundance and community structure of multiple species of prey. However, our experiment generated a relatively simple predator–prey relationship, with C. boenak primarily responding to the most abundant species of prey. Three responses of predators were observed: aggregative, functional and developmental. Thirty-one per cent of individuals moved between patch reefs during the experiment, all from areas of relatively low to high prey density. Feeding rates were higher on patch reefs of high prey density, while growth rates of fish that remained on low prey density reefs throughout the experiment were lower. Growth rates of C. boenak on the experimental reefs were also much higher than for those living on natural patch reefs over the same time period, corresponding with overall differences in prey abundance. These results suggest that local abundance, feeding rate and growth of C. boenak were closely linked to the abundance of their main prey. This combination of predatory responses is a potential mechanism behind recent observations of density-dependent mortality and population regulation of prey in coral reef fish communities.     

Changes in Morphology and Elemental Composition of <Emphasis Type="Italic">Vibrio splendidus</Emphasis> Along a Gradient from Carbon-limited to Phosphate-limited Growth

We examined morphology, elemental composition (C, N, P), and orthophosphate-uptake efficiency in the marine heterotrophic bacterium Vibrio splendidus grown in continuous cultures. Eight chemostats were arranged along a gradient of increasing glucose concentrations in the reservoirs, shifting the limiting factor from glucose to phosphate. The content of carbon, nitrogen, and phosphorus was measured in individual cells by x-ray microanalysis using a transmission electron microscope (TEM). Cell volumes (V) were estimated from length and width measurements of unfixed, air-dried cells in TEM. There was a transition from coccoid cells in C-limited cultures toward rod-shaped cells in P-limited cultures. Cells in P-limited cultures with free glucose in the media were significantly larger than cells in glucose-depleted cultures (P < 0.0001). We found functional allometry between cellular C-, N-, and P content (in femtograms) and V (in cubic micrometers) in V. splendidus (C = 224 × V ^0.89, N = 52.5 × V ^0.80, P = 2 × V ^0.65); i.e., larger bacteria had less elemental C, N, and P per V than smaller cells, and also less P relative to C. Biomass-specific affinity for orthophosphate uptake in large P-limited V. splendidus approached theoretical maxima predicted for uptake limited by molecular diffusion toward the cells. Comparing these theoretical values to respective values for the smaller, coccoid, C-limited V. splendidus indicated, contrary to the traditional view, that large size did not represent a trade-off when competing for the non-C-limiting nutrients.