Differential grazing of two heterotrophic nanoflagellates on marine Synechococcus strains

Grazing of heterotrophic nanoflagellates on marine picophytoplankton presents a major mortality factor for this important group of primary producers. However, little is known of the selectivity of the grazing process, often merely being thought of as a general feature of cell size and motility. In this study, we tested grazing of two heterotrophic nanoflagellates, Paraphysomonas imperforata and Pteridomonas danica , on strains of marine Synechococcus . Both nanoflagellates proved to be selective in their grazing, with Paraphysomonas being able to grow on 5, and Pteridomonas on 11, of 37 Synechococcus strains tested. Additionally, a number of strains (11 for Paraphysomonas , 9 for Pteridomonas ) were shown to be ingested, but not digested (and thus did not support growth of the grazer). Both the range of prey strains that supported growth as well as those that were ingested but not digested was very similar for the two grazers, suggesting a common property of these prey strains that lent them susceptible to grazing. Subsequent experiments on selected Synechococcus strains showed a pronounced difference in grazing susceptibility between wild-type Synechococcus sp. WH7803 and a spontaneous phage-resistant mutant derivative, WH7803PHR, suggesting that cell surface properties of the Synechococcus prey are an important attribute influencing grazing vulnerability.     

Effects of UV radiation on grazing by two marine heterotrophic nanoflagellates on autotrophic picoplankton

Phytoplankton <4-3 µm in diameter, or autotrophic picoplankton,can constitute the majority of the biomass and productivityof photosynthetic organisms in marine and freshwater systems.Indirect evidence has indicated that mortality of autotrophicpicoplankton occurs principally at night in the open ocean,but continuously in coastal water. The predominant view of thefate of autotrophic picoplankton production in the ocean isthat they are consumed by heterotrophic nanoflagellates. A possiblemechanism to explain these observations is that grazing of heterotrophicnanoflagellates on autotrophic picoplankton is inhibited byultraviolet radiation (W), at least in clear open-ocean environments.A series of laboratory experiments was conducted to examinethe effects of UV radiation on the grazing impact of two heterotrophicnanoflagellates on Synechococcus spp., a commonly occurringgenus of autotrophic picoplankton. The two nanoflagellates usedwere Paraphysomonas bandaieensis and Paraphysomonas imperforata.For both nanoflagellates, there was an inverse relationshipbetween the grazing mortality of Synechococcus and UV irradiance.The grazing mortality of Synechococcus was reduced less withP.imperforata than with P.bandaiensis. In some experiments,the effect of UV on the grazing impact of the nanoflagellatepopulations was caused in part by UV-related reductions in nanoflagellatesurvival. However, UV reduced the grazing impact of nanoflagellatesprimarily by reducing the rates of consumption of Synechococcusby individual nanoflagellates, to a degree directly relatedto UV irradiance. The results suggat that UV radiation may bean important selection factor in clear open-ocean water, andthat in order to predict the effect of increasing UV radiationon marine microbial plankton communities, we must consider interactionsbetween trophic levels as well as effects on single trophiclevels.     

An approach to measure ciliate grazing on living heterotrophic nanoflagellates

The complicated routes by which organic material is channelled up to higher trophic levels via bacteria and protozoans is a major issue in aquatic microbial ecology. Because of the fragile nature of protists it is not straightforward to perform experimental studies of prey–predator interactions. Here we present an approach for the assessment of ciliate grazing on living heterotrophic nanoflagellates. Stationary phase cultures of a heterotrophic nanoflagellate (Cafeteria sp.) were live-stained by allowing them to take up fluorescently labelled macromolecules. Controls revealed that this label persisted for several hours. Fluorescently labelled living flagellates (FLLF) were added into enriched natural assemblages of marine oligotrich ciliates and uptake of FLLF was monitored over time. Oligotrich ciliates did not incorporate fluorescent-labelled macromolecules but a linear FLLF uptake over time was observed for 20–30 min at 20°C. Ingestion rates were 21–46 FLLF h^–1 at a concentration of about 2×10⁴ FLLF ml^–1, which corresponded to clearance rates of 0.7–0.8 l ciliate^–1 h^–1. These results are in the same order of reported ciliate grazing on phytoplankton of similar size. This method represents a direct approach to measure ciliate grazing specifically on living heterotrophic nanoflagellates.     

Direct and indirect effects of zooplankton on algal composition in in situ grazing experiments

To examine both direct and indirect effects of macrozooplankton on phytoplankton species in Lake Biwa, we conducted in situ grazer-gradient experiments under different nutrient levels in summer, when Daphnia galeata dominated, and in autumn, when Eodiaptomus japonicus dominated. The experiments revealed that grazing pressure on phytoplankton was highly dependent on zooplankton species composition. Smaller phytoplankton species such as Stephanodiscus carconensis were more grazed when D. galeata was abundant, whereas large colonial diatom species such as Aulacoseira granulata were preferentially grazed when E. japonicus dominated. In addition, indirect effect of macrozooplankton through nutrient regeneration was suggested, although the magnitude of nutrient regeneration effects seemed to differ between D. galeata and E. japonicus. Specifically, growth rates of Sphaerocystis schroeteri were stimulated more by E. japonicus than by D. galeata. Macrozooplankton also enhanced the growth rates of colonial cyanobacteria such as Microcystis incerta, probably through decreasing the density of microzooplankton grazers (ciliates and rotifers). The results suggest that the effects of large zooplankton on phytoplankton populations are species-specific and cannot be understood without consideration of changes in abundance of other components of plankton communities.     

Direct and indirect evidence for polypeptide absorption by the teleost gastrointestinal tract

The ability of the gastrointestinal tract of chinook salmon, Oncorhynchus tshawytscha , to absorb polypeptides in vivo was investigated by reference to the appearance of orally administered adrenocorticotropic hormone (ACTH) within the blood of fish previously treated with dexamethasone (3μg g^−l body weight) in order to suppress endogenous ACTH secretion. Further, since cortisol presence within plasma is dependent upon the availability of ACTH, dexamethasone blockade of endogenous ACTH secretion, in conjunction with subsequent measurements of plasma cortisol levels, provides a means by which biological patency of absorbed exogenous ACTH may be demonstrated. Levels of ACTH and cortisol in plasma of dexamethasone-treated salmon were therefore measured for a period of 360 min immediately following oral intubation of ACTH (15μg g⁻¹ body weight). Peak plasma presence of ACTH-like immunoreactivity (676.6 ± 33.6 pg ml⁻¹ plasma) and cortisol (227.1 ± 29.0 ng ml⁻¹ plasma) were recorded 120 min after ACTH administration. Results from the experimental groups were compared to those of 15 control treatments. Since administration of ACTH to chinook salmon elicited a consistent and significant elevation in not only plasma ACTH but also cortisol presence, it is contended that the salmonid gut expresses an ability to absorb polypeptides of dietary origin. The significance of these findings with respect to the oral administration of biologically active peptides and proteins to fish of importance to aquaculture is discussed.     

Direct and indirect effects of grazing on the phytoplankton seasonal succession in an oligotrophic lake

We examined the simultaneous dynamics of phytoplankton and zooplanktonin an oligotrophic lake, as well as the algal response to experimentalmanipulations of herbivore population structure and density.The seasonal succession of phytoplankton is characterized bya shift in dominance from eukaryotic species to prokaryoticcyanobacteria, as in a eutrophic lake. This unusual patternfor an oligotrophic lake is related to the release of zooplanktonphosphorus, rather than to the amount of total phosphorus. Thehighest estimated values of this released phosphorus occur duringthe co-dominance of small-sized species and of one calanoidpopulation, both under natural and experimental conditions.Our experimental results clearly reveal that a blue-green speciesreplaces non-blue-green ones when environmental conditions renderlow or intermediate values for released phosphorus, irrespectiveof the grazing pressure. The absence of predators or high levelsof released phosphorus stimulate non-blue-green growth. Theseresults agree with the successional pattern observed for thealgal assemblage under natural conditions. Nutrient recyclingplays an essential role in the maintenance of the metabolismof the pelagic system, and therefore the prevalence of a topdownregulation mechanism as proposed for such low-nutrient environmentsshould be reconsidered.     

Direct and indirect effects of predators on the dominant invertebrates of two freshwater littoral communities

Summary Two congeneric damselfly species, Enallagma traviatum and E. aspersum, dominate the littoral macroinvertebrates of Bays Mountain Lake and of the adjacent fish-free Ecology Pond, respectively (northeastern Tennessee, USA). Extending previous experimental studies, we test seven hypotheses concerning the role of fish (bluegill sunfish, Lepomis macrochirus) and larvaldragonfly (Anax junius) predation, competitive effects on damselflies, and the interaction between competition and predation, in determining invertebrate dominance in these communities. Three types of experiments were conducted: an enclosure experiment within Ecology Pond, an outdoor replicated tub experiment, and a laboratory behavior experiment. The in-situ enclosure experiment showed that E. traviatum larvae were more susceptible to Anax predation than were E. aspersum larvae; a tendency toward greater vulnerability to fish of E. aspersum compared with E. traviatum was not statistically significant. The outdoor tub experiment confirmed both of these trends with statistically significant results. In the tubs, both predators inhibited feeding of both zygopterans (as indicated by reduced fecal mass), particularly for E. aspersum in the presence of fish. This effect appears to have been primarily indirect, mediated through exploitation of the zooplankton. We also detected competitive effects of E. traviatum on E. aspersum: E. traviatum reduced the emergence and increased the exposure above the substrate of E. aspersum. In the absence of predators, E. traviatum inhibited feeding of E. aspersum via interference. In the laboratory behavior experiment, predators inhibited crawling by E. aspersum. E. aspersum was more exposed than was E. traviatum; it swam and crawled more than did E. traviatum, considerably increasing these movements at night. Over all, E. traviatum consistently appeared to be the more cryptic of the two species, and E. aspersum appeared to be much more active. Our results suggest an explanation for the clear difference in structure between communities like Bays Mountain Lake and Ecology Pond: predaceous fish eliminate large invertebrate predators and shift the community toward cryptic forms at relatively low densities, reflecting the effects of both predation and exploitation competition. In the absence of fish, large invertebrate predators are less able to deplete littoral invertebrates but may favor the more active forms, perhaps because these are better able to avoid invertebrate predators.     

Impact of violacein-producing bacteria on survival and feeding of bacterivorous nanoflagellates

We studied the role of bacterial secondary metabolites in the context of grazing protection against protozoans. A model system was used to examine the impact of violacein-producing bacteria on feeding rates, growth, and survival of three common bacterivorous nanoflagellates. Freshwater isolates of Janthinobacterium lividum and Chromobacterium violaceum produced the purple pigment violacein and exhibited acute toxicity to the nanoflagellates tested. High-resolution video microscopy revealed that these bacteria were ingested by the flagellates at high rates. The uptake of less than three bacteria resulted in rapid flagellate cell death after about 20 min and cell lysis within 1 to 2 h. In selectivity experiments with nontoxic Pseudomonas putida MM1, flagellates did not discriminate against pigmented strains. Purified violacein from cell extracts of C. violaceum showed high toxicity to nanoflagellates. In addition, antiprotozoal activity was found to positively correlate with the violacein content of the bacterial strains. Pigment synthesis in C. violaceum is regulated by an N-acylhomoserine lactone (AHL)-dependent quorum-sensing system. An AHL-deficient, nonpigmented mutant provided high flagellate growth rates, while the addition of the natural C. violaceum AHL could restore toxicity. Moreover, it was shown that the presence of violacein-producing bacteria in an otherwise nontoxic bacterial diet considerably inhibited flagellate population growth. Our results suggest that violacein-producing bacteria possess a highly effective survival mechanism which may exemplify the potential of some bacterial secondary metabolites to undermine protozoan grazing pressure and population dynamics.     

Short-term productivity responses of algae and bacteria to zooplankton grazing in two freshwater lakes

SUMMARY. 1. The specific productivities of algae and bacteria were measured in short-term (4 day) experiments consisting of enclosures with natural or reduced zooplankton biomass. Experiments were repeated five times over a season in each of two lakes that differed in the background concentration of dissolved organic carbon (DOC).
2. Algal biomass as estimated by chlorophyll a was suppressed in enclosures with ambient grazer levels in six of ten experiments and enhanced in one experiment. Distribution of chlorophyll among net and nanoplankton was not significantly affected by grazing.
3. Relative to enclosures with reduced zooplankton, normal grazer biomass (97–466μg 1⁻¹ dry weight) enhanced specific algal productivity in only one of five experiments in the low DOC take and had no effect in all five experiments in the high DOC lake. The main effects of grazers on algae was through removal of biomass rather than through indirect changes in turnover rate.
4. Between experiments, bacterial density was either unaffected, or mildly enhanced (4–87%) in enclosures with ambient macrozooplankton compared to those with reduced levels. Bacterial productivity and turnover estimated by incorporation of [³H]thymidine into DNA showed different responses across experiments; increasing, declining or remaining the same with grazer minipulation. This variability was not related to differences in dissolved primary production or to background DOC between lakes or experiments. Comparison of bacterial productivities based on thymidine incorporation rates with changes in cell densities indicated that control of bacterial loss processes by macrozooplankton is more important than control of growth rates.     

Short communication. Direct and indirect effects of strong grazing by Daphnia galeata on bacterial production in an enclosure experiment

Bacterial production was stimulated in Daphnia enclosures at the beginning of the experiment due to the release of dissolved organic carbon during Daphnia grazing of algae (sloppy feeding). In contrast, bacterial production decreased in the treatments at the end of the experiment. The decline was not caused by reduction of bacterial biomass, but by decreasing biomass-specific production due to a decline of the resources for bacterial growth (phytoplankton biomass and production).      

Grazing of bacteria and phytoplankton by heterotrophic nanoflagellates in a Baltic Sea sample

Grazing by heterotrophic nanoflagellates on bacteria and phytoplankton was studied in a laboratory experiment, using a natural pelagic community originating from the Tvärminne sea area off the southern coast of Finland. Water was prescreened to remove larger grazers. Four experimental treatments were used: light and dark, with and without added nutrients. The growth of the large heterotrophic flagellates was stimulated by increased production of < 3 m phytoplankton. Clearance rates for heterotrophic nanoflagellates were estimated and were found to be within the range of previously reported values.     

Heterotrophic nanoflagellates in water column and bottom sediments of the Rybinsk Reservoir: Species composition,abundance, biomass and their grazing impact on bacteria

Abundance, biomass, and taxonomic composition of heterotrophic nanoflagellates (HNFs) have been determined in the water column and bottom sediments of the large lowland meso-eutrophic reservoir (Rybinsk Reservoir, Upper Volga) in summer. The role of HNFs in the consumption of the bacterial production is estimated. In the reservoir, 55 species from 15 large taxa, including 35 species from the plankton, are identified and 45 species are from benthos samples. The orders Kinetoplastida, Choanomonada, and Chrysomonadida are distinguished by the highest species diversity. Abundance and biomass of HNFs in the water column average 991 ± 326 cells/mL and 41.4 ± 14.1 mg/m³, while in the bottom sediments they are (236 ± 61) × 10³ cells/mL and 10.7 ± 4.0 μg/mL, respectively. The biomass of HNFs average 11.2% of the bacterial biomass in the water column and only 0.8% of that in the sediments. Flagellates are found to be a major factor which control the development of bacterioplankton grazing, on average, 32.3% of its daily production, whereas their impact on bacteriobenthos is insignificant, as they consume, on average, only 2.0% of its production.     

Direct and indirect responses of a freshwater food web to a potent synthetic oestrogen

Endocrine-disrupting chemicals (EDCs) in municipal effluents directly affect the sexual development and reproductive success of fishes, but indirect effects on invertebrate prey or fish predators through reduced predation or prey availability, respectively, are unknown. At the Experimental Lakes Area in northwestern Ontario, Canada, a long-term, whole-lake experiment was conducted using a before-after-control-impact design to determine both direct and indirect effects of the synthetic oestrogen used in the birth control pill, 17α-ethynyloestradiol (EE2). Algal, microbial, zooplankton and benthic invertebrate communities showed no declines in abundance during three summers of EE2 additions (5–6 ng l⁻¹), indicating no direct toxic effects. Recruitment of fathead minnow (Pimephales promelas) failed, leading to a near-extirpation of this species both 2 years during (young-of-year, YOY) and 2 years following (adults and YOY) EE2 additions. Body condition of male lake trout (Salvelinus namaycush) and male and female white sucker (Catostomus commersonii) declined before changes in prey abundance, suggesting direct effects of EE2 on this endpoint. Evidence of indirect effects of EE2 was also observed. Increases in zooplankton, Chaoborus, and emerging insects were observed after 2 or 3 years of EE2 additions, strongly suggesting indirect effects mediated through the reduced abundance of several small-bodied fishes. Biomass of top predator lake trout declined by 23–42% during and after EE2 additions, most probably an indirect effect from the loss of its prey species, the fathead minnow and slimy sculpin (Cottus cognatus). Our results demonstrate that small-scale studies focusing solely on direct effects are likely to underestimate the true environmental impacts of oestrogens in municipal wastewaters and provide further evidence of the value of whole-ecosystem experiments for understanding indirect effects of EDCs and other aquatic stressors.     

Experimental evidence for long-term coexistence of copiotrophic and oligotrophic bacteria in pelagic surface seawater

Most marine copiotrophic bacteria can produce extracellular enzymes to degrade biopolymers into bio-available smaller solutes, while oligotrophic bacteria usually cannot. Bacterial extracellular enzymes and enzymatic products can be a common resource that could be utilized by both copiotrophs and oligotrophs; when present, oligotrophs may outcompete the enzyme-producing copiotrophs. However, copiotrophs and oligotrophs consistently coexist in the ocean. How they maintain coexistence has still not been experimentally studied. In this study, the interaction and coexistence of a copiotroph and an oligotroph, isolated from the same surface seawater sample and utilizing the same proteinaceous substrate, were experimentally investigated. The copiotroph could secrete extracellular proteases to degrade and then utilize the proteinaceous substrate. The oligotroph was unable to utilize the proteinaceous substrate by itself, but could grow by using the hydrolysate amino acids. The copiotroph outcompeted the oligotroph by adsorbing the amino acids quickly and having a higher growth rate in the rich medium. The oligotroph survived by adapting to low concentration of nutrients. The copiotroph and oligotroph were able to maintain long-term (up to 142 days) coexistence in the laboratory. This study indicates that differences in the utilization of different concentrations of nutrients can drive the coexistence of marine copiotrophs and oligotrophs.     

Cadmium sorption by bacteria and freshwater sediment

Summary Sorption of cadmium by sediment bacteria and freshwater sediment was investigated using diffusion chambers to simulate the water-sediment interface. Diffusion chambers were constructed to provide two compartments separated by a dialysis membrane. Diffusion of cadmium across the membrane was monitored after pure cultures of sediment bacteria or lake sediments were added to the sediment side of a diffusion chamber. Cellular accumulation of cadmium by cadmium-sensitive and cadmium-resistant bacteria removed between 20% and 80% of the dissolved cadmium from the simulated water column and pore water. Cellular accumulation of cadmium was greatest for cadmium-sensitive isolates that were tested. Sediment with an intact microbial community sequestered 80% of the cadmium added to sediment, whereas autoclaved sediment retained 97% of the metal that was added. Addition of glucose to cadmium-amended sediment decreased retention of cadmium by untreated and autoclaved sediments, resulting in elevated concentrations of dissolved cadmium in the simulated water column.     

Direct and indirect effects of grazing constrain shrub encroachment in semi‐arid Patagonian steppes

Question: What are the long‐term effects of grazing exclusion on the population structure and dynamics of, and interactions among, three dominant shrub species? Location: Grass‐shrub Patagonian steppe, Chubut, Argentina. Methods: Permanent plots were established in grazed paddocks and paddocks excluded from grazing in representative Patagonian rangelands. Shrub abundance, population size‐structure, short‐term (two 3‐yr periods) and long‐term (matrix models) population dynamics, and neighborhood interactions of three native and codominant shrub species (Mulinum spinosum, Senecio filaginoides and Adesmia volckmanni) were measured and analysed using different statistical approaches. Results: The total density of shrubs was 74% higher in paddocks excluded from grazing, owing mainly to increases in Mulinum (80%) and Senecio (68%) species. However, differences in size structure between ungrazed and grazed paddocks were only detected in Mulinum. Demographic rates differed between shrub species, time‐periods and grazing conditions. In particular, recruitment in the short term (especially in wet years) and population growth rate in the long term (λ) were higher in paddocks excluded from grazing only in Mulinum populations. Senecio populations showed a marginal increase in recruitment and mortality independent of the grazing condition in the wet and dry period. Grazing exclusion modified the balance of neighborhood interactions among the three shrub species. In grazing‐exclusion paddocks, there was a balance between positive and negative interspecific interactions, while in grazed paddocks there were more negative intraspecific and interspecific interactions, resulting in a net negative balance of neighborhood interactions. Conclusions: Our understanding of woody encroachment in arid rangelands can be informed through evaluation of direct and indirect effects of grazing exclusion on the abundance and demography of dominant woody species. In Patagonian arid steppes, the occurrence of woody encroachment in rangelands excluded from grazing can be explained by altered responses in plant‐animal and plant‐plant interactions among shrub species.     

Direct versus indirect effects of p-chloromercuribenzenesulphonic acid on sucrose uptake by plant tissues: The electrophysiological evidence

The short-term effects of p -chloromercuribenzenesulphonic acid (PCMBS) on the transmembrane potential difference (PD) of broad bean ( Vicia faba L. cv. Aguadulce) cotyledon cells and on sugar beet ( Beta vulgaris L. cv. Klein E.) leaf cells were studied by the electrophysiological method. These effects were compared with that of the permeant thiol reagents N-ethylmaleimide and HgCl₂. N-Ethylmaleimide and HgCl₂ markedly and rapidly depolarised the PD of all the material studied, while PCMBS caused either a slight depolarisation (cotyledon cells) or no depolarisation (leaf cells) during the first 30 min of treatment. In cotyledons, PCMBS markedly inhibited sucrose uptake (89%) and the sucrose-induced depolarisation associated with the proton-sucrose symport (67%), while it decreased the proton-motive force only marginally (7%). It is concluded that during short treatments (30 min or less). PCMBS inhibits sucrose uptake directly by blocking the sucrose carrier, and not the proton pump. For longer treatments, indirect effects cannot be excluded.     

Zooplankton community size structure and taxonomic composition affects size-selective grazing in natural communities

The body size of an individual zooplankton is well related to its grazing rate and to the range of particle sizes it can ingest, and since cladocerans and copepods feed differently, they follow different relationships. Based on these general patterns in individual organisms, we tested whether the size structure and taxonomic composition of more complex natural zooplankton communities are related to their in situ grazing rate and to the range of algal sizes they graze. We compared community grazing rates on individual algal taxa in two communities dominated by small cladocerans, three communities dominated by large cladocerans and three copepod-dominated communities. Small algae were usually grazed most intensively, but grazing rates were poorly related to algal size alone. The range in size of grazed algae increased with increasing mean zooplankton body size, but differed systematically with their taxonomic composition. Communities dominated by Ceriodaphnia or Holopedium grazed a narrower size range of algae [maximum greatest axial length dimension (GALD)=16–36 μm)] than communities with large biomasses of Bosmina or Daphnia (maximum GALD=28–78 μm). Copepod-dominated communities followed the same general relationship as cladocerans. Daphnia-dominated communities grazed the broadest range of algal sizes, and their total grazing rates were up to 2.4 times their grazing rates on small (<35 μm) “highly edible” algae, a difference of similar magnitude to those found in successful trophic cascade biomanipulations. Received: 31 March 1998 / Accepted: 19 October 1998     

Top–down control of benthic heterotrophic nanoflagellates by oligochaetes and microcrustaceans in a littoral freshwater habitat

1. We investigated trophic interactions between benthic heterotrophic nanoflagellates (HNF) and oligochaetes and microcrustaceans (cladocerans and copepods) transferred from a silty, littoral freshwater habitat to laboratory microcosms. With a newly adapted experimental design we were able to compute (i) predation rates on benthic HNF by the tested metazoan organisms and (ii) growth rates of the natural benthic HNF population when losses because of this predation were excluded. 2. The experiments covered a temperature range of 4–27 °C and a fivefold variation of predator densities (September 2000–February 2002). For 60% of these experiments, significant predation of oligochaetes and microcrustaceans on benthic HNF was revealed. Predation rates on HNF ranged from 0 to 0.256 day^?1. Growth rates of the benthic HNF assemblage varied from ?0.098 to 0.353 day^?1; they were used to estimate the significance of the measured losses in comparison with possible other loss factors. 3. The data suggested that during the major part of the year a high percentage of the HNF production was consumed in the surficial sediment of the investigated system, resulting in a relatively constant and low HNF standing stock and an uncoupling of benthic bacteria and their protistan grazers. Top–down control by microcrustaceans and oligochaetes was identified as one significant, frequently prevailing regulatory factor, while other parameters responsible for the control of benthic HNF densities remain to be examined.