Concentrations and annual loading of dissolved organic matter in a small moorland stream

SUMMARY. 1. Dissolved organic matter (DOM) was measured at 8h intervals over a 1 year period in a stream draining 51 ha of moorland with peaty soils.
2. DOM concentrations increased with increasing stream discharge from low flow values of 0–3 mg 1⁻¹ to maximum values of 30 mg 1⁻¹. There were also seasonal differences of up to 13mgl⁻¹ between August (maximum) and February, and differences of about 2.5 mg l⁻¹ between rising and falling stage samples.
3. Seasonal variation was closely related to mean temperature.
4. Total loss over the year was 168 kg ha⁻¹ DOM (84 kg ha⁻¹ C), larger than previous estimates for upland sites.     

Microscale vertical distribution of algal and bacterial plankton in Lake Vechten (The Netherlands)

Abstract Depth profiles of horizontal light-beam attenuation and in situ fluorescence recorded at 1 to 2 cm depth intervals in Lake Vechten contained pronounced maxima related to phytoplankton and bacterioplankton layers. This was confirmed by microscopy and HPLC analysis of pigments in discrete samples. Algae were sparse throughout the oxic water column compared with green and purple sulphur bacteria in the anoxic zone of the lake, but plankton layers comprising diatoms, green algae, euglenophytes and cryptomonads were observed. Maximum concentrations of phototrophic bacteria, with dominant representatives Synechococcus, Thiopedia, Chloronema and Chlorobium , occurred very close to one another and within the anoxic zone at depths of pigment maxima and major fluorescence peaks. Plankton layers affected downwelling irradiance in two ways: first, by markedly increasing overall light extinction in the vicinity of the layers, e.g. between 4 m and 8 m the minimum wavelength-specific extinction coefficient, which was at 580 nm ( k ₅₈₀) increased from 0.5 m⁻¹ to 1.5 m⁻¹; secondly, changes in the shape of extinction ( k ) spectra corresponded to in vivo absorbance characteristics of photosynthetic pigments of plankton in the layers. It is inferred that field microbial ecological studies may greatly benefit from simultaneous use of in situ optical techniques.     

Cadmium toxicity to the freshwater amphipod Gammarus pulex (L.) during the moult cycle

SUMMARY. 1. The toxicity of cadmium to mature Gammarus pulex at different stages in the moult cycle is described.
2. Immediate post-moult animals are significantly more sensitive than intermoult specimens at cadmium concentrations between 1.0 and 0.1 mg 1⁻¹ but not at 0.03 or 0.01 mg Cd 1⁻¹.
3. At a calcium concentration of 40mgl⁻¹, post-moult animals undergo recalcification within 7 days and thereafter there is little variation in their response to cadmium.
4. External calcium concentrations of 40 and 115mg 1⁻¹ do not affect cadmium toxicity but at 180 mg Ca1⁻¹ the sensitivity of immediate post-moult specimens is significantly reduced.
5. The results are discussed with regard to the protection of G. pulex by present water quality standards.     

The effects of temperature, pH and water hardness on winter starvation of young-of-the-year smallmouth bass, Micropterus dolomieui Lacepede

Year-class strength in northern populations of smallmouth bass is strongly influenced by winter starvation of young-of-the-year. We examined starvation among young bass under both winter and summer light and temperature conditions. During starvation, body condition declines to a specific level and then the fish dies. Body condition at death is a well defined function of body size that remains relatively constant over a wide range of environmental conditions. Starvation rate varies systematically with body size, temperature, pH and water hardness. Available stored energy increases more rapidly with body size than starvation rate. Therefore, lifetime under starvation conditions tends to increase with increasing body size. The Q₁₀ for starvation rate over the temperature range 2.5-8° C is 2.2. Starvation rate increases as pH declines from 7.0-4.9: the rate at pH 4.9 is ∼ 1.25 times the rate at pH 7 Starvation rate decreases as Ca concentration increases from 1 mgl⁻¹ to 80mgl⁻¹: the rate at 80 mg Ca 1⁻¹ is ∼0.80 times the rate at 1 mgl⁻¹.     

Microdistribution of immature African blackflies resulting from water velocity and turbulence preferences

Abstract. 1. The velocity preferences of larval and pupal blackflies were studied experimentally by comparing the colonization of plastic strings placed in different velocity ranges; and also by investigating the simuliid microdistribution under natural conditions in the river. This required a novel method described for taking spot measurements of water velocity.
2. Simulium mcmahoni de Meillon, S.hirsutum Pomeroy and S.cervicor-nutum Pomeroy were most abundant in slow velocities (0.3-1.0 m s^_1), S.colasbelcouri Grenier & Ovazza and S.hargreavesi Gibbins preferred 1.1-1.8 m s^_1, S.squamosum Enderlein and S.vorax Pomeroy 1.9-2.2 m s^_1.
3. The larvae of S.hargreavesi and S.cervicornutum preferred a higher velocity range compared with their pupae.
4. In moderate velocities (1.1-1.4 m s^_1), the abundance of S.hargreavesi was greater under turbulent conditions than in smooth-flowing water, but this was reversed at very high velocities (2.3-2.6 m s^_1).
5. Velocity had no apparent effect on substrate preference when substrates of different flexibilities were compared for three blackfly species. All avoided the most flexible substrates and preferred ones consisting of rigid articulating plates. Anisopteran predators were found to have a similar substrate choice.     

Histological changes in the gill, kidney and liver of Lahontan cutthroat trout, Salmo clarki henshawi, living in lakes of different salinity-alkalinity

Lahontan cutthroat trout thrive in saline-alkaline lakes, where other trout species often cannot survive. We examined Lahontan cutthroat trout from nine lakes in which salinity and alkalinity ranged from about 90 to 12 000 mg1⁻¹ and 60 to 3500mgl⁻¹ as HCO₃⁻ respectively, for sublethal histological changes in gill, kidney, and liver tissues. Gill chloride cell hyperplasia, gill lamellar epithelial separation, kidney glomerular swelling, blood congestion in kidneys, and deposition of hyalin droplets in kidney glomeruli, tubules, and hemopoietic tissues were the histological alterations statistically associated with differences in lakewater chemistry.
Deposition of hyalin in kidney tubules was the only histological change judged pathological and whose severity appeared sufficient to jeopardize normal organ function. Differences in lakewater chemistry explained nearly 90% of the variability observed in severity of tubular hyalin degeneration, and SO₄²⁻ was the ion most positively correlated with increasing tubular hyalin. Our results suggest that Lahontan cutthroat trout will develop slight to moderate hyalin degeneration in kidney tubules if stocked into lakes where salinity and SO₄²⁻ concentrations equal or exceed 5000 mgl⁻¹ and 2000mgl⁻¹, respectively.     

The nutrient budget of a small eutrophic loch and the effectiveness of straw bales in controlling algal blooms

1. A 1-year intensive study of nutrient flows in Airthrey Loch, central Scotland, a small eutrophic [mean total phosphorus (TP) = 61.1 μg P l^–1] well-flushed freshwater body (area, 6.9 ha; mean depth, 1.85 m; volume, 1.274 × 10⁵ m³; retention time, 0.44 yr), was undertaken.
2. The nutrient budget was dominated by large allochthonous P inputs, equivalent to an areal load to the loch of 8.56 kg P ha^–1 yr^–1, which occurred predominantly during winter. In the summer, when TP inputs were low, water column levels of TP still increased, as a result of aerobic P release from sediments.
3. Sorption experiments indicated potential for sediment P release at water P concentrations of up to 200 μg P l^–1.
4. Aerobic release rate of P from sediments to the water column of Airthrey Loch was estimated to be of the order of 1 mg P m^–2 day^–1, and occurred during periods of elevated water column pH.
5. Straw bales placed in the loch to retard algal blooms were found not to have any demonstrable impact on algal concentrations observed during the study.     

The nutrient budget of a small eutrophic loch and the effectiveness of straw bales in controlling algal blooms

1. A 1-year intensive study of nutrient flows in Airthrey Loch, central Scotland, a small eutrophic [mean total phosphorus (TP) = 61.1 μg P l^–1] well-flushed freshwater body (area, 6.9 ha; mean depth, 1.85 m; volume, 1.274 × 10⁵ m³; retention time, 0.44 yr), was undertaken.
2. The nutrient budget was dominated by large allochthonous P inputs, equivalent to an areal load to the loch of 8.56 kg P ha^–1 yr^–1, which occurred predominantly during winter. In the summer, when TP inputs were low, water column levels of TP still increased, as a result of aerobic P release from sediments.
3. Sorption experiments indicated potential for sediment P release at water P concentrations of up to 200 μg P l^–1.
4. Aerobic release rate of P from sediments to the water column of Airthrey Loch was estimated to be of the order of 1 mg P m^–2 day^–1, and occurred during periods of elevated water column pH.
5. Straw bales placed in the loch to retard algal blooms were found not to have any demonstrable impact on algal concentrations observed during the study.     

Continuous drift samples of macroinvertebrates in a large river, the Danube in Austria

SUMMARY 1. A new method is described for sampling the drift in large rivers continuously and automatically, using equipment based on the marine, continuous plankton sampler of Hardy (1936).
2. Between April and November 1989, about 790 h were sampled continuously, equivalent to a water volume of 2937m³ and the total catch was 527 drifting macroinvertebrates, in forty-nine taxa. Twenty-five of these taxa are recorded in the drift for the first time and twenty-eight of the drifting taxa were present in benthos samples from the Danube.
3. Total drift density varied between 12 and 31 animals per 100m³ of water sampled throughout the year, with a maximum of 31 animals in May, The composition of major faunal groups showed a significant seasonal pattern, with Oligochaeta and Diptera predominant in spring, Crustacea and Insecta in summer and only Crustacea in autumn. The overall density of the macrozoobenthos from October 1986 to December 1987 was about 19360 animals m⁻² and the proportion of total benthos animals, drifting at any instant in time, ranged from 0.0026 to 0.0064%.
4. The relationship between drift density day⁻¹ and mean daily discharge was described by a power-function. Total mean drift rate of macroinvertebrates in the Danube was estimated to be 13 600 000 animals per 24 h and the mean drift distance was estimated to vary between about 4 and 31 m, dependent on the animal group and the water velocity.
5. No obvious consistent diel pattern could be established from the continuous samples, and no marked diel rhythm could be detected for Oligochaeta, Diptera larvae and Crustacea.     

The effects of water hardness upon the uptake, accumulation and excretion of zinc in the brown trout, Salmo trutta L.

The effects of water hardness (9 and 220 mgl⁻¹ as CaCO₃) upon zinc exchange in brown trout exposed to 0.77 μmol Zn 1⁻¹ have been investigated using artificial soft water (<49.9 μmol Ca ^l-1, <40.1 μmol Mg 1⁻¹) and mains hard water (1671.7 μmol Ca 1⁻¹, 493.6 μmol Mg 1⁻¹) of known composition. Both hard and soft water-adapted fish exhibited a bimodal pattern of net zinc influx. Net zinc influxes during both fast and slow uptake phases were significantly greater ( P <0.001) in soft (82.9 and 6.2 μmol Zn 100 g⁻¹ h⁻¹) than in hard water (46.3 and 2.4 μmol Zn 100 g h⁻¹). Zinc efflux (- 0.2 μmol Zn 100 g⁻¹ h⁻¹) was enhanced only in hard water during the slow net influx phase.
Brown trout exposed to zinc in hard water and placed in metal-free media exhibited a greater net efflux (- 25.6 μmol Zn 100 g⁻¹ h⁻¹) of the metal than did fish in soft water (-4.2 μmol Zn 100 g⁻¹ h⁻¹) treated in the same manner. Tissue ⁶⁵Zn activities reflected both the differences in uptake and excretion rates of the metal between hard and soft water fish. During zinc exposure (0.77 μmol Zn 1⁻¹) high water hardness reduced tissue burdens of the metal by reducing net branchial influx, and enhancing efflux of the metal in hard water fish.     

Effect of organic loading on nitrification and denitrification in a marine sediment microcosm

Abstract The effects of organic additions on nitrification and dentrification were examined in sediment microcosms. The organic material, heat killed yeast, had a C/N ratio of 7.5 and was added to sieved, homogenized sediments. Four treatments were compared: no addition (control), 30 g dry weight (dw) m⁻² mixed throughout the 10 cm sediment column (30M), 100 g dw m⁻² mixed throughout sediments (100M), and 100 g dw m⁻² mixed into top 1 cm (100S). After the microcosms had been established for 7–11 days, depth of O₂ penetration, sediment-water fluxes and nitrification rates were measured. Nitrification rates were measured using three different techniques: N-serve and acetylene inhibition in intact cores, and nitrification potentials in slurris. Increased organic additions decreased O₂ penetration from 2.7 to 0.2 mm while increasing both O₂ consumption, from 30 to 70 mmol O₂ m⁻² d⁻¹, and NO₃⁻ flux into sediments. Nitrification rates in intact cores were similar for the two methods. Highest rates occurred in the 30M treatment, while the lowest rate was measured in the 100S treatment. Total denitrification rates (estimated from nitrification and nitrate fluxes) increased with increased organic addition, because of the high concentrations of NO₃⁻ (40 μM) in the overlaying water. The ratio of nitrification: denitrification was used as an indication of the importance of nitrification as the NO₃⁻ supply for denitrificaion. This ratio decreased from 1.55 to 0.05 iwth increase organic addition.     

Contribution of grazing in phytoplankton overall losses in a shallow French lake

SUMMARY 1. Based on data for ¹⁴C-primary production and biomass changes in a small and shallow lake (Créteil Lake, France), overall phytoplankton losses were calculated through an annual cycle (November 1985-October 1986). The summer period in 1986 is compared with two other summer periods in 1985 and 1980, these two years corresponding to extreme levels of algal biomass.
2. Independent from the trophic state of the lake, phytoplankton populations were dominated by small-sized species (<20 μm); their high growth rate (maximal in May and June: 0–8 day⁻¹) was characteristic of nanoplanktonic natural populations.
3. The positive correlation between phytoplankton losses and production indicates a close coupling between growth and loss processes.
4. With a high filtering rate (0.22 day⁻¹ as an annual average), zoo-plankton impact is considerable at any time of the year but especially in late summer, when grazing losses exceeded primary production.
5. Despite the uncertainty concerning the meaning of ¹⁴C-primary production, the persistence of small algae throughout the year implies that a great part of the phytoplankton production was harvested by grazers which led to a recycling of organic matter within the water column.     

Lough Ennell: laboratory studies on sediment phosphorus release under varying mixing, aerobic and anaerobic conditions

SUMMARY. 1. The exchange of phosphorus between the epilimnetic (shallow zone) sediment and water column in Lough Ennell was investigated in laboratory experiments using five intact cores.
2. Variations in water mixing, sediment suspension and aerobic–anaerobic oxygen status in the water column and its effects on sediment phosphorus release rates were determined.
3. Experimental results indicated that phosphorus release is possible under both aerobic and anaerobic conditions. Aerobic release (0.025 mg P l⁻¹ over 5 days) was possible up to the point when mass resuspension of sediment occurred. Anaerobic release for the same period and mixing conditions was 0.183 mg Pl⁻¹.
4. The release rate under aerobic conditions at 10°C equates to an internal areal loading of 0.134 g P m⁻² yr^−1, which is approximately 17% and 30% of the average total phosphorus and orthophosphate loadings respectively for the period 1974–79.
5. The results clearly implicate aerobic inorganic phosphorus release from the epilimnetic sediments as a significant source of this nutrient to the overlying water column and is likely a major factor in the continuing eutrophic status in the lake.     

Can filter‐feeding fishes improve water quality in lakes?

1.  In this paper we examine the potential of a cichlid fish species ( Sarotherodon galilaeus ) to both maintain positive growth rates through filter-feeding on phytoplankton and improve water quality in Lake Kinneret through suppression of dinoflagellate ( Peridinium gatunense ) blooms.
2.  Seasonal plankton consumption by S. galilaeus from Lake Kinneret was examined experimentally by monitoring changes in plankton assemblages during 24 h in 5-m³ mesocosms containing varying densities of fish. Taxon-specific grazing rates ranged from 0 to 17 mg g_fish day^–1, with mean total consumption of 1.6% fish body weight per day. During the spring bloom of P. gatunense , S. galilaeus consumed mostly (94%) netphytoplankton (≥20 μm). The remaining 6% consisted mostly of nanophytoplankton (<20 μm). During the summer and fall, net- and nanophytoplankton accounted for 54 and 42%, respectively, of the diet of S. galilaeus . Zooplankton and flagellated and ciliated protozoans made up the remaining 4%.
3.  Simulations using a fish bioenergetics model indicated that consumption rates ( C ) were near maximum in spring (90% C _max), while consumption was reduced in summer-fall (59% C _max). Sarotherodon galilaeus obtains sufficient energy through filter-feeding year-round, although most growth (≥60%) occurs during the spring P. gatunense bloom.
4.  Despite efficient feeding on P. gatunense and nanophytoplankton by S. galilaeus , estimates of instantaneous plankton mortality caused by ingestion were two orders of magnitude lower than maximum potential plankton growth rates. Thus the potential for the S. galilaeus population in Lake Kinneret to positively affect water quality through algal suppression is low.     

The distances travelled by downstream-moving trout fry, Salmo trutta, in a Lake District stream

SUMMARY. 1. Field experiments were performed in the day and night at six modal water velocities (range 10–52cm s⁻¹), using: (i) newly- emerged fry without neutral buoyancy; (ii) older fry in poor condition (weight well below that expected for resident fry); (iii) older fry in good condition (weight similar to that of resident fry); (iv) dead fry.
2. An exponential model described the return rate of fry to the stream bottom; the mean distance travelled downstream varied considerably between the four fry categories, but always increased linearly with increasing water velocity.
3. Results were similar for dead fry and newly-emerged fry released at night; 50% of the fry returned to the bottom in 10–11 s and nearly all returned in c . 70s, the maximum distance travelled ranging from c . 7 m at 10 cm s⁻¹ to c . 37m at 52cm s⁻¹, Newly-emerged fry released in the day returned slightly faster (54s for 99% return to bottom).
4. Older fry in poor condition returned to the bottom slightly faster in the day than at night, but took about 2 min and travelled about twice the distance covered by dead fry. Older fry in good condition returned to the bottom at the fastest rate (3–6s for 50% and c . 30s for the rest), and travelled only about half (at night) or a third (in day) of the distance covered by dead fry.
5. The implications of this investigation are discussed and it is concluded that, apart from water velocity, the age and condition of the fry were the two most important factors affecting their downstream movement.     

Denitrification in river sediments: relationship between process rate and properties of water and sediment

1. A survey was made of denitrification and nitrous oxide (N₂O) production in river sediments at fifty sites in north‐east England during one season in order to investigate the relationship between rates and environmental factors likely to influence these processes. The sites were chosen to represent a wide range of physical and chemical conditions. Denitrification rate and N₂O production were measured within 5 h of sampling using the slurry acetylene blockage technique.
2. Denitrification rate ranged from less than 0.005–260 nmol N g^–1 DW h^–1, tending to increase in a downstream direction. N₂O production ranged from negative values (net consumption) to 13 nmol N₂O‐N g^–1 DW h^–1 and accounted for 0–115% of the N gases produced.
3. Denitrification rate and N₂O concentration in the sediment were correlated positively with nitrate concentration in the water column, water content of the sediment and percentage of fine (< 100 μm) particles in the sediment.
4. The variation in denitrification rate was satisfactorily explained (64% total variance) by a model employing measurements of water nitrate and water content of sediments. No simple or multiple relationship was found for N₂O production.     

Influence of host plant vs. natural enemies on the spatial distribution of a pine sawfly, Neodiprion autumnalis

Abstract .1. The pine sawfly, Neodiprion autumnalis , infests ponderosa pine, Pinus ponderosa , growing at low densities near the bottom of an altitudinal gradient in Arizona, U.S.A. The relative importance of host-plant quality vs. natural-enemy effects in determining the spatial distribution of this sawfly was examined over a 3-year period.
2. Field and laboratory bioassays were conducted on all life stages of N. autumnalis at two forest stand densities (high ≥ 23 m² ha^–1, low ≤ 7 m² ha^–1) and at two elevations (bottom slope = 2390 m, top slope = 2540 m). These experiments were used for constructing life tables of N. autumnalis that compared the effects of host-plant quality on oviposition preference and progeny performance with the effects of natural enemies at different tree densities and elevations.
3. Life-table analyses determined that mortality attributed to host-plant effects during the egg and larval stages had the largest impact on fitness between tree densities and elevations.
4. Natural enemies caused a significant reduction in progeny survival, but their effects were similar across all tree densities and elevations during egg and larval life stages. However, cocoon-stage survival did vary between tree densities and elevations due to natural-enemy effects.
5. It was concluded that the observed oviposition preference for, and higher progeny performance on, trees at low densities and bottom slope elevations were caused primarily by host-plant effects.
6. These results further the argument that heterogeneity at the resource level (i.e. bottom-up forces) determines potential outcomes of multitrophic level interactions.     

Orientation of carp, Cyprinm carpio L., to free amino acids from Tubifex extract in an olfactometer

We analysed the effects of L-amino acid combinations, tested at the same relative concentrations as in 500 mgl¹ of a filtrated Tubifex extract, on attraction and exploratory behaviour of juvenile carp, Cyprinus curpio L., in an olfactometer. Tubifex filtrate was significantly active in the range of concentrations tested (5 mgl⁻¹, 50 mgl⁻¹, 500 mgl⁻¹, 5 gl⁻¹). Maximum attraction was obtained in response to a concentration of 500 mg l⁻¹. Soluble TCA fraction and a mixture of the 17 synthetic amino acids found in this fraction also showed significant effects on attraction and exploratory behaviour. Tests of the four chemical groups of amino acids gave the following results: acidic amino acids (aspartic and glutamic) did not produce significant activity. Basic amino acids (histidine, arginine and lysine) and polar, uncharged amino acids (glycine, serine, threonine, tyrosine, asparagine and glutamine) were ineffective as attractants but significantly increased exploration. Non-polar amino acids (alanine, valine, leucine, isoleucine, phenylalanine, and methionine) showed significant effects on both attraction and exploration. In tests where different pairs of these amino acid groups were mixed it was shown that a combination of non-polar amino acids and polar uncharged amino acids was the most effective in inducing both attraction and exploration. The simplest combination to have a significant effect on both attraction and exploration was alanine, valine and glycine.
These results are discussed in the light of recent data on the role of free amino acids in the behaviour of fishes.     

The affect of temperature on the metabolism and behaviour of an endemic amphipod, Hyalella montezuma, from Montezuma Well, Arizona, U.S.A.

1. Hyalella montezuma is endemic to Montezuma Well, Arizona, and is exposed to minimal diel and seasonal temperature fluctuations in the pelagic zone (21 ± 4 °C). Juvenile H . montezuma feed in the pelagic zone during the day and migrate into the littoral vegetation at night, while adults remain primarily in the littoral vegetation.
2. Oxygen consumption ( V O₂) of adult and juvenile H . montezuma was measured at 20, 25 and 30 °C. The V O₂ of both adult and juvenile H . montezuma increased with temperature. However, the V O₂ of juveniles was significantly greater than that of adults at all temperatures, with greatest divergence at 30 °C where mean juvenile V O₂ (6.31 μl mg^–1 dry weight (DW) h^–1) was almost twice that of adults (3.60 μl mg^–1 DW h^–1).
3. Survivorship of juveniles was significantly lower (54%) at 30 °C than at 27.5 °C (95%) after 4 h, whereas adults showed at least a 93% survivorship at both temperatures.
4. Our data suggest that temperature may have been the proximate cue that elicited the diel horizontal migration of juvenile H . montezuma in Montezuma Well, with the behaviour maintained and enhanced by intensive invertebrate predation in the pelagic and littoral zones.     

The affect of temperature on the metabolism and behaviour of an endemic amphipod, Hyalella montezuma, from Montezuma Well, Arizona, U.S.A.

1. Hyalella montezuma is endemic to Montezuma Well, Arizona, and is exposed to minimal diel and seasonal temperature fluctuations in the pelagic zone (21 ± 4 °C). Juvenile H . montezuma feed in the pelagic zone during the day and migrate into the littoral vegetation at night, while adults remain primarily in the littoral vegetation.
2. Oxygen consumption ( V O₂) of adult and juvenile H . montezuma was measured at 20, 25 and 30 °C. The V O₂ of both adult and juvenile H . montezuma increased with temperature. However, the V O₂ of juveniles was significantly greater than that of adults at all temperatures, with greatest divergence at 30 °C where mean juvenile V O₂ (6.31 μl mg^–1 dry weight (DW) h^–1) was almost twice that of adults (3.60 μl mg^–1 DW h^–1).
3. Survivorship of juveniles was significantly lower (54%) at 30 °C than at 27.5 °C (95%) after 4 h, whereas adults showed at least a 93% survivorship at both temperatures.
4. Our data suggest that temperature may have been the proximate cue that elicited the diel horizontal migration of juvenile H . montezuma in Montezuma Well, with the behaviour maintained and enhanced by intensive invertebrate predation in the pelagic and littoral zones.