The biological degradation of arginine and glutamic acid in seawater in relation to the growth of phytoplankton

The responses of natural summer coastal plankton communities to low-level additions (10^-5 to 10^-7 M) of arginine and glutamic acid has been followed by in vivo measurement of chlorophyll fluorescence. This technique is capable of detecting a response to a 10^-6 M enrichment under most conditions. The time sequence of the response varied with the amino acid used and with the enantiomeric form of the amino acid. Ammonia and carbon dioxide were liberated before the increase in chlorophyll fluorescence occurred. Liberation of ammonia in a dark bottle from L-arginine was from 75 to 85% of the theoretical yield. Microautoradiography using ¹⁴C L-arginine or L-glutamic acid at 10^-7 M showed heavy labeling associated with fecal pellets and detrital aggregates. Phytoplankton cells were not appreciably labeled. The evidence suggests that bacteria are important in the cycling of these compunds.     

Allelopathic inhibition on red tide microalgae Skeletonema costatum by five macroalgal extracts

This study aims to identify effective antialgal allelochemicals from marine macroalgae that inhibit the growth of red tide microalgae. Practically, new algicidal agents were developed to control red tide. The growth inhibitory effects of 5 marine macroalgae Porphyra tenera, Laminaria japonica, Ulva pertusa, Enteromorpha clathrata, and Undaria pinnatifida on Skeletonema costatum were evaluated by adding crude seawater extracts of macroalgal dry tissue into the culture medium containing S. costatum. The half-effective concentrations at 120 h (EC_{50, 120 h}) of the seawater extracts were 0.6, 0.9, 1.0, 1.0, and 4.7 g/L for the five macroalgae above, respectively. E. clathrata, L. japonica and U. pertusa showed strong allelopathic effect on the growth of S. costatum. There have been no previous reports with regard to the allelopathic effects of the former two macroalgae so far. The possible allelochemicals of 21 compounds of the E. clathrata were detected using Gas chromatography-mass spectrometry (GC-MS) analysis. Unsaturated fatty acids, acrylic acid (C₃H₄O₂), and linolenic acid (C₁₈H₃₀O₂) were the most likely allelochemicals in E. clathrata.     

High rates of nitrogen fixation on coral skeletons after predation by the crown of thorns starfish Acanthaster planci

At One Tree Reef, Great Barrier Reef, Australia, between 1983 and 1985, corals killed by the crown of thorns seastar Acanthaster planci L. gave rise to skeletons which were colonised rapidly by blue-green and other algae. For the next 3 to 9 mo these coral skeletons showed over three times more nitrogen fixation (acetylene reduction) than control substratum rates (9 to 32 nmol vs 3 to 10 nmol C₂H₂ cm^-2 h^-1, over all seasons). These values convert to relatively high annual fixation rates of 37 to 127 kg N ha^-1 yr^-1 but, at the low densities of A. planci on One Tree Reef (ca. 0.65 ha^-1), this has little impact on the total nitrogen fixation rate and, as a result, on the level of organic nitrogen in the system. However, it is suggested that on reefs subjected to high aggregations of a. planci such an effect would enhance the level of organic nitrogen and lead to greater primary and secondary production throughout the reef system.     

Comparative study of vanadium biokinetics in three species of echinoderms

Vanadium-48 (as vanadate) was used to study the uptake, tissue distribution, depuration and food-chain transfer of vanadium through 3 species of echinoderms: the seastar Marthasterias glacialis L., the sea urchin Paracentrotus lividus Lmk. and the holothurian Holothuria forskali D.Ch.; all were collected from the littoral zone near Monaco. Uptake by all species was relativelyslow; after 3 wk exposure, isotopic equilibrium had not been reached and whole-body concentration factors ranged from 5 and 7 in the holothurian and sea urchin, respectively, to 18 in the seastar. Sixty-three to 77% of the incorporated radiotracer was associated with the body wall or test, suggesting surface sorption as the principal mechanism governing uptake from water. Stable vanadium measurements confirmed the preponderance of this element in the external hard parts of the echinoderms; however, concentration factors based on stable vanadium levels were significantly higher than those measured experimentally. Subsequent vanadium depuration rates were also species-dependent, with biological half-times for loss ranging from approximately 50 d in the sea urchin and holothurian to 123 d in the seastar. Food-chain transfer experiments indicated that seastars can assimilate and retain a large fraction of the vanadium ingested with food whereas sea urchins appear to lack this capability. The relative importance of the water and food input pathway in achieving vanadium levels in echinoderms is discussed in light of results of ⁴⁸V distribution in experimental individuals and stable vanadium distribution in samples from the natural environment.     

Escape responses of four Chilean intertidal limpets to seastars

The Chilean intertidal Lottiidae limpet fauna consists of at least nine species, grouped into the monophyletic taxon Scurria and a single taxon preliminarily designated as `Lottia' orbignyi (Dall). Within the Scurria clade, the most basal species are Scurria variabilis (Sowerby), S. viridula (Lamarck), and S.zebrina (Lesson), and among the most crown groups are S. ceciliana (Orbigny) and S. araucana (Orbigny). This phylogenetic information, obtained by the analysis of molecular characters (16S mtDNA), provides a framework for a comparative study of behavioral characters. In this study, we analyze the escape responses of two basal limpet species (`L.' orbignyi and S. viridula) and two derived limpet species (S. araucana and S. ceciliana) in the presence of the limpet predatory seastar Heliaster helianthus (Lamarck) and the limpet non-predatory seastars Stichaster striatus Muller and Troschel and Patiria chilensis Lutken. Neither P.chilensis nor Stichaster striatus induced escape responses such as those observed with the predatory H. helianthus. Moreover, in the presence of H. helianthus, basal and derived limpet species differed significantly in the percentage of individuals responding, reaction time, and duration of the response. The basal species `L.' orbignyi and Scurriaviridula exhibited instantaneous and vigorous locomotor responses, whereas the derived species S. araucana and S. ceciliana displayed no locomotor responses. The same pattern was found in responses to H. helianthus homogenates. The results strongly suggest a correlation between phylogeny and anti-predator escape responses to the seastar H. helianthus. Alternative escape responses have evolved for basal and derived species within the Chilean clade. Received: 27 April 2000 / Accepted: 16 August 2000     

Avoidance behaviour of Asterias rubens to extracts of Mytilus edulis,solutions of bacteriological peptone,and selected amino acids

The effect of some homogenates, peptones and amino acids upon the positive rheotactic behaviour of Asterias rubens L. is studied. It was found that homogenates of Mytilus edulis, solutions of bacteriological peptone L 37, L-glutamic acid and a mixture of 10 common amino acids acted as repellents and altered the positive rheotactic behaviour of A. rubens. The experimental evidence suggests that the active compound(s) of the homogenates of M. edulis and of the solution of L 37 is a heat stable substance whose molecular weight is lower than ca. 30,000. It was found that A. rubens sensed L-glutamic acid in concentrations of the part per million range. Sinergistic interaction was found between L-glutamic acid and a mixture of one or more other 9 common amino acids. It is shown that the sensitivity of A. rubens to one of the repellents, bacteriological peptone L 37, changes during the year. Among other factors, seasonal changes in sea-water temperature could account for this.     

Intracellular localization and binding of technetium-95m in the seastar Marthasterias glacialis

The subcellular distribution of technetium was studied using the seastar Marthasterias glacialis which had previously ingested prey labelled with ^95mTc either as pertechnetate (VII) or in the reduced state (IV). The majority of the Tc [98% (IV) and 96% (VII)] was taken up and retained in the pyloric caeca, the gland responsible for digestive and food storage processes. Differential centrifugation and separation techniques have demonstrated that, at the subcellular level, technetium in the pyloric caeca was largely associated with the lysosomes. ^95mTc (IV) was distributed between the lysosomes and the soluble fraction to roughly the same degree, whereas pertechnetate was more strongly associated with the lysosomal fraction. Chromatography of the cytosol by gel filtration indicated the presence of two distinct protein compounds which were responsible for binding virtually all of the technetium in the soluble fraction. Technetium had a greater relative affinity for the low molecular weight compound, more so for Tc (VII) than for Tc (IV). This anionic protein had an apparent molecular weight of approximately 10 000, and thus may play a role in metal detoxification mechanisms at the cellular level.     

Sterols in coral-reef animals

The sterol constituents of both coral and non-coral-reef animals (mollusks, coelenterates, echinoderms) have been investigated, and determined by gas-liquid chromatography and spectral analyses. All coral-reef animals examined contained larger amounts of C₂₈-sterols, such as 24-methylcholesterol, than non-coral-reef animals. The seastar Acanthaster planci contains the same sterol constituents as some coral-reef coelenterates and mollusks, differing only in the position of the double bond in a steroid ring. This observation suggests the transport of dietary ⁵-sterols to ⁷-sterols through the food-chain in A. planci.     

Responses of estuarine crab megalopae to pressure,salinity and light: Implications for flood-tide transport

The megalopal larval stage of many estuarine brachyuran crabs appears to return to adult habitats by undergoing rhythmic vertical migrations which result in saltatory up-estuary transport on flood tides. Larval ascent into the water column during rising tides may be cued by changing hydrologic variables. To test this hypothesis, we investigated the responses of field-caught megalopae of the blue crab Callinectes sapidus and the fiddler crab Uca spp. to constant rates of pressure and salinity change under laboratory conditions. For both genera, pressure changes resulted in increased movement (barokinesis) and upward migration in the test chamber, with C. sapidus megalopae having a lower response threshold (2.8×10^-2 mbar s^-1) than Uca spp. larvae (5×10^-2 mbar s^-1). Similarly, larvae ascended in response to increasing salinity, with C. sapidus larvae being more sensitive. Larvae were negatively phototactic and failed to respond to pressure increases at light levels above 1.0×10¹⁵ and 1.0×10¹³ photons m^-2 s^-1 for C. sapidus and Uca spp. megalopae, respectively. Such responses are thought to explain the low abundances of larvae in the water column during daytime flood tides. Nevertheless, threshold sensitivities to increasing pressure for both genera were above levels experienced during floodtide conditions in the field. Similarly, it is unlikely that increasing salinity is sufficient to induce ascent in Uca spp. postlarvae. However, rates of salinity increase during midflood tide typically reach levels necessary to induce an ascent in C. sapidus megalopae. These results are consistent with the hypothesis that fiddler crab megalopae utilize an endogenous activity rhythm for flood-tide transport, while blue crab megalopae rely upon external cues, especially salinity changes, to time their sojourns in the water column.     

Acute toxicity of 30 pharmaceutically active compounds to freshwater planarians,Dugesia japonica

Pharmaceutically active compounds are produced worldwide and consumed in large quantities, so these chemicals are frequently detected in limnic environments posing potential ecological risks. Thirty pharmaceutically active compounds were selected for examination of their acute toxicity for freshwater planarians (Dugesia japonica). Among the 30 compounds tested, diclofenac, mefenamic acid, naproxen, propranolol HCl, and diphenhydramine HCl had a 48-h nominal LC₅₀ below10 mg L^?1, and for 18 chemicals, it exceeded 100 mg L^?1. The 96-h nominal LC₅₀ was below 10 mg L^?1 for diclofenac, mefenamic acid, naproxen, propranolol HCl, diltiazem HCl, diphenhydramine HCl, hydroxyzine HCl, and triprolidine HCl, and for 15 chemicals, it exceeded 100 mg L^?1. Among different therapeutic groups, analgesics were most toxic to planarians, and antibiotics were least toxic. Antihistamines and beta blockers varied in their acute toxicity to planarians. At the current environmental levels, none of the tested pharmaceutically active compounds may have acutely harmful impacts on aquatic invertebrates. To answer the question whether chronic, long-term exposure to pharmaceutically active compounds may entail ecological risks for aquatic ecosystems, further investigations with different end points in multiple species tests are needed.     

Energetic costs and efficiencies of ciliary filter feeding

The relationship between activity of the lateral cilia and oxygen consumption was studied in fragments of the gills of Mytilus edulis L., and the effects of stimulation with the nerve transmitters serotonin and dopamine were measured. Beat frequency, f, and oxygen consumption, M_{O 2}, increased with increasing doses of the transmitters. Serotonin increased f up to 26 Hz and the relationship with M_{O 2} could be described by M_{O 2} (l cm^-2 gill area h^-1)=0.44+0.35 f (r=0.99). The relationship after dopamine stimulation did not differ from that established by serotonin, but the maximum value of f was only 19 Hz. At the frequency of 10 Hz, characteristic of undisturbed pumping, the cells carrying the lateral cilia were estimated to account for one third of the total oxygen consumed, equivalent to 1.05×10^-10 mol ATP cm^-2 s^-1. This rate of synthesis of ATP was compared with the rate at which the active cilia hydrolyse ATP. ATP hydrolysis ranged from 58 to 87% of synthesis as the assumed ratio between number of ATP molecules hydrolysed per beat and number of dynein molecules ranged from 2 to 3. The rate at which ATP is hydrolysed by the active cilia was compared with the work done in pumping water. At two estimates available of the pumping pressure, 1.4 and 2.5 mm H₂O, the work amounted to 1.4×10^-6 or 2.5×10^-6 joule cm^-2 s^-1, equivalent to 2.9×10^-11 and 5.2×10^-11 mol ATP cm^-2 s^-1. The efficiencies of work thus corresponded to 48 or 86% at the ATP/dynein ratio of 2. The overall efficiencies of the mussel pump amounted to 18 or 32%.     

Depression of feeding and growth rates of the seastar Evasterias troschelii during long-term exposure to the water-soluble fraction of crude oil

To test the effect of petroleum hydrocarbons on predation by the seastar Evasterias troschelii (Stimpson, 1862) on the mussel Mytilus edulis (L.), we exposed the predator with the prey to six concentrations of the water-soluble fraction (WSF) of Cook Inlet crude oil. Seastars and mussels were collected at Auke Bay, Alaska, in November 1980. During a 28 d exposure in a flow-through system, seastars were more sensitive to the WSF than mussels: the LC₅₀ for the seastars was 0.82 ppm at Day 19 and, although no mussels were exposed to WSF for more than 12 d, none died. Daily feeding rates (whether in terms of number of mussels seastar^-1 d^-1 or dry weight of mussels seastar^-1 d^-1) were significantly reduced at all concentrations above 0.12 ppm. At 0.20, 0.28 and 0.72 ppm WSF, daily feeding rates (in terms of dry weight of mussels) were, respectively, 53, 37, and 5% of the control rate; at the two highest concentrations (0.97 and 1.31 ppm WSF), the seastars did not feed. Seastars at concentrations greater than 0.12 ppm WSF grew slower than individuals from the control group and the 0.12 ppm-treatment group combined. These laboratory results show that E. troschelii is more sensitive to chronic low levels of the WSF of crude oil. The possibility that such oil pollution could reduce predation and permit M. edulis to monopolize the low intertidal zone of southern Alaska remains to be studied.     

Cycling of selenite and selenate in marine phytoplankton

The marine phytoplankton Dunaliella tertiolecta, Cachonina niei, Thalassiosira nordenskioldii, Phaeodactylum tricornutum, and Chaetoceros sp. were incubated with a range of molar concentrations of sodium-selenite (Na₂-SeIVO₃) and sodium-selenate (Na₂-SeVIO₄) to examine further their role in metabolic cycling of selenium in ocean waters. At low selenium concentrations, approaching those found naturally in seawater (10^-10 to 10^-9 M), all species distinguished between selenite and selenate, and actively concentrated selenite from the incubating medium while only marginally accumulating selenate. At much higher concentrations (10^-8 to 10^-6 M), selenate was also taken up. At the highest concentration tested, i.e., 10^-5 M with C. niei, after an immediate rapid uptake in the first 24 h, the intracellular selenite and selenate levels dropped to about 35 to 50% of the initial peak values. These observations suggest an uptake mechanism in these algae which, at normal ambient concentrations of selenium (10^-9 M), preferentially selects selenite and excludes selenate. At much higher concentrations (10^-8 M), the mechanism becomes overloaded and both selenium species enter the cells. Intracellularly, selenite became associated primarily with protein and amino acid fractions, in approximately equal proportions, while only ca. 4% of total intracellular selenium was found in the lipid fraction. Trace amounts of selenate that entered the cells, mainly during the first minutes of exposure, also entered the protein and amino acid components, but over time were increasingly associated with the protein fraction only. At the end of a 10-d incubation of algal cells in selenite-spiked medium, less than 25% of total Se in the medium could in fact be identified analytically as selenite. This suggests the presence of a non-selenite metabolite, possibly released back into the medium from the algae.     

Effect of low salinity on the activity,feeding, growth and absorption efficiency of Luidia clathrata (Echinodermata: Asteroidea)

Exposure of Luidia clathrata (Say) to low salinity (17 S) results in a decrease in activity and rate of feeding, but has little effect on the absorption efficiency. Growth, as indicated by changes in body size and energy content of the body wall and pyloric caeca, is reduced in low salinity due to a decrease in feeding rate (energy consumption) and to a decrease in efficiency of utilization of material and energy absorbed. The low efficiency indicates that either additional energy is required for maintenance or energy is used inefficiently in low salinity. Thus, L. clathrata is limited energetically in low salinities, although it lives and reproduces there.     

In vitro kinetic characterization of inhibition of acetylcholinesterase by organophosphate and carbamate compounds in food animals

Organophosphate, dichlorvos (2,2-dichlorovinyldimethylphosphate, DDVP), and diazinon (DZN) as well as carbamates are currently found in contaminated water, dust, soil, agricultural waste, and elsewhere in the environment, and can be harmful after accidental or deliberate exposure. Intoxication by these compounds causes a generalized cholinergic crisis due to the inhibition of acetylcholinesterase (AChE), whose major physiological role in mammalian tissues is in nervous transmission. The half-maximal inhibitory concentration (IC₅₀) was found to be higher for carbaryl than DDVP and DZN, about 3–4 times in liver and about 4–6 times in muscles. The half time (t _1/2) of inhibition (8?µmol?L^?1 DDVP or DZN; 40?µmol?L^?1 carbaryl) ranged between 4 and 16?min and decreased as follows: pig?>?cattle?>?sheep for liver tissue, and cattle?>?sheep?>?pig for muscle. Rate constants of inhibition (k _i) ranged between 43?×?10^?3 and 168?×?10^?3?min^?1 for liver and from 46?×?10^?3 to 115?×?10^?3?min^?1 for muscles. Very little residual AChE activity (<6.1%) was seen in liver, but more was seen in muscle (<17%).     

Biomarkers as tools to assess the chronic toxicity of ammonia in the juvenile Mugil cephalus

With juvenile fish as the subject, the effects of low concentration ammonia on antioxidant system were studied using Mugil cephalus. Samples of gill and liver tissue were obtained from 0.35, 0.70, 1.5 and 3?mg/L ammonia groups at 0, 5, 10, 15 and 20 days of exposure, at which times the biomarkers were measured. Results showed that gill malondialdehyde (MDA) content exhibited an initial significant increase (p?≤?0.05) at unionised ammonia concentrations of 0.70, 1.5 and 3.0?mg/L on day 5, followed by subsequent declines, while liver MDA levels exhibited significant increases (p?≤?0.05) at unionised ammonia concentrations of 1.5?mg/L starting on day 10 and at 3.0?mg/L starting on day 5. With exposure to ammonia at different concentrations, Na⁺-K⁺-ATPase activity in liver and gill decreased over time. The Na⁺-K⁺-ATPase activity was negatively related to ammonia concentration from 0.70 to 3.0?mg/L. Overall, our results show that MDA and Na⁺-K⁺-ATPase, evaluated here as potential biomarkers of ammonia exposure, exhibited responses to sublethal concentrations of ammonia that were concentration dependent.     

The nature of zinc and copper complexes in the oyster Ostrea edulis

The zinc and copper associated with the soft tissues of the oyster Ostrea edulis Linnaeus have been separated into a soluble component and a tissue-residue, cell-debris bound component. In the case of zinc, the tissue-bound component was found to contain at least two species of complex; a firmly-bound species, exchangeable with ⁶⁵Zn²⁺ and a less-firmly, reversibly-bound species, exchangeable with ⁶⁵Zn²⁺. The soluble component, which constitutes some 40% of the total zinc and copper, was fractionated on Sephadex G-25 and the zinc and copper shown to be weakly-complexed to the small molecular weight compounds, taurine, lysine, ATP and possibly homarine (N-methyl--picolinic acid) and to be fully exchangeable with ⁶⁵Zn²⁺. These soluble complexes can act as a freely available mobile reserve of metal to ensure a constant saturation of metal-dependent enzyme systems operating under adverse environments. Sephadex G-25 acts as a weak ion-exchange resin, which can cause a translocation of zinc and copper from its soluble weak complexes and result in the spurious association of the metals with other compounds.     

The role of asteroid predators in the organization of a sessile community on pier pilings

The diets of 4 species of asteroids were observed for `9 months on the piles of a pier at Rapid Bay, South Australia.Coscinasterias calamaria (Gray) fed on molluscs, crustaceans and moribund items.Patiriella brevispina H.L. Clark fed on compound ascidians, sponges and moribund items.Tosia australis Gray andPetricia vernicina (Lamarck) ate mainly sponges, bryozoans and compound ascidians. A short-term (6 months) caging experiment showed that 5 common species of sessile animals-3 sponges and 2 compound ascidians-were significantly reduced by seastar predation. Nevertheless, the amount eaten, considered with seastar densities and distributions, indicates that the seastars are unimportant in influencing the utilization of space by sessile fauna. From comparing caged and uncaged controls, we tentatively suggest that other predators are also unimportant, and that competition for substrate space and for access to the water column is (and, has been during the evolution of these sessile organisms) far more influential.     

An energy budget for the free-swimming and metamorphosing larvae of Balanus balanoides (Crustacea: Cirripedia)

Analysis of biochemical components and measurements of oxygen consumption rates of cypris larvae of Balanus balanoides (L.) maintained in the laboratory at 10°C for up to 5 weeks after capture shows that lipid is the primary energy reserve, although later protein is utilised. Initially, the cyprids swim freely with an oxygen consumption rate of ca. 37×10^-3 l O₂ h^-1 cyprid^-1, but within a few days the rate falls to ca. 21×10^-3 l O₂ h^-1 cyprid^-1 when they cease swimming and explore the substratum. The cost of metamorphosis was calculated both from the loss of biochemical components and oxygen consumption rates during metamorphosis; the values were 2.8×10^-2 and 3.2×10^-2 cal cyprid^-1, respectively. A budget was collated from the data on respiration and biochemical composition, whereby the energy per cyprid was partitioned into that required for essential structural components (6.8×10^-2 cal), that needed for metamorphosis (3.0×10^-2 cal) and an excess available for swimming and exploring, which in the batches studied was about 5.0×10^-2 cal. This excess is mainly derived from the utilisation of lipid reserves and is used up usually 2 1/2 to 4 weeks after capture. During these measurements, samples of cyprids were taken at weekly intervals to test the rate of settlement and success of metamorphosis. The results showed that they lose their competence to metamorphose successfully approximately at the same time (3 to 4 weeks) that the energy supply for swimming and exploration is used up.     

Heterotrophic nitrogen fixation (acetylene reduction) during leaf-litter decomposition of two mangrove species from South Florida, USA

Heterotrophic nitrogen-fixation (acetylene reduction) was measured during decomposition (under dark conditions) of Rhizophora mangle L. and Avicennia germinans (L.) Stearn leaf litter. Nitrogen-fixation rates in leaf litter increased following 24 d incubation, then decreased after ≃44 d for both species. Maximum rates of 66.2 and 64.6 nmol C₂H₄ g⁻¹ dry wt h⁻¹ were reached by R. mangle and A. germinans leaf litter, respectively. Higher fixation rates of leaf litter were associated with an increase in water content and sediment particles on leaf surfaces of both species. Rates of nitrogen fixation by diazotrophs attached to sediment particles were not significantly different from zero. With additions of d-glucose, ethylene production rates increased by factors of 625-, 34- and 7-fold for sediment, R. mangle and A.␣germinans leaf litter, respectively, compared to rates prior to enrichment. These organically enhanced rates of nitrogen fixation on leaves could be accounted for by increased activity associated with attached sediment particles and not the leaf material. Total phenolics [reported as tannic acid equivalent (TAE) units] decreased nitrogen-fixation rates when added to d-glucose-enriched sediment at >20 mg TAE l⁻¹. Phenolic compounds could explain the initial lag in rates of nitrogen fixation during leaf-litter decomposition of R. mangle (initial content of 110.8 mg TAE g⁻¹ dry wt), but not of A. germinans (initial content of 23.4 mg TAE g⁻¹ dry wt). The higher phenolic content and reportedly lower carbon substrate of R. mangle did not result in species-specific differences in either the magnitude or temporal pattern of nitrogen fixation compared to A. germinans leaf litter. We conclude that the availability of organic substrates leached from the leaf litter along with colonization by the heterotrophic diazotrophs (as indicated by sediment accumulation) controls nitrogen-fixation rates in a similar manner in the leaf litter of both species. Received: 8 August 1997 / Accepted: 4 December 1997