Fecal Pellet Production Research Articles

New Insights into Polychaete Traces and Fecal Pellets: Another Complex Ichnotaxon?

Neoichnological observations help refine paleoichnological records. The present study reports extensive observations on the distribution, morphology, occurrence and association of burrows and fecal pellets of the polychaete Nereis diversicolor in the Kundalika Estuary on the west coast of India. Our holistic study of these modern-day traces suggests it to be a complex trace arising from domichnial, fodinichnial and possibly pascichnial behavior of polychaetes. The study for the first time reports extensive fecal pellet production, distribution and their preservation as thick stacks in modern estuarine environment. These observations testify the fossilization potential of pellets and provide an explanation to their origin in the geological record. Their occurrence as strings associated with mounds not only suggests pascichnial behaviour of polychaetes but also allows the assignment of post-Paleozoic Tomaculum to the activity of polychaete worms. The production of fecal pellets in such large quantities plays a major role in increasing the average grain size of the substrate of these estuarine tidal flats, thereby improving aeration within the substrate.

Size as the master trait in modeled copepod fecal pellet carbon flux

AbstractZooplankton fecal pellet flux is a highly variable component of the biological carbon pump. While fecal pellets can comprise 0 to nearly 100% of particulate organic carbon collected in sediment traps, mechanisms for this variability remain poorly understood. Fecal pellet carbon flux is a complex function of several variables. We present a model that incorporates individual‐scale metabolic processes to determine fecal pellet production rate, the relationship between body size and fecal pellet size, the relationship between fecal pellet size and sinking rate, and a function representing the breakdown of particles in the water column. When applied to copepod communities sampled by the continuous plankton recorder in the Gulf of Maine over 25 years, a seasonal pattern of fecal pellet carbon flux emerges. The interannual flux time series produced by the model reflects known oceanographic perturbations and shows how organism‐scale processes can be scaled up to explain ecosystem level variability. We conclude that fecal pellet carbon flux in the Gulf of Maine is driven by copepod community size structure and copepod abundance, and that the fraction of fecal pellet carbon that reaches depth is a function of copepod size, rather than abundance. Changes in the physical environment which alter the size composition of the copepod community lead to variability in fecal pellet carbon flux. Our results indicate that incorporating size composition into biogeochemical models can more accurately constrain zooplankton‐mediated carbon flux.

Copepod summer grazing and fecal pellet production along the Western Antarctic Peninsula

Copepods are important grazers on phytoplankton and contributors to carbon export, but their role is poorly understood in the Western Antarctic Peninsula (WAP), a region of high productivity and rapid climate warming. We conducted grazing and egestion experiments with large, dominant copepods each January from 2012 to 2014. We found higher gut evacuation rates (k), initial gut pigment and ingestion rates (I) for Calanus propinquus and Rhincalanus gigas compared with Calanoides acutus. Since k and I linearly increased with chlorophyll a for most species, ingestion rates were 4–70 times greater in more productive coastal regions than offshore, slope waters. Copepods have a low grazing impact on phytoplankton biomass (<1%) and productivity (1%, up to 11%) compared with the dominant WAP macro- and microzooplankton. Egestion rates were high (0.8–37.3 µgC ind.−1 day−1); however, ∼58% of fecal pellets are retained in the upper water column. Daily carbon rations of ∼1% indicated feeding on other carbon sources (protozoans and metazoans) to meet metabolic demands. However, during a coastal phytoplankton bloom, daily C rations increased to up to 13%, indicating increased reliance on phytoplankton. Future changes in the WAP plankton community may affect food web carbon flow and export.

Domestication as a Novel Approach for Improving the Cultivation of Calanoid Copepods: A Case Study with Parvocalanus crassirostris.

Calanoid copepods are an important food source for most fish larvae. Their role as a natural prey item means that it is important to develop culture technology for copepods to meet the requirements of larvae culture in aquaculture hatcheries. Copepods have been cultured successfully for some time; however, the implications of long-term cultivation or domestication on copepod life history traits have not yet been assessed. Therefore, the aim of this study was to determine if the domesticated and wild populations of Parvocalanus crassirostris are phenotypically or physiologically different. Wild-caught P. crassirostris were compared to a long-held domesticated strain (>2 years) for size of developmental stage, fecundity and lifespan of adult females, culture density, ingestion rates, faecal pellet production and fatty acid profiles. The domesticated strain was significantly different from the wild strain in size (eggs, nauplii, copepodites and adults were larger in the domesticated strain), egg production (112.3 ± 1.8 eggs female-1 vs. 64.6 ± 3.3 eggs female-1) and adult female lifespan (8.8 ± 0.1 days vs., 7.5 ± 0.1 days). At 1, 3 and 5 adults mL-1, the domesticated strain performed significantly better than the wild strain in egg production (4189.8 ± 61.2, 11224.0 ± 71.7 and 21860.6 ± 103.6 eggs vs. 1319.5 ± 54.3, 2374.5 ± 80.9 and 4933.8 ± 269.5 eggs, respectively) and mean daily mortality rate (5.6% across all densities vs. 22.9 ± 1.6, 29.8 ± 1.2 and 31.3 ± 1.3%, respectively). The domesticated strain had significantly higher ingestion rates than the wild stain (888.4 ± 9.9 ng C l-1 and 775.3 ± 11.2 ng C l-1, respectively), while faecal pellet production was not significantly different between strains. Fatty acid profiles indicated higher levels (as % of total fatty acid) of long-chain polyunsaturated fatty acids in the domesticated strain (36.2±0.4%) than the wild strain (16.1±0.1%). Overall, this study found that the reproductive capacity and tolerance to the culture environment of the calanoid P. crassirostris have improved significantly due to domestication.

Resistance of modified wood to marine borers

The resistance of differently modified wood to the common shipworm, Teredo navalis, and the wood boring crustacean, Limnoria quadripunctata, was assessed in a field trial and by means of a short term laboratory assay, respectively. Scots pine (Pinus sylvestris) sapwood was treated with TEOS (tetra-ethoxy-ortho-silane) and different thermosetting resins, namely phenol formaldehyde (PF) and methylated melamine formaldehyde (MMF). Additionally, acetylated and untreated Radiata pine (Pinus radiata) was included. In the field trial according to EN 275 in the Baltic Sea over a period of six years the specimens were exclusively attacked by T. navalis. For the laboratory assay, matchstick-sized samples cut from spare panels prepared for the field trial were subjected to individuals of L. quadripunctata; faecal pellet production served as a measure of feeding rate. Treatments that prevented shipworm attack in the field also reduced feeding of L. quadripunctata in the laboratory assay: efficacy of resin treatments was enhanced by parameters that increase the amount of resin in the cell wall (i.e. high WPG and dry curing conditions); acetylation resulted in high resistance; and TEOS treatment was not effective. The results suggest that modification on cell wall level is required to impart marine borer resistance.

Wood Warbler Phylloscopus sibilatrix nest provisioning rates are correlated with seasonal caterpillar availability in British Oak Quercus woodlands

Capsule Maximum provisioning rates occurred during the seasonal peak in caterpillar availability, controlling for brood age and time of day.Aims To determine if the seasonal availability of caterpillars influenced Wood Warbler Phylloscopus sibilatrix nest provisioning rates.Methods Remote cameras were used to monitor nest visits at 14 nests in 8 British oak Quercus woodlands during 2013. The timing of the caterpillar peak was estimated by monitoring the production of caterpillar faecal pellets under oak trees.Results Maximum provisioning rates occurred when the date of provisioning coincided with the date of peak caterpillar availability, but the percentage change in provisioning rates during asynchrony was dependent on brood age. Younger broods experienced a greater decline in provisioning rates than older broods during asynchrony. Provisioning rates peaked in the morning and declined during the day, but when caterpillar availability was low there was little within-day variation.Conclusion Provisioning rates show seasonal variation that is correlated with caterpillar availability. Previous studies suggest that Wood Warbler reproductive fitness is independent of synchrony between the breeding cycle and the caterpillar peak, however, the implications for adult fitness are unknown and should be investigated further.

The contribution of zooplankton faecal pellets to deep-carbon transport in the Scotia Sea (Southern Ocean)

Abstract. The northern Scotia Sea contains the largest seasonal uptake of atmospheric carbon dioxide yet measured in the Southern Ocean. This study examines one of the main routes by which this carbon fluxes to the deep ocean: through the production of faecal pellets (FPs) by the zooplankton community. Deep sediment traps were deployed at two sites with contrasting ocean productivity regimes (P3, naturally iron-fertilized, and P2, iron-limited) within the same water mass. The magnitude and seasonal pattern of particulate organic carbon (POC) and FPs in the traps was markedly different between the two sites. Maximum fluxes at P3 (22.91 mg C m−2 d−1; 2534 FP m−2 d1) were 1 order of magnitude higher than at P2 (4.01 mg C m−2 d−1; 915 FP m−2 d1, with flux at P3 exhibiting a double seasonal peak, compared to a single flatter peak at P2. The maximum contribution of FP carbon to the total amount of POC was twice as high at P3 (91%) compared to P2 (40%). The dominant FP category at P3 varied between round, ovoidal, cylindrical and tabular over the course of the year, while, at P2, ovoidal FPs were consistently dominant, always making up more than 60% of the FP assemblage. There was also a difference in the FP state between the two sites, with FPs being relatively intact at P3, while FPs were often fragmented with broken peritrophic membranes at P2. The exception was ovoidal FPs, which were relatively intact at both sites. Our observations suggest that there was a community shift from a herbivorous to an omnivorous diet from spring through to autumn at P3, while detritivores had a higher relative importance over the year at P2. Furthermore, the flux was mainly a product of the vertically migrating zooplankton community at P3, while the FP flux was more likely to be generated by deeper-dwelling zooplankton feeding on recycled material at P2. The results demonstrate that the feeding behaviour and vertical distribution of the zooplankton community plays a critical role in controlling the magnitude of carbon export to the deep ocean in this region.

Thermal response of ingestion and egestion rates in the Arctic copepod Calanus glacialis and possible metabolic consequences in a warming ocean

Climate change is particularly rapid in the Arctic, where water temperatures are predicted to increase substantially with implications for Arctic marine organisms, especially ectotherms such as the calanoid copepod Calanus glacialis, a key herbivore in the Arctic marine ecosystem. Feeding depends on temperature, and recent studies indicate different thermal responses in ingestion and respiration implying a possible metabolic mismatch with increasing temperatures. We investigated the thermal response of ingestion and faecal pellet production as an indicator of egestion of the Arctic copepod C. glacialis in incubation experiments at five temperatures ranging from 0 to 10 °C and compared the obtained data with published results on temperature dependence of respiration. Copepods were fed ad libitum with the diatom Thalassiosira gravida, and algae concentration was assessed prior and after 4 h feeding experiments. Egested faecal pellets were collected and counted. Ingestion and faecal pellet production rates increased linearly (Q 10 coefficient ~1.4–1.7 and ~1.8–4.1, respectively). No pronounced effect of feeding history (fed vs. starved for 3 days prior to experiment) was found, but responses in both rates were generally less dependent on temperature in the pre-starved experiment. Q 10 values for ingestion rates were lower than Q 10 values for published respiration rates (~1.8–4.6), indicating that metabolic losses increase stronger with increasing temperature than metabolic gains by ingestion. A persistent imbalance between metabolic losses and energy uptake could lead to reduced fitness for C. glacialis, thereby affecting the population dynamics and distribution of this important species in the Arctic.

Degradation of discarded appendicularian houses by oncaeid copepods

AbstractWe examined the degradation of discarded appendicularian houses due to feeding by oncaeid copepods in surface waters of the Kuroshio region. Vertical distributions of oncaeid copepods and discarded houses were investigated by optical sampling using a video plankton recorder (VPR). In situ VPR observations revealed that oncaeid copepods were commonly associated with discarded houses: the proportion of individuals attached to discarded houses reached saturated values of 25‐39% at house concentrations above 2000 houses m−3, suggesting that feeding by oncaeids was not limited by encounters at house concentrations typically observed in the upper 50 m. Laboratory observations confirmed that oncaeid copepods attached to the surface of discarded houses and ingested both house membranes and adhering particles. Individual feeding rates of the dominant Oncaea species, O. venusta and O. media, determined by measuring fecal pellet production and assimilation efficiency, varied from 0.13 μg C ind.−1 d−1 to 1.3 μg C ind.−1 d−1 (10‐37% body C d−1). Our results indicate that the oncaeid copepod community consumed up to 10% of house biomass each day in the upper 100 m of the water column, and show that through their close trophic coupling with discarded appendicularian houses, the oncaeids contribute significantly to house degradation in surface waters.

Particle flux on the continental shelf in the Amundsen Sea Polynya and Western Antarctic Peninsula

Abstract We report results from a yearlong, moored sediment trap in the Amundsen Sea Polynya (ASP), the first such time series in this remote and productive ecosystem. Results are compared to a long-term (1992–2013) time series from the western Antarctic Peninsula (WAP). The ASP trap was deployed from December 2010 to December 2011 at 350 m depth. We observed two brief, but high flux events, peaking at 8 and 5 mmol C m−2 d−1 in January and December 2011, respectively, with a total annual capture of 315 mmol C m−2. Both peak fluxes and annual capture exceeded the comparable WAP observations. Like the overlying phytoplankton bloom observed during the cruise in the ASP (December 2010 to January 2011), particle flux was dominated by Phaeocystis antarctica, which produced phytodetrital aggregates. Particles at the start of the bloom were highly depleted in 13C, indicating their origin in the cold, CO2-rich winter waters exposed by retreating sea ice. As the bloom progressed, microscope visualization and stable isotopic composition provided evidence for an increasing contribution by zooplankton fecal material. Incubation experiments and zooplankton observations suggested that fecal pellet production likely contributed 10–40% of the total flux during the first flux event, and could be very high during episodic krill swarms. Independent estimates of export from the surface (100 m) were about 5–10 times that captured in the trap at 350 m. Estimated bacterial respiration was sufficient to account for much of the decline in the flux between 50 and 350 m, whereas zooplankton respiration was much lower. The ASP system appears to export only a small fraction of its production deeper than 350 m within the polynya region. The export efficiency was comparable to other polar regions where phytoplankton blooms were not dominated by diatoms.

Infection of the planktonic copepod Calanus finmarchicus by the parasitic dinoflagellate, Blastodinium spp: effects on grazing, respiration, fecundity and fecal pellet production

Several dinoflagellate species in the genus Blastodinium are gut parasites of marine planktonic copepods. However,there is only limited information on the occurrence and infection frequencies of Blastodinium spp. in the field andalmost no information on the functional impact on their hosts. We report upon the effects of Blastodinium sp. infectionon Calanus finmarchicus from the northeastern Atlantic coast off southern Norway during April 2013 and 2014. Up to58% of C. finmarchicus were infected near the coast, while ,5% were infected several kilometers offshore. Ingestionrates of infected females were below detection limits and significantly lower than uninfected females. Blastodinium sp.-infected females showed characteristic symptoms of starvation, including lower respiration rates (implying a lowermetabolic rate), production of smaller and fewer fecal pellets and significantly fewer eggs than uninfected females. Afew females in this study were able to void the infection, however the extended period of starvation is likely to havelonger-term repercussions on egg production rates well after the copepod clears the infection. The degree to whichthe infection affects C. finmarchicus recruitment depends on the extent of the spatial distribution of the infection.Monitoring of parasitic infection during routine field surveys will be required in order to clarify this.KEYWORDS: zooplankton; secondary production; gut parasite; starvation; egg production; sterility; metabolism;ingestion; population dynamics; biogeochemical cycles

Validation of a radioimmunoassay-based fecal corticosteroid assay for Richardson’s ground squirrels Urocitellus richard-sonii and behavioural correlates of stress

Abstract We validated a radioimmunoassay-based method quantifying fecal glucocorticoid metabolites (FGMs) from captive male and female Richardson’s ground squirrels Urocitellus richardsonii. Blood samples were drawn to explore the correlation between plasma cortisol and FGM concentrations. We also injected groups of squirrels with normal saline (CTL; control), adre-nocorticotropic hormone (ACTH; stimulating adrenal activity), or dexamethasone (DEX; suppressing adrenal activity). Potential correlations between stress and behaviour were explored through quantification of fecal pellet production and the intervention necessary to elicit defecation, as well as the behaviour of subjects in the context of handling. Changes in plasma cortisol concentration between capture (baseline), and following handling (stress-induced) were also quantified for free-living squirrels. While glucocorticoid concentrations recovered from feces during our captive-animal study were not well correlated with plasma cortisol concentrations, and uncorrelated with defecation or behaviour, FGM concentrations did reflect the activation of the hypothalamic-pituitary-adrenal (HPA) axis. FGM concentrations increased significantly during initial captivity, but declined to baseline level as individuals acclimated to the novel environment. Injection of subjects with ACTH increased FGMs above baseline, confirming activation of the HPA axis. Plasma cortisol concentrations increased significantly with induced stress, indicating that capture and handling activated the glucocorticoid stress response even among previously handled, free-living subjects. Our findings validate a non-invasive tool that will afford new insight into the physiological processes underlying social, reproductive and antipredator behaviour of Richardson’s ground squirrels.

Ingestion and sublethal effects of physically and chemically dispersed crude oil on marine planktonic copepods.

Planktonic copepods play a key function in marine ecosystems, however, little is known about the effects of dispersants and chemically dispersed crude oil on these important planktonic organisms. We examined the potential for the copepods Acartia tonsa, Temora turbinata and Parvocalanus crassirostris to ingest crude oil droplets and determined the acute toxicity of the dispersant Corexit® 9500A, and physically and chemically dispersed crude oil to these copepods. We detected ingestion of crude oil droplets by adults and nauplii of the three copepod species. Exposure to crude oil alone (1 µL L−1, 48 h) caused a reduction of egg production rates (EPRs) by 26–39 %, fecal pellet production rates (PPRs) by 11–27 %, and egg hatching (EH) by 1–38 % compared to the controls, depending on the species. Dispersant alone (0.05 µL L−1, 48 h) produced a reduction in EPR, PPR and EH by 20–35, 12–23 and 2–11 %, respectively. Dispersant-treated crude oil was the most toxic treatment, ~1.6 times more toxic than crude oil alone, causing a reduction in EPR, PPR and EH by 45–54, 28–41 and 11–31 %, respectively. Our results indicate that low concentrations of dispersant Corexit 9500A and chemically dispersed crude oil are toxic to marine zooplankton, and that the ingestion of crude oil droplets by copepods may be an important route by which crude oil pollution can enter marine food webs.

How to increase productivity of the copepodAcartia tonsa(Dana): effects of population density and food concentration

In this study, we analysed the effect of population density and food concentration on the fecundity of a Mediterranean strain of Acartia tonsa to maximize egg production. During 4-day feeding experiments, egg hatching success and faecal pellet production were also followed. The algae Rhinomonas reticulata was supplied at different concentrations corresponding to 250, 500, 1000, 1500, 2000 and 3000 μg C L−1 day−1 at the following adult copepod density: 40, 80 and 160 ind. L−1. Our results show a positive relationship between algal concentration and egg production under all experimental conditions confirming that the quantity of food strongly limits A. tonsa fecundity. Maximum egg production (57 eggs per female) was reached at the lowest density and at the maximum food concentration. Percentage of egg hatching success was not dependent on the quantity of food used. At the same food concentration, an increase in population density from 40 to 80 ind. L−1 induced an increase in faecal pellet production per couple which did not correspond to an increase in egg production, suggesting that higher energetic costs were shifted to swimming activity. Productivity of the A. tonsa Mediterranean strain is mainly limited by the quantity of food rather than by crowding conditions.

Distinct Effects of Repeated Restraint Stress on Basolateral Amygdala Neuronal Membrane Properties in Resilient Adolescent and Adult Rats

Severe and repeated stress has damaging effects on health, including initiation of depression and anxiety. Stress that occurs during development has long-lasting and particularly damaging effects on emotion. The basolateral amygdala (BLA) plays a key role in many affective behaviors, and repeated stress causes different forms of BLA hyperactivity in adolescent and adult rats. However, the mechanism is not known. Furthermore, not every individual is susceptible to the negative consequences of stress. Differences in the effects of stress on the BLA might contribute to determine whether an individual will be vulnerable or resilient to the effects of stress on emotion. The purpose of this study is to test the cellular underpinnings for age dependency of BLA hyperactivity after stress, and whether protective changes occur in resilient individuals. To test this, the effects of repeated stress on membrane excitability and other membrane properties of BLA principal neurons were compared between adult and adolescent rats, and between vulnerable and resilient rats, using in vitro whole-cell recordings. Vulnerability was defined by adrenal gland weight, and verified by body weight gain after repeated restraint stress, and fecal pellet production during repeated restraint sessions. We found that repeated stress increased the excitability of BLA neurons, but in a manner that depended on age and BLA subnucleus. Furthermore, stress resilience was associated with an opposite pattern of change, with increased slow afterhyperpolarization (AHP) potential, whereas vulnerability was associated with decreased medium AHP. The opposite outcomes in these two populations were further distinguished by differences of anxiety-like behavior in the elevated plus maze that were correlated with BLA neuronal excitability and AHP. These results demonstrate a substrate for BLA hyperactivity after repeated stress, with distinct membrane properties to target, as well as age-dependent factors that contribute to resilience to the effects of stress.

Impact of living with kin/non-kin on the life history traits of Tetranychus urticae (Acari: Tetranychidae)

In many vertebrates and invertebrates, living in a group may influence the life history traits, physiology and behaviour of its individual members, whereas genetic relatedness affects social interactions among individuals in a group. The two-spotted spider mite Tetranychus urticae is characterised by a communal organization, in which silk production plays a key role. A silken web protects the colony against biotic and abiotic agents such as predators, competitors, humidity, wind, rain and acaricides. To evaluate the potential costs and benefits of being associated with genetically distant vs genetically close individuals in T. urticae, we assessed various fitness indicators (faecal pellet production, fecundity, death rate) in pure and mixed groups of two distinct populations of T. urticae: a red-form population from Tunisia and a green-form population from Belgium. If genetic origin had no influence, the values of fitness indicators in mixed groups composed of green and red individuals, would be intermediate between those of the pure green-form and red-form groups. Our results show that in a mixed group, faecal pellet production and death rate were statistically similar to the values obtained in the pure group of green-form individuals. Therefore, our study suggests that strain recognition ability may occur in T. urticae and that the genetic background of an individual may have a great impact on several of its life history traits.

Evaluating pyrene toxicity on Arctic key copepod species Calanus hyperboreus

Calanus hyperboreus is a key species in the Arctic regions because of its abundance and role in the Arctic food web. Exploitation of the off shore oil reserves along Western Greenland is expected in the near future, and it is important to evaluate the acute and chronic effects of oil emissions to the ecosystem. In this study C. hyperboreus females were exposed to concentrations of 0, 0.1, 1, 10 and 100nM pyrene and saturated concentrations measured to ~300nM. Daily quantification of egg and faecal pellet production showed significant decreases in the pellet production, while the egg production was unaffected. The hatching success was also unaffected, although the total reproductive output was reduced with increased pyrene concentrations. Accumulation of pyrene in the copepods was higher in feeding than starving females and only trace amounts of the phase I metabolite 1-hydroxypyrene, were found. Lowered reproductive output, reduced grazing, and reduced ability to metabolize pyrene suggest that oil contamination may constitute a risk to C. hyperboreus recruitment, energy transfer in the food web and transfer of pyrene to higher trophic levels.

Vertical Carbon Flux of Biogenic Matter in a Coastal Area of the Aegean Sea: The Importance of Appendicularians

This work focuses on the direct measurement of the vertical flux of appendicularian houses in order to assess their importance as a component of vertical carbon flux in coastal areas. For this purpose, arrays of cylindrical sediment traps were deployed for 5 to 8 days at two depths in a coastal area of the northern Aegean Sea (inner Thermaikos Gulf) during spring. The data support the contention that resuspension was minimal. Fecal pellet (FP) production and grazing experiments with the dominant copepods (Acartia clausi) were conducted to provide additional information on the potential FP contribution to the total carbon flux. The magnitude of the vertical flux of particulate organic carbon (POC) ranged between 310 and 724 mg C m−2 day−1. The proportion of phytoplankton carbon in the POC vertical flux was up to 45 %. The contribution of zooplankton FPs to the total carbon never exceeded 5 %. On the contrary, appendicularian houses were an essential component of the biogenic carbon flux contributing up to 55.3 % of the total vertical carbon flux. Consequently, both phytoplankton and appendicularian houses contributed equally to the biogenic carbon flux exceeding 80 % of the total sinking POC. Taking into account the sinking speed of the particles and the environment in the area, all this carbon probably reaches the seafloor, thus indicating a strong pelagic–benthic coupling.

Macroarthropod-microorganism interactions during the decomposition of Mediterranean shrub litter at different moisture levels

Saprophagous macroarthropods reach high population densities in many Mediterranean terrestrial ecosystems of southern Europe but their impact on decomposition processes remains unclear. We studied the direct and indirect effects of the millipede Ommatoiulus sabulosus on the decomposition of litter from four shrub species (Cistus albidus, Quercus coccifera, Rosmarinus officinalis and Ulex parviflorus) in a 1-month laboratory experiment. Litter mass loss, substrate-induced respiration (SIR) in litter and faecal pellets, and microbial community structure and biomass in litter (using PLFA profiles) were determined under three sets of conditions: (1) litter and microorganisms; (2) litter, microorganisms and millipedes; (3) litter, microorganisms and millipede faecal pellets regularly added to litter. Two watering frequencies were used to study the effects of simulated drought on the measured variables. Millipedes consumed large amounts of shrub litter with low assimilation efficiency. The largest effect was observed for Cistus that showed also the highest rate of microbial-driven decomposition. Conversion of litter into faecal pellets by millipedes did not increase organic matter decomposition and there was no evidence for a stimulation of microbial activity in faecal pellets compared to uningested litter. Faecal pellet deposition on litter, which was maximal in Cistus, did not change mass loss and SIR rates of litter but significantly changed microbial community structure. Bacterial biomass was lower and fungal:bacterial ratio was higher in moist, but not in dry Cistus litter, after 1 month of regular faeces addition. Simulated drought strongly decreased microbial-driven decomposition but not millipede feeding. The resulting sustained production of faecal pellets, however, did not offset the slower microbial-driven decomposition under dry conditions. We conclude that the transformation of large amounts of shrub litter into faeces by macroarthropods and the resultant interactions with microorganisms do not enhance carbon mineralization in the short term, regardless of litter humidity. We however expect that faeces could be incorporated into the soil more easily than intact plant litter, with important consequences for carbon and nutrient cycling in Mediterranean ecosystems under increasing drought.

Modelling the influence of copepod behaviour on faecal pellet export at high latitudes

The contribution of faecal pellet (FP) production by zooplankton to the downward flux of particulate organic carbon (POC) can vary from 70 m) occurred when diel VM took place. Simulated ADCP vertical velocity fields from the foray-type scenario resembled field observations, particularly with regard to the occurrence of positive anomalies in deeper waters and negative anomalies in shallower waters. The model illustrates that active vertical flux of zooplankton FP can occur at high latitudes even when no synchronised VM is taking place.