Species aggregation: the influence of detritus in a benthic invertebrate community

Species aggregation with detritus was examined in a zoobenthic stream community. Detritus was quantified (dry weight in particle size categories) in contiguously arranged quadrats along four transects within a 40m stream section. Total organism density was found to be positively related to increasing detrital biomass (dry weight) in all transects, although the magnitude of this association was influenced by the detrital particle size distribution in each quadrat. Eliminating certain particle size categories of detritus resulted in substantial changes in the magnitude of association. For example, in one transect, total organism density and total detrital biomass showed no association. However, when one particle size category (greater than 10 mm) was removed a highly significant relationship was found. Total detrital biomass was found to be a useful predictor of organism distribution, however, inordinately high detrital biomass and variance within larger particle size categories can mask a relationship. This result suggests that the use of discrete size categories is necessary when considering species/ detrital relationships.Significant associations were found between the density of particular species and total biomass of detritus. The magnitude, as well as significance, of these associations were found to vary when correlations were calculated with restricted particle size categories of detritus. Species associations were often not consistent between transects, possibly due to local characteristics of detritus or overriding environmental factors. Current velocity was found to be unimportant in determining the detrital biomass held in the substrate. Likewise, few associations were found between current velocity and organism density in three out of four transects. Results of this study suggest that habitat selection by some taxa is influenced by local variation in detrital biomass, as well as detrital particle size composition.     

Selective ingestion of detritus by a north temperate omnivorous fish, the juvenile white sucker, Catostomus commersoni

Synopsis The capacity of the juvenile white sucker, Catostomus commersoni, to feed selectively on detritus was examined by analyzing gut contents of fish feeding in a northern Michigan pond and in laboratory aquaria. Suckers selected fine particle detritus. The median length of detritus particles consumed by pond suckers ranged from 26–38 m for all seasons sampled. Although suckers in the pond ingested invertebrates > 215 m they never ingested detritus particles > 215 m. Suckers fed pond detritus in laboratory aquaria selectively ingested the fine particle fraction (Kolmogorov-Smirnov test, p < 0.0001). Suckers excluded particles > 220 m and ingested a median particle size of 54 m. Amorphous detritus particles comprised 100% of the detritus particles in 93% of the foreguts analyzed. Suckers fed fine particle (< 45 ) pond detritus in laboratory aquaria rejected the inorganic fraction and selectively ingested organicmaterial with the highest protein and energy content. Ash free dry mass (AFDM) as a percent of dry-mass was 2.6 times greater in the foregut contents than in the detritus (p < 0.001). The amino acid concentration and energy density of the ingested organic matter were 1.9 times higher (p < 0.044) and 1.3 times higher (p < 0.025) than the respective amino acid and energy content of the organic fraction of the detritus. These results indicate that suckers have the capacity to consume selectively fine particle detritus with the highest nutritional value.     

Enzymic and chemical analysis of particulate organic matter from a boreal river

SUMMARY. 1. Benthic particulate organic matter (POM) was collected from a shallow pool of a fourth order boreal stream and sorted into seven size fractions ranging from 63 to >4000μm. Each size fraction was analysed for fibre, total phosphorus, and total Kjeldahl nitrogen. Microbial activity was measured by oxygen consumption and characterized by assaying for eleven classes of exoenzymes including cellulase, phenol oxidase, peroxidase, phosphatase and sulphatase.
2. Indices of detritus quality such as C/N, C/P, percent lignin, and microbial respiration showed improvement with decreasing particle size. Three covarying exoenzyme groups were identified: a carbohydrase-phosphatase group that included eight enzymes, a phenol oxidase-peroxidase group, and sulphatase. The activity of the carbohydrase-phosphatase group, was significantly correlated with microbial respiration and the carbohydrate content of the POM. Phenol oxidase-peroxidase activity was correlated with lignin content for POM greater than 250 μm, but activity increased markedly in the two smallest size fractions even though the lignin content of the POM continued to decline, Sulphatase activity was inversely related to particle size over the entire range.
3. The changes in microbial activity with particle size were attributed to the increasing surface area to volume ratio of smaller particles and to an ecological succession in the microbial community.     

Size selection of latex beads by blackfly larvae (Diptera: Simuliidae) in the laboratory

In laboratory experiments, blackfly larvae collected from a lake outlet, a woodland and a meadow stream were tested for size selection of latex beads of < Ito > 100 μm diameters. 3 suspensions of varying proportions for each size category were supplied to these blackfly larvae in common experiments. Comparisons between the size frequency distributions of particles supplied and the particle compositions in the larval guts showed intra- and interspecific differences and were quantified by calculating Jacobs' electivity index. In all species selection of larger particles increased with the larger larval instars. Although there was a positive selectivity of small particles in some cases, the ingested proportion of large particles increases volumes and biomasses of gut content and may be more important for larval growth than small particles.     

Substrate selection for pit making and oviposition in an antlion, Myrmeleon bore Tjeder, in terms of sand particle size

The larvae of the pit‐making antlion Myrmeleon bore Tjeder live in open sand in riverbeds with a substratum consisting of various particle sizes. We analyzed the spatial distribution of their pits in a sandy floodplain to determine their larval and adult responses to the heterogeneous substrate. The spatial distribution pattern of their pits had an aggregated distribution, and there was a significant positive correlation between pit density and the ratio of medium‐size sand particles to total weight of sand. We examined the size of sand particles selected in the larval pit‐building behavior and the oviposition behavior of the adult. Both larvae and adults selected medium‐size sand particles. The larvae of M. bore are relatively sedentary predators and rarely move great distances. Thus, the present results suggest that habitat selection by adult females is a major factor causing the aggregative distribution of the pits.     

Utilization of larval and pupal detritus by Aedes aegypti and Aedes albopictus

The utilization of detritus sources by mosquito larvae during development may significantly affect adult life history traits and mosquito population growth. Many studies have shown invertebrate carcasses to be an important detritus source in larval habitats, but little is known regarding how invertebrate carcasses are utilized by mosquito larvae. We conducted two studies to investigate the rate of detritus consumption and its effect on larval development and life history traits. Overall, we found that Aedes aegypti and Aedes albopictus larvae rapidly consumed larval detritus, while pupal detritus was consumed at a significantly slower rate. We also found that the consumption of larval detritus significantly increased larval survivorship and decreased male development time but did not significantly influence female development time or pupal cephalothorax length for either sex. Our results suggest that the direct consumption of larval detritus can support the production of adults in larval habitats that lack allochthonous detritus inputs or where such organic inputs are insufficient. These studies indicate that different forms of invertebrate detritus are utilized in distinct ways by mosquito larvae, and therefore different forms of invertebrate detritus may have distinct effects on larval development and adult life history traits.     

Factors influencing the longitudinal distribution of larval Hydropsychidae (Trichoptera) in a southern Appalachian stream system (U.S.A.)

The influence of physical habitat variables and suspended particulate organic matter (seston) on the distribution and production of eight species of larval Hydropsychidae was studied along a 6.4 km section of a southern Appalachian stream. Samples were collected at six stations encompassing stream orders 1–4 and an elevation range of 610 m. Multivariate analysis of covariance (using time as the covariable) and discriminant function analysis were used to examine habitat differences between the sampling stations due to the following variables: current velocity; coarse benthic detritus; substrate composition (by particle size); substrate heterogeneity; degree-days; and diel temperature fluctuation. The associations of these variables with the abundance (and production) of larval hydropsychids was also examined using the same statistical procedures. The six sampling stations represented three or four distinct habitats based on patterns of change in the variables along the stream continuum. Diel temperature fluctuation, median substrate particle size, the proportion of sand substrate, and substrate heterogeneity were most closely associated with the overall difference between the sampling stations. Hydropsychid species distribution along the stream system followed subfamily lines, i.e., Arctopsychinae and Diplectroninae were more abundant and productive in the upper 4.5 km of the stream, while Hydropsychinae were dominant in the lower 1.9 km. Diel temperature fluctuation was the habitat variable most highly correlated with patterns of hydropsychid abundance and production. The longitudinal pattern of species distribution, i.e., larger particle feeding Arctopsychinae being replaced downstream by smaller particle feeding Hydropsychinae, also coincided with the distribution of seston particle size classes along the stream. Mean seston particle size generally declined downstream, as particles <42 µm increased in relative abundance while those between 43 µm and 5 mm decreased. Temperature, seston, and substrate composition all undoubtedly exerted an important influence on the distribution and production of Hydropsychidae and other filter feeding insects in this stream. The validity and generality of these results depend upon the scope of the sampling effort. Conclusions drawn from data collected over the entire range (and limits) of a species' distribution are more sound than those based on data from a limited area.     

Life histories as sampling devices: optimum egg size in pelagic fishes*

In teleosts, egg size is an important determinant of larval size. The latter determines where larvae start feeding in the size spectrum of biomass particles. Egg and larval size also determine the size range of predators exploiting these early ontogenetic stages. Thus, egg size is important in determining the particle size range sampled during ontogeny. However, egg size also largely determines how many can be produced by a female. Optimum egg size for marine, pelagic fishes was estimated using a modification of a model developed by Ware. A numerical analysis using empirical information on size and temperature dependent mortality and growth in pelagic, marine systems suggests that allometry of size-dependent mortality and temperature are significant factors in determining optimum egg size. Allometry of mortality will depend on the frequency of encounters between particles of different sizes. Both empirical and modelling studies of such frequencies in pelagic and other environments are required.     

Spatial variability of marine bacterial and archaeal communities along the particulate matter continuum

Biotic and abiotic particles shape the microspatial architecture that defines the microbial aquatic habitat, being particles highly variable in size and quality along oceanic horizontal and vertical gradients. We analysed the prokaryotic (bacterial and archaeal) diversity and community composition present in six distinct particle size classes ranging from the pico‐ to the microscale (0.2 to 200 μm). Further, we studied their variations along oceanographic horizontal (from the coast to open oceanic waters) and vertical (from the ocean surface into the meso‐ and bathypelagic ocean) gradients. In general, prokaryotic community composition was more variable with depth than in the transition from the coast to the open ocean. Comparing the six size‐fractions, distinct prokaryotic communities were detected in each size‐fraction, and whereas bacteria were more diverse in the larger size‐fractions, archaea were more diverse in the smaller size‐fractions. Comparison of prokaryotic community composition among particle size‐fractions showed that most, but not all, taxonomic groups have a preference for a certain size‐fraction sustained with depth. Species sorting, or the presence of diverse ecotypes with distinct size‐fraction preferences, may explain why this trend is not conserved in all taxa.     

Can an optical plankton counter produce reasonable estimates of zooplankton abundance and biovolume in water with high detritus?

The Optical Plankton Counter (OPC) has been used in oceanicand fresh waters to estimate zooplankton abundance and biovolume.However, it is not clear whether the OPC can produce accurateestimates of zooplankton abundance and biovolume in waters withhigh detritus. In order to test the capability of the OPC toestimate zooplankton abundance and biovolume in Chesapeake Bay,two sets of laboratory experiments were conducted using waterwith high detritus concentrations collected from the upper Choptankestuary of Chesapeake Bay and laboratory cultured Artemia. Ourresults suggest that the OPC is able to produce accurate estimatesof zooplankton biovolume after correcting for the influenceof background detritus in all the detritus concentrations used,but accurate estimates of zooplankton abundance only in waterwith background detritus <100 particles l^–1. The relationshipbetween light attenuation and OPC background particle concentrationsprovides a useful way to estimate OPC background particle concentrationswhen direct OPC background particle measurements are not available.Light attenuation corrected OPC particle abundance and particlevolume gave accurate estimates of zooplankton abundance andbiovolume. However, the accuracy of the corrected OPC measurementsby the estimated background particle concentrations was notas high as the corrected OPC measurements by the direct backgroundparticle measurements.     

Microbial diversity along a sediment detrital particle size gradient

We studied the structural organization of microbial decomposer communities by comparing patterns of genetic complexity over a template defined by site, season and detrital particle size. Epibenthic sediment samples were collected monthly from a Lake Erie coastal wetland and a small woodland stream, and sieved into five fine particulate organic matter (FPOM) size ranges: 1000–500 (500), 500–250 (250), 250–125 (125), 125–63 (63) and 63–38 (38) m. Whole community DNA-DNA hybridizations were used to compare the structural similarity of the microbial communities associated with each sample. Microbial community heterogeneity increased as particle size decreased, and declined from a summer maximum to a late winter minimum. Cluster analysis of hybridization scores partitioned the communities into two groups: one associated with the 500, 250 and 125 m fractions and a second with the 63 and 38 m fractions. The larger particles were easily recognized as comminuted plant detritus; the smaller particles were amorphous, presumably formed through the aggregation of dissolved organic carbon. This disjunction in particle morphology and microbial community diversity that occurs at about 100 m appears to delineate two trophic resources whose origin and fate may be largely independent.     

Ingestion of Large Centric Diatoms, Mangrove Detritus, and Zooplankton by Zoeae of Aratus pisonii (Crustacea: Brachyura: Grapsidae)

Ingestion rates of zoeae of Aratus pisonii Milne Edwards (Brachyura: Grapsidae) were determined offering natural plankton-detritus mixtures in laboratory food selection experiments. The food mixtures were sampled in the Itamaracá estuary, north-eastern Brazil, and standardised to a size range of 50–200 μm. Zoeae ingested significant amounts of large centric diatoms (Coscinodiscus spp.), mangrove detritus, tintinnids (Favella ehrenbergi) and adult copepods during feeding experiments. Diatoms were positively selected by A. pisonii zoeae in all three experiments, with ingestion rates of 3.3–21.3 cells zoea⁻¹ day⁻¹. Detritus particles were always more abundant than phytoplankton and zooplankton in the particle size spectrum offered. Detritus was ingested in two of three experiments, with ingestion rates of up to 34.1 particles zoea⁻¹ day⁻¹, being the most important food item during one experiment. Adult copepods (up to 1.8 ind. zoea⁻¹ day⁻¹) and tintinnids (up to 0.4 ind. zoea⁻¹ day⁻¹) were ingested by A. pisonii zoeae during one experiment each. In spite of a wide range of zoeal density, food particle composition, and density, zoeae of A. pisonii displayed a consistent pattern of food selectivity. This hints at a consistent sensory and behavioural mechanism related to capture and handling of food particles, that most likely also affects larval feeding under natural conditions. Although detritus showed to be quantitatively ingested under estuarine conditions, zoeae of A. pisonii preferred large diatoms and ingested zooplankton only occasionally.     

THE INFLUENCE OF SOIL MOISTURE ON THE EMERGENCE OF LARVAE FROM CYSTS OF THE BEET EELWORM, HETERODERA SCHACHTII SCHMIDT

The significance of soil moisture in relation to aeration and larval emergence from cysts of Heterodera schachtii Schmidt is discussed. The rate of larval emergence increased as oxygen concentration increased. A comparison of the moisture characteristics of a mass of cysts and of sand of about the same particle size showed that water was removed from between both the cysts and the sand particles at 12–16 cm. of water-pressure deficiency. There was an indication that water was removed from between the eggs within the cysts at 100–135 cm - of water-pressure deficiency. The rate of larval emergence showed a gradual decline is suction was increased beyond 20 cm. of water-pressure deficiency, approaching zero at 175 cm. of water-pressure deficiency. Results suggest that egg hatch is not directly dependent on pressure deficiency and that the presence of free larvae within the cyst inhibits further egg hatch. A technique is described for measuring larval motility in sand. There is a decline in motility when most of the water has been removed from the sand pore-spaces. It is suggested that the relatively low rates of larval emergence at high-pressure deficiencies are due to inhibition of larval migration from the cyst by the surface forces of the water film.     

Size Effects on the Uptake of Particles by Populations of Tetrahymena pyriformis Cells

Flow cytometry has been used to make direct measurements of rates of uptake of latex microspheres from dilute, monodisperse suspensions by Tetrahymena pyriformis. Measurements were made for five different sizes of microspheres, ranging from 1.09 to 6.17 μm diameter. Fractions of cells in the population that did not ingest the microspheres offered were also determined. In addition, the size distributions, as indicated by the forward angle light scattering intensity which is measured by the instrument, were determined for the whole population and for the subpopulations of cells that did and did not ingest the particles, for each particle size used. It was found that the fraction of cells that did not ingest the particles was small and independent of particle size when this was less than about 2.7 μm, but increased with particle size when particle size was increased above this value. The so-called maximum clearance rate, which can be calculated from the data, was found to increase monotonically with particle size if it were based only on those cells which actually ingested the particles offered. However, a plot of maximum clearance rate vs. particle size exhibited a maximum if the clearance rate were based on all cells present in the population.     

Chironomid microdistribution in gravel of an English chalk river

1. The study aimed to determine the relative importance of certain environmental variables to the distribution of instars of chironomid species in mid-stream gravels of the River Pang. 2. Core samples of gravel substrata were collected over 1 year. Water depth, current, particle size distribution, substratum porosity and detritus were recorded for each sample. The four principal components from a linear ordination of the environmental variables were used to constrain the ordination of chironomid data using the program CANOCO. 3. Microdistribution was principally related to a temporal gradient of water depth, current and substratum heterogeneity. There was significant spatial separation of instars along a gradient of fine detritus, silt and substratum porosity. The distributions of early instars of three taxa were also significantly related to particle size. There was a significant relationship between the size distribution of larvae and the spatial distribution of fine sediments and detritus. This relationship was strengthened when restricted to larvae classified as scrapers.     

Agglomeration behaviour of magnetic microparticles during separation and recycling processes in mRNA purification

Purification of mRNA with oligo(dT)‐functionalized magnetic particles involves a series of magnetic separations for buffer exchange and washing. Magnetic particles interact and agglomerate with each other when a magnetic field is applied, which can result in a decreased total surface area and thus a decreased yield of mRNA. In addition, agglomeration may also be caused by mRNA loading on the magnetic particles. Therefore, it is of interest how the individual steps of magnetic separation and subsequent redispersion in the buffers used affect the particle size distribution. The lysis/binding buffer is the most important buffer for the separation of mRNA from the multicomponent suspension of cell lysate. Therefore, monodisperse magnetic particles loaded with mRNA were dispersed in the lysis/binding buffer and in the reference system deionized water, and the particle size distributions were measured. A concentration‐dependent agglomeration tendency was observed in deionized water. In contrast, no significant agglomeration was detected in the lysis/binding buffer. With regard to magnetic particle recycling, the influence of different storage and drying processes on particle size distribution was investigated. Agglomeration occurred in all process alternatives. For de‐agglomeration, ultrasonic treatment was examined. It represents a suitable method for reproducible restoration of the original particle size distribution.     

Detritus as a potential food source for protozoans: utilization of fine particulate plant detritus by a heterotrophic flagellate, <Emphasis Type="Italic">Chilomonas paramecium</Emphasis>, and a ciliate, <Emphasis Type="Italic">Tetrahymena pyriformis</Emphasis>

We investigated the direct utilization of fine particulate detritus (dried and homogenized plant material in the size range of bacteria) as a food source for protozoans using axenic cultures of the cryptomonad, heterotrophic flagellate, Chilomonas paramecium, and the hymenostome ciliate, Tetrahymena pyriformis. When fed media containing only particulate detritus, these species revealed growth rates similar to those reported for field populations. The growth rates of Chilomonas fed exclusively particulate detritus were similar to those obtained on a bacterial diet. Considering the high percentage of detritus particles in the size range of bacteria in lakes, our results imply that direct utilization of detritus by protozoans may form an additional pathway of carbon in aquatic food webs that has generally been overlooked.     

Manipulation of habitat quality: effects on chironomid life history traits

1. To understand how habitat heterogeneity affects organisms, the present authors measured the response of chironomid life-histories to single patch types in a controlled laboratory experiment. The objective was to determine whether the size and type of leaf particles in the substratum affect development rate and survival of larvae, and the fecundity of the resulting adults.
2. Chironomus riparius larvae were raised in sand supplemented with dogwood, Cornus florida, (a rapid decomposer) or American sycamore, Platanus occidentalis, (a refractory decomposer) leaves fully crossed in a factorial design with four leaf particle sizes (0.2, 0.5, 1 and 5 cm²), or in sand only.
3. Development time was shorter in dogwood habitats than sycamore habitats, and shorter in sycamore habitats than in sand habitats. Survival was higher and larval head capsule lengths of instars III and IV were larger in dogwood than in sycamore or sand habitats. Development time was longer and head capsule lengths of instars III and IV were smaller in habitats with 5-cm² leaf particles than in 0.2-, 0.5- and 1-cm² particle size treatments. Female thorax length (a measure of fecundity) did not differ among treatments.
4. The present authors conclude that, for C. riparius confined to a single patch type, fitness is significantly enhanced when the patch contains small particles of dogwood (labile) leaves.     

Benthic detritus dynamics in a mountain stream

The dynamics of sedimentary detritus was examined over a 17-month period in a third-order mountain stream in Colorado. Autumnal and vernal peaks in total detritus standing crop (197 g and 165 g AFDW m⁻², respectively) were associated with leaf abscission and ice out. The flushing action of snowmelt runoff rapidly reduced benthic detritus to 14 g AFDW m⁻². Detrital size classes differentially contributed to temporal fluctuations in total detritus. The autumnal maximum was largely comprised of coarse particulate organic matter (CPOM), primarily leaf litter. The large increase following ice out was mainly due to fine detritus (FPOM, 0.45–75 μm) which accounted for > 50% of benthic detritus during 9 months of the year. Evidence suggests that changes in standing crop and particle size distribution during the extended period of ice cover (November-March) resulted from in situ processing rather than transport. Despite major differences in climate, geochemistry and hydrology, the Colorado stream exhibited patterns of detrital dynamics remarkably similar to an eastern deciduous forest stream.     

Implications for the mesopelagic microbial gardening hypothesis as determined by experimental fragmentation of Antarctic krill fecal pellets

1. Detritivores need to upgrade their food to increase its nutritional value. One method is to fragment detritus promoting the colonization of nutrient‐rich microbes, which consumers then ingest along with the detritus; so‐called microbial gardening. Observations and numerical models of the detritus‐dominated ocean mesopelagic zone have suggested microbial gardening by zooplankton is a fundamental process in the ocean carbon cycle leading to increased respiration of carbon‐rich detritus. However, no experimental evidence exists to demonstrate that microbial respiration rates are higher on recently fragmented sinking detrital particles.
2. Using aquaria‐reared Antarctic krill fecal pellets, we showed fragmentation increased microbial particulate organic carbon (POC) turnover by 1.9×, but only on brown fecal pellets, formed from the consumption of other pellets. Microbial POC turnover on un‐ and fragmented green fecal pellets, formed from consuming fresh phytoplankton, was equal. Thus, POC content, fragmentation, and potentially nutritional value together drive POC turnover rates.
3. Mesopelagic microbial gardening could be a risky strategy, as the dominant detrital food source is settling particles; even though fragmentation decreases particle size and sinking rate, it is unlikely that an organism would remain with the particle long enough to nutritionally benefit from attached microbes. We propose “communal gardening” occurs whereby additional mesopelagic organisms nearby or below the site of fragmentation consume the particle and the colonized microbes.
4. To determine how fragmentation impacts the remineralization of sinking carbon‐rich detritus and to parameterize microbial gardening in mesopelagic carbon models, three key metrics from further controlled experiments and observations are needed; how particle composition (here, pellet color/krill diet) impacts the response of microbes to the fragmentation of particles; the nutritional benefit to zooplankton from ingesting microbes after fragmentation along with identification of which essential nutrients are being targeted; how both these factors vary between physical (shear) and biological particle fragmentation.