Download UvA-DARE (Digital Academic Repository) Feeding of detritivores in

UvA-DARE (Digital Academic Repository)
Feeding of detritivores in freshwater sediments
Vos, J.H.
Link to publication
Citation for published version (APA):
Vos, J. H. (2001). Feeding of detritivores in freshwater sediments Amsterdam
General rights
It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s),
other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons).
Disclaimer/Complaints regulations
If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating
your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask
the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam,
The Netherlands. You will be contacted as soon as possible.
UvA-DARE is a service provided by the library of the University of Amsterdam (http://dare.uva.nl)
Download date: 18 Jun 2017
CHAPTERR 1
GENERALL INTRODUCTION
9 9
ChapterChapter 1
Inn all natural waters particles of non-living organic matter are present, which are
derivedd from autotrophic plants or dying animals. These non-living particles, or detritus,
supportt distinct assemblages of animals which use the particles as a food source. When
thiss food source is suspended it requires filter feeding to be consumed, but when the
detrituss particles are sedimented in aquatic systems, the bottom material serves as both
habitatt and food for detritivores. Detritivory is defined as the intake of nonliving particulatee organic matter together with the microorganisms that are associated with it
(Cumminss 1973). Although the general view of food webs supposes a dominance of
grazerss cropping plant material, these herbivores being eaten by carnivores, in fact as
muchh as 90% of primary plant production in aquatic systems enters the detritus food
webb (Pomeroy 1980). Detritivores form a major link in the foodweb processing
nutritionallyy low quality dead organic matter. Through their activity they stimulate
degradationn of organic matter by microorganisms (Ten Winkel et al. 1982, Harris 1983,
Shepardd & Minshall 1984, van de Bund & Davids 1993, van den Bund et al. 1994,
Goedkoopp 1994). In aquatic systems detritivores in soft sediments are one of the main
foodd sources for predators, like fish and birds, or carnivorous invertebrates such as
mitess and damselfly larvae (Healey 1984, Walde & Davies 1984, Hershey 1985a & b,
Hersheyy & Dodson 1987, Ten Winkel et al. 1989, review Wallace & Webster 1996).
Benthicc detritivores can be partioned into functional feeding groups based on
food-acquiringg mechanisms. Deposit feeders, also termed collector gatherers are bulkfeeders,, processing large volumes of sediment at high rate. Deposit feeders typically
processs at least one body weight of sediment daily. A way to attain a high rate of
sedimentt processing is to sort the sediment but reject most before ingestion as pseudofaeces.. Collector gatherers feed on fine particle detritus (Fine Particular Organic Matter,
FPOM,, < 1 mm) deposited on the substrate surface. Filtering gatherers feed on particles
inn suspension and therefore are also called suspension feeders. Shredders are
detritivorouss species that use plant debris with somewhat bigger particle size (Coarse
Particularr Organic Matter, CPOM, > 1 mm) compared to the gatherers. Shredders only
processs 0.01 to 0.4 times their body weights daily. Quite often taxa can not be assigned
too one particular functional feeding group only. For instance, some species of polychaetess have the ability to switch between suspension feeding and deposit feeding,
dependingg on the availability of suspended particles, while certain species of bivalves
10 0
GeneralGeneral Introduction
simultaneouslyy display both suspension and deposit feeding. Shredders often ingest
FPOMM while feeding on leaf litter (Cummins 1973, Cummins & Klug 1979, review
Lopezz & Levinton 1987).
Thee principal food sources for detritivores are materials that are derived from
algaee and macrophytes, but detritus may also include fragments of dead animals and
terrestriall run-off introducing soil particles and leaf litter. The nutritional value of the
foodd sources of detritivores varies spatially and temporally. Spatial differences depend
onn the contribution of the different sources to detritus. Detritus of large lakes will
containn more algal material formed in the water column, whereas detritus of small
streamss is likely to contain more material from macrophytes, eroded soil particles, and
leaff litter.
Detritivoress are confronted with different quantities of organic matter admixed
withh mineral particles with different nutritional qualities throughout the year, depending
onn a number of factors such as seasonal input of leaf litter, algal blooms, and macrophytess growth, on the depth of the watersystem, and the degradation rate of phytodetriruss (Johannson & Beaver 1983, Johnson 1987, Lopez & Levinton 1987, Moore
1987,, Marsh & Tenore 1990, Hill et al. 1992, Cheng et al. 1993, Ahlgren et al. 1997).
Seasonalityy of algal input to the benthic ecosystem is illustrated by a study of Ahlgren et
al.. (1997). Plankton net samples and sedimenting matter in traps from mesotrophic Lake
Erkenn were analysed for carbon, nitrogen, phosphorus, total lipids and fatty acid content
too follow seasonal changes in food availibility. Organic matter abundance in the pelagic
andd the benthic zone depended on respectively the presence and sedimentation of
phytoplankton.. Biochemical analyses of plankton and sedimenting matter showed that
thee benthic fauna have access to high-quality food only during spring and autumn due
thee dominance of diatoms during these periods. Studies on benthic-pelagic coupling in
thee field reported a rapid response of the macrofauna community to an increase of the
inputt from the pelagic zone, suggesting food limitation during much of the year (Graf et
al.. 1982, Graf 1989, Lopez & Levinton 1987, Goedkoop & Johnson 1996). Field studies
alsoo showed that algal blooms composed of different algal species caused different
responsess of local macrofauna communities (Marsh et al. 1989, Marsh & Tenore 1990,
Chengg et al. 1993) indicating regulation of macroinvertebrates by organic matter
composition.. Noting that food is a primary need for heterotrophic animals, the supply
11 1
ChapterChapter 1
ratee and quality of detritus particles reaching the benthos is a key limiting factor. It is
hypothesizedd that temporal and spatial differences in the availibility of food sources for
detritivoress are main regulating factors for communities of benthic invertebrates. This
regulationn is the main subject of this thesis.
Compositionn of organic matter in sediments
Compositionn of organic matter is dependent of its source and of the degree of
degradation.. During the complex process of decomposition the most labile components
aree degraded first. Conversely, the refractory matter is slowly broken down and tends to
accumulatee in the sediment (Fry 1987, Kemp & Johnston 1979). Therefore, depositfeederss mostly depend on low-quality organic matter as food bulk compared to organismss that consume selectively other food items (Bowen 1987, Ahlgren et al. 1997).
Decayy of the organic matter encompasses fragmentation, cell leaching, bacterial
decomposition,, and chemical oxidation. In the pelagic column zooplankters strip the
sestonn of useful biochemical compounds (Cavaletto & Gardner 1999). After sedimentation,, oxidation and fragmentation of organic matter continues on the lake bottom.
Bioturbationn of surface sediments may prolong the exposure of the sedimented matter to
oxidation-reductionn cycles.
Microorganismss are mostly the first to take advantage of the most labile
componentss that are released from decaying organic matter. Benthic microbial activity
andd biomass are observed to increase within hours after sedimentation of algae (Graf
1992).. In aerobic environments, aquatic hyphomycetes are dominant during the early
stagess of decomposition of plant matter. The fungal hyphae invade the structure of the
plantt and fungal exoenzymes break down the structural cellulose. This weakens the
plantt tissue. Bacteria typically colonize the remaining fragments.
Microbiall conditioning enhances the palatibility of leaf material to detritivores
(Barlocherr & Kendrick 1975, McGrath & Matthews 2000). Being part of detrital aggregatess microorganisms contribute to the potential food sources of detritivores. However,
bacteriaa are often estimated to contribute less than 1% to the weight of fine detrital
particles.. A number of studies showed that microorganisms' biomass consumed by
detritivoress accounts for less than 10% of the detritivore's growth (Bowen et al. 1984,
Findlayy et al. 1984). Other studies, however, stress that microbes are efficiently digested
12 2
GeneralGeneral Introduction
andd provide a nutrient rich diet (Martin et al. 1980, review Bowen 1987, review Graf
1992).. Even though the microorganisms' biomass constitutes a low share of the ingested
food,, small quantities of microorganisms may supply significant quantities of nutrients
suchh as vitamins and amino acids (review Phillips 1984a, Wolf et al. 1997). Thus,
colonizationn by microorganisms may increase overall nutritional value of originally
refractivee organic matter significantly, which is called microbial enrichment. The term
enrichmentt is also used because the microorganisms convert soluble components that
otherwisee would be lost for further consumption by benthic invertebrates into biomass.
Inn summary, degradation of organic matter by microorganisms is based on at least two
mechanisms.. 1) Microorganisms convert refractory material and soluble components
intoo microbial biomass which is more easy to digest and more nutritious compared to
thee degradating organic matter. 2) Microbial degradation breaks organic matter into
subunitss digestible for detritus-feeders.
Duringg the complex process of degradation the originally nutritious organic
materiall (e.g. from recently died algae) is deprived of the most nutritious components
andd only part of the lost biomass is replaced by microbial biomass. Therefore, detritivoress probably have to deal with food sources of low nutritional value which will vary
withh states of degradation and microbial conditioning. The present study aims to investigatee the variety in composition of food exploited by detritivores and thereby to relate
thee nutritional value of the food source for detritivores to its degradation state.
Nutritionall requirements of invertebrates
Assimilationn may follow basically similar patterns among different animals. The
needss of heterotrophic organisms in regard to nutrition can roughly be divided into two
classes.. Firstly, the organism needs energy for activity and internal maintenance. The
needd for energy can be satisfied by a variety of compounds that are oxidized and
thereforee is called a non-specific need. Secondly, heterotrophic organisms need a supply
off specific substances for synthesis of new tissue. Such specific needs can only be satisfiedd by a limited suite of organic compounds: the essential nutrients.
Influencee of food abundance, and hence of available energy, has been examined
inn several studies on a number of invertebrate species. Manipulation of food availability
inn laboratory experiments show that life history responses to food limitation range from
13 3
ChapterChapter 1
retardedd growth and an increase in longevity to lowered fertility and reproduction rate
andd as an extreme consequence, death. Changes in life history parameters due to food
limitationn lower the intrinsic rate of population growth.
Requirementss for some essential nutrients (e.g., polyunsaturated fatty acids and
vitamins)) are fairly consistent among vertebrates and invertebrates, whereas the
requirementss for others vary, depending on the particular taxonomie or physiological
groupp or developmental stage of the animal (Downer 1981, Phillips
1984A).
Polyun-
saturatedd fatty acids (PUFA) are a relatively well studied group of essential components.. PUFA are responsible for regulation of animal cell membrane physiology and
servee as precursors to eicosanoids. Eicosanoids are critical in a wide range of
physiologicall processes in invertebrates. Deficiencies in PUFA can impair functioning
off membranes and membrane-bound enzyme systems and are needed for optimal eggproduction,, egg-laying, spawning, and hatching (Brett & Müller-Navarra 1995).
Animalss can convert one form of PUFA to another through elongation and
desaturation,, but very few species can synthesize PUFA de novo (review Blomquist et
al.. 1991). Polyunsaturated fatty acids are almost exlusively synthesized by plants.
Consequently,, most freshwater organisms exhibit a strong dietary demand for PUFA.
Thee vast body of literature on aquaculture shows that diets rich in PUFA are essential
forr fishes, molluscs, crustaceans, and zooplankton. This literature is reviewed by Brett
&& Müller-Navarra (1995). Prawns are most thoroughly studied because of their
economicc importance as highly valued food for humans. Studies show that PUFA
contentt of artificial and natural diets impacts survival, growth, fecundity, egg
hatchability,, molting and osmotic stress tolerance of shrimps (D'Abramo & Sheen 1993,
Xuu et al. 1993, Rees et al. 1994). Prawns are primarily carnivores, but literature on
pelagicc herbivores also documents a strong dietary demand for PUFA-rich phytoplankton.. All herbivorous insects require PUFA of plant origin (Blomquist et al. 1991).
Addingg emulsions of PUFA to algae cultures can markedly increase the growth rates of
herbivorouss zooplankton (Brett & Müller-Navarra 1995).
Literaturee on PUFA-demands of benthic detritivores is scarce. Although overall
nutritionall demands of organisms may be similar among taxa it should be noted that
adaptationss in specific species may have taken place due to existence of special environmentall conditions or specific modes of behavior. In regard to modes of feeding physio-
14 4
GeneralGeneral Introduction
logicall adaptations may have occurred to be able to digest certain food sources. Hanson
ett al. (1985) presented a study on the fatty acid composition of aquatic insects from
differentt orders and with different modes of feeding. Fatty acid composition differed
predictablyy among orders and functional feeding groups, most noticeably for the
polyunsaturatedd fatty acids. Collector gatherers of the order Diptera contained relatively
lowerr levels of PUFA than filterers, predators, scrapers, and shredders from the same
order,, implying that the collector gatherers use diets poor in PUFA compared to the
foodd sources of the Diptera taxa with other modes of feeding. Collector gatherers from
thee order Trichoptera, however, contained levels of PUFA comparable to PUFA levels
measuredd in Trichoptera specimen with other feeding modes. Thus, the effect of food
sourcess on detritivores cannot be predicted with precision by means of data on other
orderss of insects with similar modes of feeding or data on insects of the same order but
withh different feeding modes.
Studiess on detritivores
NutritionalNutritional demands
Literaturee on the influence of food composition on detritivores is limited and
concentratess on the effect of microbial enrichment (Martin et al. 1980, Bowen 1987),
foodd particle size selection, and ingestion rate as a function of food value (Taghon
1982),, but mostly does not take biochemical composition into consideration. Food
particlee size selection often is the result of the limitation of most detritivores in their
abilityy to handle or ingest certain particle sizes (Jumars et al. 1982, Taghon 1982). Some
studiess on detritivores indicate that particle size selection originates from the aim to
maximizee net rate of energy gain. Two schools of thought have arisen on the effects of
foodd quality on ingestion rate. One group reports that ingestion rate vary inversely with
foodd quality as mechanism to maintain constant intake rate of some food component,
suchh as energy (Calow 1975, Cammen 1980, Phillips 19846). Conversely, others find
ingestionn rate to be positively related to food quality (refs. in Taghon 1981).
DigestionDigestion of refractory food sources
Detritivoress seem to have adapted their digestion system to the low palatibility
off their food. Bjarnov (1972) compared the digestion of saccharides by Chironomus
15 5
ChapterChapter 1
plumosusplumosus and C. antracinus, Gammarus pulex, and various Trichoptera which have
differentt feeding modes and different food sources. Significant differences between
speciess was only found for the degradation of polysaccharides. All a- and fi -glucosides
andd galactosides were degraded by all tested species. Marked species differences existed
inn the degradation of pectin, xylan and chitin. Digestion of pectine and xylan was
clearlyy observed for 5 species of Trichoptera, which were all shredders or suspension
feeders.. The carnivores were better adapted to digest chitin. The results suggest that
mostt invertebrates are not able to digest the long chain cellulose and other structural
polysaccharidess on their own, but also show that aquatic invertebrates have adapted
theirr digestion to their particular food source.
AA number of studies focussed on the digestive systems in stream detritivores,
e.g.. several Gammarus species, Hydropsyche betteni, Tipula caloptera, T. abdominalis,
PteronarcysPteronarcys proteus, and Pycnopsyche luculenta (Martin et al. 1980, Sinsabaugh et al.
1985,, Barlocher & Porter 1986, Chamier & Willoughby 1986, Chamier 1991, McGrath
&& Matthews 2000). The ability to digest major plant polysaccharides was restricted to
thee Gammarus species, Pteronarcys proteus, and Pycnopsyche luculenta, but enzymes
capablee of breaking glycosidic linkages, similar to enzymes that are released during
microbiall breakdown of leaf polysaccharides, were present in the gut fluid of all
animals.. Ingested fungi are partly responsible for the cellulase activity in the gut of
GammarusGammarus species, but production of an endogenous cellulase system by the insects has
alsoo been noted (Sinsabaugh et al. 1985, Chamier & Willoughby 1986, Chamier 1991,
Harriss 1993).
Forr grazers percentages of 10 to 50 % of the plant material that is actually
consumedd are mentioned not to be digested but excreted as faeces or pseudofaeces
(Pomeroyy 1980). Since detritivores cope with more refractory food than grazers the
percentagee of ingested food that is not digested probably will be higher. Furthermore, a
rapidd rate of passage through the gut probably allows little time for most detritivores to
exercisee extensive digestion and therefore will not be able to thoroughly digest cellulose
orr other polysaccharides (Bjarnov 1972).
16 6
GeneralGeneral Introduction
SedimentSediment as habitat
Sedimentt constitutes both the food source and the physical environment of
benthicc detritivores. The physical characteristics of sediments are mainly determined by
particlee size distribution and organic matter. The impact of organic matter content and
grainn size distribution on detritivores in the field is difficult to distinguish, because they
oftenn covary in natural sediments (e.g., Rabeni & Minshall 1977, Pinder 1986, 1995,
Suedell & Rodgers 1994, Maxon et al. 1997, Reinhold-Dudok van Heel & den Besten
1999). .
Severall ways have been proposed in which particle size could affect preference
andd performance of a sediment inhabitant. Particle size distribution of a substrate may
influencee the suitability of a substratum to borrow in as reflected by penetration rate by
thee organisms (Wiley 1981a, Winnell & Jude 1984). Particle size distribution determiness if the organism is able to construct a living tube, because many insects are limited
inn their ability to handle particles of certain sizes (Brennan & McLachlan 1979).
Inhabitingg a tube diminishes the risk of predation by damselflies, stoneflies, mites and
fishfish as is found in several laboratory and field studies (Hershey 1985 & 1987, Ten
Winkell 1987, Macchiusi & Baker 1991, Baker & Ball 1995). Particle size distribution
andd organic matter also influence oxygen concentrations within the substrate, consequentlyy selecting the species that are able to sustain themselves in the substrate
(Verdonschott 1990, Heinis 1993).
Detritivoress may show preference for a distinct type of habitat. Such preference
forr a habitat is often related to the options for the organisms to develop (Wiley 1981ft).
Suitabilityy of a sediment type can be tested through the migration of an organism from a
referencee substrate, although migration towards a suitable substrate has hardly been
observedd over larger distances (Wiley 1981a, Butman 1987, Rosillon 1987). During a
laboratoryy study by Sibley et al. (1998) on preferences of Chironomus tentans larvae the
interactionn of physical characteristics with organic matter content was obviated by using
minerall particle substrates with different particle size ranges and by imposing uniform
oxygenn conditions. C. tentans larvae consistently selected the smaller particle size range
off two substrates when they only needed to travel short distances (cms).
Nott all detritivores will be able to handle the broad range of particle sizes as
chironomidd larvae do. In addition, chironomid larvae are probably more tolerant to low
17 7
ChapterChapter 1
oxygenn levels in substrates than other detritivores. One other possible mode of action of
particlee size distribution on detritivores has still been unexplored. Deposit-feeders ingest
inorganicc particles together with the organic detritus (Rasmussen 1984, Lopez &
Levintonn 1987). I expected that ingestion of inorganic material obstructs food uptake
andd therefore may hamper growth of detritivores.
Biologyy and ecology of the test organism Chironomus riparius
Too study the impact of nutritional value of sediments we choose C riparius as
modell species being a well-known deposit-feeder (Rasmussen 1984). C. riparius
belongss to the dipteran family of Chironomidae which encompasses at least 15,000
differentt species. The family is the most widely distributed group of insects, having
adaptedd to nearly every type of aquatic or semiaquatic environment, ranging from large
lakess to small streams (Armitage et al. 1995, Lindegaard & Brodersen 1995, Batzer &
Wissingerr 1996, Silver Botts 1997). The midges account for most of the macroinvertebratee numbers in freshwater environments. In many aquatic habitats this group
constitutess more than half of the total number of macroinvertebrate species present.
Chironomidss are often the earliest colonizers to arrive in newly formed or disturbed
habitatss (Sheldon 1984, Layton & Voshell 1991, Batzer & Wissinger 1996).
Abundancess of certain species or species groups of Chironomidae are often used to
characterizee types of watersystems (Saether 1975, Resh & Rosenberg 1984, Johnson
1995)) and as biological indicators of water quality.
Deposit-feedingg is the most common feeding mode exhibited by chironomids.
Mostt detritivorous and tube-dwelling chironomids feed by extending the head and
anteriorr part of the body outside the tube while using the posterior prolegs to maintain
contactt with the inner surface of the tube. Therefore, foraging areas are restricted to a
regionn immediately surrounding the tube. Detritivores cope with special feeding conditions.. Firstly, the organisms mostly deal with organic matter of low nutritious value as
wass argued above. The second special condition is the high mineral particle content of
mostt sediments. Mineral particles may be ingested along with organic matter, thereby
potentiallyy reducing the nutritional value of the food intake.
Thee larvae of C. riparius are found in both lentic and lotic environments. The
speciess favours eutrophic conditions or conditions with organic loading (Armitage et al.
18 8
GeneralGeneral
Introduction
1995),, where it can reach densities upto 50,000 individuals per square meter (Köhn &
Frankk 1980, Rasmussen 1984). Larvae of C. riparius are known as "bloodworms"
especiallyy to fishermen due to their red colour. The red colour is caused by haemoglobinee which aids the chironomids to tolerate reduced levels of dissolved oxygen. Low
oxygenn levels are rarely a problem for invertebrates living on stones or submerged
macrophytess but may cause problems for species such as chironomid larvae that live in
softt sediments with a high content of organic matter. Oxygen consumption within mud
cann create sharp gradients of reducing oxygen concentration in the few millimetres or
centimetress above the sediment (Watling 1991, Heinis 1993, Armitage et al. 1995).
Thee ability of chironomids to construct tubes decreases the risk of predation by
vertebratess and invertebrates and may minimize dislodgement by currents. C. riparius is
knownn to build protective tubes from detritus, algae and other sediment particles. Particless are joined together with the larvae's saliva (Edgar & Meadows 1969). Larvae are
capablee of handling only a certain range of particle size for tube construction. Similarly,
penetrationn of a substrate by chironomid larvae is dependent of particle size distribution
(Armitagee et al. 1995, Wiley 1981a). Thus, the suitability of a substrate for chironomids
too settle in depends on the particle size distribution.
Thee life-cycle of chironomids comprises an egg stage, four larval stages, and a
pupall stage, which all live in the aquatic environment. In principal the first instar larvae
aree planktonic, while older individuals inhabit the upper layer of the sediment.
Winteringg of C. riparius occurs in the 3rd or 4th larval stage. The pupal stage lasts a few
Fourthh instar larvae of Chironomus riparius.
19 9
ChapterChapter 1
dayss and takes place in the mud. These stages of aquatic life are followed by a terrestrial
adultt stage which does not feed. Adults often emerge simultaneously and form vast
matingg clouds. After swarming and mating of male and female adults, the females
depositt egg masses at the water surface and attach them to some kind of substrate. Egg
massess may contain upto 600 eggs. At 20 °C hatching occurs within three days after egg
deposition.. In temperate regions C. riparius displays multivoltine life cycles (Groenendijkk et al. 1996) although the number of consecutive generations per year is strongly
relatedd to the water temperature (Mackey 1977).
Lifee cycle of Chironomidae displaying the egg stage, the four larval instars, the pupal
stagee and the terrestrial imago (adopted from Timmermans 1991).
20 0
GeneralGeneral Introduction
Objectivess of this study
Althoughh food is recognized as potentially important factor for macrofauna
communities,, the influence of sedimentary organic matter and its biochemical compositionn on benthic detritivores has been neglected so far. Therefore, this study aims to
clarifyy the influence of the nutritional value of sediments on benthic detritivores. Focal
pointss are the influence of sedimentary organic matter and its biochemical composition
onn the survival and growth of benthic detritivores and the interaction between particle
sizee distribution and nutritional value of sediments.
Duringg the present study efforts were made to answer the following questions:
1)) Which biochemical components of potential food influence growth and survival of
ChironomusChironomus riparius under controlled conditions and which components
influencee growth of this species in natural sediments?
2)) Do physical characteristics of sediments, which serve simultaneously as food and
habitat,, alter behaviour, growth, and survival of C. riparius!
3)) How do differences in nutritional value of sediments lead to differences in multispeciess communities of detritus feeders in the field?
21 1
ChapterChapter 1
Outlinee of this thesis
Thee present study aims to clarify the influence of nutritional value of sediments
onn detritivores. Firstly, the effects of organic matter abundance and composition on
detritivorouss invertebrates were assessed. For this purpose a set of artificial food items
weree analyzed biochemically and offered in concentration series to first instar larvae of
thee model species Chironomus riparius in standardized mineral substrate (Chapter 2).
Growthh after one week at limiting food levels and at excess of food was correlated to
biochemicall composition of the food and
keyy parameters for food limitation and
saturationn of growth were derived. The
resultss of this study were verified for the
fieldfield situation in Chapter 3. In this
chapterr sediments were sampled in the
fieldfield and growth of chironomid larvae on
thesee substrates was determined in the laboratory. The sediments were analyzed for a set
off biochemical variables, water content, and particle size distribution. Correlations were
soughtt between the biochemical variables and larval growth. The generally sub-optimal
growthh of chironomid larvae measured in natural sediments was shown not to be caused
byy physical characteristics of the substrates tested. Yet, results suggested that
indigestiblee sediment particles which were ingested indiscriminately with food reduced
thee growth potential. Therefore, Chapter 4 focused on the effect of particle size distributionn on chironomid larval growth using artificial mineral substrates. Simultaneously,
preferencee of the chironomid larvae for the particle size distribution was examined.
Becausee suitability of substrates may be expressed in preference of organisms, the
activee search strategies of the larvae for sediment substrates with different food
concentrationss was also explored in Chapter 4. In Chapter 5 the observations on the
effectt of nutritional value of sediments on the model species C. riparius is extrapolated
too the field. Biochemical composition of a number of sediments is correlated to the
densitiess of taxa, with reference to modes of feeding of the benthic invertebrates. In
Chapterr 6 the main findings of this thesis are summarized and reviewed.
22 2