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Feeding of detritivores in freshwater sediments

Vos, J.H.

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Citation for published version (APA):Vos, J. H. (2001). Feeding of detritivores in freshwater sediments. Amsterdam.

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Download date: 16 Aug 2020

CHAPTERR 1

GENERALL INTRODUCTION

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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 parti-

culatee 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 bulk-

feeders,, 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 pseudo-

faeces.. 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 poly-

chaetess 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 macro-

phytess growth, on the depth of the watersystem, and the degradation rate of phyto-

detriruss (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, deposit-

feederss mostly depend on low-quality organic matter as food bulk compared to orga-

nismss 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 sedimen-

tation,, 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 aggre-

gatess 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, detriti-

voress 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 inves-

tigatee 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 satis-

fiedd 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 compo-

nents.. 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 egg-

production,, 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 phyto-

plankton.. 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 environ-

mentall 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 littl e 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 deter-

miness 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, conse-

quentlyy 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 macro-

invertebratee 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 condi-

tions.. 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 haemo-

globinee 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. Parti-

cless 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 (Groenen-

dijkk 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 compo-

sitionn 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 multi-

speciess 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 distri-

butionn 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