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FIFTIETH ANNUAL REPORT
REVIEW ARTICLES
ORGANIC DETRITUS AND ITS ROLE AS A FOOD-SOURCE IN CHALK
STREAMS
M. LADLE
Introduction
Organic detritus consists of the products of plant and animal death and
decomposition (including droppings, cast skins etc.), and is often a
substantial part of the material carried by rivers.
At the River Laboratory of the FBA, work on the role of organic detritus
in chalk streams falls into two categories. The first of these is concerned
with the mass movements of material carried by the flowing water. The
second deals with the processing of this material, particularly as food, by
the animals of the river.
The chalk stream ecosystem (Westlake et al. 1972) included a speculative
annual budget of organic matter for a small chalk stream. At that time,
the annual net production by invertebrates was tentatively set at 130 g dry
wt m~2, or about 0.3% of the throughput of fine organic detritus. A large
question mark was assigned to these values. No consideration was given
to variation in populations of invertebrates or to the relationships between
invertebrate production and detrital food material. This review deals
with these topics.
Feeding types
The available food in the form of detritus is considered to occur in three
forms: fine particulate matter (<63 um, equivalent to the silt and clay
fractions of sediment analysis) suspended in the water; fine particulate
matter in sediment deposits; and coarse particulate matter including dead
leaves and aquatic plant fragments. The corresponding feeding types of
invertebrates are suspension feeders, such as simuliid (blackfly) and
chironomid (midge) larvae, fine-particle deposit feeders such as oligochaetes (worms), and coarse-particle feeders or shredders including Gammarus pulex (shrimp). Members of each type have been studied. These
types are more or less equivalent to the functional groups of Anderson &
Sedell (1979) and others. There are obvious limitations to such classifications. The same species of organism may span more than one feeding
type or shift from one to another in different life stages or seasons. In
addition to these detritus feeders there is a large group of grazing organisms
eating aufwuchs (organic films on surfaces). At times this material
consists mainly of living algae. Some species of detritus-feeding invertebrates may even be facultative carnivores.
REPORT OF THE DIRECTOR
31
Food sources
The relationships between food supply and rates of processing by the
animals of chalk streams changes both with time and with position in the
stream. Such changes are in accordance with the rates of input and
translocation of food materials, the population structure and abundance of
consumers, and the influences of environmental conditions on feeding
rates. These will now be considered in turn.
Coarse particulate matter originates chiefly from the leaves, flowers and
seeds of riverside trees, shrubs and herbs and from the remains of aquatic
plants (Ladle & Bass 1981). Allochthonous input (from outside the
stream) varies with the extent of tree canopy cover and can be as much as
400-500 g dry wt m-2 annually. The relative contribution of this material
generally decreases with increasing stream width (Dawson 1976a). This
source shows marked increases in autumn and winter. The biomass of
living Ranunculus penicillatus (water crowfoot)
in chalk streams attains
similar maxima of about 400 g dry wt m-2 (Ladle, Bass & Jenkins
1972). Most of the losses of Ranunculus as large suspended material also
occur in the autumn and winter (Dawson 1976b, 1981). These inputs of
large particles are available as food only when resting on the river bed or
trapped by obstructions such as growing plants.
Suspended fine particles of organic matter are present, in small quantities, in chalk-spring and borehole-source water (Baker & Farr 1977).
At first there is usually a rapid increase in concentrations as the water
passes downstream, although variations are great. Subsequently, downstream of the main groundwater inflows, there is a tendency for a decrease
in mean concentration and in the frequency and amplitude of
variations. Most fine suspended matter is moved during winter floods,
which represent only a small part of the annual timespan (Baker & Farr
1977).
Deposits of fine particulate organic matter (80-85% less than 63 um
diameter) are conspicuous in streambed sediments near the sources of
chalk streams. Much of the larger material of between 125 um and
500 (um in diameter is clearly in the form of invertebrate faecal remains,
which are often characteristic of specific taxa (Ladle & Griffiths 1980).
Apart from local concentrations of organic matter, the proportion of the
sediment which is inorganic increases downstream. The quantity of fine
organic deposits in the Bere Stream in summer and autumn was roughly
double the amount present in winter (Ladle 1972). Similar fluctuations
were noted in the Tadnoll Brook (Welton 1980).
It should be noted that chalk-stream water contains dissolved organic
matter, with dissolved organic carbon concentrations of 0.6-1.6 mg l-1
(Baker, Bartlett, Farr & Williams 1974). The importance of this material
as a food source is unknown. The throughput of dissolved organic matter
is one to three times that of suspended organic matter. Bacterial activity
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FIFTIETH ANNUAL REPORT
in the water column could not take up more than a small fraction of the
dissolved organic matter present (Baker 1981).
Consumer distribution
The food is eaten by successions of species which can be broadly
regarded as feeding on similar materials, each species being adapted to a
particular reach of stream. Thus, of the fine suspension feeders, the
blackflies Simulium spinosum and S. costatum inhabit the extreme upper
reaches of chalk streams often within a few tens of metres of the source; S.
latipes and Metacnephia amphora favour small intermittent streams (Ladle
& Bass 1975); S. vernum, S. equinum, S. lineatum and S. erythrocephalum
inhabit successive, overlapping downstream sections (Ladle & Esmat 1972;
Ladle, Bass, Philpott & Jeffery 1977); while S. ornatum andS.angustipes
are more or less ubiquitous. S. nolleri (syn. argyreatum) and S. posticatum
(syn. austeni) have specialized habitat preferences associated with lake
outflows and river impoundments respectively (Hansford & Ladle 1979;
Hansford 1978). (Fig. 1).
Fine particle deposit feeders and grazers, such as tubificid and lumbriculid worms, show similar successions. From source to lower reaches of
the River Frome the sequence is: Rhyacodrilus coccineus, Stylodrilus heringianus, Limnodrilus hoffmeisteri, Stylodrilus brachystylus (G. J. Bird pers.
comm.), Psammoryelides barbatus and Tubifex ignotus (Ladle & Bird 1980).
The shrimp Gammarus pulex, one of the major consumers of coarse
particles, is widespread, but caddis, some of which eat similar material,
are known to be longitudinally zoned. Similar distribution patterns are
demonstrable in most other groups of chalk-stream invertebrates.
Changes with time
Changes with time include variations in the numbers and life stages of
each species. The suspension feeder Simulium ornatum normally has four
successive generations of larvae annually in the Bere Stream at Bere
Heath. The annual cycles of this and other species are not constant and
may differ in different regions of the river (Ladle, Bass, Philpott & Jeffery
1977). Many species of chalk stream insects including simuliids, chironomids and Ephemeroptera (mayflies) have several generations in a
year. In summer the growth rates of these insects are comparatively high
and metamorphosis frequently takes place at a smaller size than in the
colder months (Ladle, Bass & Jenkins 1972; Welton, Ladle & Bass 1982).
Oligochaete worms appear to have less well-defined life cycles. Characteristically, prolonged periods of reproductive activity are succeeded by
several months in which only immature worms are present (Ladle 1971;
Bird 1982). Lastly, the egg-bearing females of Gammarus pulex, a
coarse-particle feeder, are present throughout the year, indicating that
breeding is more or less continuous (Welton 1979). Longevity of this
REPORT OF THE DIRECTOR
33
species ranged from 17-23 months for females and up to 30 months for
males (Welton & Clarke 1980).
Seasonal variations in discharge (flow) may modify the distribution and
abundance of invertebrate detritus feeders. This phenomenon is clearly
demonstrated in the case of the lumbriculid worm Stylodrilus, which is
replaced by tubificids as the predominant form in the gravel bed of Bere
Stream following flood disturbance (Ladle 1971).
Chalk stream invertebrate numbers and production
Ladle (1972) gives examples of the distribution and densities of some
major groups of invertebrates in chalk stream habitats. Eight major taxa
exhibited
population densities of between 1700 and 97 300 individuals
m-2. In an account of a two-year study of Waterston Stream, a small
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FIFTIETH ANNUAL REPORT
headwater, Ladle & Bass (1981)
tabulate 48 taxa which attained maximum
densities greater than 1000 m-2.
Production estimates for major groups of detritus feeders include a value
of 5.5 g dry wt m-2 for Simuliidae and one of 12.9 g dry wt m-2 for
Gammarus pulex (Ladle, Bass & Jenkins 1972; Welton 1979). Because of
the problems of interpreting life histories it is difficult to obtain meaningful
production figures for tubificid worms, but a series of biomass estimates
based on over 200 core samples from the Bere Stream over a period of 15
months ranged from 8.5 to 12.5 g dry wt m-2.
Feeding rates and digestion
Estimates of detritus ingestion rates have been obtained by a variety of
field and laboratory methods. The results of all approaches are difficult
to evaluate because interference with either food or conditions of feeding
are liable to affect ingestion rates. Suspension feeders are comparatively
easy to study in the field by means of simple marking techniques involving
a brief introduction of charcoal powder to the flowing water (Ladle, Bass
& Jenkins 1972). There is little risk of misinterpretation because the
measurements are made on animals feeding essentially on natural concentrations of suspended particles. Deposit feeders and coarse-particle feeders are more difficult to study but the application of both field and
laboratory methods provides the following ranges of2 maximum
estimates
1
of food ingested: Simuliidae
13-75
&
dry
wt
mday,
Oligochaeta
43-162 g dry wt m-2 day-1 and Gammarus pulex 2-13 g dry wt m~2 day--1
(Ladle, Bass & Jenkins 1972; Ladle & Griffiths 1981; McCall & Fisher
1980; Welton 1979; Sutcliffe, Carrick & Willoughby 1981).
Little is known of digestive processes in invertebrate detritivores although it is apparent that, at the very least, those elements of ingested
food which require little or no digestion prior to assimilation, will be
utilized. In theory, studies on the chemical composition of food together
with information on the digestive capabilities of the animals, should
provide conclusive answers regarding the quantitative role of detritus as
food. In practice it is much more difficult to separate true detritus from
the living organisms associated with it. Even the relatively large protozoa
and the young stages of large invertebrates are not easily distinguished;
total separation from the detritus therefore is difficult.
Few studies on the utilization of the microbial component of organic
detritus as food have been attempted, but a number of assumptions
concerning the nature of ingested food can be made. Firstly, material
ingested from suspension in the water is likely to be more homogeneous
than that grazed from surfaces or fine sediment deposits, because of the
well-mixed nature of the turbulent water. This is borne out by the studies
of Baker & Bradnam (1976) and Bradnam (pers. comm.) who found that
Simulium (a suspension feeder) showed a more consistent relationship
REPORT OF THE DIRECTOR
35
between the fore and hind regions of the midgut in the nature of the
microbial content of the food. In contrast the results from grazing
organisms (e.g. Ephemerella ignita) and shredders (e.g. Gammarus pulex)
were more variable, probably indicating a patchy intake (Fig. 2).
The actual gain of material from ingestion of bacteria is insufficient for
the energetic requirements of most of the feeding organisms although some
nutrients may be supplied largely by bacteria. Of those species so far
examined, only Chironomus appears likely to find bacteria an adequate
source of food. Much more work is needed before the sources of food
which are actually used by important chalk stream detritivores can be
resolved.
Conclusion
There is no evidence to suggest that the invertebrate production guesstimate (130 g m-2 year-1) given by Westlake et al. (1972) was much in
error. In view of the enormous variation between times and places, it
would require a massive research effort to make effective comparisons of
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FIFTIETH ANNUAL REPORT
even a few chalk-stream sites. Nonetheless it is only by such comparisons
that we can assess how river communities are adapted in terms of their
composition, development and production to exploit the available food
resources.
It is clear that the total quantity of detrital material processed by
invertebrate consumers is many times greater than the production of these
consumers. In fact the amount of detritus, associated algae, protozoa and
microorganisms ingested each year by chalk stream invertebrates may well
be similar to the annual input of autochthonous primary production plus
that from allochthonous tree cover.
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